A portfolio effect of shrub canopy height on species richness in both stressful and competitive environments

Facilitating effects of benefactor plants on plant species richness have been commonly tested in stressful habitats because competitive effects are assumed to predominate in more productive habitats. Here, we examine this assumption by testing whether benefactor plants can nonetheless be facilitating in competitive environments. We provide a conceptual framework describing how a trait of benefactor plants, canopy height of shrubs, can have a portfolio of facilitative effects on species richness in more competitive environments, and we provide an empirical assessment of this portfolio effect in tundra plant communities. Across tundra plant communities representing an extensive gradient in aboveground live biomass ranging from 11 to above 800 grams per m2, we found that species richness exhibited a humped-back relationship. Increasing canopy height of shrubs to a maximum height of what defines the dwarf shrub tundra, that is 40 cm, consistently and significantly increased species richness...