Mycorrhizal associations of temperate forest seedlings mediate rhizodeposition, but not soil carbon storage, under elevated nitrogen availability.

Tree-mycorrhizal associations are associated with patterns in nitrogen (N) availability and soil organic matter storage; however, we still lack a mechanistic understanding of what tree and fungal traits drive these patterns and how they will respond to global changes in soil N availability. To address this knowledge gap, we investigated how arbuscular mycorrhizal (AM)- and ectomycorrhizal (EcM)-associated seedlings alter rhizodeposition in response to increased seedling inorganic N acquisition. We grew four species each of EcM and AM seedlings from forests of the eastern United States in a continuously 13C-labeled atmosphere within an environmentally controlled chamber and subjected to three levels of 15N-labeled fertilizer. We traced seedling 15N uptake from, and 13C-labeled inputs (net rhizodeposition) into, root-excluded or -included soil over a 5-month growing season. N uptake by seedlings was positively related to rhizodeposition for EcM- but not AM-associated seedlings in root-included soils. Despite this contrast in rhizodeposition, there was no difference in soil C storage between mycorrhizal types over the course of the experiment. Instead root-inclusive soils lost C, while root-exclusive soils gained C. Our findings suggest that mycorrhizal associations mediate tree belowground C investment in response to inorganic N availability, but these differences do not affect C storage. Continued soil warming and N deposition under global change will increase soil inorganic N availability and our seedling results indicate this could lead to greater belowground C investment by EcM-associated trees. This potential for less efficient N uptake by EcM-trees could contribute to AM-tree success and a shift toward more AM-dominated temperate forests.

Herbivory and Soil Water Availability Induce Changes in Arbuscular Mycorrhizal Fungal Abundance and Composition.

We tested the prediction that abundance and composition of arbuscular mycorrhizal fungi (AMF) in Ipomopsis aggregata roots and soils are influenced by ungulate herbivory and drought conditions by examining the effects in a field setting over two years. We used a multi-metric approach to quantify AMF root colonization, AMF reproduction, and AMF community composition in roots and soils. We incorporated complimentary community characterization assays by morphologically identifying spores from trap cultures and the use of terminal restriction fragment length polymorphism (T-RFLP) fingerprinting. Herbivory caused a twofold increase in spore production, an increase in AMF taxa diversity in roots, and a shift in AMF species composition in rhizosphere soils. The impact of herbivory was dependent on water availability, which differed in the two contrasting years. This study demonstrates that both soil water availability and herbivory shape arbuscular mycorrhizal fungi communities. The changes to mycorrhizal communities may help in understanding mycorrhizal function in changing climates.

Role of Yeasts in the Cranberry Fruit Rot Disease Complex.

Cranberry fruit rot (CFR) is an economically important disease caused by at least 10 species of filamentous fungi. Despite the application of fungicides, incidence of CFR is sometimes high, raising the possibility of a role for microbes other than fungi in the CFR complex. Isolation of microbes from rotten berries on culture media that favor either bacteria or yeasts resulted in mucoid colonies from <15% of dry-harvested rotten berries but up to 60% of wet-harvested berries. The mucoid colonies were identified as yeasts, primarily in the family Saccharomycetaceae. Inoculation of sound berries with three yeasts, Hanseniaspora uvarum, Pichia fermentans, and Pichia terricola, resulted in significantly higher incidence and severity of rot symptoms compared with mock-inoculated controls; these yeasts were recovered from inoculated berries, providing evidence of their pathogenicity. The minimum concentrations of azoxystrobin, chlorothalonil, and prothioconazole that resulted in 80% inhibition of growth compared with untreated controls (MIC80) were determined for a subset of yeasts. In general, MIC80s were higher for azoxystrobin and prothioconazole (usually >64 µg/ml) than for chlorothalonil (usually ≤1 µg/ml). To complement culture-dependent studies, DNA was isolated from wet- and dry-harvested rotten berries, and fungi were identified to the level of family by high-throughput sequencing of the fungal internal transcribed spacer region. There were no fungal families consistently detected among samples by one method (culturing or high-throughput sequencing) and missed by the other that have not previously been reported in cranberry; however, some fungal families were found to be more abundant by one method versus the other. Harvest method (wet or dry) had a significant effect on the composition of fungal communities of rotten berries (P < 0.001), and operational taxonomic units representing the Saccharomycetaceae were more abundant in wet- than dry-harvested berries. Taken together, the results suggest that some yeasts are pathogenic to cranberry and may be especially relevant in wet-harvested berries.

