High-carbon primary forests as conservation targets in Ontario, Canada

Ontario’s forests are changing rapidly under the pressure of industrial logging, coupled with the effects of climate change, which is facilitating increased wildfires. The reduction of mature and old-growth forests negatively affects ecosystem functions, including wildlife habitat suitability and long-term carbon storage. Here, we present an operational mapping tool to identify and monitor the extent of high-carbon primary forests (HCPF) to support their protection and track progress toward the Kunming-Montréal Global Biodiversity Framework’s Target 3. We define HCPF as mature and old-growth, naturally regenerated forests that have not experienced tree canopy disturbance over the past 50 years and are located within unfragmented landscape patches with a minimum area of 1,000 ha. By integrating existing maps, satellite-based products, and detailed manual image interpretation, we mapped 28.4 Mha of HCPF in Ontario for 2020. HCPF comprised 80.7% of all forests outside industrial timber concessions, whereas within private and industrial forests, their percentage was much lower (29.7%). They represented less than 2% of all forests within the Mixedwood Plains ecozone of southern Ontario. The HCPF vegetation carbon stock density is higher than that of fragmented and recently disturbed forests by 7% within the entire province, and by 18% within the region of industrial forestry. Ontario’s unfragmented landscapes contain more than 72% of the total soil organic carbon stock of the province, illustrating their importance for long-term carbon storage. Unfragmented landscapes are essential for the survival of Ontario’s woodland caribou populations; however, their percentage within most caribou population ranges was below the management threshold of 65% and continued to decline. From 2021 to 2025, Ontario’s HCPF area decreased by 3.5% (0.2 Mha/yr) due to wildfires and logging. Within the region of industrial forestry, the proportion of 2021–2025 logging within HCPF exceeded that in non-HCPF forests, and the annual logging area within HCPF increased by 55%. This acceleration of logging and associated road construction illustrates that HCPF, as opposed to secondary forests, continue to be the major source of timber in Ontario. The HCPF method provides the tools and data required to accurately assess and monitor mature and old-growth primary forests, and to develop effective conservation strategies and policies at the provincial and national levels. As demonstrated for Ontario, the HCPF method can be applied to all provinces to facilitate the protection of some of the last remaining primary forests in Canada.