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Biotic interactions play primary roles in major theories of the distribution and abundance of species, yet the nature of these biotic interactions can depend upon the larger ecological community. Leguminous plants, for example, commonly associate with both arbuscular mycorrhizal fungi (AMF) and rhizobia bacteria, and the pairwise interactions may depend upon the presence or identity of the third partner. To determine if the dynamics of plant-AMF and plant-rhizobia interactions are affected by the alternate symbiont, we manipulated the presence and identity of each symbiont, as well as levels of the nutrients supplied by each symbiont (nitrogen and phosphorus), on the growth of prairie legume Amorpha canescens . We found strong synergistic effects of AMF and rhizobia inoculation on plant biomass production that were independent of nutrient levels. AMF and rhizobia responses were each influenced by the other, but not in the same direction. AMF infection increased root nodule number and mass, but rhizobia inoculation decreased AMF hyphal colonization of roots. The relative benefits of each combination of symbionts depended upon phosphorus level. The effect of nitrogen was also contingent on the biotic environment where nitrogen addition decreased nodulation, but this decrease was reduced with coinfection by AMF. Our results demonstrate a strong contingency on the co-occurrence of AMF and rhizobia for the long-term fitness of A. canescens , and suggest that the belowground community is critical for the success of this species in tallgrass prairies.

Conspecific negative density-dependent establishment, in which local abundance negatively affects establishment of conspecific seedlings through host-specific enemies, can influence species diversity of plant communities, but the generality of this process is not well understood. We tested the strength of density dependence using the United States Forest Service's Forest Inventory and Analysis database containing 151 species from more than 200,000 forest plots spanning 4,000,000 square kilometers. We found that most species experienced conspecific negative density dependence (CNDD), but there was little effect of heterospecific density. Additionally, abundant species exhibited weaker CNDD than rarer species, and species-rich regions exhibited stronger CNDD than species-poor regions. Collectively, our results provide evidence that CNDD is a pervasive mechanism driving diversity across a gradient from boreal to subtropical forests.

It is increasingly recognized that microbiota affect host health and physiology. However, it is unclear what factors shape microbiome community assembly in nature, and how microbiome assembly can be manipulated to improve host health. All plant leaves host foliar endophytic fungi, which make up a diverse, environmentally acquired fungal microbiota. Here, we experimentally manipulated assembly of the cacao tree (Theobroma cacao) fungal microbiome in nature and tested the effect of assembly outcome on host health. Using next-generation sequencing, as well as culture-based methods coupled with Sanger sequencing, we found that manipulating leaf litter exposure and location within the forest canopy significantly altered microbiome composition in cacao. Exposing cacao seedlings to leaf litter from healthy conspecific adults enriched the seedling microbiome with Colletotrichum tropicale, a fungal endophyte known to enhance pathogen resistance of cacao seedlings by upregulating host defensive pathways. As a result, seedlings exposed to healthy conspecific litter experienced reduced pathogen damage. Our results link processes that affect the assembly and composition of microbiome communities to their functional consequences for host success, and have broad implications for understanding plant–microbe interactions. Deliberate manipulation of the plant–fungal microbiome also has potentially important applications for cacao production and other agricultural systems in general.

Plant-soil feedbacks are known to play a central role in species co-existence, but conceptual frameworks for predicting their magnitude and direction are lacking. We ask whether co-occurring trees that associate with different types of mycorrhizal fungi, which are hypothesized to differ in terms of nutrient use and plant-soil feedbacks, differ in sapling establishment densities and probability of co-occurrence. Given that ectomycorrhizal (ECM) trees typically have fungal structures that protect roots from pathogens whereas arbuscular mycorrhizal (AM) trees do not, we hypothesized that ECM saplings would be clustered around ECM trees, while AM saplings would be suppressed near AM trees. Most previous studies have focused on seedlings, but here we examine whether the spatial signal is evident in later life stages. We measured the spatial associations of ~ 28,000 trees using point pattern analysis in a 25-ha old-growth forest where ECM trees comprised 72 % of total basal area and 42 % of the total stems, while AM trees comprised the remainder. Supporting our hypothesis, AM saplings were more inhibited by AM trees, while ECM saplings were more clustered around ECM trees. The spatial patterns of AM and ECM trees on saplings of the alternate mycorrhizal type were inhibited. To the extent that similar types of feedbacks occur for other AM and ECM trees, our results suggest that fundamental differences in the nature of local-scale biotic interactions between trees and their fungal symbionts may influence forest community assembly and ecosystem dynamics.

