Mixing tree species associated with arbuscular or ectotrophic mycorrhizae reveals dual mycorrhization and interactive effects on the fungal partners

1. Recent studies found that the majority of shrub and tree species are associated with both arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi. However, our knowledge on how different mycorrhizal types interact with each other is still limited. We asked whether the combination of hosts with a preferred association with either AM or EM fungi increases the host tree roots’ mycorrhization rate and affects AM and EM fungal richness and community composition.
2. We established a tree diversity experiment, where five tree species of each of the two mycorrhiza types were planted in monocultures, two‐species and four‐species mixtures. We applied morphological assessment to estimate mycorrhization rates and next‐generation molecular sequencing to quantify mycobiont richness.
3. Both the morphological and molecular assessment revealed dual‐mycorrhizal colonization in 79% and 100% of the samples, respectively. OTU community composition strongly differed between AM and EM trees. While host tree species richness did not affect mycorrhization rates, we observed significant effects of mixing AM‐ and EM‐associated hosts in AM mycorrhization rate. Glomeromycota richness was larger in monotypic AM tree combinations than in AM‐EM mixtures, pointing to a dilution or suppression effect of AM by EM trees. We found a strong match between morphological quantification of AM mycorrhization rate and Glomeromycota richness.
4. Synthesis. We provide evidence that the combination of hosts differing in their preferred mycorrhiza association affects the host''s fungal community composition, thus revealing important biotic interactions among trees and their associated fungi.

     

Herbivore and pathogen effects on tree growth are additive,but mediated by tree diversity and plant traits

Herbivores and fungal pathogens are key drivers of plant community composition and functioning. The effects of herbivores and pathogens are mediated by the diversity and functional characteristics of their host plants. However, the combined effects of herbivory and pathogen damage, and their consequences for plant performance, have not yet been addressed in the context of biodiversity–ecosystem functioning research. We analyzed the relationships between herbivory, fungal pathogen damage and their effects on tree growth in a large‐scale forest‐biodiversity experiment. Moreover, we tested whether variation in leaf trait and climatic niche characteristics among tree species influenced these relationships. We found significant positive effects of herbivory on pathogen damage, and vice versa. These effects were attenuated by tree species richness—because herbivory increased and pathogen damage decreased with increasing richness—and were most pronounced for species with soft leaves and narrow climatic niches. However, herbivory and pathogens had contrasting, independent effects on tree growth, with pathogens decreasing and herbivory increasing growth. The positive herbivory effects indicate that trees might be able to (over‐)compensate for local damage at the level of the whole tree. Nevertheless, we found a dependence of these effects on richness, leaf traits and climatic niche characteristics of the tree species. This could mean that the ability for compensation is influenced by both biodiversity loss and tree species identity—including effects of larger‐scale climatic adaptations that have been rarely considered in this context. Our results suggest that herbivory and pathogens have additive but contrasting effects on tree growth. Considering effects of both herbivory and pathogens may thus help to better understand the net effects of damage on tree performance in communities differing in diversity. Moreover, our study shows how species richness and species characteristics (leaf traits and climatic niches) can modify tree growth responses to leaf damage under real‐world conditions.     

Plant quality and predation risk mediated by plant ontogeny: consequences for herbivores and plants

Bottom‐up and top‐down impacts on herbivores can be influenced by plant productivity, structural complexity, vigor and size. Although these traits are likely to vary with plant development, the influence of plant ontogeny on the relative importance of plant quality (i.e. bottom‐up forces) and predation risk (i.e. top‐down forces) has been the focus of little previous investigation. We evaluated the role of plant ontogeny for the relative importance of bottom‐up and top‐down forces on insect herbivore abundance, species richness, and species diversity attacking the tropical tree Casearia nitida. We also quantified the cascading effects on herbivory, growth and reproduction of this plant species. Plant quality traits (nitrogen and phenolic compounds) were assessed in saplings and reproductive trees. Bottom‐up forces were manipulated by fertilizing plants from both ontogenetic stages. Top‐down forces were manipulated by excluding insectivorous birds from saplings and reproductive trees. Plant ontogeny influenced foliage quality in terms of total phenolics, which were in greater concentration in reproductive trees than in saplings; however, it did not influence bottom‐up forces as modified by fertilization. Bird exclusion increased herbivore density with the same magnitude on both stages. Ontogeny influenced species diversity, which was greater in reproductive trees than in saplings, and also influenced treatment impacts on species richness and diversity. Although top‐down forces increased herbivory equally on plants of each ontogenetic stage, the two stages showed different overcompensation responses to increased damage: caged saplings produced greater leaf biomass than non‐caged saplings, whereas caged trees increased in height proportionally more than non‐caged trees. In sum, plant ontogeny influenced the impact of bird predation on herbivore density, species richness, and species diversity, and the growth variables affected by increased damage in caged plants. We suggest that plant ontogeny can contribute to some extent to the influence of plant quality and the third trophic level on herbivores in this system.     