Management and Soil Conditions Influence Common Scab Severity on Potato Tubers Via Indirect Effects on Soil Microbial Communities.

Common scab, caused by Streptomyces scabies and related species, is a potato tuber blemish disease that causes reductions in marketable yield worldwide. Evidence of suppression of common scab by indigenous soil microbial populations has been found in several studies. However, we lack a comprehensive understanding of how common scab severity relates functionally to potato varieties, farming systems, soil physical and chemical properties, and soil microbial communities. These factors may affect disease directly or indirectly by affecting one of the other variables. We performed a survey of 30 sampling locations across 12 fields in Wisconsin and used structural equation modeling to disentangle the direct effects of potato market classes, farm management (conventional versus organic), and soil physiochemical properties on common scab severity from their indirect effects mediated through soil bacterial and fungal communities. We found that, although potato market classes affected disease severity directly, the effects of farm management and soil physiochemistry were best explained as indirect, mediated by their impacts on soil bacterial communities. This suggests that evaluating the consequences of specific management practices for soil microbial communities may be useful for understanding disease pressure across fields.

Simultaneous adaptation and maladaptation of tree populations to local rhizosphere microbial communities at different taxonomic scales.

Plant populations are often adapted to their local conditions, but the specific selective forces creating this adaptation are often unclear. All plants interact with diverse microbial communities, but we know little about how these microbial communities as a whole shape the evolutionary trajectory of plant populations. We tested whether tree populations were adapted or maladapted to their local rhizosphere microbial communities by growing seedlings sourced from multiple locations with soil microbial communities from all locations in a fully reciprocal design, using seedling growth as a proxy for fitness. In addition, we compared the microbial composition of the experimental inocula with that of the communities we detected associating with naturally occurring trees at the seedling source populations. We found that seedlings grew similarly when inoculated with local vs foreign microbial communities, but this neutral response derived from conflicting patterns - plant populations appeared to be adapted to the presence or absence of whole taxonomic groups in their local microbial community, but were simultaneously maladapted to the particular microbial populations present in their local site. As rapid climate change and other factors push tree populations into new areas, the successful establishment of seedlings may depend critically on the balance between the novelty and familiarity of the microbial communities they encounter.

Ectomycorrhizal fungal richness declines towards the host species' range edge.

Plant range boundaries are generally considered to reflect abiotic conditions; however, a rise in negative or decline in positive species interactions at range margins may contribute to these stable boundaries. While evidence suggests that pollinator mutualisms may decline near range boundaries, little is known about other important plant mutualisms, including microbial root symbionts. Here, we used molecular methods to characterize root-associated fungal communities in populations of two related temperate tree species from across the species' range in the eastern United States. We found that ectomycorrhizal fungal richness on plant roots declined with distance from the centre of the host species range. These patterns were not evident in nonmycorrhizal fungal communities on roots nor in fungal communities in bulk soil. Climatic and soil chemical variables could not explain these biogeographic patterns, although these abiotic gradients affected other components of the bulk soil and rhizosphere fungal community. Depauperate ectomycorrhizal fungal communities may represent an underappreciated challenge to marginal tree populations, especially as rapid climate change pushes these populations outside their current climate niche.

Coevolution between invasive and native plants driven by chemical competition and soil biota.

