Contrasting effects of intra- and interspecific identity and richness of ectomycorrhizal fungi on host plants, nutrient retention and multifunctionality

Maintenance of host specialisation gradients in ectomycorrhizal symbionts

Many fungi that form ectomycorrhizas exhibit a degree of host specialisation, and individual trees are frequently colonised by communities of mycorrhizal fungi comprising species that fall on a gradient of specialisation along genetic, functional and taxonomic axes of variation. By contrast, arbuscular mycorrhizal fungi exhibit little specialisation. Here, we propose that host tree root morphology is a key factor that gives host plants fine-scale control over colonisation and therefore opportunities for driving specialisation and speciation of ectomycorrhizal fungi. A gradient in host specialisation is likely driven by four proximate mechanistic 'filters' comprising partner availability, signalling recognition, competition for colonisation, and symbiotic function (trade, rewards and sanctions), and the spatially restricted colonisation seen in heterorhizic roots enables these mechanisms, especially symbiotic function, to be more effective in driving the evolution of specialisation. We encourage manipulation experiments that integrate molecular genetics and isotope tracers to test these mechanisms, alongside mathematical simulations of eco-evolutionary dynamics in mycorrhizal symbioses.

Tropical tree ectomycorrhiza are distributed independently of soil nutrients

Mycorrhizae, a form of plant-fungal symbioses, mediate vegetation impacts on ecosystem functioning. Climatic effects on decomposition and soil quality are suggested to drive mycorrhizal distributions, with arbuscular mycorrhizal plants prevailing in low-latitude/high-soil-quality areas and ectomycorrhizal (EcM) plants in high-latitude/low-soil-quality areas. However, these generalizations, based on coarse-resolution data, obscure finer-scale variations and result in high uncertainties in the predicted distributions of mycorrhizal types and their drivers. Using data from 31 lowland tropical forests, both at a coarse scale (mean-plot-level data) and fine scale (20 × 20 metres from a subset of 16 sites), we demonstrate that the distribution and abundance of EcM-associated trees are independent of soil quality. Resource exchange differences among mycorrhizal partners, stemming from diverse evolutionary origins of mycorrhizal fungi, may decouple soil fertility from the advantage provided by mycorrhizal associations. Additionally, distinct historical biogeographies and diversification patterns have led to differences in forest composition and nutrient-acquisition strategies across three major tropical regions. Notably, Africa and Asia's lowland tropical forests have abundant EcM trees, whereas they are relatively scarce in lowland neotropical forests. A greater understanding of the functional biology of mycorrhizal symbiosis is required, especially in the lowland tropics, to overcome biases from assuming similarity to temperate and boreal regions.

Symbiotic control of canopy dominance in subtropical and tropical forests

Subtropical and tropical forests in Asia often comprise canopy dominant trees that form symbioses with ectomycorrhizal fungi, and species-rich understorey trees that form symbioses with arbuscular mycorrhizal fungi. We propose a virtuous phosphorus acquisition hypothesis to explain this distinct structure. The hypothesis is based on (i) seedlings being rapidly colonised by ectomycorrhizal fungi from established mycelial networks that generates positive feedback and resistance to pathogens, (ii) ectomycorrhizal fungi having evolved a suite of morphological, physiological, and molecular traits to enable them to capture phosphorus from a diversity of chemical forms, including organic forms, and (iii) allocation of photosynthate carbon from adult host plants to provide the energy needed to undertake these processes.

Ectomycorrhizal diversity, taxon-specific traits and root N uptake in temperate beech forests

Roots of forest trees are colonized by a diverse spectrum of ectomycorrhizal (EM) fungal species differing in their nitrogen (N) acquisition abilities. Here, we hypothesized that root N gain is the result of EM fungal diversity or related to taxon-specific traits for N uptake. To test our hypotheses, we traced ¹⁵ N enrichment in fine roots, coarse roots and taxon-specific ectomycorrhizas in temperate beech forests in two regions and three seasons, feeding 1 mM NH₄ NO₃ labelled with either ¹⁵ NH₄ ⁺ or ¹⁵ NO₃ ^- . We morphotyped > 45 000 vital root tips and identified 51 of 53 detected EM species by sequencing. EM root tips exhibited strong, fungal taxon-specific variation in ¹⁵ N enrichment with higher NH₄ ⁺ than NO₃ ^- enrichment. The translocation of N into the upper parts of the root system increased with increasing EM fungal diversity. Across the growth season, influential EM species predicting root N gain were not identified, probably due to high temporal dynamics of the species composition of EM assemblages. Our results support that root N acquisition is related to EM fungal community-level traits and highlight the importance of EM diversity for tree N nutrition.

