THE DYNAMICS OF TROPICAL VEGETATIONAL CHANGE DURING THE FINAL STAGE OF THE LATE PALEOZOIC ICE AGE AND SUBSEQUENT DEGLACIATION

Vegetational change in tropical latitudes during the late Paleozoic is hierarchical and can be characterized as within-biome and between-biome. Within-biome change consists largely of persistent dominance-diversity patterns with gradual species turnover, via background extinction and origination, at levels of less than 10% between stratigraphically successive, isotaphonomic sampling points (presumed glacial-interglacial events during some time intervals). Rarely, within-biome change may involve highly elevated short-term levels of extinction that result in restructuring of dominance-diversity. Between biome change results in widespread spatial replacement of one major species pool by another. Within-biome gradual species turnover occurs in those biomes for which there are sufficient long-term data to document it (Pennsylvanian wetlands and Early Permian seasonally dry environments). Both within-biome major turnovers and between-biome replacements appear to be driven by geologically short-term climatic shifts in paleoatmospheric pCO2, temperature, and precipitation patterns between presumed cool and warm earth conditions. Major turnovers reflect ecosystem reassembly after disruption. Replacements reflect ecosystem tracking of climatic change. The best documented major within-biome change occurs at the Middle-Late Pennsylvanian transition (tree ferns replace lycopsids and seed ferns in most wetland habitats). At least three between-biome changes occur during the Early Permian and record an overall trend toward drying in the tropics interrupted by shorter-lived wetter and possibly colder intervals. With each event, the new vegetation consists of progressively more evolutionarily derived species, suggesting long periods of independent evolution in extrabasinal areas before its appearance in the lowlands.