PI requests funds to collect volcanic rocks suitable for precise dating of 19 fossil floras, chiefly in the region from northern Nevada to Washington and central Idaho. This is required to complete a comparative snythesis of vegetation and climate during the time 14-12 Ma over the region from eastern Washington to the Salton Sea, some 1,700 km (1,000 mi.) southward. All the floras under study record environments directly following a major climatic crisis as revealed by comparison with floras 16-15 Ma. The younger floras show a drastic reduction or virtual elimination of exotic genera, those now in eastern US or Asia, a change reflecting a marked decrease in summer rainfall over the interior. As a result, most species in younger floras closely resemble those in modern vegetation in the western U.S. However, the change was the greatest over the central interior for exotic taxa persisted longer under more favorable climate in coastal regions and to the north. To complete this regional synthesis, radiometric dating of volcanic rocks associated with previously described floras is mandatory so that those of closely similar age will be compared. Research will improve understanding of how vegetation changes in response to climate change.
