EFFECTS OF THE FREQUENCY, TIMING, AND INTENSITY OF SIMULATED BROWSINGON GROWTH AND MORTALITY OF TREE SEEDLINGS

Field exclosure studies have shown that mammalian browsers such as whi te-tailed deer (Odocoileus virginianus) can have pervasive effects on forest dynamics in eastern North America. Direct experimental tests of the effects of browsing on growth and survival of a wide range of tre e species, however, have yielded conflicting results. This study was d esigned to assess the effects of variation in the frequency, seasonal timing, and intensity of browsing (simulated by mechanical clipping) o n the growth and mortality of three of the major tree species of the H udson Valley, New York. The clipping treatments were applied to seedli ngs grown under two different light regimes (full sun and 8% of full s un) to examine seedling responses under different levels of shade-indu ced carbon stress. Our results demonstrate that even 2 successive year s of heavy winter clipping (75% of new shoot growth removed) has littl e immediate effect on growth or survival of any of the three species. It is possible that winter browsing only has significant negative effe cts when seedlings are browsed repeatedly over long periods of time. H owever, comparable levels of summer browsing for only 2 years signific antly reduced both growth and survival of all three species. While mos t natural browsing occurs in the dormant season, our results suggest t hat it is the less frequent browsing during late spring and early summ er that has the greatest immediate effect on tree seedlings. Shading r educed growth and increased mortality in all three species; however, t here was only a limited interaction between light level and the simula ted browsing treatments. The effects of browsing on survival were simi lar in all three species; however, the effects of browsing on cumulati ve height and annual growth varied enough among the species to suggest that browsing could cause significant variation among these species i n their rate of invasion in old fields and rights of way, and their ra te of regeneration following logging or disturbance of forests.