The patterns and mechanisms of precipitation use efficiency in alpine grasslands on the Tibetan Plateau

Abstract

Precipitation use efficiency (PUE), the ratio of aboveground net primary productivity (ANPP) to annual mean precipitation, is a key indicator for assessing the differential response of the water and C cycles to environmental change. Although the alpine grassland of the Tibetan Plateau (TP) is sensitive and vulnerable to climate change, a generalized pattern of PUE along the water and heat availability gradients is still unavailable. Here, a transect survey was conducted across grasslands on the TP to measure ANPP in the community and further explored the patterns of PUE along aridity index (AI) gradients with “Changepoint” analysis method. The results indicated that two patterns of PUE were detected, with significantly (P < 0.05) lower PUE (0.15 g m−2 mm-1) in arid environments than that in semiarid and humid environments (0.30 g m−2 mm-1). These PUE patterns were largely due to the differences in vegetative ANPP and coverage (COV). Specifically, the relatively low annual mean precipitation (210.90 mm) and soil nutrients limited the vegetative ANPP and COV in arid environments as a result of lower PUE. In contrast, the increased precipitation and soil nutrients induced significantly (P < 0.05) positive effects on ANPP and COV. Consequently, the PUE was significantly (P < 0.05) promoted by soil C, ANPP, and COV, with standard total effects of 0.19, 0.89 and 0.29 in the structural equation model, respectively. Notably, a negative relationship between AI and PUE was detected (AI>55), this phenomenon was probably explained by plants via losing water resources to take up more soil nutrients in humid environments. Because the higher stomatal conductance of plant in humid environments leads to higher transpiration (water loss) and therefore can improve soil nutrient uptake. Our findings highlight that environmental factors regulate the variation in PUE through their effects on vegetative ANPP and COV.