Long-term changes in Southwest tree growth. Millennia-length tree-ring width index chronologies from the Southwest showing anomalous post-1976 growth surge in trees that are primarily sensitive to cool season precipitation. Trees included in these chronologies grow in isolated high-elevation sites with harsh growing conditions where direct human-related impacts, such as logging or livestock grazing, have not occurred.

The upper plot shows average tree-ring growth for the six high-elevation sites in Arizona and New Mexico compared with cool-season precipitation records from meteorological stations in the Southwest, 1910-1990. The lower plot is a simple average of the six high-elevation chronologies for the past 1000 years. The post-1976 growth increase is unprecedented during the past 1000 years, and it is obvious that conditions as dry as the 1950s drought have occurred several times during the past 1000 years. When fire history records are compared with precipitation-sensitive tree growth records, analyses reveal predictable patterns of wet and dry years prior to fire occurrence.