Although reciprocal evolutionary responses between interacting species are a driving force behind the diversity of life, pairwise coevolution between plant competitors has received less attention than other species interactions and has been considered relatively less important in explaining ecological patterns. However, the success of species transported across biogeographic boundaries suggests a stronger role for evolutionary relationships in shaping plant interactions. Alliaria petiolata is a Eurasian species that has invaded North American forest understories, where it competes with native understory species in part by producing compounds that directly and indirectly slow the growth of competing species. Here I show that populations of A. petiolata from areas with a greater density of interspecific competitors invest more in a toxic allelochemical under common conditions. Furthermore, populations of a native competitor from areas with highly toxic invaders are more tolerant to competition from the invader, suggesting coevolutionary dynamics between the species. Field reciprocal transplants confirmed that native populations more tolerant to the invader had higher fitness when the invader was common, but these traits came at a cost when the invader was rare. Exotic species are often detrimentally dominant in their new range due to their evolutionary novelty; however, the development of new coevolutionary relationships may act to integrate exotic species into native communities.

An exotic invader drives the evolution of plant traits that determine mycorrhizal fungal diversity in a native competitor.

The symbiosis between land plants and arbuscular mycorrhizal fungi (AMF) is one of the most widespread and ancient mutualisms on the planet. However, relatively little is known about the evolution of these symbiotic plant-fungal interactions in natural communities. In this study, we investigated the symbiotic AMF communities of populations of the native plant species Pilea pumila (Urticaceae) with varying histories of coexistence with a nonmycorrhizal invasive species, Alliaria petiolata (Brassicaceae), known to affect mycorrhizal communities. We found that native populations of P. pumila with a long history of coexistence with the invasive species developed more diverse symbiotic AMF communities. This effect was strongest when A. petiolata plants were actively growing with the natives, and in soils with the longest history of A. petiolata growth. These results suggest that despite the ancient and widespread nature of the plant-AMF symbiosis, the plant traits responsible for symbiotic preferences can, nevertheless, evolve rapidly in response to environmental changes.

Species invasion alters local adaptation to soil communities in a native plant.

Plant populations are often adapted to their local conditions, including abiotic factors as well as the biotic communities with which they interact. Soil communities, in particular, have strong effects on both the ecology and evolution of plant populations. Many invasive plant species alter the ecological relationships between native plants and soil communities; however, whether invaders also alter the evolutionary dynamics between native plants and soils is less well known. Here I show that populations of a native annual, Pilea pumila, shift from being maladapted to adapted to their local soil community with increasing history of invasion by Alliaria petiolata, an invader known to alter microbial communities. Additionally, native populations showed a signal of adaptation to soils of particular invasion stages, independent of local coevolutionary dynamics. These results suggest that invasive species affect not only the ecological, but also the evolutionary relationships of native species.

Shifting microbial communities can enhance tree tolerance to changing climates.

Climate change is pushing species outside of their evolved tolerances. Plant populations must acclimate, adapt, or migrate to avoid extinction. However, because plants associate with diverse microbial communities that shape their phenotypes, shifts in microbial associations may provide an alternative source of climate tolerance. Here, we show that tree seedlings inoculated with microbial communities sourced from drier, warmer, or colder sites displayed higher survival when faced with drought, heat, or cold stress, respectively. Microbially mediated drought tolerance was associated with increased diversity of arbuscular mycorrhizal fungi, whereas cold tolerance was associated with lower fungal richness, likely reflecting a reduced burden of nonadapted fungal taxa. Understanding microbially mediated climate tolerance may enhance our ability to predict and manage the adaptability of forest ecosystems to changing climates.

Evolutionary limits ameliorate the negative impact of an invasive plant.

Invasive species can quickly transform biological communities due to their high abundance and strong impacts on native species, in part because they can be released from the ecological forces that limit native populations. However, little is known about the long-term dynamics of invasions; do invaders maintain their dominant status over long time spans, or do new ecological and evolutionary forces eventually develop to limit their populations? Alliaria petiolata is a Eurasian species that aggressively invades North American forest understories, in part due to the production of toxic phytochemicals. Here we document a marked decline in its phytotoxin production and a consequent decline in their impact on three native species, across a 50+ year chronosequence of Alliaria petiolata invasion. Genetic evidence suggests that these patterns result from natural selection for decreased phytotoxin production rather than founder effects during introduction and spread. These patterns are consistent with the finding of slowing A. petiolata population growth and rebounding native species abundance across a separate chronosequence in Illinois, U.S. These results suggest that this invader is developing evolutionary limits in its introduced range and highlight the importance of understanding the long-term processes that shape species invasions and their impacts.

