| Ordination procedures were used to reveal a hypothetical moisture gradient which is associated with a community gradient of various desert shrub species. This proposed, synthetic gradient involves a complex interaction of soil and site factors including soil depth, percent clay, percent sand, and drainage position. Unlike past studies conducted in similar communities, factors of salinity and sodicity have little to do with the species distributions in this study. Communities at the mesic extreme of the proposed gradient are dominated by Sarcobatus vermiculatus and Grayia spinosa. Occupying a portion of the gradient which is only slightly less mesic, are communities dominated by Artemisia tridentata. The next more xeric portion of the proposed gradient is dominated by Artriplex confertifolia which is succeeded by a more xeric community of Artemisia spinescens, Atriplex gardneri, Ceratoides lanata, and Kochia americana. Near the xeric extreme of the gradient Tanacetum nuttalli is the only shrub species which can be referred to as dominant whereas nonshrub species such as Machaeranthera grindelioides, Elymus salina, and Pteryxia terebinthina assume importance. The ordination process verifies that reciprocal averaging and polar ordination using percent distance are superior techniques to other polar ordinations or variants of principal components analysis. It was found that a management oriented sampling procedure, using sample units consisting of mapped vegetation types, can be used to carry out ordinations even though this does not meet the accepted sample requirement for homogeneity. |
