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Effects of clearcutting on landscape structure and bird species diversity and abundance in the Rocky Mountains Varnier, Pierre Raoul Paul


Even-aged, short-rotation forest practices are increasingly replacing wildfires as the dominant large-scale disturbance process in many forest landscapes of British Columbia. The resulting landscape patterns are often dramatically different from historical patterns with consequences for forest birds and other wildlife species that are poorly understood, especially at the landscape level. The goal of this study was to assess the effects of clearcutting on the structure of forest landscapes and to describe the response of birds to these structural patterns. The study was conducted in the managed and protected landscapes within the montane spruce biogeoclimatic zone on the western slopes of the Rocky Mountains. Forest cover maps, biophysical habitat maps, aerial photographs, and a geographic information system were used to create a habitat patch map of the montane spruce zone. The effects of clearcutting on landscape structure were assessed by comparing current managed landscape patterns to patterns in adjacent protected areas and to historical conditions within the same landscape. Landscapes were quantified using a variety of indices measuring compositional and configurational aspects of spatial structure. The results of a bird survey undertaken in 117 plots located in the same area were then overlaid on the habitat patch map to describe bird responses to current landscape conditions. Multiple regression analysis was used to model the relations between birds and surrounding habitat patterns measured in concentric circles ranging in area from 0.8 ha to 314.2 ha. The spatio-temporal analysis of landscape patterns revealed several differences between 'managed' and 'unmanaged' forest landscapes which can be attributed to clearcutting. In particular, clearcutting has (1) increased the number of early-, mid-, and late-seral forest patches, (2) increased the total area of early-seral habitat at the expense of late-seral forest habitat and to a lesser extent of mid-seral forest habitat, (3) reduced the total area of mature interior forest habitat while increasing the number of core areas, (4) increased the total area of mature forest edge habitat, (5) increased the density of high contrast edges, (6) reduced mean patch size and variability of mid- and late-seral forest patches, (7) simplified the overall shape of patches in the landscape while increasing the complexity of late-seral forest patches, (8) increased patch diversity, and (9) reduced patch contagion. Bird responses to surrounding habitat patterns varied with respect to the strength and nature of the relationships. Landscape variables explained between 43% and 51% of the variation in bird species richness, diversity, and total abundance. Mature forest edge habitat was the strongest predictor variable in each case. Landscape variables explained between 25% and 49% of the variation in the abundance of 10 bird species analysed. One group of birds (Chipping Sparrow, Dark-eyed Junco, Vesper's Sparrow, and Orange-crowned Warbler) was associated with the proportion of clearcuts in the surrounding landscape. Another group (Red-breasted Nuthatch, Golden-crowned Kinglet, Townsend's Sparrow, Brown Creeper, and Swainson's Thrush) was associated with the proportion of mature forest edge and interior habitat. One species, Yellow-rumped Warbler, was best predicted by the amount of young forest and edge habitat. The strengths of the relationships were greatest for the 12.6 ha and 19.6 ha concentric circles. Many more observational studies of this type, repeated in different locations, will be necessary to improve our understanding of the interactions among landscape patterns, natural disturbance processes, and human activities. Moreover, a more profound understanding of the influence of landscape structure on birds will depend on more analytical investigations which rely on better quality landscape-level habitat data collected for that purpose.

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