Photosynthetic Responses of Three Codominant Species from the North-western Sonoran Desert - a C3 Deciduous Sub-shrub, a C4 Deciduous Bunchgrass, and a CAM Evergreen Leaf Succulent

To investigate seasonal and annual influences of environmental conditions on leaf net CO2 uptake (A), three codominant species from the north-western Sonoran Desert differing in photosynthetic pathway and leaf phenology were examined: the C3 deciduous sub-shrub Encelia farinosa, the C4 deciduous bunchgrass Pleuraphis rigida, and the CAM evergreen leaf succulent Agave deserti. To allow interspecific comparisons and to predict field responses from 1974 through 1995, an environmental productivity index (EPI) model previously developed for CAM plants was used, which scaled the responses of A to water, temperature, and photosynthetic photon flux (PPF) over 24-h periods to individual dimensionless values. The net CO2 uptake predicted using the EPI approach agreed well with field measurements. Agave deserti was the most drought-tolerant and E. farinosa was the least; the optimum day/night air temperatures and the PPF requirement for A were highest for P. rigida and lowest for A. deserti. For 1974 through 1995, daily EPI averaged over a year was highest for E. farinosa, indicating that it operates closest to its photosynthetic optimum. However, the predicted A was highest for P. rigida. Variations in A were annually bimodal, with the greatest differences among the three species in wet years. Afor all three species increased linearly as annual rainfall increased. Leaf area per plant for E. farinosa was highest in the winter and early spring and did not respond appreciably to summer rainfall; leaf area for P. rigida was also highest in the winter. For the evergreen A. deserti, which based on ground cover is the dominant species at the field site, new leaves unfolded in response to both winter and summer rainfall but most photosynthetic area was contributed by older leaves, leading to the highest annual plant net CO2 uptake.