Facilitation of sub-alpine fir and aspen regeneration by overstory aspen

Background/Question/Methods

Forest inventory and analysis data show a 60% decline of aspen in the Interior West relative to historic highs.  Succession to conifer has been a major contributor to aspen habitat 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 (pure conifer, aspen-conifer mix, pure aspen, and gap).

Results/Conclusions

Subalpine fir establishment was 2-3 fold higher under pure aspen and aspen-fir mixed stands compared to pure sublapine fir stands.  Subalpine fir did not establish in canopy gaps adjacent to the transition zones.  High density of taller height classes (> 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 (> 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).  A follow-up greenhouse study demonstrated the height growth of aspen was twice as high under soil and light conditions found in pure aspen stands compared to the soil and light conditions underneath mixed or pure sublapine fir stands.  This suggests that shifts in soil chemistry and light environment as more sublapine fir establish favors subalpine fir regeneration.