Exploring succession within aspen communities using a habitat-based modeling approach

Quaking aspen (Populus tremuloides Michx.) forest communities play a crucial ecological role across western North America. However, there is increasing evidence that these communities have diverging ecological roles across aspen's expansive range. Previous studies show evidence for both “seral” and “stable” aspen functional types. This leads us to believe that the pathway of these systems may not always lead to a climax conifer sere, but in many cases results in a stable community dominated by aspen. This study is an attempt to use a static model, based on large-scale environmental variables, to account for successional dynamics within aspen–conifer systems and predict distributions of aspen functional types across large landscapes. Environmental factors influencing aspen–conifer succession have been observed in past research but not fully explored. Our study methodologies and application of model results were specifically designed to aid land managers in identifying extent and function of aspen forest communities in order to plan restoration projects. Four study sites were chosen within Utah in order to capture the widest geographic variance. Photointerpretation of National Agriculture Imagery Program (NAIP) color infrared imagery was used to classify dominant forest cover at approximately 250 plots within each site. At each plot, variables were calculated and derived from DAYMET data, digital elevation models, and soil surveys and assessed for precision and ability to model forest type distributions. A generalized linear model was used to assess habitat overlap between aspen and conifer in order to explore successional dynamics and predict areas where stable aspen communities are likely to occur. Model results indicate an interaction between topographic position and moisture influence the probability of conifer encroachment but do not preclude conifers entirely. The highest probability for stable aspen communities occurs between 60 and 90cm of total annual precipitation on topographic positions receiving greater than 4500Wh/m2/d of solar radiation. Prediction-conditioned fallout-rates were used to partition the continuous model output into a “hard” classification. These results were applied in an overlay analysis with Southwest Regional Gap landcover data, indicating 19% of aspen forests across Utah are potentially stable functional types, whereas the remaining 81% are vulnerable to conifer encroachment.