Forest tree growth is linked to mycorrhizal fungal composition and function across Europe

Most trees form symbioses with ectomycorrhizal fungi (EMF) which influence access to growth-limiting soil resources. Mesocosm experiments repeatedly show that EMF species differentially affect plant development, yet whether these effects ripple up to influence the growth of entire forests remains unknown. Here we tested the effects of EMF composition and functional genes relative to variation in well-known drivers of tree growth by combining paired molecular EMF surveys with high-resolution forest inventory data across 15 European countries. We show that EMF composition was linked to a three-fold difference in tree growth rate even when controlling for the primary abiotic drivers of tree growth. Fast tree growth was associated with EMF communities harboring high inorganic but low organic nitrogen acquisition gene proportions and EMF which form contact versus medium-distance fringe exploration types. These findings suggest that EMF composition is a strong bio-indicator of underlying drivers of tree growth and/or that variation of forest EMF communities causes differences in tree growth. While it may be too early to assign causality or directionality, our study is one of the first to link fine-scale variation within a key component of the forest microbiome to ecosystem functioning at a continental scale.

Does resource exchange in ectomycorrhizal symbiosis vary with competitive context and nitrogen addition?

Ectomycorrhizal symbiosis is essential for the nutrition of most temperate forest trees and helps regulate the movement of carbon (C) and nitrogen (N) through forested ecosystems. The factors governing the exchange of plant C for fungal N, however, remain obscure. Because competition and soil resources may influence ectomycorrhizal resource movement, we performed a 10-month split-root microcosm study using Pinus muricata seedlings with Thelephora terrestris, Suillus pungens, or no ectomycorrhizal fungus, under two N concentrations in artificial soil. Fungi competed directly with roots and indirectly with each other. We used stable isotope enrichment to track plant photosynthate and fungal N. For T. terrestris, plants received N commensurate with the C given to their fungal partners. Thelephora terrestris was a superior mutualist under high-N conditions. For S. pungens, plant C and fungal N exchange were not coupled. However, in low-N conditions, plants preferentially allocated C to S. pungens rather than T. terrestris. Our results suggest that ectomycorrhizal resource transfer depends on competitive and nutritional context. Plants can exchange C for fungal N, but coupling of these resources can depend on the fungal species and soil N. Understanding the diversity of fungal strategies, and how they change with environmental context, reveals mechanisms driving this important symbiosis.

Macrofungal conservation in Canada and target species for assessment: a starting point

Despite the ecological importance of fungi, we still know little about their diversity in Canada. One of the largest hurdles to implementing fungal conservation initiatives is the lack of fungal distribution data. As anthropogenic impacts accelerate the speed of environmental change, it is imperative that we fill this major information gap, critical for fungal protection. To gain insight on the conservation status of Canadian macrofungi, we took advantage of the large and growing body of fungal biodiversity data from government research ( Wild Species 2020), citizen science, trained independent mycologists, university, and museum biodiversity research. The majority of macrofungi are data deficient; we do not know their geographic distribution or habitat requirements, occurrence, or abundance in Canada. For mushrooms that fruit only a few days of the year and are often difficult to positively identify, there is a lot of work to overcome the uncertainty of distinguishing under-sampling from rarity. Our work stresses the importance of building a strong network of professional and amateur mycologists to develop resources, disseminate information to make educated decisions, and advance conservation actions. We found that several fungi can be prioritized; we present a short list for consideration for formal conservation assessment.

Ectomycorrhizal and Dark Septate Fungal Associations of Pinyon Pine Are Differentially Affected by Experimental Drought and Warming

Changing climates can cause shifts in temperature and precipitation, resulting in warming and drought in some regions. Although each of these factors has been shown to detrimentally affect forest ecosystems worldwide, information on the impacts of the combined effects of warming and drought is lacking. Forest trees rely on mutualistic root-associated fungi that contribute significantly to plant health and protection against climate stresses. We used a six-year, ecosystem-scale temperature and precipitation manipulation experiment targeted to simulate the climate in 2100 in the Southwestern United States to quantify the effects of drought, warming and combined drought and warming on the root colonization (abundance), species composition and diversity of ectomycorrhizal fungi (EMF), and dark septate fungal endophytes in a widespread woodland tree, pinyon pine (Pinus edulis E.). Our results show that pinyon shoot growth after 6 years of these treatments was reduced more by drought than warming. The combined drought and warming treatment reduced the abundance and diversity of EMF more than either treatment alone. Individual ectomycorrhizal fungal taxa, including the drought tolerant Cenococcum geophilum, were present in all treatments but the combined drought and warming treatment. The combined drought and warming treatment also reduced the abundance of dark septate endophytes (DSE), but did not affect their diversity or species composition. The current year shoot growth of the trees correlated positively with ectomycorrhizal fungal diversity, highlighting the importance of diversity in mutualistic relationships to plant growth. Our results suggest that EMF may be more important than DSE to aboveground growth in P. edulis, but also more susceptible to the negative effects of combined climate stressors.

