Estimating carbon assimilation rates from fossil leaves and application to the mid-Miocene Clarkia forest.

PREMISE: The rate of carbon assimilation in leaves (A) is a key trait central to a plant's economic strategy that has downstream impacts on the regional and global cycling of carbon and other nutrients. Most previous paleoecological studies estimate A from nearest living relatives or leaf vein density.

METHODS

We present a method for reconstructing A using gas-exchange modeling that requires both measured (stomatal size and density, leaf δ13C) and inferred (e.g., atmospheric CO2 concentration) inputs. We apply this method to ten extant taxa and nine fossil taxa representing common angiosperms of the exquisitely preserved mid-Miocene (~15.9 Ma) flora at Clarkia in northern Idaho, USA.

RESULTS

Application to extant taxa produces estimates of A that are near measured values on the same leaves (R2 = 0.89 across all taxa). Median reconstructed A for fossil taxa range from 9.5-21.7 µmol m-2 s-1 with 95% confidence intervals ~+51%/-38% indicating that most species are statistically indistinguishable. Sensitivity tests show that our method is most reliable when CO2 is well-constrained, but when that is impractical, taxa within single sampling horizons (with a presumed fixed CO2 concentration) can be organized by A into a relative rank order with tighter confidence intervals (~+16%/-14%).

CONCLUSIONS

Following this relative approach at Clarkia, we reconstruct high A for taxa whose modern relatives are characterized by rapid growth and/or riparian habitats (Castanea and Platanus) and corroborate previous interpretations on the ecology of taxa whose modern relatives are less known (Quercus simulata).