Featured

Teal Acorn WristTwist & Tree

Price: £15.00
View Product

Teal Acorn WristTwist & Tree

Our Trees:Tree Species

In 1996 Trees for Life purchased Plodda Lodge.

Plodda Lodge is near the village of Tomich, just outside Glen Affric, and within the Trees for Life 600 square mile target area.  This large three bedroom house is located in a quiet secluded area surrounded by trees, and was built in the 1850s as a laundry for the Guisachan Estate. Its remote location in the forest means that it is ideally suited for use as a field base for our increasing practical work in the area, and the house will play an important role in the future development of our project. It was purchased with the aid of a substantial donation from one of our members, financial assistance from the Chris Brasher Trust and funds raised by our appeal in February 1995.

Neil Armstrong and Jill Hodge are our staff members based at Plodda Lodge - Neil is the field base warden and Jill is the tree nursery manager. Conservation Holidays groups are based at Plodda in the spring and autumn each year, and they help with work in the nursery, as well as carrying out restoration work in the Caledonian Forest in nearby areas such as Glen Affric and Corrimony.

Aspen propagation unit and tree nursery

Trees for Life has now established an aspen propagation unit in a polytunnel at Plodda lodge, complete with propagation equipment. This new facility, together with the increased number of sites to collect root sections from (following the near completion of an exhaustive survey of aspen stands in Glens Affric, Cannich and Moriston) should result in many more young aspens being available for planting in the years ahead.

 

 

Here is a list of the tree species Trees for Life work with in the Caledonian Forest.  At any time, our planting could include any one of these species:

 

1. Alder

2. Aspen

3. Birch (silver birch and downy birch)

4. Bird cherry

5. Dwarf birch

6. Eared willow

7. Hazel

8. Holly

9. Juniper

10.  Oak

11.  Rowan

12.  Scots Pine

 

Please see below for detail on each species.  Information has been given to us from Trees for Life, our tree planting partners in the Caledonian Forest.  All the links embedded in the copy and highlighted in green send you to the Trees for Life website.

 

If you would like to know specifically what species of tree we are planting on any given week, please do not hesitate to contact us on info@treetwist.co.uk


1.  Common or black alder

(Alnus glutinosa)

Alder is the most common tree in riparian forests, and it improves soil fertility through its ability to fix nitrogen from the air.


Alder tree beside the Abhainn Gleann nam Fiadh stream in Glen Affric, showing the typical river-edge habitat of this species.

Worldwide distribution

Common or black alder occurs throughout most of Europe and across Russia to Siberia. Its range also includes the Caucasus, Iran, Turkey and North Africa, where it is native to Tunisia, Algeria and Morocco. It has been introduced to North America, probably during colonial times, and has become naturalised in eastern Canada and the northeast of the USA.

Distribution in Scotland

Alder is found throughout Scotland, although its presence in Shetland is due to planting. Today it is rare in the Outer Hebrides and the far northwest of the mainland, but this is the result of deforestation, rather than a limit in the tree's range. Alder occurs at elevations of up to 500 metres and typically grows alongside streams and rivers, or on wet ground. As a result it is often found in narrow bands along watercourses, although in the right conditions it can form alder carr, which is a dense thicket or stand of alder, usually small in height, growing on wet swampy soils.

Physical characteristics

Drawing of alder cones and leaves

Alder is a member of the birch family of trees, Betulaceae, and can reach 25 metres in height, although in Scotland it is rarely more than 20 metres tall. Like the birches, it is a pioneer species which grows quickly and is relatively short-lived, with the maximum age typically being 150 years. The annual rate of growth can be up to 90 cm. a year when the tree is young, and after the death of the original trunk, new shoots can sprout from the base, forming a multi-stemmed clump of new growth.

Alder bark is dark grey and fissured, while the young branches are smooth and slightly green in colour. Branches are ascending in their growth form, and trees with two or three main trunks are common. The twigs are sometimes sticky to the touch (which gives rise to the second part of its binomial name, glutinosa, which means sticky), and the buds are 7 mm. in length. Alder is deciduous, and the new leaves open out in April. They are up to 10 cm. in size with a waved margin and are almost circular in shape, sometimes with a notch at the apex, and tapering to a slight point on the leaf stem. The leaves have shallow irregular teeth and remain on the tree quite late in autumn. However, they do not change to any dramatic colours, but instead just darken and begin to wither before being shed in late October or November.


Male catkins and female flowers of alder



Ripening cones and leaf of alder, in late summer.

Alder is monoecious, so each tree bears both male and female flowers. Male catkins are dark yellow-brown in colour, and are up to 5 cm. long when they are fully open. At 6 mm. in length, the female flowers are much smaller in size, and are red, erect and cone-like in shape. The flowers appear before the new leaves, in March (or early April in the Highlands), and pollination is by the wind. Pollinated female flowers grow into ovoid fruits about 1.5 cm. in length, which are green in colour and grow in clusters of up to 4 at the end of twigs. These ripen and turn woody by October, and release a number of small flat red-brown seeds, each weighing about 0.004 gm. The seeds have small 'wings', which are air-filled membranes that enable them to float on water, and dispersal is by both wind and water. Seeds have been recorded as germinating on the surface of water, and then rooting successfully when they are washed up on land. The empty cones can persist on the tree until the following spring and are a distinctive feature of the alder tree in winter.

Ecological relationships of alder

Alder is the most common tree species in riparian forests, and it plays a crucial role in these stream- and loch-edge woodlands. As a deep-rooted species, it helps to maintain the soil in river banks and reduces the effects of erosion. Alder root systems which are exposed in the water give shelter to fish during times of high water flow, and provide a safe refuge from predators. Alder foliage provides shade which is beneficial to fish, including salmon (Salmo salar) and brown trout (Salmo trutta), and its leaves, which are relatively quick to decompose in water, provide nutrients for invertebrates such as the larvae of caddisflies, stoneflies and water beetles. These in turn form part of the aquatic food web, and are eaten by larger organisms, including salmonid fish.

Alder is particularly noted for its important symbiotic relationship with a bacterium (Frankia alni), which forms nodules on the tree's roots. This nitrogen-fixing bacterium absorbs nitrogen from the air and makes it available to the tree, with the rate of fixation estimated at up to 125 kg. of nitrogen per hectare per year. Alder, in turn, provides the bacterium with carbon, which it produces through photosynthesis. As a result of this mutually-beneficial relationship, alder improves the fertility of the soils where it grows, and as a pioneer species, it helps provide additional nutrients for the successional species which follow.

A mature alder supports a variety of moss and lichen species on its bark and branches. Because it grows by rivers and streams, where there is often a higher humidity in the air due to spray, moisture-loving lichens such as tree lungwort (Lobaria pulmonaria) are relatively common on alder. At least one lichen species (Stenocybe pullatula) only occurs on alder, while another (Menegazzia terebrata) is more common on alder than on any other tree species.

47 species of mycorrhizal fungi have been recorded as growing with alder, and in the symbiotic relationships formed by these fungi and the tree, both the partners benefit through an exchange of nutrients, which each organism cannot access directly itself. Notable species forming this relationship with alder include a rare Russula (Russula pumila), two milk caps (Lactarius obscuratus and L. cyathula) and the brown roll-rim (Paxillus filamentosus), all of which are restricted to alder, and several species in the genus Naucoria, which are mainly restricted to alder. The fruits of the brown cup fungus (Ciboria amentacea) appear in spring, on alder catkins after they have fallen to the ground. In recent years, a rare fungus (Taphrina amentorum) which produces curved, tongue-like galls on alder cones, has been discovered in Britain. Previously known only from mainland Europe, this species grows in the flowers and seeds of young alder catkins, before producing the galls, which harden as the cones ripen.


These galls on the upper surface of an alder leaf were induced by the mite (Eriophyes laevis inangulis).

Galls are also induced on the leaves of alder by a mite (Eriophyes laevis inangulis), and these take the form of raised pustules on the upper surface of the leaves. The galls vary in colour from pale yellow-green to deep red, and the mite itself lives on sap, which it sucks from the cell tissue of the tree.

Over 140 phytophagous (ie plant eating) insects have been recorded on alder. These include the striped alder sawfly (Hemichroa crocea) and another sawfly (Cimbex connatus), although the latter is rare in the UK. In comparison to birches (Betula spp.), there are relatively few moths in Scotland which feed exclusively on alder, but the May highflyer (Hydriomena impluviata) is specific to alder, and its larva lives in a shelter which is formed by two leaves that have been sewn together by silk. Another moth whose larva is a specialist feeder on alder is the dingy shell (Euchoeca nebulata), while 4 species of micromoths in the genus Phyllonorycter make blister mines on alder leaves. In England a wider range of moths are associated with alder, including the alder kitten moth (Furcula bicuspis), but that species is absent from Scotland.

As with most tree species in Scotland, alder is browsed upon by red deer (Cervus elaphus), and this prevents the natural regeneration of the tree in many parts of the Highlands, while domestic sheep have a similar effect throughout the country. However, other potentially more serious threats to the survival of alder have recently been noted in the UK. One is a fungus (Phytophthora sp.) which grows upwards from the bottom of the tree, killing the roots and bark. This has become a widespread problem in England and Wales, where over 10% of riparian alders are either dead or infected with the fungus, but there have only been a few confirmed reports of this in Scotland so far.

 

The other problem affecting alders is crown dieback, which results in the tree dying from the top downwards. This condition was first noted in the northwest of Scotland in the 1980s and has subsequently spread throughout the Highlands: in some riparian areas of Glen Affric, for example, most of the alders are affected. The cause of crown dieback is still unknown, and research into the problem is ongoing.

Because it is the most common tree in riparian woodlands, which often form the ecological linkages between different forest patches, alder is an important species in the Caledonian Forest, and its survival and expansion is essential to the health of the land and rivers alike.

2.  Aspen

(Populus tremula)

Characterised by its shimmering foliage in summer, the aspen tree is a beautiful but rare feature of the Caledonian Forest.

 Distribution

European aspen is one of the most widely distributed trees in the world, with a natural range that stretches from the Arctic Circle in Scandinavia to North Africa, and from Britain across most of Europe and north Asia to China and Japan.

Distribution in Scotland

Although aspen occurs throughout Britain, it is most common in the north and west of Scotland, and is also known from Shetland and the Hebridean Islands. In the Highlands it will grow at elevations of up to 550 metres, and in the Glen Affric area it is most often found on rocky slopes or cliffs with a southerly aspect. However, because it can tolerate a wide range of soil types and climatic conditions, it is likely that its present distribution is due to the effects ofdeforestation, with these sites representing the only places where it has been able to survive out of the reach of grazing animals.

Physical characteristics

In good situations, aspen can reach a height of twenty metres, but in most locations in the Highlands it will only grow to about ten metres tall. The bark is grey, or sometimes greenish-grey, and is either smooth or in some cases is pitted with diamond-shaped lenticels. On old, mature trees the bark is often covered with a dark-coloured lichen (Collema nigrescens), which gives the trunk a black appearance. Aspen has a distinctive branching pattern, which is most visible in winter when the tree is leafless, and in mature trees the topmost branches are often bent over horizontally.

Aspen leaves are round, measuring between 2.5 and 6 cm across, and have irregular blunt teeth on their margins. On young aspen suckers, or ramets, the leaves are usually elongated and triangular in shape. The leaf stalks, or petioles, of aspen are flattened (on other trees they are round in cross-section) and very flexible near the leaf blade. This gives rise to the characteristic fluttering of its leaves in the slightest breeze. When the leaves first open in spring, they are a distinctive coppery colour, before turning green. In autumn, the leaves turn a brilliant yellow, or in some cases red, with each separate aspen clone having its own individual colouration.


Female catkins


Male catkins

Aspen is dioecious, so individual trees are either male or female (in contrast to most trees, such as Scots pine, for example, where male and female flowers occur on the same tree). Trees flower in March and April, before the leaves appear, with both the male and female trees producing catkins. Pollinated female catkins ripen in early summer and release tiny seeds (each weighing about one ten-thousandth of a gram!) which are tufted with hairs. Seed production by aspen rarely occurs in Scotland. The reasons for this are not fully understood, but the fragmented and scattered distribution may be a factor, restricting the scope forpollination between male and female trees. The occurrence of appropriate climatic conditions for seed development may also play a part.

