Paper No. 1
Presentation Time: 1:40 PM

FOSSIL LEAF VENATION AS AN INDICATOR OF PRIMARY PRODUCTIVITY THROUGH TIME


BOYCE, C. Kevin, School of Earth, Energy, and Environmental Sciences, Stanford University, 450 Serra Mall, Bldg. 320, Stanford, CA 94305 and ZWIENIECKI, Maciej A., University of California, Davis, Davis, CA 95616, ckboyce@stanford.edu

As long as underlying physiological mechanisms are understood, the failure of a proxy to perform as expected can be as informative as its success. Leaf gas exchange rates, including transpiration and assimilation, have been linked to leaf vein density in living plants and vein density has been used as a readily measurable proxy of physiology for fossil plants. However, problems arise when physiological expectations are compared to details of the fossil record. Declining CO2 over the Cretaceous has been suggested as an evolutionary driver of the high leaf vein densities (7 to 28 mm mm-2) that are unique to the angiosperms throughout all of Earth history. Photosynthetic modeling indicated the link between high vein density and productivity documented in the modern low-CO2 regime would be lost as CO2 concentrations increased, but also implied that plants with very low vein densities (less than 3 mm mm-2) should experience substantial disadvantages with high CO2. Thus, the hypothesized relationship between CO2 and plant evolution can be tested through analysis of the concurrent histories of alternative lineages since an extrinsic driver like atmospheric CO2 should affect all plants, not just the flowering plants. No such relationship is seen. Regardless of CO2 concentrations, low vein densities are equally common among non-angiosperms throughout history and common enough to include forest canopies and not just obligate shade species that will always be of limited productivity. Modeling results can be reconciled with the fossil record if maximum assimilation rates of non-flowering plants are capped well below those of flowering plants, capturing biochemical and physiological differences that would be consistent with extant plants but previously unrecognized in the fossil record. Although previous photosynthetic modeling suggested that productivity would double or triple with each Phanerozoic transition from low to high CO2, productivity changes are likely to have been limited before a substantial increase accompanying the evolution of flowering plants.