Seasonal warming responses of the ecosystem CO2 sink of northern forests are sensitive to stand age

Abstract Northern forests (forests north of 30°N) are major terrestrial CO2 sinks, while rapid warming in these regions can disturb their CO2 sink function. The age-dependent nature of the CO2 sink in forests has been widely reported, but the impact of tree age on the response of forests to rapid climate change remains unclear. Here we use multi-year net CO2 exchange (NEE) observations from 65 northern forest sites to show that warmer springs are associated with increased net CO2 uptake (negative change in NEE (ΔNEE)), whereas warmer summers and autumns are associated with decreased net CO2 uptake. Annually, net CO2 uptake decreased by 5 g C m− 2 yr− 1 across all sites on average. When considering stand age differences, the increased net CO2 uptake during warmer springs was more pronounced in old forests (> 90 years old) compared to young (< 40 years old) and mid-aged (40–90 years old) forests. In addition, the decreased net CO2 uptake during warmer summers and autumns was more pronounced in young forests compared to mid- and old-aged forests. Annually, this resulted in an increase in net CO2 uptake due to seasonal warming for old forests (ΔNEE of -4.8 g C m− 2 yr− 1) and a decrease in young- and mid-aged forests (ΔNEE of 3.2 and 0.8 g C m− 2 yr− 1, respectively). The carbon sink of northern forests would be underestimated by approximately 8% if the impacts of stand age on the responses of NEE to seasonal warming are not taken into account, comparing scenarios of seasonal warming with and without accounting for stand age. In future projections, increasingly uneven seasonal warming may amplify the impacts of stand age on CO2 sinks of northern forests.