Disentangling effects of an experimentally imposed extreme temperature event and naturally associated desiccation on Arctic tundra

1. Climate projections suggest that extreme events will increase in frequency during this century As tundra is recognized to be among the most vulnerable biomes, we exposed patches of arctic tundra vegetation to an experimental heatwave (by infrared irradiation), followed by a recovery period. The heating increased the surface temperature with an average of 7-6 degrees C during 13 days, which slightly exceeded the longest climatic episode with such a temperature deviation since 1961. 2. The heatwave decreased stomatal conductance (g(s)) and PSII maximum efficiency (F-V/F-m), although there were differences in response among the four target species. Salix arctica Pall. (shrub) was affected during the heatwave and could not recover. In Carex bigelowii Tor. ex Schwein (sedge) and Pyrola grandiflora Radius (forb), on the other hand, the effects on g(s) and F-V/F-m became clear, particularly in the aftermath of the heatwave, whereas Polygonum viviparum L. (forb) was never stressed. 3. Effects of the heat on g, were mainly indirect, through increased desiccation, whereas effects on F-V/F-m, were more related to leaf temperature (although not in all species). The observed changes can therefore probably be ascribed to a combination of heat and drought causing dysfunctions that ultimately led to senescence. 4. Two conclusions of this study, species-specific responses and increased leaf mortality, indicate that more frequent extreme temperature events accompanied by desiccation might alter/endanger tundra communities in a future climate. Predictions of global change effects on arctic ecosystems should therefore take into account the impact of extremes.