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Of all terrestrial ecosystems, the productivity of deserts has been suggested to be the most responsive to increasing atmospheric CO2. The extent to which this prediction holds will depend in part on plant responses to elevated CO2 under the highly variable conditions characteristic of arid regions. The photosynthetic responses of Larrea tridentata, an evergreen shrub, to a step-increase in atmospheric CO2 (to 550 mu mol mol(-1)) were examined in the field using Free-Air CO2 Enrichment (FACE) under seasonally varying moisture conditions. Elevated CO2 substantially increased net assimilation rate (A(net)) in Larrea during both moist and dry periods of the potential growing season, while stomatal conductance (g(s)) did not differ between elevated and ambient CO2 treatments. Seasonal and diurnal gas exchange dynamics in elevated CO2 mirrored patterns in ambient CO2, indicating that elevated CO2 did not extend photosynthetic activity longer into the dry season or during more stressful times of the day. Net assimilation vs. internal CO2 (A/C-i) responses showed no evidence of photosynthetic down-regulation during the dry season. In contrast, after significant autumn rains, A(max) (the CO2 saturated rate of photosynthesis) and CE (carboxylation efficiency) were lower in Larrea under elevated CO2. In situ chlorophyll fluorescence estimation of Larrea Photosystem II efficiency (F-v/F-m) responded more to water limitation than to elevated CO2. These findings suggest that predictions regarding desert plant responses to elevated CO2 should account for seasonal patterns of photosynthetic regulatory responses, which may vary across species and plant functional types. (C) 2000 Academic Press.