Ectomycorrhizal mediation of competition between coniferous tree species

The influence of tree genus, phylogeny, and richness on the specificity, rarity, and diversity of ectomycorrhizal fungi

Partner specificity is a well-documented phenomenon in biotic interactions, yet the factors that determine specificity in plant-fungal associations remain largely unknown. By utilizing composite soil samples, we identified the predictors that drive partner specificity in both plants and fungi, with a particular focus on ectomycorrhizal associations. Fungal guilds exhibited significant differences in overall partner preference and avoidance, richness, and specificity to specific tree genera. The highest level of specificity was observed in root endophytic and ectomycorrhizal associations, while the lowest was found in arbuscular mycorrhizal associations. The majority of ectomycorrhizal fungal species showed a preference for one of their partner trees, primarily at the plant genus level. Specialist ectomycorrhizal fungi were dominant in belowground communities in terms of species richness and relative abundance. Moreover, all tree genera (and occasionally species) demonstrated a preference for certain fungal groups. Partner specificity was not related to the rarity of fungi or plants or environmental conditions, except for soil pH. Depending on the partner tree genus, specific fungi became more prevalent and relatively more abundant with increasing stand age, tree dominance, and soil pH conditions optimal for the partner tree genus. The richness of partner tree species and increased evenness of ectomycorrhizal fungi in multi-host communities enhanced the species richness of ectomycorrhizal fungi. However, it was primarily the partner-generalist fungi that contributed to the high diversity of ectomycorrhizal fungi in mixed forests.

Common Mycorrhizae Network: A Review of the Theories and Mechanisms Behind Underground Interactions

Most terrestrial plants establish symbiotic associations with mycorrhizal fungi for accessing essential plant nutrients. Mycorrhizal fungi have been frequently reported to interconnect plants via a common mycelial network (CMN), in which nutrients and signaling compounds can be exchanged between the connected plants. Several studies have been performed to demonstrate the potential effects of the CMN mediating resource transfer and its importance for plant fitness. Due to several contrasting results, different theories have been developed to predict benefits or disadvantages for host plants involved in the network and how it might affect plant communities. However, the importance of the mycelium connections for resources translocation compared to other indirect pathways, such as leakage of fungi hyphae and subsequent uptake by neighboring plant roots, is hard to distinguish and quantify. If resources can be translocated via mycelial connections in significant amounts that could affect plant fitness, it would represent an important tactic for plants co-existence and it could shape community composition and dynamics. Here, we report and critically discuss the most recent findings on studies aiming to evaluate and quantify resources translocation between plants sharing a CMN and predict the pattern that drives the movement of such resources into the CMN. We aim to point gaps and define open questions to guide upcoming studies in the area for a prospect better understanding of possible plant-to-plant interactions via CMN and its effect in shaping plants communities. We also propose new experiment set-ups and technologies that could be used to improve previous experiments. For example, the use of mutant lines plants with manipulation of genes involved in the symbiotic associations, coupled with labeling techniques to track resources translocation between connected plants, could provide a more accurate idea about resource allocation and plant physiological responses that are truly accountable to CMN.

Successional Change of the Fungal Microbiome Pine Seedling Roots Inoculated With Tricholoma matsutake

The pine mushroom (Tricholoma matsutake; Agaricales, Tricholomataceae) is an ectomycorrhizal fungus that produces a commercially valuable, edible mushrooms. Attempts to artificially cultivate T. matsutake has so far been unsuccessful. One method used to induce T. matsutake to produce fruiting bodies of in the wild is shiro (mycelial aggregations of T. matsutake) transplantation. In vitro ectomycorrhization of T. matsutake with seedlings of Pinus densiflora has been successful, but field trials showed limited production of fruiting bodies. Few studies have been done to test what happens after transplantation in the wild, whether T. matsutake persists on the pine seedling roots or gets replaced by other fungi. Here, we investigated the composition and the interaction of the root fungal microbiome of P. densiflora seedlings inoculated with T. matsutake over a 3 year period after field transplantation, using high-throughput sequencing. We found a decline of T. matsutake colonization on pine roots and succession of mycorrhizal fungi as P. densiflora seedlings grew. Early on, roots were colonized by fast-growing, saprotrophic Ascomycota, then later replaced by early stage ectomycorrhiza such as Wilcoxina. At the end, more competitive Suillus species dominated the host roots. Most of the major OTUs had negative or neutral correlation with T. matsutake, but several saprotrophic/plant pathogenic/mycoparasitic species in genera Fusarium, Oidiodendron, and Trichoderma had positive correlation with T. matsutake. Four keystone species were identified during succession; two species (Fusarium oxysporum, and F. trincintum) had a positive correlation with T. matsutake, while the other two had a negative correlation (Suillus granulatus, Cylindrocarpon pauciseptatum). These findings have important implications for further studies on the artificial cultivation of T. matsutake.

