Two markedly different geologic formations, the Baum limestone and Trinity sand and an area of intermixed talus were compared for several physical attributes and these correlated with the structure of the plant communities occupying them and with the variability of a conspicuous constituent for each of the three habitats. (1) Juniperus Ashei occupies the Baum Limestone in nearly pure stands, but with wide spacing, while the general cover consists of Bouteloua hirsula and Andropogon scoparius. The limestone has a high carbonate content, high hygroscopic coefficient, low soil moisture. (2) Juniperus virginiana occupies the Trinity sand on cleared, reverted land normally supporting a post-oak, black-jack oak forest. The junipers are widely spaced with oak forest in various stages of regeneration. Woody cover as well as ground cover is greater on the Trinity than on the Baum. The Trinity has low carbonate, low hygroscopic coefficient, high soil moisture. (3) The juniper population on the mixed talus sites are hybrid but closer to Juniperus Ashei. The various factors measured are intermediate between the other two habitats. (4) The data suggest that in southern Oklahoma J. Ashei develops plentifully in areas where the moisture content is low enough to limit J. virginiana, but as the habitats of the two species intergrade hybridization occurs, i.e., currently as well as historically, with only those hybrids surviving which are adapted to the intermediate habitats. (5) These mixed habitats are rich sources for continued hybridization and introgression until succession closes both the mixed habitats and the Trinity sand to junipers.