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The Biodiversity and Management of Aspen Woodlands
The Trees for Life Aspen Project

The Biodiversity and Management of Aspen woodlands: Proceedings of a one-day conference held in Kingussie, Scotland, on 25th May 2001

Alan Watson Featherstone
Trees for Life, The Park, Findhorn Bay, Forres IV36 3TZ. Email: info@treesforlife.org.uk


Introduction


Aspen tree in early autumn, overlooking Loch Beinn a Mheadhoin in Glen Affric.

Trees for Life initiated an aspen project in 1991 as part of our work to help restore the Caledonian Forest in Glen Affric and the surrounding areas. Our observations had shown that the species only occurred in small stands, often widely separated from each other, and that little regeneration was taking place, because of grazing pressure, primarily by red deer. In addition, in the light of aspen's relatively poor ability to reproduce from seed (Worrell, 1995a, b), we recognised that, unlike other tree species such as birch, rowan etc., it was very unlikely to spread beyond the sites where it was already established.

To address these concerns, our aspen project has four main elements to it: surveying and mapping of existing stands; protection of ramets or suckers at existing stands to facilitate natural regeneration; propagation and planting of young aspens; and research into the ecology of aspen.

Surveying and mapping of existing stands

Commencing in 1991, we began recording data on aspen stands in Glen Affric, and subsequently expanded this work to other locations, such as Glens Cannich, Strathfarrar and Moriston, where Trees for Life also seeks to restore native forest. 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; and associated vegetation and tree species. This information is stored in a database, and to date, 195 aspen stands have been surveyed in this way, with the mapping continuing on an ongoing basis.

Throughout this work, it has become apparent that aspen occurs more widely than we had first thought, and 53 aspen stands have now been identified in Glen Affric alone. The stand sizes vary from some which contain only 1 tree to others with more than 300, and with heights which range up to over 20 metres. Some of the most extensive aspen stands, with the tallest trees, are in riparian areas in Glen Moriston and on the RSPB's Corrimony Nature Reserve. All the data we've collected has now been transferred on to a GIS data set for us by a student from Bangor University, and this will provide a valuable tool for planning the strategic expansion of aspen in the areas where we work.


Aspen and Scots pines in autumn in Strathconon.

The surveys have also produced some interesting data. For example, most of the aspen stands in Glen Affric are in south-facing rocky locations, whereas, by contrast, the majority of sites we've observed in Strathconon are on north-facing rocky sites. However, rather than indicating preferential growing conditions, it is likely that these observations reflect the topography in each glen, with the sites forming the least accessible parts of aspen's former distribution, where some trees have been able to survive because of the reduced incidence of grazing there.

Another result of our surveys has been the discovery of an aspen site in Glen Affric which consists only 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's 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 (Borset,1976; Barring, 1988), and in the case of the example in Glen Affric, we suspect that the parent tree(s) may have been drowned when the construction of the Benevean dam in the 1950s raised the level of Loch Benevean by 6 metres, with the roots and ramets surviving on higher ground.

The surveys have also recorded seasonal phenomena associated with aspen. In the summer of 1999, for example, we documented widespread blackening and dying back of leaves at various stands in Glen Affric. This was subsequently identified by Dr. Adrian Newton and colleagues at Edinburgh University's Institute of Ecology and Resource Management as being caused by Venturia, an ascomycetes fungus which non-lethally infects aspen, particularly in years of stressful conditions such as drought. Flowering of aspen is known to occur irregularly, and during 10 years of surveying in Glen Affric, we have observed it twice - in 1996, when aspen flowered profusely throughout Scotland (Worrell, R. et al, 1999), and on a single tree in 2001.

Protection of ramets or suckers at existing aspen stands

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

Beginning in 1992, therefore, 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 preferential. 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 routed the fence lines for larger exclosures which were planned to achieve regeneration of a variety of tree species to specifically include aspen stands.

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.


