COMPLEXITIES OF MODERN LEAF MORPHOLOGY, CLIMATE PROXIES, AND APPLICABILITY IN THE FOSSIL RECORD

Leaf morphological traits including roundness, toothiness, and stomatal densities have been used as indicators of paleoclimate with some success. However, interpretation of these indicators is complicated by a range of evolutionary and environmental interactions. This research investigates a modern species analogue relevant to taphonomic bias and its implications for further study with application to the fossil record. Betula occidentalis (western water birch) was studied at three sites over an 1100 m elevation gradient in southwestern Utah to observe morphological changes in response to variations in temperatures and CO₂ concentrations. Random leaf samples were collected from shrubs growing adjacent to perennial stream channels, thus limiting water stress variability. Leaf samples were analyzed for roundness and toothiness using the ImageJ plugin ObjectJ 1.03p and stomatal densities were calculated across each lamina perpendicular to the midvein. Statistical analyses were completed using R 3.1.0. Results indicate negative correlations between toothiness and elevation (p=0.00004) and stomatal density and elevation (p=0.0002). Conversely, a positive relationship between roundness and elevation (p=0.001) was observed. Due to the responses observed within B. occidentalis and the taphonomic bias in the fossil record for water facultative and obligate species, the complexity and interactions of physiological responses should be considered. Although contrasting results were observed in this study, combinations of continuous morphological traits might provide stronger resolution for paleoenvironmental interpretations.