Reconstructing the early Paleocene light environments using fossil Platanites from the San Juan Basin, New Mexico, and its implications for stomatal pCO2 reconstructions.

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In the last two decades, stomatal proxies have been extensively used to reconstruct the concentrations of paleoatmospheric CO2 (pCO2) based on the well-established negative correlation between stomatal density (SD) and pCO2. However, various light environments within a fossil flora, known to influence many plant traits, including SD, were rarely discussed in most previous pCO2 reconstructions. We collected one well-preserved early Paleocene flora from the San Juan Basin, New Mexico, that is dominated by Platanites. We then analyzed the light environment of this flora based on leaf epidermal cell wall undulations, quantified by undulation index (UI), and the range of leaf carbon isotope. We found a negative correlation between UI and SD, which indicates that leaves under higher light intensities produce higher SD. Importantly, we observed a positive correlation between UI and reconstructed pCO2 using a leaf gas-exchange model, and we recommend future pCO2 reconstructions using stomatal proxies also assess variations caused by canopy light environments.

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