However, even in other parts of its range where it is more abundant, aspen's main method of reproduction is vegetative, with new suckers, or ramets, growing off the roots of mature trees. The numbers of new shoots produced in this way can be very prolific, especially after a majordisturbance such as fire, with the density of ramets reaching 70,000 per hectare. Aspen has an extensive root system, and ramets have been recorded growing up to 40 metres from a parent tree. Because of their access to nutrients through the parent tree's root system, aspen ramets can grow very quickly - up to a metre per year for the first few years. As the ramets grow, they remain joined through their roots, and all the interconnected trees are called a clone. They are all the same individual organism and are therefore all single-sexed, either male or female. Each clone exhibits synchronous behaviour, with, for example, all the component trees coming into leaf at the same time in the spring. A clone can also sometimes be identified by the specific colour its leaves change to in the autumn.

Aspen roots also have the unusual ability of staying alive underground for many years after the death of the parent tree. This leads to the appearance of aspen ramets in areas where there are no mature trees, and there is a good example of this at a site on the north shore of Loch Beinn a'Mheadhion (L. Benevean) in Glen Affric.

Like birch (Betula spp.) and rowan (Sorbus aucuparia), aspen is a pioneer species - it is fast growing and regenerates profusely after disturbance. As in other pioneer species, an individual tree is short-lived, surviving for perhaps only 50-100 years, but the clone to which it belongs will live for much longer than this, if vegetative reproduction takes place. Little research has been done on the age of clones for European aspen, but in North America scientists studying the closely-related trembling aspen (Populus tremuloides) have concluded that individual clones can survive for 10,000 years or more, making them possibly the longest lived organisms on the planet.

Ecological relationships of aspen

In comparison with other Caledonian Forest trees such as Scots pine (Pinus sylvestris) or birch, aspen has comparatively few mycorrhizal associations with fungi - just nine species are known to have this mutually beneficial relationship. One of these, Leccinum aurantiacum, a member of the Bolete group of fungi, only occurs in association with aspen. The ground vegetation where aspen grows is usually characterised by various grass species and bracken (Pteridium aquilinum), although it also occurs in areas where heather (Calluna vulgaris) and blaeberry (Vaccinium myrtillus) predominate. Three species of lichen commonly occur on aspens: Collema nigrescens (see above), Xanthoria parietinaand Physcia ciliaris.

Over 60 species of insect have been recorded as feeding on aspen foliage in Scotland. These include two gall midges(Harmandiola globuli) and (Syndiplosis petioli) which are restricted to aspen. Recent research has also shown that seventeen insect species are associated with dead aspen wood, including the aspen hoverfly (Hammerschmidtia ferruginea) which is classified as endangered and is restricted to mature aspen stands greater than 4.5 hectares in area. Amongst the other saproxylic insects (those which feed on dead and decaying wood) which are specifically dependent on aspen in Scotland, is a newly identified species of fly, Ectaetia christiei, and two other endangered fly species,Homalocephala biumbratum and Strongylopthalmyla ustulata.

Because of its current, limited distribution in Scotland, there is little information available on the role which aspen plays for bird and mammal species here. However, its foliage is readily eaten by red deer (Cervus elaphus), and overgrazing in this way is the main factor preventing natural regeneration in existing stands of aspen. In Scandinavia, where aspen is more abundant, it is reported as being an important species for birds such as woodpeckers, which nest in holes in trees. It is also an important tree for theEuropean beaver (Castor fiber), which feeds extensively on aspen, particularly in winter. The proposed reintroduction of beavers to Scotland in the near future adds another important reason for the instigation of measures to regenerate and restore aspen, a tree which formerly flourished to a much greater extent in the Highlands.


3.  Birch

Silver Birch (Betula pendula)
Downy Birch (Betula pubescens)

Birches, the most common native trees in Scotland, are a vital part of the Caledonian Forest, both as pioneer species in the pinewoods and through forming extensive stands of their own.

 

Birch in autumn

Silver birch in autumn, showing the pendulous shape of its branches

Distribution

Silver birch is distributed throughout almost all of Europe and in Asia Minor. Downy birch also occurs throughout much of Europe and in north Asia, and is one of the very few native trees in Iceland.

Distribution in Scotland

Both species of birch are widespread in Scotland, with silver birch occurring principally on well-drained, drier soils and downy birch preferring wetter locations. In many parts of the country, this results in a longitudinal transition, with silver birch in the east giving way to downy birch in the west.

Birches occur within other forest types, such as pine (Pinus sylvestris) and oak woods (Quercus spp.), and as largely monospecific stands, or birchwoods, in which they are the predominant trees. Birches are the most common native tree in Scotland. However, prior to the deforestation of most of the country by humans, when much larger areas of Scots pine andoak forests flourished, it is likely that birch was proportionately less abundant than it is today.

Physical characteristics

Both species of birch are fast-growing pioneer trees which readily colonise open ground. Silver birch is the faster growing of the two, and also the taller, reaching a height of up to 30 metres, whereas downy birch seldom exceeds 21 metres. As pioneer species, they are short-lived, with typical lifespans being between 60 and 90 years old, although some individuals can live up to 150 years. The trees are slender, with their trunks not normally exceeding a diameter of 40 cm. at breast height.

Birches in the mist

Silver birch trees in fog near Badger Falls in Glen Affric

In young trees the bark is reddish-brown, but this changes to white as they mature. The white bark is most prominent on silver birch, where it is interspersed with conspicuous black patches. By contrast, the bark of downy birch is more greyish-white, with horizontal grooves on it. On old silver birches the bark can become corklike and deeply fissured, with parts covered by large colonies of the yellow foliose lichen, Candelaria concolor.

Birches are deciduous, and before their new leaves appear in spring the twigs and buds exhibit a characteristic reddish-purple colour, which is especially apparent after rain. The new leaves emerge in April and are bright green at first, with the colour darkening to a duller green after a week or two. The colour changes to yellow or brown in autumn, with the colours becoming more intense after sharp frosts. Silver birch leaves tend to turn a brighter yellow than those of downy birch, which are usually dull or brownish. The leaves are dropped at the end of October or early November, although this can be earlier, and the appearance of the new leaves in spring later, at higher elevations, where the climate is harsher.

Silver and Downy birch leaves

Top: Silver birch leaf
Bottom: Downy birch leaf

The two species can be distinguished by their leaves, with those of downy birch being rounder in shape than silver birch, and having a single row of teeth on the leaf margin, in contrast to the double row of teeth on silver birch leaves. They can also be identified by their twigs, which in silver birch have small white warts, whereas those on downy birch are covered in small hairs or 'down'. In general, silver birch has an overall drooping, pendulous shape to its branches, which gives rise to its specific name, whilst the branches on downy birch tend to be more upright in their growth form. However, intermediate forms exist between the two species, with various combinations of these characteristics, and this can make the identification of individual trees difficult.

Birches are monoecious, meaning that each individual tree has both male and female flowers. The trees can begin flowering when they are as young as 5 - 10 years old, and the flowers appear in the spring at the same time as the new leaves. The male flowers are drooping catkins, up to 3 cm. in length, whilst the female flowers are upright and 1.2 - 2 cm. in height. Pollination is by wind, and the female flowers ripen to form hanging catkins up to 3 cm. long in late summer or early autumn. The catkins contain hundreds of tiny seeds, each a two-thousandth of a gram in weight and having 2 transparent wings, which facilitate their dispersal by the wind. A large tree can produce up to 1 million seeds in a year, but only a few of these will germinate and grow into mature trees. The majority of seedling trees become established within 100 metres of their parent, but some seeds can travel long distances in the wind.

Ecological relationships of birch

As pioneer species, one of the important functions which birch trees fulfil in ecosystems is that of improving soils. They are deep-rooted, and their roots draw up nutrients into their branches and leaves, which the trees use for their growth. Some of these nutrients are returned to the surface of the soil each year when the leaves fall in the autumn, thereby becoming available for other organisms in the forest community. An indication of the scale and significance of this nutrient cycling can be drawn from the estimate that birch trees will produce between 3 and 4 tonnes of leaf litter per hectare per year. In an undisturbed forest ecosystem, birches would be replaced by slower growing species such as oak and Scots pine, but in Scotland today this successional process has been interrupted in most places by human exploitation of the land.

Tinder fungus on dead birch

tinder fungus (Fomes fomentarius) growing on a dead silver birch trunk

The roots of birch trees have mycorrhizal associations with various species of fungi. In these mutualistic or symbiotic relationships between trees and fungi, both partners in the association benefit from their interactions. One of the best known fungi associated with birch trees is the fly agaric (Amanita muscaria), while two edible boletes (Leccinum scabrum, L. versipelle) and the chanterelle (Cantharellus cibarius) also form mycorrhizal relationships with them. The names of some other fungi reflect a similar association with birches - Russula betularum and Cortinarius betuletorum - while the polypores Lenzites betulina and Piptoporus betulinus grow on the wood of dead birches. Another polypore, thetinder fungus (Fomes fomentarius) also grows on dead birches, with its hard, wood-like fruiting bodies reaching a large size and persisting for many years. These polypores all help to break down the tough cellulose of the wood, and by doing so make the nutrients in it available for other organisms. Not all fungi have mutually beneficial relationships with trees, however, and the witches' broom fungus (Taphrina betulina) is parasitic on birch trees, causing an abnormally dense growth of small twigs, which radiate from a point on a branch.

A number of different flowers are associated with birchwoods, including primroses (Primula vulgaris) and violets (Viola riviniana) which flower in early spring, before the trees' new leaves limit the light reaching the forest floor. Other common flowers in birchwoods include bluebells (Hyacinthoides non-scripta), wood anemones (Anemone nemorosa) and wood sorrel (Oxalis acetosella). Birch trees also provide the shade for typical understorey plants of the Caledonian Forest, such as blaeberry (Vaccinium myrtillus) and cowberries (Vaccinium vitis-idaea), to flourish on the forest floor beneath them.

Catkins

Leaves and male flowers (catkins) of silver birch in spring

Birches support a large community of insects and other invertebrates, with 334 species known to feed on them - more than any other trees native to Scotland, except for oaks and willows. These include the caterpillars of the pebble hook-tip moth (Drepana falcataria) and the Kentish glory moth (Endromis versicolora). The invertebrates in turn are food for various bird species, whilst other birds such as the siskin (Carduelis spinus) feed on the seeds in autumn.Red deer (Cervus elaphus) eat the leaves of young birches, and their grazing is the main limiting factor which has prevented the return of the birches, and other Scottish native trees, to more of their original habitat in the Highlands.

Now, however, with all the regeneration measures initiated for the Caledonian Forest since the mid-1980s, birchwoods are once again expanding. This means that not only are these trees recovering more of their former territory, but also that all the species which depend on them have an opportunity to flourish once more in greater numbers.


4.  Bird cherry

(Prunus padus)

Bird cherry in flower

Although it is relatively scarce, bird cherry’s spectacular spring flowering display makes it one of the most 
flamboyant trees in the Caledonian Forest.

Worldwide distribution

Bird cherry has a wide geographic distribution in Europe and northern Asia, and has also been reported from Morocco in North Africa. Its range in Europe extends from northern Scandinavia, where it reaches the shore of the Arctic Ocean, to the northwest of Italy and the mountains of Spain and Portugal, although it is absent from the Mediterranean coastal region and the west of France. It occurs from Ireland and the UK eastwards to Croatia and Bulgaria, and then through northern Russia, the Caucasus region and the Himalayas to western Siberia.

Bird cherry in flower.

Bird cherry in flower in the birchwood on 
the lower slopes of Binnilidh Bheag on Dundreggan.

Bird cherry has been introduced to North America, where it occurs in Alaska and parts of the northeastern USA and eastern Canada, but it is not considered to be a problematic or invasive species there.

