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Catastrophic and Non-catastrophic Modes of Aspen Regeneration in Western Colorado
Dominik Kulakowski, Clark University, Worcester, MA, Thomas T. Veblen, University of Colorado, Boulder, CO, and Brian P. Kurzel, Colorado State Parks, Denver, CO
The decline of quaking aspen across the western United States has received considerable research attention. This decline has been attributed to fire exclusion as well as other causes. To assess long-term changes in the extent of aspen in western Colorado, we compared historical (1898) maps of vegetation and fires to current maps of forest cover. Based on these comparisons, a larger portion of the current landscape is dominated by quaking aspen, relative to the late 19th century. During the 20th century, aspen was persistent over most of its extent, even in the absence of fire. Fires of the late 19th century also increased aspen cover in stands that were previously dominated by spruce and fir.
To investigate regeneration dynamics of both persistent and seral aspen stands, we reconstructed patterns of stand development in 26 aspen stands. Stand-level age structures were determined from 1919 increment cores and size structures from counts and diameters of all stems in 40 m × 40 m plots. Stand structures were interpreted to determine modes of tree regeneration and patterns of stand development. In the eight seral stands aspen regeneration was generally catastrophic (depended on coarse-scale, severe disturbance by fire). However, most aspen stands showed signs of aspen self-replacement despite presence of conifers. In the 16 persistent aspen stands showing no conifer invasion, aspen were able to regenerate through a variety of regeneration modes that did not appear to require severe disturbance. Over 70% of the persistent aspen stands sampled did not require coarse-scale disturbance to regenerate. Two (11%) of the persistent stands regenerated continuously through time, and 11 (61%) had aspen cohorts that appeared to develop episodically without being triggered by severe fire or other severe disturbance. In the persistent stands sampled in our study, episodic regeneration not dependent on severe fire is the main mode of stand re-initiation.