The ecology and history of Aspen woodlands

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

Peter Quelch
Native Woodland Adviser, Forestry Commission, Whitegates, Lochgilphead, Argyll, PA31 8RS. Email: peter.quelch@forestry.gov.uk

Aspen landscape © Ernest and Valerie Emmett Aspen has a striking appearance in the landscape and is very characteristic of areas in the Scottish Highlands like Strathspey. Despite appearing locally common in some areas, only 160ha of Aspen woodland remains in Great Britain.

Introduction

This paper gives a quick overview of Aspen, both as a tree species and as a rare woodland type in Scotland. The ecology of Aspen is well covered by Rick Worrell, along with other selected papers and booklets. Much has been written about the folklore of Aspen (Elliott, this volume) and why it is such an enigmatic and well loved tree.

This paper will examine where Aspen occurs naturally in today's landscape, and then to ask if we are satisfied with the status quo. If we are not, what greater part could Aspen play in Scotland's woods and forests, and what actions should be considered on its behalf? At present, Aspen is a well liked but 'Cinderella' species, somewhat neglected, and yet with an unrealised potential. Recognition of its values for biodiversity have emphasised Aspen's importance, and this gives the context for this paper.

Aspen in the Biodiversity Action Plan (BAP) process

Simple woodland classifications tend to label woodland types by their dominant tree species, i.e. Oakwoods, Birchwoods, Pinewoods and so on. We can therefore recognise 'Aspen woods' where locally Aspen predominates in certain stands of semi-natural broadleaves in Badenoch and Strathspey, alongside Birch, Rowan, Hazel, Sallow and Alder.

The UK BAP process uses such a classification in selecting the main native woodland types and allocating targets for action (for an overview of woodland classifications see Hall and Kirby, 1998). But what is the status of those native woodland types which are not given BAP plans?

• Birchwoods - after many years of discussion it has now been agreed that upland Birchwoods should have their own Habitat Action Plan (HAP).

• Hazel is not covered separately in the BAP process, despite some lobbying on behalf of the western coastal hazelwoods, which are exceptionally rich habitats for oceanic bryophytes and lichens. Certain rare lichens characteristic of this habitat, (e.g. Arthothelium macounii, or Pseudocyphellaria norvegica) then become surrogates in the BAP process for the habitat they depend on, since they have been given Species Action Plans (SAP).

• Juniper - this native shrub species is covered by having its own SAP, but no HAP.

• Aspen is mentioned in the SAPs for three invertebrate species which depend on it as a habitat, (Hammerschmidtia ferruginea, Byctiscus populi and Epione parallelaria), and two bryophytes (Orthotrichium sp.), but Aspen has neither its own HAP or SAP. Aspen woodlands are however recognised as important habitats in some Local Biodiversity Action Plans (LBAP), e.g. the Cairngorms LBAP.

Aspen in woodland classifications

In the National Vegetation Classification (NVC) (Rodwell 1991), Aspen is described as a component of upland Ashwood (W9b), but even then only occurring rarely. Aspen is also mentioned as an infrequent component in several lowland woodland types: W5, W6, W8, W10, and W16.

Aspen woodlands are not recognised as a distinct woodland type in either NVC, or in the Peterken Stand Type classification (Peterken 1993), where Aspen is associated with the Rowan/Birch stands of Type 12A. Rackham (1986) recognises Aspen woodlands as a subset of Peterken's Birch/Hazel woods, at least for East England. In their classic survey of native pinewoods, Steven and Carlisle (1959) record Aspen as rare or occasional in most of the pinewoods they surveyed, but never abundant. Interestingly they record more than usual Aspen in Glen Strathfarrar pinewoods, which appear to be one of the most natural woodland remnants in the country today.

