PHYSIOLOGICAL FUNCTIONING OF SEASONALLY DRY ADAPTED FLORAS IN RESPONSE TO CHANGES IN LATE PALEOZOIC ATMOSPHERIC COMPOSITION

The Late Paleozoic Ice Age (LPIA; 340–290 Ma) archives repeated climate changes within an icehouse and is the only record of a permanent turnover from icehouse to fully greenhouse conditions since the evolution of metazoans and vascular plants. The LPIA has long been compared to the Pleistocene glacial state given that both were characterized by very low atmospheric pCO₂, eccentricity-scale glacial-interglacial cycles, and extensive, long-lived continental ice sheets. During the LPIA, Carboniferous forest ecosystems underwent repeated restructurings on glacial-interglacial (i.e. long eccentricity; ~405 kyr) to million year timescales in response to changes in water availability and CO₂. Although temporal links between climate and the vegetation restructurings have been suggested, less is known of the physiological thresholds of the individual plant groups, how such thresholds led to changes in communities and biomes through time, or how vegetation restructurings affected climatic and geologic processes through vegetation-climate feedbacks. Utilizing leaf morphologic traits and isotopic values from eight seasonally dry-adapted fossil floras, time-specific atmospheric CO₂, O₂, and O₂:CO₂ values, modeled late Paleozoic meteorology, and a process-based ecosystem model (Biome-BGC 4.2), we evaluate the floras in terms of annual net primary productivity, net biome productivity, leaf area index, evapotranspiration, runoff, and runoff ratio (i.e. runoff/precipitation). Preliminary results show marked changes in the parameters of interest, especially in those related to water cycling and terrestrial runoff, in concert with changes in atmospheric composition. In addition, our results indicate that changes in the composition of the floras (i.e. the gradual disappearance of wet-adapted elements and subsequent rise of plants that were physiologically adapted to lower water availability, culminating in the dominance of conifers) have a great effect of community-wide functioning. These results document the dominant influence of whole assemblages (rather than individual plants) on regional climate and ecosystem dynamics. Our findings provide further evidence that application of a modern plant model to fossil plants communities has the power to illuminate physiological constraints on vegetation shifts.