Differences in prey personality mediate trophic cascades

Functional trait approaches in ecology chiefly assume the mean trait value of a population adequately predicts the outcome of species interactions. Yet this assumption ignores substantial trait variation among individuals within a population, which can have a profound effect on community structure and function. We explored individual trait variation through the lens of animal personality to test whether among-individual variation in prey behavior mediates trophic interactions. We quantified the structure of personalities within a population of generalist grasshoppers and examined, through a number of field and laboratory-based experiments, how personality types could impact tri-trophic interactions in a food chain. Unlike other studies of this nature, we used spatial habitat domains to evaluate how personality types mechanistically map to behaviors relevant in predator-prey dynamics and found shy and bold individuals differed in both their habitat use and foraging strategy under predation risk by a sit-and-wait spider predator. In the field-based mesocosm portion of our study, we found experimental populations of personality types differed in their trophic impact, demonstrating that prey personality can mediate trophic cascades. We found no differences in respiration rates or body size between personality types used in the mesocosm experiment, indicating relative differences in trophic impact were not due to variation in prey physiology but rather variation in behavioral strategies. Our work demonstrates how embracing the complexity of individual trait variation can offer mechanistically richer understanding of the processes underlying trophic interactions.

How mycorrhizal associations drive plant population and community biology

Mycorrhizal fungi provide plants with a range of benefits, including mineral nutrients and protection from stress and pathogens. Here we synthesize current information about how the presence and type of mycorrhizal association affect plant communities. We argue that mycorrhizal fungi regulate seedling establishment and species coexistence through stabilizing and equalizing mechanisms such as soil nutrient partitioning, feedback to soil antagonists, differential mycorrhizal benefits, and nutrient trade. Mycorrhizal fungi have strong effects on plant population and community biology, with mycorrhizal type-specific effects on seed dispersal, seedling establishment, and soil niche differentiation, as well as interspecific and intraspecific competition and hence plant diversity.

From field sampling to pneumatic bioreactor mycelia production of the ectomycorrhizal mushroom Laccaria trichodermophora

In order to increase survival rates of greenhouse seedlings destined for restoration and conservation programs, successful mycorrhization of the seedlings is necessary. To reforest forest ecosystems, host trees must be inoculated with ectomycorrhizal fungi and, in order to guarantee a sufficient supply of ectomycorrhizal inoculum, it is necessary to develop technologies for the mass production of ectomycorrhizal fungi mycelia. We selected the ectomycorrhizal fungus Laccaria trichodermophora, due to its ecological traits and feasible mycelia production in asymbiotic conditions. Here, we report the field sampling of genetic resources, as well as the highly productive nutritional media and cultivation parameters in solid cultures. Furthermore, in order to achieve high mycelial production, we used strain screening and evaluated pH, carbon source concentration, and culture conditions of submerged cultures in normal and baffled shake flasks. The higher productivity culture conditions in shake flasks were selected for evaluation in a pneumatic bioreactor, using modified BAF media with a 10 g/L glucose, pH 5.5, 25 °C, and a volumetric oxygen transfer coefficient (K_La) of 36 h^-1. Under those conditions less biomass (12-37 %) was produced in the pneumatic bioreactor compared with the baffled shake flasks. This approach shows that L. trichodermophora can generate a large biomass concentration and constitute the biotechnological foundation of its mycelia mass production.