The Janzen–Connell hypothesis 1 , 2 proposes that host-specific, distance- and/or density-dependent predators and herbivores maintain high tree diversity in tropical forests. Negative feedback between plant and soil communities could be a more effective mechanism promoting species coexistence because soil pathogens can increase rapidly in the presence of their host 3 , causing conditions unfavourable for local conspecific recruitment 4 , 5 , 6 . Here we show that a soil pathogen leads to patterns of seedling mortality in a temperate tree ( Prunus serotina ) as predicted by the Janzen–Connell hypothesis. In the field, the mean distance to parent of seedling cohorts shifted away from maternal trees over a period of 3 years. Seedlings were grown in soil collected 0–5 m or 25–30 m from Prunus trees. Sterilization of soil collected beneath trees improved seedling survival relative to unsterilized soil, whereas sterilization of distant soil did not affect survival. Pythium spp., isolated from roots of dying seedlings and used to inoculate healthy seedlings, decreased survival by 65% relative to controls. Our results provide the most complete evidence that native pathogens influence tree distributions, as predicted by the Janzen–Connell hypothesis, and suggest that similar ecological mechanisms operate in tropical and temperate forests.

Factors that control tree seedling dynamics are critical determinants of forest diversity. We examined the role of density-dependent mortality and abiotic factors in the differential establishment and survival of tree seedlings at three, large, mapped forest plots in Indiana, Virginia, and Wisconsin, USA. We tested whether seedling densities and seedling survival are related to local biotic and abiotic factors with generalized linear mixed models. Spatial point pattern analyses were utilized to determine if the distribution patterns of seedlings and saplings are consistent with a pattern generated by negative density-dependent mortality with respect to conspecific trees. Initial sampled seedling density for nearly a third of species showed a positive correlation with increasing conspecific basal area, indicating dispersal limitation, but few had any association with abiotic variables. By contrast, survival of seedlings over one year significantly declined with increasing conspecific basal area. Point pattern analyses indicated that nearly one-third of tree species had significantly over-dispersed point patterns of conspecific seedlings and saplings relative to adult densities; the majority of other species exhibited random spatial arrangements. Our results demonstrate-that negative conspecific density-dependent mortality of seedlings could generate the spatial patterns observed at later life stages. By differentially favoring seedlings of other species, this process may contribute to the maintenance of tree diversity in temperate forests, just as others have demonstrated for tropical forests.

In nature, plants often associate with multiple symbionts concurrently, yet the effects of tripartite symbioses are not well understood. We expected synergistic growth responses from plants associating with functionally distinct symbionts. In contrast, symbionts providing similar benefits to a host may reduce host plant growth. We reviewed studies investigating the effect of multiple interactions on host plant performance. Additionally, we conducted a meta-analysis on the studies that performed controlled manipulations of the presence of two microbial symbionts. Using response ratios, we investigated the effects on plants of pairs of symbionts (mycorrhizal fungi, fungal endophytes, and nitrogen-fixers). The results did not support the view that arbuscular mycorrhizal (AM) fungi and rhizobia should interact synergistically. In contrast, we found the joint effects of fungal endophytes and arbuscular mycorrhizal fungi to be greater than expected given their independent effects. This increase in plant performance only held for antagonistic endophytes, whose negative effects were alleviated when in association with AM fungi, while the impact of beneficial endophytes was not altered by infection with AM fungi. Generalizations from the meta-analysis were limited by the substantial variation within types of interactions and the data available, highlighting the need for more research on a range of plant systems.

Increasing evidence suggests that microbial interactions are important determinants of plant biodiversity. The hypothesis that fungal endophyte symbiosis reduces diversity in successional fields was tested by manipulating infection of tall fescue, the most abundant perennial grass in the eastern United States. Over a 4-year period. species richness declined and tall fescue dominance increased in infected plots relative to uninfected plots without difference in total productivity. A host-specific endophyte, with negligible biomass, altered plant community structure in this long-term field experiment and may be reducing plant diversity throughout its expanding range.

Phylogenetic distance among host species represents a proxy for host traits that act as biotic filters to shape host‐associated microbiome community structure. However, teasing apart potential biotic assembly mechanisms, such as host specificity or local species interactions, from abiotic factors, such as environmental specificity or dispersal barriers, in hyperdiverse, horizontally transmitted microbiomes remains a challenge. In this study, we tested whether host phylogenetic relatedness among 18 native Asteraceae plant species and spatial distance between replicated plots in a common garden affects foliar fungal endophyte (FFE) community structure. We found that FFE community structure varied significantly among host species, as well as host tribes, but not among host subfamilies. However, FFE community dissimilarity between host individuals was not significantly correlated with phylogenetic distance between host species. There was a significant effect of spatial distance among host individuals on FFE community dissimilarity within the common garden. The significant differences in FFE community structure among host species, but lack of a significant host phylogenetic effect, suggest functional differences among host species not accounted for by host phylogenetic distance, such as metabolic traits or phenology, may drive FFE community dissimilarity. Overall, our results indicate that host species identity and the spatial distance between plants can determine the similarity of their microbiomes, even across a single experimental field, but that host phylogeny is not closely tied to FFE community divergence in native Asteraceae. We tested whether foliar fungal endophyte communities are structured by host phylogenetic relatedness using a common garden experimental design and an explicit selection of host species from a defined phylogenetic range. We found that host species identity was a significant driver of foliar fungal endophyte community structure. However, we did not find that pairwise phylogenetic distance between host species was significantly correlated with foliar fungal endophyte community dissimilarity between host individuals.