Carbon and nitrogen dynamics of temperate and subarctic heath ecosystems with emphasis on cold-season processes

Large amounts of carbon are stored in terrestrial ecosystems and the annual carbon exchange with the atmosphere due to photosynthesis and respiration is high. Terrestrial ecosystems may therefore represent major positive or negative feedbacks to the carbon dioxide concentration of the atmosphere and thus to future climate change.

In order to assess the impacts of global changes we need to understand the controls of important ecosystem processes under the current climate. However, recent research has made it clear that our knowledge of some processes, including the cold season carbon and nitrogen dynamics, is still limited. In this thesis, I investigated the ecosystem respiration and photosynthesis in a temperate heath ecosystem at Mols Bjerge, Denmark, and in subarctic heath and birch understory ecosystems at Abisko, Northern Sweden.

I focused on the cold season fluxes in order to estimate the contribution of cold season respiration and photosynthesis to the annual carbon budget. At the sites in Abisko, possible future changes in snow depth and in the freeze-thaw regime were simulated in situ, to investigate the ecosystem responsiveness to such changes.

Isotopic tracer studies were also performed at both the temperate and the subarctic sites in order to investigate plant nitrogen uptake during the cold season. The main findings include: 1) Cold-season ecosystem respiration and, more surprisingly, also photosynthesis were considerable and important in the annual carbon budget in both the temperate and subarctic ecosystems (Papers II and III). 2) Increased freeze-thaw frequency at the subarctic heath site had little effect on ecosystem carbon exchange and no effect on ecosystem nitrogen exchange (Papers II and IV, respectively) suggesting that the ecosystem will respond slowly to future changes in the freeze-thaw regime. 3) All investigated plant groups at the temperate heath had significant nitrogen uptake throughout the winter, while evergreen dwarf shrubs as the only plant group showed a considerable nitrogen uptake immediately after snowmelt at the subarctic heath site (Paper IV). 4) For the snowmelt period at a subarctic heath and birch understory, a classic temperature-dependent ecosystem respiration model was improved when incorporating a measure of substrate supply, in the form of dissolved organic carbon or nitrogen into the model (Paper I). 5) At the temperate heath, a better model fit, as well as a lower and more realistic temperature sensitivity, was achieved when photosynthetic rates where incorporated into the temperature-dependent model (Paper III).

The results from these model approaches support the recent critique of the wide-spread use of respiration models, which only depend on temperature, and highlight the need for incorporating other potentially important factors into the models.