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Displacement of diverse native plant communities by low-diversity invasive communities is a global problem. In the western United States, the displacement of sagebrush-dominated communities by cheatgrass has increased since the 1920s. Restoration outcomes are poor, potentially due to soil alteration by cheatgrass. We explored the poorly understood role of plant–soil feedbacks in the dominance of cheatgrass in a greenhouse study where uninvaded sagebrush soils were conditioned with either cheatgrass, a native bunchgrass or sagebrush. Sagebrush seedlings were grown in the soils that remained following the removal of conditioning plants. We expected cheatgrass to strongly suppress sagebrush due to a change in belowground microbial communities, conspecifics to have neutral effects and the native bunchgrass to have intermediate effects as it coevolved with sagebrush but belongs to a different functional group. We assessed the effects of conditioning on sagebrush growth, tissue nutrients, and carbon allocation. We also characterized the abundance, diversity and community composition of root microbial associates. Cheatgrass strongly suppressed sagebrush growth at high and low conditioning densities, the native bunchgrass showed suppression at high conditioning densities only and conspecific effects were neutral. Tissue nutrients, amount of root colonization by soil fungi or root microbial community composition were not associated with these plant–soil feedbacks. Although we did not identify the precise mechanism, our results provide key evidence that rapid soil alteration by cheatgrass results in negative plant–soil feedbacks on sagebrush growth. These feedbacks likely contribute to cheatgrass dominance and the poor success of sagebrush restoration. [ABSTRACT FROM AUTHOR]

An important factor controlling tree species diversity is conspecific density dependence (CDD). Adult trees associated with arbuscular mycorrhiza (AM) and ectomycorrhiza (ECM) can exhibit negative and positive CDD effects on conspecific recruitment, respectively. However, the extent to which these mycorrhizal associations affect spatial distributions of individual trees and their relative abundances within forests through CDD remains uncertain. We analysed changes in spatial correlations between adults and conspecific juveniles at different growth stages of five hardwood species in a 6-ha plot of an old-growth forest using a point pattern analysis. The clump sizes of large individuals were also evaluated using the Iδ index (a measure of individual dispersion) in 24 species. In two AM-associated species, juveniles were distributed at greater distances with increasing size or were always distributed at a distance from adults, resulting in small clumps of adults. In contrast, juveniles of two ECM-associated species were distributed close to adults during early or late growth stage, resulting in large clumps of adults. Juveniles of an ECM-associated species disappeared with increasing size, probably due to shade intolerance. In 24 tree species with large numbers of individuals within a plot, the relative basal area was related to both mycorrhizal type and maximum diameter, suggesting that the relative abundance of a species is largely related to its mycorrhizal associations and maximum plant size. This study strongly demonstrated that mycorrhizal associations play an important role in determining the spatial distribution patterns and community structure of tree species through CDD. [ABSTRACT FROM AUTHOR]

The relationship between forest productivity and tree species diversity has been described in detail, but the underlying processes have yet to be identified. One important issue is to understand which processes are at the origin of observed aboveground overyielding in some mixed forests. We used a beech-maple plantation exhibiting aboveground overyielding to test whether belowground processes could explain this pattern. Soil cores were collected to determine fine root (FR) biomass and vertical distribution. Correlograms were used to detect spatial arrangement. Near-infrared reflectance spectroscopy was used to identify the tree species proportion in the FR samples and spatial root segregation. An isotopic approach was used to identify water acquisition patterns. The structure and the composition of the ectomycorrhizal fungal community were determined by high-throughput sequencing of DNA in the soil samples. We found no spatial pattern for FR biomass or for its vertical distribution along the gradients. No vertical root segregation was found, as FR density for both species decreased with depth in a similar way. The two species displayed similar vertical water acquisition profiles as well, mainly absorbing water from shallow soil layers; hence, niche differentiation for water acquisition was not highlighted here. Significant alterations in the fungal community compositions were detected in function of the percentage of maple in the vicinity of beech. Our findings do not support the commonly suggested drivers of aboveground overyielding in species-diverse forests and suggest that competition reduction or between-species facilitation of belowground resource acquisition may not explain the observed aboveground overyielding. [ABSTRACT FROM AUTHOR]

Interactions between species can have cascading effects that shape subsequent interactions. For example, herbivory can induce plant defenses that affect subsequent interactions with herbivores, pathogens, mycorrhizae, and pollinators. Parasitic plants are present in most ecosystems, and play important roles in structuring communities. However, the effects of host herbivory on parasitic plants, and the potential mechanisms underlying such effects, are not well known. We conducted a greenhouse study to ask whether gypsy moth ( Lymantria dispar) damage, host cultivar, and their interaction affected preference of the stem parasite dodder ( Cuscuta spp.) on cranberry hosts ( Vaccinium macrocarpum). We then assessed the mechanisms that could underlie such effects by measuring induced changes in phytohormones and secondary compounds. We found that damage by gypsy moths delayed dodder attachment by approximately 0.3 days when dodder stems were added 2 days after damage, and reduced attachment by more than 50% when dodder stems were added 1 week after host plant damage. Gypsy moth damage significantly increased jasmonic acid (JA) levels, total volatile emissions, and the flavonol, quercetin aglycone, suggesting possible mechanisms underlying variation in dodder ability to locate or attach to hosts. Dodder preference also differed between cranberry cultivars, with the highest attachment on the cultivar that had significantly lower levels of total volatile emissions and total phenolic acids, suggesting that volatile composition and phenolics may mediate dodder preference. Our results indicate that herbivory can reduce subsequent attachment by a highly damaging parasitic plant, demonstrating the potential importance of early damage for shaping subsequent species interactions. [ABSTRACT FROM AUTHOR]

