Changes in vegetation composition and diversity in relation to morphometry, soil and grazing on a hyper-arid watershed in the central Saudi Arabia

The structure and distribution of vegetation are critical components in the function of watersheds that provide many important ecosystem services in the arid regions of the World. Although the majority of these watersheds are highly disturbed by livestock grazing, few studies have examined the interactive effects of such disturbance with morphometry and soil on plant community structure and diversity. In this study, 54 vegetation stands were surveyed along 23 transects placed at set distances, ranging from 300. m to 25. km, from human settlement in Altwiqi watershed, central Saudi Arabia. For each stand, we measured morphometric and soil factors as well as grazing intensity indicators. Vegetation stands were classified into five plant communities based on relative abundances of different growth forms as well as palatable and unpalatable species. Non-metric Multidimensional Scaling (NMDS) ordination and Canonical Correspondence Analysis (CCA) revealed that community structure may be influenced primarily by a combination of morphometric and soil variables of the watershed, and grazing pressure. This combination may determine the distribution and abundance of moisture and nutrients, and perhaps promote habitat specialization and/or competitive exclusion. Those communities occurring on distant sites, with higher elevation, stream order, and fine textured soil close to the ephemeral water channels were more diverse in both species richness and growth forms. However, under heavy grazing near the village, in sites with lower elevation and stream order, the vegetation is dominated by a few unpalatable species on sandy textured soil. The findings of the study have implications for the design of rehabilitation programs through incorporating biotic and abiotic variables that optimize productivity and conserve biodiversity of ephemeral streams in arid regions. © 2012 Elsevier B.V.