Herbivore attack in Casearia nitida influenced by plant ontogenetic variation in foliage quality and plant architecture

Traits influencing plant quality as food and/or shelter for herbivores may change during plant ontogeny, and as a consequence, influence the amount of herbivory that plants receive as they develop. In this study, differences in herbivore density and herbivory were evaluated for two ontogenetic stages of the tropical tree Casearia nitida. To assess plant ontogenetic differences in foliage quality as food for herbivores, nutritional and defensive traits were evaluated in saplings and reproductive trees. Predatory arthropods were quantified and the foraging preferences of a parasitoid wasp of the genus Zacremnops were assessed. In addition, survival rates of lepidopteran herbivores (Geometridae) were evaluated experimentally. Herbivore density was three times higher and herbivory was 66% greater in saplings than in reproductive trees. Accordingly, concentrations of total foliar phenolics were higher in reproductive trees than in saplings, whereas leaf toughness, water and nitrogen concentration did not vary between ontogenetic stages. Survival rates of lepidopteran larvae exposed to natural enemies were equivalent in reproductive trees and saplings. Given the greater herbivore density on saplings, equal survival rates implied a greater foraging effort of predators on reproductive trees. Furthermore, observed foraging of parasitoid wasps was restricted to reproductive trees. I propose that herbivore density, and as a consequence, leaf damage were lower in reproductive trees than in saplings due to both traits influencing food quality, and architectural or unmeasured indirect defensive traits influencing foraging preference of natural enemies of herbivores.     

The mycorrhizal status and colonization of 26 tree species growing in urban and rural environments

Urban environments are highly disturbed and fragmented ecosystems that commonly have lower mycorrhizal fungal species richness and diversity compared to rural or natural ecosystems. In this study, we assessed whether the mycorrhizal status and colonization of trees are influenced by the overall environment (rural vs. urban) they are growing in. Soil cores were collected from the rhizosphere of trees growing in urban and rural environments around southern Ontario. Roots were extracted from the soil cores to determine whether the trees were colonized by arbuscular mycorrhizal fungi, ectomycorrhizal fungi, or both, and to quantify the percent colonization of each type of mycorrhizal fungi. All 26 tree species were colonized by arbuscular mycorrhizal fungi, and seven tree species were dually colonized by arbuscular mycorrhizal and ectomycorrhizal fungi. Overall, arbuscular mycorrhizal and ectomycorrhizal fungal colonization was significantly (p < 0.001) lower in trees growing in urban compared to rural environments. It is not clear what ‘urban’ factors are responsible for the reduction in mycorrhizal fungal colonization; more research is needed to determine whether inoculating urban trees with mycorrhizal fungi would increase colonization levels and growth of the trees.     

Dominant forest tree mycorrhizal type mediates understory plant invasions

Forest mycorrhizal type mediates nutrient dynamics, which in turn can influence forest community structure and processes. Using forest inventory data, we explored how dominant forest tree mycorrhizal type affects understory plant invasions with consideration of forest structure and soil properties. We found that arbuscular mycorrhizal (AM) dominant forests, which are characterised by thin forest floors and low soil C : N ratio, were invaded to a greater extent by non‐native invasive species than ectomycorrhizal (ECM) dominant forests. Understory native species cover and richness had no strong associations with AM tree dominance. We also found no difference in the mycorrhizal type composition of understory invaders between AM and ECM dominant forests. Our results indicate that dominant forest tree mycorrhizal type is closely linked with understory invasions. The increased invader abundance in AM dominant forests can further facilitate nutrient cycling, leading to the alteration of ecosystem structure and functions.     

