By Paul Rogers, Utah State University
Aspen "decline" comes in two flavors: long-term decline related to conifer encroachment and short-term decline (a.k.a., Sudden Aspen Decline or SAD) initiated by drought. For decades we have been hearing about great losses in aspen coverage in the western states due to advancing conifer succession. Some of these changes were caused by past management practices, such as fire suppression and active removal of aspen or management for higher value conifer timber species. More recent evidence, however, suggests that a regionally moist 20th century generally kept fire events limited, allowing conifer expansion and aspen decline. In other locations, aspen actually expanded its range so we must be careful not to oversimplify regional patterns. This interaction with conifers does not explain losses in stable (nearly pure) aspen forests. Thus, we must understand at least two aspen "types" which depend on different ecological mechanisms to persist.
SAD is defined as the rapid die-off of both overstory canopy trees and supporting root systems where the end result is the death of complete aspen clones. Strong evidence has been presented that this type of rapid die-off has occurred in southwest Colorado, but corroborating support is limited elsewhere. There has been much broader patterns of mature tree mortality across the West and in southern Canada associated with the 2001-2004 drought. In many instances, combined effects of overstory drought-induced die-off and intense herbivory of young aspen by browsers has led to "de facto SAD" where we witness complete aspen community collapse.
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