Dilution of specialist pathogens drives productivity benefits from diversity in plant mixtures.

Productivity benefits from diversity can arise when compatible pathogen hosts are buffered by unrelated neighbors, diluting pathogen impacts. However, the generality of pathogen dilution has been controversial and rarely tested within biodiversity manipulations. Here, we test whether soil pathogen dilution generates diversity- productivity relationships using a field biodiversity-manipulation experiment, greenhouse assays, and feedback modeling. We find that the accumulation of specialist pathogens in monocultures decreases host plant yields and that pathogen dilution predicts plant productivity gains derived from diversity. Pathogen specialization predicts the strength of the negative feedback between plant species in greenhouse assays. These feedbacks significantly predict the overyielding measured in the field the following year. This relationship strengthens when accounting for the expected dilution of pathogens in mixtures. Using a feedback model, we corroborate that pathogen dilution drives overyielding. Combined empirical and theoretical evidence indicate that specialist pathogen dilution generates overyielding and suggests that the risk of losing productivity benefits from diversity may be highest where environmental change decouples plant-microbe interactions.

Arbuscular Mycorrhizal Fungi Taxa Show Variable Patterns of Micro-Scale Dispersal in Prairie Restorations.

Human land use disturbance is a major contributor to the loss of natural plant communities, and this is particularly true in areas used for agriculture, such as the Midwestern tallgrass prairies of the United States. Previous work has shown that arbuscular mycorrhizal fungi (AMF) additions can increase native plant survival and success in plant community restorations, but the dispersal of AMF in these systems is poorly understood. In this study, we examined the dispersal of AMF taxa inoculated into four tallgrass prairie restorations. At each site, we inoculated native plant species with greenhouse-cultured native AMF taxa or whole soil collected from a nearby unplowed prairie. We monitored AMF dispersal, AMF biomass, plant growth, and plant community composition, at different distances from inoculation. In two sites, we assessed the role of plant hosts in dispersal, by placing known AMF hosts in a "bridge" and "island" pattern on either side of the inoculation points. We found that AMF taxa differ in their dispersal ability, with some taxa spreading to 2-m in the first year and others remaining closer to the inoculation point. We also found evidence that AMF spread altered non-inoculated neighboring plant growth and community composition in certain sites. These results represent the most comprehensive attempt to date to evaluate AMF spread.

Mycorrhizal types influence island biogeography of plants.

Plant colonization of islands may be limited by the availability of symbionts, particularly arbuscular mycorrhizal (AM) fungi, which have limited dispersal ability compared to ectomycorrhizal and ericoid (EEM) as well as orchid mycorrhizal (ORC) fungi. We tested for such differential island colonization within contemporary angiosperm floras worldwide. We found evidence that AM plants experience a stronger mycorrhizal filter than other mycorrhizal or non-mycorrhizal (NM) plant species, with decreased proportions of native AM plant species on islands relative to mainlands. This effect intensified with island isolation, particularly for non-endemic plant species. The proportion of endemic AM plant species increased with island isolation, consistent with diversification filling niches left open by the mycorrhizal filter. We further found evidence of humans overcoming the initial mycorrhizal filter. Naturalized floras showed higher proportions of AM plant species than native floras, a pattern that increased with increasing isolation and land-use intensity. This work provides evidence that mycorrhizal fungal symbionts shape plant colonization of islands and subsequent diversification.

Celebrating INVAM: 35 years of the largest living culture collection of arbuscular mycorrhizal fungi.

The International Culture Collection of (Vesicular-) Arbuscular Mycorrhizal Fungi-INVAM-the largest living culture collection of arbuscular mycorrhizal fungi (AMF) celebrated its 35th year in 2020. The authors record here the mission and goals of INVAM, its contribution as a living culture collection, some historical aspects of INVAM, and describe the advances in mycorrhizology and AMF systematics after INVAM moved to West Virginia University. This commentary emphasizes the importance of a living culture collection to preserve germplasm and to educate and assist researchers in mycorrhizal science.

Symbionts as Filters of Plant Colonization of Islands: Tests of Expected Patterns and Environmental Consequences in the Galapagos.

