Introduction

Climate is a principle factor that controls where species occur in nature (Woodward 1987).

As the climate changes so then does the distribution of species. Long-lived plant species have adapted repeatedly to past climate change (see Ackerly 2003). When climate change exceeds species tolerance limits, continued survival is dependent on the species ability to genetically adapt or migrate, or both, to suitable climate. These processes have contributed successfully to the persistence of long-lived plant species in response to past climate change (Davis and Shaw 2001). Their effectiveness under future climate change may be exacerbated by the increased rate of change predicted to occur over the present century. Projections for change in global climate rival historic periods of climate change at an accelerated rate (Houghton and others 2001). The accelerated rate of change threatens to displace current plant species distributions into climatic disequilibrium resulting in an increased potential for extinction of all or portions of species’ ranges (Thomas and others 2001).

The objective for these analyses was to develop bioclimatic models that predict the occurrence of species with small natural distributions in the Western United States and project where suitable climates for the natural occurrence of these species may occur in the future in response to global warming. Three species were selected for the purposes of this analysis. The first was subalpine larch (Larix lyallii), a high-elevation, deciduous conifer inhabiting the Pacific Northwest. The second included two subspecies of Arizona cypress (Cupressus arizonica), the smooth Arizona cypress (C. arizonica ssp. glabra), which is endemic to central Arizona, and the Piute cypress (C. arizonica ssp. nevadensis), which is endemic to the southern tip of the Sierra Nevada. The third was Macfarlane’s four-o’clock (Mirabilis macfarlanei), a long-lived, deep-rooted perennial species, which inhabits mid-to low-level canyon grasslands near the Oregon-Idaho northern border and is listed as endangered by the Bureau of Land Management (BLM) (USFWS 1996).