Utah State University, Forest Ecology, M.S. Thesis
Quaking aspen is the most widely distributed tree species in North America and an asset to sociological, ecological, and hydrological land values in the western United States. In recognition of these values, land managers seek means to oppose a regional decline of aspen in the Intermountain West—a decline apparently in progress since the close of the Pleistocene and driven by climate change, fire suppression, and increasing ungulate densities. One location of special relevance to this decline is the Utah-Colorado Book Cliffs, a discrete boundary of quaking aspen’s geographic range and a potential biogeographic corridor between the sub-alpine communities of the Utah and Colorado Rocky Mountains. To inform land management plans for the Book Cliffs and develop methods applicable on public lands across the region, a spatial database of aspen concentration, potential, and threat was created for a 10 450-ha pilot section of the area. Raster GIS data layers of fuzzy landcover concentration and monthly potential evapotranspiration (pET) were created from field measurements and satellite remotely sensed imagery. The Hutchinsonian niche concept was generalized with fuzzy-sets and likelihood theories to accommodate measurements of continuous landcover-class membership, and values of aspen’s fuzzy Hutchinsonian niche were regressed over a ~10% sample of the aspen-pET cells using regression trees. Applying the niche model over the cell population created an aspen pET-habitat map, and subtracting measured from expected concentrations created a map of potential aspen concentration change. Validation of aspen’s model pET niche showed that aspen concentration is strongly constrained by unmanageable water relations, but aspen’s high spatial aggregation suggests that net-positive local feedbacks maintain aspen concentration above expectation at many locations. Also, under-occupation of aspen habitat is strongly correlated with Douglas-fir concentration, and by manipulating the latter two factors within the limits imposed by water balance, aspen physiology, and management logistics, the landscape may be coarsely managed to increase aspen concentrations. These results are to be used in the Book Cliffs as spatially explicit hypotheses for adaptive ecosystem management, these methods are under consideration for application in similar ecoregions, and these interpretations may better accommodate niche theory to geographic ecology.
Sexton, Joseph O. Models for the distribution of quaking aspen in geographic and potential evapotranspiration spaces relevent to the Book Cliffs (Utah), 2000-2002. Utah State University, Forest Ecology, M.S. Theses.