Aspen Project

Aspen is one of the lesser-known tree species in the Caledonian Forest. However, ecologically it is very important, as it provides the habitat for a suite of associated organisms, including mosses, lichens and invertebrates, many of which are now rare and endangered in Scotland.

These range from the aspen hoverfly (Hammerschmidtia ferruginea), which is a priority species for conservation in the UK and whose larvae feed in dead aspen wood, to the European beaver (Castor fiber), which relies on aspen as one of its main food sources in winter.

Aspen has probably suffered more from deforestation than any other native tree in Scotland.  This is because it rarely produces seeds, so that when it is lost from an area, it is highly unlikely to recolonise it again of its own accord. Instead, aspen reproduces mainly by new shoots, called ramets or suckers, growing from the root systems of mature trees. As a result, it is constrained to producing new trees only on the periphery of existing aspen stands, where the roots of established trees can extend up to 50 metres from their source trunk. Adding to its challenges in reproducing, aspen is one of the most palatable of all tree species for red deer (Cervus elaphus), so any new shoots are eaten, unless they occur in locations that are out of reach of the deer.

The consequence of this is that aspen has been reduced to small fragmented stands, sometimes consisting of a handful of old trees all growing off the same root system, that are geographically isolated from each other. As such, they are unable to provide the habitat for the species that depend on aspen. In many cases the aspens that do survive are growing in difficult and inaccessible locations, such as the edge of cliffs or in steep rocky gullies. Those are the only sites where deer can’t reach them – they do not necessarily represent the best sites for aspen to grow in.

Recognising both the ecological importance of aspen, and the difficulties the species faces in re-establishing itself in the forest, we launched a project in 1991 to help restore aspen in the Highlands. There are four main elements to the project: surveying and mapping of existing aspen stands; protection of ramets or suckers at existing stands to facilitate their natural regeneration; propagation and planting of young aspens to establish new stands; and research into the ecology of aspen and its associated organisms.

We began recording data on aspen stands in 1991, initially in Glen Affric, and subsequently throughout a 1,000 square mile area to the west of Inverness and Loch Ness. The information gathered at each site includes data on: soil types; aspect; elevation; number, height and stem diameter of trees; numbers of ramets observed; evidence of grazing; accessibility of roots for collection and propagation; and associated vegetation and tree species. This information is stored in a database, and to date, over 450 aspen stands have been surveyed, with the mapping now expanding to sites outside our original project area.

Throughout this work, it has become apparent that aspen occurs more widely than we had first thought, and 68 aspen stands have now been identified in Glen Affric alone. The stand sizes vary from some that contain only 1 tree to others with more than 200, and with heights that range up to over 20 metres. Extensive aspen stands, with some of the tallest trees, are in riparian areas in Glenmoriston and on the RSPB's Corrimony Nature Reserve. The largest stand surveyed to date is in the gorge of the Cannich River in Glen Cannich, where there are 330 trees, with one measuring 2.7 metres in girth, giving it an approximate age of 270 years. All the data we collect is transferred on to our GIS dataset, and this provides a valuable tool for planning the strategic expansion of aspen in the areas where we work.

The surveys have also produced some interesting data, including the discovery of an aspen site in Glen Affric that consists solely of young ramets measuring 30 centimetres or less in height, with no evidence of a parent tree anywhere nearby. This is an example of the ability of aspen roots to survive underground for many years after the death of the parent tree, with the photosynthesis from the ramets' leaves providing enough nourishment to keep them alive. This phenomenon has been known of since the 19th century, and in the case of this example in Glen Affric, we suspect that the parent tree(s) may have been drowned when the construction of the Beinn a’Mheadhoin dam in the 1950s raised the level of Loch Beinn a’Mheadhoin by 6 metres, with only the roots and ramets surviving on higher ground.

The surveys have also recorded seasonal phenomena associated with aspen. These include the widespread blackening and dying back of leaves that happens in some years, and is caused by Venturia, an ascomycetes fungus which non-lethally infects aspen, particularly under stressful conditions such as drought. Flowering of aspen is known to occur irregularly, and we have built up records of this at some sites during the past two decades.

Our surveys of aspen stands confirmed our casual observations that very little natural regeneration of aspen was occurring in them, and the ramets showed clear evidence of grazing damage from deer that was preventing their growth. Many stands had profuse numbers of sprouting ramets a few centimetres in height, but no young trees whatsoever, indicating that recruitment, and therefore successful regeneration of the stands over time, was being completely inhibited.

