Frequencies, Microclimate and Root Properties for Three Codominant Perennials in the Northwestern Sonoran Desert on North- vs. South-facing Slopes

At a site in the northwestern Sonoran Desert the percent ground cover for the C $ subshrub Encelia farinosa was eighttimes higher on more arid 20° south-facing slopes than on 20° north-facing slopes at 820 m elevation, and was sixtimes higher on north-facing slopes at a 300-m-lower elevation, also the more arid condition. The ground cover of the C % bunchgrass Pleuraphis rigida decreased over 50% from 20° north-facing slopes to the more arid conditions of a 36° north-facing slope, a 20° south-facing slope and a 20° north-facing slope at a 300-m-lower elevation. The CAM leaf succulent AgaŠe deserti also had greater ground cover for the 20° north-facing slopes at 820 m compared with 520 m. For these three codominants that averaged 58% of the total ground cover, the key for the relative frequency of E. farinosa was apparently its greater root growth on the warmer slopes during the winter. The key for the other two species was most likely soil water availability, especially during the seedling stage for A. deserti. The wetter soil conditions on 20° north-facing slopes at 820 m apparently led to individual plants of P. rigida that were twice as large as on south-facing slopes. Thus root properties may exert the primary influence on relative plant frequency in this desert ecosystem for which soil temperature and water availability are crucial. # 1997 Annals of Botany Company