Distribution in Scotland

Bird cherry is common and widespread in Scotland, occurring throughout the mainland, although it is scarcer in the mountainous core of the Highlands and in the far southwest. It is found on Skye and Mull, but is absent from the Outer Hebrides and Orkney and Shetland.

Bird cherry grows at elevations of up to 600 metres in the UK, but does not reach so high in Scotland’s mountains, because of the harsher climate. South of the border, it is common in northern England, Wales and parts of East Anglia, but is absent from the south of the country. Its close relative, the wild cherry or gean (Prunus avium), is the only other cherry native to the UK, and has a similar distribution, but also occurs in the south of England.

The preferred habitat of bird cherry is damp, base-rich soils or on limestone, and it often occurs in wet flushed areas, frequently with alder trees (Alnus glutinosa). It also grows alongside roads and paths, and is usually seen growing amongst other native broad-leaved trees such as birch (Betula spp.) and rowan (Sorbus aucuparia).

Physical characteristics

Bird cherry flowers.

Flower detail of a bird cherry, showing a raceme and the individual blossoms.

Bird cherry is a relatively small deciduous tree in the Rosaceae family, and in Scotland it is also known as hagberry or hackberry, with its fruits being called ‘hags’. It can grow to a height of 15 metres, but in Scotland it’s usually only up to 10 metres tall. It branches readily, forming a spreading and sometimes multi-trunked shape, and has smooth grey-brown or dark grey bark.

The leaves grow in an alternate pattern on the branches, and are 6 – 10 cm. long. They are lanceolate (meaning that they are elliptical in shape, tapering to a pronounced point at the end) and are finely toothed or serrated on their margin. In the Highlands, the new leaves appear in late April and are dark green on the upper surface and slightly lighter in colour on the underside. There are two small nectar glands on the petiole, or leaf stem, at the base of the leaf, and these exude a chemical that attracts beneficial insects, in response to attacks by pests such as greenfly.

Bird cherry buds on twig.

The tree’s flowers open from greenish-yellow buds in May or early June and grow on upright spikes or racemes that are 15 cm. long. The racemes don’t always stay upright for long though, and each one contains up to 35 individual blossoms. The flowers have 5 white petals, with pale yellow stamens, are 1 cm. in size and smell of almonds. Pollination is carried out by insects such as bees, butterflies and beetles that are attracted by the fragrance of the blossoms.

After pollination, fertilised flowers develop into fruits that are spheroid in shape and 8 mm. in diameter. These ripen from red to black in August and contain a hard stone or pit, surrounded by fleshy fruit that is very bitter. The seeds are dispersed by birds that eat the fruit, and by small mammals that consume fallen fruits.

In October the leaves turn a yellow-orange colour before being shed, and the sap is drawn downwards, as the tree enters a state of dormancy for the winter. Bird cherry can live up to an estimated maximum of 200 years, but most trees in the Highlands today are younger than that.

Fruit on a bird cherry on Dundreggan.

Fruit on a bird cherry on Dundreggan in late summer.

Ecological relationships of bird cherry

Unlike most other trees, bird cherry has relatively few fungi that grow in mycorrhizal association with it. There are however a number of inconspicuous fungi in the order Glomerales that form a vesicular-arbuscular mycorrhizal or endomycorrhizal association with bird cherry, in which the fungal hyphae penetrate the cell walls of the tree’s roots, so that an exchange of nutrients takes place. This symbiotic relationship provides benefits to both organisms, with the tree gaining nutrients that the fungus accesses in the soil, while the fungus in turn receives sugars and carbohydrates that the tree produces through the process of photosynthesis.

Multi-stemmed trunk of a bird cherry on Dundreggan, in spring.

Multi-stemmed trunk of a bird cherry on Dundreggan, in spring.

Other fungi occurring with bird cherry include coral spot (Nectria cinnabarina), which grows on dead twigs, and saprotrophic, or dead-wood decomposing, species such as turkeytail fungus (Trametes versicolor) and candlesnuff fungus (Xylaria hypoxylon). Bird cherry is particularly susceptible to infection by honey fungus (Armillaria mellea), a parasitic species that weakens and kills its host.

In the UK, at least 30 species of insects have been recorded in association with bird cherry, including 10 different moths and 10 species of beetles. Amongst the beetles is a blossom-feeding weevil (Anthonomus rectirostris) that has been found on Dundreggan. Of the moths that occur on bird cherry, the most common is the bird-cherry ermine (Yponomeuta evonymella). The larvae or caterpillars of this species feed on the leaves inside communal silken tents that they weave, and can reach pest proportions, completing stripping a tree. However, the caterpillars are parasitised by the larvae of a predatory sarcophagid fly (Agria mamillata), and this acts as a natural control on their population in some cases.

Adults of the aspen hoverfly (Hammerschmidtia ferruginea), a rare saproxylic insect that breeds in the wood of dead aspens and is a Priority Species in the UK’s Biodiversity Action Plan (BAP), are thought to feed on the flowers of bird cherry. This hoverfly only occurs at a few sites in Scotland, so the presence of bird cherry in close proximity to aspensmay be important for its survival.

Galls induced by a mite on the leaf of a bird cherry on Dundreggan.

Galls induced by a mite (Phytoptus padi) on the leaf of a bird cherry on Dundreggan.

Galls, in the form of raised pustules, are induced on the leaves of bird cherry by the larvae of a mite (Phytoptus padi), while the bird cherry-oat aphid (Rhopalosiphum padi) induces galls that cause the leaves to curl and wrinkle. This aphid over-winters on bird cherry but the adults can then become a serious pest of cereal crops such as barley and wheat in the summer. Galls are induced on the fruit of bird cherry by a fungus (Taphrina padi). These are known as pocket plum galls, and the affected fruits become elongated and distorted in shape.

The insects such as beetles and bees that pollinate bird cherry benefit from the nutrients in the flowers’ nectar, while at the same time providing the essential service of pollination to the tree. Despite their bitter taste, the fruits of bird cherry are eaten by birds, especially robins (Erithacus rubecula), starlings (Sturnus vulgaris) and members of the thrush family (Turdus spp.), and are relished as some of the earliest fruit to be available in the summer. While the birds benefit from this food, the tree also gains, by having its seeds dispersed in the birds’ droppings.


5. Dwarf birch

(Betula nana)

Dwarf birch is a little-known but important component of the montane shrub community in the Highlands.

 

Dwarf birch in flower

Dwarf birch with female flowers

Worldwide distribution

Dwarf birch is circumpolar in distribution - it occurs in Greenland and Iceland and in the northern latitudes of Europe, Asia and North America. Two distinct subspecies are recognised: Betula nana ssp. nana, which grows in western coastal Greenland, Iceland, Scotland, Scandinavia, the Alps and across northern Asia to western Siberia; and Betula nana ssp. exilis, which is found in Siberia, Alaska, northern Canada and Greenland. The principal difference between these two subspecies is that exilis has shorter leaf blades thanssp. nana. In Scandinavia, Betula nana hybridises naturally with downy birchBetula pubescens, to form B. pubescens ssp. tortuosa, which has a similar habit and growth pattern. Another closely related species of dwarf birch, Betula glandulosa, occurs in Alaska, northern Canada and Greenland.

Distribution in Scotland

Dwarf birch is restricted to the Highlands in Scotland, and it occurs from Perthshire northwards through the Cairngorms and the hills of Inverness- and Ross-shire to the Flow Country of Caithness and Sutherland. It grows in areas with wet and cold climates, and at altitudes ranging from as low as 120 metres up to 835 metres. Further south in Britain, there are two outlying populations in northern England, in the North Pennines and at Kielder in Northumberland.

Over-grazed dwarf birch

Overgrazed dwarf birch on the Wester Guisachan Estate in Glen Affric

In Scotland, dwarf birch is classified as being nationally scarce, and it exists in scattered pockets throughout its range, usually in a considerably suppressed form, because of overgrazing by deer and sheep.

Formerly it was much more abundant within its range, but it has declined because of deforestation, overgrazing and burning of heather moorland.

Surveys carried out by Trees for Life volunteers have found that it is quite widely but patchily distributed on the hills between Glen Affric and Glen Moriston. Outside the UK, dwarf birch commonly forms thickets and extensive stands.

Physical characteristics

Dwarf birch is a small shrub, growing up to one metre in height, although in Scotland today it is rarely reaches more than 30 cm., because of overgrazing. Its branches or twigs are a dull dark brown colour and are quite stiff. As with downy birch, the twigs are covered in a fine down.

Detail of dwarf birch leaf Dwarf birch with male catkins Dwarf birch with female flowers

The leaves are small, reaching a maximum width of 2 cm., round in shape and have a few large, rounded and regularly-spaced teeth on their margins. The top surfaces of the leaves are dark green in colour, while the undersides are a slightly lighter shade.

Like other birches, dwarf birch is deciduous and the new leaves appear in spring, in April or early May, and turn a dull yellow colour before being shed in October.

Dwarf birch is monoecious, so each individual plant has both male and female flowers. The male flowers are catkins up to 0.8 cm. long, which are erect when young, but then droop as they mature. The female flowers are slightly larger and broader, and are pollinated by the wind, becoming cone-like in appearance as they mature.

Ctakin scale, female flower and seed

Top: Catkin scale 
Middle: Detail of female flower
Bottom: Dwarf birch seed, with crescent-shaped wings

The tiny seeds, each weighing less than 0.2 milligrammes, ripen by September or October and have a small crescent-shaped wing on each side of them.

The seeds are dispersed by the wind and usually germinate in mid-May in Scotland, triggered by rising temperatures and increasing daylight. The initial growth rate is very slow and seedlings reach a maximum height of 3 cm. in their first year, with 4-6 true leaves.

Dwarf birch can also reproduce vegetatively, with new shoots growing from underground root sections (which have been recorded in Greenland as being up to 147 years old), or via adventitious roots which grow off stems that have been overgrown by moss.

In Scotland dwarf birch grows exclusively on blanket peat, where the soil conditions are typically wet, acidic and lacking in nutrients. Elsewhere in its range, however, it also occurs in more rocky sub-arctic and alpine locations, growing on steeper slopes and where the soils are better drained.

The vegetation community in which dwarf birch most commonly occurs in the Highlands is that characterised by heather (Calluna vulgaris) and hare's tail cottongrass (Eriophorum vaginatum). Other plants commonly found in this community include blaeberry (Vaccinium myrtillus), cowberry (Vaccinium vitis-idaea), feather moss (Hylocomium splendens) and sphagnum moss (Sphagnum spp.).

Ecological relationships of dwarf birch

In common with the larger species of birch, dwarf birch has mycorrhizal associations with various fungi. In these mutualistic or symbiotic relationships between trees and fungi, both partners in the association benefit from their interactions, through an exchange of nutrients between their root systems. However, because of its limited distribution in Scotland, and the fact that until recently it has been little-studied here, not much is known about mycorrhizal fungi which are associated with dwarf birch. The fungal species which have been recorded with it are Lactarius helvus, L. rufus, Leccinum holopus and an unidentified Russula sp.

Elsewhere within its circumpolar distribution, a much larger number of fungi have been recorded with dwarf birch. Some of those species, such as the edible boletes (Leccinum scabrum, L. versipelle and Boletus edulis), also occur in Scotland, so it is likely that they would form mycorrhizal relationships with dwarf birch, if it was more widespread and abundant, as in the past.

Non-mycorrhizal fungi occurring with dwarf birch in Scotland are a parasitic rust fungus (Melampsoridium betulinum) and Mycena epipterygia, which is saprotrophic, meaning that it grows from dead organic matter. A greater variety of both parasitic and saprotrophic fungi grow on dwarf birch elsewhere in its range.

Although dwarf birch has quite a number of host-specific insects (ie insects that feed on it and nothing else) outside the UK, there are only two such insects recorded in Scotland: the larvae of the moths Swammerdamia passerella and Stigmella nanivora. Other insects which feed on dwarf birch, as well as other plants, include the larvae of a moorland moth (Argyrotaenia pulchellana) and the northern winter moth (Operophtera fagata). In other parts of its range, a number of aphid species suck the sap of dwarf birch.

