| Landscape responses of the Siberian flying squirrel (Pteromys volans) in northern Finland: The effect of scale on habitat patterns and species incidence | ||
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In landscape ecological studies it is of great importance to consider the history and natural characteristics of a study area but also the extent and intensity of human influence in that particular landscape. For forest-dwelling species northern boreal forest landscape is by no means a homogenous large block of forest with uniform, predictable landscape structure. On the contrary, northern taiga forest is literally a vast mosaic of several landscape elements that are in a constant dynamic state of change (Sousa 1984, Pickett & White 1985). Some elements such as water bodies and wetland areas are practically permanent parts in this mosaic but forested land is dynamic and under repeated disturbance and recovery processes (Esseen et al. 1997). Common disturbances in boreal taiga ranges from forest fires and storm winds that operate at a large scale to small scale patch dynamics (White 1979). Subsequent forest succession in a disturbed area follows patterns that are typical of regional climatic conditions and local soil characteristics (Bonan & Shugart 1989). These recovery processes result in a spatially heterogeneous mixture of different forest types and seral stages in a forest landscape. This dynamic framework forms a starting point to studies on all organisms that have adapted to boreal forest setting (Tiebout & Anderson 1997). The important outcome of the dynamics in boreal forest ecosystem is that at a large scale taiga forest landscape is regularly fragmented by natural non-forested areas and, secondly, forested habitats such as late seral forest patches, for instance, are spatially segregated and their location tend to change in time.
In northern Finland man has interacted with his immediate environment ever since the first settlers invaded the large tracts of forest. In the course of history, human influence in terms of the exploitation of forests has become more extensive and intensive (Kimmins 1997). Before modern times human induced changes in boreal forest ecosystem were subtle and local. Compared with natural disturbances, they tended to have a minor effect on overall landscape structure. At a forest stand level, household harvesting or slash-and-burn-cultivation altered profoundly only forest stand structure but, on the other hand, at the same time these activities were a source of e.g. deciduous-rich forest sites (Aarnio 2001). When forest harvesting in Finland was organised in the middle of the 1800’s and forests became an important raw material reserve for the growing saw and paper industry systematic forest management began at a larger scale (e.g. Cajander 1910). In the 20th century forest practises were pursued throughout the forest land in Finland and harvesting operations were extended to the Finnish Lapland. However, although forestry now operated at a large scale and the whole forest landscape was subject to a human influence, “old-fashioned” forest harvesting did not bring about dramatic changes in a landscape structure. The age structure of stands was mostly affected by selective cutting of large diameter timber but composition of forest types and their configuration or large scale connectivity in forest landscapes altered only relatively little. Since the 1950’s in parallel with the modernisation of forest harvesting methods, intensity and efficiency of forest practises became a new paradigm (Leikola 1983). New methods such as clear cutting, regeneration of stands by pine plantations, suppression of broad-leaved trees, and soil preparation shaped the forest landscape in a new fashion. Gradually but inevitably modern forestry altered landscape structure and converted it into economic forest landscape (Franklin & Forman 1987, Hunter 1990). A consequence of this trend was that landscape composition on public land turned towards pine dominance and homogenous even-aged forest stand structure (Anon. 1998). Landscape configuration resulted from rational forest planning and natural landscape connectivity was often decimated by large clear cuts and dense sapling stands that fragmented previously continuous forest blocks (Mykrä et al. 2000). The present day landscape structure is largely a product of relatively short term intensive forest planning and management and it is likely that this will have an impact on ecological processes in forest landscapes still in the future (Rolstad 1991).