Interspecific differences among woody species in shade tolerance are considered the primary driving forces that underlie forest succession. Variation in shade tolerance, however, may be only one of many interspecific differences that cause species turnover. For example, tree species may differ in their tolerance to herbivory and these differences could also partly drive regeneration dynamics. Nonetheless existing conceptual models of forest dynamics rarely explicitly consider the impact of herbivores. In this study we address the question: Does deer browsing alter the relationship between light availability, growth and survival of tree species? We monitored growth (height) and survival for seedlings of six woody species over two years within six windthrow gaps and nearby intact forest in the presence and absence of deer.
Results/Conclusions
Browsing decreased seedling height growth for all species except Fagus grandifolia. More notably, growth rankings among species differed between exclosed and non-exclosed seedlings. There was a significant positive relationship between mean canopy openness and growth rate for three of the six species when deer were excluded, but browsing obscured this relationship for all three species by either negating the relationship entirely (birch) or weakening it substantially (beech and hemlock). Browsing reduced survivorship for three species, however, survival rankings did not significantly differ between herbivory treatments. Our results indicate that deer browsing and light availability operate simultaneously to influence plant growth and mortality. Existing models of forest dynamics may make inaccurate predictions of the timing and composition of the species that reach the canopy unless they can account for how herbivores change patterns of relative growth rates under varying patterns of herbivore pressure.