Precipitation and grazing intensity jointly shape plant compensatory growth and productivity in a semi-arid steppe ecosystem

Abstract

Plant compensatory growth and productivity under animal grazing and their relations with climate have been one of the central topics in the ecology of natural grasslands. However, no consistent and general framework exists for the relationships of plant compensatory growth with grazing intensity and precipitation, especially in the semi-arid steppes of central Asia. We studied plant aboveground net primary productivity (ANPP) and compensatory growth (CG) of a semi-arid typical steppe grassland under different experimental grazing intensities, and over six years that were hugely variable with respect to precipitation in central Inner Mongolia (China). The grazing experiment used a random block-design consisting of four grazing intensities of nil (NG), light (LG), moderate (MG) and heavy grazing (HG), and replicated in four blocks. We found that (i) grazing with a defoliation rate ranging from 20 % (LG) to 60 % (HG) induced a plant over-compensatory growth, which significantly increased grassland ANPP, though the HG grassland showed a significantly lower potential production of plant aboveground biomass (AGB_p, i.e., peak AGB of the grazed grassland but not subject to grazing in the measurement year) than the NG grassland. (ii) Precipitation and grazing intensity interactively determined plant growth and production. Plant over-compensatory growth and ANPP showed a humpback curve on the grazing gradient, and grazing intensity with the highest over-compensatory growth and ANPP increased with annual precipitation. (iii) Plant community over-compensatory growth under grazing was mostly attributed to over-compensatory growth of dominant species Leymus chinensis and some subordinate annual species, while the C₄ grass Cleistogenes squarrosa had an under-compensatory growth at HG intensity. (iv) Precipitation enhanced grassland ANPP by promoting both AGB_p and the over-compensatory growth after grazing. Our study demonstrates a framework showing the interrelations of plant compensatory growth or ANPP with grazing intensity and annual precipitation, which has important implications for the development of sustainable grassland management systems to adapt to climate and land use changes.