Moderate Grazing Can Promote Aboveground Primary Production of Grassland under Water Stress

Grazing is a key disturbance that shapes the structure and function of grassland communities. However, comprehensive understanding on the effects of the principle functional processes controlling the response of net primary production (NPP) to grazing is still elusive. The mechanism between aboveground net primary productivity (ANPP) and grazing activities under various ecological conditions or grazing intensities is poorly understood.

Differences in ANPP between grazed and ungrazed treatments were more sensitive to varying environmental variables than to varying grazing variables. So it is necessary to choose a suitable study area with significant differences in climate landform, and diverse grasslands and grazing systems/intensity, which is helpful to better understand plant–herbivore interactions and what ecological conditions or level of ecological organization NPP may increase, and to further clarify some controversy in whether or not herbivory can increase net primary production.

Tianshan Mountains-Junggar Basin (TMJB), located in the centre of the Central Eurasia continent, was a representative grassland type in Central Eurasia continent. In recent years, overgrazing became more and more evident due to increased intensive grassland use in this region. As the vast extent of arid area over Central Eurasia continent, TMJB is of great importance to investigate the response of ANPP to grazing intensity (GI) for better understanding the interactions between grass and herbivore communities and sustainably maintaining the ecological function in grassland ecosystems.

Therefore, Prof. LUO Geping et al. 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 ANPP in TMJB 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 GI scenarios indicated that ANPP decreased with increasing GI at AM and MMFM. But at LMDG and PDG, ANPP increased when GI was smaller than optimal GI (GIopt). After GIopt, ANPP decreased with the increasing GI. This implied that appropriate GI 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. It is concluded that the response of ANPP to GI highly depended on the climatic conditions in grassland ecosystems over Central Asia, and moderate grazing can promote ANPP under water stress.

The result has been published on Ecological Complexity, 2012, 11: 126-136. The paper can be downloaded from http://www.sciencedirect.com/science/article/pii/S1476945X12000438.