Spatial expression of ENSO, drought, and summer monsoon in seasonal δ13C of ponderosa pine tree rings in southern Arizona and New Mexico

Abstract

Major climatic impacts from ENSO and the North American Monsoon in the U.S. Southwest provide fortuitous circumstances for exploring the suitability of stable‐carbon isotope (δ13C) composition of tree rings to represent and reconstruct interannual climate variability. We sampled ponderosa pine tree rings at eight sites in seven mountain ranges over a 500‐km transect in southern Arizona and New Mexico. We developed δ13C chronologies for 1985–1995 by subdividing rings into pre‐ and post‐monsoon‐onset segments based on the presence of a false‐latewood band in each ring induced by the arid period immediately preceding the summer monsoon, pooling these segments from multiple trees at each site, and analyzing the cellulose component. Chronologies from two subsets of trees from the same site and separate chronologies from two different sites in the same mountain range indicate strong coherence and signal reproducibility. Additionally, the seasonal δ13C patterns showed remarkable coherence across the transect, with the spatially disjunct easternmost chronology tending to be most frequently dissimilar. The δ13C values of ring subdivisions before the false‐latewood band are generally positively correlated with winter Southern Oscillation Indices (SOI), resulting from the ENSO teleconnection with southwestern winter/early spring precipitation. The δ13C from these sites correlates with monthly Palmer Drought Severity Indices (PDSI), with the strongest correlations occurring in progressively later months from first‐formed to last‐formed subdivisions. The post‐false latewood subdivision δ13C was particularly strongly correlated with summer precipitation amount. These results confirm the strong influence of moisture on tree ring δ13C and support their use in both temporal and spatial modes to infer past climate variability.