亚热带幼林树木功能性状与叶片氮磷重吸收率的关系

本研究以亚热带29种3年生人工纯林为对象,研究了29个树种功能性状与氮磷重吸收效率的关系。结果表明: 29种幼林平均氮、磷重吸收效率分别为50.5%和57.3%。22种丛枝菌根树种的氮重吸收效率平均为52.7%,显著高于7种外生菌根树种(45.1%)。29个树种的细根组织密度与氮重吸收效率呈显著正相关,7种外生菌根树种细根直径与磷重吸收效率呈显著正相关,22种丛枝菌根树种的功能性状对氮重吸收效率和磷重吸收效率无显著影响。在29个树种中,菌根类型、比叶面积、细根组织密度、叶厚度及叶厚度与菌根类型的相互作用共同解释氮重吸收效率变异的27%,比根长、细根碳含量、细根碳氮比、菌根类型、叶片碳含量及叶片碳含量与菌根类型的相互作用共同解释磷重吸收效率变异的35%。因此,亚热带树种根系功能性状能较好地预测了氮、磷养分重吸收效率,综合多个功能性状可以更好地揭示不同生物因子对养分重吸收的相对重要性。     

Dual colonization of Eucalyptus urophylla S.T. Blake by arbuscular and ectomycorrhizal fungi affects levels of insect herbivore attack

Abstract 1 Eucalypts are an important part of plantation forestry in Asia but, in south China, productivity is very low. This is due to infertile soils and lack of indigenous symbiotic mycorrhizal fungi. The genus Eucalyptus is unusual because it forms both arbuscular (AM) and ectomycorrhizal (ECM) associations. 2 Eucalyptus urophylla saplings were grown with and without AM (Glomus caledonium) and ECM (Laccaria laccata) fungi in a factorial design. Two experiments were performed: one to simulate nursery conditions and the other to simulate the early stages of plantation establishment. Plant growth was measured over 18 weeks and levels of insect attack were recorded. 3 The AM fungus reduced tree growth in the early stages, but the effect appeared to be transient. No effects of ECM were detected on tree growth, but the ectomycorrhiza reduced colonization by the arbuscular mycorrhiza. AM fungi appear to be rapid invaders of the root system, gradually being replaced by ECM. 4 Both fungal types affected levels of damage by insect herbivores. Most importantly, herbivory by the pest insects Anomala cupripes (Coleoptera) and Strepsicrates spp. (Lepidoptera) was decreased by ECM. 5 It is suggested that mycorrhizal effects on eucalypt insects may be determined by carbon allocation within the plant. Future studies of eucalypt mycorrhizas need to take into account the effects of the fungi on foliar‐feeding insects and also the effects of insect herbivory on mycorrhizal establishment.     

Aboveground resource allocation in response to root herbivory as affected by the arbuscular mycorrhizal symbiosis

Aims

Arbuscular mycorrhizal (AM) fungi associate with the majority of terrestrial plants, influencing their growth, nutrient uptake and defence chemistry. Consequently, AM fungi can significantly impact plant-herbivore interactions, yet surprisingly few studies have investigated how AM fungi affect plant responses to root herbivores. This study aimed to investigate how AM fungi affect plant tolerance mechanisms to belowground herbivory.

Methods

We examined how AM fungi affect plant (Saccharum spp. hybrid) growth, nutrient dynamics and secondary chemistry (phenolics) in response to attack from a root-feeding insect (Dermolepida albohirtum).

Results

Root herbivory reduced root mass by almost 27%. In response, plants augmented investment in aboveground biomass by 25%, as well as increasing carbon concentrations. The AM fungi increased aboveground biomass, phosphorus and carbon. Meanwhile, root herbivory increased foliar phenolics by 31% in mycorrhizal plants, and increased arbuscular colonisation of roots by 75% overall. AM fungi also decreased herbivore performance, potentially via increasing root silicon concentrations.

Conclusions

Our results suggest that AM fungi may be able to augment plant tolerance to root herbivory via resource allocation aboveground and, at the same time, enhance plant root resistance by increasing root silicon. The ability of AM fungi to facilitate resource allocation aboveground in this way may be a more widespread strategy for plants to cope with belowground herbivory.

