Ring-width chronologies in Douglas-fir, pinyon pine, and Utah juniper show some distinctly different characteristics and exhibit highly predictable relationships with variations in climate. Narrow rings in Douglas-fir are largely the result of low precipitation and high temperatures of the previous June, low precipitation during August through February, low precipitation and low temperatures during March through May, and low precipitation and high temperatures of the current June. Narrow rings in pinyon pine are largely a function of low precipitation from October through May, but high July temperatures near the end of the growing season may also exert an influence. Narrow rings in Utah juniper are the result of low precipitation and high temperatures during the previous October through November, low precipitation during December through February, and low precipitation and high temperatures during March through May. A biological model for these relationships is proposed. The tree-ring chronology from A.D. 1273 through 1285 exhibits a clearly defined drought which exceeds in length and intensity any dry period occurring since A.D. 1673. A comparison of the chronologies from species which are influenced differently by summer precipitation indicates that during this period both summers and winters must have been dry. However, the A.D. 1273-1285 drought at Mesa Verde was surpassed by six other droughts of greater intensity during the period A.D. 500–1300. The A.D. 1273–1285 drought may be only one of several factors in a chain of events which led to the decline of prehistoric population in the Mesa Verde.