The persistence of quaking aspen (Populus tremuloides) in the Grand Mesa area, Colorado

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Human activities have caused the decline of numerous species and ecosystems. To promote ecosystem resilience, recent management efforts aim to maintain ecosystem patterns and processes within their historical range of variability. There has been substantial concern that quaking aspen, the most widely distributed tree in North America and the most important deciduous tree in the subalpine forests of the Rocky Mountains, has declined significantly in the western landscape during the 20th century. This reported decline has been attributed to conifer encroachment associated with fire exclusion, as well as other causes. To assess long-term changes in the extent of quaking aspen in a 175 000-ha study area in western Colorado, we used a Geographic Information System to compare an 1898 map of vegetation and fires to a modern map of present forest cover types. Based on this comparison, a larger portion of the current landscape is dominated by quaking aspen relative to the late 19th century, a period of extensive burning in this area. 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. The trend toward increased aspen was greater at lower elevations. The total area where spruce and fir have replaced aspen is small in comparison to the area where aspen has increased or has persisted. The successional replacement of aspen by conifers is more pronounced at higher elevations and where the predisturbance vegetation was dominated by conifers. The net effect of severe disturbances during and after the late 19th century has also increased aspen cover relative to the period that preceded these disturbances. Thus, where the successional replacement of aspen by conifers is occurring today, such a trend may be within the range of historical variation. Long intervals between large, severe natural disturbances in ecosystems can result in a broad range of natural ecological conditions, including dominance by particular species.

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Ecological Applications

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Aspen, Colorado, USA, fire, Geographic Information System (GIS), historical range of variability, Populus, reference period, vegetation change