Mycorrhizal suppression and phosphorus addition influence the stability of plant community composition and function in a temperate steppe

Nutrient enrichment can reduce ecosystem stability, typically measured as temporal stability of a single function, e.g. plant productivity. Moreover, nutrient enrichment can alter plant–soil interactions (e.g. mycorrhizal symbiosis) that determine plant community composition and productivity. Thus, it is likely that nutrient enrichment and interactions between plants and their soil communities co-determine the stability in plant community composition and productivity. Yet our understanding as to how nutrient enrichment affects multiple facets of ecosystem stability, such as functional and compositional stability, and the role of above–belowground interactions are still lacking. We tested how mycorrhizal suppression and phosphorus (P) addition influenced multiple facets of ecosystem stability in a three-year field study in a temperate steppe. Here we focused on the functional and compositional stability of plant community; functional stability is the temporal community variance in primary productivity; compositional stability is represented by compositional resistance, turnover, species extinction and invasion. Community variance was partitioned into population variance defined as community productivity weighted average of the species temporal variance in performance, and species synchrony defined as the degree of temporal positive covariation among species. Compared to treatments with mycorrhizal suppression, the intact AM fungal communities reduced community variance in primary productivity by reducing species synchrony at high levels of P addition. Species synchrony and population variance were linearly associated with community variance with the intact AM fungal communities, while these relationships were decoupled or weakened by mycorrhizal suppression. The intact AM fungal communities promoted the compositional resistance of plant communities by reducing compositional turnover, but this effect was suppressed by P addition. P addition increased the number of species extinctions and thus promoted compositional turnover. Our study shows P addition and AM fungal communities can jointly and independently modify the various components of ecosystem stability in terms of plant community productivity and composition.     

Context dependent effects of ectomycorrhizal species richness on tree seedling productivity

While there has been much recent interest about the relationships between plant diversity and plant productivity, much remains unknown about how the diversity of mycorrhizal fungi affects plant productivity. We investigated the effects of ectomycorrhizal fungal community composition and diversity on the productivity and growth characteristics of seedlings of two tree species ( Pinus sylvetris and Betula pendula ) as well as their interactions with each other. This involved setting up a mycorrhizal fungal diversity gradient from one to eight species using a design previously demonstrated to be able to separate diversity effects from compositional effects. We found that the eight mycorrhizal fungal species differed in their effects on seedling productivity and that the nature of effects was determined by the fertility of the substrate. Fungal species richness effects were also important in affecting seedling productivity over and above what could be explained by "sampling effect" but only in some situations. For B. pendula in a low fertility substrate there were clear positive causative effects between fungal species richness and productivity with the eight species treatment having over double the productivity of any of the eight monoculture treatments; no diversity effects were, however, detected in a high fertility substrate. For P. sylvestris in a high fertility substrate there were significant negative effects of fungal diversity on productivity while in a low fertility substrate no effects were apparent. The possible mechanistic bases for these results are discussed. The growth of P. sylvestris relative to that of B. pendula when grown in combination was unaffected by mycorrhizal treatments. Our results provide clear evidence that effects of mycorrhizal fungal diversity on productivity are context dependent and may be positive, negative or neutral depending on the situation considered.     

The role of plant–mycorrhizal mutualisms in deterring plant invasions: Insights from an individual‐based model

