Aspen is a medium-sized tree in the poplar family. The Eurasian Aspen Populus tremula and the North American Populus tremuloides are closely related. Together, they have the widest natural range of any tree species in the world. Although it is found throughout Scotland, Aspen is generally rather scarce. It is most common in the Highlands, but even here it survives mainly as small remnants, widely scattered and generally unmanaged.
Aspen is found on a wide range of sites from sea-level to the tree-line. It occurs in most types of woodland, but most commonly associates with birch. It is often found in sites inaccessible to grazing animals, such as crags and riparian woodlands. Aspen’s importance for biodiversity has become increasingly recognised. It supports many rare animals and plants. It is also a preferred foodplant for beavers, whose reintroduction to Scotland is currently under trial.
The pollen record shows that Aspen was one of the first trees to colonise Britain after the end of the last Ice Age. It shares many characteristics with other pioneer tree species, such as willows, pine, birch and alder. It is tolerant of a wide-range of soils and climatic conditions, and it disperses readily by wind. Aspen is light-demanding and appears to establish and persist most successfully on open sites which experience periodic disturbance. It is also highly palatable to herbivores. The most important feature of Aspen ecology is its capacity for regeneration. Unlike most trees, Aspen infrequently flowers and sets seed. When it does so, it yields many light seeds which can be carried great distances by the wind.
However, Aspen readily produces root suckers which can develop into new trees or ‘ramets’. This allows a single tree to grow into a grove in which all the stems belong to a single clone. In temperate forests, this ability is shared only with a few trees and shrubs in the genus Prunus (such as cherry and blackthorn). This ability can confer virtual immortality on some Aspen clones.
Throughout its range, Aspen is valued for the contribution it makes to woodland biodiversity. In scotland, Aspen has been found to play host to a remarkably diverse flora and fauna. This includes fungi, lichens, mosses, flies, beetles and moths, many of which have Red Data Book status. Five UK Biodiversity Action Plan (BAP) Priority Species depend on Aspen. Aspen also provides an important source of food for beavers, especially in winter.
Many Aspen-dependent species have very particular requirements. The larvae of the Dark-bordered beauty moth, for instance, only feed on the leaves of young Aspen suckers. Aspen hoverfly requires fallen Aspen deadwood. Such requirements have implications for the management of Aspen stands. The fragmented nature of our Aspen woodlands also impacts on biodiversity. Networks of Aspen habitat are likely to support greater biodiversity than scattered fragments.