Effects of CO₂-fertilization on Evapotranspiration

Summary

Concentrations of greenhouse gases are increasing rapidly. It is most likely that the present CO₂ concentrations will be doubled by about the middle of the next century. An important question to answer, when considering the role of water and the hydrological cycle in global change, is to what extent the stomata of plants, whole plants or entire plant communities adapt to changing CO₂ concentrations. Plant response could alter the temporal or spatial distribution of evaporation from terrestrial systems.

Changes in the ambient CO₂ concentrations primarily affect the net photosynthesis of C3-plants, the majority of the plant species. Higher ambient concentrations also increase the concentration gradient which is the driving force for diffusion of CO₂ into the stomatal cavities. This larger gradient and the concurrent higher diffusion rate are counteracted to some extent by a partial closure of the stomata, thus reducing the transpirational loss of water from the leaves. The instantaneous transpiration efficiency, the ratio between assimilation and transpiration rates, thus increases both by enhanced carbon assimilation and simultaneous reduced transpiration.

An enhanced carbon assimilation rate induces a stimulated growth. For annual crops increased assimilation causes a faster canopy development in spring thus reducing soil evaporation and increasing plant transpiration. However, the carbon allocation in the plant not only depends on the growth stage but also on stress factors that limit growth. Plants with limited water or nutrient availability show increased root:shoot ratios when grown at higher CO₂ levels. Plants tend to adjust their carbon allocation so that limitation of growth by different resources is equalized as much as possible.

Long term changes of natural ecosystems are difficult to predict. It is the combination of stress factors, and their spatial and temporal variability, on the one hand and differences of sensitivity among plant species to these factors on the other hand that determine the outcome of competition and the resulting biological diversity.