Patterns and mechanisms of facilitation in aspen-conifer forests

Background/Question/Methods

Conifer and broadleaf tree species intermix to form vast forests in the continents of the northern hemisphere. While it is well understood how competitive interactions influence these forest systems, field observations of conifer seedlings established aggregated at the base of aspen trees suggest there is a strong facilitative effect between aspen and conifers. The objective of this study was to characterize patterns and mechanisms of plant-plant interactions between aspen and subalpine fir along stand composition gradients in aspen-fir forests.  Subalpine fir seeds were placed at distances of 1 cm and 25 cm in each cardinal direction at the base of mature aspen and subalpine fir trees and in interspaces in each of the three stand types (aspen dominated,  mixed, conifer dominated stands) and in adjacent meadows at seven field locations on the Fishlake National Forest, UT. Fir seedling emergence and mortality were tracked. Soil moisture content, light and temperature were measured. Germination trials for subalpine fir were conducted under a range of water potential and temperature conditions to determine if microenvironment conditions near aspen trees or underneath different stand types explain differences in germination success.

Results/Conclusions

Germination of subalpine fir in aspen stands was 9 and 13 fold greater than in mixed and conifer dominated stands. Germination was twice as high at the base of aspen trees compared to fir trees and stand interspaces, while seedlings failed to germinate in adjacent meadows. Seedling mortality was lowest in aspen stands but was not significantly influenced by proximity to mature trees. Soil moisture was highest in aspen dominated stands, with the highest soil moisture conditions occurring at the base of aspen trees and in interspaces.  Water relations were better in fir seedlings growing next to aspen trees but did not differ significantly between stand types. Because of aspen’s primary role in initiating secondary succession through post-disturbance sucker regeneration, and the subsequent dependence of fir on aspen for establishment shown in this study, aspen mortality via competition with conifers, drought, or intensive ungulate browsing may limit forest regeneration in some areas.