Comparative field water relations of three Mediterranean shrub species co-occurring at a natural CO2 vent

Annual variations in the water relations and stomatal response of Erica arb orea, Myrtus communis and Juniperus communis occurring at a natural CO2 ven t were analysed under Mediterranean field conditions, A distinct gradient o f CO2 concentration ([CO2]) exists between two sites near a natural CO2-emi tting vent, with higher [CO2] (700 mu mol mol(-1)) in the proximity of the CO2 spring. Plants at the CO2 spring site have been growing for generations at elevated [CO2]. At both sites, maximum leaf conductance was related to predawn shoot water potential. The effects of water deficits during the sum mer drought were severe. Leaf conductance and water potential recovered aft er major rainfalls in September to predrought values. Strong relationships between leaf conductance, predawn water potential, and leaf-specific hydrau lic resistance are consistent with the role of stomata in regulating plant water status. Considerable between-species variation in sensitivity of wate r potentials and stomatal characters to elevated [CO2] were observed. Commo n to all the shrubs were a reduction in leaf conductance and an increase in water potentials in response to elevated [CO2]. Elevated [CO2] decreased t he sensitivity of leaf conductance to vapour pressure deficit. Morphologica l characters (including stomatal density and degree of sclerophylly) showed site-dependent variations, but degree and sign of such changes varied with the species and/or the season. Measurements of discrimination against C-13 provided evidence for long-term decreases of water use efficiency in CO2 s pring plants, Analysis of C isotope composition suggested that a downward a djustment of photosynthetic capacity may have occurred under elevated [CO2] . Elevated [CO2] effects on water relations and leaf morphology persisted i n the long term, but the three shrubs growing in the same environment showe d species-specific responses.