LATITUDINAL PATTERNS IN CONSUMPTION OF WOODY GIANTS BY SNOWSHOE HARESIN THE EASTERN UNITED-STATES

Previous studies have documented large-scale spatial variation in pala tability of woody plants to snowshoe hares, Lepus americanus, but clea r latitudinal gradients have not been demonstrated. We conducted a ser ies of feeding trials designed to compare palatabilities of woody plan ts that differed either in latitude of collection, geographic distribu tion, or stage of development. Tests were conducted using three specie s of birches (Betula) and two species of aspens (Populus) found in the eastern United States within the range of snow/shoe hares. When prese nted with conspecific juvenile- and mature-stage twigs of species with predominantly boreal distributions, hares generally avoided juvenile- stage twigs, whereas little discrimination was evident between juvenil e- and mature-stage twigs with nonboreal distributions. When presented with a choice of congeneric juvenile-stage aspens, hares avoided the twigs of the boreal species. No differences in consumption were noted among birches; however, Bryant et al. have observed an avoidance of bo real birches by hares in Alaska and Connecticut. Finally, when present ed with conspecific twigs collected at different latitudes, hares did not discriminate among mature-stage twigs. However, discrimination did occur when hares were offered juvenile-stage twigs; twigs from northe rn latitudes were eaten less than twigs from southern latitudes, and t his trend was more pronounced for species with predominantly boreal di stributions.It is doubtful that the observed patterns were generated s olely, or even primarily, by latitudinal gradients in temperature affe cting either bioenergetics of hares or chemicals associated with cold hardiness in plants. Rather, the available evidence suggests that the elevated levels of avoidance exhibited by hares for juvenile plants wi th predominantly boreal distributions have resulted from more intense selection for chemical defense against herbivory in winter-dormant pla nts occupying boreal areas. Colder temperatures, by limiting nutrient availability, also could increase the selection pressure for antiherbi vore defenses in boreal areas by magnifying the cost of herbivory.