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• middle-latitude steppe climate. Average annual precipitation <br />is between 10 and 14 inches, although surrounding mountain <br />areas may receive up to 25 inches. Temperatures average 46 to <br />48 degrees F. and there is a frost-free period of 110 to 130 <br />days (Stover 1991:2.04-18 to 2.04-31). <br />Paleoclimate <br />Reconstruction of paleoenvironmental conditions is <br />essential to the understanding of population movement and <br />cultural change in prehistoric times (Euler et al. 1979). <br />Changing environmental conditions altered the exploitative <br />potential of an area and put stress upon aboriginal cultures <br />by requiring adjustments in their subsistence patterns. To <br />interpret whatever changes are seen in the archaeological <br />record, an account of fluctuations in past climatic conditions <br />must be available or inferences must be made from studies done <br />in surrounding areas. Generally, only gross climatic trends <br />have been established for western North America prior to 2000 <br />B.P. (Antevs 1955; Mehringer 1967; Madsen 1982; Wendlund and <br />Bryson 1974; Peterson 1981). Scientific data derived from <br />investigations of prehistoric cultures and geoclimatic and <br />• bioclimatic conditions on the southern Colorado Plateau over <br />the past two millennia have achieved a much greater degree of <br />resolution (Dean et al. 1985). <br />A study by Berry and Berry (1983:126-130 draft) <br />summarizes the gross climatic episodes of the Colorado Plateau <br />and Basin and Range provinces over the past 13,000 years, as <br />determined from radiocarbon-dated pollen samples. Between <br />11,000 and 8,000 B.C., conditions shifted from glacial to non- <br />glacial; i.e., there was an overall decrease in effective <br />moisture and an increase in temperature. The ensuing pre- <br />Boreal/Boreal period (8000-6500 B.C.) brought cooler, drier <br />conditions to the region. The Atlantic period (6500-3100 <br />B.C.) was one of complexity. In large part, it corresponds to <br />Antev's Altithermal, but indicates two comparatively short <br />phases (6500-5500 and 4750-3950 B.C.) of increased coolness. <br />Between 3100 and 800 B.C., the Sub-Boreal episode saw an <br />increase in effective moisture and, on the Plateau, a <br />corresponding increase in pinyon pine forest. It is notable <br />that Stiger (1981:107-108) suggests pinyon pine disappeared <br />from the Gunnison Basin during this time (possibly corres- <br />ponding to the latest Triple Lakes glacial advance ca 1700- <br />1300 B.C.) and was replaced, in part, by ponderosa pine. <br />Pinyon was apparently never able to regain the higher <br />• elevations of the Gunnison Basin after that period. <br />4 <br />