Moderate grazing can promote aboveground primary production of grassland under water stress

Comprehensive understanding on the response of net primary production (NPP) to grazing is still elusive. It is necessary to choose a suitable study area with significant differences in climate, landform, diverse grasslands and grazing intensity (GI), to better understand plant-herbivore interactions and what environmental conditions NPP may increase. This study used Biome-BGC model with inclusion of a grazing process and the effects of the excreta on vegetation to investigate the effects of grazing on the aboveground NPP (ANPP) in Tianshan Mountains-Junggar Basin with four grasslands along a climatic gradient from mountain to plain, i.e., alpine meadow (AM), mid-mountain forest-meadow (MMFM), low-mountain dry grassland (LMDG), and plain desert grassland (PDG). The model simulated ANPP agreed well with the measured values for both non-grazed and grazed experiments at four sites, suggesting that the model successfully captured the effects of grazing on ANPP. The model results based on different Cl scenarios indicated that ANPP decreased with increasing Cl at AM and MMFM. But at LMDG and PDG, ANPP increased when GI was smaller than optimal GI (GI(opt)). After GI(opt), ANPP decreased with the increasing GI. This implied that appropriate Cl stimulated ANPP at LMDG and PDG, with magnitude of 4.1-22% at LMDG and 6.6-15.7% at PDG. By investigating the annual evapotranspiration (ET) and soil volumetric water content under non-grazed and grazed conditions, it was found that grazing reduced ET and hence improved soil water at sites LMDG and PDG, which explained the different response of ANPP to GI in different grasslands. We concluded that the response of ANPP to Cl highly depended on the climatic conditions in grassland ecosystems over Central Asia, and moderate grazing can promote ANPP under water stress. (C) 2012 Elsevier B.V. All rights reserved.