BRINGING LEAF SHADE PROXIES INTO THE LIGHT: WHAT DO UNDULATION INDEX AND LEAF δ13C MEAN IN A FOSSIL FOREST?

The structure of a forest’s canopy and its degree of closure are a key character of any forest ecosystem. Canopy structure impacts community composition and structure, hydrology, and nutrient cycling. But canopy structure is not preserved directly by fossil leaf assemblages and must be measured indirectly from the fossil record. Paleobotanical analyses have focused on light exposure or the degree of shading on fossil leaves as a means of reconstructing forest canopy structure. Forests with completely closed, multi-level canopies will contain leaves grown in a wider variety of light environments (from full sunlight to deep shade) than forests or woodlands with open, rather than overlapping, canopies. Two leaf-based proxies for light exposure are cell wall undulation (undulation index, UI) and leaf carbon isotope composition (δ¹³C). We examined both UI and leaf δ¹³C values from the leaves of two legume species, as well as sediment and leaf δ¹³C values, from a lagerstätte-quality fossil locality, the Mush Valley, from the early Miocene in central Ethiopia. Paleobotanical evidence, including leaf area and nearest living relatives, suggests that this was a wet, warm forest, and the ranges in observed values for both UI and leaf δ¹³C are consistent with leaves grown in a wide spectrum of light regimes. However, if light regime is the primary driver of both UI and leaf δ¹³C, then the two proxies would be expected to correlate with one another, and they do not. This lack of correlation indicates that there are other drivers and/or that the two proxies are recording light exposure at different times in the leaf’s life (i.e., growth versus senescence). We can use this type of multi-proxy comparison and available studies from modern plants to explore the possible drivers of each, especially leaf δ¹³C, but it also points to the need for more extensive modern calibrations.