A key question in savanna ecology is how trees and grasses coexist under N limitation. We used N stable isotopes and N content to study N source partitioning across seasons from trees and associated grasses in a semi-arid savanna. We also used N tracer additions to investigate possible redistribution of N by trees to grasses. Foliar stable N isotope ratio (δN) values were consistent with trees and grasses using mycorrhiza-supplied N in all seasons except in the wet season when they switched to microbially fixed N. The dependence of trees and grasses on mineralized soil N seemed highly unlikely based on seasonal variation in mineralization rates in the Kruger Park region. Remarkably, foliar δN values were similar for all three tree species differing in the potential for N fixation through nodulation. The tracer experiment showed that N was redistributed by trees to understory grasses in all seasons. Our results suggest that the redistribution of N from trees to grasses and uptake of N was independent of water redistribution. Although there is overlap of N sources between trees and grasses, dependence on biological sources of N coupled with redistribution of subsoil N by trees may contribute to the coexistence of trees and grasses in semi-arid savannas. [ABSTRACT FROM AUTHOR]

Foliar nitrogen isotope (δN) composition patterns have been linked to soil N, mycorrhizal fractionation, and within-plant fractionations. However, few studies have examined the potential importance of the direct foliar uptake of gaseous reactive N on foliar δN. Using an experimental set-up in which the rate of mycorrhizal infection was reduced using a fungicide, we examined the influence of mycorrhizae on foliar δN in potted red maple ( Acer rubrum) seedlings along a regional N deposition gradient in New York State. Mycorrhizal associations altered foliar δN values in red maple seedlings from 0.06 to 0.74 ‰ across sites. At the same sites, we explored the predictive roles of direct foliar N uptake, soil δN, and mycorrhizae on foliar δN in adult stands of A. rubrum, American beech ( Fagus grandifolia), black birch ( Betula lenta), and red oak ( Quercus rubra). Multiple regression analysis indicated that ambient atmospheric nitrogen dioxide (NO) concentration explained 0, 69, 23, and 45 % of the variation in foliar δN in American beech, red maple, red oak, and black birch, respectively, after accounting for the influence of soil δN. There was no correlation between foliar δC and foliar %N with increasing atmospheric NO concentration in most species. Our findings suggest that total canopy uptake, and likely direct foliar N uptake, of pollution-derived atmospheric N deposition may significantly impact foliar δN in several dominant species occurring in temperate forest ecosystems. [ABSTRACT FROM AUTHOR]

Positive feedbacks have been suggested as a means for non-indigenous species to successfully invade novel environments. Frequency-dependent feedbacks refer to a species performance being dependent on its local abundance in the population; however, frequency dependence is often described as a monolithic trait of a species rather than examining the variation in response for individual genotypes and fitness traits. Here, we investigate frequency-dependent outcomes for individual genotypes and fitness-related traits for the invasive grass Phalaris arundinacea. We tested for competition-mediated frequency dependence by establishing hexagonal arrays with the center target plant surrounded by either same, different or no genotype neighbors to determine how changing the small-scale frequency neighborhood-influenced invasion success. We used a Bayesian ANOVA approach which allowed us to easily accommodate our non-normal dataset and found that same neighbor plots had greater biomass production than different neighbor plots. Target plants also had greater stem height and aboveground biomass when surrounded by same genotype neighbors. A greenhouse experiment did not support the hypothesis that increased mycorrhizal associations were the cause of positive frequency dependence. We devised a frequency-dependent metric to quantify the extent of fitness-related differences for individual genotypes and found that individual genotypes showed a range of both positive and negative responses to different frequency treatments; however, only positive responses were statistically significant. The small-scale genotypic neighborhood had no effect for the fitness-related traits of leaf number, belowground biomass and total biomass. We demonstrate that individual invasive genotypes respond differently to changing frequency neighborhoods and that growth responses do not respond with the same direction and magnitude. A range of frequency-dependent responses may allow genotypes to invade a wide range of environments. [ABSTRACT FROM AUTHOR]

