Influences of infrequent fire, elevation and pre-fire vegetation on the persistence of quaking aspen (Populus tremuloides Michx.) in the Flat Tops area, Colorado, USA

Aim: The recent concern that quaking aspen (Populus tremuloides Michx.) has been declining in parts of western North America due to fire suppression is largely based on trends during the latter part of the 20th century. The aim of the current study was to compare the extent of aspen in the modern landscape with its extent in the late 19th century prior to fire suppression, and to assess the effects of elevation, late-19th century fires, and pre-fire forest composition on the successional status of aspen. Location: North-west Colorado, USA. Methods: We used a georeferenced 1898 map and modern maps to examine trends in aspen dominance since the late 19th century in a 348,586 ha area of White River and Routt National Forests in north-western Colorado. Stand age and structure were sampled in 30 stands. Results: We found no evidence of overall aspen decline over this period. In fact, aspen distribution has increased in parts of the study area following severe fires in the late 19th century in forests that were previously dominated by conifers. Aspen persistence and increase was especially pronounced at elevations below 3000 m a.s.l. Most 120-year-old post-fire stands that are presently being successionally replaced by conifers were dominated by conifers prior to the last severe fire. Main conclusions: Human perceptions of ecosystems are often on time scales that are shorter than the cycles of natural variation within those ecosystems. This disparity may lead to an underestimation of the range of natural variability of ecosystem patterns and processes. The appropriate temporal scale of inquiry is necessary for the correct understanding of natural variation in ecosystems. © 2006 The Authors