Aspects of Aspen ecology

Like Birch and other successful colonisers, Aspen can tolerate a wide range of soil types, from lime-rich sites to acidic heaths (for example, Aspen suckers are spreading onto acidic heath at Crannach pinewood, Bridge of Orchy). Like Oak and Ash, Aspen actually prefers good well-drained mineral soils, a site type that it finds in greater areas in Badenoch, Strathspey and Deeside. While sites that it occupies are often moist, it is not a wet woodland species in the same way as Alder or the Willows, or even Bird cherry. Aspen grows at a wide range of altitudes, from sea level (coastal Aspen at Assynt and on Rum) to high altitude gullies almost to the tree-line.

Aspen history

Aspen has an ancient history in Scottish woodlands, being a very early coloniser, arriving with Birch, Sallow and Rowan during the pre-Boreal period 10,000 years ago, earlier than Hazel, and before Scots pine began to dominate. All this happened well before Oak, Alder, Ash, Elm and holly joined the flora. I see Aspen not so much as a rare and neglected woodland type, but more as a tree species which is now under-represented as a component of natural woodland types in Scotland, despite its ancient lineage. I also find it significant that Aspen is host to so many specialist species, despite the fact that the tree itself is not now very common or in extensive stands. To me this dependency indicates a very long ecological association, and this is backed up by the history of Aspen in Scotland.

Aspen and ancient woodlands

Aspen seems to be strongly linked to ancient woodland sites, both in Scotland and in England where it is also a somewhat rare component of usually ancient woodlands (Rackham 1986, 1990). Indeed, I would go further and suggest that Aspen in Scotland is actually an ancient woodland indicator species. Most examples that I find are linked to ancient woodlands, large or small. For example, I recently came across Aspen in the Ryvoan Pass (Glenmore Forest) in a very mixed old-growth stand at high elevation, alongside veteran Scots pine, Juniper and Rowan, as well as very old grey Sallow and Alder. Aspen has strong connections, not only with ancient woodland patches, but sometimes to the tiny woodland refugia of the most natural origins.

Aspen in Europe

In Europe and Scandinavia, where Aspen is more abundant, it is usually as a component species of the northern sub-boreal temperate forest zone (Worrell 1996), rather than as a woodland dominant on certain soil types (compared to say, Oak or Beech). Its natural place seems to be in the small group of broadleaved associates in northern coniferous forests, along with Birch, Rowan, Sallow, and Alder, where together they typically occupy about 15-20% of the forest, alongside the Pine and Spruce (Peterken 1996).

Reasons for current distribution of Aspen

Why has Aspen survived where it is to be found today even sometimes after all other tree species have gone? The reasons for Aspen's ability to survive, albeit in low numbers, include:

• Aspen is actually a poor coloniser (in modern times at least), for while it can produce viable seed this is a rare occurrence.

• Aspen trees are dioecious, so individual trees and even whole clones are either male or female. As individuals become separated from the opposite sex, it is not surprising that Aspen does not reproduce well in its currently fragmented condition.

• Nevertheless, Aspen is very good at self-perpetuation on a local scale by producing masses of suckers in response to felling, windthrow, fire or other disturbance. It seems that vegetative reproduction keeps Aspen going in the same locality almost indefinitely.

• While grazing animals do eat the suckers (it is more palatable than Alder, but less so than Ash and Elm), sufficient survive to grow into new trees, unless grazing pressure is kept at very high levels.

• Aspen is not an inherently rare species like the various Whitebeams for example, partly because it has wide soil and altitude tolerance.

• Aspen has not traditionally been a valuable species for its timber, bark, or coppice shoots (unlike Oak and Hazel) and so has not been deliberately protected or cultivated.

• Aspen has probably been reduced in status partly through poor seeding ability (compared to Birch and Sallow), combined with susceptibility to grazing, but also an inability to form veteran trees (unlike Oak, Holly, Ash, Pine and Alder, which can all survive as stems of many centuries age). Long-lived trees have more time in which to set viable seed and produce new generations during lulls in grazing pressure. The reason why Aspen cannot live a long time and form a huge hollow and ancient stem must surely be that the soft white wood is not durable against rot fungi (unlike Oak and Pine for example).

• If it were not for its suckering ability, Aspen may well have been lost entirely from Scotland.

Are we happy with Aspen's current distribution?

