The Effects of Fossorial Mammals on Alpine Treeline Dynamics in the American West
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Alpine treelines are often used to monitor climate change. However, caution should be exercised when these ecotone proxies are used to infer climate change. Although climate has been shown to have a considerable influence on treeline location, many fine-scale processes are also accountable for treeline dynamics. The geomorphic effects of burrowing mammals may provide conifer seedlings with refuge in previously inhospitable treeline environments and may result in treeline dynamics that are not directly associated with climate change. In Olympic National Park, Olympic marmot (Marmota olympus) populations have been declining (Griffin et al. 2008) and abandoned burrows may provide ideal sites for conifer establishment. Similarly, in Glacier National Park, pocket gophers (Thomomys talpoides) have disturbed large areas of alpine meadows at treeline, which may facilitate establishment. Substrate altered by gopher and marmot activity, when compared to adjacent, undisturbed areas, may provide ideal conditions for conifer seedling germination, emergence, and ultimate establishment. Gopher and marmot activity reduced soil compaction and such soils were typically drier than adjacent, undisturbed soils. Soils disturbed by gophers in GNP were cooler than undisturbed soils, whereas marmot-disturbed soils were warmer. Seed germination was higher on marmot-disturbed soils than on gopher-disturbed soils. Despite these experimental findings, no statistical difference existed in seed germination within control soils compacted to gopher and marmot rates observed in the field. It is likely, therefore, that although soil compaction appears to influence soil moisture and soil temperature, it may not be the ultimate driving force determining seed germination in mammal-disturbed soils. Additional research is needed to individually evaluate additional variables that may influence seed germination and survival in an alpine environment.