Long-term aspen cover change in the western US

Quaking aspen (Populus tremuloides Michx.) is one of the most important tree species in the western United States due to its role in biodiversity, tourism, and other ecological and aesthetic values. This paper provides an overview of the drivers of long-term aspen cover change in the western US and how these drivers operate on diverse spatial and temporal scales. There has been substantial concern that aspen has been declining in the western US, but trends of aspen persistence vary both geographically and temporally. One important goal for future research is to better understand long-term and broad-scale changes in aspen cover across its range. Inferences about aspen dynamics are contingent on the spatial and temporal scales of inquiry, thus differences in scope and design among studies partly explain variation among conclusions. For example, major aspen decline has been noted when the spatial scale of inquiry is relatively small and the temporal scale of inquiry is relatively short. Thus, it is important to consider the scale of research when addressing aspen dynamics.Successional replacement of aspen by conifer species is most pronounced in systems shaped by long fire intervals and can thus be seen as part of a normal, long-term fluctuation in forest composition. Aspen decline was initially reported primarily at the margins of aspen’s distribution, but may be becoming more ubiquitous due to the direct effects of climate (e.g. drought). In contrast, the indirect effects of recent climate (e.g. forest fires, bark beetle outbreaks, and compounded disturbances) appear to favor aspen and may facilitate expansion of this forest type. Thus, future aspen trends are likely to depend on the net result of the direct and indirect effects of altered climate.

► Conclusions about aspen dynamics depend on the spatial and temporal scales of inquiry.
► Infrequent disturbances lead to normal, long-term fluctuations in aspen cover.
► The direct effects of recent climate (drought) conribute to aspen decline.
► But indirect effects of recent climate (altered disturbance regimes) appear to favor aspen.
► Future aspen cover will depend on the net result of direct and indirect effects of altered climate.
► Aspen persistence, decline, regeneration dynamics vary across space and time.