Abstract
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.
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.
| Original language | English |
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| Publisher | University of Copenhagen |
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| Publication status | Published - 2006 |