Larger herbivores which graze on dwarf birch include sheep and red deer (Cervus elaphus), and their feeding is one of the main factors limiting the natural spread of this species in Scotland. In Scandinavia and North America, dwarf birch is also eaten by moose (Alces alces) and reindeer or caribou (Rangifer tarandus), so it is likely that they also grazed on it in Scotland, before they were exterminated. Siskins (Carduelis spinus) feed on birch seed in winter, and will eat dwarf birch seed where it is available.

It is humans, though, who have largest impact on dwarf birch in Scotland, through past deforestation and the continuing effects of overgrazing and the burning of heather moorland. With a growing interest in dwarf birch today, and a recognition of its important role within the montane shrub community (which has largely disappeared from the Highlands), measures are now being taken at some sites to counter these problems. As a result, this scarce species should begin to make a comeback in the near future to more of its former range.


6. Eared willow

(Salix aurita)

Eared willow is a large, multi-branched woody shrub and a pioneer species, commonly occurring in wet sites in the Caledonian Forest.

Worldwide distribution

Eared willow sketch

 

Eared willow is native to Europe, and is distributed over much of the continent, from Scandinavia to Spain, and from the UK to Russia. It is most common in central and northern regions, but is also found in Albania, Bulgaria and the island of Corsica, although it is absent from Italy and Greece. It has been introduced to the state of Pennsylvania in the USA.

Distribution in Scotland

Eared willow occurs throughout Scotland, including the Western Isles and Orkney, and it is one of the few trees that grow as far north as North Voe on the mainland of Shetland. Because it prefers moist sites and slightly acid soils, it is more common in the north and west of the country and occurs especially in damp woods, heaths and moors. It also forms part of the montane scrub community at the treeline, and has been recorded at elevations of up to 780 metres in the UK.

Physical characteristics

Eared willow is a member of the willow family, Salicaceae, in which it is grouped with the sallows - in fact an alternative name sometimes used for the species is eared sallow. It is considered to be a shrub, rather than a tree, and it can reach 3 metres in height with stems up to 20 cm. in diameter. It is typically rounded and bushy in shape, with multiple stems spreading out at, or near, ground level. The bark is grey and smooth, although in older individuals there can be large cracks or splits running longitudinally on the stems. Twigs are a dark reddish-brown in colour, and the scales which cover the buds are yellow-brown and darker at their tips.

Eared willow is deciduous, so it is leafless in winter, with the new leaves appearing in April or early May. The leaves are grey-green in colour, and are wrinkled, or rugose, in texture. They are slightly hairy, or pubescent, especially on the underside. The shape of the leaves is elliptic or lanceolate, meaning lance-like, and the widest point is closer to the tip than the base of the leaf. The leaves measure up to 4 or occasionally 6 cm in length and 3 or sometimes 4 cm in width. The most distinctive feature is the 'ears' or stipules which give the species its common name. These are small leafy appendages that are somewhat ear-like in shape, and grow on each side of the leaf stem, near the base of, but separate from, the leaf itself. The leaves turn yellow in October before being shed for the winter.

Female eared willow flowers

Female flowers of eared willow.

Like the other members of the genus Salix, eared willow is dioecious, meaning that individual trees are either male or female (in contrast to most trees, such as Scots pine, for example, where male and female flowers occur on the same tree). The flowers appear just before the leaves open in spring, with the male flowers or catkins, which are about 1.5 cm. long, opening to reveal numerous white stamens tipped with yellow pollen. The female flowers are more upright and develop thicker pale green carpels which receive the pollen. Pollination is carried out by insects, mainly by bees, but also by hoverflies. Pollinated female flowers produce tiny seeds that ripen in May or June, and are dispersed by the wind, aided by white cottony tufts attached to them. Eared willow also propagates easily from branch cuttings that are inserted directly into the ground.

Eared willow is a pioneer species, meaning that it readily colonises open ground, and it can often be the first woody species to get established on wetter areas. It can grow rapidly, branching profusely to produce its characteristic spreading bushy shape. Like other willows, it readily hybridises, particularly with grey willow (Salix cinerea), tea-leaved willow (Salix phylicifolia) and creeping willow (Salix repens), and this can make identification of the species problematical at times.

Ecological relationships of eared willow

Although it is not as large or conspicuous as the trees in the forest, eared willow is nonetheless a significant part of the ecosystem, especially where the ground is damp or wet, and a range of other organisms are associated with it. Underground, eared willow forms ectomycorrhizal relationships with fungi, in which the roots of the fungi surround the plant's roots without penetrating them, and a mutually beneficial exchange of nutrients takes place. Other fungi are more visible and less beneficial, and eared willow is susceptible to infection by a rust fungus (Melampsora capraearum) which appears as spots on the leaves.

The stems and branches provide the habitat for a number of mosses and lichens, and on the lower sections mosses such as common tamarisc-moss (Thuidium tamariscinum), and cypress-leaved plait-moss (Hypnum cupressiforme) can be found. Higher up, generally only pincushion mosses, such as crisped pincushion (Ulota crispa) and Bruch's pincushion (Ulota bruchii) occur. Lichens which occur on eared willow include widely-distributed and abundant species such asPlatismatia glauca and puffed shield lichen (Hypogymnia physodes), while a rare species (Pseudocyphellaria crocata) has been found on it at Balmacara in Lochalsh.

Moth caterpillar on willow

Northern eggar moth caterpillar (Lasiocampa quercus callunae) feeding on eared willow.

The Salix genus as a whole in the UK supports 450 species of invertebrates, so unsurprisingly, eared willow has a range of insects which feed on it. The caterpillars of a number of moths feed on eared willow, and two which are particularly associated with it in Scotland are the ruddy highflyer (Hydriomena ruberata) and a tortrix moth (Epinotia subocellana). The larvae of cousin German (Protolampra sobrina), a rare moth that is the subject of a UK Biodiversity Action Plan, have recently been discovered feeding on eared willow in Speyside. The larvae of a sawfly (Nematus miliaris) also feed on the leaves.

A rare species in Britain that is particularly associated with eared willow is the 10 spotted pot beetle (Cryptocephalus decemmaculatus). The adult beetles feed on the leaves, and the larvae feed on fallen leaves. This beetle is a Priority Species in the UK's Biodiversity Action Plan, and is only known to occur at Rannoch and Braemar in the Highlands, and at some sites in Staffordshire and Cheshire in England.

Galls are induced on the leaves of eared willow by various invertebrates. These include pustules on the upper surface, caused by a mite (Aculus laevis), and harder galls caused by two midges (Iteomyia capreae and I. major). Another midge (Rabdophaga cinerearum) induces the formation of a tiny rosette of leaves at the tip of the twig. Elsewhere in Europe, a sawfly (Euura auritae) causes the formation of spindle-shaped galls on the stems.

Eared willow is an important food plant in winter for the European beaver (Castor fiber), which was hunted to extinction in Scotland by the 16th century. Because of its preference for growing in wet sites and its ability to naturally coppice (ie to send out new stems from the base of one that is damaged), eared willow is well-suited to thrive in the presence of beavers.

Together with some of its close relatives, such as grey willow (Salix cinerea) and goat willow (Salix caprea), it also forms part of willow carr (wet woodland dominated by willows), which is an important habitat for invertebrates.


7. Hazel

(Corylus avellana)

Hazel is an important understorey tree in the Caledonian Forest, providing nuts for squirrels and other rodents, and it also supports a rare lichen community.

 

Hazel tree

Hazel tree in the Affric River gorge in Glen Affric.

Worldwide Distribution

Hazel is widely distributed throughout much of Europe, from Britain and Scandinavia eastwards to the Ural Mountains in Russia, and as far south as Spain, Italy and Greece. It also occurs in Morocco, Algeria, Turkey, Iran and the Caucasus region.

Within this large range its distribution is uneven and it typically grows as an understorey component of deciduous forest, especially with oaks (Quercus spp.), although it also occurs with conifers.

Archaeological evidence from pollen analysis has shown there was a rapid expansion in the range of hazel during the Mesolithic period (from 11,000 to 6,000 years ago). Because the large nuts are not dispersed over great distances by small mammals, this has led to speculation that Mesolithic peoples may have transported the nuts with them as a food source, and thereby aided the expansion of the tree's range.

Distribution in Scotland

Hazel is known from all parts of Scotland, including the Outer Hebrides, Orkney and Shetland, and was formerly much more abundant, especially on some of the Hebridean islands. On the mainland, it is common in the western Highlands, and there are stands of almost pure hazel in Argyll. Elsewhere, it occurs in the more fertile, lower-lying parts of the country, usually in association with oak (Quercus spp.), ash (Fraxinus excelsior) and also birch (Betula spp.). It is rarely found on acid soils, such as those of the pinewoods of the Caledonian Forest, and is also uncommon in the Southern Uplands, but this is most likely due to the extensive deforestation of that region, rather than a natural absence.

In Glen Affric, hazel is found primarily in the Affric River gorge, between Dog Falls and Badger Falls, growing amongst birch on south-facing slopes with relatively rich soils. However, it also occurs on the north shore of Loch Beinn a'Mheadhoin and at the east end of Loch Affric, indicating that it was formerly more widespread in the glen.


Hazel in Glen Affric showing the large base or stool which develops in old trees.



Hazel catkins

The male catkins open in February.



Parts of hazel

Hazelnuts; hazel leaves; male catkins and the tuft-like female flowers.



Hazel in autumn

Hazel tree in autumn, near Dog Falls in Glen Affric.

Physical characteristics

Hazel is a member of the birch family of trees, Betulaceae, and can grow to a height of 10 metres, although in Scotland it is usually no more than 6 metres tall. Typically it has a number of shoots or trunks branching out at, or just above, ground level, and this growth habit has led to some people referring to it as a bush rather than a tree, because it doesn't meet the strict definition for a tree, of having a single stem that is unbranched near the ground.

Hazel's ability to produce multiple stems gives it a dense, spreading appearance and has led to its extensive use for coppicing. It is a short-lived tree, reaching 50-70 years in age, but if it is coppiced, either by people or naturally through damage to its trunks, it will live much longer.

The growth of successive new stems leads to the formation of a large base, or stool, which can be up to 2 metres in diameter, and in this way coppiced hazels can live for several hundred years.

The bark is smooth and shiny, and is greyish-brown in colour. It peels off in strips as the tree gets older, and the trunks are often covered in mosses, liverworts and lichens, especially in the wetter parts of its range. The twigs are covered in long stiff hairs, and the buds are smooth and ovoid in shape.

The leaves are roundish in shape, with a point at the end, and are about 10 cm. across. Leaf edges are doubly serrate, or double toothed, and the leaves are hairy, which gives them a rough texture. Hazel is deciduous, with the new leaves appearing in April each year, and turning a bright yellow before being shed in October.

Male flowers are in the form of catkins, which are pale yellow in colour and up to 5 cm. long. They open in February, when hazel and its companion deciduous trees are all leafless, so they are one of the first obvious signs of spring in the forest. The female flowers are tiny red tufts, growing out of what look like swollen buds, and are visible on the same branches as the male catkins. Pollination is by wind, and hazel is self-incompatible - successful pollination only occurs between different trees, as a single tree cannot pollinate itself.

Fertilised female flowers grow into nuts which are up to 2 cm. in size and occur in clusters of 1 to 4. Each nut is partially enclosed by a cup-shaped sheath of papery bracts, or modified leaves. The nuts ripen to a brown colour in September and October, with the nut itself enclosed by a tough woody shell. Empty nuts are an occasional occurrence.

The nuts are distributed by red squirrels (Sciurus vulgaris), woodpeckers (Dendrocopos major,), and, after falling to the ground, by other small rodents. Most of the nuts are consumed by these dispersers, but some of those which are hoarded for winter, or are overlooked, germinate and grow the following spring.

Ecological relationships of hazel

In Scotland hazel normally occurs as an understorey component in deciduous forests characterised by oaks, ash or birch. However, in comparison to oak and birch it has relatively few mycorrhizal relationships with fungi - only 21 species of fungi are recorded as having this mutualistic association with hazel. Of these, one species, the fiery milk mushroom or hazel milk-cap (Lactarius pyrogalus) is largely restricted to growing with hazel.

