Share the wealth: Trees with greater ectomycorrhizal species overlap share more carbon

Extramatrical mycelial biomass is mediated by fine root mass and ectomycorrhizal fungal community composition across tree species

In many ecosystems, a large fraction of gross primary production is invested in mycorrhiza. Ectomycorrhizal (ECM) mycelium is involved in regulating soil carbon and nutrient cycling. However, little is known about how mycelial biomass, production and turnover differ depending on ECM fungal community composition and associated tree species. We quantified fine root biomass and length using soil cores, and mycelial traits (biomass, production, and turnover) using mesh-bags and ergosterol analysis, and identified ECM exploration types by Illumina MiSeq sequencing of four ECM-dominated tree species (Picea asperata, Larix gmelinii, Quercus aquifolioides and Betula albosinensis) in subalpine forest. The ECM fungal community composition separated between needle-leaved and broadleaved species, and between evergreen and deciduous species. The ratio of mycelial to fine root biomass was similar across the species regardless of genus-scale community composition and the relative abundance of exploration types. Compared to the other species, Q. aquifolioides displayed higher fine root biomass and mycelial biomass and production, dominated by contact-short exploration type. Mycelial turnover rate tended to be lowest in P. asperata, dominated by medium-long exploration type. Much higher production of mycelium and only slightly higher turnover rate in Q. aquifolioides suggests that its steady-state mycelial biomass would be higher than of the other species. Moreover, compared to the two deciduous species, with similar production but somewhat lower turnover rate, the standing crop of mycelium in P. asperata may stabilize at a higher value. Our findings, that exploration type may affect production and turnover, highlight the importance of characterizing ECM fungal communities by exploration types when estimating the contribution of mycelium biomass to forest carbon sink and storage.

Ectomycorrhizal fungi of Douglas-fir retain newly assimilated carbon derived from neighboring European beech

Ectomycorrhizal (ECM) fungi distribute tree-derived carbon (C) via belowground hyphal networks in forest ecosystems. Here, we asked the following: (1) Is C transferred belowground to a neighboring tree retained in fungal structures or transported within the recipient tree? (2) Is the overlap of ectomycorrhizal fungi in mycorrhizal networks related to the amount of belowground C transfer? We used potted sapling pairs of European beech (Fagus sylvatica) and North-American Douglas-fir (Pseudotsuga menziesii) for 13CO2 pulse-labeling. We compared 13C transfer from beech (donor) to either beech or Douglas-fir (recipient) and identified the ECM species. We measured the 13C enrichment in soil, plant tissues, and ECM fractions of fungal-containing parts and plant transport tissues. In recipients, only fungal-containing tissue of ectomycorrhizas was significantly enriched in 13C and not the plant tissue. Douglas-fir recipients shared on average one ECM species with donors and had a lower 13C enrichment than beech recipients, which shared on average three species with donors. Our results support that recently assimilated C transferred belowground is shared among fungi colonizing tree roots but not among trees. In mixed forests with beech and Douglas-fir, the links for C movement might be hampered due to low mycorrhizal overlap with consequences for soil C cycling.

Differential Strategies of Ectomycorrhizal Development between Suillus luteus and Pinus massoniana in Response to Nutrient Changes

Ectomycorrhizal fungi employ different strategies for mycelial growth and host colonization under varying nutrient conditions. However, key genes associated with mycorrhizal interaction should be influenced solely by the inoculation treatment and not by nutrient variations. To utilize subtle nutrient differences and rapidly screen for key genes related to the interaction between Suillus luteus and Pinus massoniana, we performed an inoculation experiment using culture bottles containing high- and low-nutrient media. Interestingly, S. luteus LS88 promoted the growth of P. massoniana seedlings without mature ectomycorrhiza, and the impact of LS88 inoculation on P. massoniana roots was greater than that of nutrient changes. In this study, the resequenced genome of the LS88 strain was utilized for transcriptome analysis of the strain. The analysis indicated that a unique gene encoding glutathione S-transferase (GST) in LS88 is likely involved in colonizing P. massoniana roots. In this study, the GST gene expression was independent of nutrient levels. It was probably induced by P. massoniana and could be used as a marker for S. luteus colonization degree.

