Effect of defoliation intensity on aboveground and belowground relative growth rates.

According to a simple growth model, grazed and ungrazed plants may have equal absolute growth rates provided that the relative growth rate (RGR) of grazed plants increases exponentially with grazing intensity (proportion of biomass removed). This paper reports results from an experiment designed to determine whether plants of two grass species subjected to a gradient of defoliation intensities, from 0 to 100% aboveground biomass removal, showed such a response. The relationship between aboveground RGR and defoliation intensity was exponential and closely matched the theoretical relationship of equal absolute growth rate. Thus, plants showed the same aboveground growth regardless of defoliation intensity thanks to an exponential stimulation of RGR by defoliation. Belowground RGR was depressed by defoliation of more than 20% of the above-ground biomass. In spite of the drastic modification imposed by the treatments on the relative proportions of different plant parts, after a 42-day recovery period basic allometric relationships, such as root:shoot and leafarea: weight ratios, were not affected by defoliation intensity. Exponential aboveground compensatory responses represent a key feedback process resulting in constant aboveground growth regardless of defoliation intensity and appear to be a simple consequence of strong commitments to certain allometric relationships.