Spatial variability of CO 2 uptake in polygonal tundra: assessing low-frequency disturbances in eddy covariance flux estimates

• Published in: Biogeosciences
• Main Authors: N. Pirk, J. Sievers, J. Mertes, F.-J. W. Parmentier, M. Mastepanov, T. R. Christensen
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2017
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Arctic; Svalbard; Climate change; Tundra
• Online Access: https://doi.org/10.5194/bg-14-3157-2017; https://doaj.org/article/34b4ac0d330a4c9ba2ccc789d68c944e

The large spatial variability in Arctic tundra complicates the representative assessment of CO 2 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 CO 2 . The analysis of spectral distributions showed that conventional EC methods do not accurately capture the turbulent CO 2 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 gC m −2 to the annual CO 2 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 CO 2 in 2015 (−82 gC m −2 ). Due to differences in light-use efficiency, wetter areas with low-centered polygons sequestered 47 % more CO 2 than drier areas with flat-centered polygons. While Svalbard has experienced a strong increase in mean annual air temperature of more than 2 K 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 CO 2 sink in polygonal tundra.