Tree—ring samples of Pseudotsuga menziesii (Mirb.) Franco, Pinus ponderosa Laws., and Pinus edulis Engelm. for 1860 through 1962 were obtained from sites along a vegetational gradient, ranging from forest interior sites to semiarid lower forest border sites. Samples were analyzed using a stratified nested plot design. Near the lower forest border tree rings are narrowest, the variability in the relative ring—width response from year to year is greatest, and the variance in common or the correlation between radii and between trees is highest. This variability is more highly related to climatic fluctuations than the variability in highest. This variability is more highly related to climatic fluctuations that the variability in forest interior trees and may be attributed to longer and more frequent periods during which water stress is limiting to physiological processes in the trees. Hence, at the arid forest border tree—ring chronologies contain the most consistent, but variable, growth responses and provide the best record of climatic fluctuations. The semiarid lower forest border trees exhibit a high frequency of partial growth layers. Chronologies from the forest interior exhibit very few partial growth layers. Crossdating between both types of sites and among many individuals of different species makes absolute dating not only possible but exceedingly reliable. Such tree—ring analyses may have considerable application to the evaluation of ecological forest gradients.
Three trees which began growing in the 12th century lean in a manner not characteristic of neighboring Douglas-firs and have tree-ring patterns showing changes that do not appear to reflect climatic influences. Two old trees, now dead, have limb stubs that were cut by stone tools. It is possible the prehistoric Indians cultivated trees so that in a relatively short time a single root system would produce several limbs suitable for use as construction timbers.
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.
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