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- Pirk, Norbert, et al.
(författare)
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Spatial variability of CO2 uptake in polygonal tundra : Assessing low-frequency disturbances in eddy covariance flux estimates
- 2017
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 14:12, s. 3157-3169
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Tidskriftsartikel (refereegranskat)abstract
- The large spatial variability in Arctic tundra complicates the representative assessment of CO2 budgets. Accurate measurements of these heterogeneous landscapes are, however, essential to understanding their vulnerability to climate change. We surveyed a polygonal tundra lowland on Svalbard with an unmanned aerial vehicle (UAV) that mapped ice-wedge morphology to complement eddy covariance (EC) flux measurements of CO2. The analysis of spectral distributions showed that conventional EC methods do not accurately capture the turbulent CO2 exchange with a spatially heterogeneous surface that typically features small flux magnitudes. Nonlocal (low-frequency) flux contributions were especially pronounced during snowmelt and introduced a large bias of -46 gCm-2 to the annual CO22 budget in conventional methods (the minus sign indicates a higher uptake by the ecosystem). Our improved flux calculations with the ogive optimization method indicated that the site was a strong sink for CO2 in 2015 (82 gCm2). Due to differences in light-use efficiency, wetter areas with lowcentered polygons sequestered 47% more CO2 than drier areas with flat-centered polygons. While Svalbard has experienced a strong increase in mean annual air temperature of more than 2K in the last few decades, historical aerial photographs from the site indicated stable ice-wedge morphology over the last 7 decades. Apparently, warming has thus far not been sufficient to initiate strong ice-wedge degradation, possibly due to the absence of extreme heat episodes in the maritime climate on Svalbard. However, in Arctic regions where ice-wedge degradation has already initiated the associated drying of landscapes, our results suggest a weakening of the CO2 sink in polygonal tundra.
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| 2. |
- Pirk, Norbert, et al.
(författare)
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Toward a statistical description of methane emissions from arctic wetlands
- 2017
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Ingår i: Ambio: a Journal of Human Environment. - : Springer Science and Business Media LLC. - 0044-7447. ; 46, s. 70-80
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Tidskriftsartikel (refereegranskat)abstract
- Methane (CH4) emissions from arctic tundra typically follow relations with soil temperature and water table depth, but these process-based descriptions can be difficult to apply to areas where no measurements exist. We formulated a description of the broader temporal flux pattern in the growing season based on two distinct CH4 source components from slow and fast-turnover carbon. We used automatic closed chamber flux measurements from NE Greenland (74°N), W Greenland (64°N), and Svalbard (78°N) to identify and discuss these components. The temporal separation was well-suited in NE Greenland, where the hypothesized slow-turnover carbon peaked at a time significantly related to the timing of snowmelt. The temporally wider component from fast-turnover carbon dominated the emissions in W Greenland and Svalbard. Altogether, we found no dependence of the total seasonal CH4 budget to the timing of snowmelt, and warmer sites and years tended to yield higher CH4 emissions.
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| 3. |
- Pirk, Norbert
(författare)
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Tundra meets atmosphere : Seasonal dynamics of trace gas exchange in the High Arctic
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Arctic environments have experienced strong warming in recent decades, which is affecting the carbon cycle of tundra ecosystems.Degrading permafrost, diminishing snow cover, and changing hydrology are examples of ongoing processes that affect the land-atmosphere interactions and seasonal ecosystem dynamics.Since a number of the affected processes involve the exchange of atmospheric greenhouse gases, such as the trace gases methane (CH4) and carbon dioxide (CO2), there is a potential for these interactions to lead to climate feedbacks.The impact this feedback could have on the global climate is currently not well known due to gaps in our knowledge about the involved processes.One reason for this mismatch is the scarcity of direct, continuous and comparable measurements of CH4 and CO2 fluxes in the high Arctic tundra.The relatively remote, harsh, and low-flux conditions dominating these environments for most of the year pose challenges for flux measurement techniques that have proven to work well at lower latitudes.The present study is, therefore, not only aiming to advance our understanding of trace gas exchanges in the Arctic tundra, but also trying to improve commonly-used flux measurement techniques to yield new insights.The two main study sites of this work are located in permafrost-underlain wetlands in Adventdalen, Svalbard and Zackenberg, NE Greenland.These sites show distinctly different processes that govern the trace gas exchange throughout the different seasons:The snow-free season is characterized by high CH4 emissions, which seem to follow predictable spatial and temporal patterns. Large CO2 uptake by photosynthesis and release by respiration give rise to a large amplitude of net ecosystem exchange during the growing season.The autumnal freeze-in period can feature the highest gas emissions, most likely due to physical mechanisms connected to the soil freezing that release a part of the soil gas reservoir.During winter and spring a low level of microbial activity is sustained but the gas transport capability of the frozen soil is relatively low. Still, snowpack gas concentrations indicate consistent emissions of CH4 and CO2 from the soil.Around the snowmelt period, emissions of stored gases in the snowpack and soil are superimposed on the fast increase of biological activity. The flooded and heterogeneous conditions make the representative flux estimation extremely challenging around this time of year.The diverse range of processes governing the seasonal flux dynamics at these two study sites exemplifies the complexity and possibilities of predicting the resilience and vulnerability of Arctic tundra ecosystems to climate change.
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