Summary
Interspecific facilitation is often invoked in explanations of biodiversity-ecosystem function relationships in plant communities, but it is seldom clear how it occurs. Physiological experiments show that excess light causes stress that may depress long-term carbon assimilation. If shading by a plant’s neighbors reduces light stress, it may facilitate that plant’s growth. On the other hand, when light is a limiting factor for growth, shading will often have a net negative, competitive effect.
In a tree diversity experiment, we measured growth rates and photosynthetic physiology of 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).
Although most species had lower growth with larger neighbors, implying a net competitive effect, the two most shade-tolerant species (Tilia americana and Acer negundo) had positive responses to neighbor size. Compared to the others, these two species were especially susceptible to photoinhibition (reduced dark-acclimated Fv/Fm) in full sun. While most species had lower assimilation rates in shaded bicultures, T. americana had carbon assimilation rates up to 25% higher. T. americana also dropped its leaves 3-4 weeks later in the shaded biculture, 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 deepen our understanding of the species interactions that underlie biodiversity-ecosystem function relationships.
