Invasive plant suppresses the growth of native tree seedlings by disrupting belowground mutualisms

Occurrence of Arbuscular Mycorrhizal Herbs Decreases Selectively in Communities Dominated by Invasive Tree Acer negundo

We tested whether one of the consequences predicted for alien plant invasion by the mutualism disruption hypothesis was true in the case of the ash-leaved maple Acer negundo L. The study aimed to determine whether the occurrences of mycorrhizal and nonmycorrhizal herbs varied similarly or differently in communities with varying degrees of A. negundo dominance. The analysis included the results of 78 vegetation descriptions carried out in Belarusian Polesia, the Middle Volga region, and the Middle Urals. Communities with or without A. negundo dominance were described in each region. The mycorrhizal status of plant species was determined using the FungalRoot Database. Species that are more likely to form arbuscular mycorrhiza were found to occur less frequently in A. negundo thickets. On the contrary, a higher probability of the nonmycorrhizal status was associated with a lower frequency of detection in A. negundo thickets. Therefore, the occurrence of arbuscular mycorrhizal herbs was found to selectively decrease in communities dominated by A. negundo.

The root microlandscape of arbuscular mycorrhizal fungi

Understanding the drivers of assemblages of arbuscular mycorrhizal fungi (AMF) is essential to leverage the benefits of AMF for plant growth and health. Arbuscular mycorrhizal fungi are heterogeneously distributed in space even at small scale. We review the role of plant distribution in driving AMF assemblages (the passenger hypothesis), using a transposition of the conceptual framework of landscape ecology. Because rooting systems correspond to habitat patches with limited carrying capacity that differ in quality due to host-preference effects, we suggest considering plant communities as mosaics of AMF microhabitats. We review how predictions from landscape ecology apply to plant community effects on AMF, and the existing evidence that tests these predictions. Although many studies have been conducted on the effect of plant compositional heterogeneity on AMF assemblages, they mostly focused on the effect of plant richness, while only a few investigated the effect of configurational heterogeneity, plant connectivity or plant community temporal dynamics. We propose key predictions and future prospects to fill these gaps. Considering plant communities as landscapes extends the passenger hypothesis by including a spatially explicit dimension and its associated ecological processes and may help understand and manipulate AMF assemblages at small spatial scales.

Artemisia smithii patches form fertile islands and lead to heterogeneity of soil bacteria and fungi within and around the patches in alpine meadows of the Qinghai-Tibetan Plateau

Herbivore-avoided plant patches are one of the initial characteristics of natural grassland degradation. These vegetation patches can intensify the spatial heterogeneity of soil nutrients within these grasslands. However, the effects of non-edible plant patches patches on the spatial heterogeneity of microorganisms have not been sufficiently studied in alpine meadows of the Qinghai-Tibetan Plateau, especially patches formed by herbaceous plants. To answer this question, soil nutrients, plant assembly, and microbial communities were measured inside, around, and outside of Artemisia smithii patches. These were 0 m (within the patch), 0-1 m (one meter from the edge of the patch), 1-2 m (two meters from the edge of the patch), 2-3 m (three meters from the edge of the patch), and >30 m (non-patch grassland more than thirty meters from the edge of the patch). Our results showed that A. smithii patches accumulated more aboveground biomass (AGB) within the patches (0 m), and formed fertile islands with the soil around the patches. Additionally, A. smithii patches increased soil bacterial diversity within (0 m) and around (0-1 m) the patches by primarily enriching copiotrophic bacteria (Actinobacteria), while the diversity of fungal communities increased mainly in the 0-1 m area but not within the patches. Bacterial community diversity was driven by pH, urease, nitrate nitrogen (NO3 --N), and microbial biomass carbon (MBC). The contents of soil water (SWC), soil organic matter (SOM), urease, NO3 --N, and MBC were the main factors influencing the diversity of the fungal community. This study elucidates the vegetation, nutrients, and microbial heterogeneity and their interrelationships, which are observed in fertile islands of herbivore-avoided plant patches in alpine meadows, and provides further insights into the spatial pattern of nutrients in patchy degraded grasslands.

Exotic plantations differ in "nursing" an understory invader: A probe into invasional meltdown

Forest plantations most likely promote exotic plant invasion. Using an in situ monitoring method, this study investigated the traits correlated with growth and reproduction of an understory invader, Phytolacca americana L., and ecological factors including understory irradiance, soil stoichiometry and microbial patterns associated with these traits in different exotic plantations of Robinia pseudoacacia L. and Pinus thunbergii Parl. at Mount Lao, Qingdao, China. We found that the traits of P. americana underneath the R. pseudoacacia stand might be situated at the fast side of the trait economic spectrum. The R. pseudoacacia stand appeared to "nurse" P. americana. Furthermore, we intended to explain the nurse effects of R. pseudoacacia stands by examining their ecological factors. First, the R. pseudoacacia stand created understory light attenuation, which matched the sciophilous feature of P. americana. Second, the soil beneath the R. pseudoacacia stand might benefit P. americana more since the soil has greater resource availability. Third, a higher microbial diversity was found in the soil derived from P. americana underneath the R. pseudoacacia stand. A greater abundance of plant pathogens was detected in the soil derived from P. americana in the R. pseudoacacia stand, while more abundant mycorrhizal fungi were detected in the P. thunbergii stand. We speculate that plant pathogens can defend P. americana from aggression from other understory competitors. The mycorrhizal fungi in the P. thunbergii stand might benefit P. americana while simultaneously benefiting other understory plants. Intensive competition from other plants might interfere with P. americana. The potential relationships between plant performance and ecological factors may explain the invasion mechanism of P. americana. The present study provides a novel insight on the facilitative effects of exotic tree plantation on an exotic herb through the modification of soil biota, with implications for the biocontrol of invasive species and forest management and conservation.