Highly invasive tree species are more dependent on mutualisms

Why some species become invasive while others do not remains an elusive question. It has been proposed that invasive species should depend less on mutualisms, because their spread would then be less constrained by the availability of mutualistic partners. We tested this idea with the genus Pinus, whose degree of invasiveness is known at the species level (being highly and negatively correlated with seed size), and which forms obligate mutualistic associations with ectomycorrhizal fungi (EMF). Mycorrhizal dependence is defined as the degree to which a plant needs the mycorrhizal fungi to show the maximum growth. In this regard, we use plant growth response to mycorrhizal fungi as a proxy for mycorrhizal dependence. We assessed the responsiveness of Pinus species to EMF using 1,206 contrasts published on 34 species, and matched these data with data on Pinus species invasiveness. Surprisingly, we found that species that are more invasive depend more on mutualisms (EMF). Seedling growth of species with smaller seeds benefited more from mutualisms, indicating a higher dependence. A higher reliance on EMF could be part of a strategy in which small-seeded species produce more seeds that can disperse further, and these species are likely to establish only if facilitated by mycorrhizal fungi. On the contrary, big-seeded species showed a lower dependence on EMF, which may be explained by their tolerance to stressful conditions during establishment. However, the limited dispersal of larger seeds may limit the spread of these species. We present strong evidence against a venerable belief in ecology that species that rely more on mutualisms are less prone to invade, and suggest that in certain circumstances greater reliance on mutualists can increase spread capacity.

Hi-sAFe: A 3D Agroforestry Model for Integrating Dynamic Tree–Crop Interactions

Agroforestry, the intentional integration of trees with crops and/or livestock, can lead to multiple economic and ecological benefits compared to trees and crops/livestock grown separately. Field experimentation has been the primary approach to understanding the tree–crop interactions inherent in agroforestry. However, the number of field experiments has been limited by slow tree maturation and difficulty in obtaining consistent funding. Models have the potential to overcome these hurdles and rapidly advance understanding of agroforestry systems. Hi-sAFe is a mechanistic, biophysical model designed to explore the interactions within agroforestry systems that mix trees with crops. The model couples the pre-existing STICS crop model to a new tree model that includes several plasticity mechanisms responsive to tree–tree and tree–crop competition for light, water, and nitrogen. Monoculture crop and tree systems can also be simulated, enabling calculation of the land equivalent ratio. The model’s 3D and spatially explicit form is key for accurately representing many competition and facilitation processes. Hi-sAFe is a novel tool for exploring agroforestry designs (e.g., tree spacing, crop type, tree row orientation), management strategies (e.g., thinning, branch pruning, root pruning, fertilization, irrigation), and responses to environmental variation (e.g., latitude, climate change, soil depth, soil structure and fertility, fluctuating water table). By improving our understanding of the complex interactions within agroforestry systems, Hi-sAFe can ultimately facilitate adoption of agroforestry as a sustainable land-use practice.

Ectomycorrhizal host specificity in a changing world: can legacy effects explain anomalous current associations?

Despite the importance of ectomycorrhizal (ECM) fungi in forest ecosystems, knowledge about the ecological and co-evolutionary mechanisms underlying ECM host associations remains limited. Using a widely distributed group of ECM fungi known to form tight associations with trees in the family Pinaceae, we characterized host specificity among three unique Suillus-host species pairs using a combination of field root tip sampling and experimental bioassays. We demonstrate that the ECM fungus S. subaureus can successfully colonize Quercus hosts in both field and glasshouse settings, making this species unique in an otherwise Pinaceae-specific clade. Importantly, however, we found that the colonization of Quercus by S. subaureus required co-planting with a Pinaceae host. While our experimental results indicate that gymnosperms are required for the establishment of new S. subaureus colonies, Pineaceae hosts are locally absent at both our field sites. Given the historical presence of Pineaceae hosts before human alteration, it appears the current S. subaureus-Quercus associations represent carryover from past host presence. Collectively, our results suggest that patterns of ECM specificity should be viewed not only in light of current forest community composition, but also as a legacy effect of host community change over time.