The establishments of new organisms that arrive naturally or with anthropogenic assistance depend primarily on local conditions, including biotic interactions. We hypothesized that plants that rely on fungal symbionts are less likely to successfully colonize remote environments such as oceanic islands, and this can shape subsequent island ecology. We analyzed the mycorrhizal status of Santa Cruz Island, Galapagos flora compared with the mainland Ecuador flora of origin. We experimentally determined plant responsiveness and plant-soil feedback of the island flora and assessed mycorrhizal density and soil aggregate stability of island sites. We found that a greater proportion of the native island flora species belongs to families that typically do not associate with mycorrhizal fungi than expected based upon the mainland flora of origin and the naturalized flora of the island. Native plants benefited significantly less from soil fungi and had weaker negative soil feedbacks than introduced species. This is consistent with the observation that field sites dominated by native plant species had lower arbuscular mycorrhizal (AM) fungal density and lower soil aggregate stability than invaded field sites at the island. We found support for a mycorrhizal filter to the initial colonization of the Galapagos.

Soil microbiome mediates positive plant diversity-productivity relationships in late successional grassland species.

Which processes drive the productivity benefits of biodiversity remain a critical, but unanswered question in ecology. We tested whether the soil microbiome mediates the diversity-productivity relationships among late successional plant species. We found that productivity increased with plant richness in diverse soil communities, but not with low-diversity mixtures of arbuscular mycorrhizal fungi or in pasteurised soils. Diversity-interaction modelling revealed that pairwise interactions among species best explained the positive diversity-productivity relationships, and that transgressive overyielding resulting from positive complementarity was only observed with the late successional soil microbiome, which was both the most diverse and exhibited the strongest community differentiation among plant species. We found evidence that both dilution/suppression from host-specific pathogens and microbiome-mediated resource partitioning contributed to positive diversity-productivity relationships and overyielding. Our results suggest that re-establishment of a diverse, late successional soil microbiome may be critical to the restoration of the functional benefits of plant diversity following anthropogenic disturbance.

Mycorrhizal fungi influence global plant biogeography.

Island biogeography has traditionally focused primarily on abiotic drivers of colonization, extinction and speciation. However, establishment on islands could also be limited by biotic drivers, such as the absence of symbionts. Most plants, for example, form symbioses with mycorrhizal fungi, whose limited dispersal to islands could act as a colonization filter for plants. We tested this hypothesis using global-scale analyses of ~1.4 million plant occurrences, including ~200,000 plant species across ~1,100 regions. We find evidence for a mycorrhizal filter (that is, the filtering out of mycorrhizal plants on islands), with mycorrhizal associations less common among native island plants than native mainland plants. Furthermore, the proportion of native mycorrhizal plants in island floras decreased with isolation, possibly as a consequence of a decline in symbiont establishment. We also show that mycorrhizal plants contribute disproportionately to the classic latitudinal gradient of plant species diversity, with the proportion of mycorrhizal plants being highest near the equator and decreasing towards the poles. Anthropogenic pressure and land use alter these plant biogeographical patterns. Naturalized floras show a greater proportion of mycorrhizal plant species on islands than in mainland regions, as expected from the anthropogenic co-introduction of plants with their symbionts to islands and anthropogenic disturbance of symbionts in mainland regions. We identify the mycorrhizal association as an overlooked driver of global plant biogeographical patterns with implications for contemporary island biogeography and our understanding of plant invasions.

Non-native plants and soil microbes: potential contributors to the consistent reduction in soil aggregate stability caused by the disturbance of North American grasslands.

• Soil aggregate stability is an important ecosystem property that is altered by anthropogenic disturbance. Yet, the generalization of these alterations and the identification of the main contributors are limited by the absence of cross-site comparisons and the application of inconsistent methodologies across regions. • We assessed aggregate stability in paired remnant and post-disturbance grasslands across California, shortgrass and tallgrass prairies, and in manipulative experiments of plant composition and soil microbial inoculation. • Grasslands recovering from anthropogenic disturbance consistently had lower aggregate stability than remnants. Across all grasslands, non-native plant diversity was significantly associated with reduced soil aggregate stability. A negative effect of non-native plants on aggregate stability was also observed in a mesocosm experiment comparing native and non-native plants from California grasslands. Moreover, an inoculation study demonstrated that the degradation of the microbial community also contributes to the decline in soil aggregate stability in disturbed grasslands. • Anthropogenic disturbance consistently reduced water-stable aggregates. The stability of aggregates was reduced by non-native plants and the degradation of the native soil microbial community. This latter effect might contribute to the sustained decline in aggregate stability following anthropogenic disturbance. Further exploration is advocated to understand the generality of these potential mechanisms.