Wildfire severity reduces richness and alters composition of soil fungal communities in boreal forests of western Canada

Wildfire is the dominant disturbance in boreal forests and fire activity is increasing in these regions. Soil fungal communities are important for plant growth and nutrient cycling postfire but there is little understanding of how fires impact fungal communities across landscapes, fire severity gradients, and stand types in boreal forests. Understanding relationships between fungal community composition, particularly mycorrhizas, and understory plant composition is therefore important in predicting how future fire regimes may affect vegetation. We used an extreme wildfire event in boreal forests of Canada's Northwest Territories to test drivers of fungal communities and assess relationships with plant communities. We sampled soils from 39 plots 1 year after fire and 8 unburned plots. High‐throughput sequencing (MiSeq, ITS) revealed 2,034 fungal operational taxonomic units. We found soil pH and fire severity (proportion soil organic layer combusted), and interactions between these drivers were important for fungal community structure (composition, richness, diversity, functional groups). Where fire severity was low, samples with low pH had higher total fungal, mycorrhizal, and saprotroph richness compared to where severity was high. Increased fire severity caused declines in richness of total fungi, mycorrhizas, and saprotrophs, and declines in diversity of total fungi and mycorrhizas. The importance of stand age (a surrogate for fire return interval) for fungal composition suggests we could detect long‐term successional patterns even after fire. Mycorrhizal and plant community composition, richness, and diversity were weakly but significantly correlated. These weak relationships and the distribution of fungi across plots suggest that the underlying driver of fungal community structure is pH, which is modified by fire severity. This study shows the importance of edaphic factors in determining fungal community structure at large scales, but suggests these patterns are mediated by interactions between fire and forest stand composition.     

Mycorrhizas influence functional traits of two tallgrass prairie species

Over the past decade, functional traits that influence plant performance and thus, population, community, and ecosystem biology have garnered increasing attention. Generally lacking, however, has been consideration of how ubiquitous arbuscular mycorrhizas influence plant allometric and stoichiometric functional traits. We assessed how plant dependence on and responsiveness to mycorrhizas influence plant functional traits of a warm‐season, C4 grass, Andropogon gerardii Vitman, and the contrasting, cool‐season, C3 grass, Elymus canadensis L. We grew both host species with and without inoculation with mycorrhizal fungi, across a broad gradient of soil phosphorus availabilities. Both host species were facultatively mycotrophic, able to grow without mycorrhizas at high soil phosphorus availability. A. gerardii was most dependent upon mycorrhizas and E. canadensis was weakly dependent, but highly responsive to mycorrhizas. The high dependence of A. gerardii on mycorrhizas resulted in higher tissue P and N concentrations of inoculated than noninoculated plants. When not inoculated, E. canadensis was able to take up both P and N in similar amounts to inoculated plants because of its weak dependence on mycorrhizas for nutrient uptake and its pronounced ability to change root‐to‐shoot ratios. Unlike other highly dependent species, A. gerardii had a high root‐to‐shoot ratio and was able to suppress colonization by mycorrhizal fungi at high soil fertilities. E. canadensis, however, was unable to suppress colonization and had a lower root‐to shoot ratio than A. gerardii. The mycorrhiza‐related functional traits of both host species likely influence their performance in nature: both species attained the maximum responsiveness from mycorrhizas at soil phosphorus availabilities similar to those of tallgrass prairies. Dependence upon mycorrhizas affects performance in the absence of mycorrhizas. Responsiveness to mycorrhizal fungi is also a function of the environment and can be influenced by both mycorrhizal fungus species and soil fertility.     

