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  1. The loss of plant productivity with declining diversity is well established, exceeding other global change drivers including drought. These patterns are most clearly established for aboveground productivity, it remains poorly understood whether productivity increases associated with diversity are replicated belowground. To address this gap, we established a plant diversity-manipulation experiment in 2018. It is a full factorial manipulation of plant species richness and community composition, and precipitation. Three and five years post-establishment, two bulk soil cores (20cm depth) were collected and composited from each plot and were processed for roots to determine belowground biomass as root standing crop. We observed a strong positive relationship between richness and aboveground production and belowground biomass, generating positive combined above and belowground with diversity. Root standing crop increased 1.4-fold from years three to five. Grass communities produced more root biomass (monoculture mean 463.9 ± 410.3g m−2), and the magnitude of the relationship between richness and root standing crop was greatest within those communities. Legume communities produced the fewest roots (monoculture mean 212.2 ± 155.1g m−2), and belowground standing crop was not affected by diversity. Root standing crops in year three were 1.8 times higher under low precipitation conditions, while in year five we observed comparable root standing crops between precipitation treatments. Plant family was a strong mediator of increased belowground biomass observed with diversity, with single family grass and aster families generating 1.7 times greater root standing crops in six compared to single species communities, relationships between diversity and aboveground production were consistently observed in both single-family and multiple family communities. Diverse communities with species from multiple families generated only 1.3 times the root standing crop compared to monoculture average root biomass. We surprisingly observe diverse single family communities can generate increases in root standing crops that exceed those generated by diverse multiple family communities, highlighting the importance of plant richness within plant family for a given community. These patterns have potential implications for understanding the interactions of multiple global change drivers as changes in both precipitation and plant community composition do alter whether plant production aboveground is translated belowground biomass.

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    Free, publicly-accessible full text available November 27, 2024
  2. Abstract

    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.

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    Free, publicly-accessible full text available December 1, 2024
  3. This paper investigates the response of five tomato and five pepper varieties to native arbuscular mycorrhizal (AM) fungal inoculation in an organic farming system. The field experiment was conducted across a growing season at a working organic farm in Lawrence, KS, USA. The researchers hypothesized that native AM fungi inoculation would improve crop biomass production for both crop species, but that the magnitude of response would depend on crop cultivar. The results showed that both crops were significantly positively affected by inoculation. AM fungal inoculation consistently improved total pepper biomass throughout the experiment (range of +2% to +8% depending on the harvest date), with a +3.7% improvement at the final harvest for inoculated plants. An interaction between pepper variety and inoculation treatment was sometimes observed, indicating that some pepper varieties were more responsive to AM fungi than others. Beginning at the first harvest, tomatoes showed a consistent positive response to AM fungal inoculation among varieties. Across the experiment, AM fungi-inoculated tomatoes had +10% greater fruit biomass, which was driven by a +20% increase in fruit number. The study highlights the potential benefits of using native AM fungi as a soil amendment in organic farmed soils to improve pepper and tomato productivity.

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    Free, publicly-accessible full text available August 1, 2024
  4. Although several studies have shown increased native plant establishment with native microbe soil amendments, few studies have investigated how microbes can alter seedling recruitment and establishment in the presence of a non-native competitor. In this study, the effect of microbial communities on seedling biomass and diversity was assessed by seeding pots with both native prairie seeds and a non-native grass that commonly invades US grassland restorations, Setaria faberi. Soil in the pots was inoculated with whole soil collections from ex-arable land, late successional arbuscular mycorrhizal (AM) fungi isolated from a nearby tallgrass prairie, with both prairie AM fungi and ex-arable whole soil, or with a sterile soil (control). We hypothesized (1) late successional plants would benefit from native AM fungi, (2) that non-native plants would outcompete native plants in ex-arable soils, and (3) early successional plants would be unresponsive to microbes. Overall, native plant abundance, late successional plant abundance, and total diversity were greatest in the native AM fungi+ ex-arable soil treatment. These increases led to decreased abundance of the non-native grass S. faberi. These results highlight the importance of late successional native microbes on native seed establishment and demonstrate that microbes can be harnessed to improve both plant community diversity and resistance to invasion during the nascent stages of restoration. 
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  5. Abstract

