Joseph O. Sexton

    Joseph O. Sexton

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    Joseph SextonHabitone analysis of quaking aspen in the Book Cliffs (Utah, USA): effects of site water demand and conifers on aspen cover

    Joseph O. Sexton, Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC; R. Douglas Ramsey, Department of Forest, Range, and Wildlife Sciences, Utah State University, Logan, UT; Dale L. Bartos, Rocky Mountain Research Station, USDA Forest Service, Logan, UT

    Quaking aspen (Populus tremuloides Michx.) is the most widely distributed tree species in North America, but its presence is declining across much of the western United States. Aspen decline is complex, but results largely from two factors: 1) regional Holocene climatic drying has led to water limitation of aspen seedling recruitment, and 2) anthropogenic fire suppression during the 20th century has allowed shading of aspen clones by fire-intolerant conifers. These processes interact variously and often diffusely, but traditional, binary habitat mapping approaches can only resolve their net effect after complete loss of aspen patches. To inform land management in the Book Cliffs—a biogeographic link between the Utah and Colorado Rocky Mountains and a location experiencing typical aspen decline—we developed a regression-based generalization of habitat analysis that is both usable in GIS and capable of detecting anomalies in cover before complete patch conversion. We estimated the realized niche of quaking aspen to potential evapotranspiration (PET) with regression trees, projected aspen’s niche expectation and uncertainty geographically, and correlated differences between observed and expected aspen cover to remotely sensed conifer cover. Results confirm the strong constraint of site water demand on aspen cover and suggest that conifer cover decreases aspen cover beneath its expectation given the PET environment. Compared to sites without quaking aspen, our aspen sites had lower PET in every month of the growing season, but the difference increases over the growing season as drought effects become more extreme. Conifer cover displaces aspen cover and shows a positive correlation with niche-model deviance (r = 0.344). Ultimately, the thematic information conserved by our approach allowed us to resolve detailed rasters of management potential and map a modest potential increase of aspen cover—14.63 ha (0.14%) of the study area, or +2.46% of current aspen cover—within one management cycle.


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