Investigation of the Allelopathic Effect of Two Invasive Plant Species in Rhizotron System

A key question in plant invasion biology is why invasive plants are more competitive in their introduced habitat than in their native habitat. Studies show that invasive species exhibit allelopathy, influencing other plants by releasing chemicals. Research on allelopathy uses in vitro tests, investigating effects on seed germination and seedling development. Although soil plays a role in modifying allelopathic effects, observations with soil are rare and almost nothing is known about the root development of test plants developing in soil and the effects of allelopathic compounds on root architecture. Our study evaluates the allelopathic effects of false indigo-bush (Amorpha fruticosa L.) and common milkweed (Asclepias syriaca L.) on oilseed rape growth as a model plant. The rhizotron system was used to study the effect of morphology and root architecture. Leaf-soil mixtures at 0.5%, 1%, and 5% concentrations were used. Shoot and root development was strongly inhibited at 5%. But there was no difference between the allelopathy of the two species, and the application of lower concentrations did not show any effect, demonstrating that soil has a significant modifying effect on their allelopathy. Our results highlight that the development of roots growing in the soil is also worth investigating in connection with allelopathy, which can strengthen the ecological importance of allelochemicals during successful invasions.

No evidence that modification of soil microbiota by woody invader facilitates subsequent invasion by herbaceous species

Many terrestrial ecosystems are co-invaded by multiple exotic species. The "invasional meltdown" hypothesis predicts that an initial invasive species will facilitate secondary invasions. In the plant kingdom, the potential underlying mechanisms of this hypothesis may be that modification of the soil properties by the initial invaders benefits for the subsequent exotic species invasion. In this study, we analyzed the composition of soil microbial communities and soil chemical properties from sites invaded by woody Rhus typhina, as well as uninvaded sites, to assess the impact of R. typhina invasion. Furthermore, we conducted a greenhouse experiment with multiple native-invasive pairs of herbaceous species to test whether R. typhina invasion facilitates subsequent exotic herb invasion. Our results showed that R. typhina invasion significantly altered the composition of soil fungal communities, especially pathogenic, endophytic, and arbuscular mycorrhizal fungi. However, this change in microbial composition led to neither direction nor magnitude changes in negative plant-soil feedback effects on both native and invasive species. This indicates that initial R. typhina invasion does not facilitate subsequent herb invasion, which does not support the "invasional meltdown" hypothesis. Additionally, R. typhina invasion significantly decreased soil total nitrogen and organic carbon contents, which may explain the significantly lower biomass of herbaceous roots grown in invaded soils compared with uninvaded soils. Alternately, although invasive herb growth was significantly more inhibited by soil microbiota compared with native herb growth, such inhibition cannot completely eliminate the risk of exotic herb invasion because of their innate growth advantages. Therefore, microbial biocontrol agents for plant invasion management should be combined with another approach to suppress the innate growth advantages of exotic species.

Multiple invasive species affect germination, growth, and photosynthesis of native weeds and crops in experiments

Alien plant species regularly and simultaneously invade agricultural landscapes and ecosystems; however, the effects of co-invasion on crop production and native biodiversity have rarely been studied. Secondary metabolites produced by alien plants may be allelopathic; if they enter the soil, they may be transported by agricultural activities, negatively affecting crop yield and biodiversity. It is unknown whether substances from different alien species in combination have a greater impact on crops and wild plants than if they are from only one of the alien species. In this study, we used a set of common garden experiments to test the hypothesis that mixed extracts from two common invasive species have synergistic effects on crops and weeds (defined as all non-crop plants) in European agricultural fields compared to single-species extracts. We found that both the combined and individual extracts had detrimental effects on the seed germination, seedling growth, biomass, and photosynthetic performance of both crops and weeds. We found that the negative effect of mixed extracts was not additive and that crop plants were more strongly affected by invasive species extracts than the weeds. Our results are important for managing invasive species in unique ecosystems on agricultural land and preventing economic losses in yield production.

Allelopathic Activity of the Invasive Plant Polygonum chinense Linn. and Its Allelopathic Substances

Polygonum chinense Linn., belonging to the Polygonaceae family, is distributed mostly in northern temperate climates. This species is a high-risk invasive plant and is thought to possess allelopathic potential. This study aimed to isolate and identify the allelopathic substances from P. chinense. Aqueous methanol extracts of P. chinense significantly inhibited the growth of alfalfa and Italian ryegrass seedlings in a species- and concentration-dependent manner. Activity-guided fractionation led to the isolation of two active compounds: dehydrovomifoliol and loliolide. A cress bioassay was used to determine the biological activity of dehydrovomifoliol, and cress, alfalfa, and Italian ryegrass were used to determine loliolide. Dehydrovomifoliol significantly suppressed the seedling growth of cress at the concentration of 1 mM, and the concentrations necessary for 50% growth inhibition (I50 values) of the roots and shoots were 1.2 and 2 mM, respectively. Loliolide significantly suppressed the shoot growth of cress, alfalfa, and Italian ryegrass at the concentration of 1 mM, and the concentrations necessary for I50 values of the shoots and roots were 0.15 to 2.33 and 0.33 to 2.23 mM, respectively. The findings of our study suggest the extracts of P. chinense might have growth-inhibitory potential and that dehydrovomifoliol and loliolide might contribute as allelopathic agents.

Perspectives on Computation in Plants

Plants thrive in virtually all natural and human-adapted environments and are becoming popular models for developing robotics systems because of their strategies of morphological and behavioral adaptation. Such adaptation and high plasticity offer new approaches for designing, modeling, and controlling artificial systems acting in unstructured scenarios. At the same time, the development of artifacts based on their working principles reveals how plants promote innovative approaches for preservation and management plans and opens new applications for engineering-driven plant science. Environmentally mediated growth patterns (e.g., tropisms) are clear examples of adaptive behaviors displayed through morphological phenotyping. Plants also create networks with other plants through subterranean roots-fungi symbiosis and use these networks to exchange resources or warning signals. This article discusses the functional behaviors of plants and shows the close similarities with a perceptron-like model that could act as a behavior-based control model in plants. We begin by analyzing communication rules and growth behaviors of plants; we then show how we translated plant behaviors into algorithmic solutions for bioinspired robot controllers; and finally, we discuss how those solutions can be extended to embrace original approaches to networking and robotics control architectures.

