Variability of Seasonal δ13 C Patterns in Apache Pine from Southern Arizona, USA

Abstract

Seasonal δ13 C changes observed in tree rings offer the potential of reconstructing environmental conditions at finer than annual resolution. In the American Southwest, an opportunity to better expose environmental influences on tree-ring δ13 C at seasonal scales is fortuitously afforded by the presence of a time marker within rings. The strong winter-summer bimodal precipitation distribution is conducive to formation of a false-latewood band in the middle of the growing season, approximately June, after which normal growth usually resumes to the end of the growing season. The variability in seasonal δ13 C patterns in the 1991-1993 growth rings of two Apache pine (Pinus engelmannii) containing these false rings was investigated by descriptive comparison (1) between radii in a tree, (2) between different vertical heights in the trunks of both trees, and (3) between trees. The patterns of seasonal δ13 C change in tree-ring cellulose were broadly similar between radii, but with differences in amplitude and differences in absolute values of up to nearly 2%. Between trees, the isotopic patterns were quite similar: concave downward for 1991 and 1993, and continuously increasing in 1992. There were differences of ca . 0.5‰ among patterns at different heights within a tree, but there was no common gradient in the isotopic change with height. Comparison of the seasonal patterns with environmental variations suggests they are more tightly linked to moisture conditions than to temperature or changes in atmospheric δ13 C.