Aspen seedlings in a cold frame at Trees for Life's field base at Plodda Lodge, near Glen Affric. These seedlings have been propagated from root cuttings in a special aspen propagation facility in a polytunnel.

Propagation and planting

In early 1992, we began work on the propagation of aspen from root cuttings, using methods described by the Forestry Commission (Hollingsworth and Mason, 1991). 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. Following successful bids for funding from the International Tree Foundation and the Millennium Forest for Scotland Trust, a custom aspen propagation facility, consisting of a polytunnel containing a mist propagation unit and adjacent cold frames, was established at Plodda Lodge, our field base near Glen Affric. Using this facility, the production of young aspens has been increased to its current level of between 1,000 and 2,000 plants per year.

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 (Worrell, 1995b) 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, as mentioned above, these conditions may not be not be the preferential ones for aspen, but just where the species has been able to survive.

We have an ongoing monitoring programme for the trees we've planted, and results in December 2000 showed that in one area where 66 small aspens had been planted as in 1996, the average height after 4 years of growth was 94.8 cm., with individual heights ranging from 13 to 180 cm.. In another location one tree planted in 1993 had reached a height of 350 cm. by December 2000. In future, we intend to refine our monitoring system to track the relative growth rates of aspens grown from different parent stands, to identify the variation between them.


Aspen sapling in Glen Affric, 6 years after it was by planted by Trees for Life staff. This plant was propagated from root cuttings taken from an aspen stand elsewhere in the glen.


Alan Watson Featherstone beside an aspen sapling protected by a Netlon tree guard in Glen Affric. This aspen sucker was unable to grow because of overgrazing by red deer until it was protected with this guard. Now that the tree is large enough to not be damaged by deer, the guard has been removed.

Research into the ecology of aspen

With aspen having been little studied in Scotland until recently, research into the ecology of the species is an important component of our project. In the past few years, several students from Edinburgh University have carried out research into aspen in Glen Affric for us. Isozyme analysis has been used to 'fingerprint' the different aspen clones in a number of stands, and a clear relationship was found between the size of the stand and the number of clones it contained. Contrary to the assertions of some authors (Easton, 1998), this research found that the small stands of aspen studied in Glen Affric generally consisted of more than one clone. Other research has documented the extent of grazing damage by red deer on ramets, and we've used the results from this to plan and implement protection measures for a number of aspen stands in Glen Affric.

In recent years a unique community of saproxylic insects has been identified as being associated with aspen stands, and specifically with dead aspen wood (MacGowan, 1991; Watt, 1998). The key criteria for the survival of this community are aspen stands with a minimum area of 5 hectares and a regular supply of dead aspen wood. Although none of the stands we've surveyed achieve the 5 hectare criterion, we commissioned a survey in May 2001 by members of the Malloch Society to look for evidence of the aspen-dependent saproxylic insects at selected aspen stands in Glen Affric and on the RSPB's Corrimony Nature Reserve in upper Glen Urquhart. Although the results of the study were not available at the time of writing, some of the saproxylic insect species, consistent with the size of the stands, were recorded, although the full range of species were not present. However, we anticipate that the study will produce recommendations for management measures to expand and link up nearby stands, so that in time there will be adequate habitat for the whole saproxylic insect community.

During the Malloch Society's study, a rare fungus, Phellinus tremulae, which grows on aspen trees (Emmett & Emmett, this volume) was observed for the first time in Glen Affric. This highlights the need for further research and study to be carried out in the area, and indeed on aspen throughout much of its range in Scotland.

Current work and future directions

Trees for Life has identified an area of approximately 1000 square miles, to the west of Inverness and bounded by the two main roads from Inverness to Kyle of Lochalsh, where we aim to promote the restoration of the Caledonian Forest and all its constituent species. To this end, one of our immediate goals is to complete the surveying and mapping of all the existing aspen stands in this area. By adding them to our aspen GIS data set, we will be able to prioritise areas for both regeneration measures and the possible establishment of new aspen sites.

