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3 Mar 2022

Trembling in the breeze – aspen conservation

Written by Rob Dewar (Natural Heritage Adviser) with contribution from Aidan Bell (Inverewe Estate Supervisor)
A close-up view of some bright green aspen leaves, with tree branches silhouetted against a clear blue sky behind.
The trembling leaves of an aspen tree
The aspen tree is often overlooked but it harbours a distinct range of wildlife. The National Trust for Scotland is nurturing and conserving this sensory tree at our aspen nursey at Inverewe.

The ‘quaking aspen’ is often heard before it is seen, as the tree makes a distinctive sound with its shimmering leaves that tremble in the breeze. This phenomenon is reflected in its botanical name: Populus tremula. Why does the tree tremble so? There is a story that it is due to sadness or awful guilt, since wood from the tree was used to make the cross that Jesus was crucified on. The factual answer is due to the length and the flexible square-shape of the leaf stalk (petiole), which produces a response in the lightest of wind. The aspen is truly a tree to delight the senses. Even the bark of aspen is interesting – the diamond-shaped lenticels (raised pores) form dark and distinct patterns on the smooth grey bark.

A close-up photo of a section of aspen bark, which has diamond-shaped markings on its speckled, silvery grey trunk.
Aspen bark

The crenulated leaves of aspen trees burst into life in spring and are a copper colour before turning green with chlorophyll. In autumn the yellow leaves shine radiant, the tree a jewel to behold as it shimmers gold to those that pass by. Occasionally, some aspen trees turn an equally stunning vibrant red in autumn. This is an unusual genetic trait of certain trees and is something to look out for!

The aspen almost became a forgotten tree as it has largely disappeared from the landscape, but recent conservation work has raised awareness of the importance of this tree species. We are still learning about the ecological role the tree plays.

“Over 60 species of insect have been recorded as feeding on aspen foliage and at least 17 species of insect are associated with dead aspen wood.”
Rob Dewar
Natural Heritage Adviser
A half portrait photograph of a man, standing in leafy woodland. He has short grey hair and wears a black outdoors jacket. He is smiling at the camera.

The tree is also an important species for beavers in Europe; now this ‘ecosystem engineer’ is back in Scotland, we may look forward to a future where aspen and beaver interact as nature always intended them to do.

However, one of the reasons why aspen was disappearing from our landscapes is that aspen seed production rarely occurs in Scotland, unlike in Scandinavia. The reasons for this are not yet fully understood. It may be related to our climate – we do know that the rare years when seed production has occurred in Scotland have been after long hot summers. Aspen flowered prolifically in 1996 after the hot summer of 1995, and then again in 2019 after another hot summer. Aspen trees are also readily browsed by herbivores, so the scattered and fragmented nature of the tree explains its localised distribution where it often only survives on rocky slopes and cliffs, away from grazing animals.

The main way in which aspen propagates in Scotland is vegetative, with new suckers or ramets growing off the roots of mature trees. This process has allowed aspen clones to survive on sites after deforestation – nurtured by the nutrients provided by the parent tree. Some aspen clones may be ancient and can burst into regenerative growth from ramets if grazing pressure is relaxed.

A mix of young trees grow on a low hillside beside a loch.
An example of suckering

Aspen is a diecious species, so individual trees are either male or female. As aspen rarely flowers in Scotland, most remnant colonies of aspen are of a single sex plant, likely to have established from a single cloned tree that has reproduced by suckering. This situation, along with the relatively rare distribution, has led to the initiative to conserve and help this tree thrive in the Highlands once again, an initiative that the National Trust for Scotland is very much involved in.

I have always enjoyed seeing aspen trees and find the tree enigmatic in the Highland landscape. When I heard about the work to conserve aspen trees, I felt we would be well placed to be part of such an endeavour. I initially approached Coille Alba and Trees for Life, who are the leading exponents of aspen conservation work, to see how the Trust could work in partnership with them. We then received a vital donation from the National Trust for Scotland Foundation USA that paid for a substantial polytunnel, and so the scene was set to begin our own aspen project. The idea was also met with great enthusiasm by my colleagues at Inverewe Garden who were keen to develop a tree nursery. Inverewe gardeners Jim and Maddie have been instrumental in developing the nursery – volunteering their time and steering the project through the difficult COVID period.

When looking to propagate and plant trees into the wild, it is important to find local or regional genetic sources of seed or cuttings – this is known as local provenance. The first stage of our aspen project involved collecting cuttings from regional aspen trees that were then grafted onto root stock that was provided by the Trees for Life nursery. The cuttings (or scions) are taken from the fresh growth on the terminal highest branches of the aspen tree.

Local knowledge and distribution maps were used to locate the scattered populations of aspen in the north-west region of the Highlands, ensuring genetic local provenance was maintained. Trust staff and volunteers also received training from Coille Alba in the delicate art of grafting the collected scions on to the root stock.

One of the great benefits of the project is the involvement of the local community. The GALE Centre in Gairloch are involved with our regular volunteers, collecting scions and grafting to rootstock; the Highland group of the Trust’s Conservation Volunteers helped with fitting out the polytunnel. We intend to provide interpretation at the polytunnel to explain to visitors and members what we’re doing, the reasons for it and the conservation benefits of the project.

Once the scions have been grafted, the root stock is then potted into air pots that enable the trees to grow without the roots becoming pot-bound. An automated irrigation system has been installed in the polytunnel nursery, which helps with consistent watering of the trees. The polytunnel is large enough to allow the trees to grow to sufficient maturity to produce seed, so they can become a seed orchard – the next phase in this exciting conservation project.

Aidan Bell has been a key member of the Inverewe staff – collecting scions from trees and coordinating local volunteers in the project. He explains: ‘This is a long-term project, so the measurable outcomes will be that we can conserve the local genetic provenance of specific and mapped aspen trees; new trees can be planted out in the environment to augment the local landscape; awareness and skills can be shared with our members and volunteers; and surplus trees can begin to fill the gap in the provision and availability of north-west provenance aspen.’

“The aspen project is yet another example of pioneering conservation work at Inverewe. The expertise and passion of the Inverewe staff have been instrumental in the success of this project so far.”
Aidan Bell
Inverewe Estate Supervisor
A man stands beside a large, moss-covered tree stump. He is holding out one hand, with some stripped pine cones in his palm. He is smiling at the camera.

Involving the local community and volunteers adds even more value in raising awareness of the aspen tree and our conservation work. I very much look forward to the fruition of the propagation work, when we begin to plant aspen into the wild.

The Trust would like to acknowledge and thank Coille Alba and Trees for Life who have been our partners in this project. They have supported the Inverewe aspen nursey and have done much to conserve aspen in the Highlands.

Further information about aspen conservation work can be found at the project page on the Trees for Life website.