Modeling aspen community types using a remote sensing and GIS-based approach

Background/Question/Methods

Quaking aspen (Populus tremuloides Michx.) is the most widespread broadleaf tree species of North America, and as such, plays a crucial ecological role; however, there is increasing evidence that aspen does not fulfill the same ecological role across its expansive range.  Many studies show evidence for both “seral” and “stable” aspen community types leading us to believe that the successional pathway of aspen may not always lead to a climax conifer sere, but may in some cases consist of persisting stands of pure aspen. Factors influencing aspen-conifer succession have not been fully explored at the landscape or regional level.  This study is an attempt to understand the relationship of aspen community types to climatic, physical, and biophysical variables by detecting patterns of aspen and conifer distribution across large scales using remote sensing and GIS technology.  Four ecologically unique sites were chosen within Utah in order to capture the geographic and climatic range in which aspen inhabits.  Landsat TM and high resolution NAIP imagery were used to systematically sample and interpret forest cover at approximately 430 plots within each site.  At each plot, variables were calculated and derived from Daymet data, digital elevation models, and soil surveys.  A multivariate cluster analysis was done for each site and across sites and a chi-square test for goodness-of-fit was conducted to determine if aspen and conifer were distributed at random within environmental space.

Results/Conclusions

Results show broad environmental ranges of tolerance for aspen and conifer species, however, aspen tends to exhibit a realized niche free from conifer encroachment according to abiotic variables.  Chi-square test indicates that aspen and conifer are not occurring at random within environmental space (d.f. =2, chi-square 11.69, p-value 0.003).  In addition to this, cluster analysis shows that pure aspen plots tend to be proportionately higher than plots with a conifer component at sites with relatively higher solar radiation and soil water availability.   We conclude that distributional patterns of aspen and conifer at the landscape scale indicate a realized niche for both seral and stable aspen, which can be modeled by remote sensing and GIS-based variables.  A landscape-scale model of this sort has the potential to aid land managers in appropriately treating aspen stands in the face of conifer encroachment or stand decline.