Hazel is important, however, in providing the main habitat for an ascomycete fungus (Hypocreopsis rhododendri) which was only known from rhododendrons in North America until it was discovered on hazel in Mull in the 1970s, and then elsewhere in western Scotland. It mainly grows on standing dead stems of hazel, but also has been found on living branches. Because of its rarity in Britain, a Species Action Plan has been prepared for this fungus.

Hazel is very important for lichens, and is the best host species in the UK for Graphidion lichens - those which grow on smooth-barked trees. Several of these lichens are rare and endangered, and are the subject of Species Action Plans, including one (Arthothelium macounii) which is the only host for a parasitic fungus (Arthonia cohabitans). Another lichen (Graphis alboscripta) is also almost entirely restricted to hazel, and is only known from Scotland. Hazel is also a good host for the Lobarion group of lichens - the larger, leafy lichens, which include tree lungwort(Lobaria pulmonaria,) - particularly in western Scotland.

Unlike trees such as birch, hazel has relatively few insect species associated with it. However, there are 5 species of moths which are specialist feeders on hazel, including a narrow-winged leaf miner (Parornix devoniella), whose larvae live under a folded down leaf edge, and a nepticulid moth (Phyllonorycter coryli,), whose larvae form 'blotch' mines in the leaves. A few beetles, especially weevils, and some flies are also known to use hazel, while there is a range of insects associated with the nuts, particularly in continental Europe.

Because of its growth as a densely-branched understorey component in forests, hazel plays a significant role in increasing the vertical structure within woodland, which is important for bird diversity. Hazel leaves are eaten by roe deer (Capreolus capreolus) and red deer (Cervus elaphus), and the nuts, which are rich in fats and protein, are eaten by the wood mouse (Apodemus sylvaticus) and the red squirrel. Squirrels split the shell of the nut in two halves to get the kernel inside, whereas wood mice will gnaw a hole through the shell. In England, hazel is an important tree for the dormouse (Muscardinus avellanarius), but this species is absent from Scotland.

Until recently hazel has received less conservation attention than some other tree species, but this is changing now that its importance has been recognised, particularly in the temperate rainforests of western Scotland.


8. Holly (Ilex aquifolium)

With its prickly, evergreen leaves and bright red berries, holly is a distinctive but uncommon small tree in the understorey of the Caledonian Forest.

Worldwide distribution

Drawing of holly leaves and berries

Holly, or European holly as it sometimes known, occurs naturally in western, central and southern Europe, where its range includes the coastal region of Norway, Denmark, Germany, Britain, Ireland, Portugal, Spain and across the Mediterranean to Bulgaria and Turkey. It also occurs in North Africa, in Morocco, Algeria and Tunisia, and further east, in the Caucasus Mountains and northern Iran.

Holly has also been introduced to a number of other countries, including New Zealand, temperate parts of Australia and the Pacific Northwest of North America, in all of which it has caused problems as an invasive species.

Distribution in Scotland

Holly is widely distributed in Scotland, and occurs all throughout the mainland, except for Caithness, and in Orkney, Lewis, Harris and Skye. It is most common as an understorey species in oakwoods, particularly on the west coast, but also occurs in other forest types, including amongst the native pinewoods of the Caledonian Forest.

Physical characteristics

Holly is an evergreen, broadleaved tree in the Aquifoliaceae family, which consists of the single genus, Ilex, with over 400 species worldwide. In Scotland it typically grows up to 10 metres in height, although it can be as tall as 25 metres. Its trunk can be up to 40-80 cm. in diameter, but it frequently divides near the ground, giving rise to multiple smaller stems growing close together. The lower branches from these are often tangled amongst each other, and, where they touch the ground will take root, making the central part of the tree relatively inaccessible.

Female holly flowers

Female flowers of holly.

Male holly flowers

Male flowers of holly.

The bark of holly is grey and smooth, becoming somewhat fissured with age. The leaves are from 5-12 cm long and 2-6 cm wide, and deep green in colour above, but paler underneath. The leaves are leathery and shiny, and, on the lower part of the tree, have three to seven sharply pointed spines on each margin, that alternate in pointing upwards and downwards. Higher up, where they are out of reach of grazing herbivores, the leaves tend to have fewer or no spines. Individual leaves persist on a tree for 2 - 3 years, and when they are shed (usually in the spring) they take a long time to decay, so that skeleton leaves can often be seen on the ground.

Holly is dioecious, so individual trees are either male or female (in contrast to most trees, such as Scots pine or oak, for example, where male and female flowers occur on the same tree). Flowering begins when a tree is about 20 years old, and the flowers appear in clusters near the base of the leaves. They are pinkish-red as buds, opening in May or June to reveal white flowers with 4 petals. The male flowers have stamens covered in pollen and sterile stigma that are reduced in size, whilst the females have larger stigma and smaller, sterile stamens with no pollen.

Pollination is mainly by bees, which are attracted by the fragrance of the flowers. Fertilised female flowers develop into berries 6-10 mm. in diameter, and these are initially green in colour, ripening to a bright red by mid-winter. Each berry contains four seeds, and the birds that eat the fruitdisperse these in their droppings. However, the berries are initially very bitter, and it is only after frost that they become softer and more palatable, so they can persist on a tree for many months. The seeds will not germinate in their first year, and in nurseries they need to be stratified in sand for two winters in order to break down the tough seed coat. Seed production is greatest in trees over 40 years old, especially in a mast year, when large quantities of berries will be produced. Such a year is usually followed by one or more years when the tree has few fruits on it.

Holly can also reproduce vegetatively, by means of shoots growing off the root system of a tree. This appears to be more common in dense woodland, where pollination is perhaps less likely to occur. A holly tree can live up to 250 or 300 years, although the oldest hollies in the UK, in the Shropshire Hills in England, are thought to be possibly up to 400 years old.

Ecological relationships of holly

Unlike many other tree species in the Caledonian Forest, holly has relatively few fungi associated with it. However, one saprotrophic species that grows exclusively with holly is the holly parachute fungus (Marasmius hudsonii), which occurs on the dead leaves, helping to break down their tough, leathery structure. Various microfungi are associated with holly, and the most commonly seen is the holly speckle (Trochila ilicina), which appears as small black spots on fallen leaves.

There are also comparatively few phytophagous (plant-eating) insects that feed on holly, with less than a dozen recorded in the UK. Of these, the holly leaf miner (Phytomyza ilicis) is the most common, and the larvae of this fly produce distinctive trails, or mines, in the leaves. The larvae of the micro-moth, holly tortrix (Rhopobota naevana), specialise in feeding on the tender young shoots of holly, while the larvae of two other moth species, the double-striped pug (Gymnoscelis rufifasciata) and the yellow-barred brindle (Acasis viretata), feed on the flowers of holly, as well as on other plants. Caterpillars of the holly blue butterfly (Celastrina argiolus) also feed on the flowers and buds. However, this species is mainly confined to the southern half of the UK, where its numbers exhibit dramatic fluctuations every few years, due, it is thought, to the impact of a wasp (Listrodomus nycthemerus) that parasitises the butterfly's larvae.

Through its nectar, holly provides a source of food to the bees and other insects that pollinate its flowers, and the berries are food for a variety of birds, especially thrushes such as the redwing (Turdus iliacus), fieldfare (Turdus pilaris) and mistle thrush (Turdus viscivorus). Because of its dense foliage and the prickly nature of its leaves, holly also provides a good nesting site for birds.

As one of the only evergreen broadleaved trees in the UK, holly is an important food source in winter, and, despite its spiked, leathery leaves, it is browsed by mammals including red deer (Cervus elaphus) and domesticated herbivores such as sheep. Considered to have protective powers in the Gaelic culture, holly has been widely cultivated for a long time, and as a result many different forms now exist with variegated leaves etc.

9. Juniper

Juniper (Juniperus communis)

Juniper, an important understorey shrub or small tree in the Caledonian Forest, has declined recently and is now the subject of conservation concern.

 

Worldwide distribution

Common juniper has the largest geographic range of any woody plant in the world. It is circumboreal in distribution, occurring from western Alaska throughout Canada and northern parts of the USA, in coastal areas of Greenland, in Iceland, throughout Europe and in northern Asia and Japan. It is widespread in Europe, except for some low-lying areas around the Mediterranean, and it also occurs in North Africa. In North America it extends from beyond the northern limit of trees south to the Carolinas and the mountains of California, Arizona and New Mexico.

Throughout its range, juniper occurs at varying elevations, and at its southernmost extent it has been recorded at altitudes of up to 3,500 metres. Several subspecies or varieties have been described, but further research is needed to clarify the taxonomic details of these.

Distribution in Scotland

Juniper occurs throughout most of Scotland, including the Borders area, Orkney and some of the Hebridean islands, but it is only common in the Highlands. Two subspecies are found in Scotland, of which the erect, shrubby form, Juniperus communis ssp. communis, is most widespread, while the prostrate form, Juniperus communis ssp. nana, occurs mainly as a component of the montane scrub vegetation community, at the tree line in the mountains, where it can reach an elevation of 1,000 metres.

In the Highlands, juniper is more abundant in the drier, eastern part of the country, where it reaches a larger size. Good examples of large junipers can be seen in the Cairngorms area, for example at Rothiemurchus, while some of the best examples of prostrate juniper occur on Beinn Eighe in Wester Ross.

Conservation Status

Because of its vast geographic range, juniper is not considered threatened at an international level. However, in Britain there has been a substantial decline in both the distribution of juniper and the size of juniper colonies, particularly in England. As a result, juniper is now the subject of a Biodiversity Action Plan (BAP), under the government's response to the Convention on Biological Diversity agreed at the 1992 Earth Summit.

Physical characteristics

Juniper is an evergreen shrub or small tree in the cypress, or Cupressaceae, family. It is slow-growing and in Scotland few specimens are taller than 5 metres, although heights of up to 10 metres have been recorded elsewhere in its range. It displays several different growth forms, which vary from erect and columnar to bushy, spreading or mat-forming and shrublike. Juniper is unusual in being able to grow on both acid and alkaline soils.

The foliage consists of small blue-green needles which are up to 1 cm. long and have a broad white stripe, or stomatal band, on their inside surface. The needles grow in alternating whorls of three on the twigs, and are prickly to the touch. The bark is brown on young plants, but turns grey as it gets older, and is shed in thin strips.

Juniper is dioecious, which means that individual plants are either male or female, unlike most tree species, where both male and female flowers occur on the same tree. Male flowers appear as yellow blossoms near the ends of the twigs in spring and release pollen, which is wind-dispersed. Female flowers are in the form of very small clusters of scales, and after pollination by the wind, these grow on to become berry-like cones. Shaped like irregularly-sided spheres, the berries are green at first, but ripen after 18 months to a dark, blue-purple colour and are 0.6 cm. in diameter. Each berry contains 3 - 6 seeds, although some of these are often infertile, due to insect infestations. The seeds are triangular, hard and black, and are dispersed by birds which eat the berries.

Because of their long ripening period, berries occur on juniper throughout the year, and it is usually possible to see them at different stages of development on the same plant. The seeds are slow to germinate and normally require two winters of dormancy before they will sprout and begin growing. Juniper is a slow-growing species, and in Scotland it typically grows about 3-5 cm. per year. In good growing conditions, it can grow up to 28 cm. in a year, and flowering and seed production begins when the plants are 7-10 years old. Bushes live on average for about 100-120 years, while the oldest recorded juniper in the UK was aged at 255 years, based on counting the annual growth rings.

Juniper can also reproduce vegetatively, with this typically occurring more in old, dying stands. Old bushes sometimes collapse, and where their branches touch the ground, they will form roots and produce new growth, so that a circular pattern of plants forms around the original bush. As a shade-intolerant species, juniper is found in more open types of woodland, typically birch woods or pine woods in the Highlands.

Ecological relationships of juniper

In contrast to many tree species, juniper has comparatively few mycorrhizal fungi associated with it. In these mutualistic or symbiotic relationships between trees and fungi, both partners in the association benefit from their interactions, through an exchange of nutrients between their root systems. In the case of juniper, 5 arbuscular mycorrhizal species (those where the fungal hyphae penetrate the cells of the tree's roots) and 2 ectomycorrhizal species (where the fungus surrounds the tree roots, without penetrating them) are known.