Understanding the factors that determine invasion success for non‐native plants is crucial for maintaining global biodiversity and ecosystem functioning. One hypothesized mechanism by which many exotic plants can become invasive is through the disruption of key plant–mycorrhizal mutualisms, yet few studies have investigated how these disruptions can lead to invader success. We present an individual‐based model to examine how mutualism strengths between a native plant (Impatiens capensis) and mycorrhizal fungus can influence invasion success for a widespread plant invader, Alliaria petiolata (garlic mustard). Two questions were investigated as follows: (a) How does the strength of the mutualism between the native I. capensis and a mycorrhizal fungus affect resistance (i.e., native plant maintaining >60% of final equilibrium plant density) to garlic mustard invasion? (b) Is there a non‐linear relationship between initial garlic mustard density and invasiveness (i.e., garlic mustard representing >60% of final equilibrium plant density)? Our findings indicate that either low (i.e., facultative) or high (i.e., obligate) mutualism strengths between the native plant and mycorrhizal fungus were more likely to lead to garlic mustard invasiveness than intermediate levels, which resulted in higher resistance to garlic mustard invasion. Intermediate mutualism strengths allowed I. capensis to take advantage of increased fitness when the fungus was present but remained competitive enough to sustain high numbers without the fungus. Though strong mutualisms had the highest fitness without the invader, they proved most susceptible to invasion because the loss of the mycorrhizal fungus resulted in a reproductive output too low to compete with garlic mustard. Weak mutualisms were more competitive than strong mutualisms but still led to garlic mustard invasion. Furthermore, we found that under intermediate mutualism strengths, the initial density of garlic mustard (as a proxy for different levels of plant invasion) did not influence its invasion success, as high initial densities of garlic mustard did not lead to it becoming dominant. Our results indicate that plants that form weak or strong mutualisms with mycorrhizal fungi are most vulnerable to invasion, whereas intermediate mutualisms provide the highest resistance to an allelopathic invader.     

The effects of above- and belowground mutualisms on orchid speciation and coexistence

Both pollination by animals and mycorrhizal symbioses with fungi are believed to have been important for the diversification of flowering plants. However, the mechanisms by which these above- and belowground mutualisms affect plant speciation and coexistence remain obscure. We provide evidence that shifts in pollination traits are important for both speciation and coexistence in a diverse group of orchids, whereas shifts in fungal partner are important for coexistence but not for speciation. Phylogenetic analyses show that recently diverged orchid species tend either to use different pollinator species or to place pollen on different body parts of the same species, consistent with the role of pollination-mode shifts in speciation. Field experiments provide support for the hypothesis that colonization of new geographical areas requires adaptation to new pollinator species, whereas co-occurring orchid species share pollinator species by placing pollen on different body parts. In contrast to pollinators, fungal partners are conserved between closely related orchid species, and orchids recruit the same fungal species even when transplanted to different areas. However, co-occurring orchid species tend to use different fungal partners, consistent with their expected role in reducing competition for nutrients. Our results demonstrate that the two dominant mutualisms in terrestrial ecosystems can play major but contrasting roles in plant community assembly and speciation.     

Links between tree species, symbiotic fungal diversity and ecosystem functioning in simplified tropical ecosystems

We studied the relationships among plant and arbuscular mycorrhizal (AM) fungal diversity, and their effects on ecosystem function, in a series of replicate tropical forestry plots in the La Selva Biological Station, Costa Rica. Forestry plots were 12 yr old and were either monocultures of three tree species, or polycultures of the tree species with two additional understory species. Relationships among the AM fungal spore community, host species, plant community diversity and ecosystem phosphorus-use efficiency (PUE) and net primary productivity (NPP) were assessed. Analysis of the relative abundance of AM fungal spores found that host tree species had a significant effect on the AM fungal community, as did host plant community diversity (monocultures vs polycultures). The Shannon diversity index of the AM fungal spore community differed significantly among the three host tree species, but was not significantly different between monoculture and polyculture plots. Over all the plots, significant positive relationships were found between AM fungal diversity and ecosystem NPP, and between AM fungal community evenness and PUE. Relative abundance of two of the dominant AM fungal species also showed significant correlations with NPP and PUE. We conclude that the AM fungal community composition in tropical forests is sensitive to host species, and provide evidence supporting the hypothesis that the diversity of AM fungi in tropical forests and ecosystem NPP covaries.     

