From Cells to Canopies: Reconstructing Vegetation Structure and its Response to Climate Change in the Middle Cenozoic of Patagonia Using Phytolith Morphology

Abstract

Reconstructing ancient ecosystems to answer outstanding questions in paleoecology requires a multidisciplinary approach. This dissertation research integrates geochronology and conventional phytolith analysis, with a new method for reconstructing vegetation structure (Leaf Area Index) based on the response of epidermal cell (and phytolith) morphology to light. First, a highly resolved U-Pb dated chronology provides precise ages for a densely sampled phytolith record of fossil samples ranging from 42-18 Ma. Second, two experiments were conducted to examine epidermal cell response to varying levels of light. Based on the observation that epidermal cells of shade leaves are more undulated and larger than those of sun leaves in several species of dicots, the first study tested whether this same pattern occurs in grasses in a controlled growth experiment. We found that grass cell undulation is not affected by light, but their morphology changes in other ways. For instance, grass cells tend to have longer epidermal cells in the shade treatments, though this pattern was not found in all species. The second experiment tests the same idea, but uses a different approach. Instead, it tests whether non-grass phytolith assemblages extracted from soils vary with canopy complexity, measured as Leaf Area Index (LAI). We hypothesized that phytoliths from soils under denser canopies will be larger and more undulated than phytoliths growing in more open habitats. Soils were collected across an LAI gradient in Costa Rican habitats and phytoliths were extracted and measured. We found that mean phytolith undulation and mean phytolith area are highly correlated to LAI. We constructed a robust model to predict LAI and using this model, we reconstructed LAI (rLAI) from 44 fossil phytolith assemblages from Patagonia. The synthesis of this research is a long deep-time record of environmental change that relates to climate in southern South America spanning from 50-11 Ma. The results reveal new details about the environmentmental history of temperature and rainfall, and how LAI responds to these variables. Additionally, the rLAI record may track Patagonian habitat response to major climatic changes associated with the opening of the Drake Passage.