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Horizontal and vertical zones of influence for root systems of four Mojave Desert shrubs were characterized using P-32 as a nutrient tracer. Larrea tridentata’s horizontal zone of influence was sparse near the plant’s stem base, with a maximum probability of accessin P-32 (P-max) of 41%. However, its horizontal zone of influence extended beyond 5 m, and the distance from the stem base at which the probability of accessing P-32 was half P-max (L-50 similar to 3 m) was significantly greater than the other three shrubs. Ambrosia dumosa’s zone of influence was dense near the plant’s stem base (P-max similar to 78%), but was rare at distances > 2m (L-50 similar to 1 M)Zones of influence for Lycium andersonii and Lycium pallidum were intermediate between those of L. tridentata and A. dumosa. For vertical zones of influence, L. tridentata was more likely to obtain P-32 from 5 m soil depths than A. dumosa, but L. pallidum was not significantly different from either A. dumosa or L. tridentata. Horizontal zones of influence did not change with treatments that altered soil water and nitrogen availability, but vertical zones of influence increased with a flood irrigation treatment that increased water availability to 5 m soil depth. These differences among species likely reflect compromises between their shoot growth strategies and their need to acquire spatially and temporally limited soil resources, especially through competitive interactions. (c) 2005 Elsevier Ltd. All rights reserved.