ROOT DISTRIBUTION AND SEASONAL PRODUCTION IN THE NORTHWESTERN SONORANDESERT FOR A C-3 SUBSHRUB, A C-4 BUNCHGRASS, AND A CAM LEAF SUCCULENT

To investigate root distribution with depth, which can affect competit ion for water, surface areas of young and old roots were determined in 4-cm-thick soil layers for the C-3 subshrub Encelia farinosa Torrey a nd A. Gray, the C-4 bunchgrass Pleuraphis rigida Thurber, and the CAM (crassulacean acid metabolism) leaf succulent Agave deserti Engelm. At a site in the northwestern Sonoran Desert these codominant perennials had mean rooting depths of only 9-10 cm for isolated plants. Young ro ots had mean depths of 5-6 cm after a winter wet period, but 11-13 cm after a summer wet period. Young roots were most profuse in the winter for E. farinosa, which has the lowest optimum temperature for root gr owth, and in the summer for P. rigida, which has the highest optimum t emperature. Roots for interspecific pairs in close proximity averaged 2-3 cm shallower for A. deserti and a similar distance deeper for the other two species compared with isolated plants, suggesting partial sp atial separation of their root niches when the plants are in a competi tive situation. For plants with a similar root surface area, the twofo ld greater leaf area and twofold higher maximal transpiration rate of E. farinosa were consistent with its higher root hydraulic conductivit y, leading to a fourfold higher estimated maximal water uptake rate th an for P. rigida. Continuous water uptake accounted for the shoot wate r loss by A. deserti, which has a high shoot water storage capacity. A lower minimum leaf water potential for P. rigida than for A. deserti indicates greater ability to extract water from a drying soil, suggest ing that temporal niche separation for water uptake also occurs.