Allelochemicals defend plants against herbivore and pathogen attack aboveground and belowground. Whether such plant defenses incur ecological costs by reducing benefits from plant mutualistic symbionts is largely unknown. We explored a potential trade-off between inherent plant chemical defense and belowground mutualism with arbuscular mycorrhizal fungi (AMF) in Plantago lanceolata L., using plant genotypes from lines selected for low and high constitutive levels of the iridoid glycosides (IG) aucubin and catalpol. As selection was based on IG concentrations in leaves, we first examined whether IG concentrations covaried in roots. Root and leaf IG concentrations were strongly positively correlated among genotypes, indicating genetic interdependence of leaf and root defense. We then found that root AMF arbuscule colonization was negatively correlated with root aucubin concentration. This negative correlation was observed both in plants grown with monocultures of Glomus intraradices and in plants colonized from whole-field soil inoculum. Overall, AMF did not affect total biomass of plants; an enhancement of initial shoot biomass was offset by a lower root biomass and reduced regrowth after defoliation. Although the precise effects of AMF on plant biomass varied among genotypes, plants with high IG levels and low AMF arbuscule colonization in roots did not produce less biomass than plants with low IG and high AMF arbuscule colonization. Therefore, although an apparent trade-off was observed between high root chemical defense and AMF arbuscule colonization, this did not negatively affect the growth responses of the plants to AMF. Interestingly, AMF induced an increase in root aucubin concentration in the high root IG genotype of P. lanceolata. We conclude that AMF does not necessarily stimulate plant growth, that direct plant defense by secondary metabolites does not necessarily reduce potential benefits from AMF, and that AMF can enhance concentrations of root chemical defenses, but that these responses are plant genotype-dependent. [ABSTRACT FROM AUTHOR]

Plant communities can be affected both by arbuscular mycorrhizal fungi (AMF) and hemiparasitic plants. However, little is known about the interactive effects of these two biotic factors on the productivity and diversity of plant communities. To address this question, we set up a greenhouse study in which different AMF inocula and a hemiparasitic plant ( Rhinanthus minor) were added to experimental grassland communities in a fully factorial design. In addition, single plants of each species in the grassland community were grown with the same treatments to distinguish direct AMF effects from indirect effects via plant competition. We found that AMF changed plant community structure by influencing the plant species differently. At the community level, AMF decreased the productivity by 15–24%, depending on the particular AMF treatment, mainly because two dominant species, Holcus lanatus and Plantago lanceolata, showed a negative mycorrhizal dependency. Concomitantly, plant diversity increased due to AMF inoculation and was highest in the treatment with a combination of two commercial AM strains. AMF had a positive effect on growth of the hemiparasite, and thereby induced a negative impact of the hemiparasite on host plant biomass which was not found in non-inoculated communities. However, the hemiparasite did not increase plant diversity. Our results highlight the importance of interactions with soil microbes for plant community structure and that these indirect effects can vary among AMF treatments. We conclude that mutualistic interactions with AMF, but not antagonistic interactions with a root hemiparasite, promote plant diversity in this grassland community. [ABSTRACT FROM AUTHOR]

The distribution of dry Douglas-fir forests in western North America is expected to shift northward with climate change and disappear from the grassland interface in the southern interior of British Columbia. This shift may be accentuated by clearcutting, a common harvesting practice that aims to reduce the competitive effects of residual mature trees on new regeneration, but in so doing, ignores their facilitative effects. In this study, we investigated the net effects of competition from and facilitation by mature trees retained on harvested sites on seedling establishment in the dry interface Douglas-fir forests. We demonstrate that access to a mycorrhizal network (MN) and proximity to trees have important influences on seedling performance. On six sites, we established trenched plots around 24 mature Pseudotsuga menziesii var. glauca (Douglas-fir) trees, then planted Douglas-fir seedlings into four mesh treatments that served to restrict MN access (i.e., planted into mesh bags with 0.5-, 35-, or 250-μm pores, or without mesh) or into impermeable bags (grown in isolation) at four distances (0.5, 1.0, 2.5, or 5.0 m). Seedling survival tended to be greater and water stress lower where seedlings had full access to the MN. Seedling height, shoot biomass, needle biomass, and nutrient uptake peaked at 2.5–5.0 m from mature trees. Seedlings 0.5 m from mature trees had lower CO2 assimilation rates and wood δ13C compared to seedlings 5.0 m away. Competition for soil resources was highest near mature trees but facilitation was relatively greater at further distances, resulting in a zone of net benefit for seedlings. These results show that intraspecific tree-seedling interactions are both competitive and facilitative in dry Douglas-fir forests, and that they are spatially dependent. After disturbance, maintaining residual mature trees may be important for their beneficial regeneration zones. [ABSTRACT FROM AUTHOR]

Ectomycorrhizal (EM) basidiomycete fungi are obligate mutualists of pines and hardwoods that receive fixed C from the host tree. Though they often share most recent common ancestors with wood-rotting fungi, it is unclear to what extent EM fungi retain the ability to express enzymes that break down woody substrates. In this study, we tested the hypothesis that the dominant EM fungus in a pure pine system retains the ability to produce enzymes that break down woody substrates in a natural setting, and that this ability is inducible by reduction of host photosynthetic potential via partial defoliation. To achieve this, pines in replicate blocks were defoliated 50% by needle removal, and enzyme activities were measured in individual EM root tips that had been treated with antibiotics to prevent possible bacterial activity. Results indicate that the dominant EM fungal species ( Suillus granulatus) expressed all enzymes tested (endocellulase D-glucosidase, laccase, manganese peroxidase, lignin peroxidase, phosphatase and protease), and that activities of these enzymes increased significantly ( P < 0.001) in response to defoliation. Thus, this EM fungus (one of the more specialized mutualists of pine) has the potential to play a significant role in C, N and P cycling in this forested ecosystem. Therefore, many above-ground factors that reduce photosynthetic potential or divert fixed C from roots may have wide-reaching ecosystem effects. [ABSTRACT FROM AUTHOR]

