Tree-ring δ13C tracks flux tower ecosystem productivity estimates in a NE temperate forest

We investigated relationships between tree-ring delta C-13 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 (Delta C-13) and leaf intercellular CO2 concentration (c(i)) showed significant increasing trends for the dominant deciduous and coniferous species. Delta C-13 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 Delta C-13 over the 1992-2010 period is attributed to increasing annual and summer water availability identified at Harvard Forest and across the region. Higher Delta C-13 is coincident with an enhancement in growth and ecosystem-level net carbon uptake. This work suggests that tree-ring delta C-13 could serve as a measure of forest GPP and be used to improve the calibration and predictive skill of ecosystem and carbon cycle models.