Aspen facilitates subalpine fir regeneration by increasing germination success

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 very few studies have documented facilitation between deciduous and conifer species or explored how it may shapes successional outcomes in the development of mixed conifer-deciduous forests. The objective of this study was to characterize patterns and mechanisms of facilitation along stand composition gradients in aspen-fir forests.

Subalpine fir seeds were placed during the fall of 2010, at distances of 0cm and 25cm in into 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 à mixed à conifer dominated stands) and in adjacent meadows at six field locations on the Fishlake National Forest, UT. Seedling emergence was recorded at the beginning of the summer of 2011 and seedling mortality was recorded in October 2011. Soil moisture content was measured and the water relations and foliar nutrient content of fir seedling growing next to aspen trees and in interspaces was compared.

Results/Conclusions

Subalpine fir germination in aspen stands was 11 times greater than mixed or conifer dominated stands. Germination was 2.3 fold greater at the base of aspen trees than fir trees and two fold greater at the base of aspen trees than interspaces. Seedling mortality was lower 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 with no significant effects of stand type.  In contrast, fir seedlings in aspen stands had higher foliar nutrients status, while proximity to aspen trees had no influence on foliar nutrient status.

This study illustrates a strong facilitative interaction in which both aspen dominated stands and aspen trees increase the likelihood of subalpine fir seedling establishment by drastically increasing rates of subalpine fir germination.  Because of aspen’s primary role in initiating secondary succession through post-disturbance sucker regeneration, and the subsequent dependence of conifers on aspen for establishment, aspen mortality via competition with conifers, drought, or intensive ungulate browsing may result in loss of some aspen-conifer forest communities.