Defoliation Effects on Pasture Photosynthesis and Respiration

Abstract

Ecosystem C gain or loss from managed grasslands depends on management practices. However, limited information is available at the field scale on how the type of defoliation, specifically grazing vs. cutting, affects gross primary productivity (GPP) and ecosystem respiration (RE) immediately after defoliation and during the regrowth process. This study takes advantage of daily field‐scale micrometeorological measurements of GPP and RE made over a 9‐yr period on two pastures that were grazed or cut approximately five times per year to examine the relative effects of grazing and cutting on GPP and RE. The null hypotheses were that the initial reduction in GPP and RE following defoliation and the time required for recovery to pre‐defoliation levels did not differ in response to defoliation method. Overall, RE was greater in cut pastures than in grazed pastures. Regardless of defoliation method, RE did not change during the first week after defoliation, or during the 5 wk following. The initial reduction in GPP2000 (GPP in full sunlight) was greater when pastures were cut (70%) than grazed (46%) and recovery to pre‐defoliation levels took 5 wk for cut pastures vs. 3 wk when grazed. This study suggests, all other management practices being similar, that grazed pastures should be stronger annual C sinks than cut pastures. However, cut and grazed pastures are rarely managed in the same way such that the realized balance between GPP and RE will depend on both physiological responses to defoliation and management intensity.