Facilitation of Subalpine Fir and Aspen Regeneration by Overstory Aspen

Forest inventory and analysis data show a 60% decline of aspen in the Interior West relative to historic highs. Succession to conifer is hypothesized to be the major contributor to aspen loss. Fire suppression has likely accelerated conifer displacement of aspen but there are important knowledge gaps in our understanding of how stable aspen communities transition to conifer dominated communities. Regeneration success is a predictor of future overstory composition and structure. The objective of this study was to characterize regeneration dynamics of aspen and subalpine fir as influenced by overstory composition in aspen-subalpine fir transition zones. We measured regeneration density and height classes of aspen ramets and subalpine fir seedlings at seven field sites across the state of Utah that were characterized as having a clearly demarcated overstory transition zone (dominant conifer, aspen-conifer mix, dominant aspen, gap). Subalpine fir establishment was 2-3 fold higher under pure aspen and aspen-fir mixed stands compared to pure subalpine fir stands. Subalpine fir did not establish in canopy gaps adjacent to the transition zones. High density of taller height classes (\u3e 1 meter) were only observed in aspen-conifer mixed stands. Aspen regenerated readily in pure aspen, aspen-conifer mixed stands and in adjacent gaps but did not regenerate under pure subalpine fir stands. The highest density of taller height classes (\u3e 1 meter) for aspen regeneration was achieved in pure aspen stands. The data suggest that a dominant presence of overstory aspen facilitates the establishment of subalpine fir and aspen but that greater height is obtained under different conditions (aspen under pure aspen stands, subalpine fir in mixed stands)