Regional expression of the 1988 U.S. Midwest drought in seasonal δ13C of tree rings

Abstract

This study analyzes δ13C of subdivisions of the 1988 and 1989 growth rings from three conifer species (Pinus resinosa, Pinus strobus, and Picea glauca) across a network of nine sites over four states in the upper U.S. Midwest to infer seasonal and interannual environmental moisture conditions. Both years were characterized by different spatial patterns of drought (expressed through rainfall and Palmer Drought Indices), with the area of 1989 drought being smaller and shifting westward relative to 1988. Rings were subdivided into four equal segments from which holocellulose was separated. The range of δ13C variation within each ring was typically 1–2‰ for both years, and the single site where all three species co‐occurred revealed coherent seasonal δ13C patterns for the pines but no correlation of the spruce δ13C patterns with those of the pines. Various averaged combinations of monthly Palmer Drought Severity Index (PDSI) and precipitation assigned to each subdivision versus both δ13C and discrimination (Δ ≈ δ13Cair − δ13Cplant) yielded significant correlations (generally higher with PDSI) and even stronger correlations when the spruce data were removed. The fraction of average monthly cumulative precipitation (relative to 1900–2001 means) for these 2 years was more strongly related to δ13C than either monthly precipitation or monthly fraction of average precipitation, but was not as strongly related to δ13C as was PDSI. Furthermore, the pattern of increasing δ13C in 1988 observed at several sites is an indicator of the increasing moisture stress of that year, and the difference between the 1988 and 1989 average isotopic compositions of the third and fourth subdivisions matched the regional moisture shift well.