Competition – that is, negative plant-plant interaction – has been well-studied and is known to influence community structure and composition. Facilitation – positive plant-plant interaction – is less well-understood, but has been found to affect community richness and functional diversity in some instances by alleviating environmental stress for the recipient species or community. There are two types of facilitation: direct and indirect. Direct facilitation occurs when a nearby plant directly improves environmental conditions for a community. Indirect facilitation occurs through interactions among plant species via shared competitors.
In a new article published in Annals of Botany, lead author Xiangtai Wang and colleagues studied both direct and indirect facilitation by a dominant shrub species and how it impacts the productivity of the local forb community. The authors studied a Tibetan alpine meadow dominated by the shrub Dasiphora fruticosa. They experimented with the presence of the shrub in the presence and absence of graminoids and in both enclosed and open, grazed meadows in order to measure its effect on forb biomass. Both herbivores and graminoids served as mediators of indirect facilitation.
The principal effect of the shrub on forb community biomass was positive. The facilitation was indirect, acting through a reduction in herbivory, both with and without graminoids present. This is the first evidence of indirect facilitation via protection against grazing outside of tropical peatlands. In enclosed plots with grazers excluded, there was also direct facilitation, though the presence of graminoids reduced this effect. Overall, facilitation by the shrub increased the productivity of the understory through an increase in its functional diversity.
“Our findings have important consequences on how we think environmental filters act on the dispersion of functional trait values when facilitation exists,” write the authors. “Moreover, the effect of facilitators on trait values can affect ecosystem functioning,” they explain, through the expanded realized niche of species that have been on the receiving end of facilitation, which in turn benefits community productivity in marginal environments.