Tree-ring isotopic pooling without regard to mass: No difference from averaging δ13C values of each tree

Pooling of growth rings from several trees before subsequent isotopic analysis of the mixture may offer the advantage of obtaining “representative” isotopic values fairly rapidly. In principle, however, unless equal masses are taken from the individuals to make the composite sample, the measured isotope value will be biased by the relative mass contribution of the trees, each of which likely has a different isotopic composition. Although biasing associated with direct pooling without regard to mass may happen, this study provides evidence (at least for stable-carbon isotope composition) from three sites in the U.S. Midwest and Southwest that the “error” (probably more appropriately called “difference”) is negligible with respect to error of preparation and analysis, and variability of isotopic composition within and among trees at a site.