Stomatal pore length change in leaves of Eotrigonobalanus furcinervis (Fagaceae) from the Late Eocene to the Latest Oligocene and its impact on gas exchange and CO2 reconstruction

Abstract

It has repeatedly been reported that stomatal pore length changes with changing atmospheric CO2 concentration. Since stomatal pore length affects stomatal conductance, the question arises of how pore length change interacts with the application of stomatal density as a CO2 proxy. In this study, stomatal density, pore length and maximum stomatal conductance are evaluated for Eotrigonobalanus furcinervis (Fagaceae) from the late Eocene, late Oligocene and latest Oligocene. Then a model that simulates plant gas exchange by treating transpiration and photosynthesis as coupled processes and allows also for CO2 reconstruction was applied to the data of E. furcinervis. The model is based on an optimization approach which calculates plant gas exchange as providing a maximum amount of photosynthesis rate under given climate conditions. Stomatal density increases from the late Eocene onwards in E. furcinervis and stomatal pore length decreases. Maximum stomatal conductance which results from both stomatal pore length and density is highest for the late Eocene and lowest for the latest Oligocene. Since both stomatal conductance and stomatal density are usually negatively correlated with CO2, the stomatal data of E. furcinervis appear to be in conflict with a CO2 decrease over that period, as inferred from other evidence. The model results indicate that the stomatal data of E. furcinervis are in fact linked to a decrease of CO2 from the late Eocene to the Oligocene, in accordance with other proxy data although the late Eocene CO2values of this study are lower than usually found for this time period (range between 259 and 640ppm for the late Eocene, 213–581 ppm for the late Oligocene and 258–611 ppm for the latest Oligocene). The seemingly inconsistency between changes of stomatal density, stomatal pore length and stomatal conductance as found in this study is due to the effect on gas exchange exerted by other climate parameters than CO2. The results of this study strongly suggest that any interpretation of stomatal parameters as indicators of CO2 should consider the influence of climate on stomatal conductance and gas exchange.