Implications of treeline shift and increasing wetland cover for regional climate in northwestern Canada

Abstract:

Forest, wetland, lake and tundra ecosystems of northern ecoclimatic regions are undergoing rapid changes in extent and spatial distribution in response to increasingly warmer air temperatures. Through various feedback mechanisms these changes are projected to have consequences for how northern ecoclimatic regions function as part of the climate system. A wealth of studies has documented a northward shift of the treeline into areas presently occupied by tundra. This expansion in forest cover is contrasted by a decline in forest cover along the southern limit of permafrost where forest with degrading permafrost is replaced by permafrost-free, treeless wetlands and lakes. Both types of land cover change are expected to alter the surface energy balance and thus regional climate given the distinct differences in surface characteristics between forests and tundra, and between forests and wetlands. We measured latent and sensible heat fluxes using the eddy covariance technique together with net radiation and ground heat fluxes as part of an evolving meso-network of micrometeorological towers in the Northwest Territories. Experiencing similar meteorological conditions each, one pair of instrumentation was installed over a “northerly” boreal forest (68° 19’ N; 133° 31’ W) and a nearby tundra site (68° 44’ N; 133° 31’ W) and one pair over a “southerly” boreal forest-wetland and a nested wetland site (61° 18’ N; 121° 18’ W). Here, we contrast differences in surface characteristics and energy balance components between northerly boreal forest expansion and southerly increasing wetland cover to discuss respective implications for regional climate in northwestern Canada. Peak sensible heat fluxes at the tundra and wetland sites were ~150 W m^-2 and ~200 W m^-2 smaller than at the paired boreal forest and boreal forest-wetland sites, respectively. Differences in latent heat fluxes during the snow cover period were negligible. The tundra and wetland sites showed ~100 W m^-2 and ~50 W m^-2 higher peak latent heat fluxes, respectively, compared to their corresponding boreal forest and boreal forest-wetland sites. These results suggest that the warming trend along the southern limit of permafrost might be attenuated, whereas additional regional warming might occur through the northward shift of the treeline.