SummaryThe Alpine region is warming fast, and concurrently, the frequency and intensity of climate extremes are increasing. It is currently unclear whether alpine ecosystems are sensitive or resistant to such extremes.We subjected Swiss alpine grassland communities to heat waves with varying intensity by transplanting 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 additionally induce a drought at each site.Heat waves had no significant impacts on fluorescence (F v /F m , a stress indicator), senescence and aboveground productivity if irrigation was provided. However, when heat waves coincided with drought, the plants showed clear signs of stress, resulting in vegetation browning and reduced phytomass production. This likely resulted from direct drought effects, but also, as measurements of stomatal conductance and canopy temperatures suggest, from increased high-temperature stress as water scarcity decreased heat mitigation through transpiration.The immediate responses to heat waves (with or without droughts) recorded in these alpine grasslands were similar to those observed in the more extensively studied grasslands from temperate climates. Responses following climate extremes may differ in alpine environments, however, because the short growing season likely constrains recovery.
Summary• A field experiment was established at 2000 m above sea level (asl) in the central Swiss Alps with the aim of investigating the effects of elevated ozone (O 3 ) and nitrogen deposition (N), and of their combination, on above-ground productivity and species composition of subalpine grassland.• One hundred and eighty monoliths were extracted from a species-rich GeoMontani-Nardetum pasture and exposed in a free-air O 3 -fumigation system to one of three concentrations of O 3 (ambient, 1.2 × ambient, 1.6 × ambient) and five concentrations of additional N. Above-ground biomass, proportion of functional groups and normalized difference vegetation index (NDVI) were measured annually.• After 3 yr of treatment, the vegetation responded to the N input with an increase in above-ground productivity and altered species composition, but without changes resulting from elevated O 3 . N input > 10 kg N ha -1 yr -1 was sufficient to affect the composition of functional groups, with sedges benefiting over-proportionally. No interaction of O 3 × N was observed, except for NDVI; positive effects of N addition on canopy greenness were counteracted by accelerated leaf senescence in the highest O 3 treatment.• The results suggest that effects of elevated O 3 on the productivity and floristic composition of subalpine grassland may develop slowly, regardless of the sensitive response to increasing N.
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