A Model for Assimilation and Transpiration In Extinct Plants
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 modelsuch as carbon assimilation and evapotranspiration ratesmay be substituted into regional and global climate models, grounding these models in individual plant anatomy.