Cumulative disturbance converts regional forests into a substantial carbon source

Abstract

British Columbia’s interior forests (∼400 000 km2) have experienced severe cumulative disturbance from harvesting, wildfires, and mountain pine beetle (MPB). Estimating their impacts on carbon dynamics is critical for effective forest management and climate-change mitigation strategies. This study quantifies the magnitude of historical cumulative forest disturbances and models the effect on regional carbon stocks and emissions using the Carbon Budget Model of the Canadian Forest Service from 1980 to 2018. The study region has been a sustained carbon source since 2003, with an estimated net biome production of −18.6 ± 4.6 gC m−2 yr−1 from 2003 to 2016, dropping to −90.4 ± 8.6 gC m−2 yr−1 in 2017 and 2018 due to large-scale wildfires. MPB affected areas emitted an estimated 268 ± 28 Mt C from 2000 to 2018. Harvesting transferred an estimated 153 ± 14 Mt C to forest products and these areas also emitted 343 ± 27 Mt C in 2000–2018. Areas disturbed by wildfire from 2000 to 2018 generated an estimated 100 ± 8 Mt C of emissions, 73% of which were from 2017 and 2018. Of the area burned between 2014 and 2018, 38% had been previously affected by MPB, highlighting landscape-level interactions of cumulative forest disturbance. Approximately half of decomposition carbon emissions from disturbances in 2000–2018 were calculated as incremental to the decomposition that would have otherwise occurred without MPB disturbance. The average net primary production was reduced by 10% to 335 ± 31 gC m−2 yr−1 from 2000 to 2018. We conclude that cumulative forest disturbance has driven the region’s forests to become a sustained carbon source over the past two decades. While MPB and harvesting were dominant and consistent drivers, recent severe wildfires have prolonged and strengthened the carbon source. Increased disturbances, driven in part by climate change, may limit the ability of regional forests to meet national carbon emission reduction targets.