Resistance and recovery of soil microbial communities in the face of Alliaria petiolata invasions.

Invaders can gain ecological advantages because of their evolutionary novelty, but little is known about how these novel advantages will change over time as the invader and invaded community evolve in response to each other. Invasive plants often gain such an advantage through alteration of soil microbial communities. In soil communities sampled from sites along a gradient of invasion history with Alliaria petiolata, microbial richness tended to decline, but the community's resistance to A. petiolata's effects generally increased with increasing history of invasion. However, sensitive microbial taxa appeared to recover in the two oldest sites, leading to an increase in richness, but consequent decrease in resistance. This may be because of evolutionary changes in the A. petiolata populations, which tend to reduce their investment to allelopathic compounds over time. These results show that, over time, microbial communities can develop resistance to an invasive plant but at the cost of lower richness. However, over longer time-scales evolution in the invasive species may allow for the recovery of soil microbial communities.

Introduced Brassica nigra populations exhibit greater growth and herbivore resistance but less tolerance than native populations in the native range.

Rapid post-introduction evolution has been found in many invasive plant species, and includes changes in defence (resistance and tolerance) and competitive ability traits. Here, we explored the post-introduction evolution of a trade-off between resistance to and tolerance of herbivory, which has received little attention. In a common garden experiment in a native range, nine invasive and 16 native populations of Brassica nigra were compared for growth and defence traits. Invasive populations had higher resistance to, but lower tolerance of, herbivore damage than native populations. Invasive populations survived better and produced more seeds than native ones when released from herbivores; but fitness was equivalent between the regions under ambient herbivory. The invasive populations grew taller, and produced more biomass and lighter seeds than natives, irrespective of insecticide treatment. In addition to supporting the idea of post-introduction rapid evolution of plant traits, our results also contribute to an emerging pattern of both increasing resistance and growth in invasive populations, contrary to the predictions of earlier theories of resistance-growth trade-offs.

Conflicts in maintaining biodiversity at multiple scales.

Biodiversity consists of multiple scales, including functional diversity in ecological traits, species diversity and genetic diversity within species, and is declining across the globe, largely in response to human activities. While species extinctions are the most obvious aspect of this, there has also been a more insidious loss of genetic diversity within species. While a vast literature concerns each of these scales of biodiversity, less is known about how different scales affect one another. In particular, genetic and species diversity may influence each other in numerous ways, both positively and negatively. However, we know little about the mechanism behind these patterns. In this issue of Molecular Ecology, Nestmann et al. (2011) experimentally explore the effect of species and functional diversity and composition of grassland plant communities on the genetic structure of one of the component species. Increasing species richness led to greater changes in the genetic composition of the focal populations over 4 years, primarily because of genetic drift in smaller population sizes. However, there were also genetic changes in response to particular plant functional groups, indicating selective differences driven by plant community composition. These results suggest that different levels of biodiversity can trade-off in communities, which may prove a challenge for conservation biologists seeking to preserve all aspects of biodiversity.

Intraspecific variation in allelochemistry determines an invasive species' impact on soil microbial communities.

Invasive species can benefit from altered species interactions in their new range, and by interfering with species interactions among native competitors. Since exotic invasions are generally studied at the species level, relatively little is known about intraspecific variation in the traits that determine an invader's effect on native species. Alliaria petiolata is a widespread and aggressive invader of forest understories that succeeds in part by interfering with mutualistic interactions between native plants and soil fungi. Here, I show that the impact of A. petiolata on soil microbial communities varied among individuals due to variation in their allelochemical concentrations. The differential impacts translated into varied effects on native tree growth, partly because A. petiolata's allelochemicals preferentially affected the most mutualistic fungal taxa. These results highlight the importance of considering the spatial and temporal variation in an invasive species' impacts for understanding and managing the invasion process.

Genetic variation promotes long-term coexistence of Brassica nigra and its competitors.