Frontiers in alley cropping: Transformative solutions for temperate agriculture

Annual row crops dominate agriculture around the world and have considerable negative environmental impacts, including significant greenhouse gas emissions. Transformative land-use solutions are necessary to mitigate climate change and restore critical ecosystem services. Alley cropping (AC)-the integration of trees with crops-is an agroforestry practice that has been studied as a transformative, multifunctional land-use solution. In the temperate zone, AC has strong potential for climate change mitigation through direct emissions reductions and increases in land-use efficiency via overyielding compared to trees and crops grown separately. In addition, AC provides climate change adaptation potential and ecological benefits by buffering alley crops to weather extremes, diversifying income to hedge financial risk, increasing biodiversity, reducing soil erosion, and improving nutrient- and water-use efficiency. The scope of temperate AC research and application has been largely limited to simple systems that combine one timber tree species with an annual grain. We propose two frontiers in temperate AC that expand this scope and could transform its climate-related benefits: (i) diversification via woody polyculture and (ii) expanded use of tree crops for food and fodder. While AC is ready now for implementation on marginal lands, we discuss key considerations that could enhance the scalability of the two proposed frontiers and catalyze widespread adoption.

Into and out of the tropics: global diversification patterns in a hyperdiverse clade of ectomycorrhizal fungi

Ectomycorrhizal (ECM) fungi, symbiotic mutualists of many dominant tree and shrub species, exhibit a biogeographic pattern counter to the established latitudinal diversity gradient of most macroflora and fauna. However, an evolutionary basis for this pattern has not been explicitly tested in a diverse lineage. In this study, we reconstructed a mega-phylogeny of a cosmopolitan and hyperdiverse genus of ECM fungi, Russula, sampling from annotated collections and utilizing publically available sequences deposited in GenBank. Metadata from molecular operational taxonomic unit cluster sets were examined to infer the distribution and plant association of the genus. This allowed us to test for differences in patterns of diversification between tropical and extratropical taxa, as well as how their associations with different plant lineages may be a driver of diversification. Results show that Russula is most species-rich at temperate latitudes and ancestral state reconstruction shows that the genus initially diversified in temperate areas. Migration into and out of the tropics characterizes the early evolution of the genus, and these transitions have been frequent since this time. We propose the 'generalized diversification rate' hypothesis to explain the reversed latitudinal diversity gradient pattern in Russula as we detect a higher net diversification rate in extratropical lineages. Patterns of diversification with plant associates support host switching and host expansion as driving diversification, with a higher diversification rate in lineages associated with Pinaceae and frequent transitions to association with angiosperms.

Inter-plant communication through mycorrhizal networks mediates complex adaptive behaviour in plant communities

Adaptive behaviour of plants, including rapid changes in physiology, gene regulation and defence response, can be altered when linked to neighbouring plants by a mycorrhizal network (MN). Mechanisms underlying the behavioural changes include mycorrhizal fungal colonization by the MN or interplant communication via transfer of nutrients, defence signals or allelochemicals. We focus this review on our new findings in ectomycorrhizal ecosystems, and also review recent advances in arbuscular mycorrhizal systems. We have found that the behavioural changes in ectomycorrhizal plants depend on environmental cues, the identity of the plant neighbour and the characteristics of the MN. The hierarchical integration of this phenomenon with other biological networks at broader scales in forest ecosystems, and the consequences we have observed when it is interrupted, indicate that underground 'tree talk' is a foundational process in the complex adaptive nature of forest ecosystems.

Defoliation of interior Douglas-fir elicits carbon transfer and stress signalling to ponderosa pine neighbors through ectomycorrhizal networks

Extensive regions of interior Douglas-fir (Pseudotsuga menziesii var. glauca, IDF) forests in North America are being damaged by drought and western spruce budworm (Choristoneura occidentalis). This damage is resulting from warmer and drier summers associated with climate change. To test whether defoliated IDF can directly transfer resources to ponderosa pine (Pinus ponderosae) regenerating nearby, thus aiding in forest recovery, we examined photosynthetic carbon transfer and defense enzyme response. We grew pairs of ectomycorrhizal IDF 'donor' and ponderosa pine 'receiver' seedlings in pots and isolated transfer pathways by comparing 35 μm, 0.5 μm and no mesh treatments; we then stressed IDF donors either through manual defoliation or infestation by the budworm. We found that manual defoliation of IDF donors led to transfer of photosynthetic carbon to neighboring receivers through mycorrhizal networks, but not through soil or root pathways. Both manual and insect defoliation of donors led to increased activity of peroxidase, polyphenol oxidase and superoxide dismutase in the ponderosa pine receivers, via a mechanism primarily dependent on the mycorrhizal network. These findings indicate that IDF can transfer resources and stress signals to interspecific neighbors, suggesting ectomycorrhizal networks can serve as agents of interspecific communication facilitating recovery and succession of forests after disturbance.

