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.

促进作用（facilitation）——即正相互作用——已成为众多生态系统中占主导地位的生态学机制。近年来其重要性范畴已拓展至种内相互作用，为物种内更高层级的自然选择创造了可能性。本研究通过多维度证据证实，假山毛榉（Nothofagus pumilio）种群内的同种促进作用，似乎可在两个生活史阶段克服竞争压力。在一项针对胁迫与种植密度效应的幼苗实验中，我们发现：无胁迫条件下幼苗死亡率最低（约0%），且不同密度组间死亡率无显著差异。此外，在45年生的成熟林中，无明显胁迫区域内，具有遗传变异的融合个体死亡率较非融合个体降低了50%。因此，若要全面理解促进作用的发生机制，或许需要一套比广为引用的胁迫梯度假说（Stress Gradient Hypothesis）更为普适的资源改良模型。此外，融合林木在种群水平上呈现出密度依赖性的死亡率模式。本研究数据表明，通过茎干融合，促进作用这一潜在机制可推动该层级的自然选择。这类融合而成的‘超个体’将验证相关理论预测：促进作用可作为驱动群体选择的生态学机制。