Legume effects in a native community invaded by alien Asteraceae in a multi-species comparison

Facilitation has been a long-neglected type of interaction but received more attention recently. Legumes are commonly involved in facilitative interactions due to their nitrogen fixation. Facilitative interactions are so far underappreciated yet potentially important for biological invasions, especially given increasing numbers of alien species. In a common garden experiment using 30 annual Asteraceae species (neophytes, archaeophytes, plus some natives), grown in communities with or without legume presence, we measured functional traits and fitness in focal Asteraceae, as well as nitrogen characteristics of Asteraceae and two native community phytometer species. We investigated how legume presence affects relationships between trait and nitrogen concentration and Asteraceae fitness; and whether mechanisms of facilitation in legume presence and its effects on aboveground performance differ among native phytometer, neophyte, and archaeophyte Asteraceae using the δ15N natural abundance method. Lower specific leaf area was associated with higher aboveground biomass and seed production, with a stronger effect in legume absence. Nitrogen concentration had a positive relationship with biomass, but did not generally increase seed production. Our results hint at N facilitation for the native grass phytometer Festuca rupicola when growing in legume presence, whereas the forb Potentilla argentea and 27 alien Asteraceae species did not indicate facilitative effects. Intriguingly, direct legume facilitation in native phytometer species was only detected when growing with archaeophytes neighbors, not with neophytes. This hints at varied mechanisms of competition for nitrogen between natives and alien species of different residence time and deepens the understanding of altered facilitative leguminous effects in alien species presence.

Parasitism Shifts the Effects of Native Soil Microbes on the Growth of the Invasive Plant Alternanthera philoxeroides

Soil microbes play an important role in plant invasion, and parasitic plants regulate the growth of invasive plants. However, the mechanisms by which parasitic plants regulate the effects of soil microbes on invasive plants have not been investigated. Here, we used the invasive plant Alternanthera philoxeroides and the holoparasitic plant Cuscuta grovonii to test whether and how C. grovonii parasitism shifts the effect of native soil microbes on the growth of A. philoxeroides. In a factorial setup, A. philoxeroides was grown in pots with the presence versus absence of parasitism and the presence versus absence of native soil microbes. The findings showed that native soil microbes increased the biomass and clonal growth of A. philoxeroides only in the absence of a parasite, whereas parasitism decreased the biomass and clonal growth of A. philoxeroides only in the presence of soil microbes. In addition, the presence of soil microbes increased the deleterious effects of the parasite on A. philoxeroides. These results indicate that parasitism can shift the effects of native soil microbes on the growth of the invasive plant A. philoxeroides. Our results enrich the understanding of the mechanisms underlying the success of plant invasion.

Effects of Mustard Invasions on Soil Microbial Abundances and Fungal Assemblages in Southern California

Although mustards (family, Brassicaceae) are common across southern California, research has not focused on the effects of type-conversion of native California sage scrub (CSS) to areas dominated by invasive mustards. To better understand how mustard invasions, primarily the short-pod mustard, Hirschfeldia incana, impact soil microbial assemblages, we examined microbial abundance and assemblages from intact CSS and adjacent mustard-dominated soils at three sites. We also explored if germination rates for various plant species differed between CSS and mustard soils. We found that mustard invasions reduce soil microbial abundances by more than 50% and alter soil fungal assemblages. Fungal richness, diversity, and evenness did not differ between habitats, highlighting that these habitats harbor unique microbial assemblages. While mustard allelopathy is predicted to be the primary driver of these changes, mustard invasions also increased soil pH. Although functional consequences of these shifts are unknown, low mustard germination in CSS soils supports biological resistance to mustard invasion in CSS. Overall, our results demonstrate that mustard invasions, H. incana in particular, exert a strong selecting force on soil microbial assemblages, which can influence effective CSS restoration and preservation of ecosystem services.

Impacts of an invasive filter-feeder on bacterial biodiversity are context dependent

Bacteria represent most of the biodiversity and play key roles in virtually every ecosystem. In doing so, bacteria act as part of complex communities shaped by interactions across all domains of life. Here, we report on direct interactions between bacteria and dreissenid mussels, a group of invasive filter-feeders threatening global aquatic systems due to high filtration rates. Previous studies showed that dreissenids can impact bacterial community structure by changing trait distributions and abundances of specific taxa. However, studies on bacterial community effects were conducted using water from Lake Michigan (an oligotrophic lake) only, and it is unknown whether similar patterns are observed in systems with differing nutrient regimes. We conducted ten short-term dreissenid grazing experiments in 2019 using water from two eutrophic lake regions-the western basin of Lake Erie and Saginaw Bay in Lake Huron. Predation by dreissenids led to decline in overall bacterial abundance and diversity in both lakes. However, feeding on bacteria was not observed during every experiment. We also found that traits related to feeding resistance are less phylogenetically conserved than previously thought. Our results highlight the role of temporal, spatial, and genomic heterogeneity in bacterial response dynamics to a globally important invasive filter feeder.

Is allelochemical synthesis in Casuarina equisetifolia plantation related to litter microorganisms?

Productivity decline of Casuarina equisetifolia plantation and difficulty in natural regeneration remains a serious problem because of allelopathy. Previous studies have confirmed that 2,4-di-tert-butylphenol (2,4-DTBP) are the major allelochemicals of the C. equisetifolia litter exudates. The production of these allelochemicals may derive from decomposition of litter or from the litter endophyte and microorganisms adhering to litter surfaces. In the present study, we aimed to evaluate the correlation between allelochemicals in litter and endophytic and epiphytic fungi and bacteria from litter. A total of 100 fungi and 116 bacteria were isolated from the interior and surface of litter of different forest ages (young, half-mature, and mature plantation). Results showed that the fermentation broth of fungal genera Mycosphaerella sp. and Pestalotiopsis sp., and bacterial genera Bacillus amyloliquefaciens, Burkholderia-Paraburkholderia, and Pantoea ananatis had the strongest allelopathic effect on C. equisetifolia seeds. Allelochemicals, such as 2,4-DTBP and its analogs were identified in the fermentation broths of these microorganisms using GC/MS analysis. These results indicate that endophytic and epiphytic fungi and bacteria in litters are involved in the synthesis of allelochemicals of C. equisetifolia. To further determine the abundance of the allelopathic fungi and bacteria, Illumina MiSeq high-throughput sequencing was performed. The results showed that bacterial genera with strong allelopathic potential were mainly distributed in the young and half-mature plantation with low abundance, while the abundance of fungal genera Mycosphaerella sp. and Pestalotiopsis sp. were higher in the young and mature plantations. In particular, the abundance of Mycosphaerella sp. in the young and mature plantations were 501.20% and 192.63% higher than in the half-mature plantation, respectively. Overall, our study demonstrates that the litter fungi with higher abundance in the young and mature plantation were involved in the synthesis of the allelochemical 2,4-DTBP of C. equisetifolia. This finding may be important for understanding the relationship between autotoxicity and microorganism and clarifying the natural regeneration problem of C. equisetifolia.