Sequestrate fungi from Patagonian Nothofagus forests: Cystangium (Russulaceae, Basidiomycota)

Six species of Cystangium, a genus of sequestrate taxa related to Russula, were collected in Patagonia (Argentina and Chile) during autumn 2001. Two species, C. depauperatum Singer & A.H. Sm. and C. nothofagi (E. Horak) Trappe, Castellano & T. Lebel, were already known from this region, while four new species, C. domingueziae, C. gamundiae, C. grandihyphatum and C. longisterigmatum, are described, illustrated and a key to the species is provided. In addition, sequences of the ITS (rDNA) region were obtained to explore the phylogenetic relationships of our South American Cystangium species.

Hijacking common mycorrhizal networks for herbivore-induced defence signal transfer between tomato plants

Common mycorrhizal networks (CMNs) link multiple plants together. We hypothesized that CMNs can serve as an underground conduit for transferring herbivore-induced defence signals. We established CMN between two tomato plants in pots with mycorrhizal fungus Funneliformis mosseae, challenged a 'donor' plant with caterpillar Spodoptera litura, and investigated defence responses and insect resistance in neighbouring CMN-connected 'receiver' plants. After CMN establishment caterpillar infestation on 'donor' plant led to increased insect resistance and activities of putative defensive enzymes, induction of defence-related genes and activation of jasmonate (JA) pathway in the 'receiver' plant. However, use of a JA biosynthesis defective mutant spr2 as 'donor' plants resulted in no induction of defence responses and no change in insect resistance in 'receiver' plants, suggesting that JA signalling is required for CMN-mediated interplant communication. These results indicate that plants are able to hijack CMNs for herbivore-induced defence signal transfer and interplant defence communication.

Self-organizing systems across scales

Over the past few years, ecologists have increasingly recognized the existence of strong self-reinforcing (or self-organizing) interactions within systems at a variety of scales. Positive feedback within food chains has been reported from terrestrial and aquatic ecosystems. Accumulating evidence supports the existence within communities of cooperative guilds - tit-for-tat relationships based on diffuse mutualisms and favored by environmental unpredictability. At the landscape level, both real world experience and models indicate that processes such as hydrology and the propagation of disturbance can be strongly self-reinforcing (i.e. the landscape structure supports the process, and vice versa). Hence the picture emerges of a hierarchy of self-organizing systems that span food chains, communities and landscapes/regions.

Below-ground carbon transfer among Betula nana may increase with warming in Arctic tundra

• Shrubs are expanding in Arctic tundra, but the role of mycorrhizal fungi in this process is unknown. We tested the hypothesis that mycorrhizal networks are involved in interplant carbon (C) transfer within a tundra plant community. • Here, we installed below-ground treatments to control for C transfer pathways and conducted a (13)CO(2)-pulse-chase labelling experiment to examine C transfer among and within plant species. • We showed that mycorrhizal networks exist in tundra, and facilitate below-ground transfer of C among Betula nana individuals, but not between or within the other tundra species examined. Total C transfer among conspecific B. nana pairs was 10.7 ± 2.4% of photosynthesis, with the majority of C transferred through rhizomes or root grafts (5.2 ± 5.3%) and mycorrhizal network pathways (4.1 ± 3.3%) and very little through soil pathways (1.4 ± 0.35%). • Below-ground C transfer was of sufficient magnitude to potentially alter plant interactions in Arctic tundra, increasing the competitive ability and mono-dominance of B. nana. C transfer was significantly positively related to ambient temperatures, suggesting that it may act as a positive feedback to ecosystem change as climate warms.

Root-associated ectomycorrhizal fungi shared by various boreal forest seedlings naturally regenerating after a fire in interior alaska and correlation of different fungi with host growth responses

The role of common mycorrhizal networks (CMNs) in postfire boreal forest successional trajectories is unknown. We investigated this issue by sampling a 50-m by 40-m area of naturally regenerating black spruce (Picea mariana), trembling aspen (Populus tremuloides), and paper birch (Betula papyrifera) seedlings at various distances from alder (Alnus viridis subsp. crispa), a nitrogen-fixing shrub, 5 years after wildfire in an Alaskan interior boreal forest. Shoot biomasses and stem diameters of 4-year-old seedlings were recorded, and the fungal community associated with ectomycorrhizal (ECM) root tips from each seedling was profiled using molecular techniques. We found distinct assemblages of fungi associated with alder compared with those associated with the other tree species, making the formation of CMNs between them unlikely. However, among the spruce, aspen, and birch seedlings, there were many shared fungi (including members of the Pezoloma ericae [Hymenoscyphus ericae] species aggregate, Thelephora terrestris, and Russula spp.), raising the possibility that these regenerating seedlings may form interspecies CMNs. Distance between samples did not influence how similar ECM root tip-associated fungal communities were, and of the fungal groups identified, only one of them was more likely to be shared between seedlings that were closer together, suggesting that the majority of fungi surveyed did not have a clumped distribution across the small scale of this study. The presence of some fungal ribotypes was associated with larger or smaller seedlings, suggesting that these fungi may play a role in the promotion or inhibition of seedling growth. The fungal ribotypes associated with larger seedlings were different between spruce, aspen, and birch, suggesting differential impacts of some host-fungus combinations. One may speculate that wildfire-induced shifts in a given soil fungal community could result in variation in the growth response of different plant species after fire and a shift in regenerating vegetation.

