Canada’s forests are shifting from a recovery-driven carbon sink to a disturbance-driven carbon source

Canada’s terrestrial ecosystems are critical to the global carbon cycle and are responding to unprecedented climate change and wildfire disturbance. However, our understanding of Canada’s historical (~1920 - present) carbon cycle is incomplete. There are also no published physically coherent (i.e., those that respect conservation laws) wall-to-wall estimates of all major carbon pools and fluxes for Canada. Existing assessments vary in spatial scale and methodology, yielding notable differences in the magnitude of Canada’s land carbon sink. Moreover, inversions and data-driven estimates do not disentangle the relative influence of disturbance, CO2 fertilization, or climate change on Canada’s carbon cycle. Here, we synthesize information from the site to Canada-wide scale with a land surface model and the most comprehensive wildfire and wood harvest estimates available to provide the first physically coherent wall-to-wall estimates of all major carbon pools and fluxes for Canada. Using factorial model runs, we show that Canada’s terrestrial ecosystems have been a carbon sink since the mid-20th-century, due to wildfire and timber harvest before 1940. Since the early 2000s, wildfire disturbance has been driving Canadian forests towards becoming a carbon source. Continued increases in wildfire activity will further weaken, and may ultimately reverse, Canada’s role as a carbon sink.