Great Basin Naturalist Memoirs


The existence of a floristic transition zone can be inferred by the fact that a high proportion of indigenous plant species reach a distributional limit within the area. The vascular flora of Washington County, Utah, exhibits this character to a marked degree with 53.6 percent of the native flora reaching a distributional limit within the county. By looking at geographic distributions, ecological preferences, and range termination information for the component species, first approximations are made as to the probable factors mediating plant distributions within the county, particularly of the range-terminating species. The high proportion of range-terminating species in the flora may be accounted for mainly by limiting factors associated with abrupt shifts along two environmental gradients. The first factor is climatic and is mediated largely by altitude; change occurs primarily along a north-south gradient. The second is both climatic and edaphic and is mediated by factors other than altitude per se; it is oriented in an essentially east–west direction. Species with narrower tolerances are shown to be more sensitive to these environmental shifts. Some of the species distributions are better explained by a model involving the effects of interactions between habitat mosaic and genetic homogeneity of given populations on relative migration rates in the transition area. These species may have the capacity to migrate farther, but differences in migration rates give their distributional limits a quasi-stable aspect. These data suggest that species cannot simply be divided into those which are environmentally limited in their present distributions and those which are not. It seems more fruitful to regard these two conditions as extremes on a continuum which can be expressed as migration rate.