Inter-generational consistency of the ectomycorrhizal fungal community in a mixed pine-cedar post-fire stand

The mutualistic interaction between trees and ectomycorrhizal fungi (EMF) can have a major effect on forest dynamics and specifically on seedling establishment. Here, we compared the EMF community composition associated with the roots of young saplings and mature trees of two co-habiting Pinaceae: Pinus halepensis and Cedrus deodara growing together in a post-fire forest plot, using fungal ITS metabarcoding. We found that the differences in the EMF community between the two sapling groups were mostly attributed to changes in the relative abundance of specific fungal species, with little species turnover. Specifically, Tomentella showed high abundance on pine roots, while Tuber, Russula and Sebacina were more common on the roots of cedars. The physical proximity to a specific host species was correlated with the EMF community composition of young saplings. Specifically, regardless of the sapling's own identity, the roots of saplings growing next to mature cedars had higher abundance of Tuber species, while Tomentella coerulea (Höhn. & Litsch), Russula densifolia (Secr. ex Gillet) and Tuber nitidum (Vittadini) dominated saplings next to mature pines. Cedar saplings' shoot structure was correlated with a specific EMF species. Overall, these results suggest that when germinating next to mature trees, the EMF community of saplings could be determined by extrinsic factors such as the small-scale distribution of mature trees in the forest.

The mycorrhizal symbiosis: research frontiers in genomics, ecology, and agricultural application

Mycorrhizal symbioses between plants and fungi are vital for the soil structure, nutrient cycling, plant diversity, and ecosystem sustainability. More than 250 000 plant species are associated with mycorrhizal fungi. Recent advances in genomics and related approaches have revolutionized our understanding of the biology and ecology of mycorrhizal associations. The genomes of 250+ mycorrhizal fungi have been released and hundreds of genes that play pivotal roles in regulating symbiosis development and metabolism have been characterized. rDNA metabarcoding and metatranscriptomics provide novel insights into the ecological cues driving mycorrhizal communities and functions expressed by these associations, linking genes to ecological traits such as nutrient acquisition and soil organic matter decomposition. Here, we review genomic studies that have revealed genes involved in nutrient uptake and symbiosis development, and discuss adaptations that are fundamental to the evolution of mycorrhizal lifestyles. We also evaluated the ecosystem services provided by mycorrhizal networks and discuss how mycorrhizal symbioses hold promise for sustainable agriculture and forestry by enhancing nutrient acquisition and stress tolerance. Overall, unraveling the intricate dynamics of mycorrhizal symbioses is paramount for promoting ecological sustainability and addressing current pressing environmental concerns. This review ends with major frontiers for further research.

Age-Related Conservation in Plant-Soil Feedback Accompanied by Ectomycorrhizal Domination in Temperate Forests in Northeast China

This study investigates the effects of forest aging on ectomycorrhizal (EcM) fungal community and foraging behavior and their interactions with plant-soil attributes. We explored EcM fungal communities and hyphal exploration types via rDNA sequencing and investigated their associations with plant-soil traits by comparing younger (~120 years) and older (~250 years) temperate forest stands in Northeast China. The results revealed increases in the EcM fungal richness and abundance with forest aging, paralleled by plant-soil feedback shifting from explorative to conservative nutrient use strategies. In the younger stands, Tomentella species were prevalent and showed positive correlations with nutrient availability in both the soil and leaves, alongside rapid increases in woody productivity. However, the older stands were marked by the dominance of the genera Inocybe, Hymenogaster, and Otidea which were significantly and positively correlated with soil nutrient contents and plant structural attributes such as the community-weighted mean height and standing biomass. Notably, the ratios of longer-to-shorter distance EcM fungal exploration types tended to decrease along with forest aging. Our findings underscore the integral role of EcM fungi in the aging processes of temperate forests, highlighting the EcM symbiont-mediated mechanisms adapting to nutrient scarcity and promoting sustainability in plant-soil consortia.