How multiple species coexist in the face of limiting resources remains one of the central questions in ecology. Recent theoretical and empirical studies have documented the importance of evolutionary forces in species coexistence. However, there remains a disconnect between these two approaches, as empirical studies are generally too short to explore long-term coexistence and theoretical studies are rarely specific enough to allow for meaningful comparisons with natural systems. Here I combine field data with simulation modeling to test how a genetic trade-off between intra- and interspecific competitive ability alters the long-term coexistence of plant species. In two of the three species combinations tested, coexistence was possible only in models that included evolutionary processes. Additionally, genetic variation and the resultant evolutionary change allowed for coexistence under a much wider range of ecological conditions by both increasing equalizing (neutral) effects and providing a novel evolutionary stabilizing (niche) effect. Biodiversity is declining at both the species and the genetic levels. These results suggest that conserving species diversity may depend critically on our ability to conserve the genetic diversity within species.

Community complexity drives patterns of natural selection on a chemical defense of Brassica nigra.

Plants interact with many different species throughout their life cycle. Recent work has shown that the ecological effects of multispecies interactions are often not predictable from studies of the component pairwise interactions. Little is known about how multispecies interactions affect the evolution of ecologically important traits. We tested the direct and interactive effects of inter- and intraspecific competition, as well as of two abundant herbivore species (a generalist folivore and a specialist aphid), on the selective value of a defensive chemical compound in Brassica nigra. We found that investment in chemical defense was favored in interspecific competition but disfavored in intraspecific competition and that this pattern of selection was dependent on the presence of both herbivores, suggesting that selection will depend on the rarity or commonness of these species. These results show that the selective value of ecologically important traits depends on the complicated web of interactions present in diverse natural communities and that fluctuations in community composition may maintain genetic variation in such traits.

Declining survival across invasion history for Microstegium vimineum.

Many alien species become invasive because they lack coevolutionary history with the native community; for instance, they may lack specialized enemies. These evolutionary advantages may allow the invader to establish and persist when rare within a community and lead to its monodominance through positive frequency dependence, i.e. increasing per capita population growth rate with increasing frequency of conspecifics. However, this advantage could degrade through time due to evolutionary and ecological changes in the invasive and native plant and microbial communities. We investigated survival rates and individual biomass as proxies for per capita population growth rates for the invasive grass, Microstegium vimineum, across a gradient of conspecific frequencies (10-100% relative cover of M. vimineum) within 12 sites that varied in time since invasion. We expected M. vimineum frequency dependence to become more negative and its proxies for population growth at low conspecific frequency to decline across invasion history. We also explored the belowground fungal community associated with M. vimineum, since we hypothesized that changes in M. vimineum population dynamics may result from shifting microbial interactions over time. Microstegium vimineum frequency dependence changed from negative to neutral across invasion history and the shift was driven by a decline in survival at low frequency. Changes in M. vimineum root fungal community were associated with time since invasion. Our results do not support a shift in frequency dependence from positive to negative across invasion history. However, our results suggest M. vimineum populations may be less prone to persist at older invaded sites and thus more vulnerable to management intervention.

Soil-mediated eco-evolutionary feedbacks in the invasive plant Alliaria petiolata.

Ecological and evolutionary processes historically have been assumed to operate on significantly different time-scales. We know now from theory and work in experimental and model systems that these processes can feed back on each other on mutually relevant time-scales.Here, we present evidence of a soil-mediated eco-evolutionary feedback on the population dynamics of an invasive biennial plant, Alliaria petiolata.As populations age, natural selection drives down production of A. petiolata's important antimycorrhizal allelochemical, sinigrin. This occurs due to density-dependent selection on sinigrin, which is favoured under interspecific, but disfavoured under intraspecific, competition.We show that population stochastic growth rates (λS) and plant densities are positively related to sinigrin concentration measured in seedling roots. This interaction is mediated by sinigrin's positive effect on seedling and summer survival, which are important drivers of λS.Together, these illustrate how the evolution of a trait shaped by natural selection can influence the ecology of a species over a period of just years to decades, altering its trajectory of population growth and interactions with the species in the soil and plant communities it invades.Our findings confirm the predictions that eco-evolutionary feedbacks occur in natural populations. Furthermore, they improve our conceptual framework for projecting future population growth by linking the variation in plant demography to a critical competitive trait (sinigrin) whose selective advantages decrease as populations age.