Characterised by its shimmering foliage in summer, the aspen tree is a beautiful but rare feature of the Caledonian Forest.
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.
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 of deforestation, with these sites representing the only places where it has been able to survive out of the reach of grazing animals.
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.
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.
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 for pollination 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 major disturbance 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.
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 the European 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.
Alan Watson Featherstone