    Research suggests that microbiomes play a major role in structuring plant communities and influencing ecosystem processes, however, the relative roles and strength of change of microbial components have not been identified. We measured the response of fungal, arbuscular mycorrhizal fungal (AMF), bacteria, and oomycete composition 4 months after planting of field plots that varied in plant composition and diversity. Plots were planted using 18 prairie plant species from three plant families (Poaceae, Fabaceae, and Asteraceae) in monoculture, 2, 3, or 6 species richness mixtures and either species within multiple families or one family. Soil cores were collected and homogenized per plot and DNA were extracted from soil and roots of each plot. We found that all microbial groups responded to the planting design, indicating rapid microbiome response to plant composition. Fungal pathogen communities were strongly affected by plant diversity. We identified OTUs from genera of putatively pathogenic fungi that increased with plant family, indicating likely pathogen specificity. Bacteria were strongly differentiated by plant family in roots but not soil. Fungal pathogen diversity increased with planted species richness, while oomycete diversity, as well as bacterial diversity in roots, decreased. AMF differentiation in roots was detected with individual plant species, but not plant family or richness. Fungal saprotroph composition differentiated between plant family composition in plots, providing evidence for decomposer home-field advantage. The observed patterns are consistent with rapid microbiome differentiation with plant composition, which could generate rapid feedbacks on plant growth in the field, thereby potentially influencing plant community structure, and influence ecosystem processes. These findings highlight the importance of native microbial inoculation in restoration.

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  6. Choosing restoration strategies may depend on ecosystem's stability properties. When degraded ecosystems do not self‐perpetuate, natural regeneration can lead to system recovery, and restoration interventions are often designed to accelerate the natural regeneration process. However, when degraded systems self‐perpetuate, reestablishing functional ecosystems depends on overcoming resistance thresholds that impede recovery. In both scenarios, concentrating restoration efforts in patches of the desired state may enhance ecosystem recovery. Introducing patches of a desired state has been motivated by two frameworks: autocatalytic nucleation and the analogy to nucleation. When restoration depends on overcoming resistance thresholds, autocatalytic nucleation lowers restoration barriers by initiating a local positive feedback mechanism that is only successful when desired patches are introduced above a critical patch size. In contrast, the analogy to nucleation accelerates natural regeneration whereby desired patches interact with landscape scale factors often through directed dispersal. We compare nucleation frameworks, and discuss their applications for restoration practices.

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  7. Abstract

    Plants affect associated biotic and abiotic edaphic factors, with reciprocal feedbacks from soil characteristics affecting plants. These two‐way interactions between plants and soils are collectively known as plant–soil feedbacks (PSFs). The role of phylogenetic relatedness and evolutionary histories have recently emerged as a potential driver of PSFs, although the strength and direction of feedbacks among sympatric congeners are not well‐understood. We examined plant–soil feedback responses ofAsclepias syriaca, a common clonal milkweed species, with several sympatric congeners across a gradient of increasing phylogenetic distances (A. tuberosa,A. viridis,A. sullivantii, andA. verticillata, respectively). Plant–soil feedbacks were measured through productivity and colonization by arbuscular mycorrhizal (AM) fungi.Asclepias syriacaproduced less biomass in soils conditioned by the most phylogenetically distant species (A. verticillata), relative to conspecific‐conditioned soils. Similarly, arbuscular mycorrhizal (AM) fungal colonization ofA. syriacaroots was reduced when grown in soils conditioned byA. verticillata, compared with colonization in plants grown in soil conditioned by any of the other threeAsclepiasspecies, indicating mycorrhizal associations are a potential mechanism of observed positive PSFs. This display of differences between the most phylogenetically distant, but not close or intermediate, paring(s) suggests a potential phylogenetic threshold, although other exogenous factors cannot be ruled out. Overall, these results highlight the potential role of phylogenetic distance in influencing positive PSFs through mutualists. The role of phylogenetic relatedness and evolutionary histories have recently emerged as a potential driver of plant–soil feedbacks (PSFs), although the strength and direction of feedbacks among sympatric congeners are not well‐understood. Congeneric, sympatric milkweeds typically generated positive PSFs in terms of productivity and AM fungal colonization, suggesting the low likelihood of coexistence among tested pairs, with a strength of feedback increasing as the phylogenetic distance increases.

