ASSESSING THE EFFECTS OF SHADE ON SYCAMORE ECOPHYSIOLOGY: PRESENT AND PAST

Plant community-based approaches of leaf carbon isotopes and epidermal cell wall undulation are two increasingly common methods used to reconstruct light regime and canopy architecture throughout the geological record. The implicit assumption of these approaches is that plant taxa will respond in a predictable manner to changes in light environments. However, most species-level studies looking at cell wall undulation only consider two qualitative factors, “sun” or “shade” leaves, hindering a fully quantitative taxon-specific method. Furthermore, the relationship, if any, between leaf carbon isotopes and epidermal cell wall undulation is unknown. Here we use two experiments to characterize the response of Platanus occidentalis leaves to changes in light environment. In experiment one, saplings were grown under controlled conditions in growth chambers with two different light intensities (~230 ± 82 and ~20.4 ± 18 µmol m^-2 s^-1). However, changes in light quantity in a natural canopy are concomitant with changes in temperature, humidity, and light quality (e.g., decreasing red: far red ratio). In experiment two, saplings were grown at the Waco Wetlands in Waco, Texas with six experimental light conditions: natural light, black shade cloth (30%, 60%, 90%), and green shade cloth (60%, 87%). Leaf morphology, epidermal cell wall undulation (undulation index, UI), epidermal cell area, and carbon isotope values (δ¹³C) were measured on leaves from both experiments. In experiment two, leaf area (R²=0.56), cell area (R²=0.38), and UI (R²=0.88) all increased with decreasing light quantity while δ¹³C became more depleted (R²=0.53). However, light quality did not affect leaf or cell morphology. Our experimental results indicate a clear response in P. occidentalis cell morphology to changes in light quantity that can be used to develop a taxon-specific quantitative proxy for reconstructing the light environment from Platanaceae in the fossil record. Using this proxy, we will then compare the cell anatomy and δ¹³C response of modern P. occidentalis to fossil Platanites raynoldsii from the lower Paleocene Nacimiento Formation (~65.8-62.2 million years ago) in the San Juan Basin, New Mexico to look for evidence of varying light conditions through time.