丛枝菌根真菌的生态分布及其影响因子研究进展

丛枝菌根(arbuscular mycorrhiza,AM)共生体系对于植物适应各种逆境胁迫具有重要积极作用。AM真菌还能够通过根外菌丝网络调节植物群落结构和演替,深刻影响生态系统结构和功能的稳定性。AM真菌生态生理功能的发挥主要取决于其生态适应性,明确AM真菌在不同环境中的多样性、生态适应性以及对各种生态因子的响应机制,是AM真菌资源管理、功能发掘与利用的前提。迄今为止,有关各种生态因子对AM真菌多样性的影响已有不少研究,但是AM真菌生态分布及其形成机制仍缺乏系统的研究和理论分析。综述了生物因子和非生物因子对AM真菌生态分布的影响,结合大型生物空间分布理论探讨了AM真菌生态分布规律和建成机制,分析了当前本研究领域所存在的问题和动向,以期推动相关研究进展。     

全球变化下菌根真菌的作用及其作用机制

全球气候、环境、经济与社会的发展变化,对环境与资源造成严重挑战和新的发展机遇。菌根真菌是陆地生态系统中的重要生物组份,占据不可替代的重要地位,充当调控生态系统稳定和保持可持续发展的多重角色。分析了全球变化对菌根真菌的影响,探讨了全球变化下菌根真菌的地位、角色和作用,以及菌根真菌应对全球变化的可能作用机制,旨在为加强全面应对全球变化提供新的思路和途径。     

Mycorrhizal fungal growth responds to soil characteristics,but not host plant identity,during a primary lacustrine dune succession

Soil factors and host plant identity can both affect the growth and functioning of mycorrhizal fungi. Both components change during primary succession, but it is unknown if their relative importance to mycorrhizas also changes. This research tested how soil type and host plant differences among primary successional stages determine the growth and plant effects of arbuscular mycorrhizal (AM) fungal communities. Mycorrhizal fungal community, plant identity, and soil conditions were manipulated among three stages of a lacustrine sand dune successional series in a fully factorial greenhouse experiment. Late succession AM fungi produced more arbuscules and soil hyphae when grown in late succession soils, although the community was from the same narrow phylogenetic group as those in intermediate succession. AM fungal growth did not differ between host species, and plant growth was similarly unaffected by different AM fungal communities. These results indicate that though ecological filtering and/or adaptation of AM fungi occurs during this primary dune succession, it more strongly reflects matching between fungi and soils, rather than interactions between fungi and host plants. Thus, AM fungal performance during this succession may not depend directly on the sequence of plant community succession.     

An ecological view of the formation of VA mycorrhizas

In spite of the major advances in understanding the functioning of symbioses between plants and arbuscular mycorrhizal fungi, details of the ecology of mycorrhizal fungi are not well documented. The benefits of the association are related to the timing and extent of colonization of roots, and fungi differ in their contribution to plant growth and presumably to soil aggregation. Knowledge of the processes that lead to successful colonization of roots by beneficial fungi at appropriate times for the host plants will form the basis of guidelines for soil management to maximize the benefits from the symbiosis. Fungi differ in the manner and extent to which they colonize roots. They also differ in their capacity to form propagules. The importance of hyphae, spores and propagules within living or dead mycorrhizal roots also differs among species and for the same species in different habitats. The relationships between colonization of roots and propagule formation, and between propagule distribution and abundance and subsequent mycorrhiza formation, for different fungi in field environments, are not well understood. Methods for quantifying mycorrhizal fungi are not especially suitable for distinguishing among different fungi within roots. Consequenctly, the dynamics of colonization of roots by different fungi, within and between seasons, have been little studied. Research is required that focuses on the dynamics of fungi within roots as well as on changes in the abundance of propagules of different fungi within soil. Interactions between fungi during the colonization of roots, the colonization of soil by hyphae and sporulation are all poorly understood. Without knowledge of these processes, it will by difficult to predict the likely success of inoculation with introduced fungi. Such knowledge is also required for selecting soil management procedures to enhance growth and survival of key species within the population. The relative tolerance of various fungi to perturbations in their surroundings will provide a basis for identifying those fungi that are likely to persist in specific environments. The processes that influence mycorrhizal fungi in field soils can be identified in controlled studies. However, greater emphasis is required on studying these processes with mixed populations of fungi. The role played by diversity within populations of mycorrhizal fungi is virtually unexplored.     