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  8. Abstract Island biogeography has classically focused on abiotic drivers of species distributions. However, recent work has highlighted the importance of mutualistic biotic interactions in structuring island floras. The limited occurrence of specialist pollinators and mycorrhizal fungi have been found to restrict plant colonization on oceanic islands. Another important mutualistic association occurs between nearly 15,000 plant species and nitrogen-fixing (N-fixing) bacteria. Here, we look for evidence that N-fixing bacteria limit establishment of plants that associate with them. Globally, we find that plants associating with N-fixing bacteria are disproportionately underrepresented on islands, with a 22% decline. Further, the probability of N-fixing plants occurring on islands decreases with island isolation and, where present, the proportion of N-fixing plant species decreases with distance for large, but not small islands. These findings suggest that N-fixing bacteria serve as a filter to plant establishment on islands, altering global plant biogeography, with implications for ecosystem development and introduction risks. 
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  9. Abstract

    Symbiont diversity can have large effects on plant growth but the mechanisms generating this relationship remain opaque. We identify three potential mechanisms underlying symbiont diversity–plant productivity relationships: provisioning with complementary resources, differential impact of symbionts of varying quality and interference between symbionts. We connect these mechanisms to descriptive representations of plant responses to symbiont diversity, develop analytical tests differentiating these patterns and test them using meta‐analysis. We find generally positive symbiont diversity–plant productivity relationships, with relationship strength varying with symbiont type. Inoculation with symbionts from different guilds (e.g. mycorrhizal fungi and rhizobia) yields strongly positive relationships, consistent with complementary benefits from functionally distinct symbionts. In contrast, inoculation with symbionts from the same guild yields weak relationships, with co‐inoculation not consistently generating greater growth than the best individual symbiont, consistent with sampling effects. The statistical approaches we outline, along with our conceptual framework, can be used to further explore plant productivity and community responses to symbiont diversity, and we identify critical needs for additional research to explore context dependency in these relationships.

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  10. Abstract

    Meeting restoration targets may require active strategies to accelerate natural regeneration rates or overcome the resilience associated with degraded ecosystem states. Introducing desired ecosystem patches in degraded landscapes constitutes a promising active restoration strategy, with various mechanisms potentially causing these patches to become foci from which desired species can re‐establish throughout the landscape. This study considers three mechanisms previously identified as potential drivers of introduced patch dynamics: autocatalytic nucleation, directed dispersal, and resource concentration. These mechanisms reflect qualitatively different positive feedbacks. We developed an ecological model framework that compared how the occurrence of each mechanism was reflected in spatio‐temporal patch dynamics. We then analyzed the implications of these relationships for optimal restoration design. We found that patch expansion accelerated over time when driven by the autocatalytic nucleation mechanism, while patch expansion driven by the directed dispersal or resource concentration mechanisms decelerated over time. Additionally, when driven by autocatalytic nucleation, patch expansion was independent of patch position in the landscape. However, the proximity of other patches affected patch expansion either positively or negatively when driven by directed dispersal or resource concentration. For autocatalytic nucleation, introducing many small patches was a favorable strategy, provided that each individual patch exceeded a critical patch size. Introducing a single patch or a few large patches was the most effective restoration strategy to initiate the directed dispersal mechanism. Introducing many small patches was the most effective strategy for reaching restored ecosystem states driven by a resource concentration mechanism. Our model results suggest that introducing desirable patches can substantially accelerate ecosystem restoration, or even induce a critical transition from an otherwise stable degraded state toward a desired ecosystem state. However, the potential of this type of restoration strategy for a particular ecosystem may strongly depend on the mechanism driving patch dynamics. In turn, which mechanism drives patch dynamics may affect the optimal spatial design of an active restoration strategy. Each of the three mechanisms considered reflects distinct spatio‐temporal patch dynamics, providing novel opportunities for empirically identifying key mechanisms, and restoration designs that introduce desired patches in degraded landscapes according to these patch dynamics.

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