Beginning in 1992, we instigated a programme of protection for ramets at selected sites, using a variety of methods. These include the protection of individual ramets with tree guards, and we've used both solid tubes and open mesh Netlon guards for this, with the latter being more effective. In other locations, small areas containing ramets have been protected with stock fencing, with a typical size of such an exclosure being about 3 metres by 6 metres. Deer are unlikely to jump into such a small, confined space, and these exclosures are readily erected by groups of volunteers, using recycled fencing materials, thereby keeping the costs to a minimum. In some situations we have also used small areas of deer fencing to protect aspen stands, or have planned the fence lines for larger exclosures to specifically include aspen stands.

Ramets that are protected from overgrazing are able to grow very quickly – much faster than a newly-planted tree seedling is able to do. This is because an individual ramet grows off the root system of a mature tree, and is able to draw on all the food resources produced by the tree. As a result, some ramets we’ve protected have grown by just over a metre in height in 10 weeks!

Our intention with this work is to ensure that successful regeneration takes place in as many stands as possible, and also to extend the aspen stands. In addition, another objective, where stands are located near one another, is to link them up to form larger contiguous areas – this is crucial in creating aspen stands that are large enough to support some of the associated species, which require a minimum habitat of 4.5 hectares of aspen woodland to live in.

In 1992, we began work on the propagation of aspen from root cuttings, using methods first developed by the Forestry Commission. Roots were initially collected from a few sites in Glen Affric, and propagation trials were on a small scale, until the technique had been mastered. We’ve subsequently developed a custom aspen propagation facility, which now consists of two polytunnels, a mist propagation unit and adjacent standing areas in our native Tree Nursery at Dundreggan. Using this facility, the production of young aspens from root cuttings has been increased to its current level of between 3,000 and 4,000 plants per year, making us the largest producer of aspens by this method in Scotland.

Roots are collected from aspen stands listed in our database, with any given stand being left for several years to recover before further roots are collected. The young aspens grown from the root cuttings are all tracked with regard to which parent stand they are derived from, and when they are planted out, this is done in groups containing representatives of at least six or seven parent stands. This not only provides genetic diversity within each planted group of aspens, but also, by statistical averages, should ensure that both male and female plants are represented in each planting. This, in turn, should help to facilitate pollination and seed production when the trees reach reproductive age. To ensure that the local provenance of aspen is maintained, only young trees grown from parent stands within a particular glen, such as Glen Affric for example, will be used for planting out in that glen.

Planting has been done to both establish new aspen stands and to enrich the clonal diversity of existing stands, by planting trees sourced from parent stands elsewhere in the same glen. Because aspen is one of the most palatable trees for herbivores such as red deer, all the aspens we've planted have been protected, either with individual tree guards, or inside fenced exclosures. The planting sites have generally been selected on the basis of exhibiting similar characteristics to the extant stands of aspen, with the caveat that these conditions may not be not be the preferential ones for aspen, but just where the species has been able to survive.

Our planting has also been targeted at linking up existing stands where possible, and in some cases, at creating a future habitat for European beavers, should further reintroductions of that species be approved in Scotland. At Dundreggan we have planted a substantial area of aspen in the riparian zone of the River Moriston, with a view to creating adequate habitat in future for the aspen-dependent invertebrates that require large areas of aspen to live in.

With aspen having been little known and studied in Scotland in the past, research into the ecology of the species and its associated organisms is an important component of our project. There are several different aspects to this, with one element consisting of research projects carried out by students from various universities in Scotland. Past projects have included one focusing on isozyme analysis, to 'fingerprint' the different aspen clones in a number of stands, and another documenting the extent of grazing damage by red deer on aspen ramets, the results of which we used to plan and implement protection measures for a number of aspen stands in Glen Affric.

At Dundreggan, our ongoing partnership with Plymouth University for research into the biodiversity in the canopy of the Caledonian Forest has included an ascent into the canopy of a large aspen tree, and this will be developed further in future. In our tree nursery, in partnership with the charity, Coille Alba, we are carrying out experimental work on grafting aspens, to encourage more regular flowering of the trees, and are planning to establish an aspen seed orchard, to facilitate greater seed production of the species.

We have a programme of biodiversity surveys at Dundreggan and other sites where we work, and this has produced a number of important discoveries regarding species associated with aspen. These include mapping of the distribution of the aspen bracket fungus (Phellinus tremulae), the first record in Scotland of the palisade sawfly (Stauronematus platycerus) that feeds on aspen trees, and the discovery of a fungus (Calathella eruciformis) that grows on dead aspen wood that had not been known from  anywhere in the UK before.

Trees for Life is a founder member of the Highland Aspen Group, which brings together aspen enthusiasts and experts from throughout northern Scotland to share their knowledge, experiences and skills, and we contributed to the two Aspen Conferences, held in the Highlands in 2001 and 2008. 

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