Hybridization between Juniperus ashei Buchholz and Juniperus pinchoti Sudworth in Southwestern Texas
Hall, M.T.; Mccormick, J.F.; Fogg, G.C.
Butler Univ Bot Stud 14(1): 9-28
Juniperus ashei of central Texas and J. pinchoti in western Texas hybridize where they meet to form hybrid swarms and introgress as far west as Palo Duro Canyon, and Alpine, Texas. J. ashei is the more mesic species, occupying aquiferous limestones; J. pinchoti occupies dry clayey lowlands where it may successfully compete with grasses. Hybrid swarms are best developed in intermediate habitats where the species meet, e. g., Dryden, Sheffield, upper Devils River, and at the Callahan Divide from Blackwell to Cross Plains, Texas. Introgression of genes of J. ashei into J. pinchoti is measurable from the western Edwards Plateau becoming progressively lower in intensity to the western slopes of the Sacramento Mountains of New Mexico. Bimodal variation is typical of stands growing in habitats where nearly knifeline differences are observed. Where limestone washes (intermittent) with clayey banks sloping to clayey talus support junipers, variants close to J. ashei occupy the limy creek bed, while those close to J. pinchoti occur on the clay slopes. Differential selection in these habitats is quite strong. The results of hybridization and differential selection between these species are marked ecotypic differentiation, enhancement of variability, and increase in survival value as evidenced by recent expansion of stands of introgressants.