Water use of two Mojave Desert shrubs under elevated CO2

Plant responses to elevated atmospheric CO2 have been characterized general ly by stomatal closure and enhanced growth rates. These responses are being increasingly incorporated into global climate models that quantify interac tions between the biosphere and atmosphere, altering climate predictions fr om simpler physically based models. However, current information on CO2 res ponses has been gathered primarily from studies of crop and temperate fores t species. In order to apply responses of vegetation to global predictions, CO2 responses in other commonly occurring biomes must be studied. A Free A ir CO2 Enrichment (FACE) study is currently underway to examine plant respo nses to high CO2 in a natural, undisturbed Mojave Desert ecosystem in Nevad a, USA. Here we present findings from this study, and its companion glassho use 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 st omatal conductance remained constant or was increased under elevated CO2. F ield-grown Larrea did not show a reduced LSR under high CO2, and stomatal c onductance was reduced in the high CO2 treatment, although the effect was a pparent only under conditions of unusually high soil moisture. Both finding s suggest that the common paradigm of 20-50% reductions in stomatal conduct ance under high CO2 may not be applicable to arid ecosystems under most con ditions.