Effects of exclosure on aboveground biomass, vegetation constitution, and midday gross primary productivity in semi-arid Mongolian steppe

摘要

We examined the effects of grazing on aboveground biomass (AGB), vegetation constitution, and gross primary productivity (GPP) at midday in midsummer on a Mongolian steppe with mixed vegetation consisting of C3 and C4 plants. A grazed area (GA) and anongrazed area (NGA) created by an exclosure were established in 2002 on land with a history of conventional grazing management. AGB of the mixed C_3-C_4 vegetation was determined in the GA and NGA from 2003 to 2006. Nongrazing resulted in increased AGBand a decreased ratio of C_4 AGB to total AGB. In 2003 and 2006 a dynamic closed-chamber method was used to measure midday GPP; there was no significant difference between the GA and NGA in either year. Partitioning analysis of midday GPP to C_3 and C_4photosynthesis indicated that exclosure exerted little influence on the midday GPP in midsummer, because the enhancement of GPP by increased AGB was offset by the reduced AGB of C_4 plants.