EFFECTS OF SIMULATED HERBIVORY AND INTRASPECIFIC COMPETITION ON THE COMPENSATORY ABILITY OF BIRCHES

To improve our knowledge regarding plant responses to herbivory we sub jected juvenile birches (Betula pubescens) to different types of simul ated herbivory, i.e., removal of the top (I cm) of the main stem or de foliation (including removal of main stem top), and to different level s of intraspecific competition (i.e., three stand densities). Treatmen ts were also repeated during the following year. Birch responses to th ese treatments were measured as changes in plant height and dry mass. Defoliated plants showed reduced growth irrespective of the level of i ntraspecific competition. Topped plants in the low-density plots showe d enhanced growth, i.e., over-compensation, whereas topped plants grow ing in the high- and medium-density plots were unaffected by the treat ments. Our results indicate that we should expect a continuum of plant responses to herbivory. Thus, there is little value in asking whether herbivory has a positive or negative effect on plant growth. Instead, emphasis should be placed on identifying conditions under which we wo uld expect different responses. Further, we suggest that overcompensat ion should not generally be considered as an adaptive response that ha s evolved as a consequence of natural selection due to herbivory, but rather as an indirect consequence of selection for competitive ability . Because apical dominance (assumed to be an adaptation to competition ) restricts growth, the growth mte will increase if apical dominance i s removed.