Late Quaternary vegetation and climate of the Klamath Mountains (California, Oregon)

The Klamath Mountains region of northwestern California and southwestern Oregon is a center of coniferous tree diversity for western North America. Species with divergent climatic and geographic affinities contribute to exceptional local and regional species richness, leading some researchers to believe that forests of the region represent remnants of Tertiary floras that have persisted due to relatively stable and equable regional climate. Despite the floristic importance of the region, its Quaternary history as it relates to contemporary diversity is largely unknown. This study presents evidence of the timing and nature of major environmental changes in the region over the last 23,000 years in order to enhance understanding of the reasons behind the exceptional conifer diversity of the Klamath Mountains.; A sediment core from Twin Lake, in the western Klamath Mountains, yielded a record of environmental change from the last glacial maximum to the present. Sedimentary pollen indicates that open forests of mountain hemlock, fir, and western hemlock dominated the Klamath Mountains throughout the late glacial period. Analyses of modern pollen from 83 lakes in the region indicate that similar forests do not exist in the Klamath Mountains today. In fact, the modern forest composition at Twin Lake developed only in the last 7,000 years. Variations in sedimentary charcoal in the Twin Lake core indicate that rapid changes in forest composition were often accompanied by increased fire activity. It is likely that fire has been an important agent of forest change throughout the late glacial period in the Klamath Mountains.; Taken together, the data indicate that forests of the Klamath Mountains do not represent remnants of Tertiary floras. Late Quaternary climate changes affected rapid and widespread changes in the distributions of conifer species. The region's contemporary forest diversity is in large measure due to topographic, geologic, and climatic diversity that has allowed once common species to survive under an unfavorable regional climate alongside more drought-adapted species. Both regional and local diversity are maximized today because of, rather than in spite of, unstable Quaternary climate.