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 brows ing by hares (Lepus) in winter than the juvenile-stage of congeners fr om other subarctic regions. Our objectives were (1) to determine if si milar biogeographical variation in woody plant defense occurs in subar ctic North America, and (2) to evaluate some possible causes of this v ariation. To achieve these objectives we (1) conducted feeding trials that compared snowshoe hare (L. americanus) preferences for winter-dor mant twigs of juvenile-stage tree birch and aspen from Alaska with har e preferences for the juvenile-stage of congeners and conspecifics fro m eastern North America (Maine and Connecticut), and (2) in the case o f birch related hare preferences to twig defensive chemistry. We found that hares preferred eastern North American plants, and preferences f or birch were related to defensive chemistry. Two historical explanati ons for such biogeographical variation in the chemical defense of juve nile-stage subarctic woody plants against browsing by hares have been suggested by Bryant et al.: (1) It is a consequence of geographic vari ation in the intensity of browsing by Pleistocene megaherbivores; or ( 2) it is a consequence of very large-scale spatial variation in intens ity of browsing by hares and associated extant fire-adapted mammals. U sing the glacial history and fire history of subarctic North America, we developed scenarios that allowed us to evaluate these historical hy potheses. We also considered the possibility that biogeographical vari ation in defense of subarctic woody plants against browsing by mammals is a result of ecological responses of plants to the physical environ ment. While fully recognizing that all three processes may have contri buted to the biogeographical pattern in plant defense we documented, w e have concluded that browsing by hares and other extant fire-adapted mammals is likely to be the most important cause. This conclusion indi cates that the climatic variation that developed across subarctic Nort h America after the ice age has resulted in a geographical pattern in North American wildfire history, which through effects on vegetation h as influenced the intensity of selective browsing by mammals in winter and thereby resulted in biogeographical variation in the chemical def ense of woody plants against browsing.