Trait-based ecology of terrestrial arthropods

In focusing on how organisms' generalizable functional properties (traits) interact mechanistically with environments across spatial scales and levels of biological organization, trait-based approaches provide a powerful framework for attaining synthesis, generality and prediction. Trait-based research has considerably improved understanding of the assembly, structure and functioning of plant communities. Further advances in ecology may be achieved by exploring the trait-environment relationships of non-sessile, heterotrophic organisms such as terrestrial arthropods, which are geographically ubiquitous, ecologically diverse, and often important functional components of ecosystems. Trait-based studies and trait databases have recently been compiled for groups such as ants, bees, beetles, butterflies, spiders and many others; however, the explicit justification, conceptual framework, and primary-evidence base for the burgeoning field of 'terrestrial arthropod trait-based ecology' have not been well established. Consequently, there is some confusion over the scope and relevance of this field, as well as a tendency for studies to overlook important assumptions of the trait-based approach. Here we aim to provide a broad and accessible overview of the trait-based ecology of terrestrial arthropods. We first define and illustrate foundational concepts in trait-based ecology with respect to terrestrial arthropods, and justify the application of trait-based approaches to the study of their ecology. Next, we review studies in community ecology where trait-based approaches have been used to elucidate how assembly processes for terrestrial arthropod communities are influenced by niche filtering along environmental gradients (e.g. climatic, structural, and land-use gradients) and by abiotic and biotic disturbances (e.g. fire, floods, and biological invasions). We also review studies in ecosystem ecology where trait-based approaches have been used to investigate biodiversity-ecosystem function relationships: how the functional diversity of arthropod communities relates to a host of ecosystem functions and services that they mediate, such as decomposition, pollination and predation. We then suggest how future work can address fundamental assumptions and limitations by investigating trait functionality and the effects of intraspecific variation, assessing the potential for sampling methods to bias the traits and trait values observed, and enhancing the quality and consolidation of trait information in databases. A roadmap to guide observational trait-based studies is also presented. Lastly, we highlight new areas where trait-based studies on terrestrial arthropods are well positioned to advance ecological understanding and application. These include examining the roles of competitive, non-competitive and (multi-)trophic interactions in shaping coexistence, and macro-scaling trait-environment relationships to explain and predict patterns in biodiversity and ecosystem functions across space and time. We hope this review will spur and guide future applications of the trait-based framework to advance ecological insights from the most diverse eukaryotic organisms on Earth.

The interplay between propagule pressure, seed predation and ectomycorrhizal fungi in plant invasion

There are many hypotheses aiming to explain invasion success, but evaluating individual hypotheses in isolation may hinder our ability to understand why some species invade and others fail. Here we evaluate the interaction between propagule pressure, seed predation and missed mutualism in the invasion success of the pine, Pinusponderosa. We evaluated the independent and interactive effects of propagule pressure and seed predation at increasing distances from a pine plantation. Additionally, because pines are obligate mutualists with ectomycorrhizal fungi (EMF) and pine invasions fail in the absence of their EMF symbionts, we evaluated EMF availability through a growth chamber bioassay. In this bioassay we measured root colonization by EMF with soil samples collected from the different distances from the plantation. We found that propagule pressure overwhelms seed predation only at the edge of the pine plantation, while seed predation overcomes propagule pressure at 25 m and further distances from the plantation. We also found that EMF root colonization decreases with distance from the plantation. However, pine roots were colonized up to 200 m from the plantation, suggesting that EMF may not be hindering invasion, at least not on the scale of this experiment. Taken together our results demonstrate that seed predation may be limiting the invasion of P.ponderosa in the study region as propagule pressure only overcomes seed predation at the plantation edge. Here we provide evidence of how strong biotic resistance can suppress an invasion, regardless of the variation in propagule pressure and the availability of mutualists.

Ectomycorrhizal fungal diversity increases phosphorus uptake efficiency of European beech

Increases in summer droughts and nitrogen (N) deposition have raised concerns of widespread biodiversity loss and nutrient imbalances, but our understanding of the ecological role of ectomycorrhizal fungal (ECMF) diversity in mediating root functions remains a major knowledge gap. We used different global change scenarios to experimentally alter the composition of ECMF communities colonizing European beech saplings and examined the consequences for phosphorus (P) uptake (H₃³³ PO₄ feeding experiment) and use efficiencies of trees. Specifically, we simulated increases in temperature and N deposition and decreases in soil moisture and P availability in a factorial experiment. Here, we show that ECMF α diversity is a major factor contributing to root functioning under global change. P uptake efficiency of beech significantly increased with increasing ECMF species richness and diversity, as well as with decreasing P availability. As a consequence of decreases in ECMF diversity, P uptake efficiency decreased when soil moisture was limiting. By contrast, P use efficiencies were a direct (negative) function of P availability and not of ECMF diversity. We conclude that increasing summer droughts may reduce ECMF diversity and the complementarity of P uptake by ECMF species, which will add to negative growth effects expected from nutrient imbalances under global change.

Does genotypic and species diversity of mycorrhizal plants and fungi affect ecosystem function?

