Physiological and morphological responses of grassland species to elevated atmospheric CO <i>2</i> concentrations in FACE-systems and natural CO <i>2</i> springs

Abstract

Stomatal density, leaf conductance and water relations can be affected by an increase in the concentration of atmospheric CO₂, and thus affect plant productivity. However, there is uncertainty about the effects of elevated CO₂ on stomatal behaviour, water relations and plant productivity, owing to the lack of long-term experiments in representative natural 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 can complement 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 to 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. Above-ground 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 capacity. Considerable between-species variation in response to elevated CO₂ may provide a mechanism for changing competitive interactions among plant species.