Micronutrient transport in mycorrhizal symbiosis; zinc steals the show

Mycorrhizas are mutually beneficial associations between soil-borne fungi and plant roots. Mycorrhizal fungi provide their host plant with essential nutrients in exchange for sugars and/or lipids. Traditionally, transport and translocation of macronutrients, including nitrogen and phosphorus, throughout the fungal mycelium and towards the host plant are well studied. However, the regulation of nutrient exchange and their contribution in the morphogenesis and development of mycorrhizas remains unclear. In this Opinion, we argue that adding micronutrients in the current models of symbiotic transport is essential to fully understand the establishment, maintenance, and functioning of mycorrhizal associations. Homeostatic mechanisms at the cellular level and the first transport proteins involved have been recently documented for zinc (Zn) in arbuscular mycorrhizal, ectomycorrhizal, and ericoid mycorrhizal fungi. Mycorrhizal plants benefit from an improved Zn status in control conditions and are better protected when environmental Zn availability fluctuates. These recent progresses are paving the way for a better understanding of micronutrient allocation in mycorrhizas. Revising our vision on the role of micronutrients, particularly of Zn, in these interactions will allow a better use of mycorrhizal fungi in sustainable agriculture and forestry, and will increase management practices in waste land, as well as in agricultural and natural ecosystems.     

菌根真菌在生态系统中的作用

 菌根是一种植物营养根与土壤真菌形成的共生体,在自然界中分布广泛。本文着重从以下几个方面介绍相关的研究进展：1) 菌根真菌作为生态系统的重要组成部分,具有不可忽视的生物量,并成为连接绿色植物和食真菌者食物链的重要一环;2) 菌根真菌通过参与凋落物的酶降解过程影响有机物的循环,通过促进生物固氮、加速土壤磷的风化、提高土壤溶液离子的有效性以及直接吸收等过程影响氮、磷、钾、钙、镁等元素的无机循环;3) 菌根真菌与土壤微生物间存在有益的或拮抗的相互作用,并可以直接或间接地影响根际生物区系的组成和数量;菌根真菌通过对宿主植物的有益作用而影响植物的种间竞争,通过菌根网络而形成的种团可以在同种或不同种植物间实现资源的重新分配和共享;由于对种间关系的作用和对食物链的影响,菌根真菌对群落的物种构成和多样性的维持具有重要的作用;菌根真菌是群落演替过程的指示者,也是这一过程的参与者和推动者,并且菌根真菌的存在也有利于提高土壤团聚体的稳定性及促进灰壤的形成;4) 菌根真菌的种类和数量可以指示生态系统中自然的或人类活动引起的变化,并可以在生态系统的保护、恢复或重建过程中发挥重要作用。文章的最后还介绍了最新的研究热点和发展趋势。     

Arbuscular Mycorrhizas: Drivers or Passengers of Alien Plant Invasion

Observational and manipulative studies have revealed that alien plant invasions are an outcome of interplay between a myriad of biotic and abiotic factors operating at various spatio-temporal stages and scales. Despite the salient role of ubiquitous arbuscular mycorrhizal (AM) fungi in plant interactions, studies exploring the role of such symbionts in invasiveness of alien species and invasibility of communities are limited, in part because of difficult-culturablilty of AM fungi on artificial media and apparent complexities in manipulations of AM-plant interactions in field and laboratory experiments. Moreover, analysis of the AM-plant invasion studies conducted so far have yielded contradictory results with some indicating facilitation of invasion by AM fungi and others its inhibition. Other studies have indicated that arbuscular mycorrhizal symbiosis has no effect on invasiveness of alien plants. While arbuscular mycorrhizas may facilitate invasiveness of some alien plants, such plants may also potentially impact mycorrhizal community structure and functions in the invaded habitats in different ways. The present review addresses these paradoxically conflicting observations in the context of mutualism-commensalism-parasitism gradient that characterizes the relationship between AM fungi and their alien vs. native hosts and also discusses the influence of alien invasive plants on mycorrhizal community structure of invaded ecosystems. Through critical analysis of costs and benefits for invasive plants that associate with AM fungi in their introduced range, invasion-induced shifts in AM mutualism are evaluated in the context of their impact on native biodiversity. Underlining limitations of methodologies and experimental designs usually employed to understand AM-mediated plant invasiveness, we proposes herein some alternative frameworks and experimental approaches to overcome these limitations.     