Acquisition and evolution of enhanced mutualism-an underappreciated mechanism for invasive success?

Soil biota can determine plant invasiveness, yet biogeographical comparisons of microbial community composition and function across ranges are rare. We compared interactions between Conyza canadensis, a global plant invader, and arbuscular mycorrhizal (AM) fungi in 17 plant populations in each native and non-native range spanning similar climate and soil fertility gradients. We then grew seedlings in the greenhouse inoculated with AM fungi from the native range. In the field, Conyza plants were larger, more fecund, and associated with a richer community of more closely related AM fungal taxa in the non-native range. Fungal taxa that were more abundant in the non-native range also correlated positively with plant biomass, whereas taxa that were more abundant in the native range appeared parasitic. These patterns persisted when populations from both ranges were grown together in a greenhouse; non-native populations cultured a richer and more diverse AM fungal community and selected AM fungi that appeared to be more mutualistic. Our results provide experimental support for evolution toward enhanced mutualism in non-native ranges. Such novel relationships and the rapid evolution of mutualisms may contribute to the disproportionate abundance and impact of some non-native plant species.

Plant invasions facilitated by suppression of root nutrient acquisition rather than by disruption of mycorrhizal association in the native plant

Invasive species have profound negative impacts on native ranges. Unraveling the mechanisms employed by invasive plant species is crucial to controlling invasions. One important approach that invasive plants use to outcompete native plants is to disrupt mutualistic interactions between native roots and mycorrhizal fungi. However, it remains unclear how differences in the competitive ability of invasive plants affect native plant associations with mycorrhizae. Here, we examined how a native plant, Xanthium strumarium, responds to invasive plants that differed in competitive abilities (i.e., as represented by aboveground plant biomass) by measuring changes in root nitrogen concentration (root nutrient acquisition) and mycorrhizal colonization rate. We found that both root nitrogen concentration and mycorrhizal colonization rate in the native plant were reduced by invasive plants. The change in mycorrhizal colonization rate of the native plant was negatively correlated with both aboveground plant biomass of the invasive plants and the change in aboveground plant biomass of the native plant in monocultures relative to mixed plantings. In contrast, the change in root nitrogen concentration of the native plant was positively correlated with aboveground plant biomass of the invasive plants and the change in aboveground plant biomass of the native plant. When we compared the changes in mycorrhizal colonization rate and root nitrogen concentration in the native plant grown in monocultures with those of native plants grown with invasive plants, we observed a significant tradeoff. Our study shows that invasive plants can suppress native plants by reducing root nutrient acquisition rather than by disrupting symbiotic mycorrhizal associations, a novel finding likely attributable to a low dependence of the native plant on mycorrhizal fungi.

Invasive Grass Dominance over Native Forbs Is Linked to Shifts in the Bacterial Rhizosphere Microbiome

Rhizosphere microbiomes have received growing attention in recent years for their role in plant health, stress tolerance, soil nutrition, and invasion. Still, relatively little is known about how these microbial communities are altered under plant competition, and even less about whether these shifts are tied to competitive outcomes between native and invasive plants. We investigated the structure and diversity of rhizosphere bacterial and fungal microbiomes of native annual forbs and invasive annual grasses grown in a shade-house both individually and in competition using high-throughput amplicon sequencing of the bacterial 16S rRNA gene and the fungal ITS region. We assessed how differentially abundant microbial families correlate to plant biomass under competition. We find that bacterial diversity and structure differ between native forbs and invasive grasses, but fungal diversity and structure do not. Furthermore, bacterial community structures under competition are distinct from individual bacterial community structures. We also identified five bacterial families that varied in normalized abundance between treatments and that were correlated with plant biomass under competition. We speculate that invasive grass dominance over these natives may be partially due to effects on the rhizosphere community, with changes in specific bacterial families potentially benefiting invaders at the expense of natives.

Evidence for the evolution of native plant response to mycorrhizal fungi in post-agricultural grasslands

Plant-microbe interactions play an important role in structuring plant communities. Arbuscular mycorrhizal fungi (AMF) are particularly important. Nonetheless, increasing anthropogenic disturbance will lead to novel plant-AMF interactions, altering longstanding co-evolutionary trajectories between plants and their associated AMF. Although emerging work shows that plant-AMF response can evolve over relatively short time scales due to anthropogenic change, little work has evaluated how plant AMF response specificity may evolve due to novel plant-mycorrhizal interactions. Here, we examine changes in plant-AMF interactions in novel grassland systems by comparing the mycorrhizal response of plant populations from unplowed native prairies with populations from post-agricultural grasslands to inoculation with both native prairie AMF and non-native novel AMF. Across four plant species, we find support for evolution of differential responses to mycorrhizal inocula types, that is, mycorrhizal response specificity, consistent with expectations of local adaptation, with plants from native populations responding most to native AMF and plants from post-agricultural populations responding most to non-native AMF. We also find evidence of evolution of mycorrhizal response in two of the four plant species, as overall responsiveness to AMF changed from native to post-agricultural populations. Finally, across all four plant species, roots from native prairie populations had lower levels of mycorrhizal colonization than those of post-agricultural populations. Our results report on one of the first multispecies assessment of local adaptation to AMF. The consistency of the responses in our experiment among four species provides evidence that anthropogenic disturbance may have unintended impacts on native plant species' association with AMF, causing evolutionary change in the benefit native plant species gain from native symbioses.

