Boreal and sub-arctic forest dynamics in the face of climate warming
The boreal forms a circumpolar belt of forest between 45˚ and 70˚N and is the second largest forested biome. It is important in both climate regulation and the global carbon cycle. Boreal forests occupy latitudes that are expected to warm most dramatically over the coming decades, and evidence indicates that changes are already underway in these systems. The boreal extends over a large climatic gradient thus mechanisms underlying boreal ecosystem response are likely variable, necessitating a process-based understanding across latitudes. Because permafrost thaw is one of the most immediate and system-altering manifestations of climate change across the entire boreal, our focus is on documenting responses of forests impacted by the presence of and changes in permafrost coverage and/or depth and the linkages and feedbacks of these changes with hydrology (Quinton and Hayashi) and gas flux (Sonnentag).
The southern margin of permafrost is especially susceptible to thaw-induced land-cover change, since in this region, the permafrost is discontinuous, relatively thin (< 10 m), warm and ice-rich. Climate warming appears to have disrupted the balance between degradation and aggradation processes in these systems. In zones of discontinuous permafrost, permafrost forms the physical foundation on which trees develop, forming tree-covered peat plateaus where trees are likely to contribute to permafrost maintenance and aggradation processes through the reductions in radiation load and changes in snow accumulation. Evidence suggests that warming is leading to permafrost thaw and surface subsidence, which decreases forest cover while increasing wetland hydrological connectivity. Despite dramatic impacts of warming on the hydrology of these systems, we know little about corresponding changes in plant communities, interactions or feedbacks between vegetation and hydrological responses or impacts on ecosystem productivity.
Northern boreal forests near the forest-tundra transition have a markedly different set of limitations but are also likely to be highly sensitive to warming processes. Here, permafrost is continuous but the northern extent and spatial distribution of the forest is considered temperature-limited. The permafrost at the forest-tundra transition is continuous and relatively thick (>100 m). As such, permafrost degradation is characterized by a thickening active (seasonally thawed) layer, with changes to soil moisture and carbon cycling. The implications of these changes for plant communities have received much less study than warming-related distributional shifts. Quantification of dynamic responses of trees near the transition zone indicate reduced growth and decoupling of growth-climate relationships; however, data aimed at quantifying response of these forests to climate change are limited.
This aspect of our research focuses on the 550,000 km2 Taiga Plains ecoregion that occupies 48% of the Northwest Territories’ landmass, and spans much of the latitudinal gradient of the Territory and its associated permafrost, climate and vegetation gradients. Large forest dynamics plots will be established at the end member sites (Scotty Creek and Havikpak/Trail Valley Creek)and form the first boreal sites within the Smithsonian Institute’s Global Earth Observatory Network (SIGEO; http://www.sigeo.si.edu/). These larges plot sites will be connected by a representative and spatially extensive network of pre-existing small permanent sample plots through collaboration with the Government of the Northwest Territories ENR Forestry Division (see Laurier-GNWT Partnership website: http://www.wlu.ca/homepage.php?grp_id=12612).