Physiological responses to light explain competition and facilitation in a tree diversity experiment

Ecologists often invoke interspecific facilitation to help explain positive biodiversity-ecosystem function relationships in plant communities, but seldom test how it occurs. One mechanism through which one species may facilitate another is by ameliorating abiotic stress. Physiological experiments show that a chronic excess of light can cause stress that depresses carbon assimilation. If shading by a plant9s neighbors reduces light stress enough, it may facilitate that plant9s growth. If light is instead most often a limiting factor for photosynthesis, shading may have an adverse, competitive effect. In a temperate tree diversity experiment, we measured growth rates and photosynthetic physiology in broadleaf tree species across a gradient of light availability imposed by their neighbors. At the extremes, trees experienced nearly full sun (monoculture), or were shaded by nearby fast-growing conifers (shaded biculture). Most species had slower growth rates with larger neighbors, implying a net competitive effect. On the other hand, the two most shade-tolerant species (Tilia americana and Acer negundo) had faster growth rates with larger neighbors, implying a net facilitative effect. Compared to the others, these two species had larger increases in photoinhibition (reduced dark-acclimated Fv/Fm) across the gradient of increasing light availability, which suggests that they are more vulnerable to chronic light stress. While most species had lower carbon assimilation rates in the shaded biculture treatment, T. americana had rates up to 25% higher. T. americana also dropped its leaves 3-4 weeks later in shaded bicultures, extending its growing season. We conclude that although large neighbors can cause light limitation in shade-intolerant species, they can also increase growth through abiotic stress amelioration in shade-tolerant species. Both positive and negative species interactions in our experiment can be explained by the photosynthetic responses of trees to the light environment created by their neighbors. We show that physiological measurements can help explain the species interactions that underlie biodiversity-ecosystem function relationships. The insights that ecologists gain by searching for physiological mechanisms may help us forecast species interactions under environmental change.