Using Forest Health Monitoring to Assess Aspen Forest Cover Change in the Southern Rockies Ecoregion

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Forest Ecology and Management





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Long-term qualitative observations suggest a marked decline in quaking aspen (Populus tremuloides Michx.) primarily due to advancing succession and fire suppression. This study presents an ecoregional coarse-grid analysis of the current aspen situation using Forest Health Monitoring (FHM) data from Idaho, Wyoming, and Colorado.

A unique feature of aspen forests in western North America is regeneration primarily by asexual ''suckering'' although rare seeding events do occur. The dominant clonal process provides the basis for this analysis. In essence, the remaining aspen stems of previously large clones provide a window to the past and possibly a view of the future. The author uses baseline observations of aspen and associated tree species regeneration, forest size and structure components, stand age, tree damage, and recent disturbance to assess regional aspen conditions. Analysis of stands where aspen is dominant (aspen forest type) and where aspen merely occurs (aspen present) are presented. Basic groupings within the aspen forest type plots were obtained by cluster analysis of 10 FHM variables derived from tree- and plot-level measurements. Stable and unstable aspen forest types were verified using principal component analysis. A further criterion of at least 25% conifer species present was placed on the unstable group to render a more conservative population estimate of instability.

The unstable aspen forest types, along with the plots having only the presence of aspen, comprise the dynamic portion of the aspen community in this area. These results support the hypothesis of an aspen decline within the past 100 years. However, additional regional plots and long-term remeasurements should provide a clearer picture of the decline's extent. Altering current and future management practices may significantly affect the rate of change.