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Dry forests and grasslands: stand structures, habitat, and small mammals as indicators of biodiversity
Sullivan, Thomas P.
The dry forests of the southern interior of B.C. are represented by three biogeoclimatic zones: Interior Douglas fir (IDF), Ponderosa pine (PP), and the Bunchgrass (BG) zone. Douglas fir and ponderosa pine forests continue to provide an important source of timber for the forest industry. Forests in the IDF are usually harvested by a variety of partial cutting systems. Several operational trials were installed in IDF forests in the 1990s and now provide a relatively long-term (10+ years) opportunity to evaluate the effectiveness of residual structures, via variable retention (VR) and green-tree retention (GTR) systems, in these harvested sites to maintain biodiversity. An example is the ?Variable Retention Systems in Mixed Fir-Pine Forest? study initiated in 1996 in the IDF zone west of Summerland. All three zones have varying levels of grasslands and shrublands. To date, ?open range? habitats in these zones have received little attention in terms of maintaining biodiversity. BC?s grasslands are most prevalent in the arid valley bottoms which coincide with popular and rapidly growing urban centres such as Penticton, Kelowna, Vernon, Kamloops, and Williams Lake. The B.C. Ministry of Agriculture and Lands reports that grassland habitats represent less than 1% of BC?s landbase, and that this limited habitat is critical to the survival of an estimated 30% of BC?s threatened or endangered species. The disproportionate number of rare species found within grasslands, and the loss of habitat and disturbance caused by urbanization, livestock grazing, and farming suggests that careful management of these sensitive ecosystems is essential for the conservation of BC?s biodiversity. Grasslands, both open range and in IDF and PP forests, are very important for the ranching industry, which has been grazing livestock within BC since the 1850?s. Cattle grazing occurs throughout virtually all areas of the southern interior IDF, PP, and BG zones. In other parts of North America, many studies report that grazing disturbance significantly decreases the overall abundance and richness of small mammal communities compared to ungrazed exclosure habitats. In all of these studies, reduced herb cover was reported as the most probable cause for the observed decrease in small mammal abundance and richness following grazing. Unfortunately, there are no published data on small mammals in open range habitats in southern B.C. What seems clear is that generalist species such as deer mice and chipmunks come to dominate small mammal communities in grazed ranges. It is not clear what communities we would have in relatively ungrazed or protected ranges. Thus, two major questions are: 1) What are the appropriate targets for (a) stand level structures in dry forests (e.g., IDF zones) and for (b) open range habitats composed of grasslands (e.g., PP and BG zones) to maintain biodiversity? 2) What indicators can we use to determine if biodiversity targets are being achieved? The question of how to measure ecological sustainability and biodiversity in managed forests is a major focus of scientists and managers worldwide. This quest is critical to future productivity (timber supply) and ecological features (biodiversity) of second-growth forests. Certification processes will likely focus on the issue of sustainability of managed forests to provide timber and ecological services through time. One way to measure biodiversity and ecological sustainability is to compare specific features of managed forests with unmanaged mature and old-growth forests by way of ecological indicator species. There are several small mammal species that could be used as indicators of biodiversity maintenance in these dry forests and grasslands.
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