A Model for Assimilation and Transpiration In Extinct Plants

Plants vary substantially in physiological performance, both within and among taxonomic groups. Because many performance differences are rooted in anatomy, aspects of plant performance can be quantified in extinct plants. I present a forward model of whole-plant physiology derived from fossil plant anatomy and anchored by a fundamental tradeoff of terrestrial photoautotrophs: plants must lose water in order to assimilate carbon dioxide. This tradeoff imposes scaling relationships between water supply and carbon uptake capacity, observed in living plants at the tissue and whole-plant scales.

Intervein and vein-to-stomatal path lengths for fossil plants were measured and used to estimate leaf capacitance (Kleaf), assimilation rate (Amax), and evapotranspiration rate (E). Applying a range of scaling relationships, including da Vinci's rule of conservation of xylem area between stems and branches, I estimated values of leaf area, as constrained by stem xylem area. Leaf and stem parameters were combined to give whole-plant estimates for Amax and E. When coupled with environmental constraints, they permit estimates of individual plant functional responses to, and effects on, biogeochemical cycles. Preliminary analysis of two Late Paleozoic seed plants, Medullosa and Lyginopteris, shows that they differed significantly in evapotranspiration and assimilation rates at the leaf-specific level. When combined with calculations of canopy area, the two plants show differences in whole-plant assimilation and evapotranspiration rates comparable in magnitude to those between extant ferns and flowering plants.

There are two principal applications for this model. First, an integrated understanding of whole-plant physiology informs individual plant functional responses given ecotypes with different crown sizes, transpiration rates, and environmental parameters. Second, the physiological parameters that are outputs of the model—such as carbon assimilation and evapotranspiration rates—may be substituted into regional and global climate models, grounding these models in individual plant anatomy.