FREQUENCIES OF MAJOR C-3, C-4, AND CAM PERENNIALS ON DIFFERENT SLOPESIN THE NORTHWESTERN SONORAN DESERT

Of the thirty-three perennial species recorded along line transects at a site in the northwestern Sonoran Desert, nine species were succulen ts from different families (Cactaceae, Crassulaceae, and Agavaceae) an d a further eight were from the Asteraceae. The CAM succulent Agave de serti, the C-3 drought-deciduous Encelia farinosa, and the C-4 bunchgr ass Pleuraphis rigida were codominant, with 33%, 13%, and 15%, respect ively, of the total cover. Agave deserti, which is clonal and had a cl umped distribution, was the most abundant species on all slopes except south-facing ones, where the randomly distributed E. farinosa was dom inant. Encelia farinosa was infrequent on west- and north-facing slope s, were the randomly distributed P. rigida reached its maximum cover. Although intra- and interspecific competition was evident for all thre e species, their frequency was dependent predominantly on microclimate .