SIGNIFICANCE OF EOCENE-OLIGOCENE FLORAS OF THE PACIFIC COAST TO MODELS OF PALEOELEVATION AND PLANT COMMUNITY DYNAMICS

Fossil floras spanning Eocene-Oligocene transition on the pacific coast have long provided the basis for reconstructing the most significant climatic deterioration of the Tertiary Period. When compared with similar age floras from the continental interior, they also form the basis for estimating paleoelevations sand for understanding the dynamics of changing biotic community composition as a response to this major climatic change.

Estimates of paleoelevation for interior upland floras rely on estimates of paleotemperature or paleoenthalpy from isochronous sea level floras. Paleoelevation can be measured by the difference between sea level and interior mean annual temperatures multiplied by the reciprocal of the terrestrial lapse rate. The foremost example for paleoelevation modeling is the latest Eocene Florissant flora of Colorado, for which models indicates a paleoelevation of about 2000 to 3000 meters or more. Pacific coast floras such as Goshen, LaPorte, and Comstock contribute critical data for inferring that the Rocky Mountain region was uplifted to present elevations much earlier than previously thought.

The Eocene-Oligocene climatic cooling involved a decrease in mean annual temperature of about 8 to 10°C, due primarily to a significant decrease in the cold month mean. Plant communities responded to this change not en masse, but rather by the responses of individual species that either became extinct, evolved in place or were preadapted, or dispersed from areas of higher elevation or latitude. Comparisons of Late Eocene and Early Oligocene floras of the Pacific coast and Rocky Mountains provide important insights for understanding these responses of plant community composition to changing climate.