Plant invasions--the role of mutualisms

Many introduced plant species rely on mutualisms in their new habitats to overcome barriers to establishment and to become naturalized and, in some cases, invasive. Mutualisms involving animal-mediated pollination and seed dispersal, and symbioses between plant roots and microbiota often facilitate invasions. The spread of many alien plants, particularly woody ones, depends on pollinator mutualisms. Most alien plants are well served by generalist pollinators (insects and birds), and pollinator limitation does not appear to be a major barrier for the spread of introduced plants (special conditions relating to Ficus and orchids are described). Seeds of many of the most notorious plant invaders are dispersed by animals, mainly birds and mammals. Our review supports the view that tightly coevolved, plant-vertebrate seed dispersal systems are extremely rare. Vertebrate-dispersed plants are generally not limited reproductively by the lack of dispersers. Most mycorrhizal plants form associations with arbuscular mycorrhizal fungi which, because of their low specificity, do not seem to play a major role in facilitating or hindering plant invasions (except possibly on remote islands such as the Galapagos which are poor in arbuscular mycorrhizal fungi). The lack of symbionts has, however, been a major barrier for many ectomycorrhizal plants, notably for Pinus spp. in parts of the southern hemisphere. The roles of nitrogen-fixing associations between legumes and rhizobia and between actinorhizal plants and Frankia spp. in promoting or hindering invasions have been virtually ignored in the invasions literature. Symbionts required to induce nitrogen fixation in many plants are extremely widespread, but intentional introductions of symbionts have altered the invasibility of many, if not most, systems. Some of the world's worst invasive alien species only invaded after the introduction of symbionts. Mutualisms in the new environment sometimes re-unite the same species that form partnerships in the native range of the plant. Very often, however, different species are involved, emphasizing the diffuse nature of many (most) mutualisms. Mutualisms in new habitats usually duplicate functions or strategies that exist in the natural range of the plant. Occasionally, mutualisms forge totally novel combinations, with profound implications for the behaviour of the introduced plant in the new environment (examples are seed dispersal mutualisms involving wind-dispersed pines and cockatoos in Australia; and mycorrhizal associations involving plant roots and fungi). Many ecosystems are becoming more susceptible to invasion by introduced plants because: (a) they contain an increasing array of potential mutualistic partners (e.g. generalist frugivores and pollinators, mycorrhizal fungi with wide host ranges, rhizobia strains with infectivity across genera); and (b) conditions conductive for the establishment of various alien/alien synergisms are becoming more abundant. Incorporating perspectives on mutualisms in screening protocols will improve (but not perfect) our ability to predict whether a given plant species could invade a particular habitat.     

Soil fertility shifts the relative importance of saprotrophic and mycorrhizal fungi for maintaining ecosystem stability

Soil microbial communities are essential for regulating the dynamics of plant productivity. However, how soil microbes mediate temporal stability of plant productivity at large scales across various soil fertility conditions remains unclear. Here, we combined a regional survey of 51 sites in the temperate grasslands of northern China with a global grassland survey of 120 sites to assess the potential roles of soil microbial diversity in regulating ecosystem stability. The temporal stability of plant productivity was quantified as the ratio of the mean normalized difference vegetation index to its standard deviation. Soil fungal diversity, but not bacterial diversity, was positively associated with ecosystem stability, and particular fungal functional groups determined ecosystem stability under contrasting conditions of soil fertility. The richness of soil fungal saprobes was positively correlated with ecosystem stability under high-fertility conditions, while a positive relationship was observed with the richness of mycorrhizal fungi under low-fertility conditions. These relationships were maintained after accounting for plant diversity and environmental factors. Our findings highlight the essential role of fungal diversity in maintaining stable grassland productivity, and suggest that future studies incorporating fungal functional groups into biodiversity–stability relationships will advance our understanding of their linkages under different fertility conditions.     

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.     

