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The enemy release hypothesis (ERH) attributes the success of some exotic plant species to reduced top‐down effects of natural enemies in the non‐native range relative to the native range. Many studies have tested this idea, but very few have considered the simultaneous effects of multiple kinds of enemies on more than one invasive species in both the native and non‐native ranges. Here, we examined the effects of two important groups of natural enemies–insect herbivores and soil biota–on the performance of Tanacetum vulgare (native to Europe but invasive in the USA) and Solidago canadensis (native to the USA but invasive in Europe) in their native and non‐native ranges, and in the presence and absence of competition.In the field, we replicated full‐factorial experiments that crossed insecticide, T. vulgare–S. canadensis competition, and biogeographic range (Europe vs. USA) treatments. In greenhouses, we replicated full‐factorial experiments that crossed soil sterilization, plant–soil feedback, and biogeographic range treatments. We evaluated the effects of experimental treatments on T. vulgare and S. canadensis biomass.The effects of natural enemies were idiosyncratic. In the non‐native range and relative to populations in the native range, T. vulgare escaped the negative effects of insect herbivores but not soil biota, depending upon the presence of S. canadensis; and S. canadensis escaped the negative effects of soil biota but not insect herbivores, regardless of competition. Thus, biogeographic escape from natural enemies depended upon the enemies, the invader, and competition. Synthesis: By explicitly testing the ERH in terms of more than one kind of enemy, more than one invader, and more than one continent, this study enhances our nuanced perspective of how natural enemies can influence the performance of invasive species in their native and non‐native ranges., We examined a leading hypothesis of plant invasion, the enemy release hypothesis, in terms of multiple invasive species, multiple natural enemies, and multiple continents. We found that escape from natural enemies depends on the enemies, the invader, and competition.

1. Multiple drivers of environmental change pose a significant challenge for ecological restoration, including climate change, soil salinization and environmental pollution. Due to the important role that soil biota play in enabling plants to cope with a variety of abiotic stressors, there is growing interest in the use of microbial inoculations to facilitate native plant restoration in the face of such change. 2. Recently, novel methods have begun being explored in agriculture to harness stress‐conditioned soil biota for improving abiotic stress tolerance in crop species. Similar applications in ecological restoration – where plants are inoculated with indigenous soil microbial communities that are preconditioned to various abiotic stressors – could potentially increase our capacity to restore degraded ecosystems under global change. 3. In this paper, we aim to (1) outline the ways in which soil microbial communities might be conditioned in order to confer greater stress tolerance to plants that are targets for restoration; (2) highlight successful (and unsuccessful) examples where stress‐tolerant soil microbial communities were utilized to improve plant performance; (3) describe the ways in which stress‐conditioned soil biota could be deployed in order to assist ecological restoration; and (4) discuss the potential risks and outstanding questions associated with such an approach. 4. If restoration practitioners are able to harness the soil microbiome to improve plant stress tolerance as is currently being explored in agriculture, this could revolutionize methods for the restoration of degraded lands in the Anthropocene.

Plant-soil feedbacks and interspecific competition are ubiquitous interactions that strongly influence the performance of plants. Yet few studies have examined whether the strength of these interactions corresponds with the abundance of plant species in the field, or whether feedbacks and competition interact in ways that either ameliorate or exacerbate their effects in isolation. We sampled soil from two intermountain grassland communities where we also measured the relative abundance of plant species. In greenhouse experiments, we quantified the direction and magnitude of plant-soil feedbacks for 10 target species that spanned a range of abundances in the field. In soil from both sites, plant-soil feedbacks were mostly negative, with more abundant species suffering greater negative feedbacks than rare species. In contrast, the average response to competition for each species was unrelated with its abundance in the field. We also determined how competitive response varied among our target species when plants competed in live vs. sterile soil. Interspecific competition reduced plant size, but the strength of this negative effect was unchanged by plant-soil feedbacks. Finally, when plants competed interspecifically, we asked how conspecific-trained, heterospecific-trained, and sterile soil influenced the competitive responses of our target species and how this varied depending on whether target species were abundant or rare in the field. Here, we found that both abundant and rare species were not as harmed by competition when they grew in heterospecific-trained soil compared to when they grew in conspecific-cultured soil. Abundant species were also not as harmed by competition when growing in sterile vs. conspecific-trained soil, but this was not the case for rare species. Our results suggest that abundant plants accrue species-specific soil pathogens to a greater extent than rare species. Thus, negative feedbacks may be critical for preventing abundant species from becoming even more abundant than rare species. National Science FoundationNational Science Foundation (NSF) [EPS-1101342 (INSTEP 3)] Thanks to Lauren Waller for help with quantifying species abundance in the field and Kim Ledger for her assistance in setting up our greenhouse experiments. K. Baer, P. Hahn, R. Hegstad, Y. Lekberg, and L. Larios provided valuable comments on the manuscript. We thank the National Science Foundation Experimental Program to Stimulate Competitive Research Track-1 EPS-1101342 (INSTEP 3) for support. Public domain – authored by a U.S. government employee