     

Defence against vertebrate herbivores trades off into architectural and low nutrient strategies amongst savanna Fabaceae species

Herbivory contributes substantially to plant functional diversity and in ways that move far beyond direct defence trait patterns, as effective growth strategies under herbivory require modification of multiple functional traits that are indirectly related to defence. In order to understand how herbivory has shaped plant functional diversity, we need to consider the physiology and architecture of the herbivores and how this constrains effective defence strategies. Here we consider herbivory by mammals in savanna communities that range from semi‐arid to humid conditions. We posited that the saplings of savanna trees can be grouped into two contrasting defence strategies against mammals, namely architectural defence versus low nutrient defence. We provide a mechanistic explanation for these different strategies based on the fact that plants are under competing selection pressures to limit herbivore damage and outcompete neighbouring plants. Plant competitiveness depends on growth rate, itself a function of leaf mass fraction (LMF) and leaf nitrogen per unit mass (N_m). Architectural defence against vertebrates (which includes spinescence) limits herbivore access to plant leaf materials, and partly depends on leaf‐size reduction, thereby compromising LMF. Low nutrient defence requires that leaf material is of insufficient nutrient value to support vertebrate metabolic requirements, which depends on low N_m. Thus there is an enforced tradeoff between LMF and N_m, leading to distinct trait suites for each defence strategy. We demonstrate this tradeoff by showing that numerous traits can be distinguished between 28 spinescent (architectural defenders) and non‐spinescent (low nutrient defenders) Fabaceae tree species from savannas, where mammalian herbivory is an important constraint on plant growth. Distributions of the strategies along an LMF‐N_m tradeoff further provides a predictive and parsimonious explanation for the uneven distribution of spinescent and non‐spinescent species across water and nutrient gradients.     

Contrasting effects of ectomycorrhizal and arbuscular mycorrhizal tropical tree species on soil nitrogen cycling: the potential mechanisms and corresponding adaptive strategies

While it is increasingly recognized that ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) tree species vary in their effects on soil nitrogen (N) cycling, little is known about the mechanisms causing and how ECM and AM trees adapt to this variation. Using monoculture plots of six ECM and eight AM tropical trees planted in a common garden, we examined whether the contrasting effects of ECM and AM trees on soil N cycling could be explained by their differences in plant traits. Furthermore, rhizosphere effects on soil N transformations and soil exploration by fine roots were also measured to assess whether ECM and AM trees differed in N acquisition capacities. Results showed that soil NH₄⁺‐N concentration, net N mineralization and net nitrification rates were markedly lower, but soil C:N ratio was significantly higher beneath ECM trees than beneath AM trees. This more closed N cycling caused by ECM trees was attributed to their resource‐conservative traits, especially the poorer leaf litter decomposability compared with AM trees. To adapt to their induced lower soil N availability, ECM trees were found to have greater rhizosphere effects on NO₃^‐‐N concentration, net N mineralization and net nitrification rates to mine N, and higher soil exploration in terms of root length density to scavenge N from soils, indicating that these two strategies work in synergy to meet N demand of ECM trees. These findings suggest that ECM and AM trees have contrasting effects on soil N cycling owing to their differences in leaf litter decomposability and correspondingly possess different N acquisition capacities.     

Partitioning of soil phosphorus among arbuscular and ectomycorrhizal trees in tropical and subtropical forests

Partitioning of soil phosphorus (P) pools has been proposed as a key mechanism maintaining plant diversity, but experimental support is lacking. Here, we provided different chemical forms of P to 15 tree species with contrasting root symbiotic relationships to investigate plant P acquisition in both tropical and subtropical forests. Both ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) trees responded positively to addition of inorganic P, but strikingly, ECM trees acquired more P from a complex organic form (phytic acid). Most ECM tree species and all AM tree species also showed some capacity to take up simple organic P (monophosphate). Mycorrhizal colonisation was negatively correlated with soil extractable P concentration, suggesting that mycorrhizal fungi may regulate organic P acquisition among tree species. Our results support the hypothesis that ECM and AM plants partition soil P sources, which may play an ecologically important role in promoting species coexistence in tropical and subtropical forests.     

西双版纳热带雨林中丛枝菌根真菌的初步研究*

对西双版纳热带雨林中30个科的42种植物根系的丛枝菌根真菌定居情况进行了调查,并从这些植物的根际土壤中分离鉴定了分属于无梗囊霉属(Acaulospora)、球囊霉属(Glomus)和硬囊霉属(Sclerocystis)的25种丛枝菌根真菌。对热带雨林土壤中丛枝菌根真菌的孢子密度（spore density）、物种丰富度（species richness）以及已鉴定种的出现频率进行统计分析发现：热带雨林土壤中丛枝菌根真菌的孢子密度在每100g土壤116~1560个之间,平均478个;物种丰富度在2~7之间,平均为4.5;无梗囊霉属和球囊霉属真菌是热带雨林土壤中丛枝菌根真菌的优势类群。     

