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

The Late Paleozoic Ice Age (LPIA; 340–280 Ma) was climatically dynamic, with repeated warmings within a protracted icehouse and is 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, Earth’s tropical forest ecosystems underwent repeated restructurings on glacial-interglacial to million-year timescales. Although it has been long appreciated that these early vascular plants responded to climate and, in turn, influenced atmospheric CO₂ through terrestrial carbon sequestration, the role of vegetation-climate feedbacks, including the nature of physiological thresholds of individual plant groups and how such thresholds led to changes in communities and biomes through time, have not been investigated quantitatively. We evaluate the physiological functioning of eight early Pennsylvanian through latest early Permian seasonally dry-adapted fossil floras in terms of annual net primary productivity, net biome productivity, leaf area index, evapotranspiration, runoff, runoff ratio (runoff/precipitation), and Water Use Efficiency. We do so utilizing taxa-specific leaf morphologic traits and isotopic values, time-specific atmospheric CO₂, O₂, and O₂:CO₂ values, and site-specific meteorology modeled using Genesis 3.0, all applied to a modified version of the process-based ecosystem model Biome-BGC 4.2. Results document marked changes in the parameters of interest in response to changes in aridity and atmospheric composition, including thresholds that result in a complete loss of physiological viability in specific plant types. Modeled thresholds and physiological changes provide mechanisms for the observed changes in composition and dominance of Carboniferous-Permian floras (i.e. the gradual disappearance of wet-adapted elements and rise of plants that were physiologically adapted to lower water availability, culminating in the dominance of conifers) and offer insight into the effect of these changes on regional climate and carbon and water cycling. Our findings highlight the unique insight that can be gleaned for extinct plant communities through process-based ecosystem modeling and illuminate the potential for placing physiological constraints on paleo-vegetation shifts.