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

Canada’s terrestrial ecosystems play a critical role in the global carbon cycle and are being affected by unprecedented climate change and wildfire disturbance. However, we have an incomplete understanding of Canada’s historical carbon cycle. Existing assessments, conducted at varying spatial scales, use a wide range of data sources and methodologies, which lead to significant differences in the estimated strength of Canada’s land carbon sink over recent decades. Moreover, many approaches (e.g., inversions and data-driven estimates) have a limited ability to disentangle the relative contributions of different processes to the carbon sink over the recent past (1700 - 2022). We addressed this gap using a land surface model recently tailored to Canada and the most comprehensive information depicting wildfire disturbance and timber harvest available to make, to our knowledge, the first physically coherent wall-to-wall estimates of all major carbon pools and fluxes for Canada. We show that Canada’s terrestrial ecosystems have been a carbon sink since the mid-20th-century, due to the influence of wildfire and timber harvest before 1940. Since the early 2000s, wildfire disturbance has been driving Canadian forests towards becoming a carbon source. Based on our findings from a purely process-oriented perspective, projected increases in wildfire activity will further impact the strength and direction of Canada's terrestrial carbon sink.