BIOGEOGRAPHY OF WOODY PLANT-CHEMICAL DEFENSE AGAINST SNOWSHOE HARE BROWSING - COMPARISON OF ALASKA AND EASTERN NORTH-AMERICA

Recent research has demonstrated that juvenile-stage woody plants from Alaska and eastern Siberia (Beringia) are more defended against browsing by hares (Lepus) in winter than the juvenile-stage of congeners from other subarctic regions. Our objectives were (1) to determine if similar biogeographical variation in woody plant defense occurs in subarctic North America, and (2) to evaluate some possible causes of this variation. To achieve these objectives we (1) conducted feeding trials that compared snowshoe hare (L. americanus) preferences for winter-dormant twigs of juvenile-stage tree birch and aspen from Alaska with hare preferences for the juvenile-stage of congeners and conspecifics from eastern North America (Maine and Connecticut), and (2) in the case of birch related hare preferences to twig defensive chemistry. We found that hares preferred eastern North American plants, and preferences for birch were related to defensive chemistry. Two historical explanations for such biogeographical variation in the chemical defense of juvenile-stage subarctic woody plants against browsing by hares have been suggested by Bryant et al.: (1) It is a consequence of geographic variation in the intensity of browsing by Pleistocene megaherbivores; or (2) it is a consequence of very large-scale spatial variation in intensity of browsing by hares and associated extant fire-adapted mammals. Using the glacial history and fire history of subarctic North America, we developed scenarios that allowed us to evaluate these historical hypotheses. We also considered the possibility that biogeographical variation in defense of subarctic woody plants against browsing by mammals is a result of ecological responses of plants to the physical environment. While fully recognizing that all three processes may have contributed to the biogeographical pattern in plant defense we documented, we have concluded that browsing by hares and other extant fire-adapted mammals is likely to be the most important cause. This conclusion indicates that the climatic variation that developed across subarctic North America after the ice age has resulted in a geographical pattern in North American wildfire history, which through effects on vegetation has influenced the intensity of selective browsing by mammals in winter and thereby resulted in biogeographical variation in the chemical defense of woody plants against browsing.