Increasing Respiration Weakens the Carbon Sink Over Two Decades in a Temperate Deciduous Forest

ABSTRACT

Temperate deciduous forests have been major carbon sinks over the past century, largely due to regrowth following historical land use. However, the persistence of this sink is uncertain as forests age under global change. Using two decades (2004–2023) of eddy covariance measurements from a deciduous forest in the northeastern United States, we assessed the drivers of interannual variability and long‐term trends in carbon fluxes. Net carbon uptake declined significantly between 2004 and 2023, decreasing at a rate of −12.5 ± 3.5 g C m ⁻²  year ⁻¹ (trend ±1 SE), driven by a sustained rise in ecosystem respiration (+15.8 ± 2.9 g C m ⁻²  year ⁻¹ ). The long‐term increase in respiration was associated with the direct effects of warming winters and the indirect effects of elevated late‐summer diffuse radiation. Principal component analysis of monthly climate anomalies showed that recurring annual climate patterns explained most of the interannual variability in carbon fluxes. Interannual variability in net ecosystem production was partly explained by climate‐driven variation in growing season length through shifts in spring and autumn timing. Variation in gross primary productivity was explained by climate anomalies that influenced both the timing of spring onset and the magnitude of carbon uptake. Cooler springs followed by dry early summers reduced gross primary productivity, and warmer summers and autumns with favorable diffuse light enhanced it. Together, four principal components explained 42%, 51%, and 45% of the interannual variability in gross primary productivity, ecosystem respiration, and net ecosystem production, respectively, and accounted for 44% and 37% of the long‐term trends in ecosystem respiration and net ecosystem production. Our findings challenge the assumption of a sustained carbon sink in mature temperate deciduous ecosystems under ongoing environmental change, despite this ecosystem experiencing sustained moderate disturbances that might otherwise maintain the net carbon sink status.