Contents Summary 1122 I. Introduction 1122 II. Are there consistent patterns in diversity of mycorrhizal fungal genotypes and species across space? 1125 III. What is the variation in functional traits and genes of mycorrhizal fungi at different taxonomic scales? 1125 IV. How will environmental change impact the relationships between genotypic and species diversity of mycorrhizal fungi and ecosystem function? 1126 V. Conclusions: considerations for future MEF research 1127 Acknowledgements 1127 References 1127 SUMMARY: Both genotypes and species of mycorrhizal fungi exhibit considerable variation in traits, and this variation can result in their diversity regulating ecosystem function. Yet, the nature of mycorrhizal fungal diversity-ecosystem function (MEF) relationships for both genotypes and species is currently poorly defined. New experiments should reflect the richness of genotypes and species in nature, but we still lack information about the extent to which fungal populations in particular are structured. Sampling designs should quantify the diversity of mycorrhizal fungal genotypes and species at three key broad spatial scales (root fragment, root system and interacting root systems) in order to inform manipulation experiments and to test how mycorrhizal fungal diversity both responds, and confers resilience to, environmental drivers.

Mycorrhizal fungi affect orchid distribution and population dynamics

Symbioses are ubiquitous in nature and influence individual plants and populations. Orchids have life history stages that depend fully or partially on fungi for carbon and other essential resources. As a result, orchid populations depend on the distribution of orchid mycorrhizal fungi (OMFs). We focused on evidence that local-scale distribution and population dynamics of orchids can be limited by the patchy distribution and abundance of OMFs, after an update of an earlier review confirmed that orchids are rarely limited by OMF distribution at geographic scales. Recent evidence points to a relationship between OMF abundance and orchid density and dormancy, which results in apparent density differences. Orchids were more abundant, less likely to enter dormancy, and more likely to re-emerge when OMF were abundant. We highlight the need for additional studies on OMF quantity, more emphasis on tropical species, and development and application of next-generation sequencing techniques to quantify OMF abundance in substrates and determine their function in association with orchids. Research is also needed to distinguish between OMFs and endophytic fungi and to determine the function of nonmycorrhizal endophytes in orchid roots. These studies will be especially important if we are to link orchids and OMFs in efforts to inform conservation.

The ecological importance of intraspecific variation

Human activity is causing wild populations to experience rapid trait change and local extirpation. The resulting effects on intraspecific variation could have substantial consequences for ecological processes and ecosystem services. Although researchers have long acknowledged that variation among species influences the surrounding environment, only recently has evidence accumulated for the ecological importance of variation within species. We conducted a meta-analysis comparing the ecological effects of variation within a species (intraspecific effects) with the effects of replacement or removal of that species (species effects). We evaluated direct and indirect ecological responses, including changes in abundance (or biomass), rates of ecological processes and changes in community composition. Our results show that intraspecific effects are often comparable to, and sometimes stronger than, species effects. Species effects tend to be larger for direct ecological responses (for example, through consumption), whereas intraspecific effects and species effects tend to be similar for indirect responses (for example, through trophic cascades). Intraspecific effects are especially strong when indirect interactions alter community composition. Our results summarize data from the first generation of studies examining the relative ecological effects of intraspecific variation. Our conclusions can help inform the design of future experiments and the formulation of strategies to quantify and conserve biodiversity.

Strain Identity of the Ectomycorrhizal Fungus Laccaria bicolor Is More Important than Richness in Regulating Plant and Fungal Performance under Nutrient Rich Conditions

Effects of biodiversity on productivity are more likely to be expressed when there is greater potential for niche complementarity. In soil, chemically complex pools of nutrient resources should provide more opportunities for niche complementarity than chemically simple pools. Ectomycorrhizal (ECM) fungal genotypes can exhibit substantial variation in nutrient acquisition traits and are key components of soil biodiversity. Here, we tested the hypothesis that increasing the chemical complexity and forms of soil nutrients would enhance the effects of intraspecific ECM diversity on host plant and fungal productivity. In pure culture, we found substantial variation in growth of strains of the ECM fungus Laccaria bicolor on a range of inorganic and organic forms of nutrients. Subsequent experiments examined the effects of intraspecific identity and richness using Scots pine (Pinus sylvestris) seedlings colonized with different strains of L. bicolor growing on substrates supplemented with either inorganic or organic forms of nitrogen and phosphorus. Intraspecific identity effects on plant productivity were only found under the inorganic nutrient amendment, whereas intraspecific identity affected fungal productivity to a similar extent under both nutrient treatments. Overall, there were no significant effects of intraspecific richness on plant and fungal productivity. Our findings suggest soil nutrient composition does not interact strongly with ECM intraspecific richness, at least under experimental conditions where mineral nutrients were not limiting. Under these conditions, intraspecific identity of ECM fungi becomes more important than richness in modulating plant and fungal performance.