The influence of arbuscular mycorrhizae on the relationship between plant diversity and productivity

Ecological theory predicts a positive and asymptotic relationship between plant diversity and ecosystem productivity based on the ability of more diverse plant communities to use limiting resources more fully. This is supported by recent empirical evidence. Additionally, in natural ecosystems, plant productivity is often a function of the presence and composition of mycorrhizal associations. Yet, the effect of mycorrhizal fungi on the relationship between plant diversity and productivity has not been investigated. We predict that in the presence of AMF, productivity will saturate at lower levels of species richness because AMF increase the ability of plant species to utilize nutrient resources. In this study we manipulated old-field plant species richness in the presence and absence of two species of AMF. We found that in the absence of AMF, the relationship between plant species richness and productivity is positive and linear. However, in the presence of AMF, the relationship is positive but asymptotic, even though the maximum plant biomass was significantly different between the two AMF treatments. This is consistent with the hypothesis that AMF increase the redundancy of plant species in the productivity of plant communities, and indicates that these symbionts must be considered in future investigations of plant biodiversity and ecosystem function.     

Invasive belowground mutualists of woody plants

Most plants require mutualistic associations to survive, which can be an important limitation on their ability to become invasive. There are four strategies that permit plants to become invasive without being limited by a lack of mutualists. One is to not be dependent on mutualists. The other three strategies are to form novel mutualisms, form associations with cosmopolitan species, or co-invade with mutualists from their native range. Historically there has been a bias to study mutualisms from a plant perspective, with little consideration of soil biota as invasive species in their own right. Here we address this by reviewing the literature on belowground invasive mutualists of woody plants. We focus on woody invaders as ecosystem-transforming plants that frequently have a high dependence on belowground mutualists. We found that co-invasions are common, with many ectomycorrhizal plant species and N-fixing species co-invading with their mutualists. Other groups, such as arbuscular mycorrhizal plants, tend to associate with cosmopolitan fungal species or to form novel associations in their exotic range. Only limited evidence exists of direct negative effects of co-invading mutualists on native mutualist communities, and effects on native plants appear to be largely driven by altered environmental conditions rather than direct interactions. Mutualists that introduce novel ecosystem functions have effects greater than would be predicted based solely on their biomass. Focusing on the belowground aspects of plant invasions provides novel insights into the impacts, processes and management of invasions of both soil organisms and woody plant species.     

Potential impacts of multiple partners on mycorrhizal community dynamics

The significance of arbuscular mycorrhizal (AM) fungi in terrestrial ecosystems is widely acknowledged, but the causes and consequences of diversity in these fungi are not well understood. A recent frequency-dependent model suggests that dynamics within mycorrhizal mutualisms could promote the coexistence of at least two competing plant species and two competing fungal species within a community. Models are developed here in which simultaneous association with multiple partners may result in elevated or depressed fitness relative to association with either partner alone. This increases the range of conditions under which negative feedback occurs and coexistence of all four species is possible. Differences between plants in the relative proportions of fungi at which maximum fitness occurs may facilitate coexistence, as may differences between fungi in their abilities to establish interplant connections. These models suggest additional mechanisms by which mutualistic interactions could promote local diversity of plants and AM fungi.     

Mycorrhizal fungal identity and richness determine the diversity and productivity of a tallgrass prairie system

We investigated the effects of arbuscular mycorrhizal fungal (AMF) species richness and composition on plant community productivity and diversity, and whether AMF mediate plant species coexistence by promoting niche differentiation in phosphorus use. Our experiment manipulated AMF species richness and identity across a range of P conditions in tallgrass prairie mesocosms. We showed that increasing AMF richness promoted plant diversity and productivity, but that this AMF richness effect was small relative to the effects of individual AMF species. We found little support for AMF-facilitated complementarity in P use. Rather, the AMF richness effect appeared to be caused by the inclusion of particular diversity- and productivity-promoting AMF (a sampling effect). Furthermore, the identity of the diversity-promoting fungi changed with P environment, as did the relationship between the diversity-promoting and productivity-promoting benefits of AMF. Our results suggest that plant diversity and productivity are more responsive to AMF identity than to AMF diversity per se, and that AMF identity and P environment can interact in complex ways to alter community-level properties.     