Mycorrhizas and mycorrhizal fungal communities throughout ecosystem development

Background and scope

Plant communities and underlying soils undergo substantial, coordinated shifts throughout ecosystem development. However, shifts in the composition and function of mycorrhizal fungi remain poorly understood, despite their role as a major interface between plants and soil. We synthesise evidence for shifts among mycorrhizal types (i.e., ectomycorrhizas, arbuscular and ericoid mycorrhizas) and in fungal communities within mycorrhizal types along long-term chronosequences that include retrogressive stages. These systems represent strong, predictable patterns of increasing, then declining soil fertility during ecosystem development, and are associated with coordinated changes in plant and fungal functional traits and ecological processes.

Conclusions

Mycorrhizal types do not demonstrate consistent shifts through ecosystem development. Rather, most mycorrhizal types can dominate at any stage of ecosystem development, driven by biogeography (i.e., availability of mycorrhizal host species), plant community assembly, climate and other factors. In contrast to coordinated shifts in soil fertility, plant traits and ecological processes throughout ecosystem development, shifts in fungal communities within and among mycorrhizal types are weak or idiosyncratic. The consequences of these changes in mycorrhizal communities and their function for plant–soil feedbacks or control over long-term nutrient depletion remain poorly understood, but could be resolved through empirical analyses of long-term soil chronosequences.     

Divergent phenologies may facilitate the coexistence of arbuscular mycorrhizal fungi in a North Carolina grassland

Interest in the diversity of arbuscular mycorrhizal (AM) fungal communities has been stimulated by recent data that demonstrate that fungal communities influence the competitive hierarchies, productivity, diversity, and successional patterns of plant communities. Although natural communities of AM fungi are diverse, we have a poor understanding of the mechanisms that promote and maintain that diversity. Plants may coexist by inhabiting disparate temporal niches; plants of many grasslands are either warm or cool season specialists. We hypothesized that AM fungi might be similarly seasonal. To test our hypothesis, we tracked the sporulation of individual AM fungal species growing within a North Carolina grassland. Data were collected in 1996 and 1997; in 1997, sampling focused on two common species. We found that AM fungi, especially Acaulospora colossica and Gigaspora gigantea, maintained different and contrasting seasonalities. Acaulospora colossica sporulated more frequently in the warm season, but Gi. gigantea sporulated more frequently in the cool season. Moreover, AM fungal species were spatially aggregated at a fine scale. Contrasting seasonal and spatial niches may facilitate the maintenance of a diverse community of AM fungi. Furthermore, these data may illuminate our understanding of the AM fungal influence on plant communities: various fungal species may preferentially associate with different plant species and thereby promote diversity in the plant community.     

不同丛枝菌根真菌侵染对土壤结构的影响

为了定量化比较研究接种丛枝菌根真菌后,根际、菌根际和菌丝际土壤结构的变化,采用四室分根装置,比较中性紫色土接种不同AM真菌后,菌根际、根际、菌丝际和非根际土壤平均重量直径(MWD)、几何平均直径(GMD)和大于0.25mm团聚体总量(R_0.25)的变化。结果表明:接种3个菌种后菌丝际EEG和有机质含量均呈高于菌根际的趋势。菌丝密度和易提取球囊霉素相关蛋白(EEG)与MWD、GMD和R_0.25呈显著正相关,菌根际和菌丝际土壤水稳性R_0.25与菌丝密度显著正相关,相关系数分别为0.777和0.671。接种G. mosseae的菌根际土壤R_0.25值显著高于其它分室土壤,而接种G.etunicatum的菌丝际土壤R_0.25值显著高于其它分室土壤。试验结果在一定程度上说明不同菌种对土壤结构均有不同程度的影响,反映了丛枝菌根真菌生态功能的多样性。     