Nitrogen isotopes (15N/14N ratios, expressed as δ15N values) are useful markers of the mycorrhizal role in plant nitrogen supply because discrimination against 15N during creation of transfer compounds within mycorrhizal fungi decreases the 15N/14N in plants (low δ15N) and increases the 15N/14N of the fungi (high δ15N). Analytical models of 15N distribution would be helpful in interpreting δ15N patterns in fungi and plants. To compare different analytical models, we measured nitrogen isotope patterns in soils, saprotrophic fungi, ectomycorrhizal fungi, and plants with different mycorrhizal habits on a glacier foreland exposed during the last 100 years of glacial retreat and on adjacent non-glaciated terrain. Since plants during early primary succession may have only limited access to propagules of mycorrhizal fungi, we hypothesized that mycorrhizal plants would initially be similar to nonmycorrhizal plants in δ15N and then decrease, if mycorrhizal colonization were an important factor influencing plant δ15N. As hypothesized, plants with different mycorrhizal habits initially showed similar δ15N values (−4 to −6‰ relative to the standard of atmospheric N2 at 0‰), corresponding to low mycorrhizal colonization in all plant species and an absence of ectomycorrhizal sporocarps. In later successional stages where ectomycorrhizal sporocarps were present, most ectomycorrhizal and ericoid mycorrhizal plants declined by 5–6‰ in δ15N, suggesting transfer of 15N-depleted N from fungi to plants. The values recorded (−8 to −11‰) are among the lowest yet observed in vascular plants. In contrast, the δ15N of nonmycorrhizal plants and arbuscular mycorrhizal plants declined only slightly or not at all. On the forefront, most ectomycorrhizal and saprotrophic fungi were similar in δ15N (−1 to −3‰), but the host-specific ectomycorrhizal fungus Cortinarius tenebricus had values of up to 7‰. Plants, fungi and soil were at least 4‰ higher in δ15N from the mature site than in recently exposed sites. On both the forefront and the mature site, host-specific ectomycorrhizal fungi had higher δ15N values than ectomycorrhizal fungi with a broad host range. From these isotopic patterns, we conclude:(1) large enrichments in 15N of many ectomycorrhizal fungi relative to co-occurring ectomycorrhizal plants are best explained by treating the plant-fungal-soil system as a closed system with a discrimination against 15N of 8–10‰ during transfer from fungi to plants, (2) based on models of 15N mass balance, ericoid and ectomycorrhizal fungi retain up to two-thirds of the N in the plant-mycorrhizal system under the N-limited conditions at forefront sites, (3) sporocarps are probably enriched in 15N by an additional 3‰ relative to available nitrogen, and (4) host-specific ectomycorrhizal fungi may transfer more N to plant hosts than non-host-specific ectomycorrhizal fungi. Our study confirms that nitrogen isotopes are a powerful tool for probing nitrogen dynamics between mycorrhizal fungi and associated plants. [ABSTRACT FROM AUTHOR]

Foliar δ15N, %N and %P in the dominant woody and herbaceous species across nutrient gradients in New Zealand restiad (family Restionaceae) raised bogs revealed marked differences in plant δ15N correlations with P. The two heath shrubs, Leptospermum scoparium (Myrtaceae) and Dracophyllum scoparium (Epacridaceae), showed considerable isotopic variation (−2.03 to −15.55‰, and −0.39 to −12.06‰, respectively) across the bogs, with foliar δ15N strongly and positively correlated with P concentrations in foliage and peat, and negatively correlated with foliar N:P ratios. For L. scoparium, the isotopic gradient was not linked to ectomycorrhizal (ECM) fractionation as ECMs occurred only on higher nutrient marginal peats where 15N depletion was least. In strong contrast, restiad species ( Empodisma minus Sporadanthus ferrugineus, S. traversii) showed little isotopic variation across the same nutrient gradients. Empodisma minus and S. traversii had δ15N levels consistently around 0‰ (means of −0.12‰ and +0.15‰ respectively), and S. ferrugineus, which co-habited with E. minus, was more depleted (mean −4.97‰). The isotopic differences between heath shrubs and restiads were similar in floristically dissimilar bogs and may be linked to contrasting nutrient demands, acquisition mechanisms, and root morphology. Leptospermum scoparium shrubs on low nutrient peats were stunted, with low tissue P concentrations, and high N:P ratios, suggesting they were P-limited, which was probably exacerbated by markedly reduced mycorrhizal colonisations. The coupling of δ15N depletion and %P in heath shrubs suggests that N fractionation is promoted by P limitation. In contrast, the constancy in δ15N of the restiad species through the N and P gradients suggests that these are not suffering from P limitation. [ABSTRACT FROM AUTHOR]