Earthworm-mycorrhiza interactions can affect the diversity, structure and functioning of establishing model grassland communities

Both earthworms and arbuscular mycorrhizal fungi (AMF) are important ecosystem engineers co-occurring in temperate grasslands. However, their combined impacts during grassland establishment are poorly understood and have never been studied. We used large mesocosms to study the effects of different functional groups of earthworms (i.e., vertically burrowing anecics vs. horizontally burrowing endogeics) and a mix of four AMF taxa on the establishment, diversity and productivity of plant communities after a simulated seed rain of 18 grassland species comprising grasses, non-leguminous forbs and legumes. Moreover, effects of earthworms and/or AMF on water infiltration and leaching of ammonium, nitrate and phosphate were determined after a simulated extreme rainfall event (40 l m(-2)). AMF colonisation of all three plant functional groups was altered by earthworms. Seedling emergence and diversity was reduced by anecic earthworms, however only when AMF were present. Plant density was decreased in AMF-free mesocosms when both anecic and endogeic earthworms were active; with AMF also anecics reduced plant density. Plant shoot and root biomass was only affected by earthworms in AMF-free mesocosms: shoot biomass increased due to the activity of either anecics or endogeics; root biomass increased only when anecics were active. Water infiltration increased when earthworms were present in the mesocosms but remained unaffected by AMF. Ammonium leaching was increased only when anecics or a mixed earthworm community was active but was unaffected by AMF; nitrate and phosphate leaching was neither affected by earthworms nor AMF. Ammonium leaching decreased with increasing plant density, nitrate leaching decreased with increasing plant diversity and density. In order to understand the underlying processes of these interactions further investigations possibly under field conditions using more diverse belowground communities are required. Nevertheless, this study demonstrates that belowground-aboveground linkages involving earthworms and AMF are important mediators of the diversity, structure and functioning of plant communities.     

Subcellular Nutrient Element Localization and Enrichment in Ecto- and Arbuscular Mycorrhizas of Field-Grown Beech and Ash Trees Indicate Functional Differences

Mycorrhizas are the chief organ for plant mineral nutrient acquisition. In temperate, mixed forests, ash roots (Fraxinus excelsior) are colonized by arbuscular mycorrhizal fungi (AM) and beech roots (Fagus sylvatica) by ectomycorrhizal fungi (EcM). Knowledge on the functions of different mycorrhizal species that coexist in the same environment is scarce. The concentrations of nutrient elements in plant and fungal cells can inform on nutrient accessibility and interspecific differences of mycorrhizal life forms. Here, we hypothesized that mycorrhizal fungal species exhibit interspecific differences in mineral nutrient concentrations and that the differences correlate with the mineral nutrient concentrations of their associated root cells. Abundant mycorrhizal fungal species of mature beech and ash trees in a long-term undisturbed forest ecosystem were the EcM Lactarius subdulcis, Clavulina cristata and Cenococcum geophilum and the AM Glomus sp. Mineral nutrient subcellular localization and quantities of the mycorrhizas were analysed after non-aqueous sample preparation by electron dispersive X-ray transmission electron microscopy. Cenococcum geophilum contained the highest sulphur, Clavulina cristata the highest calcium levels, and Glomus, in which cations and P were generally high, exhibited the highest potassium levels. Lactarius subdulcis-associated root cells contained the highest phosphorus levels. The root cell concentrations of K, Mg and P were unrelated to those of the associated fungal structures, whereas S and Ca showed significant correlations between fungal and plant concentrations of those elements. Our results support profound interspecific differences for mineral nutrient acquisition among mycorrhizas formed by different fungal taxa. The lack of correlation between some plant and fungal nutrient element concentrations may reflect different retention of mineral nutrients in the fungal part of the symbiosis. High mineral concentrations, especially of potassium, in Glomus sp. suggest that the well-known influence of tree species on chemical soil properties may be related to their mycorrhizal associates.     