This is the peer reviewed version of the following article: Jassey, V. E. J., Reczuga, M. K., Zielinska, M., Slowinska, S., Robroek, B. J. M., Mariotte, P., ... Buttler, A. (2017) Tipping point in plant–fungal interactions under severe drought causes abrupt rise in peatland ecosystem respiration. Global Change Biology, 24(3), 972-98., which has been published in final form at https://doi.org/10.1111/gcb.13928. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. Ecosystems are increasingly prone to climate extremes, such as drought, with long‐lasting effects on both plant and soil communities and, subsequently, on carbon (C) cycling. However, recent studies underlined the strong variability in ecosystem's response to droughts, raising the issue of nonlinear responses in plant and soil communities. The conundrum is what causes ecosystems to shift in response to drought. Here, we investigated the response of plant and soil fungi to drought of different intensities using a water table gradient in peatlands—a major C sink ecosystem. Using moving window structural equation models, we show that substantial changes in ecosystem respiration, plant and soil fungal communities occurred when the water level fell below a tipping point of −24 cm. As a corollary, ecosystem respiration was the greatest when graminoids and saprotrophic fungi became prevalent as a response to the extreme drought. Graminoids indirectly influenced fungal functional composition and soil enzyme activities through their direct effect on dissolved organic matter quality, while saprotrophic fungi directly influenced soil enzyme activities. In turn, increasing enzyme activities promoted ecosystem respiration. We show that functional transitions in ecosystem respiration critically depend on the degree of response of graminoids and saprotrophic fungi to drought. Our results represent a major advance in understanding the nonlinear nature of ecosystem properties to drought and pave the way towards a truly mechanistic understanding of the effects of drought on ecosystem processes.

Soil pathogens are believed to be major contributors to negative plant–soil feedbacks that regulate plant community dynamics and plant invasions. While the theoretical basis for pathogen regulation of plant communities is well established within the plant–soil feedback framework, direct experimental evidence for pathogen community responses to plants has been limited, often relying largely on indirect evidence based on above-ground plant responses. As a result, specific soil pathogen responses accompanying above-ground plant community dynamics are largely unknown. Here, we examine the oomycete pathogens in soils conditioned by established populations of native noninvasive and non-native invasive haplotypes of Phragmites australis (European common reed). Our aim was to assess whether populations of invasive plants harbor unique communities of pathogens that differ from those associated with noninvasive populations and whether the distribution of taxa within these communities may help to explain invasive success. We compared the composition and abundance of pathogenic and saprobic oomycete species over a 2-year period. Despite a diversity of oomycete taxa detected in soils from both native and non-native populations, pathogen communities from both invaded and noninvaded soils were dominated by species of Pythium. Pathogen species that contributed the most to the differences observed between invaded and noninvaded soils were distributed between invaded and noninvaded soils. However, the specific taxa in invaded soils responsible for community differences were distinct from those in noninvaded soils that contributed to community differences. Our results indicate that, despite the phylogenetic relatedness of native and non-native P. australis haplotypes, pathogen communities associated with the dominant non-native haplotype are distinct from those of the rare native haplotype. Pathogen taxa that dominate either noninvaded or invaded soils suggest different potential mechanisms of invasion facilitation. These findings are consistent with the hypothesis that non-native plant species that dominate landscapes may “cultivate” a different soil pathogen community to their rhizosphere than those of rarer native species.

Plant-plant and plant-soil interactions can help maintain plant diversity and ecosystem functions. Changes in these interactions may underlie experimentally-observed increases in biodiversity effects over time via the selection of genotypes adapted to low or high plant diversity. However, little is known about such community-history effects and particularly the role of plant-soil interactions in this process. Soil-legacy effects may occur if co-evolved interactions with soil communities either positively or negatively modify plant biodiversity effects. We tested how plant selection and soil legacy influence biodiversity effects on productivity, and whether such effects increase the resistance of the communities to invasion by weeds. We used two plant selection treatments: parental plants growing in monoculture or in mixture over 8 years in a grassland biodiversity experiment in the field, which we term monoculture types and mixture types. The two soil-legacy treatments used in this study were neutral soil inoculated with live or sterilized soil inocula collected from the same plots in the biodiversity experiment. For each of the four factorial combinations, seedlings of eight species were grown in monocultures or 4-species mixtures in pots in an experimental garden over fifteen weeks. Soil legacy (live inoculum) strongly increased biodiversity complementarity effects for communities of mixture types, and to a significantly weaker extent for communities of monoculture types. This may be attributed to negative plant-soil feedbacks suffered by mixture types in monocultures whereas monoculture types had positive plant-soil feedbacks in both monocultures and mixtures. Monocultures of mixture types were most strongly invaded by weeds, presumably due to increased pathogen susceptibility of mixture types and thereby reduced biomass and altered plant-soil interactions. These results show that biodiversity effects in experimental grassland communities can be modified by the evolution of positive vs. negative plant-soil feedbacks of plant monoculture vs. mixture types.

Seed-attacking microorganisms have an undefined potential for management of the weed seedbank, either directly through inundative inoculation of soils with effective pathogenic strains, or indirectly by managing soils in a manner that promotes native seed-decaying microorganisms. However, research in this area is still limited and not consistently successful because of technological limitations in identifying the pathogens involved and their efficacy. We suggest that these limitations can now be overcome through application of new molecular techniques to identify the microorganisms interacting with weed seeds and to decipher their functionality. However, an interdisciplinary weed management approach that includes weed scientists, microbiologists, soil ecologists and molecular biologists is required to provide new insights into physical and chemical interactions between different seed species and microorganisms. Such insight is a prerequisite to identify the best candidate organisms to consider for seedbank management and to find ways to increase weed seed suppressive soil communities.