亚热带典型树种根毛特征及其与共生真菌的关系

根毛和共生真菌增加了吸收面积,提高了植物获取磷等土壤资源的能力。由于野外原位观测根表微观结构较为困难,吸收细根、根毛、共生真菌如何相互作用并适应土壤资源供应,缺乏相应的数据和理论。该研究以受磷限制的亚热带森林为对象,选取了21种典型树种,定量了根毛存在情况、属性变异,分析了根毛形态特征与共生真菌侵染率、吸收细根功能属性之间的关系,探讨了根表结构对低磷土壤的响应和适应格局。结果表明:1)在亚热带森林根毛不是普遍存在的, 21个树种中仅发现7个树种存有根毛, 4个为丛枝菌根(AM)树种, 3个为外生菌根(ECM)树种。其中,马尾松(Pinus massoniana)根毛出现率最高,为86%;2)菌根类型是理解根-根毛-共生真菌关系的关键,AM树种根毛密度与共生真菌侵染率正相关,但ECM树种根毛直径与共生真菌侵染率负相关; 3) AM树种根毛长度和根毛直径、ECM树种根毛出现率与土壤有效磷含量呈负相关关系。该研究揭示了不同菌根类型树种根毛-共生真菌-根属性的格局及相互作用,为精细理解养分获取策略奠定了基础。     

Dual mycorrhizal colonization of forest-dominating tropical trees and the mycorrhizal status of non-dominant tree and liana species

The contribution of mycorrhizal associations to maintaining tree diversity patterns in tropical rain forests is poorly known. Many tropical monodominant trees form ectomycorrhizal (EM) associations, and there is evidence that the EM mutualism contributes to the maintenance of monodominance. It is assumed that most other tropical tree species form arbuscular mycorrhizal (AM) associations, and while many mycorrhizal surveys have been done, the mycorrhizal status of numerous tropical tree taxa remains undocumented. In this study, we tested the assumption that most tropical trees form AM associations by sampling root vouchers from tree and liana species in monodominant Dicymbe corymbosa forest and an adjacent mixed rain forest in Guyana. Roots were assessed for the presence/absence of AM and EM structures. Of the 142 species of trees and lianas surveyed, three tree species (the monodominant D. corymbosa, the grove-forming D. altsonii, and the non-dominant Aldina insignis) were EM, 137 were exclusively AM, and two were non-mycorrhizal. Both EM and AM structures were observed in D. corymbosa and D. altsonii. These results provide empirical data supporting the assumption that most tropical trees form AM associations for this region in the Guiana Shield and provide the first report of dual EM/AM colonization in Dicymbe species. Dual colonization of the Dicymbe species should be further explored to determine if this ability contributes to the establishment and maintenance of site dominance. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Short-term response of arbuscular mycorrhizal association to spider mite herbivory

We examined effects of aboveground herbivory by spider mites (Tetranychus urticae) on colonization and activity of arbuscular mycorrhizal fungi (AMF; Gigaspora margarita) using potted plants (Lotus japonicus). We evaluated changes in arbuscular mycorrhizal (AM) association two ways: (1) conventional trypan blue staining of mycorrhizal hyphae to examine AMF biomass in roots (mycorrhizal colonization) and (2) vital staining for a mycorrhizal enzyme (succinate dehydrogenase, SDH) to examine mycorrhizal activity (SDH activity). Mycorrhizal colonization and SDH activity started to increase 4 days after aboveground herbivory, and returned to the initial levels in the absence of mite herbivory in 7 and 12 days, respectively. These results suggest that the change in AM association in response to mite herbivory is a short-term response.     

Arbuscular mycorrhizal soil infectivity and spores distribution across plantations of tropical,subtropical and exotic tree species: a case study from the forest reserve of Bandia,Senegal

Several fast‐growing and multipurpose trees such as exotic and valuable native species have been widely used in West Africa to reverse the tendency of massive degradation of plant cover and restore soil productivity. Although benefic effects have been reported on soil stabilization, a lack of information about their impact on soil symbiotic microorganisms still remains. This investigation has been carried out in field trees of 28 years old in a forest reserve at Bandia. To determine the mycorrhizal inoculum potential (MIP) of soils, a mycorrhizal bioassay was conducted using seedlings of Zea mays L. Spores concentration, arbuscular mycorrhizal (AM) fungi morphotypes and mycorrhizal colonization of field plants were examined. Results showed that fungal communities were dominated in all samples by the genus Glomus. Nevertheless, the others genera Gigaspora and Scutellospora occurred preferentially out of the plantations. The number and richness of spores as well as the MIP of soils were decreased in the tree plantations. Accordingly, the amount of annual herbaceous plants kept out of the tree plantations was much greater than those under the tree plantations. The colonization was higher in field root systems of herb plants in comparison with that of the tree plants. Comparisons allowed us to conclude that vegetation type modifies the AM fungal communities, and the results suggest further adoption of management practices that could improve or sustain the development of herbaceous layers and thus promote the AM fungal communities.     