The functioning of the plant-mycorrhiza system depends on interactions with other organisms, including saprotrophic (ST) soil fungi. The interactions between mycorrhizal and ST fungi are likely affected by fungivorous soil animals, such as Collembola. In a two-factorial laboratory experiment lasting for 30 weeks we assessed the effects of an arbuscular mycorrhizal fungus (Glomus mosseae) and Collembola (Protaphorura fimata, Heteromurus nitidusandFolsomia candida) on the community composition of ST microfungi in soil planted with the invasive grassCynodon dactylon. The presence of mycorrhiza substantially reduced total plant biomass and reduced N and P availability to the soil microflora, though these effects were less pronounced in the presence of Collembola. The density of Collembola was high (corresponding to about 2×105 individuals m-2) and was not affected by the presence ofG. mosseae. In spite of the large amount of mycorrhizal mycelium in soil, it contributed little to Collembola nutrition. The presence of mycorrhiza strongly affected the community structure of ST soil fungi. In particular, mycorrhiza reduced the relative abundance ofTrichoderma harzianumandExophialasp., but increased the abundance ofRamichloridium schulzeriand several sterile forms. However, the difference between fungal communities in mycorrhizal and non-mycorrhizal treatments was much more pronounced in the presence of Collembola. Presumably, the intense grazing by Collembola destabilized the ST fungal community, thereby making it more susceptible to the influence ofG. mosseae. These results document for the first time that fungal feeding soil invertebrates can significantly affect the interactions between mycorrhizal fungi and ST soil microorganisms. [ABSTRACT FROM AUTHOR]

Extreme arctic-alpine vegetation has relatively low affinity to form mycorrhizal symbiosis. We asked whether the mycorrhizal growth benefit for the host plant is lower at low temperatures. We investigated the role of two root-associated fungi and temperature in growth, carbon–nitrogen relations and germination of an arctic-alpine herb. Seeds of Gnaphalium norvegicum were germinated at 8° or 15show $132#°show $132#C with or without arbuscular mycorrhizal (AM, Glomus claroideum) and dark septate endophytic (DSE, Phialocephala fortinii) inocula in a climate chamber. We found that germination percentage, shoot and root biomass, shoot N% and root AM colonization were lower at 8show $132#°show $132#C than at 15show $132#°show $132#C. P. fortinii inoculation had a positive impact on germination at both temperatures, whereas G. claroideum produced no effect. N% was lower in AM plants at both temperatures. Plant biomass and shoot N content were higher in AM plants than in control plants at 15show $132#°show $132#C, but not at 8show $132#°show $132#C. DSE inoculation tended also to have positive effects on plant biomass and N content at 15show $132#°show $132#C. At 15show $132#°show $132#C, rate of photosynthesis, photosynthetic nutrient use efficiency and specific leaf area were positively affected by G. claroideum, which suggests that G. claroideum formed a carbon sink and possibly enhanced the seedling water economy. The positive effects of P. fortinii were probably due to its saprotrophic function in the substrate because it did not colonize the roots. These results suggest that the effects of AM and DSE on plant growth are affected by temperature and that the mycorrhizal benefit for the host plant was lower at the lower temperature. Low saprotrophic activity and decreased mycorrhiza-mediated nutrient acquisition may thus constrain plant nutrient acquisition in cold environments. Decreased mycorrhizal benefit may be related to the comparatively low mycotrophy of cold environment vegetation. [ABSTRACT FROM AUTHOR]

The Alaskan tussock tundra is a strongly nutrient-limited ecosystem, where almost all vascular plant species are mycorrhizal. We established a long-term removal experiment to document effects of arctic plant species on ecto- and ericoid mycorrhizal fungi and to investigate whether species interactions and/or nutrient availability affect mycorrhizal colonization. The treatments applied were removal of Betula nana (Betulaceae, dominant deciduous shrub species), removal of Ledum palustre (Ericaceae, dominant evergreen shrub species), control (no removal), and each of these three treatments with the addition of fertilizer. After 3 years of Ledum removal and fertilization, we found that overall ectomycorrhizal colonization in Betula was significantly reduced. Changes in ectomycorrhizal morphotype composition in removal and fertilized treatments were also observed. These results suggest that the effect of Ledum on Betula 's mycorrhizal roots is due to sequestration of nutrients by Ledum, leading to reduced nutrient availability in the soil. In contrast, ericoid mycorrhizal colonization was not affected by fertilization, but the removal of Betula and to a lower degree of Ledum resulted in a reduction of ericoid mycorrhizal colonization suggesting a direct effect of these species on ericoid mycorrhizal colonization. Nutrient availability was only higher in fertilized treatments, but caution should be taken with the interpretation of these data as soil microbes may effectively compete with the ion exchange resins for the nutrients released by plant removal in these nutrient-limited soils. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal (AM) fungi are mutualists with plant roots that are proposed to enhance plant community diversity. Models indicate that AM fungal communities could maintain plant diversity in forests if functionally different communities are spatially separated. In this study we assess the spatial and temporal distribution of the AM fungal community in a wet tropical rainforest in Costa Rica. We test whether distinct fungal communities correlate with variation in tree life history characteristics, with host tree species, and the relative importance of soil type, seasonality and rainfall. Host tree species differ in their associated AM fungal communities, but differences in the AM community between hosts could not be generalized over life history groupings of hosts. Changes in the relative abundance of a few common AM fungal species were the cause of differences in AM fungal communities for different host tree species instead of differences in the presence and absence of AM fungal species. Thus, AM fungal communities are spatially distinguishable in the forest, even though all species are widespread. Soil fertility ranging between 5 and 9 Mg/ha phosphorus did not affect composition of AM fungal communities, although sporulation was more abundant in lower fertility soils. Sampling soils over seasons revealed that some AM fungal species sporulate profusely in the dry season compared to the rainy season. On one host tree species sampled at two sites with vastly different rainfall, relative abundance of spores from Acaulospora was lower and that of Glomus was relatively higher at the site with lower and more seasonal rainfall. [ABSTRACT FROM AUTHOR]

