The effects of subterranean rocks on shoot productivity and several root properties were investigated for the sympatric CAM succulent Agave deserti and C4 bunchgrass Pleuraphis rigida. Plants of both species were examined at a site in the northwestern Sonoran Desert along a gradient of soil rockiness, with rocks composing 5% to 60% of the total soil mass. Ten and 50 d after August rainfalls totaling 67 mm, A. deserti had 1.7-fold more newly unfolded leaves in sandy than in rocky soils. The greatest leaf productivity for P. rigida also occurred in the sandiest soils. Fifty d after the summer rain pulse, leaf number for P. rigida had decreased by 66% and 31% in sandy and rocky soils, respectively, but leaf number was still 2.5-fold higher in sandy than in rocky soils. Agave deserti had more than 2-fold greater root surface area and mean rooting depth in sandy than in rocky soils, whereas the root surface area per plant and mean rooting depth for P. rigida were not affected by soil rockiness. Moreover, the hydraulic conductance for root segments of P. rigida collected under rocks and in rock-free soil did not differ. For both species, new root surface area represented 1% to 3% of the total root surface area per plant in both sandy and rocky soils. The distribution of the thick main roots of A. deserti was significantly affected by the presence of rocks, perhaps contributing to the higher productivity of A. deserti in sandy soils. The distribution of the finer main roots of P. rigida was not affected by the presence of rocks, and its greater productivity in sandy soil may reflect both the deeper penetration of water and its greater ability to extract water from a drying soil compared to the succulent A. deserti.

Key words: root anatomy, root hydraulic conductance, root plasticity, soil heterogeneity