Water use of two Mojave Desert shrubs under elevated CO2

Plant responses to elevated atmospheric CO2 have been characterized generally by stomatal closure and enhanced growth rates. These responses are being increasingly incorporated into global climate models that quantify interactions between the biosphere and atmosphere, altering climate predictions from simpler physically based models. However, current information on CO2 responses has been gathered primarily from studies of crop and temperate forest species. In order to apply responses of vegetation to global predictions, CO2 responses in other commonly occurring biomes must be studied. A Free Air CO2 Enrichment (FACE) study is currently underway to examine plant responses to high CO2 in a natural, undisturbed Mojave Desert ecosystem in Nevada, USA. Here we present findings from this study, and its companion glasshouse experiment, demonstrating that field-grown Ephedra nevadensis and glass-house-grown Larrea tridentata responded to high CO2 with reductions in the ratio of transpirational surface area to sapwood area (LSR) of 33% and 60%, respectively. Thus, leaf-specific hydraulic conductivity increased and stomatal conductance remained constant or was increased under elevated CO2. Field-grown Larrea did not show a reduced LSR under high CO2, and stomatal conductance was reduced in the high CO2 treatment, although the effect was apparent only under conditions of unusually high soil moisture. Both findings suggest that the common paradigm of 20-50% reductions in stomatal conductance under high CO2 may not be applicable to arid ecosystems under most conditions.