Dendrochronological Modeling of the Effects of Climatic Change on Tree-Ring Width Chronologies from the Chaco Canyon Area, Southwestern United States

摘要

Hypotheses about the causes of the growth and decline of the Chacoan regional interaction system in the southwestern United States between A.D. 900 and 1200 are evaluated against tree-ring evidence and the results of an empirical model (PRECON) that computes the statistical relationships between climate and ring-width indices during the 20th century and applies the results to hypothesized precipitation or temperature changes. The statistical responses of 23 indexed conifer ring-width chronologies from New Mexico and Colorado to variations in monthly temperature and precipitation were calculated. Simulated decreases in prior autumn-winter precipitation markedly reduced ring widths, while decreased current summer precipitation was less effective, sometimes reducing ring width or having little effect. Decreased prior winter temperature slightly reduced ring width, while decreased growing season temperature usually increased or did not effect ring widths. Evaluated in terms of these results, the Chaco Canyon area tree-ring record (1) indicates that favorable climatic conditions in the 10th, 11th, and early 12th centuries fostered the growth of the Chacoan system, (2) shows that dry autumn-winter and summer conditions in the middle 1100s contributed to the downfall of the system, (3) does not support the proposition that centuries-long climatic fluctuations evident in southwestern Colorado affected Chaco Canyon, (4) does not support the idea of shifts from summer-to winter-dominant precipitation regimes, and (5) contributes little to assessing the role of anthropogenic environmental change in the collapse of the Chacoan system.