Physiological and morphological reponses of grassland species to elevated atmospheric CO(2) concentrations in FACE-systems and natural CO(2) springs

Stomatal density, leaf conductance and water relations can be affected by an increase of atmospheric CO2 concentration, thus affecting plant productivity. There is, however, uncertainty associated with the lack of long-term experiments in natural representative ecosystems. In this work, variations in stomatal density and index, leaf water relations and plant biomass of semi-natural grassland communities were analysed under field conditions by comparing plants in three different experimental set-ups (natural CO2 springs, plastic tunnels and mini-FACE systems). Natural degassing vents continuously expose the surrounding vegetation to truly long-term elevated CO2 and may join short-term manipulative experiments. Elevated CO2 concentration effects on stomata persist in the long-term, though different species growing in the same environment show species-specific responses. The general decrease in stomatal conductance after exposure to elevated CO2 was not associated with clear changes in stomatal number on leaf surfaces. The hypothesis of long-term adaptive modifications in stomatal number and distribution of plants exposed to elevated CO2 was not supported by these experiments on grassland communities. Elastic cell wall properties were affected to some extent by elevated CO2. Aboveground biomass did not vary between CO2 treatments, leaf area index did not compensate for reduced stomatal conductance, and the root system had potentially greater soil exploration ability. Considerable between-species variation to elevated CO2 may provide the mechanism for changing competitive interactions among plant species.