Heating of the lower atmosphere caused by increasing concentrations of greenhouse gases is most pronounced at higher latitudes. For a number of climatic variables Global Circulation Models (GCM) predict, not only a change of the average but also a rise in frequency and intensity of extreme values, e.g. heatwaves. The main objective of this research is a characterisation of the sensitivity of tundra to these two aspects of climate change. Because of the adaptation to low temperatures and the low metabolism of many tundra species, it can be expected that ecological communities of this biome will already be relatively strongly disturbed by small environmental fluctuations. Effects of extreme conditions on the other hand are almost completely unknown.
Increased temperature is experimentally achieved by the Free Air Temperature Increase technique (FATI) in a tundra site in Northeast Greenland. Besides continuous thermal manipulations during the growing season also shockwise infrared radiation causing higher temperature increments will be applied over shorter periods of time, in order to simulate climatological extremes. Both the changes in carbon balance of tundra-ecosystems and the vegetation dynamics associated to this will be monitored. Characteristic plant communities are irradiated in selected test plots during a complete growing season, to quantify the physical-energetical impact of warming up on vegetation, air and soil. CO2-fluxes of the whole ecosystem, and the photosynthetically active component and the soil separately will be analysed as a function of their dominant drivers (radiation and temperature), in order to reconstruct a complete carbon-balance over the growing season. Soil respiration will be analysed in detail as a function of depth, water content, temperature, C- and N-level and time. Both the direct consequences of higher temperatures on soil activity and the indirect consequences can be determined this way. Aboveground, we will use a classical `point-quadrat' method, digital camera vegetation records, normalised difference vegetation index sensors (NDVI), destructive production determinations and chlorophyl measurements. In this way it will be studied whether leaf development is earlier in spring, whether complete seasonal dynamics are fastening and whether interactions of radiation and temperature change the timespan of the potential growing season. Also changes in species composition en diversity trends are determined. In the shock-wise manipulations, attention will be focused on the components of stability of the ecosystem (resistance and resilience).
In order to extrapolate the obtained results from the continued heating experiment and the heat-wave experiment to the longer term, an existing model will be elaborated. This model (Huggett, 1993) starts with an `empty' system which gains carbon through photosynthesis and looses carbon by respiration. We will take into account measured relationships between microclimate, input and output.
In the feedback between the carbon balance of this ecosystem to the atmospheric CO2-concentrations there is potential to speed up or slow down the actual rate of C-accumulation in the atmosphere, a crucial factor of uncertainty in predicting the future climate. This research investigates whether, besides steady changes also shockwise transitions should be taken into account. Logistically there is cooperation with the Danish Polar Center (DPC), Copenhagen, which operates an international research station since 1995 in Northeast Greenland (Zackenberg).