Seasonal warming responses of the carbon dioxide sink from northern forests are sensitive to stand age

Abstract

Abstract Northern forests (forests north of 30°N) are major terrestrial carbon dioxide (CO2) sinks, while rapid warming can disturb their CO2 sink function. Here we use multi-year net CO2 exchange observations from 65 northern forest sites to show that 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 (4.8 g C m−2 yr-1) and a decrease in young- and mid-aged forests (3.2 and 0.8 g C m−2 yr-1, respectively). In future projections, increasingly uneven seasonal warming may amplify the impacts of stand age on CO2 sinks of northern forests.