With the proposed trial reintroduction of European beavers to Knapdale in Argyll during 2002, concerns have been raised about the future of Scotland's aspen population, as it is a favourite winter food for beavers (Macdonald, D.W., et al, 1995). If the trial reintroduction is determined to be successful, more widespread reintroductions may follow, and Loch Benevean in Glen Affric is a possible candidate site for this. With this perspective, we have begun to focus some of our work with aspen in Affric more specifically on increasing the extent of aspen in the area around Loch Benevean. In May and June 2001 we carried out work to establish some new stands on the north shore of the loch, to secure natural regeneration at more of the existing stands and to enhance some of those stands by planting young aspens in them, sourced from other aspen stands in the glen.

We plan to expand this programme in the next 2 years to include the south side of the loch, and our goal is to establish a robust and healthy aspen population in the vicinity of the loch, which would provide an adequate and sustainable food supply for any beavers which are reintroduced in future, without it being at risk of over-exploitation.

Similarly, we are also working to incorporate the habitat requirements of the saproxylic insect community and of the dark-bordered beauty moth (Epione vespertaria), which requires aspen regrowth of below about 1 metre in height, into our plans for regenerating and expanding aspen in the areas where we work. We hope to follow up the Malloch Society's study in 2001 with other specialist surveys in the near future for the moths, lichens, mosses and fungi associated with aspen.

Drawing all of these elements together, we aim to produce a comprehensive, integrated management plan for aspen for Glen Affric, and eventually for the other glens where we are working to facilitate natural regeneration of the native forest.

Finally, the other area which we are currently working on is the establishment of a central information resource on aspen, and our intention is to host this on the Trees for Life site on the World Wide Web, so that it is easily accessible to land managers, researchers and the public alike.

Acknowledgements

Much of the Trees for Life aspen project has been made possible thanks to funding from the Millennium Forest for Scotland Trust, the International Tree Foundation, Scottish Natural Heritage and the Mercers Charitable Foundation, as well as through the enthusiastic support of our volunteers and members. Our thanks also to the students from Edinburgh and Bangor universities who have carried out research projects on aspen in conjunction with our project, and to Dr. Adrian Newton for assistance on numerous occasions. We are also grateful to Forest Enterprise, the National Trust for Scotland, the RSPB and various private landowners, who have entered into partnerships with us, which has enabled us to carry out conservation work for aspen on their landholdings.

References

Barring, J. 1988 On the reproduction of aspen (Populus tremula L.) with emphasis on its suckering ability. Scandinavian Journal of Forest Research 3: 2, 229-240

Borset. O. 1976 Birch, aspen and alder: a guide to practical forestry. Norges Landbrukshogskole. âs, Norway. 112 pp.

Easton, E.P. 1997 'Genetic Variation and Conservation of the Native Aspen (Populus tremula L.) Resource in Scotland' Ph.D. Thesis, University of Edinburgh

Hollingsworth, M. and Mason, W.A. 1991 Vegetative propagation of aspen. Forestry Commission Research Information Note No. 200. Forestry Commission, Farnham, Surrey.

Macdonald, D.W., Tattersall, F.H., Brown, E.D. and Balharry, D. 1995 Reintroducing the European beaver to Scotland: nostalgic meddling or restoring biodiversity? Mammal Review 25: 4, 161-200

MacGowan, I. 1991 revised 1997 The Entomological Value of Aspen in the Scottish Highlands. Malloch Society Report.

Watt, K. R. 1998 The Hoverflies of Scotland: A local biodiversity action plan for the conservation of the Aspen hoverfly, Hammerschmidtia ferruginea. Malloch Society Report.

Worrell, R. 1995a European aspen (Populus tremula L.): a review with particular reference to Scotland 1: Distribution, ecology and genetic variation. Forestry 68, 94-105

Worrell, R. 1995b European aspen (Populus tremula L.): a review with particular reference to Scotland 2: Values, silviculture and utilization. Forestry 68, 231-244


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