Whole community invasions and the integration of novel ecosystems

The impact of invasion by a single non-native species on the function and structure of ecological communities can be significant, and the effects can become more drastic–and harder to predict–when multiple species invade as a group. Here we modify a dynamic Boolean model of plant-pollinator community assembly to consider the invasion of native communities by multiple invasive species that are selected either randomly or such that the invaders constitute a stable community. We show that, compared to random invasion, whole community invasion leads to final stable communities (where the initial process of species turnover has given way to a static or near-static set of species in the community) including both native and non-native species that are larger, more likely to retain native species, and which experience smaller changes to the topological measures of nestedness and connectance. We consider the relationship between the prevalence of mutualistic interactions among native and invasive species in the final stable communities and demonstrate that mutualistic interactions may act as a buffer against significant disruptions to the native community.

Plant Gender Affects Soil Fungal Microbiota Associated with Welwitschia mirabilis, an Unusual Desert Gymnosperm

In a recent study, we found a distinct soil bacterial community associated with male and female plants of the desert gymnosperm Welwitschia mirabilis. In this subsequent study, we also found that the soil fungal community associated with Welwitschia differs between male and female plants, and between unvegetated areas and the soil under plants. Site location, pH, and soil moisture also had an important influence on the composition of the fungal community. A number of Ascomycota and Chytrid species were found to be distinct indicators of male and female plants, respectively, but there was no overall difference at the phylum level or in terms of diversity. The unvegetated areas between plants also differed in terms of several Ascomycota OTUs. Network connectivity of the fungal communities was found to be higher under both male and female Welwitschia plants than in unvegetated control areas. As with the bacterial community, it is unclear what processes produce the gender-distinct fungal community, and also the more general plant-associated community, and also what the effects on the biology of the plants are. One possibility behind the gender-related difference in fungal community is that there are differences in the production of pollen or nectar between the two plant genders, affecting the below-ground soil community.

Exploring urban green packages as part of Nature-based Solutions for climate change adaptation measures in rapidly growing cities of the Global South

Given a lot of elusive information on the use and implementation of Nature-based Solutions (NbS) in the Global South, this review provides a synthesis of the evidence on the: - (1) distribution of urban green technologies in form of arboriculture and urban agriculture as a part of NbS packages for the sustainability of cities against population growth and impact of climate change; and (2) options of integrating and mainstreaming various NbS packages into city development policies, planning processes, and decision-making agendas. The sustainability of urban green as part of NbS packages and the usefulness for improvement of livelihoods is determined by the spatial (geographical location) and temporal (time of action) scales, and socio-ecological and institutional factors. Various NbS packages have shown the ability for use as climate change adaptation measures throughout the world. These functions include protection from soil erosion, protection from inland flooding, buffering natural resources against drier and more variable climates, protection from coastal hazards and sea-level rise, moderation of urban heatwaves and effects of heat island, and managing storm-water and flooding in urban areas. Furthermore, the benefits of urban agriculture and arboriculture include use as sources of food and generation of income; improve recreation and social interactions, and the sustainability of biodiversity. They also mitigate the impact of environmental pollution and climate change through reduction of gas emissions and act as carbon sinks. While the starting capital and lack of policy on urban agriculture and arboriculture in many countries, the importance of the industry is inevitably a useful agenda especially in the Global South due to vulnerability to the impact of climate change. This review also suggests the inclusion of all institutions, governments, and relevant stakeholders to emphasize gender sensitization at all levels of planning and decision-making in food production and adaptation measures to climate change.

Woody invaders are more highly colonized by arbuscular mycorrhizal fungi than congeneric native species

PREMISE

Invasive species tend to possess acquisitive plant traits that support fast growth and strong competitive ability. However, the relevance of symbioses with arbuscular mycorrhizal fungi (AMF) to the fast growing, acquisitive strategy of invasive species is still unclear.

METHODS

We measured AMF colonization in roots of five congeneric pairs of invasive and native eastern North American woody species (10 species total; 4 lianas, 6 shrubs) that were grown in a monoculture common garden experiment in Syracuse, NY. We then examined the relationships of AMF colonization to above and belowground traits of these species.

RESULTS

Total AMF colonization and arbuscule colonization were greater in invasive compared to native woody species, a pattern that was more distinct in congeneric shrubs than congeneric lianas. The level of AMF colonization was also positively correlated with traits indicative of rapid plant growth and nutrient uptake.

CONCLUSIONS

The concordance of a resource-acquisitive strategy with higher AMF colonization suggests that symbioses with AMF may be part of the strategy by which invasive woody plants of eastern North America are able to maintain fast growth rates and outcompete their native counterparts.

Environmental stress determines the colonization and impact of an endophytic fungus on invasive knotweed

There is increasing evidence that microbes play a key role in some plant invasions. A diverse and widespread but little understood group of plant-associated microbes are the fungal root endophytes of the order Sebacinales. They are associated with exotic populations of invasive knotweed (Reynoutria ssp.) in Europe, but their effects on the invaders are unknown. We used the recently isolated Sebacinales root endophyte Serendipita herbamans to experimentally inoculate invasive knotweed and study root colonisation and effects on knotweed growth under different environmental conditions. We verified the inoculation success and fungal colonisation through immunofluorescence microscopy and qPCR. We found that S. herbamans strongly colonized invasive knotweed in low-nutrient and shade environments, but much less under drought or benign conditions. At low nutrients, the endophyte had a positive effect on plant growth, whereas the opposite was true under shaded conditions. Our study demonstrates that the root endophyte S. herbamans has the potential to colonize invasive knotweed fine roots and impact its growth, and it could thus also play a role in natural populations. Our results also show that effects of fungal endophytes on plants can be strongly environment-dependent, and may only be visible under stressful environmental conditions.

