A typology of stand structure and dynamics of Quaking aspen in northwestern Colorado

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The regeneration of Quaking aspen (Populus tremuloides) in the western U.S. in some habitats occurs after severe fire which removes competing conifers and also triggers root suckering of aspen. Consequently, fire exclusion during the 20th century, sometimes in combination with elk browsing, has often been argued to have resulted in a decline in the extent of aspen relative to its pre-20th century extent. However, aspen regeneration is not always dependent on fire and many stands have structures indicative of aspen persistence even in the absence of fire. Thus, to investigate regeneration dynamics of both persistent and seral aspen stands, we reconstructed patterns of stand development in 26 aspen stands in northwestern Colorado. Seral and persistent aspen stands were initially selected based on geographic information system-based analysis of historic vegetation and fire occurrence. Stand-level age structures were determined from 1919 increment cores and size structures from counts and diameters of all juvenile, subcanopy and canopy trees in 40 m × 40 m plots. Stand structures were interpreted to determine modes of tree regeneration and patterns of stand development. As expected, in the eight seral stands aspen regeneration generally depended on coarse-scale, severe disturbance by fire. However, most aspen stands in this study showed signs of aspen self-replacement despite presence of conifers. In the 16 persistent aspen stands showing no conifer invasion, aspen are 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. At many sites in northwestern Colorado aspen regeneration occurs episodically and fits a general stand development model of cohort senescence. According to this model, many of the currently "decadent" or deteriorating aspen stands in western Colorado may be replaced by regenerating stands as the senescent stages experience high levels of canopy tree mortality, thus assuring aspen persistence in the future. © 2007 Elsevier B.V. All rights reserved.

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Forest Ecology and Management

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Aspen, regeneration, senescence, stand dynamics