Prospects for reconstruction of seasonal environment from tree-ring δ13C: baseline findings from the Great Lakes area, USA

Plant δ13C and discrimination (Δ≈δ13Cair−δ13Cplant) embody ecophysiological response to environmental conditions influencing rates of carbon fixation and stomatal conductance. This study scrutinizes the seasonal variation of δ13C and Δ in growth rings developed in 1990, as represented in trees of 11 species at three sites along a 5° latitude transect in Illinois and Wisconsin, USA. Radial stem xylem growth of the trees was monitored with monthly punch cores, and the resulting growth curves precisely identified the growth period represented in the five segments into which each growth ring was subdivided. Seasonal δ13C variation of cellulose was usually 1–1.5‰, and the seasonal patterns were similar among angiosperm and conifer trees at a site, with angiosperms generally 13C-depleted by 0.5–1.5‰. Seasonal δ13C (and Δ) patterns was different among sites, however, and regression with several environmental parameters revealed most frequent significant correlation with soil moisture (Palmer Drought Severity Index [PDSI]). The site-averaged seasonal Δ patterns track each of their divergent local seasonal drought index chronologies very well, even when isotopic curves of the different trees are simply averaged without adjustment for different growth rates. Seasonal tree-ring δ13C analysis is affirmed as a valuable method for assessing seasonal moisture status. Furthermore, results favor successful extension to ancient tree rings where different species and slope aspects may be represented in wood samples at a collection site, and where rates of ring development and seasonal δ13Cair are not likely to be known.