Common mycorrhizal network: the predominant socialist and capitalist responses of possible plant-plant and plant-microbe interactions for sustainable agriculture

Plants engage in a variety of interactions, including sharing nutrients through common mycorrhizal networks (CMNs), which are facilitated by arbuscular mycorrhizal fungi (AMF). These networks can promote the establishment, growth, and distribution of limited nutrients that are important for plant growth, which in turn benefits the entire network of plants. Interactions between plants and microbes in the rhizosphere are complex and can either be socialist or capitalist in nature, and the knowledge of these interactions is equally important for the progress of sustainable agricultural practice. In the socialist network, resources are distributed more evenly, providing benefits for all connected plants, such as symbiosis. For example, direct or indirect transfer of nutrients to plants, direct stimulation of growth through phytohormones, antagonism toward pathogenic microorganisms, and mitigation of stresses. For the capitalist network, AMF would be privately controlled for the profit of certain groups of plants, hence increasing competition between connected plants. Such plant interactions invading by microbes act as saprophytic and cause necrotrophy in the colonizing plants. In the first case, an excess of the nutritional resources may be donated to the receiver plants by direct transfer. In the second case, an unequal distribution of resources occurs, which certainly favor individual groups and increases competition between interactions. This largely depends on which of these responses is predominant ("socialist" or "capitalist") at the moment plants are connected. Therefore, some plant species might benefit from CMNs more than others, depending on the fungal species and plant species involved in the association. Nevertheless, benefits and disadvantages from the interactions between the connected plants are hard to distinguish in nature once most of the plants are colonized simultaneously by multiple fungal species, each with its own cost-benefits. Classifying plant-microbe interactions based on their habitat specificity, such as their presence on leaf surfaces (phyllospheric), within plant tissues (endophytic), on root surfaces (rhizospheric), or as surface-dwelling organisms (epiphytic), helps to highlight the dense and intricate connections between plants and microbes that occur both above and below ground. In these complex relationships, microbes often engage in mutualistic interactions where both parties derive mutual benefits, exemplifying the socialistic or capitalistic nature of these interactions. This review discusses the ubiquity, functioning, and management interventions of different types of plant-plant and plant-microbe interactions in CMNs, and how they promote plant growth and address environmental challenges for sustainable agriculture.

Mother trees, altruistic fungi, and the perils of plant personification

There are growing doubts about the true role of the common mycorrhizal networks (CMN or wood wide web) connecting the roots of trees in forests. We question the claims of a substantial carbon transfer from 'mother trees' to their offspring and nearby seedlings through the CMN. Recent reviews show that evidence for the 'mother tree concept' is inconclusive or absent. The origin of this concept seems to stem from a desire to humanize plant life but can lead to misunderstandings and false interpretations and may eventually harm rather than help the commendable cause of preserving forests. Two recent books serve as examples: The Hidden Life of Trees and Finding the Mother Tree.

Comparative transcriptome analysis of ectomycorrhizal fungus Pisolithus albus in response to individual and combined stress of copper and cadmium

An ectomycorrhizal fungus Pisolithus albus establishes the natural symbiosis with plant roots on extreme heavy metal (HM)-rich soil and enables their survival in toxic metal concentrations. Understanding P. albus key genes and pathways behind strong metal tolerance is crucial for its successful application in the rehabilitation of metal-contaminated barren lands. Therefore, this study aimed to analyze the whole transcriptome profile of P. albus under individual and combined metal stress of copper (Cu) and cadmium (Cd). At 480 µM Cu and 16 µM Cd toxic concentrations, P. albus has shown growth and survival and accumulated high metal (1.46 µg Cu and 1.13 µg Cd per mg of dry mycelia). The study found a stronger response of P. albus to single-metal stress in high concentration as compared to multi-metal stress in relatively lower concentration. Hence, the intensity of fungal response to HM stress is mainly determined by the metal concentration involved in stress. We have found a total of 11 pathways significantly associated with HM stress, among which amino acid, lipid, and carbohydrate metabolisms were highly affected. The functional enrichment of differentially expressed genes has shown the induced biosynthesis of arginine, melanin, metal chelating agents, membrane phospholipids, fatty acids, folate, pantothenate, ergothioneine, and other antioxidant agents; upregulation of zinc ion uptake, potassium transporters, and lysine degradation; and reduction of phosphatidylcholine degradation, incorrect protein folding, iron uptake, and potassium efflux as the top efficient tolerance mechanisms of P. albus against HM stress. The current study would contribute to understanding fungal HM tolerance and its further utilization in the bioremediation of metal-contaminated abandoned lands. The validation of RNA-sequencing analysis with RT-qPCR of selected genes showed the high credibility of the presented data.