Spatial elements of mortality risk in old-growth forests

For many species of long-lived organisms, such as trees, survival appears to be the most critical vital rate affecting population persistence. However, methods commonly used to quantify tree death, such as relating tree mortality risk solely to diameter growth, almost certainly do not account for important spatial processes. Our goal in this study was to detect and, if present, to quantify the relevance of such processes. For this purpose, we examined purely spatial aspects of mortality for four species, Abies concolor, Abies magnifica, Calocedrus decurrens, and Pinus lambertiana, in an old-growth conifer forest in the Sierra Nevada of California, USA. The analysis was performed using data from nine fully mapped long-term monitoring plots. In three cases, the results unequivocally supported the inclusion of spatial information in models used to predict mortality. For Abies concolor, our results suggested that growth rate may not always adequately capture increased mortality risk due to competition. We also found evidence of a facilitative effect for this species, with mortality risk decreasing with proximity to conspecific neighbors. For Pinus lambertiana, mortality risk increased with density of conspecific neighbors, in keeping with a mechanism of increased pathogen or insect pressure (i.e., a Janzen-Connell type effect). Finally, we found that models estimating risk of being crushed were strongly improved by the inclusion of a simple index of spatial proximity. Not only did spatial indices improve models, those improvements were relevant for mortality prediction. For P. lambertiana, spatial factors were important for estimation of mortality risk regardless of growth rate. For A. concolor, although most of the population fell within spatial conditions in which mortality risk was well described by growth, trees that died occurred outside those conditions in a disproportionate fashion. Furthermore, as stands of A. concolor become increasingly dense, such spatial factors are likely to become increasingly important. In general, models that fail to account for spatial pattern are at risk of failure as conditions change.

Spatial patterns of ectomycorrhizal fungal inoculum in arbuscular mycorrhizal barrens communities: implications for controlling invasion by Pinus virginiana

Invasion of globally threatened ecosystems dominated by arbuscular mycorrhizal plants, such as the alkaline prairies and serpentine barrens of eastern North America, by species of ectomycorrhizal (ECM) pine (Pinus) seriously threatens the persistence, conservation, and ongoing restoration of these rare plant communities. Using Maryland serpentine barrens and an Ohio alkaline prairie complex as model systems, we tested the hypothesis that the invasiveness of Virginia pine (Pinus virginiana L.) into such communities is regulated by the spatial pattern of ECM fungal inoculum in the soil. ECM colonization of pine seedlings can occur by (1) hyphae growing from the roots of mature ECM pines colonizing nearby seedlings (contagion model), (2) pine seedlings being infected after germinating in open areas where spores are concentrated in feces of animals that have consumed sporocarps (centers of infection model), and (3) colonization from spores that are wind-dispersed across the landscape (background model). To test these models of dispersal of ECM fungal inoculum into these barrens, we used autocorrelation and spatially explicit mapping techniques (semivariance analysis and kriging) to characterize the distribution and abundance of ECM inoculum in soil. Our results strongly suggest that ECM fungi most often disperse into open barrens by contagion, thereby facilitating rapid pine colonization in an advancing front from mature pine forests bordering the barrens. Spatial patterns consistent with the centers of infection model were present but less common. Thus, current management techniques that rely on cutting and fire to reverse pine invasion may be ineffective because they do not kill or disrupt hyphal mats attached to mature roots of neighboring pines. Management alternatives are discussed.

Below-ground interactions with arbuscular mycorrhizal shrubs decrease the performance of pinyon pine and the abundance of its ectomycorrhizas

Few studies have examined how below-ground interactions among plants affect the abundance and community composition of symbiotic mycorrhizal fungi. Here, we combined observations during drought with a removal experiment to examine the effects of below-ground interactions with arbuscular mycorrhizal (AM) shrubs on the growth of pinyon pines (Pinus edulis), and the abundance and community composition of their ectomycorrhizal (EM) fungi. Shrub density was negatively correlated with pinyon above- and below-ground growth and explained 75% of the variation in EM colonization. Consistent with competitive release, pinyon fine-root biomass, shoot length and needle length increased with shrub removal. EM colonization also doubled following shrub removal. EM communities did not respond to shrub removal, perhaps because of their strikingly low diversity. These results suggest that below-ground competition with AM shrubs negatively impacted both pinyons and EM fungi. Similar competitive effects may be observed in other ecosystems given that drought frequency and severity are predicted to increase for many land interiors.