Allelopathy as an evolutionary game

In plants, most competition is resource competition, where one plant simply preempts the resources away from its neighbors. Interference competition, as the name implies, is a form of direct interference to prevent resource access. Interference competition is common among animals that can physically fight, but in plants, one of the main mechanisms of interference competition is allelopathy. Allelopathic plants release cytotoxic chemicals into the environment which can increase their ability to compete with surrounding organisms for limited resources. The circumstances and conditions favoring the development and maintenance of allelochemicals, however, are not well understood. Particularly, despite the obvious benefits of allelopathy, current data suggest it seems to have only rarely evolved. To gain insight into the cost and benefit of allelopathy, we have developed a 2 × 2 matrix game to model the interaction between plants that produce allelochemicals and plants that do not. Production of an allelochemical introduces novel cost associated with both synthesis and detoxifying a toxic chemical but may also convey a competitive advantage. A plant that does not produce an allelochemical will suffer the cost of encountering one. Our model predicts three cases in which the evolutionarily stable strategies are different. In the first, the nonallelopathic plant is a stronger competitor, and not producing allelochemicals is the evolutionarily stable strategy. In the second, the allelopathic plant is the better competitor, and production of allelochemicals is the more beneficial strategy. In the last case, neither is the evolutionarily stable strategy. Instead, there are alternating stable states, depending on whether the allelopathic or nonallelopathic plant arrived first. The generated model reveals circumstances leading to the evolution of allelochemicals and sheds light on utilizing allelochemicals as part of weed management strategies. In particular, the wide region of alternative stable states in most parameterizations, combined with the fact that the absence of allelopathy is likely the ancestral state, provides an elegant answer to the question of why allelopathy seems to rarely evolve despite its obvious benefits. Allelopathic plants can indeed outcompete nonallelopathic plants, but this benefit is simply not great enough to allow them to go to fixation and spread through the population. Thus, most populations would remain purely nonallelopathic.

Dependence of the Distribution and Structure of the White Mulberry (Morus alba) Population in Wrocław on the Intensity of Anthropopressure and Thermal Conditions

The selection of species which show the highest possible tolerance to negative habitat conditions, also among plants of foreign origin, is a pressing issue. One of the species we would like to recommend for planting in urban areas is the white mulberry species (Morus alba) due to both its outstanding adaptability and its ecosystem services. There are no reliable studies on the distribution of this species in urbanized areas in Poland, nor sufficient analyses of the methods of its renewal, both deliberate and spontaneous spread through self-seeding. Collecting data on the population of an alien species within individual regions and forecasting potential changes in the population’s size and structure, as well as its possible impacts on other organisms, is one of the basic measures to reduce biological invasions, which is one of the six priority objectives of the European Biodiversity Strategy and an element of the Strategy on Invasive Alien Species. The aim of this study was to determine the size and structure of the white mulberry population in the city of Wrocław and to analyse the relationship between this structure and intensity of anthropopressure and thermal conditions.

OUP accepted manuscript

The invasive plant Alliaria petiolata (garlic mustard) has spread throughout forest understory and edge communities in much of North America, but its persistence, density, and impacts have varied across sites and time. Surveying the literature since 2008, we evaluated both previously proposed and new mechanisms for garlic mustard's invasion success and note how they interact and vary across ecological contexts. We analyzed how and where garlic mustard has been studied and found a lack of multisite and longitudinal studies, as well as regions that may be under- or overstudied, leading to poor representation for understanding and predicting future invasion dynamics. Inconsistencies in how sampling units are scaled and defined can also hamper our understanding of invasive species. We present new conceptual models for garlic mustard invasion from a macrosystems perspective, emphasizing the importance of synergies and feedbacks among mechanisms across spatial and temporal scales to produce variable ecological contexts.

Novel chemicals engender myriad invasion mechanisms

Non-native invasive species (NIS) release chemicals into the environment that are unique to the invaded communities, defined as novel chemicals. Novel chemicals impact competitors, soil microbial communities, mutualists, plant enemies, and soil nutrients differently than in the species' native range. Ecological functions of novel chemicals and differences in functions between the native and non-native ranges of NIS are of immense interest to ecologists. Novel chemicals can mediate different ecological, physiological, and evolutionary mechanisms underlying invasion hypotheses. Interactions amongst the NIS and resident species including competitors, soil microbes, and plant enemies, as well as abiotic factors in the invaded community are linked to novel chemicals. However, we poorly understand how these interactions might enhance NIS performance. New empirical data and analyses of how novel chemicals act in the invaded community will fill major gaps in our understanding of the chemistry of biological invasions. A novel chemical-invasion mechanism framework shows how novel chemicals engender invasion mechanisms beyond plant-plant or plant-microorganism interactions.

High canopy cover of invasive Acer negundo L. affects ground vegetation taxonomic richness

We assessed the link between canopy cover degree and ground vegetation taxonomic richness under alien ash-leaved maple (Acer negundo) and other (native or alien) tree species. We investigated urban and suburban forests in the large city of Yekaterinburg, Russia. Forests were evaluated on two spatial scales. Through an inter-habitat comparison we recorded canopy cover and plant taxonomic richness among 13 sample plots of 20 × 20 m where A. negundo dominated and 13 plots where other tree species dominated. In an intra-habitat comparison, we recorded canopy cover and ground vegetation taxonomic richness among 800 sample plots measuring 1 m² in the extended urbanised forest, which featured abundant alien (308 plots) and native trees (492 plots). We observed decreased taxonomic richness among vascular ground plant species by 40% (inter-habitat) and 20% (intra-habitat) in areas dominated by A. negundo compared to areas dominated by native tree and shrub species. An abundance of A. negundo was accompanied by increased canopy cover. We found a negative relationship between canopy cover and the number of understory herbaceous species. Thus, the interception of light and the restriction of its amount for other species is a main factor supporting the negative influence of A. negundo on native plant communities.

Meta-analysis of the impact of plant invasions on soil microbial communities

Background

One of the ecological impacts of exotic plant invasions may be alteration of the soil microbial community, which may cause changes to the diversity, richness and function of these communities. In order to explore to what extent invasive plants affect the soil microbial community, we performed a meta-analysis based on 46 scientific articles to document the effect of invasive plants on species richness and diversity of bacteria and fungi. We conducted our study across a range of invaded ecosystems including native communities, and evaluated biomass, richness and diversity. We use a random effects model to determine the increase or decrease in the values of the response variables in the presence of invasive plants.

Results

The results indicated that the response variable that changed with the invasion of plants was the diversity of bacteria. Bacterial diversity in the soil increases with the presence of invasive plants, specifically herbaceous plants producing allelopathic substances growing in forest ecosystems of temperate zones.

