Drought-wildfire synergy drives carbon-source transition and amplifies carbon budget uncertainty of boreal forests in Northeast China

Droughts are increasing in frequency and intensity, reducing photosynthesis and causing widespread tree mortality, thereby weakening the boreal forest’s ability to act as a carbon sink. They also indirectly amplify the frequency and severity of wildfires, further affecting the carbon dynamics of boreal forests. We developed a model-coupling framework integrating species-specific drought sensitivity, wildfire dynamics, and forest succession to simulate the carbon dynamics of boreal forests in Northeast China over the 21 st century. Our results show that drought substantially increases tree mortality and limits carbon sequestration. Accumulated fuel loads expand burned area and elevate fire severity. Drought-fire synergy offsets carbon sink gains from vegetation recovery and fire suppression, and they double interannual variability in the regional carbon budget. Under the ssp-5.85 scenarios, boreal forests intermittently shift from carbon sink to source in the late 21 st century. This study emphasizes the importance of considering synergistic drought-fire interactions explicitly when examining the effects of climate change on carbon dynamics of boreal forests. To mitigate these synergies, adaptive forest management should prioritize fuel load reduction measures (e.g., thinning or prescribed burning) and the promotion of drought-resistant tree species, thereby effectively reducing the probability of catastrophic wildfires and enhancing the long-term stability and carbon sequestration capacity of boreal forests.