Belowground carbon transfer across mycorrhizal networks among trees: Facts, not fantasy

The mycorrhizal symbiosis between fungi and plants is among the oldest, ubiquitous and most important interactions in terrestrial life on Earth. Carbon (C) transfer across a common mycorrhizal network (CMN) was demonstrated over half a century ago in the lab ( Reid & Woods, 1969), and later in the field ( Simard et al., 1997a). Recent years have seen ample progress in this research direction, including evidence for ecological significance of carbon transfer ( Klein et al., 2016). Furthermore, specific cases where the architecture of mycorrhizal networks have been mapped ( Beiler et al., 2015) and CMN-C transfer from mature trees to seedlings has been demonstrated ( Orrego, 2018) have suggested that trees in forests are more connected than once thought ( Simard, 2021). In a recent Perspective, Karst et al. (2023) offered a valuable critical review warning of over-interpretation and positive citation bias in CMN research. It concluded that while there is evidence for C movement among plants, the importance of CMNs remains unclear, as noted by others too ( Henriksson et al., 2023). Here we argue that while some of these claims are justified, factual evidence about belowground C transfer across CMNs is solid and accumulating.

Loss of fungal symbionts and changes in pollinator availability caused by climate change will affect the distribution and survival chances of myco-heterotrophic orchid species

The first comprehensive species distribution models for orchid, its fungal symbionts and pollinator are presented. To evaluate impact of global warming on these organisms three different projections and four various climate change scenarios were analysed. The niche modelling was based on presence-only records of Limodorum abortivum, two species of Russula and three insects pollinating orchid (Anthophora affinis, Bombus terrestris, Rhodanthidium septemdentatum). Two sets of orchid predictions were examined-the first one included only climatic data and the second one was based on climate data and data on future distribution of orchid fungal symbionts. Overall, a poleward range shift is predicted to occur as a result of climate change and apparently global warming will be favorable for L. abortivum and its potential geographical range will expand. However, due to the negative effect of global warming on fungal symbionts of L. abortivum, the actual extension of the suitable niches of the orchid will be much limited. Considering future possibility of cross-pollination, the availability of A. affinis for L. abortivum will decrease and this bee will be available in the worst case scenarios only for 21% of orchid populations. On the other hand, the overlap of orchid and the buff-tailed bumblebee will increase and as much as 86.5% of plant populations will be located within B. terrestris potential range. Also the availability of R. septemdentatum will be higher than currently observed in almost all analysed climate change projections. This study showed the importance of inclusion of ecological factors in species distribution models as the climate data itself are not enough to estimate the future distribution of plant species. Moreover, the availability of pollen vectors which is crucial for long-term survival of orchid populations should be analysed in context of climate changes.

Characterization and genome-wide sequence analysis of an ectomycorrhizal fungus Pisolithus albus, a potential source for reclamation of degraded lands

Pisolithus albus is a ubiquitous ectomycorrhizal fungus that establishes symbiosis with a wide range of woody plants around the globe. The symbiotic association of this fungus plays a crucial role in the nutrient cycling of their host plants and enables them to thrive in adverse environmental conditions. Based on its ecological importance and lack of genomic studies, whole-genome sequencing was carried out to analyze P. albus sequences through an Illumina HiSeq X system. The functional annotations were performed against various databases to explore genomic patterns and traits possibly attributing to its specialization. Comparative genomics of P. albus with phylogenetically related Pisolithus microcarpus and Pisolithus tinctorius (only available genomes of Pisolithus at NCBI till now) led to the identification of their unique and shared basic functional and stress adaptation capabilities. The de novo assembled genome of 56.15 Mb with 91.8% BUSCO completeness is predicted to encode 23,035 genes. The study is aimed to generate solid genomic data resources for P. albus, forming the theoretical basis for future transcriptomic, proteomic and metabolomic studies.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03483-5.

