PARENT SESSION
Oral Session 133: Grasslands V: Grazing.
Presiding: E Fredrickson
Friday, August 6, 8:30 AM to 12:00 PM, Meeting Room B 117.

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

Loeser, Matthew^*,1, Crews, Timothy², Sisk, Thomas³, ¹ Matthew.Loeser@nau.edu, Flagstaff, AZ² Prescott College, Prescott, AZ³ Northern Arizona University, Flagstaff, AZ

ABSTRACT- In light of the continuing uncertainty regarding overcompensation in semi-arid regions, we studied the responses of above-ground plant biomass to defoliation by clipping and livestock grazing in a high-elevation, semi-arid grassland. 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 above-ground production of high-elevation, semi-arid grasslands in the American Southwest may be temporarily increased through some grazing events. Furthermore, compensatory responses by grazing tolerant species may explain shifts in species dominance in grasslands exposed to long-term grazing by livestock.

Key words: overcompensation, clipping, ANPP, grazing