Nitrogen Uptake by Trees and Mycorrhizal Fungi in a Successional Northern Temperate Forest: Insights from Multiple Isotopic Methods

Forest succession may cause changes in nitrogen (N) availability, vegetation and fungal community composition that affect N uptake by trees and their mycorrhizal symbionts. Understanding how these changes affect the functioning of the mycorrhizal symbiosis is of interest to ecosystem ecology because of the fundamental roles mycorrhizae play in providing nutrition to trees and structuring forest ecosystems. We investigated changes in tree and mycorrhizal fungal community composition, the availability and uptake of N by trees and mycorrhizal fungi in a forest undergoing a successional transition (age-related loss of early successional tree taxa). In this system, 82–96% of mycorrhizal hyphae were ectomycorrhizal (EM). As biomass production of arbuscular mycorrhizal (AM) trees increased, AM hyphae comprised a significantly greater proportion of total fungal hyphae, and the EM contribution to the N requirement of EM-associated tree taxa declined from greater than 75% to less than 60%. Increasing N availability was associated with lower EM hyphal foraging and ¹⁵N tracer uptake, yet the EM-associated later-successional species Quercus rubra was nonetheless a stronger competitor for ¹⁵N than AM-associated Acer rubrum, likely due to the more extensive nature of the persistent EM hyphal network. These results indicate that successional increases in N availability and co-dominance by AM-associated trees have increased the importance of AM fungi in the mycorrhizal community, while down-regulating EM N acquisition and transfer processes. This work advances understanding of linkages between tree and fungal community composition, and indicates that successional changes in N availability may affect competition between tree taxa with divergent resource acquisition strategies.     

Tree‐mycorrhizal associations detected remotely from canopy spectral properties

A central challenge in global ecology is the identification of key functional processes in ecosystems that scale, but do not require, data for individual species across landscapes. Given that nearly all tree species form symbiotic relationships with one of two types of mycorrhizal fungi – arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi – and that AM‐ and ECM‐dominated forests often have distinct nutrient economies, the detection and mapping of mycorrhizae over large areas could provide valuable insights about fundamental ecosystem processes such as nutrient cycling, species interactions, and overall forest productivity. We explored remotely sensed tree canopy spectral properties to detect underlying mycorrhizal association across a gradient of AM‐ and ECM‐dominated forest plots. Statistical mining of reflectance and reflectance derivatives across moderate/high‐resolution Landsat data revealed distinctly unique phenological signals that differentiated AM and ECM associations. This approach was trained and validated against measurements of tree species and mycorrhizal association across ~130 000 trees throughout the temperate United States. We were able to predict 77% of the variation in mycorrhizal association distribution within the forest plots (P < 0.001). The implications for this work move us toward mapping mycorrhizal association globally and advancing our understanding of biogeochemical cycling and other ecosystem processes.     

Continental‐scale nitrogen pollution is shifting forest mycorrhizal associations and soil carbon stocks

Most tree roots on Earth form a symbiosis with either ecto‐ or arbuscular mycorrhizal fungi. Nitrogen fertilization is hypothesized to favor arbuscular mycorrhizal tree species at the expense of ectomycorrhizal species due to differences in fungal nitrogen acquisition strategies, and this may alter soil carbon balance, as differences in forest mycorrhizal associations are linked to differences in soil carbon pools. Combining nitrogen deposition data with continental‐scale US forest data, we show that nitrogen pollution is spatially associated with a decline in ectomycorrhizal vs. arbuscular mycorrhizal trees. Furthermore, nitrogen deposition has contrasting effects on arbuscular vs. ectomycorrhizal demographic processes, favoring arbuscular mycorrhizal trees at the expense of ectomycorrhizal trees, and is spatially correlated with reduced soil carbon stocks. This implies future changes in nitrogen deposition may alter the capacity of forests to sequester carbon and offset climate change via interactions with the forest microbiome.