Defoliation increased above-ground productivity in a semi-arid grassland

In light of the continuing debate regarding overcompensation we studied the responses of above-ground biomass in a high-elevation, semi-arid grassland to defoliation, defoliation history, and livestock grazing. The above-ground annual net primary productivity (ANPP) was measured over 2 years in one-hundred twenty, 1-m² plots that were exposed to single- and multi-year defoliation and grazing treatments. Plant communities showed an average increase in ANPP of 31%-45% due to a single defoliation event. The most conservative estimate of average ANPP of defoliated subplots was 29.4 g m^-2 greater than the non-defoliated controls. A history of defoliation, due to clipping or grazing, lessened the magnitude of the compensatory response, but above-ground overcompensation of biomass was still observed, ranging on average from 17% to 26%. One dominant species, squirreltail grass [Elymus elymoides (Raf.) Swezeyi], accounted for nearly one-third of the community-level increases in ANPP. In contrast to above-ground patterns, below-ground root production of squirreltail did not increase in response to defoliation events. These results suggest that the above-ground production of high-elevation, semi-arid grasslands in the American Southwest may be temporarily increased through certain grazing events, and may help explain shifts in species dominance in grasslands exposed to long-term grazing by livestock.