Conclusions

We provide evidence that invasive plants affect the soil biota differentially; however, it is important to consider more variables such as the N and C cycles, since these processes are mediated by soil biota and litter, and chemical compounds released by plants influence them. Changes in bacterial diversity have consequences for the nutrient cycle, enzymatic activity, mineralization rates and soil carbon and nitrogen content.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01899-2.

Impacts of plant invasions in native plant-pollinator networks

The disruption of mutualisms by invasive species has consequences for biodiversity loss and ecosystem function. Although invasive plant effects on the pollination of individual native species has been the subject of much study, their impacts on entire plant-pollinator communities are less understood. Community-level studies on plant invasion have mainly focused on two fronts: understanding the mechanisms that mediate their integration; and their effects on plant-pollinator network structure. Here we briefly review current knowledge and propose a more unified framework for evaluating invasive species integration and their effects on plant-pollinator communities. We further outline gaps in our understanding and propose ways to advance knowledge in this field. Specifically, modeling approaches have so far yielded important predictions regarding the outcome and drivers of invasive species effects on plant communities. However, experimental studies that test these predictions in the field are lacking. We further emphasize the need to understand the link between invasive plant effects on pollination network structure and their consequences for native plant population dynamics (population growth). Integrating demographic studies with those on pollination networks is thus key in order to achieve a more predictive understanding of pollinator-mediated effects of invasive species on the persistence of native plant biodiversity.

Allelopathic and competitive interactions between native and alien plants

The novel-weapons and homeland-security hypotheses are based on the idea that aliens and natives are not adapted to each other’s allelochemicals as they did not co-evolve. However, as only a few studies have tested this, it remains unclear how important co-evolutionary history is in determining the strength of allelopathic interactions between aliens and natives. Here, we tested for potential pairwise allelopathic effects on each other of five alien and five native herbaceous species in China. We did a germination experiment and a competition experiment. In the germination experiment, we tested whether aqueous extracts of the ten study species had allelopathic effects on each other’s seed germination. In the competition experiment, we tested whether the alien and native species differed in their competitive effects and responses, and whether these were changed by the presence of activated carbon—a presumed allelopathy neutralizer– in the soil. Plant extracts had negative allelopathic effects on seed germination. This was particularly the case for extracts from the native species. Moreover, aqueous extracts had slightly stronger negative effects on germination of the aliens than on germination of the natives. In the competition experiment, on the other hand, the natives suffered more from competition than the alien species did, but we could not relate this to allelopathy. Alien plants had negative competitive and allelopathic effects on native plants, but the reverse was also true. These alien-native interactions, however, were not consistently stronger or weaker than native-native or alien-alien interactions.

A Path Forward: Promoting Microbial-Based Methods in the Control of Invasive Plant Species

In this review, we discuss the unrealized potential of incorporating plant-microbe and microbe-microbe interactions into invasive plant management strategies. While the development of this as a viable strategy is in its infancy, we argue that incorporation of microbial components into management plans should be a priority and has great potential for diversifying sustainable control options. We advocate for increased research into microbial-mediated phytochemical production, microbial controls to reduce the competitiveness of invasive plants, microbial-mediated increases of herbicidal tolerance of native plants, and to facilitate increased pathogenicity of plant pathogens of invasive plants.

The role of arbuscular mycorrhizal fungi in nonnative plant invasion along mountain roads

Plant associated mutualists can mediate invasion success by affecting the ecological niche of nonnative plant species. Anthropogenic disturbance is also key in facilitating invasion success through changes in biotic and abiotic conditions, but the combined effect of these two factors in natural environments is understudied. To better understand this interaction, we investigated how disturbance and its interaction with mycorrhizas could impact range dynamics of nonnative plant species in the mountains of Norway. Therefore, we studied the root colonisation and community composition of arbuscular mycorrhizal (AM) fungi in disturbed vs undisturbed plots along mountain roads. We found that roadside disturbance strongly increases fungal diversity and richness while also promoting AM fungal root colonisation in an otherwise ecto-mycorrhiza and ericoid-mycorrhiza dominated environment. Surprisingly, AM fungi associating with nonnative plant species were present across the whole elevation gradient, even above the highest elevational limit of nonnative plants, indicating that mycorrhizal fungi are not currently limiting the upward movement of nonnative plants. We conclude that roadside disturbance has a positive effect on AM fungal colonisation and richness, possibly supporting the spread of nonnative plants, but that there is no absolute limitation of belowground mutualists, even at high elevation.

Friends, neighbours and enemies: an overview of the communal and social biology of plants

Plants were traditionally seen as rather passive actors in their environment, interacting with each other only in so far as they competed for the same resources. In the last 30 years, this view has been spectacularly overturned, with a wealth of evidence showing that plants actively detect and respond to their neighbours. Moreover, there is evidence that these responses depend on the identity of the neighbour, and that plants may cooperate with their kin, displaying social behaviour as complex as that observed in animals. These plant-plant interactions play a vital role in shaping natural ecosystems, and are also very important in determining agricultural productivity. However, in terms of mechanistic understanding, we have only just begun to scratch the surface, and many aspects of plant-plant interactions remain poorly understood. In this review, we aim to provide an overview of the field of plant-plant interactions, covering the communal interactions of plants with their neighbours as well as the social behaviour of plants towards their kin, and the consequences of these interactions. We particularly focus on the mechanisms that underpin neighbour detection and response, highlighting both progress and gaps in our understanding of these fascinating but previously overlooked interactions.

