Climate sensitivity of regional carbon stocks in Canada's western boreal forest

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Canada's western boreal forest stores vast amounts of carbon. Stretching from Manitoba to the Yukon, the managed portion of this forest region contains the equivalent of about 130 years' worth of national greenhouse gas emissions and represents a globally significant carbon pool. However, increases in aridity in recent decades have caused region-wide losses of forest carbon as moisture stress has both reduced productivity (slowed tree growth) and increased mortality. These changes pose a significant threat as warming temperatures and greater aridity together exacerbate the risk of fire, predispose trees to attack by insect pests, reduce supplies of merchantable timber, and trigger damaging climate change feedbacks when carbon stocks are released to the atmosphere. Over the next five years, we will use a coordinated set of field studies and modelling experiments to: (1) evaluate how effects of water availability on the growth and mortality of individual trees vary among species, with local competition, and with site conditions; (2) assess how climatic variation in productivity and biomass turnover within forest stands influence rates of biomass accumulation over time; (3) project how the current species composition, stem size distribution, and carbon density of forest stands across the region are expected to change under future climate and disturbance scenarios. My students and I will establish a network of 400 sites across three major boreal ecozones in western Canada. At each of these sites we will conduct aerial surveys with an unmanned aerial vehicle, and use year-to-year changes in the resulting imagery to measure annual rates of tree growth and mortality. We will use these data to develop statistical models for how climatic factors influence the productivity and mortality of individual trees and larger stands in different ways. We will then incorporate these relationships into an individual-based forest simulator to project how differences in tree growth and mortality translate into changes in forest structure and composition over time. Finally, we will run model-based experiments with this simulator to evaluate how increases in moisture stress affect regional carbon balance, forest structure, and species composition over the 21st century. This research program will provide a clearer understanding of carbon dynamics across a major Canadian forest region for which there is currently little data. Although the conventional thinking is that these forests are a net carbon sink, model projections based on extensive new data could reveal their potential to become a carbon source if moisture stress increases as expected over the coming decades. A better understanding of the sensitivity of the western boreal forest to climate change would resolve important uncertainties concerning its influence on Canada's national greenhouse gas emissions and the global carbon-climate system.