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.     

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.     

An economic approach to evaluate the role of mycorrhizas in managed ecosystems

The benefits of management of mycorrhizas in agricultural and horticultural croppiing systems remains problematic except where the indigenous fungal population is low. Most experiments have focused on the introduction of exotic fungal isolates. Promotion of plant growth by mycorrhizas can be enhanced by increasing the effectiveness of the indigenous fungi as well as by introducing more effective species. Lack of reliable methods for identification of fungal species colonizing roots is a major limitation to characterizing the change in mycorrhizal populations. Assessment of the role of mycorrhizas in commerical food production systems must include an economic analysis. To do so requires an evaluation of the response to increasing the effectiveness of the mycorrhizal symbiosis relative to increasing yield with addition of phosphorus fertilizer. Thus field experiments should be designed to measure the response to phosphorus addition with the existing mycorrhizal population as well as with the more effectively managed population. In this paper we discuss changes that may be induced in mycorrhizal fungi by management to increase their effectiveness in promoting plant growth. We then suggest an economic analysis approach to assessing the potential benefits of this increase in effectiveness. We conclude with a discussion of research approaches needed to determine, in a more objective manner, the role of mycorrhizas in managed ecosystems.     

Temporal infectivity and specificity of vesicular-arbuscular mycorrhizas in co-existing grassland species

Specificity in vesicular-arbuscular mycorrhizas (VAM), arising from selection favouring host plant/mycorrhizal fungus associations in which both organisms receive benefit, might have a significant influence on interactions between co-existing plant species. In an attempt to detect such specificity root inoculum of four tempt to detect such specificity root inoculum of four plant species, harvested from a species-rich grassland on three dates during the plant growth season, was used to infect the same plant species grown in pots. The rate and overall level of infection was different according to inoculum source and the time of year in which the inoculum was harvested, i.e. temporal variation in VAM infectivity occurs. However, there was no evidence for either specificity or mycorrhizal benefit. Inoculum produced during this experiment was used to infect bait Trifolium pratense plants and protein patterns of these roots indicated that a number of biochemically different endophytes were present, both within the inoculum of the four plant species but also within inoculum from one plant species. Temporal variation in mycorrhizal infectivity could be important for mycorrhizal propagation in the field. However, the lack of evidence, in this study, for specificity of VAM or an obvious nutritional benefit to plants with mycorrhizas make the role of mycorrhizas in this community difficult to interpret.     

A new model of carbon and phosphorus transfers in arbuscular mycorrhizas

Existing models of nutrient transfer in arbuscular mycorrhizal (AM) symbioses are inadequate as they do not explain the range of real responses seen experimentally. A computer simulation model was used to evaluate the novel hypotheses that mycorrhizal nutrient transfers were based solely on symbionts' internal needs, and that carbon and phosphorus transfers were quantitatively unlinked. To be plausible, simulated mycorrhizal plants would show a +/-50% variation in weight vs nonmycorrhizal controls, with a normal response distribution (mimicking a real data set). One plant and one arbuscular mycorrhizal fungus (AMF) growing in a soil volume were simulated, using C, P and nitrogen nutrient cycling and stoichiometry. C- and P-exchange rates were independent and could be varied at will. The model was tested at realistic nutrient concentrations and a full range of nutrient exchange rates. The model showed -20% to +55% range in mycorrhizal plant weight distributed close to normal, suggesting that the hypotheses were plausible. The model suggests that theoretical assumptions about mycorrhizas should be reassessed. The model worked only because the symbionts possessed incomplete information on their partner and environmental conditions. Conventional cost-benefit models do not work under these circumstances, but both mutualistic and parasitic interactions were successfully simulated.     

Occurrence of arbuscular mycorrhizal fungi in a phosphorus-poor wetland and mycorrhizal response to phosphorus fertilization

