Warming in northern high latitudes has increased both photosynthesis and respiration, creating uncertainty about the net CO2 balance. Using atmospheric observations (1980-2017), this study finds that early-growing season CO2 uptake trends are similar across tree cover gradients. However, late-growing season respiration increases significantly with tree cover, offsetting photosynthesis and slowing the rate of annual net CO2 uptake in areas with higher tree cover. This seasonal compensation effect explains differences in net CO2 uptake trends along vegetation-permafrost gradients and is not captured by dynamic global vegetation models, questioning projections of increasing net CO2 uptake under warming.

Zhihua Liu, John S. Kimball, Ashley P. Ballantyne, Nicholas C. Parazoo, Wen J. Wang, Ana Bastos, Nima Madani, Susan M. Natali, Jennifer D. Watts, Brendan M. Rogers, Philippe Ciais, Kailiang Yu, Anna-Maria Virkkala, Frederic Chevallier, Wouter Peters, Prabir K. Patra, Naveen Chandra