No abstract

Understanding the assembly rules of mycorrhizal fungi is crucial, given their tremendous importance in plant nutrition and health. Differentiation in plant‐associated arbuscular mycorrhizal fungi (AMF) is likely driven by a host‐preference effect. Coexisting plant species may then affect a focal plant microbiota through fungal dispersal among plants, and plant preferential recruitment of AMF. Both mechanisms are likely shaped by the plant's phylogenetic and functional strategies. We expected that (a) the structure of AMF assemblages associated with a focal plant depends on the identity of the neighbouring plant species; (b) this effect would be predicted by the phylogenetic and functional similarity between the focal and neighbouring plant species. These predictions were tested during the first stages of growth, by simulating the early development of plants within a community Using an experimental matrix‐focal plant species design testing 15 neighbouring plants from five taxonomic families, we demonstrated that the neighbouring plants provided different species pools for the focal plant, Medicago truncatula, and influenced AMF communities associated with focal plant, especially in terms of richness but not relative evenness. Medicago truncatula grown with Brassicaceae or other Poaceae species displayed respectively no or low AMF richness compared to those grown with Rosaceae and Asteraceae species. These effects were weakly dependent on the phylogenetic distance from the neighbouring plant but were predicted by the functional proximity. AMF assemblages were enriched and bore more resemblance to the neighbouring plants when the neighbouring plants were functionally dissimilar from the focal one. Functional dissimilarity was only a significant predictor when based on traits characterizing the nutrient use and uptake strategy rather than on a more integrated growing strategy of the plant. Microbiota composition was shown to be dependent on the identity of the neighbouring plant, particularly on its functional below‐ground niche. At the colonization stage, when the plant arrives in a community, plant mycobiota might be influenced by the spatial distribution of plants already present in the community. This work suggests a new view of the concept of niche partitioning in space for plants based on microorganism–plant interactions. A free Plain Language Summary can be found within the Supporting Information of this article.

Aim An individual tree resembles a living island, a small spatially distinct unit upon which colonizers maintain populations. However, several differences exist compared to oceanic islands: a tree is relatively young, is composed of numerous differently aged branches, may be phylogenetically isolated from neighbours, and some of its colonizers are specific to particular tree lineages. We suggest that these specificities strongly affect both alpha‐ and beta‐diversity within trees, including positive effects of isolation on the diversity of generalists, and strengthening of the effect of isolation with tree age. Location Rennes, Bretagne, Western France Taxon Little‐dispersive, generalist oribatid mites (Acari) and highly dispersive, specialist gall wasps (Hymenoptera: Cynipidae) on oak (Quercus sp.) trees. Methods We tested the effects of tree and branch age, tree and branch habitat diversity, and tree phylogenetic isolation on per‐branch and per‐tree alpha‐diversity, and on within‐tree beta‐diversity of both taxonomic groups. Results For gall wasps, no variable explained diversity patterns at any level. In contrast, for oribatid mites, we found that high phylogenetic isolation of trees and high branch age increased alpha‐diversity per tree and per branch (in young trees) as well as turnover among branches. High tree age decreased alpha‐diversity per branch (in phylogenetically isolated trees) and increased turnover among branches. Increasing habitat diversity increased alpha‐diversity per tree, but decreased alpha‐diversity per branch (in young trees). Main conclusions For mites, contrary to common expectation, we suggest that: (a) phylogenetically distant neighbours are a source of immigration of distinct species and (b) with the increase of tree age, species‐sorting results in a few species colonizing and dominating their preferred patches. In gall wasps, strict specialization on oaks, and efficient dispersal may render oak age or isolation unimportant. The positive relationship between isolation and within‐tree turnover is a new contribution to biogeography in general.