Respiratory loss during late-growing season determines the net carbon dioxide sink in northern permafrost regions

Warming of northern high latitude regions (NHL, > 50 °N) has increased both photosynthesis and respiration which results in considerable uncertainty regarding the net carbon dioxide (CO₂) balance of NHL ecosystems. Using estimates constrained from atmospheric observations from 1980 to 2017, we find that the increasing trends of net CO₂ uptake in the early-growing season are of similar magnitude across the tree cover gradient in the NHL. However, the trend of respiratory CO₂ loss during late-growing season increases significantly with increasing tree cover, offsetting a larger fraction of photosynthetic CO₂ uptake, and thus resulting in a slower rate of increasing annual net CO₂ uptake in areas with higher tree cover, especially in central and southern boreal forest regions. The magnitude of this seasonal compensation effect explains the difference in net CO₂ uptake trends along the NHL vegetation- permafrost gradient. Such seasonal compensation dynamics are not captured by dynamic global vegetation models, which simulate weaker respiration control on carbon exchange during the late-growing season, and thus calls into question projections of increasing net CO₂ uptake as high latitude ecosystems respond to warming climate conditions.