STRUCTURE AND FUNCTION IN DYNAMIC CARBONIFEROUS TROPICAL FORESTS: A NEW PERSPECTIVE ON PALEOZOIC PLANT FUNCTION AND PHYSIOLOGICAL FORCING OF CLIMATE

The tropical forests of the late Carboniferous Period (~330–299 Ma) were the most extensive, in space and time, until the radiation of the angiosperms during the end of the Mesozoic and early Cenozoic Eras. However, the Carboniferous tropical forests were dominated by early-diverging vascular plants, monilophytes, and seed plants, in the form of lycopsids, tree ferns, and pteridosperms (“seed ferns”). Because of pervasive extinction among land plants over the last 300 million years, few morphological analogues or close phylogenetic relatives of these once-dominant plants still exist, preventing a full understanding of how these plants responded to dramatic changes in carbon dioxide concentration, pO₂, sea level, and climate. Based on work with living lycophytes and ferns, a prevailing hypothesis has been that Carboniferous terrestrial ecosystems responded and drove changes in key environmental variables only to a limited degree, suggesting a more passive response to environmental conditions than that of modern forests.

To test this hypothesis, we integrate fossil plant anatomy of key Carboniferous plants, biophysical modeling, and stable isotope compositions of plant fossils with process-based ecological modeling and thereby show that these extinct plants were not merely responding to climate but were capable of influencing large-scale fluctuations in atmospheric CO₂ and hydrologic cycling as a consequence of their more dynamic range of physiological functioning than previously considered. Our methods reconstruct the whole-plant traits (e.g., G_cmax, G_wmax, A_max, c_i/c_a, drought tolerance) of paleotropical floral dominants that, coupled with modeling, place constraints on the physiological functioning of late Paleozoic vegetation. Our findings reveal an ecophysiological perspective on these fossil plants that diverges from the conventional view of a low level of function based on their phylogenetic relationship to nearest living relatives and suggest a major role for terrestrial plants in the Paleozoic climate system.