Moving facilitation out of its niche: Tree merging, supertrees, and the limits to the stress-gradient hypothesis

Background/Question/Methods

Facilitation (positive interactions among individuals or species) has emerged as a dominant ecological mechanism in many ecosystems. Three dominant lines of evidence in ecological research have placed facilitation as a process a) whose net effect is greater in more stressful locations, b) that is not important where niche overlap is great because strong competition overrides the facilitation effects, and c) that is often transient because of space limitation. We use multiple lines of evidence to test these points within the southern beech tree, Nothofagus pumilio. First, we established a transplant experiment of this single species across a forest-prairie stress gradient 6 years ago. We transplanted 2-year-old seedlings at two different densities (clusters of 10 and isolated) and at different distances from the forest edge (from 30 m inside the forest up to 50 m outside the forest in the prairie). Second, we evaluate mature forests using spatial analysis and a priori hypotheses to evaluate which trees are dying (merged or unmerged). Third, we test genetic identity of clusters of mature forest trees that have physically merged above and below ground level to understand longer term dynamics.  

Results/Conclusions

Three and six years after the transplants, seedlings in dense clusters had significantly higher survival than solitary seedlings at the stressful end of the gradient. This trend was reversed at the opposite end of the gradient, supporting the SGH at the intraspecific level. However, three years after planting, we found that mortality was lowest (∼0%) where there was no stress and was indistinguishable across planting densities. At six years after planting, these patterns are pushed outwards into the more stressful openings. In the analysis of the mature forest (45 years old), merged individuals were genetically variable, had much lower mortality (50%) than unmerged individuals, and overcame density dependence as a group. No merged tree existed in locations with currently identifiable stress. Revisiting the three premises above, facilitation a) occurred without stress b) occurred in resource-mediated interactions (niche overlapping) and c) was non-transient. A full understanding of the occurrence of facilitation may require a more general model of resource improvements than the commonly cited stress gradient hypothesis. These data demonstrate a potential mechanism (facilitation) driving natural selection at a the level of the group, via stem merging, and open discussion for a new model of facilitation.