The presence of arbuscular mycorrhizas in fens has received little attention, but because fen plants are often phosphorus limited, the plant-fungus interaction could be an important factor in plant competition for phosphorus. In this field study, we determined mycorrhizal colonization rates for 18 fen plant species. Also in the field, we examined the effect of four different forms of phosphorus on the percentage colonization for one fen plant species, Solidago patula. We found that in a species-rich, phosphorus-poor wetland both mycorrhizal and nonmycorrhizal species were common. Nine of ten dicotyledonous species examined formed arbuscular mycorrhizas, while all monocotyledonous species were at most very weakly mycorrhizal. A morphological explanation for this pattern is that the monocots in our study have more extensive aerenchyma, especially in coarse roots. Therefore, monocots are able to transport oxygen to their roots more effectively than dicots. In the organic wetland soil, additional oxygen in the rhizosphere promotes phosphorus mineralization and availability. Two of the monocot species (Typha latifolia and Carex lasiocarpa), which have been described previously as mycorrhizal in other wetland types, are surprisingly nonmycorrhizal in our phosphorus-poor study site, suggesting that a mycorrhizal association would not offer improved phosphorus nutrition to these species. In contrast, our field phosphorus addition decreased mycorrhizal colonization in S. patula, suggesting that one benefit to S. patula of the mycorrhizas is phosphorus uptake.     

The impact of elevated CO2 and global climate change on arbuscular mycorrhizas: a mycocentric approach

Arbuscular mycorrhizal (AM) symbioses are a potentially important link in the chain of response of ecosystems to elevated atmospheric [CO₂]. By promoting plant phosphorus uptake and acting as a sink for plant carbon, they can alleviate photosynthetic down-regulation. Because hyphal turnover is likely to be fast, especially in warmer soils, they can also act as a rapid pathway for the return of carbon to the atmosphere. However, most experiments on AM responses to [CO₂] have failed to take into account the difference in growth of mycorrhizal and non- mycorrhizal plants; those that have done so suggest that AM colonization of roots is little altered by [CO₂], although this issue remains to be resolved. Very little is known about the effects of other factors of global environmental change on mycorrhizas. These issues need urgent attention. It is also necessary to understand the potential for the various AM fungal taxa to respond differentially to environmental changes, including carbon supply and soil temperature and moisture, especially because of the differential abilities of plant and fungal species to migrate in response to changing environments. Indeed, there is a need for a new approach to the study of mycorrhizal associations, which has been too plant-centred. It is essential to regard the fungus as an organism itself, and to understand its biology both as an entity and as part of a symbiosis.     

Fungi associated with terrestrial orchid mycorrhizas,seeds and protocorms

The identity and ecological role of fungi in the mycorrhizal roots of 25 species of mature terrestrial orchids and in 17 species of field incubated orchid seedlings were examined. Isolates of symbiotic fungi from mature orchid mycorrhizas were basidiomycetes primarily in the generaCeratorhiza, Epulorhiza andMoniliopsis; a few unidentified taxa with clamped hyphae were also recovered. More than one taxon of peloton-forming fungus was often observed in the cleared and stained mycorrhizas. AlthoughCeratorhiza andEpulorhiza strains were isolated from the developing protocorms, pelotons of clamped hyphae were often presents in the cleared protocorms of several orchid species. These basidiomycetes are difficult to isolate and may be symbionts of ectotrophic plants. The higher proportion of endophytes bearing clamp connections in developing seeds than in the mycorrhizas is attributed to differences in the nutritional requirements of the fully mycotrophic protocorms and partially autotrophic plants. Most isolates ofCeratorhiza differed enzymatically fromEpulorhiza in producing polyphenol oxidases. Dual cultures with thirteen orchid isolates and five non-orchid hosts showed that some taxa can form harmless associations with non-orchid hosts. It is suggested that most terrestrial orchid mycorrhizas are relatively non-specific and that the mycobionts can be saprophytes, parasites or mycorrhizal associates of other plants.     

Root hairs and phycomycetous mycorrhizas in phosphorus-deficient soil

Summary Coprosma robusta formed phycomycetous mycorrhizas in unsteamed forest soil and grew equally well with or without added phosphate. In steamed soil it did not grow unless phosphate was added. Of the other species tested (Leptospermum scoparium, Solanum nigrum, Lolium perenne, Hakea enkiantha, Histiopteris incisa, Marchantia berteroana) most formed mycorrhizas in unsteamed soil, but all grew better in steamed soil. The dry matter of the mycorrhizal Coprosma seedlings contained the highest concentration of phosphorus, but the relatively large plants that the other species produced in steamed soil contained a greater total quantity. It is suggested that this entered mainly through their extensive root hairs (or rhizoids), and that lack of root hairs in Coprosma and other woody species explains their need for added phosphate when mycorrhizas are not formed.     

