Response of vegetation precipitation use efficiency to experimental warming in alpine grasslands of northern Tibet

Vegetation precipitation use efficiency is a key indicator for quantifying the carbon-water coupling mechanism in terrestrial ecosystems. However, it remains unclear how climate warming will affect the vegetation precipitation use efficiency of alpine grassland ecosystems on the Tibetan Plateau. A field warming experiment was performed in three alpine grassland sites (located at elevations 4313, 4513, and 4693 m) in northern Tibet in 2010. Open-top chambers were used to simulate climate warming. The normalized difference vegetation index (NDVI) and soil-adjusted vegetation index (SAVI) were measured using an agricultural camera, and then aboveground biomass (AGB) was modelled using the measured NDVI in 2014–2015 and 2017–2018. The soil temperature, soil moisture, air temperature, and relative humidity were measured using microclimate stations. The vapor pressure deficit was calculated using the measured air temperature and relative humidity. Experimental warming significantly (P < 0.001) increased air temperature (T_a), soil temperature (T_s), and vapor pressure deficit (VPD) but had no significant effects on AGB, NDVI, and SAVI. Experimental warming significantly (P < 0.05) reduced the four-year average vegetation precipitation use efficiency at an elevation of 4313 m, specifically in relatively dry years (P < 0.05), rather than relatively wet years. Experimental warming did not significantly affect vegetation precipitation use efficiency at elevations of 4513 and 4693 m. Overall, experimental warming increased the difference in vegetation precipitation use efficiency among the three elevations. Therefore, in alpine grasslands in northern Tibet, the temperature sensitivity of vegetation precipitation use efficiency can vary with elevation and measurement year. Moreover, climate warming can reconstruct the elevational distribution of vegetation precipitation use efficiency in the these grasslands. That is, climate warming can be expected to enhance the elevational heterogeneity of vegetation precipitation use efficiency in the alpine grasslands of northern Tibet.