Relationships Between Rare Plants of the White Mountains and the Late Cenozoic Geology of the Colorado Plateau

A complex geologic history has shaped the distribution of Arizona willow (Salix arizonica Dorn) and the Mogollon paintbrush (Castilleja mogollonica Pennell). These subalpine plants do not appear to be strict substrate specialists, but they do seem to favor coarse-textured and well-watered soils. Most of their occupied habitats were shaped by Quaternary glaciations, but are ultimately derived from felsic substrates formed before the Pliocene period. Populations of Arizona willow have been identified in the White Mountains of Arizona, the High Plateaus of Utah, and in the Southern Rocky Mountains of New Mexico and Colorado. Species closely related to the Mogollon paintbrush also occur in the Utah plateaus and the Southern Rocky Mountains. Genetic dissimilarity among these populations suggest that these taxa likely share an evolutionary history that extends into the Neogene, when tributaries of the ancestral Colorado River connected young volcanic highlands on the margins of the Colorado Plateau. This history points to the likelihood of additional populations of Arizona willow in the San Juan Mountains, and it suggests that these plants have survived dramatic changes in their environments. These patterns demonstrate the value of analyzing geology at a detailed level when interpreting habitat preferences and distributions of rare species.