Increased precipitation offsets the negative effect of warming on plant biomass and ecosystem respiration in a Tibetan alpine steppe

Abstract The Tibetan Plateau has become warmer and wetter in recent decades, and the warming of more than 2 °C has been considered as a risk threshold that may lead to irreversible changes in arid and semi-arid grassland ecosystems. However, few data have examined the extent to which the projected increase of precipitation may offset the negative effect of 2 °C warming on aboveground biomass (AGB) and ecosystem respiration (Re) in the vast alpine grasslands. We conducted a field manipulative experiment to investigate the combined effects of 2 °C warming plus 15% and 30% increased precipitation on AGB and Re in an alpine steppe. We observed vegetation cover, AGB, Re, soil temperature (ST), soil moisture (SM), and soil organic carbon (SOC) in the growing seasons of 2013 and 2014. Thornthwaite moisture index (MI) in the growing season (May–September) was calculated for each experimental treatment. Experimental warming generally increased ST and reduced SM and MI, and consequently caused significant reductions in vegetation cover, AGB and Re. Increased precipitation typically stimulated AGB and Re. Compared to the control, relative changes in AGB and Re increased from a negative value under warming with ambient precipitation to a positive value under warming plus 15% and 30% increased precipitation. In pooled data across treatments and years, the changes of AGB and Re showed a positive linear relation to the warming/watering induced variation of MI. The structural equation modeling indicated that AGB and SM rather than ST and SOC were the major factors driving Re during the growing season. In addition, experimental warming decreased the temperature sensitivity of Re (Q10), whereas increased precipitation had a positive effect on Q10. Our data demonstrate that the water deficit induced by 2 °C warming and its negative effect on AGB and Re can be offset by >15% increased precipitation in a semi-arid alpine grassland on the Tibetan Plateau. The change in balance of precipitation and evapotranspiration is an ecologically meaningful metric for generally describing the spatiotemporal variability of soil water availability and ecosystem function under climate change.