Trophic dynamics modify plant-soil feedbacks: Old-field herbivory shifts mycorrhizal interactions from parasitism to mutualism (sometimes)
In plant communities, keystone predation theory predicts that subordinate species can coexist with a dominant competitor due to herbivore induced mortality or increased loss rates. Do mutualists, specifically mycorrhizal fungi, buffer or intensify the effects of herbivory to change plant community dynamics? Altered rates of herbivory, or growth response following herbivory, will affect plant fitness and potentially cascade to critical community-level traits, such as reproduction, colonization rate and competitive ability. Although we generally hypothesized mycorrhizal fungi to increase plant tolerance and competitive ability relative to non-mycorrhizal plants, we also expected this relationship to be species specific. We apply this framework to help explain the consistent dominance of Solidago canadensis in old-field systems across eastern North America. We specifically tested if arbuscular mycorrhizal fungi (AMF) and herbivory interact to alter plant performance, clonal and seed reproduction, and competitive ability in 7 old-field plant species. We conducted a 3-year, in situ monoculture experiment where we factorially manipulated insect herbivory and mycorrhizal colonization by periodic applications of insecticide and fungicides.
Most plants appeared to have context dependent, conditional mutualisms with mycorrhizae, such that plants received significant AMF benefit in terms of growth, but only in the presence of herbivores. Mycorrhizal plants had similar or lower biomass than non-mycorrhizal plants when protected from insect herbivores. Mycorrhizal associations altered allocation to reproduction and flowering frequency, but these effects were varied and species-specific. However, herbivory and AMF did not affect potentially critical community-level traits such as density, clonal colonization, and light extinction. These results illustrate a possibly broad pattern of increasing mycorrhizal benefit with herbivore stress. Temporal and spatial variation in herbivory may thus also change the nature of plants-mutualist interactions, potentially providing a mechanism for long-term coexistence, even in the presence of a competitive dominant.