Data from: Facilitation within species: a possible origin of group-selected superorganisms

Facilitation—positive interactions—has emerged as a dominant ecological mechanism in many ecosystems. Its importance has recently been expanded to include intraspecific interactions, creating the potential for higher level natural selection within species. Using multiple lines of evidence we show that conspecific facilitation within the southern beech tree, Nothofagus pumilio, appears to overcome competition in two life phases. In a seedling experiment addressing stress and planting density effects, we found that mortality was lowest (~0%) where there was no stress and was indistinguishable across densities. Further, in mature forests (45 years), genetically variable, merged individuals had lower mortality (-50%) compared to unmerged individuals in locations without identifiable stress. Thus, a full understanding of the occurrence of facilitation may require a more general model of resource improvements than the commonly cited Stress Gradient Hypothesis. Additionally, the merged trees showed a density dependent mortality pattern at the level of the groups. These data demonstrate a potential mechanism (facilitation) driving natural selection at this higher level, via stem merging. These merged "superorganisms" would confirm theoretical predictions whereby facilitation acts as an ecological mechanism driving group selection.