IMPACT OF LEAF VARIATION ON THE FRANKS MODEL FOR ANCIENT CO2 RECONSTRUCTION

Metasequoia glyptostroboides is the living representative of the genus Metasequoia which has a ~100Ma history with wide and abundant fossil records. It has been widely used as a model plant for paleophysiology, paleoenvironment, and paleoclimate studies. For example, by applying the relationships of the morphological and biochemical features of this species and its growing conditions to Metasequoia fossils, paleoenvironmental and paleoclimatic parameters such as temperature, precipitation, and atmospheric CO₂ concentration of various locations with various geological ages were reconstructed.

Leaf gas-exchange models, such as the Franks model, are the most popular CO₂ reconstructing methods used in the recent decade, and Metasequoia has played a major role due to its well-studied living species and abundant fossils. Previous studies have revealed a wide variety of morphological and biochemical features of the species. To investigate the range of variety and how it can impact the accuracy of the reconstructed CO₂ by Franks model, we sampled 87 leaves from a mature Dawn Redwood tree, covering variations of branch types, orientations (sun vs shade leaves), tree heights, and locations on a branch. Besides bulk leaf carbon isotope (d¹³C) values, cleared leaf epidermis (CLE) samples made from these leaves allow us to precisely measure stomatal parameters feeding the Franks model, including whole leaf stomatal density (SD-WL), guard cell length (GCL), and guard cell width (GCW).

Our results show that there is insignificant difference of variations of GCL and GCW, while SD-WL is clearly different between sun and shade leaves. As expected, there are clear differences in the d¹³C values of sun and shade leaves (mean values of -27.58‰ and -28.43‰ respectively). Additionally, d¹³C values for leaves from different heights are distinct (-29.12‰ for the lowest and -28.35‰ for the highest). The d¹³C values bear little difference between leaves of different parts of the same branchlet. Leaves from different branch types do exhibit variation on their d¹³C values. Our data represent the first comprehensive investigation on the variations of parameters feeding the Franks model, which provide valuable insights into better reconstruction of ancient CO₂ using leaf gas-exchange models by selecting appropriate leaf samples.