Exploring the correlation of mycorrhizal host type between forest layers

Recently, research in a variety of forest ecosystems has shown that trees of the same species (conspecifics) may experience directional plant-soil feedbacks (PSFs) depending on whether they engage in arbuscular mycorrhizal (AM) symbioses or ectomycorrhizal (EM) symbioses. Some of the same processes hypothesized to be involved in conspecific feedbacks could also produce feedbacks between heterospecific species. For example, common mycorrhizal networks can connect plants of different species, and changes in the local nutrient economy can favour other species with the same host mycorrhizal type (AM or EM). Additionally, these feedbacks could extend to the understory, where the vast majority of plant diversity resides in many forest ecosystems. Whether this occurs remains unknown, as research on mycorrhizae-mediated PSFs has focused almost exclusively on canopy trees. If PSFs do occur between the canopy and understory plants, and provided they are sufficiently strong, they could conceivably influence plant community structure. For example, PSFs involving canopy and understory plants could facilitate heterospecific hosts of the same mycorrhizal type by acting as inoculum reservoirs and increasing the abundance of compatible fungal inoculum, or through mutually beneficial interactions via CMNs. Such positive PSFs could potentially increase recruitment of plants of the same mycorrhizal type into both the canopy and understory, yielding a positive correlation between the relative abundance of canopy and understory plants of the same mycorrhizal type. Alternatively, PSFs involving canopy and understory plants could be more negative among hosts of the same mycorrhizal type, perhaps through increased competition for and/or depletion of particular forms of nitrogen and phosphorus, thus favouring recruitment of hosts of a different mycorrhizal type. Such negative PSFs could potentially decrease recruitment of plants of the same mycorrhizal type into either the canopy or understory, yielding a negative correlation between the relative abundance of canopy and understory plants of the same mycorrhizal type. It is also possible that these proposed mechanisms do not occur or are too weak to noticeably influence plant community structure, resulting in patterns that are no different from what we would expect if mycorrhizal type had no bearing on recruitment within local assemblages of canopy and understory plants. Here we describe our plan to explore and quantify correlations between the relative abundance of canopy and understory plants of the same mycorrhizal types throughout forests of the United States.

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