Tree-ring δ ¹³C tracks flux tower ecosystem productivity estimates in a NE temperate forest

We investigated relationships between tree-ring δ¹³C and growth, and flux tower estimates of gross primary productivity (GPP) at Harvard Forest from 1992 to 2010. Seasonal variations of derived photosynthetic isotope discrimination (Δ¹³C) and leaf intercellular CO₂ concentration (c_i) showed significant increasing trends for the dominant deciduous and coniferous species. Δ¹³C was positively correlated to growing-season GPP and is primarily controlled by precipitation and soil moisture indicating that site conditions maintained high stomatal conductance under increasing atmospheric CO₂ levels. Increasing Δ¹³C over the 1992-2010 period is attributed to increasing annual and summer water availability identified at Harvard Forest and across the region. Higher Δ¹³C is coincident with an enhancement in growth and ecosystem-level net carbon uptake. This work suggests that tree-ring δ¹³C could serve as a measure of forest GPP and be used to improve the calibration and predictive skill of ecosystem and carbon cycle models.