Resilience of Energy and CO2 Exchange to a Summer Heatwave in an Alpine Humid Grassland on the Qinghai-Tibetan Plateau

Summer heatwaves are expected to be much more frequent and severe with negative effects on terrestrial ecosystem water and carbon budgets, while the impacts on alpine grasslands remain poorly understood. Here we analyzed eddy flux and meteorological dataset of a seven-day (July 26 to August 1) summer heatwave in an alpine humid grassland in northeastern Qinghai-Tibetan Plateau in 2015. Compared with pre-heatwave, only diurnal ecosystem respiration (RES) increased by 30.7%, evidently (P < 0.001) during the heatwave. Diurnal sensible heat fluxes (H) and latent heat fluxes (LET) increased by 18.1% (P = 0.08) and 27.5% (P = 0.02) from 9:00 to 16:00. The heatwave did not lead to substantial increments of daily H and daily LET, while daily Bowen ratio (H/LET) decreased a little (P = 0.07). Daily net ecosystem CO2 exchange increased by 76.7% (P = 0.03), mainly resulting from remarkable growth in daily RES (P<0.001) and undetectable fluctuation in daily gross primary production (GPP) (P = 0.13). Daily ecosystem water use efficiency (GPP/evapotranspiration) decreased by 20.8%. The little difference of energy and CO2 fluxes between pre-heatwave and post-heatwave indicated strong resilience to the summer heatwave in the alpine humid grassland. Our results revealed that the present-day summer heatwave exerted a limited influence on energy exchange and vegetation photosynthetic activity but did stimulate ecosystem respiration, which would provide a positive feedback to climate warming with more carbon efflux from alpine grassland.