Ectomycorrhizal networks and seedling establishment during early primary succession

Ectomycorrhizal (ECM) fungal mycelia are the main organs for nutrient uptake in many woody plants, and often connect seedlings to mature trees. While it is known that resources are shared among connected plants via common mycorrhizal networks (CMNs), the net effects of CMNs on seedling performance in the field are almost unknown. CMNs of individual ECM fungal species were produced in an early succession volcanic desert by transplanting current-year seedlings of Salix reinii with ECM mother trees that had been inoculated with one of 11 dominant ECM fungal species. Most seedlings were connected to individual CMNs without being infected by other ECM fungi. Although control seedlings showed poor growth under severe nutrient competition with larger nonmycorrhizal mother trees, nutrient acquisition and growth of seedlings connected to CMNs were improved with most fungal species. The positive effects of CMNs on seedling performance were significantly different among ECM fungal species; for example, the maximum difference in seedling nitrogen acquisition was 1 : 5.9. The net effects of individual CMNs in the field and interspecific variation among ECM fungal species are shown.

Douglas-fir ectomycorrhizae in 40- and 400-year-old stands: mycobiont availability to late successional western hemlock

We investigated ectomycorrhizal (EM) fungi in forest stands containing both early successional Douglas-fir and late successional western hemlock at two points in the typical stand development by identifying EM fungi from roots of Douglas-fir and western hemlock in mixed stands. In an early seral stage forest, EM roots of western hemlock seedlings and intermingling 40-year-old Douglas-fir were sampled. In a late seral stage forest, EM roots of trees of both species were sampled in a 400-year-old stand. We use molecular approaches to identify the symbionts from field samples in this descriptive study. In the early seral stage study, >95% of the western hemlock root tips by biomass were colonized by fungi also colonizing Douglas-fir roots. This result supports the prediction that western hemlock can associate with fungi in Douglas-fir EM networks. In the same study, fungi specific to Douglas-fir colonized 14% of its EM root tips. In the late seral stage study, 14% of the western hemlock root tips were colonized by fungi also observed in association with Douglas-fir, a result strongly influenced by sampling issues and likely represents a conservative estimate of multiple host fungi in this old growth setting. Fungi specific to Douglas-fir colonized 25% of its root tip biomass in the old growth study, in tight coralloid clusters within five of the 24 soil samples. The trends revealed in this study corroborate earlier studies suggesting a predominance of multiple host fungi in mixed communities of EM plants. The role of host-specific fungi in these stands remains unclear.

Detection of plot-level changes in ectomycorrhizal communities across years in an old-growth mixed-conifer forest

Understanding spatial and temporal patterns present in ectomycorrhizal fungal community structure is critical to understanding both the scale and duration of the potential impact these fungi have on the plant community. While recent studies consider the spatial structure of ectomycorrhizal communities, few studies consider how this changes over time. Ectomycorrhizal root biomass and the similarity of community composition were measured at scales up to 20 cm replicated in nine plots and over 3 yr. Soil cores were additionally stratified into three depths. Annual occurrence of the dominant ectomycorrhizal species was constant at larger spatial scales but varied more across years at a fine spatial scale. Turnover of ectomycorrhizal species between years was observed frequently at scales < 20 cm. The ectomycorrhizal community within a plot was more similar across years than it was to other plots sampled in the same year. Our results demonstrate the dynamic nature of the ectomycorrhizal community even in the absence of large-scale disturbances. The potential role of root turnover and drought stress is discussed.

Exploring functional definitions of mycorrhizas: Are mycorrhizas always mutualisms?

Mycorrhizas are considered to be classic mutualisms. Here, we define mutualism as a reciprocal increase in fitness of the symbionts, and we review the evidence for mycorrhizal mutualism at the community, whole-plant, and cellular scales. It is difficult to use results of most mycorrhizal studies because (i) fungal contribution to nutrient uptake is not accurately estimated, (ii) increased growth is not necessarily correlated with increased plant fecundity or survival, especially in communities, and (iii) benefits that occur only at certain times of year, or under specific extreme conditions, may not be detected. To produce the nonmycorrhizal controls required to study mutualism in the field, soil microflora and fauna must be severely perturbed; therefore, it is virtually impossible to evaluate effects of mycorrhizas on plant fitness under realistic conditions. Using the evidence available, we conclude that mycorrhizas can occupy various positions along the continuum from parasitism to mutualism, depending on the specific plant and fungal genotypes and their abiotic and biotic environments. Although we discuss the possibility of defining mycorrhizas by some physiological characteristic, we conclude that mycorrhizas should be defined on a structural or developmental basis and that any requirement to demonstrate mutualism be eliminated.Key words: mycorrhiza, mutualism, parasitism, physiology, fitness, community.