Paris-type mycorrhizas in Smilax aspera L. growing in a Mediterranean scherophyllous wood

Paris- type mycorrhiza is described in Smilax aspera L., an evergreen climbing plant of Mediterranean sclerophyllous woods. Wild plants were sampled from a protected area inside the Regional Natural Park Migliarino-S.Rossore-Massaciuccoli, on the northwestern coast of Italy, near Pisa. Mycorrhizas formed by S. aspera were identified as a variation of Paris-type arbuscular mycorrhizas. Detailed observations on stained roots and on fresh root sections showed that, after forming the appressorium, the fungus colonized the root by penetrating individual cells, growing intracellularly from cell to cell, and producing many coils and terminal arbuscules. S. aspera seedlings inoculated with the arbuscular mycorrhizal fungi Glomus mosseae and G. viscosum, which are known to form Arum-type mycorrhizas in many plant species, produced the same Paris-type-like mycorrhizas found in nature. This confirms that the type of arbuscular mycorrhizal infection is largely governed by the plant host genotype. Plants of S. aspera inoculated with G. mosseae and G. viscosum had larger growth increments than uninoculated plants. Thus Paris-type mycorrhizas produce growth responses comparable to those of Arum-type mycorrhizas. Accepted: 11 January 2000     

Phylogenetic distribution and evolution of mycorrhizas in land plants

A survey of 659 papers mostly published since 1987 was conducted to compile a checklist of mycorrhizal occurrence among 3,617 species (263 families) of land plants. A plant phylogeny was then used to map the mycorrhizal information to examine evolutionary patterns. Several findings from this survey enhance our understanding of the roles of mycorrhizas in the origin and subsequent diversification of land plants. First, 80 and 92% of surveyed land plant species and families are mycorrhizal. Second, arbuscular mycorrhiza (AM) is the predominant and ancestral type of mycorrhiza in land plants. Its occurrence in a vast majority of land plants and early-diverging lineages of liverworts suggests that the origin of AM probably coincided with the origin of land plants. Third, ectomycorrhiza (ECM) and its derived types independently evolved from AM many times through parallel evolution. Coevolution between plant and fungal partners in ECM and its derived types has probably contributed to diversification of both plant hosts and fungal symbionts. Fourth, mycoheterotrophy and loss of the mycorrhizal condition also evolved many times independently in land plants through parallel evolution.     

接种AM真菌对玉米和油菜种间竞争及土壤无机磷组分的影响

在温室盆栽条件下,分别模拟单作、间作和尼龙网分隔种植,比较接种丛枝菌根(arbuscular mycorrhizal, AM)真菌Glomus intraradices和Glomus mosseae对菌根植物玉米和非菌根植物油菜生长和磷吸收状况的影响,并分析土壤中各无机磷组分的变化。结果发现,接种AM真菌可以促进土壤中难溶性磷(Ca₁₀-P和O-P)向有效态磷转化,并显著降低总无机磷含量 (P<0.05),显著提高菌根植物玉米的生物量和磷吸收量(P<0.05),特别是在间作体系中使玉米的磷营养竞争比率显著提高了45.0%-104.1% (P<0.05),显著降低了油菜的生物量和磷吸收量(P<0.05),从而增强了了菌根植物的竞争优势,降低了非菌根植物与菌根植物的共存能力。揭示了石灰性土壤中AM真菌对植物物种多样性的影响,有助于更加全面地理解AM真菌在农业生态系统中的作用。     

Non-host plants: Are they mycorrhizal networks players?

Common mycorrhizal networks (CMNs) that connect individual plants of the same or different species together play important roles in nutrient and signal transportation, and plant community organization. However, about 10% of land plants are non-mycorrhizal species with roots that do not form any well-recognized types of mycorrhizas; and each mycorrhizal fungus can only colonize a limited number of plant species, resulting in numerous non-host plants that could not establish typical mycorrhizal symbiosis with a specific mycorrhizal fungus. If and how non-mycorrhizal or non-host plants are able to involve in CMNs remains unclear. Here we summarize studies focusing on mycorrhizal-mediated host and non-host plant interaction. Evidence has showed that some host-supported both arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) hyphae can access to non-host plant roots without forming typical mycorrhizal structures, while such non-typical mycorrhizal colonization often inhibits the growth but enhances the induced system resistance of non-host plants. Meanwhile, the host growth is also differentially affected, depending on plant and fungi species. Molecular analyses suggested that the AMF colonization to non-hosts is different from pathogenic and endophytic fungi colonization, and the hyphae in non-host roots may be alive and have some unknown functions. Thus we propose that non-host plants are also important CMNs players. Using non-mycorrhizal model species Arabidopsis, tripartite culture system and new technologies such as nanoscale secondary ion mass spectrometry and multi-omics, to study nutrient and signal transportation between host and non-host plants via CMNs may provide new insights into the mechanisms underlying benefits of intercropping and agro-forestry systems, as well as plant community establishment and stability.     

