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One of the main threats to woodland communities throughout the British Isles has been the trend towards a declining percentage of native woodland cover, which has led to our remaining woodlands becoming smaller and increasingly isolated from each other by open space - a process called fragmentation. Woodland insects associated with dead wood evolved at a time when woodland cover was almost continuous and the amount of dead wood available was probably quite large. As a general rule, they have not developed great powers of dispersal and when they do move to find a new habitat to breed in they prefer to do so at a low altitude through open native woodland habitats following a scent trail of decaying wood. In native woodlands the wind speeds are relatively low, the humidity relatively high and, with an open structure and varied flowering plants present, there are usually nectar sources available either in the ground vegetation or from the trees and shrubs to provide fuel for insects flying and dispersing. In areas of open ground, wind speeds are higher making the chances of an insect being blown away from favourable habitats much greater, scent trails from decaying wood are also dispersed and more difficult to follow, and nectar sources may be absent or infrequent. Conversely, in areas of dense conifer plantations, although wind speeds are low, the density of the trees may act as an actual physical barrier, suitable nectar sources are absent, and the overwhelming odour of conifer resins may obscure any chance of picking up a scent trail of a decaying native broadleaf. Accordingly, both areas of open ground and of conifer plantations will act as a barrier to the dispersal of woodland insects. In areas where woodland is highly fragmented, the communities of plants and insects within them are more unstable and more likely to suffer in the short-term from local extinctions and in the long-term from a more general extinction in the wider area. In an area such as Strathspey, a strategic view is required of the Aspen resource to identify which Aspen stands and areas of native woodland are fragmented, and to develop a model of how greater linkages may be achieved. On the ground, this work will probably only proceed on an opportunistic basis as applications for woodland grants are submitted or opportunities arise for the restructuring of commercial conifer woodlands, but the development of a model should predict which areas have the highest priority for such action. Habitat fragmentation case study - BalliefurthThe map (centre page insert) shows that the important Aspen stands at Grantown (shown in red) and the open Birch/Aspen wood matrix (shown in orange), through which it is probable that the rare Aspen hoverfly Hammerschmidtia ferruginea can disperse, are almost encircled by commercial forestry (shown in green) This applies in particular with regard to the route down the Spey to the south, which leads initially to the stand at Culreach and thence on to the other important Aspen stands in Strathspey. In order to improve the long-term survival prospects for Hammerschmidtia and other Aspen specialist species as part of a long-term strategic approach to creating linked woodlands in Strathspey, consideration should be given to the removal of the conifer blockage caused by Balliefurth Wood and for its replacement, at least in part, by an open woodland dominated by native broadleaf species. This, in conjunction perhaps with some additional planting of broadleaves along field boundaries, should in time create a linkage to the Culreach stand, which with a good stock of mature Aspen trees could provide suitable dead wood habitat. As a first step in a long-term process, this would allow the major Aspen stands at Grantown to be more effectively connected to other Aspen stands in Strathspey. Return to The Biodiversity and Management of Aspen Woodlands For further information about aspen, please go to the Aspen Information Resource
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Published: 3 November 2002 | |||||
