Modeling Species' Realized Climatic Niche Space and Predicting their Response to Global Warming for Several Western Forest Species with Small Geographic Distributions

The Random Forests multiple regression tree was used to develop an empirically based bioclimatic model of the presence-absence of species occupying small geographic distributions in Western North America. The species assessed were subalpine larch (Larix lyallii), smooth Arizona cypress (Cupressus arizonica ssp. glabra), Piute cypress (Cupressus arizonica ssp. nevadensis), and Macfarlane's four-o'clock (Mirabilis macfarlanei). Independent variables included 33 simple expressions of temperature and precipitation and their interactions. These climate variables were derived from a spline climate model for the Western United States that provides point estimates (latitude, longitude, and altitude). Analyses used presence-absence data largely from the Forest Inventory and Analysis, USDA Forest Service database. Overall errors of classification ranged from 1.39 percent for Macfarlane’s four-o’clock to 3.55 percent for smooth Arizona cypress. The mapped predictions of species occurrence using the estimated realized climatic niche space were more accurate than published range maps. The Hadley and Canadian general circulation models (scenario IS92a for 1 percent increase GGa/year) were then used to illustrate the potential response of the species’ contemporary realized climatic niche space to climate change. Predictions were mapped at a 1-km² resolution. The contemporary realized climatic niche space of all species declined rapidly throughout the century. These models demonstrate the heightened risk for species occupying small geographic ranges of displacement into climatic disequilibrium from rapid climate change and provide tools to assist decision makers in mitigating the threat.