The importance of integration and scale in the arbuscular mycorrhizal symbiosis

The arbuscular mycorrhizal (AM) fungus contributes to system processes and functions at various hierarchical organizational levels, through their establishment of linkages and feedbacks between whole-plants and nutrient cycles. Even though these fungal mediated feedbacks and linkages involve lower-organizational level processes (e.g. photo-assimilate partitioning, interfacial assimilate uptake and transport mechanisms, intraradical versus extraradical fungal growth), they influence higher-organizational scales that affect community and ecosystem behavior (e.g. whole-plant photosynthesis, biodiversity, nutrient and carbon cycling, soil structure). Hence, incorporating AM fungi into research directed at understanding many of the diverse environmental issues confronting society will require knowledge of how these fungi respond to or initiate changes in vegetation dynamics, soil fertility or both. Within the last few years, the rapid advancement in the development of analytical tools has increased the resolution by which we are able to quantify the mycorrhizal symbiosis. It is important that these tools are applied within a conceptual framework that is temporally and spatially relevant to fungus and host. Unfortunately, many of the studies being conducted on the mycorrhizal symbiosis at lower organizational scales are concerned with questions directed solely at understanding fungus or host without awareness of what the plant physiologist or ecologist needs for integrating the mycorrhizal association into larger organizational scales or process levels. We show by using the flow of C from plant-to-fungus-to-soil, that through thoughtful integration, we have the ability to bridge different organizational scales. Thus, an essential need of mycorrhizal research is not only to better integrate the various disciplines of mycorrhizal research, but also to identify those relevant links and scales needing further investigation for understanding the larger-organizational level responses. The U.S. Government's right to retain a non-exclusive, royalty-free licence in and to any copyright is acknowledged. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mycorrhizosphere interactions to improve plant fitness and soil quality

Arbuscular mycorrhizal fungi are key components of soil microbiota and obviously interact with other microorganisms in the rhizosphere, i.e. the zone of influence of plant roots on microbial populations and other soil constituents. Mycorrhiza formation changes several aspects of plant physiology and some nutritional and physical properties of the rhizospheric soil. These effects modify the colonization patterns of the root or mycorrhizas (mycorrhizosphere) by soil microorganisms. The rhizosphere of mycorrhizal plants, in practice a mycorrhizosphere, harbors a great array of microbial activities responsible for several key ecosystem processes. This paper summarizes the main conceptual principles and accepted statements on the microbial interactions between mycorrhizal fungi and other members of rhizosphere microbiota and discusses current developments and future trends concerning the following topics: (i) effect of soil microorganisms on mycorrhiza formation; (ii) mycorrhizosphere establishment; (iii) interactions involved in nutrient cycling and plant growth; (iv) interactions involved in the biological control of plant pathogens; and (v) interactions to improve soil quality. The main conclusion is that microbial interactions in the rhizosphere of mycorrhizal plants improve plant fitness and soil quality, critical issues for a sustainable agricultural development and ecosystem functioning. This revised version was published online in August 2006 with corrections to the Cover Date.     

Waiting for fungi: the ectomycorrhizal invasion of lowland heathlands

1.  In England, the loss of lowland heathland, a habitat of global conservation importance, is primarily due to the invasion of birch and pine. This encroachment has been researched in depth from a plant perspective but little is known about the role of mycorrhizal fungi. In lowland heathlands the resident dwarf shrubs form ericoid mycorrhizas whereas invading trees form ectomycorrhizas. Therefore, tree encroachment into heathlands can be regarded as the replacement of a resident mycorrhizal community by an invading one.
2 . This study examined how fungi form mycorrhizas with Betula and Pinus in lowland heathlands. We addressed the question of whether there are mycorrhizal fungi that mediate invasion using a molecular ecology approach to compare the mycorrhizal inoculum potential of soil at three levels of invasion (uninvaded heathland, invaded heathland and woodland) and the fungi forming mycorrhizas on tree seedlings and trees across diverse sites.
3.  We show that in lowland heathlands: (i) seedlings have severely limited access to ectomycorrhizal fungi relative to woodlands, (ii) there are few keystone spore-dispersed ectomycorrhizal fungi that can mediate tree invasion, (iii) tree seedlings can remain non-mycorrhizal for at least one year when no inoculum is present, even near saplings, and (iv) mycorrhizal seedlings achieve greater biomass than non-mycorrhizal seedlings. Within uninvaded heathland we detected only Rhizopogon luteolus , Suillus variegatus , S. bovinus ( Pinus symbionts) and Laccaria proxima (primarily a Betula symbiont).
4. Synthesis . Overall, ectomycorrhizal inoculum in lowland heathlands is rare; most tree seedlings growing in heathland soil are not mycorrhizal due to limited spore dispersal, poorly developed spore banks and weak common mycorrhizal networks. These seedlings can persist awaiting mycorrhization to boost their growth.     

