Dendrochronology relies on the seasonal changes in the wood growth of trees that result in the annual production of rings; each ring starts with large cell elements associated with spring and ends with small cell elements associated with summer and autumn growth. The age of the tree is known by counting these rings. The sequence of rings produced over the years is distinctive and shared by trees of the same species over a broad region. In the western and southwestern United States, the bristlecone pine from the White Mountains of California and the eastern Great Basin has allowed the establishment of a tree-ring chronology of 10,000 years. The California bristlecone pines are found west of the Sierra escarpment's White Mountains, on the Trans-Sierra Valley slopes. The oldest groves of the trees are at an altitude of 13,000 ft (3936 m), with a few hundred trees. The oldest living tree is "Methuselah," at 4,700 years, while some of the dead trees have ages of 8,000 years. Shaped by the wind, their silvery trunks have tightly packed ring sequences. The growth of trees, which occurs from spring to autumn, is marked each year by the formation of a new ring of wood cells. The thickness of the rings is a function of the temperature and precipitation at the time of their formation. The trees of a region experience the same variations in climate and, therefore, present the same series of growth rings for the same data (period) sequence. In 1911, an astronomer, A. E. Douglass, was studying tree rings to correlate them with s spots and climatic changes. He succeeded in establishing one of the most precise dating t hniques used in archaeology. In order for the technique to be used, the tree rings must contain an arrangement of both narrow and wide rings that vary considerably in width. Each of the rings found within the cross section is called an annual ring. A wide annual ring signifies plentiful moisture in the soil, whereas a narrow ring signifies insufficient moisture in the soil for robust growth.
