Natural Hybridization and Introgression of Pinyon Pines in Northwestern Arizona

Natural hybridization between Arizona singleleaf pinyon (Pinus monophylla subsp. fallax [Little] Zavarin) and Colorado pinyon (P. edulis Engelm.) was studied in northwestern Arizona, using morphological characters. The array of phenotypes indicated introgression of both species. In the Grand Canyon hybridization occurred between elevationally stratified populations of the taxa where flowering phenology was overlapping. Colorado pinyon showed evidence of introgression in the western Grand Canyon area and up the Colorado River canyon into southeastern Utah, as well as southeastward below the Colorado Plateau. Long-distance pollen dispersal may be responsible for introgression into southeastern Utah, and hybridization in northwestern Arizona may have started during the Late Pleistocene.