Patterns of plant naturalization show that facultative mycorrhizal plants are more likely to succeed outside their native Eurasian ranges

The naturalization of an introduced species is a key stage during the invasion process. Therefore, identifying the traits that favor the naturalization of non-native species can help understand why some species are more successful when introduced to new regions. The ability and the requirement of a plant species to form a mutualism with mycorrhizal fungi, together with the types of associations formed may play a central role in the naturalization success of different plant species. To test the relationship between plant naturalization success and their mycorrhizal associations we analysed a database composed of mycorrhizal status and type for 1981 species, covering 155 families and 822 genera of plants from Europe and Asia, and matched it with the most comprehensive database of naturalized alien species across the world (GloNAF). In mainland regions, we found that the number of naturalized regions was highest for facultative mycorrhizal, followed by obligate mycorrhizal and lowest for non-mycorrhizal plants, suggesting that the ability of forming mycorrhizas is an advantage for introduced plants. We considered the following mycorrhizal types: arbuscular, ectomycorrhizal, ericoid and orchid mycorrhizal plants. Further, dual mycorrhizal species were those that included observations of arbuscular mycorrhizas as well as observations of ectomycorrhizas. Naturalization success (based on the number of naturalized regions) was highest for arbuscular mycorrhizal and dual mycorrhizal plants, which may be related to the low host specificity of arbuscular mycorrhizal fungi and the consequent high availability of arbuscular mycorrhizal fungal partners. However, these patterns of naturalization success were erased in islands, suggesting that the ability to form mycorrhizas may not be an advantage for establishing self-sustaining populations in isolated regions. Taken together our results show that mycorrhizal status and type play a central role in the naturalization process of introduced plants in many regions, but that their effect is modulated by other factors.     

Species-specific trade-offs between regrowth and mycorrhizas in the face of defoliation and phosphorus addition

Arbuscular mycorrhizal fungi (AMF) enhance nutrient provision in exchange for carbon. However, a shift from nutrient to carbon limitation, induced by repeated or intense defoliation, can represent a trade-off between plant regrowth and the maintenance of mycorrhiza. The combined effects of AMF, phosphorus and defoliation on growth of Agropyron elongatum (C₃ grass, low mycorrhizal responsiveness) and Brachiaria brizantha (C₄ grass, high mycorrhizal responsiveness) were explored. Each species was subjected to a manipulative experiment with AMF inoculation (non-inoculated, inoculated), soluble P supply (low, high) and defoliation (non-defoliated, 60% defoliated). In A. elongatum, at low P supply, mycorrhizal plants showed increased growth rates following defoliation without substantial changes in AMF colonization. At high P supply instead, we found a clear trade-off between regrowth and the maintenance of mycorrhiza evidenced by growth depression (biomass and tillers) and lower AMF activity (reduction of arbuscules). In contrast, in B. brizantha, defoliation effects on plant regrowth were independent from AMF at any P supply (no trade-off). This indicates that cost-benefit relationship in defoliated plants is highly context-dependent and may vary with mycorrhizal responsiveness of species. This variation of responses can play a decisive role on plant recovery in pastures and natural grasslands subjected to herbivory.     

Symbiotic phosphate transport in arbuscular mycorrhizas

Arbuscular mycorrhizal fungi colonize the root systems of most land plants and modulate plant growth by enhancing the availability of nutrients, mainly phosphorus, for plant nutrition. Recently identified genes encoding mycorrhiza-specific plant phosphate transporters have enabled fundamental problems in arbuscular mycorrhizal symbiosis research to be addressed. Because phosphate transport is a key feature of this symbiosis, the study of phosphate transport mechanisms and their gene regulation will further our understanding of the intimate interaction between the two symbiotic partners.     

