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

To improve our knowledge regarding plant responses to herbivory we subjected juvenile birches (Betula pubescens) to different types of simulated herbivory, i.e., removal of the top (I cm) of the main stem or defoliation (including removal of main stem top), and to different levels of intraspecific competition (i.e., three stand densities). Treatments were also repeated during the following year. Birch responses to these treatments were measured as changes in plant height and dry mass. Defoliated plants showed reduced growth irrespective of the level of intraspecific competition. Topped plants in the low-density plots showed enhanced growth, i.e., over-compensation, whereas topped plants growing in the high- and medium-density plots were unaffected by the treatments. 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 would expect different responses. Further, we suggest that overcompensation should not generally be considered as an adaptive response that has 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 is removed.