The objective of ClimGrassThermo was to study the individual and combined effects of climate warming, elevated CO2 and summer drought on the canopy surface temperature of managed montane grassland. Two permanently monitoring thermal infrared cameras were installed in an ongoing multifactor field experiment (ClimGrass) at Raumberg-Gumpenstein. During several campaigns the continuous canopy temperature measurements were complemented and replicated using an additional manually operated camera. Canopy temperatures obtained with the thermal infrared cameras were in a similar range and showed the same dynamics as obtained with leaf temperature sensors. Canopy temperatures were significantly increased during peak drought compared to non-drought conditions, and were highest when drought occurred under future climate conditions, i.e. under warming combined with elevated CO2. Nevertheless, temperatures never exceeded a threshold of heat stress, as also confirmed by measurements of chlorophyll fluorescence on dominant grassland species. At the same time, stomatal conductance of species was strongly reduced under drought, especially under future conditions, which likely contributed to higher leaf temperatures and reduced canopy conductance to water vapor. Significant reductions of gross primary productivity under drought, most pronounced under future conditions, also caused a decline of water use efficiency. The combination of warming and elevated CO2 advanced the onset and growth and enhanced above-ground biomass production especially in spring (first cut), and also during regrowth after drought, which partly compensated for the exacerbated losses of biomass production during drought under such future conditions. In conclusion, while the drought scenarios tested for this montane grassland did not lead to any critical heat stress in the summer of 2021, even under warmer conditions combined with elevated CO2, it remains to be studied whether more extreme drought events, combined with heatwaves, could lead to critical thresholds of water and heat stress and erode the resilience of managed grassland in a future world. |