Mycorrhizal networks: a review of their extent, function, and importance

It is well known from laboratory studies that a single mycorrhizal fungal isolate can colonize different plant species, form interplant linkages, and provide a conduit for interplant transfer of isotopic carbon, nitrogen, phosphorus, or water. There is increasing laboratory and field evidence that the magnitude and direction of transfer is influenced by physiological source–sink gradients between plants. There is also evidence that mycorrhizal fungi play a role in regulating transfer through their own source–sink patterns, frequency of links, and mycorrhizal dependency. Although it is plausible that connections are extensive in nature, field studies have been hampered by our inability to observe them in situ and by belowground complexity. In future, isotopic tracers, morphological observations, microsatellite techniques, and fluorescent dyes will be useful in the study of networks in nature. Mycorrhizal networks have the potential to influence patterns of seedling establishment, interplant competition, plant diversity, and plant community dynamics, but studies in this area are just beginning. Future plant community studies would benefit from concurrent experimental use of fungal network controls, isotopic labeling, direct observation of interplant linkages, and long-term observation in the field. In this paper, we review recent literature on mycorrhizal networks and interplant carbon transfer, suggest future research directions, and highlight promising scientific approaches.Key words: common mycorrhizal network, carbon transfer, source–sink, establishment, competition, diversity.

Ectomycorrhizal community effects on hybrid spruce seedling growth and nutrition in clearcuts

A diverse community of ectomycorrhizal (ECM) fungi is generally considered beneficial to forest ecosystems, but the function of ECM communities should be considered within an ecological context. The growth of hybrid spruce (Picea glauca (Moench) Voss × Picea sitchensis (Bong.) Carrière) seedlings was compared after transplanting into recent clearcuts, where soil moisture and nitrogen are typically readily available. The seedlings had either a "forest" ECM community (taken from forest gap edges) or a "pioneer" ECM community (taken from disturbed road edges) and were planted at wide and close spacing. After 3 years, morphotype distribution and abundance (64% community similarity between "forest" and "pioneer" seedlings) overlapped considerably, but height growth was 25% greater for the "pioneer" seedlings. There was a reduction in diameter at close spacing, with little difference in competition effects between ECM communities. There were no differences detected in foliar nitrogen concentrations and no evidence of nitrogen or phosphorus deficiencies. The advantage of fungi such as Amphinema byssoides, Thelephora terrestris, and Laccaria laccata might be the proliferation of fine roots that allows for the fullest utilization of abundant soil resources. The results suggest that the ECM communities arising after clearcut disturbances are well adapted to these initial soil conditions.Key words: ectomycorrhiza, disturbance, diversity, productivity, competition, Picea glauca.

Diversity of ectomycorrhizas associated with Quercus garryana in southern Oregon

We investigated diversity of ectomycorrhizas associated with Quercus garryana Dougl. ex Hook. (Oregon white oak, or Garry oak) at Whetstone Savanna Preserve in southern Oregon. Based on morphotyping and DNA restriction fragments, we described 39 ectomycorrhizas. The most common five morphotypes were found in 5% or more of 160 soil cores. Cenococcum geophilum, the most abundant morphotype, occurred in 75% of soil cores. Another common morphotype yielded a restriction fragment length polymorphism (RFLP) pattern similar to that of Tuber species. Uncommon morphotypes were responsible for the majority of ectomycorrhizal diversity on Q. garryana. Morphotype diversity of seedlings was more similar to that of their parent tree than to seedlings under other trees. Internal transcribed spacer (ITS) – RFLP patterns of ectomycorrhizas found beneath sporocarps did not match those of the sporocarps fruiting above ground. An understanding of the diversity of the ectomycorrhizal community on Q. garryana will enable us to compare ectomycorrhizas on other oak species and habitats; determine seasonality of ectomycorrhizal growth; evaluate treatments such as fire, grazing, invasion by exotic plants, and other anthropogenic disturbances; and aid restoration protocols.Key words: biocomplexity, biodiversity, ectomycorrhizas, hypogeous fungi, morphotypes, Peziza infossa, Tuber.

