Surface energy exchange and land-atmosphere interactions of Arctic and subarctic tundra ecosystems under climate change
Abstract: The surface energy balance determines the functioning of any ecosystem on the Earth but is still poorly understood in Arctic and subarctic biomes. In a dynamic system, such as the Earth’s climate, any change in its characteristics modifies the exchange of energy, water, and greenhouse gases between the surface and the atmosphere. Therefore, this thesis aims to draw a conclusive picture of the surface energy exchange and land-atmosphere interactions of Arctic and subarctic regions under climate change. The aims are achieved by combining in-situ field measurements of surface energy balance components, snow manipulation experiments, active layer monitoring, vegetation mapping, and chamber-based carbon dioxide flux measurements from Arctic and subarctic tundra biomes in Greenland, Svalbard and northern Sweden.
Local variability in climate, surface structure, soil moisture and soil thermal regime are the main drivers of variation in the surface energy exchange and ecosystem productivity of Arctic and subarctic tundra ecosystems. At all studied locations, the magnitude of the energy fluxes of sensible heat (H), latent heat (LE) and ground heat (G) were well-correlated with net radiation (Rnet). However, evapotranspiration (ET) and LE showed a relatively strong coupling to atmospheric vapor pressure deficit (VPD), with more pronounced such control at the dry tundra sites compared to the wet-growing ecosystems. Snow and permafrost determined surface energy balance, energy partitioning and ecosystem productivity. Manipulated increase in snow accumulation at a subarctic tundra peatland complex in northern Sweden resulted in permafrost thaw, soil wetting and increased carbon sequestration. Concurrently, climate-driven increase in both snow accumulation and air temperature triggered dramatic and rapid permafrost degradation in peatland complexes and transition from dry habitats into wet-growing ecosystems, with consequent change in surface energy exchange towards both increased LE and ET at the cost of H. Interannual variability in winter snow accumulation at the high-Arctic tundra environment in Zackenberg (Northeast Greenland) prolonged the growing season during a year with low snow cover and increased the total accumulated energy balance components of the local heath and fen ecosystems. Further, energy flux partitioning at the heath was strongly determined by the reduction of soil moisture as snow is by far the main supplier of water in this region. The energy exchange of the fen, however, showed attenuated behavior due to groundwater table remaining close to the surface.
The results presented in this thesis suggest that in a future climate, accelerated permafrost thaw and increased interannual variability in snow cover may further modify the energy balance of Arctic and subarctic ecosystems, with profound impact on ecosystem adaptation capacities and the overall climate system.
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