Two species of rust fungus occur on juniper, and these induce galls to form on the stems and twigs. One of these species (Gymnosporangium clavariiforme) produces yellow or lemon fruiting bodies on the galls it gives rise to, and the spores from these in turn produce galls on hawthorn (Crataegus monogyna) which then infect juniper again. The other rust fungus (Gymnosporangium cornutum) produces brown blobs on the needles of juniper, and then also has another phase on a different tree species, in this case rowan (Sorbus aucuparia), where it produces galls in the form of yellowish pustules on the upper surface of the leaves. Juniper is susceptible to some pathogenic fungi, including juniper blight (Phomopsis juniperovora), phytophthora root rot (Phytophthora cinnamomi) and brown felt blight (Herpotrichia juniperi), which grows on the needles.

Galls are also induced on juniper by two gall midges (Oligotrophus juniperus and Oligotrophus panteli). Adults of these species lay their eggs on new terminal shoots in late spring, and the larvae burrow into the buds, where the galls develop the following spring as whorls of modified needles which protect the larval chamber. The adult midges emerge from the galls after another year.

A total of 42 species of phytophagous (ie plant-eating) insects are associated with juniper, although some of these have originated from non-native ornamental junipers which have been introduced to the UK as garden plants. These feed on different parts of the plant, such as the needles and the seeds, and insect predation on the seeds is the main causal factor for the relatively low rate of fertility amongst the seeds in the berries. Insects which damage the seeds include the juniper seed chalcid (Megastigmus kuntzei), the juniper shield bug (Elasmostethus tristriatus) and the juniper berry mite (Trisetacus quadrisetus), which has been shown in some situations to affect up to 82% of the berries.

Larvae of the juniper berry miner moth (Argyresthia praecocella) also feed on the seeds, and this is one of 5 species of Argyresthia moths whose larvae live on juniper - the other species make mines in the leaves and buds. 4 species of Geometrid moths are associated with juniper, including the juniper carpet moth (Thera juniperata) and the juniper pug (Eupithecia pusillata). The larvae of a rare moth (Dichomeris juniperella) which is associated with juniper live in tight silk webs the size of golf balls, amongst the needles. The adults of these moths are inconspicuous and rarely seen, but the species is characteristic of the Caledonian Pine Forest in Speyside and Deeside.

Juniper foliage and berries

Juniper foliage and berries

Because of its dense cover of prickly needles, juniper provides a good nesting site for birds such as the goldcrest (Regulus regulus) and the song thrush (Turdus philomelos), and it is also important in providing winter cover for theblack grouse (Tetrao tetrix). The berries are eaten, and the seeds distributed, by birds such as the fieldfare (Turdus pilais), which is a seasonal migrant from Scandinavia, and the ring ouzel (Turdus torquatus).

Juniper foliage is eaten by mammals such as red deer (Cervus elaphus) and rabbits (Oryctolagus cuniculus), and the current excessive populations of both of these species has contributed to the current decline in juniper's distribution in the UK. The burning of moorland also limits the ability of juniper to regenerate.

People have had a long association with juniper, through the use of the oil obtained from its berries for flavouring gin, and the species now requires human assistance to recover and expand its range again - this is the intention of the current Biodiversity Action Plan.

10. Oak

Sessile oak (Quercus petraea )

Pedunculate oak ( Quercus robur )

Oak is a relatively minor component of the Caledonian Forest, but is important in providing a habitat for many species of insects, lichen and birds.


Sessile oak (Quercus petraea) near Coire Loch in Glen Affric. Note the abundance of lichens on its trunk.

Worldwide distribution

Sessile or durmast oak and pedunculate or common oak have a similar distribution, occurring throughout most of Europe, from southern Scandinavia to the Mediterranean, and from Ireland and western Scotland to the Ural Mountains in Russia. The range of sessile oak does not extend quite as far east in Russia as that of pedunculate oak. About 450 species of oak have been described worldwide, of which 25 are native to Europe, but only sessile and pedunculate oak have such a broad distribution, and they are the sole species native to the north and west of the continent.

In the past, large areas of Europe were covered by temperate deciduous forests in which these two oaks predominated, but only a small proportion of those forests remain today, as the majority of the land has been converted to agriculture.

Distribution in Scotland

Both species of oak are distributed throughout much of Scotland at lower elevations, usually less than 300 metres, although pedunculate oak is more common in the south and east of the country and sessile oak predominates in the north and west. In some areas, such as the south side of the Moray Firth and in the Borders, both species are common, and the historical planting of oaks by people over the centuries has blurred the differences in their ranges.

On the west coast, the sessile oaks of the Atlantic oakwoods are particularly important, as they form bryophyte-rich temperate rainforests under the influence of the mild, wet oceanic climate. These woodlands are of international significance because of their unique assemblage of species, especially mosses, lichens and ferns, but are all greatly reduced in extent.

In the mountainous interior of the Highlands, where the native pinewoods of the Caledonian Forest predominate, oaks are less common, as the harsher climate and poorer soils limit their growth. However, in glens such as Moriston and Strathfarrar, areas of sessile oakwood do occur on the drier south-facing slopes at lower elevations. In Glen Affric a few sessile oaks can be found at the eastern end of the glen near Badger Falls. However, the presence of the flora associated with upland oakwoods suggests that, in the past, oak was more widespread on the south-facing slopes there.

Physical characteristics


 

Oaks are members of the beech family, Fagaceae, and are long-lived trees which grow relatively slowly, at least in their initial years. Both pedunculate and sessile oak are large trees, reaching up to 40 metres in height. However, because of its more north-westerly distribution and ability to grow on higher ground, sessile oak is more usually only up to 30 metres tall in Scotland. Trees of both species regularly live to be 500 years old, and individuals of 1,000 years old are known, although in some cases that age has been achieved by coppicing them over long periods of time.

The bark of both species is grey and fissured, often covered with various lichens, and old trees can reach an exceptional girth - some individuals have a circumference of 12 metres. The large spreading branches produce substantial domed crowns, sometimes resulting in the trees being wider than they are tall. The branches of the sessile oak tend to be straighter than those of the pedunculate oak, which can become twisted or even gnarled with age.

Twig

Oaks are deciduous trees, and in winter their buds are distinctive by being clustered at the end of the twigs. The buds are composed of rusty-brown overlapping scales which protect the young undeveloped leaves inside from frost and cold. In spring the new leaves appear later than those of other trees such as birch, and in the Highlands it is late April or early May when the buds burst. The leaves are up to 12 cm. long, and are characteristically lobed, with pedunculate oak having three to five pairs of lobes on each side of its leaves, while those of sessile oak have five to eight lobes per side.

One of the features which differentiates the two species is the leaf stalk or petiole. On the pedunculate oak this is very short, typically less than 0.5 cm. long, whereas on the sessile oak the petiole is 1-2 cm. in length. Further distinguishing features between the two oaks include the presence of two small lobes or auricles at the base of the leaf on pedunculate oak, in contrast to the wedge-shaped base of a sessile oak leaf. The latter also has fine hairs on the underside of the leaf, especially along the midrib, but these are absent in pedunculate oak. Sessile and pedunculate oak hybridise naturally with each other, producing trees of intermediate characteristics, and this can make it difficult to accurately identify a tree to the species level.


Male catkins and new leaves of a sessile oak near Badger Falls in Glen Affric. Note the terminal buds which haven't opened yet.

Because of the large numbers of insects and other invertebrates which feed upon oaks, many of the leaves can be tattered and have numerous holes in them by late July. Oaks then produce a new flush of leaves, especially on young trees, and this phenomenon is called lammas growth, because it occurs around the time of Lammas, the Celtic festival of first fruits, on 1st August. In autumn, the leaves turn various shades of yellow and brown, as chlorophyll is withdrawn from them and carotenoid pigments become visible instead. In the Highlands, the leaves are shed at the end of October, but in milder areas a few leaves will stay on the trees until December.

Oaks are monoecious, meaning that each individual tree has both male and female flowers on it. The flowers appear about 7-14 days after the leaves burst, and the male flowers consist of drooping, tassel-like catkins, up to 4 cm. long. The female flowers are tiny and occur at the leaf axils, where the leaf stalks join the twigs. Each catkin releases several million pollen grains which are distributed by the wind and, when pollination is successful, adhere to the receptive stigma on the female flowers. Fertilised flowers develop into acorns, which ripen in the autumn and are up to 3 cm. long. The acorns are seated in hard, warty cups called cupules, and in pedunculate oak these are borne on stalks called peduncles, which are up to 8 cm. long. By contrast, the acorns of durmast oak are either stalkless or have very short stalks less than 0.3 cm. long, and it is this sessile feature which gives this species its common name.

Acorns

An oak will produce its first good seed crop when it is 40-50 years old, and acorn production varies, with trees sometimes producing very few in a given year. Large numbers of acorns are produced intermittently in what are known as mast years, and these occur every three to five years, when a mature tree can produce up to 50,000 acorns. The acorns are shed before the leaves fall, and they begin to germinate almost immediately. The leaf litter which accumulates on top of them provides protection from frost and hides them from seed-eating animals and birds, but very few of the acorns produced by a single oak are successful in growing on to become a mature tree.


Ecological relationships of oak


Female spider (Theridion pallens) with her egg sac, beside a currant gall induced by the sexual generation of a tiny wasp (Neuroterus quercusbaccarum), on the underside of an oak leaf.

 


Common spangle galls (Neuroterus quercusbaccarum) and silk button spangle galls (Neuroterus numismalis) on the underside of an oak leaf. These galls are induced by the asexual generation of the wasps.

 


A micro moth (Diurena fagella) resting during the day on the bark of an oak tree near Coire Loch in Glen Affric.

 


Larva of the same species (Diurena fagella) on an oak leaf near Badger Falls.

 


Female orb web spider (Metellina segmentata) on an oak leaf near Badger Falls.

In the UK, oak provides a habitat for more organisms, and especially insects, than any other tree. Because of its large size and longevity, it plays a unique role in forest ecosystems and many species have adapted to live with it.

Underground, the roots of oak trees have mycorrhizal associations with various species of fungi. In these mutualistic or symbiotic relationships, both partners benefit from their interactions, with the fungi receiving sugars and carbohydrates which the tree photosynthesises using the sun's energy, while the tree receives nutrients and minerals from the fungi, which it is unable to access directly in the soil itself. Examples of mycorrhizal fungi associated with oak include several species of milk cap, for example the yellowdrop milkcap (Lactarius chrysorrheus) and the oakbug milkcap (Lactarius quietus), as well as various species of Russula, such as the purple brittlegill (Russula atropurpurea). Over a dozen species of bolete form mycorrhizal associations with oak, including the orange oak bolete (Leccinum quercinum) and the very rare Strobilomyces floccopus. There are also a number of tooth fungi which occur with oak, including the velvet tooth fungus (Hydnellum spongiosipes) and the fused tooth fungus (Phellodon confluens).

Not all fungi are beneficial though and oak mildew (Microsphaera alphitoides) is a white powdery fungus that appears on oak leaves in late summer. Another fungus with the unusual common name of nut disco (Hymenoscyphus fructigenus) grows on acorns, especially in wet weather. A number of bracket fungi are associated with oak, including the oak mazegill (Daedalea quercina), which takes its name from the wavy pattern of elongated gills on its underside and grows on dead branches and stumps. This fungus is the only home for a small rove beetle (Gyrophaena strictula), a fact which illustrates the complexity and uniqueness of habitat that an oak tree provides.

A remarkable diversity of lichens, totalling over 300 species, have been recorded growing on oaks. These range from common species such as Hypogymnia physodes and oakmoss lichen (Evernia prunastri) to various 'beard' lichens in the genus Usnea, and tree lungwort (Lobaria pulmonaria). This latter species has disappeared from much of its former range, due to atmospheric pollution, but still occurs in more pristine areas such as Glen Affric.

