Impact of aspen on subalpine fir establishment and succession

Background/Question/Methods There is evidence of aspen decline in parts of the western United States. One of the key factors hypothesized to be contributing to aspen decline is their displacement by conifers through competitive interactions. We have observed that subalpine fir seedlings preferentially establish at the base of aspen trees rather than under confer trees or in the inner spaces between trees. We characterized establishment patterns of subalpine fir seedlings and soil moisture conditions at seven field sites in the Fishlake National Forest possessing a successional gradient (pure aspen stands, pure subalpine fir stands, mixed stands, and open gap areas between stands) to test the hypothesis that aspen stands facilitate the establishment of subalpine fir seedlings by creating microsite conditions that promote higher subalpine fir seedling success. Results/Conclusions The data indicates that mature aspen trees facilitate subalpine fir establishment, which is partially driven by differences in soil moisture availability. There were significantly higher percentages of volumetric soil water content under aspen stands than adjacent conifer stands, and in addition that there is a higher percentage of volumetric soil water content at the base of an aspen tree when compared to conifer trees. It appears that aspen facilitatation of subalpine fir seedlings initiates successional processes that in time leads to mixed stand conditions prone to fire that regenerates aspen and promotes its health. However, fire suppression in the 20th century has lengthened the fire return interval leading to late successional displacement of aspen by conifers.