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

Klaus Steenberg Larsen

    Research output: Book/ReportPh.D. thesis

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    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.
    Original languageEnglish
    PublisherUniversity of Copenhagen
    Publication statusPublished - 2006

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