Bryophytes are the group of non-flowering plants which include mosses and liverworts, and 65 species grow on the trunks and branches of oaks. They are particularly abundant in the temperate rainforest ecosystems of the Atlantic oakwoods, where the trees are festooned with mosses. Common species include broom fork-moss (Dicranum scoparium) and slender mouse-tail moss (Isothecium myosuroides), while rarer species from the Atlantic oakwoods include western featherwort (Plagiochila atlantica). Frequently to be seen growing amongst the moss on the branches of oaks is the common polypody fern (Polypodium vulgare).

In the Highlands, oak grows in association with specific groups of plants in a vegetation community called upland oakwoods. This typically includes birch trees, usually downy birch (Betula pubescens) but sometimes silver birch (Betula pendula), and common plants such as wood sorrel (Oxalis acetosella), dog violet (Viola riviniana), tormentil (Potentilla erecta) and bracken (Pteridium aquilinum). Other flowering plants typically occurring in these oakwoods include wood anemone (Anemone nemorosa) and bluebells (Hyacinthoides non-scripta).

Many organisms have evolved with oak to produce the abnormal growths that are known as galls. Over 40 species, including midges, mites and wasps, are responsible for stimulating the oak, by means which are not fully understood, to produce unusual growth forms on its leaves or twigs, within which the larva of the insect lives and feeds. Some galls are well-known and easily seen, such as the marble gall - a brown spherical ball on the twigs which is induced by the gall wasp (Andricus kollari). Some of the most beautiful galls are the spangle galls which are found on the underside of the oak's leaves. Four different tiny wasps in the genus Neuroterus are the causative agents for the growth of these galls, up to a hundred of which can be found on a single leaf. The spangle galls are induced by the asexual generation of the wasps, while the subsequent sexual generation of the wasps cause different galls to form on the leaves and catkins. Some galls, such as the oak apple, which is caused by another wasp (Biorhiza pallida), support complex communities of insects that consist of parasitespredators and inquilines (insects which exhibit cuckoo-like behaviour).

Large numbers of moth larvae feed on oak leaves, and the caterpillars are usually well-camouflaged in shades of green, to reduce the risk of predation by birds. Common species include the oak hook-tip moth (Drepana binaria), the winter moth (Operophtera brunata) and micro-moths such as Phyllonorycter quercifoliella, whose larvae produce mines in the leaves. In other species, the adults are superbly camouflaged as well, including the evocatively-named merveille du jour (Dichona aprilina), which, with its green and black patterning, blends in perfectly on lichen-covered trunks. Many species of butterflies are found in oakwoods, and in Scotland these include the speckled wood (Pararge aegeria) and the pearl-bordered fritillary (Boloria euphrosyne). The purple hairstreak (Quercusia quercus) is the only butterfly whose larvae feed exclusively on oak, but it is scarce in Scotland.

Numerous beetles are associated with oak trees, including the oak bark beetle (Scolytus intricatus), whose larvae create a distinctive pattern of galleries in the tree's wood. Several nut weevils, includingCurculio glandium and Curculio venosus, lay their eggs in acorns during early summer, and the larvae feed inside the acorns, pupating after they have fallen to the ground. A rare carabid beetle (Calosoma inquisitor) which feeds on moth larvae on oaks, has recently been found for just the second time in Scotland.

Given the abundance of insects which live on oaks, it is unsurprising that spiders are common on the trees as well. Some species spin webs amongst the leaves, while others, such as the crab spiders, rely on their camouflage to ambush their prey. Common species on oaks in Scotland include the orb web spider (Metellina segmentata) and the garden spider (Araneus diadematus).

Insects are also the food source of birds, and many species take advantage of the large numbers of invertebrates on an oak tree. Common species in oakwoods in Scotland include the wood warbler (Phylloscopus sibilatrix), chaffinch (Fringilla coelebs) and tree creeper (Certhia familiaris). Other birds feed on acorns, especially the wood pigeon (Columba palumbus) and the jay (Garrulus glandarius). The latter is particularly important as a dispersal agent for acorns, as it will transport them up to a kilometre away from the tree and bury them in the ground. The jay doesn't recover all the acorns it stores in this way, so some survive to germinate and grow into new trees.


Wood mouse

Many of Britain's common mammals are found in oakwoods, ranging from the wood mouse (Apodemus sylvaticus) and bank vole (Clethrionomys glareolus) to the badger (Meles meles) and pipistrelle bat (Pipistrellus pipistrelllus). The red squirrel (Sciurus vulgaris) feeds on acorns, and like the jay, aids the tree's reproduction through its habit of caching nuts for the winter. However, it has been displaced from many oakwoods by the non-native grey squirrel (Sciurus carolinensis) which has been introduced from North America, and which is better able to feed on acorns. In the past, before they were extirpated from the UK, wild boar (Sus scrofa) would have relished the autumnal bounty of fallen acorns as well. Roe deer (Capreolus capreolus) and red deer (Cervus elaphus) both occur in oakwoods in Scotland, and their browsing of seedlings is the main factor preventing the regeneration of oak (and other trees) in ancient native woodlands such as those in Glen Affric.

Although it is not a major component of the Caledonian Forest, oak is nonetheless very important because of the habitat it provides for so many other species. It is likely to have been more abundant in the past, before it was removed by selective exploitation. Its regeneration now will add to the richness and diversity of the restored forest.

 

11.  Rowan

(Sorbus aucuparia)

Rowan is a fast-growing pioneer tree in the Caledonian Forest, characterised by its brilliant red berries at the end of summer.

 

Worldwide distribution

Rowan with berries

A rowan tree laden with berries in late summer beside the Allt Coire Ghaidheil stream on the West Affric Estate.

 

Rowan by stream

Berry-laden rowan trees beside the Allt na Imrich stream in Glen Affric.

 

A rowan tree in full blossom, showing the dense clusters or corymbs of creamy-white flowers.

 


A rowan sapling in autumn, growing in the fork of a Scots pine; its seed would have been deposited here in a bird's dropping.

 


Newly-opened rowan leaves in spring.

Rowan occurs widely throughout Europe, in western Asia in Russia and the Caucasus region, and in north Africa in the mountains of Morocco. Throughout its range it occurs in a variety of habitats, but it commonly grows in mountains and has been recorded at elevations of up to 2,000 metres in France. Together with downy birch (Betula pubescens) and some willows (Salix spp.), it grows at the most northerly limit of trees in Europe, at latitude 70° north, in Finnmark in Norway.

Five subspecies of rowan are recognised by scientists, and three of these have restricted distributions - one is limited to Bulgaria, another to northeastern Russia and a third to southern Italy, Sicily and Corsica. The other two subspecies are more widely distributed throughout the tree's range in Europe.

Distribution in Scotland

Rowan grows in most parts of Britain, but is more common in the north and west, and is found throughout Scotland. It grows at a higher altitude than any other tree in the country and occurs at elevations of almost 1,000 metres in parts of the Highlands.

This attribute, together with the similarity of its leaves to those of the ash tree (Fraxinus excelsior), gives rise to its alternative common name of mountain ash. At higher elevations it survives as small saplings which are frequently stunted in form.

In Scotland today, rowans are often found growing in inaccessible locations, such as cliffs, steep stream-sides and on top of large boulders. However, these are not the preferred locations for the species, but rather are the only places where it has been able to grow out of reach of herbivores such as red deer (Cervus elaphus) and sheep.

Physical characteristics

Rowan is a fast-growing, short-lived pioneer tree in the rose family, Rosaceae. It is typically a small tree, reaching a maximum height of 10-15 metres, or exceptionally, 20 metres. It is slender in form, although mature trees can be quite substantial - an old rowan at Carnach Mor on the West Affric Estate has a trunk which is over 40 cm. in diameter. Multi-stemmed forms are quite common, as a result of browsing by mammals and the subsequent production of basal shoots. The greyish-brown bark is smooth and shiny when wet, with dark raised dots or lenticels scattered across it. The branches are typically upward-pointing and terminate in ovoid, purplish buds, which are often covered in grey hairs.

Rowan leaves are compound and pinnate in form, meaning that each leaf is made up of matched pairs of leaflets on either side of a stem or rachis, with a terminal leaflet at the end.

Leaves are up to 20 cm. in length, and are comprised of 9-15 leaflets, which are serrated with small teeth.

Rowan is a deciduous tree, with the new leaves appearing in April, and they turn a bright orange-red colour in autumn before being shed.

The flowers blossom after the leaves have appeared, usually in May or early June, and are creamy-white in colour. Individual flowers are about 1 cm. in diameter and they grow in dense clusters or corymbs, each containing up to 250 flowers, and measuring 8-15 cm. across. The strong, sweet scent attracts pollinating insects, including many species of flies, bees and beetles.

Rowan leaves

The fertilised flowers grow into berries which are 8 mm. in diameter and these ripen to a bright red colour in August or early September. The berries are rich in ascorbic acid (vitamin C), and contain up to 8 small seeds, although 2 seeds per fruit is most common. They are eaten primarily by birds, who disperse the seeds in their droppings.

Seed production begins when the tree is about 15 years old, and in mild climates, rowan will fruit each year. However, in harsher environments such as Glen Affric, fruiting is irregular, and mast seed production, when all the trees produce a heavy crop, occurs every few years, with very little fruiting taking place in between. The tough coat of the rowan seed requires cold weather to break down, and germination usually occurs in the first or second spring after the berries have been produced. Seedlings and saplings are shade-tolerant, and are often found under the branches of large Scots pines(Pinus sylvestris), where they have grown in the droppings of birds which perched on the branches above.

For similar reasons, rowans also germinate in the forks of the trunks of pines and other trees, but in most cases there is insufficient organic matter for these seedlings to grow to more than a metre or two in size. Exceptions to this do occur, and near Glac Daraich in Glen Affric there is a good example of a rowan which became established about two metres up an alder(Alnus glutinosa) and grew into a mature tree, partly embracing the alder, after its roots reached the soil below.

Ecological relationships of rowan

Rowan is an integral part of the Caledonian Forest, where it grows in association with Scots pines, and it also occurs in oak(Quercus spp.) and birch (Betula spp.) woodlands. It rarely forms single-species stands of its own.

A variety of mycorrhizal fungi are associated with rowan, including an arbuscular mycorrhizal fungus (Glomus intradices) and an unusual ectomycorrhizal fungus (Cenoccum geophilum). In the symbiotic relationships formed by these fungi and the tree, both the partners benefit through an exchange of nutrients which each organism cannot access directly itself.

A rust fungus (Gymnosporangium cornutum) which infects juniper (Juniperus communis) spends its aecial or spore phase on rowan, where it produces galls which take the form of yellowish pustules on the upper surface of the leaves.

Rowan is a good host tree for lichens, and is the second best in the UK for Graphidion lichens (those which grow on smooth-barked trees) after hazel (Corylus avellana). Tree lungwort (Lobaria pulmonaria) is common on rowan trunks in areas with a wet climate or constant humidity, while a recently identified lichen (Arthothelium dictyosporum) which is endemic to Scotland occurs mainly on rowan.

The foliage is palatable and highly attractive to browsing animals, and red deer will eat it in preference to most other tree species in the Caledonian Forest. Mountain hares (Lepus timidus) eat the leaves, and deer also feed on the bark and stems.

In contrast to many other broadleaved trees, the leaves are not palatable to phytophagous (ie plant-eating) insects, so comparatively few of them are associated with rowan.

However, the larvae of several species of leaf-mining moths (Stigmella spp.) make mines in rowan leaves, and the caterpillars of the Welsh wave moth (Venusia cambrica) feed on the leaves. Larvae of the apple fruit moth (Argyresthia conjugella) are frequently found in the berries. A snail (Helix aspersa) has been shown to feed on the leaves, whilst a beetle (Byturus fumatus) feeds on the flowers, often completely eating the stamens.

