PHOTOSYNTHETIC RESPONSES OF 3 CODOMINANT SPECIES FROM THE NORTH-WESTERN SONORAN DESERT - A C-3 DECIDUOUS SUB-SHRUB, A C-4 DECIDUOUS BUNCHGRASS, AND A CAM EVERGREEN LEAF SUCCULENT

To investigate seasonal and annual influences of environmental conditi ons on leaf net CO2 uptake (A), three codominant species from the nort h-western Sonoran Desert differing in photosynthetic pathway and leaf phenology were examined: the C-3 deciduous sub-shrub Encelia farinosa, the C-4 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 pr oductivity index (EPI) model previously developed for CAM plants was u sed, which scaled the responses of A to water, temperature, and photos ynthetic photon flux (PPF) over 24-h periods to individual dimensionle ss values. The net CO2 uptake predicted using the EPI approach agreed well with field measurements. Agave deserti was the most drought-toler ant and E. farinosa was the least; the optimum day/night air temperatu res and the PPF requirement for A were highest for FI rigida and lowes t for A, deserti. For 1974 through 1995, daily EPI averaged over a yea r 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 dif ferences among the three species in wet years. A for 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 a lso highest in the winter. For the evergreen A. deserti, which based o n ground cover is the dominant species at the field site, new leaves u nfolded in response to both winter and summer rainfall but most photos ynthetic area was contributed by older leaves, leading to the highest annual plant net CO2 uptake.