Complexity of Successional Pathways in Subalpine Forests of the Selway-Bitterroot Wilderness Area

Abstract

We examined forest structure and composition in four watersheds in the Selway-Bitterroot Wilderness Area, Idaho and Montana, to better understand the complexity of successional processes following stand-replacing fires in subalpine forest ecosystems. Dendrochronological analyses of more than 1,100 trees were used to identify the timing of establishment of major forest species at sites that had experienced different intervals since the last fire. This was coupled with analyses of forest structure and composition using nonmetric multidimensional scaling. A conceptual model of stand development is presented to highlight our findings. Lodgepole pine (Pinus contorta var. latifolia Dougl.) dominated most overstories for the first 100–200 years but persisted as a canopy dominant for more than 250 years in some stands. As forests increased in age, Engelmann spruce (Picea engelmanii Parry) and subalpine fir (Abies lasiocarpa [Hook.] Nutt.) became more prominent in the overstory. Whitebark pine was present in some young stands and older stands but was most often represented as dead standing trees killed during twentieth-century mountain pine beetle outbreaks. Understory composition was a function of time-since-fire but showed considerable variation that is likely tied to seed arrival, environmental conditions, and establishment. Our results suggest that short fire intervals may limit the development of lodgepole pines capable of producing serotinous cones, leading to young forests dominated by spruce or fir. However, intervals longer than the lifespan of lodgepole pine (as long as 350 years) could also lead to early dominance by spruce or fir following fire. These results refine our understanding of the temporal development of subalpine communities following stand-replacing fires and have implications for the implementation of long-range management goals in these habitats.