Nitrogen Uptake, Not Transfer of Carbon and Nitrogen by CMN, Explains the Effect of AMF on the Competitive Interactions Between Flaveria bidentis and Native Species

Rhizophagus intraradices, one of the common arbuscular mycorrhizal fungi (AMF) grown in the roots of Flaveria bidentis, facilitates the invasion of this exotic plant species into China. However, it is still unknown whether nutrient transfer through the common mycorrhizal networks (CMN) between this exotic species and the native species enhances the competitive growth of F. bidentis over the native species. To elucidate this question and the related mechanism, an isotopic labeling technique was used to test the transfer of carbon (C) and nitrogen (N) by CMN. Native species like Setaria viridis and Eclipta prostrata were selected to compete with F. bidentis in a polyvinyl chloride (PVC) box. Two competitive groups (F. bidentis-S. viridis and F. bidentis- E. prostrata), three treatments (monoculture of F. bidentis, the mixture of F. bidentis and the native plant, and the monoculture of the native plant), and two levels of AMF (presence or absence) were assigned. Results showed that the corrected index of relative competition intensity (CRCI) of F. bidentis in the presence of AMF < 0 suggests that the competition facilitated the growth of F. bidentis with either S. viridis or E. prostrata. The reason was that the inoculation of R. intra radices significantly increased the C and N contents of F. bidentis in the mixtures. However, the effects of R. intra radices on the two native species were different: negative effect on the growth of S. viridis and positive effect on the growth of E. prostrata. The change of N content in S. viridis or E. prostrata was consistent with the variation of the biomass, suggesting that the N uptake explains the effects of R. intraradices on the competitive interactions between F. bidentis and the two native species. Moreover, the transfer of C and N via AMF hyphal links did occur between F. bidentis and the native species. However, the transfer of C and N by the CMN was not positively related to the competitive growth of F. bidentis.

Invasion-induced root-fungal disruptions alter plant water and nitrogen economies

Despite widespread evidence that biological invasion influences both the biotic and abiotic soil environments, the extent to which these two pathways underpin the effects of invasion on plant traits and performance remains unknown. Leveraging a long-term (14-year) field experiment, we show that an allelochemical-producing invader affects plants through biotic mechanisms, altering the soil fungal community composition, with no apparent shifts in soil nutrient availability. Changes in belowground fungal communities resulted in high costs of nutrient uptake for native perennials and a shift in plant traits linked to their water and nutrient use efficiencies. Some plants in the invaded community compensate for the disruption of nutritional symbionts and reduced nutrient provisioning by sanctioning more nitrogen to photosynthesis and expending more water, which demonstrates a trade-off in trait investment. For the first time, we show that the disruption of belowground nutritional symbionts can drive plants towards alternative regions of their trait space in order to maintain water and nutrient economics.

Soil nitrogen dynamics and competition during plant invasion: insights from Mikania micrantha invasions in China

Invasive plants often change a/biotic soil conditions to increase their competitiveness. We compared the microbially mediated soil nitrogen (N) cycle of invasive Mikania micrantha and two co-occurring native competitors, Persicaria chinensis and Paederia scandens. We assessed how differences in plant tissue N content, soil nutrients, N cycling rates, microbial biomass and activity, and diversity and abundance of N-cycling microbes associated with these species impact their competitiveness. Mikania micrantha outcompeted both native species by transferring more N to plant tissue (37.9-55.8% more than natives). We found total soil N to be at lowest, and available N highest, in M. micrantha rhizospheres, suggesting higher N cycling rates compared with both natives. Higher microbial biomass and enzyme activities in M. micrantha rhizospheres confirmed this, being positively correlated with soil N mineralization rates and available N. Mikania micrantha rhizospheres harbored highly diverse N-cycling microbes, including N-fixing, ammonia-oxidizing and denitrifying bacteria and ammonia-oxidizing archaea (AOA). Structural equation models indicated that M. micrantha obtained available N via AOA-mediated nitrification mainly. Field data mirrored our experimental findings. Nitrogen availability is elevated under M. micrantha invasion through enrichment of microbes that participate in N cycling, in turn increasing available N for plant growth, facilitating high interspecific competition.

Compatible Mycorrhizal Types Contribute to a Better Design for Mixed Eucalyptus Plantations

Mixed-species forest plantation is a sound option to facilitate ecological restoration, plant diversity and ecosystem functions. Compatible species combinations are conducive to reconstruct plant communities that can persist at a low cost without further management and even develop into natural forest communities. However, our understanding of how the compatibility of mycorrhizal types mediates species coexistence is still limited, especially in a novel agroforestry system. Here, we assessed the effects of mycorrhizal association type on the survival and growth of native woody species in mixed-species Eucalyptus plantations. To uncover how mycorrhizal type regulates plant-soil feedbacks, we first conducted a pot experiments by treating distinct mycorrhizal plants with soil microbes from their own or other mycorrhizal types. We then compared the growth response of arbuscular mycorrhizal plants and ectomycorrhizal plants to different soil microbial compositions associated with Eucalyptus plants. We found that the type of mycorrhizal association had a significant impact on the survival and growth of native tree species in the Eucalyptus plantations. The strength and direction of the plant-soil feedbacks of focal tree species depended on mycorrhizal type. Non-mycorrhizal plants had consistent negative feedbacks with the highest survival in the Eucalyptus plantations, whereas nitrogen-fixing plants had consistent positive feedbacks and the lowest survival. Arbuscular mycorrhizal and ectomycorrhizal plants performed varied feedback responses to soil microbes from distinct mycorrhizal plant species. Non-mycorrhizal plants grew better with Eucalyptus soil microbes while nitrogen-fixing plants grew worse with their own conspecific soil microbes. Different soil microbial compositions of Eucalyptus consistently increased the aboveground growth of arbuscular mycorrhizal plants, but the non-mycorrhizal microbial composition of the Eucalyptus soil resulted in greater belowground growth of ectomycorrhizal plants. Overall, Eucalyptus plants induced an unfavorable soil community, impeding coexistence with other mycorrhizal plants. Our study provides consistent observational and experimental evidence that mycorrhizal-mediated plant-microbial feedback on species coexistence among woody species. These findings are with important implications to optimize the species combinations for better design of mixed forest plantations.

Residence time determines invasiveness and performance of garlic mustard (Alliaria petiolata) in North America

While biological invasions have the potential for large negative impacts on local communities and ecological interactions, increasing evidence suggests that species once considered major problems can decline over time. Declines often appear driven by natural enemies, diseases or evolutionary adaptations that selectively reduce populations of naturalised species and their impacts. Using permanent long-term monitoring locations, we document declines of Alliaria petiolata (garlic mustard) in eastern North America with distinct local and regional dynamics as a function of patch residence time. Projected site-specific population growth rates initially indicated expanding populations, but projected population growth rates significantly decreased over time and at the majority of sites fell below 1, indicating declining populations. Negative soil feedback provides a potential mechanism for the reported disappearance of ecological dominance of A. petiolata in eastern North America.