In the American Southwest, a 6,000-year sequence based on tree rings has been used to reconstruct the history and sudden disappearance of the Anasazi Indians. The ring growth of bristlecone pines from the White Mountains, California, has shown evidence of global volcanic activity. In New England, scientists are isolating metallic ions embedded in individual rings to assess the incidence and impact of acid rain. Dendrochronology is based on the premise that the yearly growth of secondary xylem reflects local climatic conditions. The width of the annual rings varies due to changes in temperature, rainfall, light, soil conditions, water, and length of the growing season.