The berries are eaten by a variety of birds in the forest, including chaffinches (Fringilla coelebs) and siskins (Carduelis spinus), while in towns and rural areas blackbirds (Turdus merula) are the main seed dispersers. Fieldfares (Turdus pilaris) and redwings (Turdus iliacus) time their migrations from Scandinavia to the UK to coincide with the availability of rowan berries, and in good fruit years, flocks of them will descend on the trees as they pass to the south. On their way, they willdisperse the seeds, thereby enabling a new generation of young rowans to grow and take their place as a distinctive and beautiful feature of the Caledonian Forest.

12.  Scots pine

(Pinus sylvestris)

As the largest and longest-lived tree in the Caledonian Forest, the Scots pine is a keystone species (see Principles of Ecological restoration) in the ecosystem, forming the 'backbone' on which many other species depend.

 

Scots pine in the morning

A solitary, spreading Scots pine in Glen Affric.

Distribution

Scots pine is the most widely distributed conifer in the world, with a natural range that stretches from beyond the Arctic Circle in Scandinavia to southern Spain and from western Scotland to the Okhotsk Sea in eastern Siberia. Within this range it grows at elevations from sea level to 2,400 metres (8,000 feet), with the elevation generally increasing from north to south. Despite this wide distribution, the Scots pine forests in Scotland are unique and distinct from those elsewhere because of the absence of any other native conifers.

Distribution in Scotland

After the end of the last Ice Age, approximately 10,000 years ago, Scots pine, like other trees, spread northwards again from continental Europe into Britain. As the climate continued to warm, it spread into much of northern Scotland, reaching a maximum distribution about 6,000 years ago, before declining about 4,000 years ago for reasons that are not entirely understood. Today the Scots pine has a natural range confined to the Highlands in Scotland, with the native pinewoods covering approximately 17,000 hectares in a number of separate, isolated remnants - just over 1% of the estimated 1,500,000 hectare original area (see Trees for Life Vision Page for more information and map). In many of the remnant areas, the pines are growing on north-facing slopes, but the exact reason for this is not clear - the generally-wetter conditions of such northerly aspects may have provided protection from fire, which was used to clear the forest in past centuries.

Within its present-day range in Scotland, there is considerable biochemical variation in the Scots pine, and this has led to the recognition of seven different groupings of native pinewoods, characterised by these differences.

The pinewood remnants which survive today occur in some situations as stands of pure pine and in others of mixed stands of pine and birch (Betula pendula andBetula pubescens). Because of its inability to regenerate under its own canopy, it is likely that the areas where pine predominates changed over time (eg perhaps every 2-3 centuries - the lifespan of a single generation of Scots pines), making our native pinewoods a dynamic, 'mobile' forest when viewed over the millennia.


Scots pine tree in Glen Affric: note the different growth form to the tree in the other photo.



Detail of Scots pine bark, showing lichens, cowberry and blaeberry growing on it.



Resin on the trunk of a dead Scots pine in Glen Affric.

Physical characteristics

In good situations on mainland Europe, Scots pine can grow to 36 metres (120 feet) in height, but in most of the pinewood remnants in Scotland today the largest trees are about 20 metres (65 feet) tall, with exceptional trees recorded up to 27 metres (90 feet). Maximum girth at breast height is usually up to 2.4 metres (8 feet), although some trees up to 3.6 metres (12 feet) have been recorded. Scots pine usually lives up to an age of 250-300 years in Scotland, although a tree in one of the western pinewood remnants was recently discovered to be over 520 years old!

Scots pine is unusual amongst conifers in having a number of different mature growth forms, ranging from tall and straight-trunked with few side branches, to broad, spreading trees with multiple trunks. Eleven different growth forms, or habit types, have been identified for Scots pine in Scotland, and many of these can easily be seen in the pinewood remnants. Young Scots pines display the characteristically conical shape of conifers, but as the trees mature, this gives way to the flat- or round-topped shapes which are typical of the pines in the ancient Caledonian Forest remnants.

The bark of the Scots pine is also quite variable, with the young bark on small branches being papery thin and often orange-red in colour. The bark on the trunk of a mature Scots pine can vary from grey to reddish-brown and forms layered plates or flakes up to 5 cm. thick, with deep fissures in between. Several species of lichen commonly grow on the bark.

The needles grow in pairs, are blue-green in colour and about 5 cm. (2 inches) in length. They normally remain on the trees for 2-3 years, with the old needles turning yellow in September or October before they are shed. Drops of sticky resin often cover the tree's buds, and also provide a natural preservative for the wood: if a Scots pine dies while it is still standing, the skeleton can persist for 50 or even 100 years before falling down, because the high resin content in the sap makes the wood very slow to decay.


Male flowers




Mature two-year old cone and male flowers

Male and female flowers occur on the same tree. They appear in May with the females on the tips of the higher and more exposed branches and the males clustered together, often en masse, on the branches just below. Pollination is by wind, and fertilised female flowers take two years to become a fully-grown cone. The cones ripen in April, opening while they are still on the tree, and the tiny winged seeds, each weighing 0.005 grams, are dispersed by the wind. Cone production is variable, with good seasons, in which a mature tree can produce 3,000 cones, occurring every 3-5 years, while in between a tree will produce few cones, or none at all. The seeds are generally carried as far as 50-100 metres from the parent tree, although in some situations, especially when there is snow on the ground and a frozen top layer forms, the seeds have been known to travel several kilometres over the smooth, icy surface.

The seeds require a high level of light to germinate and grow, so seedlings are found in open areas and clearings; as a shade-intolerant species, Scots pine does not regenerate under its own canopy. Although germination will occur in various soil types and conditions, the preferred growing situation is on well-drained mineral soil, which in Glen Affric occurs mainly on the slopes of the glen and on the morainic mounds - raised heaps of ground-up rock left behind by the retreating glaciers of the last Ice Age - which are scattered throughout the valley bottom. In the past, it is likely that the effects of forest fires and the rooting behaviour of wild boar (Sus scrofa) both played an important role in creating the exposed mineral soil which pine seedlings grow best in.

Ecological relationships of Scots pine

As the largest and longest-lived tree in the Caledonian Forest, the Scots pine is a keystone species in the ecosystem, forming the 'backbone' on which many other species depend. In the community of organisms which makes up the forest, the Scots pine has a critical role to play, and has relationships with many plants, insects, birds and animals. Some of these live on the pine itself, particularly epiphytic lichens and mosses. These grow on the bark and branches of the pine, especially in wet areas, but do not take any nourishment from the tree. In fact, many of the lichens growing on a Scots pine add to the fertility of the forest through their ability to absorb, or fix, nitrogen from the air. This is incorporated into the body of the lichen, and when it, or the branch it is growing on, falls to the ground, the nitrogen is absorbed by the soil as the lichen decays, and then becomes available for other plants to use.

Like most trees, the Scots pine has special mycorrhizal associations with fungi, whereby the hyphae, or threadlike filaments, of the fungi wrap around the root tips of the tree, and through this an exchange of nutrients takes place. The fungi, which are unable to make direct use of the sun's energy themselves, receive carbohydrates and sugars which the pine has produced through photosynthesis, while the tree receives certain nutrients and minerals from the fungi, which it is unable to access directly in the soil. Through this mutualistic or symbiotic relationship, both the tree and the fungi benefit and are able to grow better than they would in the absence of the other. Scots pine is known to have mycorrhizal associations with over 200 species of fungi in Scotland, and these include the chanterelle (Cantharellus lutescens), a relative of the common chanterelle which only occurs in the pinewoods, and the extremely rare greenfoot tooth fungus(Sarcodon glaucopus) - Glen Affric is one of only three locations where this species has been observed in the UK.

A number of rare and special plants are particularly associated with the pinewoods of the Caledonian Forest, and these include twinflower (Linnaea borealis), one-flowered wintergreen (Moneses uniflora) and orchids such as creeping ladies tresses (Goodyera repens) and lesser twayblade (Listera cordata). The shade provided by the canopy of mature Scots pines provides a good habitat for blaeberries (Vaccinium myrtillus) and cowberries (Vaccinium vitis-idaea) to flourish in, and dense carpets of these cover the forest floor in many areas. They also play a successional role in the development of the hummocks which are commonly found in the pinewoods. These hummocks form over extended periods of time in the shade of the trees, when lichens and mosses colonise boulders or tree stumps. As these lower plants grow, humus or organic matter builds up and this allows the blaeberries and cowberries to become established. Eventually a living mat of vegetation is formed, completely covering the underlying boulder or stump, and creating the gently-rounded, hummocky forest floor which is characteristic of many of the native pinewood remnants of the Caledonian Forest.

Pine sawfly larvae

European pine sawfly larvae (Neodiprion sertifer) eating Scots pine needles.

 

Like all trees, the Scots pine attracts the attention of various insects. Some of these live in the fissures between the plates or flakes of the tree's bark, and these form a food source for birds such as the crested tit (Parus cristatus) and the treecreeper (Certhia familiaris), which specialise in winkling them out of the cracks and crevices. Larvae of the pine weevil (Hylobius abietis) burrow into the wood of the tree, and other insects live on the pine's foliage - aphids suck the sap, and caterpillars of species such as the sawfly (Neodiprion sertifer) and pine looper moth (Bupalus piniaria) eat the needles (right). Wood ants (Formica aquilonia) feed on these caterpillars, thereby helping to protect the trees from defoliation, and also `milk' the aphids for the honeydew which they produce. These ants live in large social colonies, and their mounds of fallen pine needles and forest detritus are a characteristic feature of the pinewoods. The mounds are up to a metre high, can contain as many as half a million individuals, and are generally south-facing, to take advantage of the sun's warmth.

A variety of birds are associated with the Scots pine in Scotland, ranging from common insect- or seed-eating species like the chaffinch (Fringilla coelebs) and siskin (Carduelis spinus) to large raptors such as the golden eagle (Aquila chrysaetos). Black grouse (Tetrao tetrix) and capercaillie (Tetrao urogallus) both live in the pinewoods and eat the buds and shoots of the pines. The capercaillie became extinct in Scotland in the 18th century, but was successfully reintroduced from Scandinavia in 1837 and is primarily associated with the native pinewoods today.

The only bird which is endemic to the UK (ie found here and nowhere else in the world) is the Scottish crossbill (Loxia scotica), which is confined to the pinewoods. It is sometimes called the 'Scottish parrot' because of its crossed mandibles, which it uses to prise open the tightly-fitting scales of the Scots pine's cones. The seeds inside form the mainstay of the diet for this rare bird.

Scots pine female flower

Mammals associated with the pinewoods include the red squirrel (Sciurus vulgaris), which also extracts and eats the seed from pine cones while they are still on the trees; mice and voles, which feed on pine seeds which have fallen to the ground, and thepine marten (Martes martes), which eats voles, red squirrels and small birds, and relishes blaeberries in late summer. Larger mammals found in the pinewoods include the wildcat (Felis silvestris), badger (Meles meles), fox (Vulpes vulpes), roe deer (Capreolus capreolus) and red deer (Cervus elaphus). Both roe and red deer browse on Scots pine seedlings, eating the needles and leader shoot of young trees, and the overgrazing pressure from their expanded numbers in the last 150 years has prevented the natural regeneration of the native pinewoods throughout the Highlands. Red deer also damage or kill sapling Scots pines by de-barking or thrashing them with their antlers, particularly in late spring when the new season's antlers are shedding their velvet. In a natural, healthy forest ecosystem, the deer numbers would be in balance with the regenerating trees in the forest, but the imbalance in our pinewoods has created a 'generation gap' in the Scots pines, with no trees younger than 150 years in most locations, until fencing or intensive deer-culling measures were initiated in the last 10-20 years.

In the past, the pinewoods supported a wider range of large mammals, including the wild boarEuropean beaver (Castor fiber), lynx (Felis lynx), moose (Alces alces), brown bear (Ursus arctos) and the wolf (Canis lupus), but in Scotland these have all been extirpated - the wolf was the last to disappear, when the last individual was shot in 1743.

Little-known until relatively recently, the native pinewoods of the Highlands have become the subject of various restoration and regeneration programmes, and the future prospects for this unique part of Scotland's natural heritage now look better than they have done for centuries. Many of the best remnants of the pinewoods have active restoration measures underway in them and research projects are elucidating more of the interconnections and relationships which make up this boreal forest ecosystem.