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Elevated CO2 may improve the productivity of cool-season active (’drought-deciduous’) shrubs in the deserts of southwestern North America by reducing early-season phenological constraints imposed by low leaf area when photosynthetic capacity is high and later-season physiological limitations from declining photosynthesis and midday water potentials. Altered productivity under elevated CO2 would depend on the specific responses of short-shoots that only provide early-season leaf area display, and long-shoots which determine annual growth increment in these plants. We measured plant water relations, photosynthetic gas exchange, and growth in short- and long-shoots of the drought-deciduous shrub, Lycium andersonii, under Free Air CO2 Enrichment (FACE) in the field in an intact Mojave Desert ecosystem. We were specifically interested in the differential effects CO2 enrichment would have on short-shoots and actively growing long-shoots during canopy development. Net photosynthesis (A(max)) was similar in elevated compared with ambient CO2, but stomatal conductance (g(s)) was reduced by 27% in both shoot types. L. andersonii growing in elevated CO2 had larger leaves on short-shoots, and more leaves per shoot length on long-shoots. Enhanced leaf growth did not counter lower g(s) and midday plant water potential was similar between treatments. In both short- and long-shoots, down-regulation of light-saturated photosynthetic electron transport rate (J(max)) occurred under elevated CO2. However, the balance between rubisco efficiency (estimated by the maximum carboxylation rate of rubisco, V-cmax), and electron transport capacity (V-cmax/J(max)) remained constant in short-shoots, but increased in elevated CO2 grown long-shoots. Apparent quantum requirement was similar, while light-saturated photosynthetic rates (A(max)) decreased by approximately 30% under elevated CO2 in both shoot-types. These results suggest that elevated CO2 lowered investment to photosynthetic electron transport capacity and whole-plant water use, even when leaf growth was stimulated. Such canopy dynamics are likely to enhance the ability of this drought-deciduous species to better cope with the highly variable inter- and intra-annual climate regimes characteristic of North American deserts. (C) 2002 Elsevier Science B.V. All rights reserved.