The Late Eocene (34 Ma) volcaniclastic deposits of the Florissant Fossil Beds National Monument (Colorado) contain abundant well-preserved remains of plants and a wide range of insect taxa. Paleoecological analysis of the deposits, based on integration of palynological, stratigraphic, and geochemical data, suggest that the Florissant ecosystem was subject to chronic ecological stress. The terrestrial vegetation was commonly devasted by holocaustic fires, most of which probably originated in drier communities on surrounding slopes. Precipitation was marginal, relative to the high rate of evaporation, and lake water levels fluctuated over a wide range. During low-water intervals the lake water was highly alkaline, limiting aquatic biodiversity. This is consistent with the fine preservation of fossil plant and insect material and the general absence of an indiginous lacustrine fauna. One drop in lake level lasted for several centuries, permitting the establishment of a redwood/red cedar-dominated forest on the former lake bottom. A subsequent rise in lake level resulted in the permineralization of the bases of the standing trees, today represented by the numerous petrified stumps throughout the Monument area.

Key words: Eocene, Florissant, Oligocene, paleobotany, paleoecology, palynology