Petroleum hydrocarbon contamination in boreal forest soils: a mycorrhizal ecosystems perspective

The importance of developing multi-disciplinary approaches to solving problems relating to anthropogenic pollution is now clearly appreciated by the scientific community, and this is especially evident in boreal ecosystems exposed to escalating threats of petroleum hydrocarbon (PHC) contamination through expanded natural resource extraction activities. This review aims to synthesize information regarding the fate and behaviour of PHCs in boreal forest soils in both ecological and sustainable management contexts. From this, we hope to evaluate potential management strategies, identify gaps in knowledge and guide future research. Our central premise is that mycorrhizal systems, the ubiquitous root symbiotic fungi and associated food-web communities, occupy the structural and functional interface between decomposition and primary production in northern forest ecosystems (i.e. underpin survival and productivity of the ecosystem as a whole), and, as such, are an appropriate focal point for such a synthesis. We provide pertinent basic information about mycorrhizas, followed by insights into the ecology of ecto- and ericoid mycorrhizal systems. Next, we review the fate and behaviour of PHCs in forest soils, with an emphasis on interactions with mycorrhizal fungi and associated bacteria. Finally, we summarize implications for ecosystem management. Although we have gained tremendous insights into understanding linkages between ecosystem functions and the various aspects of mycorrhizal diversity, very little is known regarding rhizosphere communities in PHC-contaminated soils. This makes it difficult to translate ecological knowledge into environmental management strategies. Further research is required to determine which fungal symbionts are likely to survive and compete in various ecosystems, whether certain fungal - plant associations gain in ecological importance following contamination events, and how PHC contamination may interfere with processes of nutrient acquisition and exchange and metabolic processes. Research is also needed to assess whether the metabolic capacity for intrinsic decomposition exists in these ecosystems, taking into account ecological variables such as presence of other organisms (and their involvement in syntrophic biodegradation), bioavailability and toxicity of mixtures of PHCs, and physical changes to the soil environment.     

Phosphorus and intraspecific density alter plant responses to arbuscular mycorrhizas

We investigated how phosphorus availability, intraspecific density, and their interaction affect plant responses to arbuscular mycorrhizas. Four facultatively mycotrophic species: chile, cilantro, tomato, and corn were examined separately in pot experiments that employed a tropical phosphorus-immobilizing soil. Each experiment comprised nine soluble phosphorus additions, two levels of intraspecific plant density, and inoculation with arbuscular mycorrhizal fungi or not. High phosphorus signi- ficantly diminished mycorrhizal colonization of corn, cilantro, and tomato, but not chile, which was highly variably colonized. Corn roots were colonized by other root-inhabiting fungi, and mycorrhizas significantly reduced colonization by these potential root parasites. High phosphorus significantly increased relative growth rates (RGR) of all species, and high density significantly reduced RGR of cilantro, tomato, and corn. Chile showed little growth at any but the highest phosphorus additions, and consequently had no RGR response to density or mycorrhizas. Mycorrhiza inoculation caused transient depression of corn growth during the first month, but mycorrhizas increased corn RGR during the second month of growth. Both RGR and dry weights at harvest, cilantro, tomato, and corn benefited from mycorrhizas at low phosphorus availability, but this benefit diminished or changed to disadvantage as phosphorus availability increased. At low phosphorus availability, high density increased the dry weight of mycorrhizal cilantro and thereby amplified the benefit of mycorrhizas. At high phosphorus availability, increased density diminished the effects of mycorrhizas on dry weight, reducing mycorrhiza benefit to tomato and chile and reducing mycorrhiza detriment to cilantro. This study demonstrates that for three of the four plant species examined, phosphorus availability, intraspecific density, and their interaction significantly modify plant responses to arbuscular mycorrhizas.     

