THE OPTIMALITY PRINCIPLE OF PLANT GAS EXCHANGE AND ITS USE IN PALEOCLIMATOLOGICAL STUDIES

Plant gas exchange shows strong interrelationships with climate. Additionally, the atmospheric CO₂ concentration interferes with the climate responses of various leaf functions. Although knowledge of climate induced changes in plant gas exchange is important because transpiration can itself influence climate, the various interactions between CO₂ and other environmental parameters with respect to their impact on plant gas exchange are still not fully understood. Plant responses to the environment are species-specific, and a straightforward assessment of paleoclimate information provided by traits of fossil plants is difficult.

The use of the optimality principle of plant gas exchange offers the possibility of a systematic evaluation of trait-climate correlations. The optimality principle is based on the circumstance that plants try to lose a minimum of water by transpiration while harvesting as much carbon as possible. It is well established in plant ecophysiology, being confirmed by numerous data. In this contribution we present the recent state of studies performed on the leaves of fossil plant taxa from various sites in Germany and Austria, spanning a time interval from the Late Eocene to the Early Miocene. The optimality principle was used to calculate plant gas exchange and CO₂, combined with paleoclimate proxies to separate influences of CO₂ and climate. Seasonality was considered by calculating temperatures during the vegetation period by using a sinus function.

Evaluation of the results showed changes in plant gas exchange to be caused both by climate and CO₂ concentration. Observed changes in functional traits showed partially unexpected behavior. For example, in Eotrigonobalanus furcinervis, stomatal pore length decreases significantly from the Late Eocene to the Early Oligocene, parallel to a decrease in CO₂. Maximum stomatal conductance, however, also decreases from the Late Eocene onwards. Furthermore, the assessment of stomatal data and application of the optimality principle resulted in quite low atmospheric CO₂ values of about 300 ppm for the Early Oligocene and higher values of about 400 ppm for the Late Oligocene and Early Miocene. The results demonstrate that the optimality principle represents a useful tool in paleoclimatology and paleoecophysiology.