Great Basin Naturalist Memoirs


About 22 percent of the pigmy conifer woodlands of the United States occur within the Great Basin. Only a very few reports of these woodlands exist in the literature. Available reports are either of general descriptive nature or specific analysis of vegetation-environmental relationships on one mountain range. In order to better understand basin-wide synecological patterns, a cooperative study was carried out by personnel at Utah State University and the University of Nevada-Reno between 1972 and 1975. Vegetation, landform, geology, and soils data obtained from 463 systematically placed stands on a randomly chosen set of 66 mountain ranges have been used to derive patterns of latitudinal, longitudinal, and altitudinal variation in the floristic diversity in juniper-pinyon dominated woodlands across the Great Basin.
The latitudinal-longitudinal patterns show greatest environmental and floristic diversity on the higher mountain ranges on the southern end of the Central Plateau portion of the study area where the Great Basin-Mojave Desert transition occurs. This is also where the elevation breadth of the woodland belt is greatest. Juniper-pinyon woodlands are largely lacking from northwestern Nevada. The lowest elevations for the type are found in the Dixie Corridor centered in southwestern Utah. The general elevation of these woodlands is highest in the west-central part of the Great Basin and declines both toward the Sierra Nevada on the west and the Wasatch Front-High Plateaus on the east.
Use of the equilibrium theory of island biography gave incomplete explanations of the diversity patterns observed. Certain conceptual and methodical problems forced by this overly simplistic theory are discussed. The best correlations obtained were between species richness and index of ecotopic diversity.
Correlations of Basin-wide patterns of woodland floristics with surficial geology, landforms and soils is nondiscriminatory. However, broad-scale, phytogeographical variations in these woodlands are closely associated with climatic differences. Although much direct climatic data are lacking, it seems likely that the relative contributions of the transitory frontal system moving inland from the Pacific, continental cyclones developing over the Great Basin, and convectional storms associated with the moist air from the Gulf of Mexico to induce precipitation at different seasons are regionally important in the causation of vegetation distribution and composition.
The instability of temperature inversions is a likely determinant of the position of woodlands along the northern boundary of the type. The Pacific frontal systems break the inversions most readily and are thought to be the major cause for the lack of this vegetation in northwestern Nevada and on exposed mountain ranges along the northern boundary of the type. Such observations provide leads for relevant ecophysiological research to support or reject these notions.