Until now, scientists assumed that most plants suffer from water stress during droughts: they close their stomata to retain water, stop growing and, in the worst case, wither. As a result, there is a decrease in evaporation and transpiration of water from vegetation, soil and water surfaces – a process that experts call evapotranspiration. “But despite dry and warm conditions, droughts are not occurring at higher altitudes in, say, forested mountain areas,” says Simone Fatichi, senior assistant at the ETH Zurich Institute of Environmental Engineering.

Analyses of observations and computer model simulations from the heatwave of summer 2003 (and recent hot and dry summers) indicate that, during droughts, mountain forests and grasslands at higher elevations release even more water into the air than in “normal” periods of growth with average temperatures and sufficient precipitation.

This is because warmth and abundant sunshine promote vegetation growth. But at the same time, the vegetation has a higher metabolism, and so it essentially sucks every last drop of water from the ground in order to grow. For that reason, evapotranspiration was much greater than expected at higher altitudes during the droughts studied.

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