Density dependent interactions between VA mycorrhizal fungi and even-aged seedlings of two perennial Fabaceae species

Summary The interaction of density and mycorrhizal effects on the growth, mineral nutrition and size distribution of seedlings of two perennial members of the Fabaceae was investigated in pot culture. Seedlings of Otholobium hirtum and Aspalathus linearis were grown at densities of 1, 4, 8 and 16 plants per 13-cm pot with or without vesicular-arbuscular (VA) mycorrhizal inoculum for 120 days. Plant mass, relative growth rates, height and leaf number all decreased with increasing plant density. This was ascribed to the decreasing availability of phosphorus per plant as density increased. O. hirtum was highly dependent on mycorrhizas for P uptake but both mycorrhizal and non-mycorrhizal A. linearis seedlings were able to extract soil P with equal ease. Plant size distribution as measured by the coefficient of variation (CV) of shoot mass was greater at higher densities. CVs of mycorrhizal O. hirtum plants were higher than those of non-mycorrhizal plants. CVs of the facultatively mycorrhizal A. linearis were similar for both mycorrhizal and non-mycorrhizal plants. Higher CVs are attributed to resource preemption by larger individuals. Individuals in populations with high CVs will probably survive stress which would result in the extinction of populations with low CVs. Mass of mycorrhizal plants of both species decreased more rapidly with increasing density than did non-mycorrhizal plant mass. It is concluded that the cost of being mycorrhizal increases as plant density increases, while the benefit decreases. The results suggest that mycorrhizas will influence density-dependent population processes of faculative and obligate mycorrhizal species.     

Does responsiveness to arbuscular mycorrhizal fungi depend on plant invasive status?

Differences in the direction and degree to which invasive alien and native plants are influenced by mycorrhizal associations could indicate a general mechanism of plant invasion, but whether or not such differences exist is unclear. Here, we tested whether mycorrhizal responsiveness varies by plant invasive status while controlling for phylogenetic relatedness among plants with two large grassland datasets. Mycorrhizal responsiveness was measured for 68 taxa from the Northern Plains, and data for 95 taxa from the Central Plains were included. Nineteen percent of taxa from the Northern Plains had greater total biomass with mycorrhizas while 61% of taxa from the Central Plains responded positively. For the Northern Plains taxa, measurable effects often depended on the response variable (i.e., total biomass, shoot biomass, and root mass ratio) suggesting varied resource allocation strategies when roots are colonized by arbuscular mycorrhizal fungi. In both datasets, invasive status was nonrandomly distributed on the phylogeny. Invasive taxa were mainly from two clades, that is, Poaceae and Asteraceae families. In contrast, mycorrhizal responsiveness was randomly distributed over the phylogeny for taxa from the Northern Plains, but nonrandomly distributed for taxa from the Central Plains. After controlling for phylogenetic similarity, we found no evidence that invasive taxa responded differently to mycorrhizas than other taxa. Although it is possible that mycorrhizal responsiveness contributes to invasiveness in particular species, we find no evidence that invasiveness in general is associated with the degree of mycorrhizal responsiveness. However, mycorrhizal responsiveness among species grown under common conditions was highly variable, and more work is needed to determine the causes of this variation.     

Transport processes at the plant-fungus interface in mycorrhizal associations: physiological studies

The roots of most plants form symbiotic associations with mycorrhizal fungi. The net flux of nutrients, particularly phosphorus (P), from the soil into the plant is greater in mycorrhizal than in comparable non-mycorrhizal plants. However despite the widespread occurrence of mycorrhizal associations the processes controlling the transfer of solutes between the symbionts are poorly understood. To understand the mechanisms regulating the transfer of solutes information about conditions at the interface between plant and fungus is needed.Measurements of apoplastic and intracellular electrical potential difference in leek roots colonised by mycorrhizal fungi and estimates of cytosolic pH in fungal hyphae are presented. These and the implications for plant/fungal mineral nutrition in vesicular-arbuscular mycorrhizas are discussed.     

Plant-plant interactions vary with different mycorrhizal fungus species

Because different species of mycorrhizal fungi have different effects on the growth of particular plant species, variation in mycorrhizal fungus species composition could cause changes in the strength of plant-plant interactions. Results are presented from a growth chamber experiment that compared the strength of interactions among seedlings of ponderosa pine (Pinus ponderosa) when the pines were colonized by two different groups of ectomycorrhizal fungi in the genus Rhizopogon. Plant density effects differed between the two groups of mycorrhizal fungi: plant growth was low regardless of density when plants were colonized with pine-specific Rhizopogon species, while plant growth declined with plant density when plants were colonized by Rhizopogon species having a broader host range. This result parallels results from previous studies showing that plant interactions are more antagonistic with mycorrhizal fungi than without, implying that plant responsiveness to beneficial mycorrhizal fungi declines with increasing plant density. If such effects are prevalent in plant communities, then variation in mycorrhizal fungus community composition is predicted to have a density-dependent effect on plants.