A large number of herbaceous and woody plants from tropical woodland, savanna, and monsoon forest were analysed to determine the impact of environmental factors (nutrient and water availability, fire) and biological factors (microbial associations, systematics) on plant δ15N values. Foliar δ15N values of herbaceous and woody species were not related to growth form or phenology, but a strong relationship existed between mycorrhizal status and plant δ15N. In woodland and savanna, woody species with ectomycorrhizal (ECM) associations and putative N2-fixing species with ECM/arbuscular (AM) associations had lowest foliar δ15N values (1.0–0.6‰), AM species had mostly intermediate δ15N values (average +0.6‰), while non-mycorrhizal Proteaceae had highest δ15N values (+2.9 to +4.1‰). Similar differences in foliar δ15N were observed between AM (average 0.1 and 0.2‰) and non-mycorrhizal (average +0.8 and +0.3‰) herbaceous species in woodland and savanna. Leguminous savanna species had significantly higher leaf N contents (1.8–2.5% N) than non-fixing species (0.9–1.2% N) indicating substantial N acquisition via N2 fixation. Monsoon forest species had similar leaf N contents (average 2.4% N) and positive δ15N values (+0.9 to +2.4‰). Soil nitrification and plant NO3- use was substantially higher in monsoon forest than in woodland or savanna. In the studied communities, higher soil N content and nitrification rates were associated with more positive soil δ15N and plant δ15N. In support of this notion, Ficus, a high NO3- using taxa associated with NO3- rich sites in the savanna, had the highest δ15N values of all AM species in the savanna. δ15N of xylem sap was examined as a tool for studying plant δ15N relations. δ15N of xylem sap varied seasonally and between differently aged Acacia and other savanna species. Plants from annually burnt savanna had significantly higher δ15N values compared to plants from less frequently burnt savanna, suggesting that foliar 15N natural abundance could be used as marker for assessing historic fire regimes. Australian woodland and savanna species had low leaf δ15N and N content compared to species from equivalent African communities indicating that Australian biota are the more N depauperate. The largest differences in leaf δ15N occurred between the dominant ECM Australian and African savanna (miombo) species, which were depleted and enriched in 15N, respectively. While the depleted δ15N of Australian ECM species are similar to those of previous reports on ECM species in natural plant communities, the 15N-enriched δ15N of African ECM species represent an anomaly. [ABSTRACT FROM AUTHOR]

Symbiotic mycorrhizal fungi play an important role in the absorption of soil nutrients and water by most plants. It has been suggested that hydraulically lifted water might maintain the integrity of the external mycorrhizal mycelium during drought. We tested this hypothesis in the obligately mycorrhizal species, coast live oak (Quercus agrifolia), using a microcosm system that separated the effects of hydraulic lift in roots from those in the external mycelium. Mycorrhizal oak seedlings were established in microcosms comprising three discrete compartments for (1) upper roots, (2) tap roots, and (3) external fungal mycelium. Eight months after planting, a drought treatment was initiated: irrigation to the upper root and fungal chambers was terminated and only irrigation to the taproot compartment was maintained. After 3, 12, 30, 50, 70 and 80 days of drought, tracers were injected into the taproot compartment at dusk. At dawn the following morning, mycorrhizal hyphae (EM and AM) and spores (AM) in upper root and fungal compartments were extensively labeled with the tracers. In contrast, no labeling was observed when tracers were injected into the taproot compartment during daytime. Nocturnal water translocation from plant to mycorrhizal fungi occurred in association with hydraulic lift. Saprotrophic/parasitic fungi in the microcosms were not labeled, suggesting a direct water transfer from plants to their mycorrhizal mutualists and not to other fungi in the soil. Even after prolonged drought (70–80 days), mycorrhizal hyphae persisted in soils with water potential values as low as –20 MPa. Maintaining mycorrhizal activity through direct water translocation could potentially improve the nutrient status of deep-rooted plants during periods when the fertile upper soil is dry. [ABSTRACT FROM AUTHOR]

We investigated the interactive effects of soil phosphorus (P) heterogeneity, plant density and mycorrhizal symbiosis on plant growth and size variability of Trifolium subterraneum. We set up mesocosms (trays 49×49 cm and 12 cm deep) with the same amount of available P, but distributed either homogeneously or heterogeneously, in randomly arranged cells (7×7 cm each) with high or low available P. The trays were planted with either 1 or 4 seedlings of T. subterraneum per cell. Half of the trays were inoculated with spores of the mycorrhizal fungus Gigaspora margarita. We harvested the plants when leaves just started to overlap, 8 weeks after planting. Plants growing in high P cells had the lowest percentage infection, but the highest mean shoot and root biomass and root length. The mean size of the plants in each cell was determined mainly by local P concentration. However, in plants growing in high density, low P cells, ca. 20% of the variability in plant biomass was explained by the number of adjacent cells with high P. Patchy trays had the highest total shoot biomass, independently of mycorrhizal infection or plant density. Inoculated trays (M) had higher total shoot biomass and relative competition intensity (measured as reduction in plant biomass due to increased density) than non-inoculated trays (NM). Plant density reduced the plant response to mycorrhizal infection, and its effect was independent of P distribution. All populations growing in patchy trays, and low density mycorrhizal ones, had the highest plant-size inequality, presumably because patchy distribution of P and mycorrhizal infection increased competitive asymmetry. We conclude that mycorrhizal symbiosis has the potential to strongly influence plant population structure when soil nutrient distribution is heterogeneous because it promotes pre-emption of limiting resources. [ABSTRACT FROM AUTHOR]