So, apart from the relatively small number of Aspen dominated woods in Badenoch, Strathspey and Deeside, Aspen is a survivor in small patches over most of Scotland. It is found on the sea cliffs of the west coast, in ancient grazed pinewoods in the central Highlands, in remote refugia like the lochside screes of Loch Muick, in the Border cleugh woodland remnants, and in the forgotten corners of many an ancient woodland.

Should Aspen be left alone to inhabit these sparse niches - the remote and craggy woodland refugia? Should Aspen continue to be treated as a somewhat enigmatic tree rarity, a minor species, mainly of interest to woodland historians and romantics as a ghost of the once great natural woodlands? Or does Aspen have a wider role in Scottish woods and forests?

A possible new scenario for Aspen?

Let's look again at the role Aspen plays in, for example, central Swedish forests, where Aspen forms a constituent of the broadleaved component of the mixed pine/spruce forests, along with Birch, Sallow and Alder.

Why could we not encourage both Birch and Aspen as a normal component of Scottish upland forests, up to a proportion of say 25%, rather than the current five or 10% normal maximum? The biodiversity and landscape benefits would be high, and Aspen timber grown in forest conditions is (like other Poplars) straight and utilisable, though not of high value (less than Birch, similar to Alder?). Birch and Sallow regenerate profusely, Alder readily coppices even in the face of moderate deer numbers, while Aspen suckers after felling. So the species in this group can perpetuate themselves at low cost, and all are relatively fast growing.

Conclusion

I think that Aspen would be sold short if we continued to confine it to woodland refugia and regard it as a rarity. There is evidence that it was once a great component of Scottish natural woodlands, and there seems to be no good reason why, with help, it could not be so again. It is time for a 'Comeback Code' for Aspen!

Actions needed to bring Aspen back into its rightful place could include the following:

• Protecting, regenerating, and expanding where possible, all existing Aspen woods, stands and trees.

• Careful planting of Aspen (of both sexes) into some degraded semi-natural woodlands where it is missing.

• Planting Aspen into forestry restock areas in sufficient numbers, that Aspen becomes a self-perpetuating component of a group of mixed broadleaves which between them would cover 15-25% of the gross area of many upland and lowland conifer forests.

Aspen seedling © Peter Quelch The future of Aspen and its dependent species in Great Britain relies on the successful regeneration of young Aspen trees.

References

Ennos R, Worrell R, Arkle P, Malcolm D, 2000. Genetic variation and conservation of British native trees and shrubs, Technical Paper 31, Forestry Commission, Edinburgh.

Hall JE, Kirby KJ, 1998. The relationship between Biodiversity Action Plan Priority and Broad Woodland Habitat Types, and other woodland classifications, JNCC Report 288, Joint Nature Conservation Committee, Peterborough.

Peterken G, 1993. Woodland conservation and management, Chapman and Hall, London.

Peterken G, 1996. Natural woodland, Cambridge University Press, Cambridge.

Rackham O, 1986. The history of the countryside, Dent, London.

Rackham O, 1990. Trees and woodland in the British landscape, Dent, London.

Ratcliffe P, 1999. Aspen woodlands: a case for conservation, paper to the Native Woodlands Advisory Panel for Scotland, Forestry Commission, Edinburgh.

Rodwell J, 1991. British Plant Communities, Vol I, Woodlands and Scrub, Cambridge University Press, Cambridge.

Steven and Carlisle, 1959. The native pinewoods of Scotland, University of Aberdeen.

Trees for Life, 2001, Aspen information and papers, on www.treesforlife.org.uk

Worrell R, 1995. European Aspen (Populus tremula L.): a review with particular reference to Scotland, I Distribution, ecology and genetic variation, Forestry 68(2), pp 93-105; II Values, silviculture and utilisation, Forestry 68(3): 231-243

Worrell R, 1996. The Boreal Forests of Scotland, Technical Paper 14, Forestry Commission, Edinburgh.

Worrell R, Gordon AG, Lee RS, McInroy A, 1999. Flowering and seed production of Aspen in Scotland during a heavy seed year, Forestry 72(1): 27-34

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