New evidence for age-related decline in carbon sinks in intact boreal forests

• We assessed age-dependent NEP changes in intact boreal forests by using gridded global datasets. • NEP magnitude and trend both declined with forest age in intact boreal forests. • Reduced forest productivity primarily drove age-related NEP declines in intact boreal forests. As climate change accelerates, understanding the carbon dynamics of intact forests is critical to climate forecasting, mitigation, and adaptation, but significant uncertainties persist regarding the long-term carbon balance of aging boreal forests. Here, we developed a space-for-time substitution approach that used seven separate net ecosystem productivity (NEP) datasets and one forest age dataset to analyze in detail the age-related changes in NEP of intact boreal forests. Our analysis of NEP dynamics in intact boreal forests uncovered two key patterns. First, old forests exhibited significantly lower NEP compared to younger forests, highlighting an age-related decline in carbon sequestration capacity. Second, temporal trends (2001–2015) revealed increasing NEP with forest age across most datasets; however, this positive trend was markedly weaker in old forests. This widespread positive trend paralleled weak, non-significant increases in air temperature and precipitation across most age classes, suggesting that concurrent climate variability may have contributed to the observed increases in NEP and partly masked age-related declines. Collectively, these findings demonstrate that both the magnitude of NEP and its rate of increase declined with forest aging. Mechanistic investigations elucidated that the observed NEP reduction in aging forests primarily stemmed from a disproportionate decline in forest productivity, which decreased more rapidly than respiratory losses, thereby driving the overall decrease in carbon sequestration capacity. We provided new evidence for previous hypotheses based on spatially separated sites using global-scale gridded NEP datasets. Our results indicate that intact boreal forests progressively lose carbon sequestration capacity as they age, and may become net carbon sources unless climate-driven gains in productivity surpass respiratory losses.