Simulated heat waves affected alpine grassland only in combination with drought

The Alpine region is warming fast, and concurrently, the frequency and intensity of climateextremes are increasing. It is currently unclear whether alpine ecosystems are sensitive orresistant to such extremes.
We subjected Swiss alpine grassland communities to heat waves with varying intensity bytransplanting monoliths to four different elevations (2440–660 m above sea level) for 17 d.Half of these were regularly irrigated while the other half were deprived of irrigation to addi-tionally induce a drought at each site.�Heat waves had no significant impacts on fluorescence (Fv/Fm, a stress indicator), senes-cence and aboveground productivity if irrigation was provided. However, when heat wavescoincided with drought, the plants showed clear signs of stress, resulting in vegetation brown-ing and reduced phytomass production. This likely resulted from direct drought effects, butalso, as measurements of stomatal conductance and canopy temperatures suggest, fromincreased high-temperature stress as water scarcity decreased heat mitigation through tran-spiration.
The immediate responses to heat waves (with or without droughts) recorded in these alpinegrasslands were similar to those observed in the more extensively studied grasslands fromtemperate climates. Responses following climate extremes may differ in alpine environments,however, because the short growing season likely constrains recovery.