Recognition of Time-Equivalent Leaf-Based Signals of Atmospheric CO2 and El Niño Variability in Florida Wetland Species

Trees are equipped with a plastic phenotype, capable of sustained adjustment of numbers of leaf stomata to changes in atmospheric CO₂ concentration. With high temporal resolution and accuracy, stomatal frequency data demonstrate that Holocene climate evolution has been influenced by century-scale CO₂ fluctuations. Apart from adapting to changes in atmospheric CO₂, leaf-epidermal properties are known to be sensitive to environmental factors such as water availability. In long-lived hygrophilous plants, epidermal tissue expansion is likely to be significantly influenced by changes in water availability. Concurrent analysis of the leaf morphology in CO₂ sensitive trees (Myrica, Quercus, Acer) and a water-stress sensitive fern species (Osmunda regalis) from leaf assemblages preserved in peat deposits in Florida (USA), reveal distinct temporal changes in epidermal properties over the past 130 years. Stomatal frequency changes in the deciduous trees reflects the human induced CO₂ increase. Epidermal-cell density changes in fern leaves, could well be interpreted in terms of El Niño / La Niña related precipitation trends. By quantifying the leaf morphological adaptation to known environmental conditions during historical times, a new paleobotanical proxy for past precipitation changes is introduced. Hence, in ENSO-sensitive regions, analysis of buried leaf assemblages offers the unique possibility of a direct recognition of time-equivalent leaf-based signals of paleo-atmospheric CO₂ and El Niño variability.