| Besides the general warming, the IPCC climate scenarios also project extreme climate events to increase in frequency during the next century. However, to our knowledge no ecological field studies have been done on the effects of extreme temperatures on the vulnerable arctic tundra. For this purpose, we exposed patches of tundra vegetation to both a small temperature increase (+2.5°C) during an entire growing season and to one or two (successive) experimental heat waves (+8°C during several days). To generate these increments we used the Free Air Temperature Increase-method, designed to homogeneously heat limited areas of short vegetation in open air. In the past decades, satellite data of Normalized Difference Vegetation Index observed a lengthening of the growing season, which has been ascribed to climate warming. By analysing digital images of the vegetation, we could support this hypothesis and document for the first time that changes in greening under simulated warming can have both a quantitative and qualitative origin. Warmer temperatures did not only promote plant growth at these high latitudes, but also retarded and/or postponed the senescence process. Because tundra eco-systems constitute large stocks of carbon, a second objective was to assess whether warming would influence the 3 main components of the carbon balance: (i) uptake of CO2 by photosynthesis, (ii) loss of CO2 by below ground respiration and (iii) loss of CO2 by canopy respiration. Simulated warming during the entire season hardly affected the total carbon balance of our high-arctic community. It acted as a relatively small net sink both under current and heated conditions. Nevertheless, the turnover increased through significant stimulation of all components. On the other hand, extreme events seem to have the potential to alter the carbon balance and shift this biome towards a source. Both below and aboveground respiration were stimulated by the instantaneous warmer soil and canopy, respectively, outweighing the increased gross photosynthesis. Another objective was to detect possible changes in the plant performance or stress level of the different species, caused by an extreme temperature event. During the first heat wave experiment (2001), plant performance of all species was improved during the extreme event, while in the aftermath the heated plants were more stressed, probably because they acclimated to warmer conditions and experienced the return to (lower) ambient as stressful. In contrast, during the second heat wave experiment (2003), which was accompanied by desiccation, the responses were highly species-specific. The increased leaf mortality indicated that heat waves might alter/endanger tundra communities. This experiment also revealed that effects of the heat on stomatal conductance were mainly indirect through increased desiccation, while effects on PSII efficiency were more related to leaf temperature (although not in all species). In the last experiment (2004), plants were exposed to two successive heat waves. Responses were also species-specific. During the second heat wave, leaf mortality suddenly increased, indicating that the heat stress lasted too long and negatively influenced the species resistance to high temperature. After the heat waves, when plants were exposed to (low) ambient temperatures again, plant performance deteriorated further, indicating possible loss of cold resistance. |
