Direct and indirect climate change effects on carbon dioxide fluxes in a thawing boreal forest-wetland landscape.

In the sporadic permafrost zone of northwestern Canada, boreal forest carbon dioxide (CO ) fluxes will be altered directly by climate change through changing meteorological forcing and indirectly through changes in landscape functioning associated with thaw-induced collapse-scar bog ('wetland') expansion. However, their combined effect on landscape-scale net ecosystem CO exchange (NEE ), resulting from changing gross primary productivity (GPP) and ecosystem respiration (ER), remains unknown. Here, we quantify indirect land cover change impacts on NEE and direct climate change impacts on modeled temperature- and light-limited NEE of a boreal forest-wetland landscape. Using nested eddy covariance flux towers, we find both GPP and ER to be larger at the landscape compared to the wetland level. However, annual NEE (-20 g C m ) and wetland NEE (-24 g C m ) were similar, suggesting negligible wetland expansion effects on NEE . In contrast, we find non-negligible direct climate change impacts when modeling NEE using projected air temperature and incoming shortwave radiation. At the end of the 21st century, modeled GPP mainly increases in spring and fall due to reduced temperature limitation, but becomes more frequently light-limited in fall. In a warmer climate, ER increases year-round in the absence of moisture stress resulting in net CO uptake increases in the shoulder seasons and decreases during the summer. Annually, landscape net CO uptake is projected to decline by 25 ± 14 g C m for a moderate and 103 ± 38 g C m for a high warming scenario, potentially reversing recently observed positive net CO uptake trends across the boreal biome. Thus, even without moisture stress, net CO uptake of boreal forest-wetland landscapes may decline, and ultimately, these landscapes may turn into net CO sources under continued anthropogenic CO emissions. We conclude that NEE changes are more likely to be driven by direct climate change rather than by indirect land cover change impacts.