丛枝菌根真菌与根围促生细菌相互作用的效应与机制

丛枝菌根(arbuscular mycorrhiza,AM)真菌是植物活体营养专性共生菌,广泛存在于陆地各生态系统中.研究表明,AM真菌与根围促生细菌(plant growth promoting rhizobacteria,PGPR)之间的相互作用,尤其是它们之间的协同作用不仅影响植物养分吸收利用、病原物发生发展、土壤理化特性与生物修复等,而且对于可持续农、林、牧业生产、稳定生态系统都具有十分重要的意义.因此,近年来给予众多关注和研究.综述了AM真菌与PGPR之间的相互影响及其可能的作用机制,以及AM真菌与PGPR协同改善植物营养和生长、协同抑制病原菌、协同修复土壤方面的作用,旨在总结AM真菌与PGPR相互作用的效应与机制方面的最新研究进展,为今后研究发展提供依据.     

Interacting influence of mycorrhizal symbiosis and competition on plant diversity in tallgrass prairie

In tallgrass prairie, plant species interactions regulated by their associated mycorrhizal fungi may be important forces that influence species coexistence and community structure; however, the mechanisms and magnitude of these interactions remain unknown. The objective of this study was to determine how interspecific competition, mycorrhizal symbiosis, and their interactions influence plant community structure. We conducted a factorial experiment, which incorporated manipulations of abundance of dominant competitors, Andropogon gerardii and Sorghastrum nutans, and suppression of mycorrhizal symbiosis using the fungicide benomyl under two fire regimes (annual and 4-year burn intervals). Removal of the two dominant C₄ grass species altered the community structure, increased plant species richness, diversity, and evenness, and increased abundance of subdominant graminoid and forb species. Suppression of mycorrhizal fungi resulted in smaller shifts in community structure, although plant species richness and diversity increased. Responses of individual plant species were associated with their degree of mycorrhizal responsiveness: highly mycorrhizal responsive species decreased in abundance and less mycorrhizal responsive species increased in abundance. The combination of dominant-grass removal and mycorrhizal suppression treatments interacted to increase synergistically the abundance of several species, indicating that both processes influence species interactions and community organization in tallgrass prairie. These results provide evidence that mycorrhizal fungi affect plant communities indirectly by influencing the pattern and strength of plant competitive interactions. Burning strongly influenced the outcome of these interactions, which suggests that plant species diversity in tallgrass prairie is influenced by a complex array of interacting processes, including both competition and mycorrhizal symbiosis. Received: 7 April 1999 / Accepted: 30 July 1999     

Linking above- and belowground responses to global change at community and ecosystem scales

Cryptic belowground organisms are difficult to observe and their responses to global changes are not well understood. Nevertheless, there is reason to believe that interactions among above- and belowground communities may mediate ecosystem responses to global change. We used grassland mesocosms to manipulate the abundance of one important group of soil organisms, arbuscular mycorrhizal (AM) fungi, and to study community and ecosystem responses to CO₂ and N enrichment. Responses of plants, AM fungi, phospholipid fatty acids and community-level physiological profiles were measured after two growing seasons. Ecosystem responses were examined by measuring net primary production (NPP), evapotranspiration, total soil organic matter (SOM), and extractable mineral N. Structural equation modeling was used to examine the causal relationships among treatments and response variables. We found that while CO₂ and N tended to directly impact ecosystem functions (evapotranspiration and NPP, respectively), AM fungi indirectly impacted ecosystem functions by influencing the community composition of plants and other root fungi, soil fungi and soil bacteria. We found that the mycotrophic status of the dominant plant species in the mesocosms determined whether the presence of AM fungi increased or decreased NPP. Mycotrophic grasses dominated the mesocosm communities during the first growing season, and the mycorrhizal treatments had the highest NPP. In contrast, nonmycotrophic forbs were dominant during the second growing season and the mycorrhizal treatments had the lowest NPP. The composition of the plant community strongly influenced soil N, and the community composition of soil organisms strongly influenced SOM accumulation in the mesocosms. These results show how linkages between above- and belowground communities can determine ecosystem responses to global change.