Scale dependency of ectomycorrhizal fungal community assembly processes in Mediterranean mixed forests

The assembly of biological communities depends on deterministic and stochastic processes whose influence varies across spatial and temporal scales. Although ectomycorrhizal (ECM) fungi play a key role in forest ecosystems, our knowledge on ECM community assembly processes and their dependency on spatial scales is still scarce. We analysed the assembly processes operating on ECM fungal communities associated with Cistus albidus L. and Quercus spp. in Mediterranean mixed forests (Southern Spain), for which root tip ECM fungi were characterized by high-throughput sequencing. The relative contribution of deterministic and stochastic processes that govern the ECM fungal community assembly was inferred by using phylogenetic and compositional turnover descriptors across spatial scales. Our results revealed that stochastic processes had a significantly higher contribution than selection on root tip ECM fungal community assembly. The strength of selection decreased at the smallest scale and it was linked to the plant host identity and the environment. Dispersal limitation increased at finer scales, whilst drift showed the opposite pattern likely suggesting a main influence of priority effects on ECM fungal community assembly. This study highlights the potential of phylogeny to infer ECM fungal community responses and brings new insights into the ecological processes affecting the structure and dynamics of Mediterranean forests.

Ectomycorrhizal fungi mediate belowground carbon transfer between pines and oaks

Inter-kingdom belowground carbon (C) transfer is a significant, yet hidden, biological phenomenon, due to the complexity and highly dynamic nature of soil ecology. Among key biotic agents influencing C allocation belowground are ectomycorrhizal fungi (EMF). EMF symbiosis can extend beyond the single tree-fungus partnership to form common mycorrhizal networks (CMNs). Despite the high prevalence of CMNs in forests, little is known about the identity of the EMF transferring the C and how these in turn affect the dynamics of C transfer. Here, Pinus halepensis and Quercus calliprinos saplings growing in forest soil were labeled using a 13CO2 labeling system. Repeated samplings were applied during 36 days to trace how 13C was distributed along the tree-fungus-tree pathway. To identify the fungal species active in the transfer, mycorrhizal fine root tips were used for DNA-stable isotope probing (SIP) with 13CO2 followed by sequencing of labeled DNA. Assimilated 13CO2 reached tree roots within four days and was then transferred to various EMF species. C was transferred across all four tree species combinations. While Tomentella ellisii was the primary fungal mediator between pines and oaks, Terfezia pini, Pustularia spp., and Tuber oligospermum controlled C transfer among pines. We demonstrate at a high temporal, quantitative, and taxonomic resolution, that C from EMF host trees moved into EMF and that C was transferred further to neighboring trees of similar and distinct phylogenies.

Asymmetric belowground carbon transfer in a diverse tree community

Mycorrhizal fungi can colonize multiple trees of a single or multiple taxa, facilitating bidirectional exchange of carbon between trees. Mycorrhiza-induced carbon transfer was shown in the forest, but it is unknown whether carbon is shared symmetrically among tree species, and if not, which tree species are better donors and which are better recipients. Here we test this question by investigating carbon transfer dynamics among five Mediterranean tree species in a microcosm system, including both ectomycorrhizal (EM) and arbuscular (AM) plants. Trees were planted together in 'community boxes' using natural soil from a mixed forest plot that serves as habitat for all five tree species and their native mycorrhizal fungi. In each box, only the trees of a single species were pulse-labeled with ¹³ CO₂ . We found that carbon transfer was asymmetric, with oak being a better donor, and pistacia and cypress better recipients. Shared mycorrhizal species may have facilitated carbon transfer, but their diversity did not affect the amount, nor timing, of the transfer. Overall, our findings in a microcosm system expose rich, but hidden, belowground interactions in a diverse population of trees and mycorrhizal fungi. The asymmetric carbon exchange among co-habiting tree species could potentially contribute to forest resilience in an uncertain future.

