A multitrophic perspective on functioning and evolution of facilitation in plant communities

P>1. Plant facilitation has been studied mostly in the context of plant-plant interactions, whereas multitrophic interactions including those that occur below ground have not yet received much attention. Here, I will discuss how above-ground and below-ground natural enemies and their predators influence plant facilitation and its evolution. 2. Specific above-ground and below-ground plant enemies and their predators play a major role in structuring the composition and dynamics of plant communities. In successional sequences, above-ground and below-ground multitrophic level interactions may tip the balance from competitive to facilitative states and vice versa. 3. Little is known about how above-ground and below-ground multitrophic interactions develop along resource or stress gradients and how the outcomes of above-ground-below-ground interactions depend on variations in these environmental conditions. 4. Facilitated plants need to fit into the above-ground-below-ground multitrophic communities of their facilitators. 5. Little is known also about the evolution of plant facilitation. The observed distance in phylogeny between facilitators and facilitated plants suggests that host-specific enemies may very well co-determine which species become facilitated by which facilitators. 6. Further, very little attention has been given to how plant strategies (allelopathy, accumulation of enemies, monopolization of symbionts) may be the result of selection against being facilitative. 7.Synthesis. Plant facilitation cannot be understood without considering a plant's natural enemies and also its enemies' enemies. Plant enemies can turn competitive interactions into facilitative interactions, whereas the enemies' enemies can turn facilitation back into competition. Below-ground interactions will have longer-lasting effects on facilitation than those above ground, because many organisms can persist in the soil, even when the host plants have disappeared.