Interactive effects of root endophytes and arbuscular mycorrhizal fungi on an experimental plant community

Plant-soil microbial interactions have moved into focus as an important mechanism for understanding plant coexistence and composition of communities. Both arbuscular mycorrhizal (AM) as well as other root endophytic fungi co-occur in plant roots, and therefore have the potential to influence relative abundances of plant species in local assemblages. However, no study has experimentally examined how these key root endosymbiont groups might interact and affect plant community composition. Here, using an assemblage of five plant species in mesocosms in a fully factorial experiment, we added an assemblage of AM fungi and/or a mixture of root endophytic fungal isolates, all obtained from the same grassland field site. The results demonstrate that the AM fungi and root endophytes interact to affect plant community composition by changing relative species abundance, and consequently aboveground productivity. Our study highlights the need to explicitly consider interactions of root-inhabiting fungal groups in studies of plant assemblages.     

Phylogeny of arbuscular mycorrhizal fungi predicts community composition of symbiosis-associated bacteria

Many physicochemical and biotic aspects of the soil environment determine the community composition of bacteria. In this study, we examined the effects of arbuscular mycorrhizal fungi, common symbionts of higher plants, on the composition of bacterial communities after long-term (7-8 years) enrichment culture in the presence of a plant host. We showed that the phylogeny of arbuscular mycorrhizal fungal isolates was a highly significant predictor of bacterial community composition, as assessed by cluster analysis, redundancy analysis and linear discriminant analysis of phospholipid fatty acid patterns. Numerous phospholipid fatty acids differed between the phylogenetic groupings; this pattern also held for fungal-origin phospholipid fatty acids and in a combined bacterial/fungal analysis, suggesting that categorizing phospholipid fatty acids into predominantly bacterial and fungal origin did not affect the overall outcome. The mechanisms underlying this observation could include substrate quality (and quantity) effects, interactions mediated by the host plant (e.g. rhizodeposition) and direct biotic interactions between arbuscular mycorrhizal fungi and bacterial populations. Our results suggest that aspects of arbuscular mycorrhizal fungal functions may be partially explained by the symbiosis-accompanying bacterial communities, a possibility that should be explicitly considered in studies examining the roles of arbuscular mycorrhizal fungal species diversity in soil and ecosystem processes.     

Balancing multiple mutualists: asymmetric interactions among plants, arbuscular mycorrhizal fungi, and fungal endophytes

Most organisms engage in beneficial interactions with other species; however, little is known regarding how individuals balance the competing demands of multiple mutualisms. Here we examine three-way interactions among a widespread grass, Schedonorus phoenix , a protective fungal endophyte aboveground, Neotyphodium coenophialum , and nutritional symbionts (arbuscular mycorrhizal fungi) belowground. In a greenhouse experiment, we manipulated the presence/absence of both fungi and applied a fertilizer treatment to individual plants. Endophyte presence in host plants strongly reduced mycorrhizal colonization of roots. Additionally, for plants with the endophyte, the density of endophyte hyphae was negatively correlated with mycorrhizal colonization, suggesting a novel role for endophyte abundance in the interaction between the symbionts. Endophyte presence increased plant biomass, and there was a positive correlation between endophyte hyphal density and plant biomass. The effects of mutualists were asymmetric: mycorrhizal fungi treatments had no significant impact on the endophyte and negligible effects on plant biomass. Fertilization affected all three species – increasing plant biomass and endophyte density, but diminishing mycorrhizal colonization. Mechanisms driving negative effects of endophytes on mycorrhizae may include inhibition via endophyte alkaloids, altered nutritional requirements of the host plant, and/or temporal and spatial priority effects in the interactions among plants and multiple symbionts.     

