Assimilation and transpiration capabilities of rhyniophytic plants from the Lower Devonian and their implications for paleoatmospheric CO2 concentration

The characteristic basic construction of early land plants with an upright posture is represented by a simple leaf- and rootless axis system with a central conducting bundle (‘rhyniophytic habit’). Variations of this simple architectural principle in different early land plant taxa probably reflect different ecophysiological requirements. In this contribution, the assimilation and transpiration of three different Rhynie Chert taxa (Pragian, Lower Devonian) which show this fundamental construction are analyzed in detail: Aglaophyton major, Rhynia gwynne-vaughanii and Nothia aphylla. The analysis was conducted by applying a simulation method based on diffusion through a porous material. The results demonstrate that the capability of gaseous exchange increases from A. major to R. gwynne-vaughanii to N. aphylla. A. major shows the most strict water-conserving strategy of the three taxa. The results are consistent with data from the fossil record concerning ecophysiological strategies and life styles. Furthermore, the results indicate clearly that the structural properties of early land plants reflect an optimization strategy with a fine-tuning of gaseous exchange. The rhyniophytic habit is strongly adapted to and dependent on high atmospheric CO2 concentrations. The results provide evidence that the atmospheric CO2 concentration of the Lower Devonian amounted to roughly 120 mmol/m3.