Water course vegetation on granitic and calcareous substrates in the Eastern Mojave Desert, California

Desert water courses are topographically distinct features with periodic flooding, which are floristically rich and structurally complex. This study was undertaken to determine if plant species and vegetation types change along desert water courses as environmental gradients are crossed. In four granitic and four calcareous water courses, vegetation patterns were classified and described for 262 samples in the eastern Mojave Desert, California. In a broad altitudinal range from mountain slope to bajada, vegetation samples were systematically taken at 300 m intervals in a 9 km distance of each water course. A total of 15 alliances and 32 associations were delineated using Ward's method cluster analysis and Twinspan and were validated by Bray-Curtis ordination. The classification included alliances of one forest, two woodlands, two intermittently flooded shrublands, two temporarily flooded shrublands, and eight shrublands. Of these alliances, ten were at the canyon position, seven were at the arroyo position, and six were at wash position. Alliances dominated by Acacia greggii, Chilopsis linearis, and Prunus fasciculata were the most widely distributed, at more than one topographic position and on both granitic and calcareous substrates; however, these alliances had more than double the number of associations found specifically on limestone as compared to granite. Environmental gradients showed strong relationships to the overall vegetation patterns as expressed by multi-response permutations procedures, Indicator Species analyses, and Bray-Curtis ordinations. Vegetation types appear to be expressions of moisture and temperature gradients, nutrient availability, and regional locations as functions of aspect, elevation, geologic substrate, geographic position, surface rock cover, and topographic position. The influence of topography and geologic substrate was apparent across all watercourses. Each water course had differential species and at least one distinct alliance per topographic position, and the water courses had differential species and vegetation types depending on geologic substrate.