树种多样性对土壤微生物群落结构和元素生物地球化学循环的影响研究进展

森林是陆地生态系统的重要组成部分,其巨大的生产力和生态服务功能对人类的生存和发展至关重要。森林树种多样性增加能够显著提高森林生产力,关于树种多样性如何影响地下生物多样性及生态功能逐渐受到国内外学者的广泛关注。从土壤微生物及其介导的元素生物地球化学循环这一视角出发,综述了树种多样性对土壤细菌和真菌多样性、群落结构及功能的影响,提出需要进一步深入研究的方向。总体来说,树种多样性有利于增加土壤细菌生物量和多样性,是预测病原性真菌和菌根真菌多样性及群落结构的重要生物因子。树种多样性能增加土壤有机碳储量,增强森林土壤的甲烷氧化能力,并提高土壤磷周转速率及有效磷含量。关于树种多样性对森林土壤氮循环的影响需考虑多样性假说和质量比假说的相对贡献。今后应加强树种多样性对多个营养级之间相互作用的研究;关注树种多样性对生态系统多功能的影响;加强学科交叉,引入微生物种群动态模型和气候模型等模型预测方法,研究树种多样性对全球气候变化的应对机制,以期促进地上植物多样性与地下生态系统功能关系的研究,增强森林生态系统应对未来全球环境变化的能力。     

<Emphasis Type="Italic">Monotropastrum humile</Emphasis> var. <Emphasis Type="Italic">humile</Emphasis> is associated with diverse ectomycorrhizal Russulaceae fungi in Japanese forests

Monotropastrum humile is nearly lacking in chlorophyll and obtains its nutrients, including carbon sources, from associated mycorrhizal fungi. We analyzed the mycorrhizal fungal affinity and species diversity of M. humile var. humile mycorrhizae to clarify how the plant population survives in Japanese forest ecosystems. We classified 78 samples of adult M. humile var. humile individuals from Hokkaido, Honshu, and Kyusyu Islands into 37 root mycorrhizal morphotypes. Of these, we identified 24 types as Russula or Lactarius fungal taxa in the Russulaceae, Basidiomycetes, but we could not identify the remaining 13 types as to their genus in the Basidiomycetes. The number of fungal species on M. humile var. humile was the highest in the plant subfamily. The diversity of fungal species revealed its increased trends in natural forests at the stand level, fagaceous vegetation, and cool-temperate climate. The most frequently observed fungus colonized mainly samples collected from sub-alpine forests; the second most frequently observed fungus colonized samples collected from sub-alpine to warm-temperate forests. These results suggest that Japanese M. humile populations are associated with specific but diverse fungi that are common ectomycorrhizal symbionts of various forest canopy trees, indicating a tripartite mycorrhizal relationship in the forest ecosystem.     

外生菌根菌与森林树木的相互关系

生态系统的每个过程都伴随着各种微生物的活动,其中最重要的功能群之一是菌根真菌(菌根菌)。一般认为,菌根菌是自然界多数植物生存最基本的组成部分,陆地上约90％以上的高等植物都具有菌根菌。这些菌类的菌丝体与植物根系结合形成菌根,使植物生长成为可能,使不同种类植物的根系联在一起。根据菌根菌入侵植物根系的方式及菌根的形态特征,菌根可分为外生菌根、内生菌根和内外生菌根3组共7种类型。外生菌根主要出现在松科、桦木科、壳斗科等树种的森林生态系统中,在根系表面形成菌丝鞘,部分菌丝进入根系皮层细胞间隙形成哈氏网表面。菌根菌剂在森林经营中得到广泛地应用。外生菌根菌对森林树木的作用可归纳为：1)促进造林或育苗成活与生长;2)提高森林生态系统中植物的多样性、稳定性和生产力;3)对森林生态系统的综合效应,主要表现在增加植物一土壤联结,改善土壤结构,促进土壤微生物,增强植物器官的功能;4)抗拮植物根部病害病原菌等。树木与菌根菌相互关系研究主要包括：1)菌根共生的机理;2)菌根菌在退化森林生态系统恢复与改造中的作用;3)菌根菌的分布格局与森林生态系统服务功能的关系;4)菌根菌对森林生态系统的综合效应,如菌根菌与森林植物群落结构、物种多样性以及森林系统稳定性和生产力的研究。