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Abstract:
With increasing air temperatures and shifts in precipitation patterns forecasted in the Arctic over the
coming decades, thawing of ice-rich permafrost is expected to change the hydrological conditions in large parts
15 of the region by creating mosaics of wetter and drier areas. The objective of this study is to investigate how
lowered water table depths of formerly wet floodplain ecosystems affect CO2 fluxes measured with a closed
chamber system, focusing on the roles of changes in vegetation community structure and soil thermal
characteristics. We found that a decade-long drainage significantly increased the abundance of shrubs but
decreased that of Eriophorum angustifolium, which subsequently made Carex species dominant. These two
20 changes had opposing influences on photosynthetic uptake during the growing season: increased abundance of
shrubs slightly increased gross primary production while replacement of Eriophorum by Carex significantly
decreased it. Drainage also diminishes the heat capacity and thermal conductivity of soil, leading to increased soil
temperatures in shallow layers during the daytime and decreased soil temperatures in deeper layers, and therefore
reduced thaw depths. This soil temperature regime can intensify growing-season ecosystem respiration by up to 93 % theoretically. Overall, drainage increased net CO2 uptake (net ecosystem 25 exchange) by 16 % over 20 days
in 2013 but decreased it by 37 % over 66 days in 2014. During the frozen season, the drained transect emitted
four times more CO2 than the undrained transect. In summary, the net effect of these complex changes recently
weakened net CO2 uptake in the drained areas.
