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

Abstract

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.

Minor contribution of overstorey transpiration to landscape evapotranspiration in boreal permafrost peatlands

“…At Scotty Creek, mean annual landscape ET estimated with the eddy covariance technique is 295 ± 43 mm·year −1 (reference periods: September to August 2013–2014, 2014–2015, and 2015–2016; Sonnentag et al, in preparation) whereas annual net ecosystem CO 2 exchange between the land surface and the atmosphere is −24.4 ± 0.9 g C·m −2 (Helbig et al, ). The sparse tree density at Scotty Creek allows plenty of light to penetrate into the black spruce canopy and to reach the understorey vegetation layer and ground surface (Chasmer et al, ), allowing ground vegetation transpiration, and ground surface evaporation to play a larger role in the landscape water balance compared with landscapes with denser canopy coverage (e.g., Lafleur, ).…”

Long‐term Impacts of Permafrost Thaw on Carbon Storage in Peatlands: Deep Losses Offset by Surficial Accumulation

“…Peat plateaus have lower apparent C accumulation rates than surrounding nonpermafrost peatlands (Robinson et al, 1999) and have increased aerobic decomposition of active layer peat (Turetsky et al, ). However, monitoring of the greenhouse gas exchange has shown peat plateaus and adjacent nonpermafrost peatlands to have similar annual net C balances (Euskirchen et al, 2014; Helbig, Chasmer, Desai, et al, ). This suggests that accumulation of less labile sylvic peat and reduced soil temperatures in the permafrost layer, compared to peat from nonpermafrost peatlands, may result in comparatively similar rates of C accumulation.…”

Predicting Wetland Distribution Changes under Climate Change and Human Activities in a Mid- and High-Latitude Region

“…Climate change and human activities are arguably the most important factors driving wetland changes [6]. Climate change affects wetland distribution through directly altering the hydrological process [7] and indirectly changing soil temperature, biogeochemical cycles, and vegetation dynamics [8,9]. Human activities such as urbanization [10,11], agricultural reclamation [12], establishing the reservoirs [13], aquaculture [14], and overgrazing [15] can lead to immediate wetland area changes.…”