Correcting tree-ring δ13C time series for tree-size effects in eight temperate tree species

Stable carbon isotope ratios (delta C-13) in tree rings have been widely used to study changes in intrinsic water-use efficiency (iWUE), sometimes with limited consideration of how C-isotope discrimination is affected by tree height and canopy position. Our goals were to quantify the relationships between tree size or tree microenvironment and wood delta C-13 for eight functionally diverse temperate tree species in northern New England, and to better understand the physical and physiological mechanisms underlying these differences. We collected short increment cores in closed-canopy stands and analyzed delta C-13 in the most recent 5 years of growth. We also sampled saplings in both shaded and sun-exposed environments. In closed-canopy stands, we found strong tree-size effects on delta C-13, with 3.7 - 7.2 parts per thousand of difference explained by linear regression vs. height (0.11 - 0.28 parts per thousand m(-1)), which in some cases is substantially stronger than the effect reported in previous studies. However, open-grown saplings were often isotopically more similar to large codominant trees than to shade-grown saplings, indicating that light exposure contributes more to the physiological and isotopic differences between small and large trees than does height. We found that in closed-canopy forests, delta C-13 correlations with DBH were nonlinear but also strong, allowing a straightforward procedure to correct tree- or stand-scale delta C-13-based iWUE chronologies for changing tree size. We demonstrate how to use such data to correct and interpret multi-decadal composite isotope chronologies in both shade-regenerated and open-grown tree cohorts, and we highlight the importance of understanding site history when interpreting delta C-13 time series.