A series of field and laboratory experiments were conducted to examine whether natural levels of insect herbivory affect the arbuscular mycorrhizal (AM) colonization of two plant species. The plant species were the highly mycorrhizal (mycotrophic) Plantago lanceolata, which suffers small amounts of insect damage continuously over a growing season and the weakly mycorrhizal (non-mycotrophic) Senecio jacobaea, which is frequently subject to rapid and total defoliation by moth larvae. Herbivory was found to reduce AM colonization in P. lanceolata, but had no effect in S. jacobaea. Similarly, AM colonization reduced the level of leaf damage in P. lanceolata, but had no such effect in S. jacobaea. AM fungi were found to increase growth of P. lanceolata, but this effect was only clearly seen when insects were absent. AM fungi reduced the growth of S. jacobaea irrespective of whether insects were present. It is concluded that the reduction of AM fungal colonization by herbivory in P. lanceolata is due to the reduced amount of photosynthate available to the symbiont. This may only become apparent at threshold levels of insect damage and, below these, increased photosynthesis elicited by the mycorrhiza is able to compensate for foliage loss to the insects. However, in S. jacobaea, the mycorrhiza appears to be an aggressive parasite and insect attack only exacerbates the reduction in biomass. In mycotrophic plants, insect herbivores may be responsible for poor functioning of the symbiosis in field conditions and there is a symmetrical interaction between insects and fungi. However, in non-mycotrophic plants, the interaction is strongly asymmetrical, being entirely in favour of the mycorrhiza. [ABSTRACT FROM AUTHOR]

Although variation in the effectiveness of the plant-mycorrhizal association has been demonstrated among plant species and among crop cultivars, the level of specificity of the plant-mycorrhizal association in natural populations and, in particular, its effect on intraspecific plant interactions have not been explored. To examine the influence of the specificity of the plant-mycorrhizal association on intraspecific interactions in Allium vineale, we surrounded target plants with either genetically identical neighbors, neighbors from the same population, or neighbors from a different population, and planted them in pots either with or without a soil fungi community. Overall, the presence of a soil fungal community was beneficial for plant growth, and thus the soil fungal community effects in this system are dominated by the mutualistic effect of mycorrhizae. In addition, both the target plant and neighbor plants had a relatively greater benefit from the mycorrhizal association if the neighbors were genetically identical or from the same population than if the neighbors came from a different population. Thus, there is specificity in the interaction between A. vineale plants and the soil fungal community at the population level, and this specificity favors intraspecific interactions among plants from the same population. In addition, in the presence of the soil fungal community, the variance in plant size within a pot was greater among neighbors from a different population. Therefore the growth advantage for plants growing with neighbors from the same population was not due to pre-emption of resources by individual plants. This finding provides indirect evidence to support the hypothesis that plants from the same population were able to share a more efficient hyphal network. [ABSTRACT FROM AUTHOR]

Results from pot and microcosm studies in the greenhouse have shown that plant growth and foliar chemistry is altered by the presence and species composition of arbuscular mycorrhizal fungi (AMF). The growth and survival of herbivores which feed on plants could, as a consequence, also be affected by these mutualistic soil fungi. Consequently, interactions between AMF, plants and herbivores could occur. To test this, larvae of the common blue butterfly, Polyommatus icarus (Lycaenidae), were fed with sprigs of Lotus corniculatus (Fabaceae) plants which were inoculated with one of two different AMF species, with a mixture of these AMF species or with sprigs of plants which were not inoculated with AMF. Survival and larval weight of third instar larvae fed with plants colonised by AMF were greater than those of larvae fed with non-mycorrhizal plants. Survival of larvae feeding on non-mycorrhizal plants was 1.6 times lower than that of larvae feeding on plants inoculated with a mixture of AMF species and 3.8 times lower than that of larvae feeding on plants inoculated with single AMF species. Furthermore, larvae fed with non-mycorrhizal plants attained only about half the weight of larvae fed with mycorrhizal plants after 11 days of growth. These differences in larval performance might be explained by differences in leaf chemistry, since mycorrhizal plants had a 3 times higher leaf P concentration and a higher C/N-ratio. Our results, thus, show that the presence of belowground mutualistic soil fungi influences the performance of aboveground herbivores by altering their food quality. Larval consumption, larval food use and adult lipid concentrations of the common blue butterfly differed between larvae which were fed with plants inoculated with different AMF species. This suggests that the performance of herbivores is not only influenced by the presence of AMF but also depends on the identity of the AMF species colonising the host plants. Moreover, a significant interaction term between AMF species and maternal identity of the larvae occurred for adult dry weight, indicating that the performance of offspring from different females was differently influenced by AMF species composition. To our knowledge, these results show for the first time that the species composition of AMF communities can influence life-history traits of butterfly larvae and possibly herbivores in general. [ABSTRACT FROM AUTHOR]

