Dynamic Carboniferous tropical forests: new views of plant function and potential for physiological forcing of climate

Abstract

Contents 1333I.1334II.1335III.1339IV.1344V.1347VI.134713481348References1348 Summary The Carboniferous, the time of Earth's penultimate icehouse and widespread coal formation, was dominated by extinct lineages of early‐diverging vascular plants. Studies of nearest living relatives of key Carboniferous plants suggest that their physiologies and growth forms differed substantially from most types of modern vegetation, particularly forests. It remains a matter of debate precisely how differently and to what degree these long‐extinct plants influenced the environment. Integrating biophysical analysis of stomatal and vascular conductivity with geochemical analysis of fossilized tissues and process‐based ecosystem‐scale modeling yields a dynamic and unique perspective on these paleoforests. This integrated approach indicates that key Carboniferous plants were capable of growth and transpiration rates that approach values found in extant crown‐group angiosperms, differing greatly from comparatively modest rates found in their closest living relatives. Ecosystem modeling suggests that divergent stomatal conductance, leaf sizes and stem life span between dominant clades would have shifted the balance of soil–atmosphere water fluxes, and thus surface runoff flux, during repeated, climate‐driven, vegetation turnovers. This synthesis highlights the importance of ‘whole plant’ physiological reconstruction of extinct plants and the potential of vascular plants to have influenced the Earth system hundreds of millions of years ago through vegetation–climate feedbacks.

The prospects for constraining productivity through time with the whole‐plant physiology of fossils

“…That does not mean the actual act of shutting the stomata will happen faster in ferns, but it does happen sooner. The hypothetical physiological scenario outlined for the Carboniferous (Wilson et al ., ) would have had the opposite environmental impact of what was suggested. To be clear, this does not diminish the undoubtedly large impact on climate of the mobilization of groundwater via transpiration since the Devonian evolution of deep‐rooting trees.…”

“…Here, we will explore the resistance mismatches that led Wilson et al . () to attribute angiosperm‐like physiological rates to Carboniferous plants and what we believe to be more reasonable interpretations of their significance.…”

“…200 to 300 μm thick and some can be as thin as 100 μm or less (Zwieniecki & Boyce, ; cf. Wilson et al ., ). In practice, however, leaf thickness cannot be considered in isolation from vein density.…”

“…As pointed out (Wilson et al ., ), low and high conductances can be associated in the same plant. However, if mismatches in resistance cannot be used to overrule a particular organ system that does not conform to a desired interpretation, then what do they mean?…”

“…Coal accumulation rates should be insensitive to incremental changes in plant biomass production, driven instead by the climate and tectonic factors that allow organic matter burial and preservation (Nelsen et al ., ; cf. Wilson et al ., ). However, changes in plant productivity are thought to be pivotal for the feedback between rock weathering and atmospheric CO 2 and, thereby, also climate.…”