EVOLUTIONARY ISOTOPE ECOLOGY OF MODERN GYMNOSPERMS WITH EXTENSIVE FOSSIL RECORDS CONSTRAINS PALEOCLIMATIC AND PALEOENVIRONMENTAL APPLICATIONS

Foundational work on the meaning of plant carbon isotope values has been ongoing for decades. Recently, meta-analyses of C₃ plants identified relationships between δ¹³C_plant or Δ_leaf (δ¹³C_atmosphere - δ¹³C_plant) and mean annual precipitation and temperature (MAP, MAT), altitude, plant-functional type, and latitude. However, most of those relationships may be too coarse to be applied quantitatively to the geologic record, much of the work has assumed that biochemistry is more important than evolution, and that universal relationships can be derived for all plants that use a given photosynthetic pathway. A number of these foundational assumptions will be tested herein using a series of vignettes that focus on modern isotope ecology of gymnosperms that have extensive Cenozoic fossil records. First, the relationship between δ¹³C_plant and MAP is investigated using both Ginkgo biloba across a wide range of sites globally and using evergreen conifers across a regional precipitation gradient; both types of plants have extenive fossil records and neither follows the “global” meta-relationships between δ¹³C_plant and MAP, wth Ginkgo exhibiting a weaker response and the conifers a stronger response. Secondly, 63 conifer species (15 genera) grown under the same natural environmental conditions exhibit small ranges (±1‰) between individuals of the same species but a large range of up to 8‰ in their Δ_leaf values between genera. The Δ_leaf differences between genera correlate strongly with phylogenetic distance, indicating that there is an evolutionary component to Δ_leaf, and calling into question the assumption of universal δ¹³C_plant-climate relationships. Third, historic records (last ~150 years) of two Thuja species show changes in leaf economics over the period of Industrialization, but constant Δ_leaf as well as δ¹³C_plant changes comparable to the Suess effect, suggesting that they are accurate recorders of δ¹³C_atmosphere, and potentially, that single species can be used as quantitative recorders of water availability. Thus, interpeting δ¹³C_plant values in the geologic record accurately requires some understanding of plant identity/evolutionary history. Taking this into account will allow for more accurate paleoclimatic and paleoenvironmental reconstructions using carbon isotopes of fossil plants.