Fine scale distribution of ectomycorrhizal fungi and roots across substrate layers including coarse woody debris in a mixed forest

• Ectomycorrhizal (ECM) fungi are widespread plant root symbionts in boreal forests, but information is lacking on the fine scale distribution of roots and fungi in substrate patches of different quality, including coarse woody debris (CWD). • Wood and soil cores were taken systematically both horizontally and vertically through decayed logs and underlying soil layers in a mixed forest. Root tips were counted and ECM fungi identified by morphotyping and sequencing. • The abundance of root tips and ECM fungi was highly variable on a 5-cm scale. Most species were replaced on a 50-cm scale. Detrended correspondence analysis demonstrated strong preference of resupinate thelephoroid and athelioid fungi and Sebacinaceae for CWD, whereas ascomycetes and euagarics appeared more frequently in mineral soil. Clavulicium delectabile was determined to be an ECM fungus for the first time. • ECM fungi occupy different niches and show variable distribution patterns. CWD plays an important role as a habitat both for roots and ECM fungi. We suggest sampling larger soil cores and selecting random root tips in future studies. Sequencing is a powerful tool in ECM community studies.

Ectomycorrhizal fungal communities in young forest stands regenerating after clearcut logging

The effects on the ectomycorrhizal fungal community of clearcut logging, which is used to harvest millions of hectares of ectomycorrhizal forest annually, has been studied for a number of years. Here, we review current knowledge of inoculum sources for ectomycorrhizal fungi in forests and then re-examine earlier studies of ectomycorrhizas on young trees in regenerating stands. We conclude that, taken separately from the effects of site preparation, the major impact of clearcut logging is to change the species composition of the ectomycorrhizal fungal community rather than to reduce the percentage of roots colonized. A thorough examination of site preparation treatments suggests that the changes in fungal species composition are driven by changes in the biology and chemistry of the soil environment after clearcutting as much as they are by loss or change in fungal inoculum. This is an important conclusion because it implies that these new ectomycorrhizal fungal communities are better adapted to the new conditions than the ones in the forest would have been. The shift in fungal species composition and diversity will have implications for seedling establishment and competition. The effects of individual fungi or diverse assemblages of fungi on seedling growth, and effects of changes in the ability of young trees to associate with a common mycelium are discussed. Contents Summary 399 I. Introduction 400 II. Population biology and inoculum potential of ectomycorrhizal fungi 401 III. Ectomycorrhiza development on seedlings regenerating after clearcut logging 402 IV. Which is the most important factor driving changes in the ECM fungal community after clearcut logging: inoculum loss or change in the below-ground environment? 406 V. Possible consequences for regenerating stands of species shifts in ectomycorrhizal fungi 414 VI. Conclusions 416.

Ectomycorrhizal community structure on western hemlock (Tsuga heterophylla) seedlings transplanted from forests into openings

This study tested whether mature-forest ectomycorrhizal (ECM) communities could be maintained in forest openings on seedlings. Naturally regenerated western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings were transplanted from mature forests into openings and the ECM fungal community was compared after 2 years with similar seedlings planted back into the forests or seedlings from openings planted back into openings. Fewer ECM morphotypes, lower average richness per seedling, and a steeper, less even species distribution curve were found, all of which suggest that the mature-forest ECM fungal community changed after transplanting forest seedlings into the openings. The increased abundance of pioneer fungi such as Thelephora terresteris suggested that many of the mature-forest ECM fungi were unable to maintain or continue root colonization in openings. Results suggest that many mature-forest ECM fungi require further stand development to maintain enough rooting density and hyphal contact to persist.Key words: ectomycorrhizal succession, disturbance, species-importance curves, multistage and late-stage fungi.

Ectomycorrhizal diversity alters growth and nutrient acquisition of grey birch (Betula populifolia) seedlings in host-symbiont culture conditions

•  The influence of ectomycorrhizal fungal diversity on plant performance was investigated by establishing a gradient of ectomycorrhizal diversity on Betula populifolia (grey birch) seedlings. •  We measured growth, as well as N and P uptake, of individual B. populifolia seedlings inoculated with replicate one, two and four species 'communities' of ectomycorrhizal fungi simultaneously and without mycorrhizas in axenic culture. •  Colonization of B. populifolia by individual species of ectomycorrhizal fungi decreased with increasing fungal diversity although total colonization increased. Shoot biomass decreased with increasing ectomycorrhizal diversity and mycorrhizal root biomass increased. Plant biomass did not differ with individual mycorrhizal species or composition. Shoot N concentration showed a small increase with increasing ectomycorrhizal diversity. Whole plant P content and concentration increased across the ectomycorrhizal diversity gradient. Despite higher mycorrhizal colonization rates with increasing fungal diversity, plant growth and nutrient responses were best explained by changes in ectomycorrhizal diversity. •  Greater ectomycorrhizal diversity per se, rather than colonization or composition, increased mycorrhizal root biomass at the expense of shoot biomass and increased P uptake of B. populifolia seedlings.