Mycorrhizal plants are commonly believed to direct much more of their photosynthates into the soil than non-mycorrhizal plants. As the growth of most organisms of the detrital food web is limited by energy, the flow of C through mycorrhizal plants into the below-ground milieu is widely assumed to nourish a variety of decomposer organisms in soils. In the current experiment, I explored whether some representatives of soil mesofauna, either fungivores or microbi-detritivores, derive benefit from the presence of ectomycorrhizal (EM) fungi growing on the roots of Scots pine (Pinus sylvestris). I also investigated whether the role of soil mesofauna in affecting pine growth depends on the presence of EM fungi in the pine rhizosphere. The study was established in microcosms with a mixture of raw humus and sand. The soil was defaunated, reinoculated with 10 species of soil bacteria and 11 species of saprophytic soil fungi, and pine seedlings, either infected or non-infected with four taxa of EM fungi, were planted in the microcosms. Five treatments with different food web configurations were established: (1) saprophytic microbes alone, (2) as (1) but with the omnivorous enchytraeid species Cognettia sphagnetorum present, (3) as (1) but with Collembola (Hypogastrura assimilis), (4) as (1) but with four species of oribatid mites (Acari) involved, and (5) as 1) but with C. sphagnetorum, H. assimilis and the Acari. The microcosms were incubated in a climate chamber with varying temperature and illumination regimes for two growing periods for the pine. After 60 weeks, pine biomass production was significantly greater in the mycorrhizal systems, the total biomass being 1.43 times higher in the presence than absence of EM fungi. Similarly, almost ten times more fungal biomass was detected on pine roots growing in the mycorrhizal than in the non-mycorrhizal systems. The presence of EM fungi was also associated with significantly lowered pH and percent organic matter of the soil. Despite the clearly larger biomass of both the pines and the fungi on the pine roots, neither the numbers nor biomasses of the mesofauna differed significantly between the EM and non-EM systems. The presence of Collembola and C. sphagnetorum had a positive influence on pine growth, particularly in the absence of EM fungi, whereas oribatid mites had no effects on pine growth. The complexity of the mesofaunal community was not related to the biomass production of the pines in a straightforward manner; for example, the complex systems with each faunal group present did not produce more pine biomass than the simple systems where C. sphagnetorum existed alone. The results of this experiment suggest that the short-term role of EM fungi in fuelling the detrital food web is less significant than generally considered, but that their role as active decomposers and/or stimulators of the activity of saprophytic microbes can be more important than is often believed. [ABSTRACT FROM AUTHOR]

Conversion of European heathlands to grassland has been reported as a response to increased nutrient availability, especially of nitrogen; a direct effect upon mycorrhizal colonization has been proposed as an likely explanation.This hypothesis was tested in a random block experiment with four blocks and four replicates on a Danish inland heath, Hjelm Hede. Ammonium nitrate was applied (0, 35, 50 and 70 kg N ha–1 year–1) to a stand of Calluna vulgaris (L.) Hull four times annually for 2 years. Calluna roots were sampled on four occasions in the 2nd year of the nitrogen treatment. The extent of ericoid mycorrhizal colonization was determined by direct observation of the roots using a line-intersection method. The nitrogen content of the current-year shoots of Calluna increased when they were treated with nitrogen. Nitrogen fertilization had no significant effects on ericoid mycorrhizal colonization of Calluna nor on root biomass. The seasonal variation in mycorrhizal colonization of the Calluna roots was highly significant. The spatial variability of mycorrhizal colonization, both in replicated plots and in the two contrasted soil horizons – the mor layer and the bleached sand – within the plots, were considerable. I conclude that heather decline under enhanced nitrogen input is unlikely to be caused by a direct impact on the ericoid mycorrhizae of Calluna. [ABSTRACT FROM AUTHOR]

N natural abundances of soil total N, roots and mycorrhizas were studied in surface soil profiles in coniferous and broadleaved forests along a transect from central to northern Europe. Under conditions of N limitation in Sweden, there was an increase in δN of soil total N of up to 9% from the uppermost horizon of the organic mor layer down to the upper 0-5 cm of the mineral soil. The δN of roots was only slightly lower than that of soil total N in the upper organic horizon, but further down roots were up to 5% depleted under such conditions. In experimentally N-enriched forest in Sweden, i.e. in plots which have received an average of c. 100 kg N ha year for 20 years and which retain less than 50% of this added N in the stand and the soil down to 20 cm depth, and in some forests in central Europe, the increase in δN with depth in soil total N was smaller. An increase in δN of the surface soil was even observed on experimentally N-enriched plots, although other data suggest that the N fertilizer added was depleted inN. In such cases roots could be enriched inN relative to soil total N, suggesting that labelling of the surface soil is via the pathway: - available pools of N-plant N-litter N. Under N-limiting conditions roots of different species sampled from the same soil horizon showed similar δN. By contrast, in experimentally N-enriched forest δN of roots increased in the sequence: ericaceous dwarf shrubs