Facilitation drives mortality patterns along succession gradients of aspen-conifer forests

Authors


  • Corresponding Editor: C. Kwit.

Abstract

While it is well established that facilitation and competition are important structuring forces in plant communities, a clear understanding of the interactions between them and how they change through the life stages of plants and affect long-term plant community development is lacking. We have observed that conifer seedlings are rarely found growing in meadows but readily establish under adjacent aspen stands, particularly at the base of aspen trees, creating the potential for antagonistic interactions in later life stages. To examine these relationships and their potential consequences on forest community development, we characterized patterns of establishment, regeneration, and overstory mortality of aspen (Populus tremuloides) and subalpine fir (Abies lasiocarpa) along a stand composition gradient (aspen dominant → mixed → conifer dominant) that develops in seral aspen forests. We found strong stand effects on the establishment of both aspen and subalpine fir regeneration. Aspen regenerated into meadows from the forest boundary, reached peak densities underneath aspen stands, and decreased significantly in mixed and conifer dominated stands. In contrast, subalpine fir seedlings failed to regenerate in meadows, but established readily underneath aspen, mixed and conifer stands. Within stands, establishing subalpine fir seedlings were strongly aggregated around mature aspen trees, which increased the proximity of the two species 2–10 fold depending on subalpine fir age class and stand type. Both stand type and increased proximity of overstory aspen and fir trees drove mortality patterns of the two species in opposite directions. Overstory aspen mortality increased sharply along the stand composition gradient: aspen (7%), mixed (17%), conifer (49%), while subalpine fir wasn't significantly influenced by stand type. Proximity of overstory aspen and subalpine fir, was associated with increased (2×) aspen mortality under all stand conditions but increased subalpine fir survival resulting in high aspen:fir mortality ratios that likely accelerate successional shifts toward conifer dominance. Our data suggest that environmental conditions that promote facilitator mortality may compromise the ability of facilitation-dependent forests to regenerate following disturbance. Maintenance of natural disturbance regimes appears to be critical in striking an ecological balance between facilitation and competition that promotes sustainability in succession-driven plant communities.

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