Past Rates of Change, Species Richness, and a Model of Vegetational Inertia in the Grand Canyon, Arizona

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Abstract
Two measures of vegetational change in the Grand Canyon illustrate that the greatest changes of the last 20,000 yr occurred between 14,000 and 6000 yr. B.P. with peak values between 12,000 and 8000 yr B.P. for species flux, and peak values between 10,000 and 600 B.P. for change relative to the modern communities. Changes in dominant species show that Wisconsin speices departed from some elevational zones before the arrival of Holocene species, suggesting lowered species richness in the interim. This inference was supported by low species numbers recorded in pack rat middens between 12,000 and 9000 yr B.P. In addition. Holocene species invaded within their modern elevational limits, suggesting that the controlling climatic changes had already occurred well before the invasion. These observations may explain the discrepancies between southwestern paleovegetation records and estimates of the timing of global changes in paleotemperatures. The observed patterns are incorporated into a model "vegetational inertia," which suggests that there was a 1000-3000-yr lag between the elimination of Wisconsin dominants from marginal habitats and the subsequent arrival of Holocene dominants.