Aspen

Aspen (Populus tremula) is one of the rarest and most enigmatic and beautiful of Scotland’s native trees and our only native poplar.  It is also regarded as the neglected tree in Scottish Forestry and there has been a recent upsurge in interest in planting this species.

Distribution

Aspen including both Populus tremula and the closely related Populus tremuloides of North America is the most common and widespread broadleaved tree in the world, found in a wide belt across the Northern Hemisphere. Aspen (Populus tremula) was the first tree to colonise the British Isles following the last Ice Age. It is the pioneer tree and is therefore very tolerant of a wide range of conditions including sites with thin, nutrient poor soils. It can tolerate wet conditions and even contaminated soil, where it can ameliorate chemical conditions through phytoremediation. As a pioneer tree, it can generate topsoil and can neutralise acid soils. Aspen is therefore an ideal nurse crop for a range of native and commercial species.

In Scotland, despite having a wide distribution, tolerating a wide range of soil types and growing from sea level up to or just beyond 550m, there are only about 300 recorded stands along the entire western half of the country.  Many of these stands may comprise of a handful of individuals and often only solitary trees are found.  This is a function of its preference of vegetative regeneration above seed production, its high palatability, its intolerance to shade and traditional land use patterns and management techniques in Scotland.  Aspen has nowhere to go and been forced to the margins on cliffs and remote river gullies.  Pure woodland stands of aspen are extremely rare.  It is estimated that only 160ha of aspen woodland remain today in Scotland, with just 25ha protected by a statutory designation.

Land useLand Management in the Scottish Uplands

Morphology

Aspen can grow to nearly 20m in height and forms a distinctive branching pattern.  Aspen has grey bark, sometimes pitted with distinctive diamond-shaped lenticels. Its leaves are small and round in shape with irregular blunt teeth on their margins.  The leaf stalks of aspen are flattened and very flexible near the leaf blade giving rise to the characteristic fluttering of its leaves in the slightest breeze. Aspen is one of the last trees to come into leaf, having a distinctive coppery coloured leaf when they first open, before turning green. In the autumn, the leaves turn a brilliant yellow or more rarely red in some individuals.  Aspen is dioecious, so individual trees can be either male or female, and trees flower in March or April before the leaves appear, with both sexes producing catkins. Pollinated female catkins ripen in summer and release tiny seeds. However seed production is rare in Scotland. Ecologists believe that this is likely due to climate change over the last 6000 years which has seen a development of a cooler and wetter maritime climate.  Seed years tend to follow a hot dry spring and summer. 

Aspen leaf

However, aspen’s main method of reproduction is vegetative and it can be very prolific at producing a mass of suckers or ramets growing off the roots of mature trees. As aspen has a very extensive root system, ramets can be found up to 40m from the parent tree and grow very quickly.  The largest living organism on earth is an aspen clone in Utah called "Pando" which is latin for "I spread".  It which extends to 106 acres, connected by a single root system and is thought to weigh about 6000 tonnes! Aspen roots can stay alive underground for many years following the death of the parent tree leading to the sudden appearance of new ramets when there is no evidence of the dead parent tree. These ramets are exact clones of the parent tree, flowering and coming into leaf at the same time and have the same appearance as the parent tree. 

Aspen SuckersAspen suckers from mature tree on right of photo.  Dumbarton Golf Course

Biodiversity Value

Aspen is a keystone woodland species, especially in the boreal forests of the north, and no species supports more biodiversity.  Aspen trees or larger stands provide a habitat for a wide range of rare or nationally important species including flies, moths, beetles, fungi, lichens and mosses. At least 100 species of fungi and 130 species of lichens have been recorded on aspen in Scotland. Five UK priority species depend exclusively upon aspen stands for their habitats including the aspen bract fungus, the aspen hoverfly and the aspen bristle-moss. However only aspen stands greater than 4.5 ha can support aspen specific insects which is extremely rare. Aspen is also the favoured food of the European beaver.  Yes there is no UK species action plan for aspen nor is there a habitat action plan for aspen woodland.  When this is taken into consideration with the fact that aspen may be a key indicator of ancient woodland, its importance in a UK and European perspective cannot be underestimated.

Aspen bristle mossAspen Bristle-Moss

Folklore

Its quivering leaves have been the foundation for the tree’s bad reputation in folklore and superstition.  In Wales, it was said that the aspen was used to make the cross of Christ and so the leaves have been shivering in horror ever since. So it has been was demonised in Christian teaching.  People even used to throw stones at the trees as they were seen as bad luck.In Scotland its quivering leaves were interpreted as gossiping – the tree was known as ‘old wives tongues’. This association with wrong-doing meant that the tree was not commonly planted until modern times.

At the same time aspen also has protective powers.  ASPIS, the aspen's Greek name, means shield and light weight aspen wood was also favoured by Celtic warriors. These shields were more than mere physical barriers - they were imbued with additional magical, protective qualities to shield the bearer from psychic as well as physical harm.  Aspen was a tree of heroes which gave them the power not only to visit the Underworld, but also to return safely.  The magically protective nature of the "shield tree" extended to the general population too.  It was thought to be good luck to plant a tree close to a house; even in recent years when an aspen tree was cut down in Kyle of Lochalsh, older residents were quite upset.  There are a small number of place names in the Highlands derived from the aspen's Gaelic name, critheann a such as Crianlarich.

Planting

Aspen is not recognised under the NVC system as a distinct woodland type although it can be a component of a number of woodland types as follows:

  • W8/W9 - upland ash woodland
  • W11 - upland conifer/birch woodland
  • W16 – oak/birch woodland
  • W18 - pinewoods

Aspen could be used as a nurse crop in woodland creation schemes for these NVC communities.

Uses

Aspen wood is used to make matches, light boxes, crates, interior parts of furniture and high quality paper.  Historically, aspen wood was used to make matches, arrows, charcoal for gunpowder, and because of the wood’s light weight, oars and paddles. Several dyes can be obtained from the leaves and bark which, if taken in concentrated form, was also reputed to have abortive properties.

Aspen is a unique tree in many ways.  Despite being one of the most neglected trees in Scottish Forestry, and on the brink of extinction, it is potentially one of the most useful tree species.  This is widely known across the world however is only just being realised in forestry and conservation circles within Scotland, with new research being undertaken into its ecological and economic uses.

There is no doubt that international evidence demands that Aspen is considered as more than just a habitat....  It is likely that it is the lack of supplies that has led to only restricted attention being paid to the silvicultural attributes of the species...Hopefully with rising interest in Aspen there will be increased quantities in Scotland”

 (Jardine, FCS, 2008). 

Eadha Enterprises aim to develop a supply chain for local provenance aspen as well as identifying superior clones from its national clone collection for certain niche uses such as:

  • Biomass energy crop (SRC/SRF)
  • Riverbank and soil/slope Stabilisation
  • Contaminated Land Remediation (Phytoremediation)
  • Soil neutralisation and topsoil creation
  • Nurse Crop
  • Wood Pasture
  • Fibreboards and fine paper making
  • Superior source for nano-cyrstalline cellulose