Andrew C. Corey, William A. Pfitsch, and Ernest H. Williams
(Biology Department, Hamilton College, 198 College Hill Road, Clinton, NY 13323)
Mammalian herbivory and compensatory growth in Aster novae-angliae
The compensatory continuum hypothesis suggests that factors such as nutrient availability and timing of herbivory can influence the degree to which plants are able to compensate for tissue lost to herbivores. We recorded evidence of mammalian grazing on more than 80% of 158 marked individuals of Aster novae-angliae growing naturally in local old fields. Plants that were grazed were significantly shorter than individuals that were never grazed (on average 39 + 1 versus 70 + 5cm). Plants grazed toward the end of the growing season did not produce flowers; plants grazed before mid-August had significantly fewer flower heads than ungrazed individuals (on average 2 + 0.5 versus 19 + 3 capitula). We also tested the effects of nutrient levels and timing of simulated herbivory on compensatory response of seedlings grown through the summer. Although nutrient level had a significant effect on nearly all parameters, there were no significant differences in performance of plants clipped in July and control plants in both high and low nutrient treatments. Therefore, A. novae-angliae could compensate for early season grazing and nutrient availability did not affect its compensatory ability. By contrast, timing of clipping did affect compensation: late season clipping resulted in significantly less total and flower mass than unclipped controls. A shorter growth chamber experiment confirmed the lack of nutrient effect on compensatory ability and suggested that increased allocation to shoot rather than root growth may enhance the compensatory growth response. Differences in compensation between field versus pot-grown plants may be related to differences in competitive environment or measurement techniques.