Role of Ectomycorrhizal Symbiosis Behind the Host Plants Ameliorated Tolerance Against Heavy Metal Stress

Soil heavy metal (HM) pollution, which arises from natural and anthropogenic sources, is a prime threat to the environment due to its accumulative property and non-biodegradability. Ectomycorrhizal (ECM) symbiosis is highly efficient in conferring enhanced metal tolerance to their host plants, enabling their regeneration on metal-contaminated lands for bioremediation programs. Numerous reports are available regarding ECM fungal potential to colonize metal-contaminated lands and various defense mechanisms of ECM fungi and plants against HM stress separately. To utilize ECM–plant symbiosis successfully for bioremediation of metal-contaminated lands, understanding the fundamental regulatory mechanisms through which ECM symbiosis develops an enhanced metal tolerance in their host plants has prime importance. As this field is highly understudied, the present review emphasizes how plant’s various defense systems and their nutrient dynamics with soil are affected by ECM fungal symbiosis under metal stress, ultimately leading to their host plants ameliorated tolerance and growth. Overall, we conclude that ECM symbiosis improves the plant growth and tolerance against metal stress by (i) preventing their roots direct exposure to toxic soil HMs, (ii) improving plant antioxidant activity and intracellular metal sequestration potential, and (iii) altering plant nutrient uptake from the soil in such a way to enhance their tolerance against metal stress. In some cases, ECM symbiosis promotes HM accumulation in metal stressed plants simultaneous to improved growth under the HM dilution effect.

Mobile forms of carbon in trees: metabolism and transport

Plants constitute 80% of the biomass on earth, and almost two-thirds of this biomass is found in wood. Wood formation is a carbon (C)-demanding process and relies on C transport from photosynthetic tissues. Thus, understanding the transport process is of major interest for understanding terrestrial biomass formation. Here, we review the molecules and mechanisms used to transport and allocate C in trees. Sucrose is the major form in which C is transported in plants, and it is found in the phloem sap of all tree species investigated so far. However, in several tree species, sucrose is accompanied by other molecules, notably polyols and the raffinose family of oligosaccharides. We describe the molecules that constitute each of these transport groups, and their distribution across different tree species. Furthermore, we detail the metabolic reactions for their synthesis, the mechanisms by which trees load and unload these compounds in and out of the vascular system, and how they are radially transported in the trunk and finally catabolized during wood formation. We also address a particular C recirculation process between phloem and xylem that occurs in trees during the annual cycle of growth and dormancy. A search of possible evolutionary drivers behind the diversity of C-carrying molecules in trees reveals no consistent differences in C transport mechanisms between angiosperm and gymnosperm trees. Furthermore, the distribution of C forms across species suggests that climate-related environmental factors will not explain the diversity of C transport forms. However, the consideration of C-transport mechanisms in relation to tree-rhizosphere coevolution deserves further attention. To conclude the review, we identify possible future lines of research in this field.

Limited evidence of vertical fine-root segregation in a subtropical forest

Vertical root segregation and the resulting niche partitioning can be a key underpinning of species coexistence. This could result from substantial interspecific variations in root profiles and rooting plasticity in response to soil heterogeneity and neighbours, but they remain largely untested in forest communities. In a diverse forest in subtropical China, we randomly sampled > 4000 root samples from 625 0-30 cm soil profiles. Using morphological and DNA-based methods, we identified 109 woody plant species, determined the degree of vertical fine-root segregation, and examined rooting plasticity in response to soil heterogeneity and neighbour structure. We found no evidence of vertical fine-root segregation among cooccurring species. By contrast, root abundance of different species tended to be positively correlated within soil zones. Underlying these findings was a lack of interspecific variation in fine-root profiles with over 90% of species concentrated in the 0-10 cm soil zone with only one species dominating in the 10-20 cm soil zone. Root profiles exhibited low responsiveness to root neighbours but tended to be shallow in soils with low phosphorus and copper content. These findings suggest that if there is niche differentiation leading to coexistence in this diverse forest, it would be occurring by mechanisms other than vertical fine-root segregation.

Distinct effects of host and neighbour tree identity on arbuscular and ectomycorrhizal fungi along a tree diversity gradient

Plant diversity and plant-related ecosystem functions have been important in biodiversity-ecosystem functioning studies. However, biotic interactions with mycorrhizal fungi have been understudied although they are crucial for plant-resource acquisition. Here, we investigated the effects of tree species richness and tree mycorrhizal type on arbuscular (AMF) and ectomycorrhizal fungal (EMF) communities. We aimed to understand how dissimilarities in taxa composition and beta-diversity are related to target trees and neighbours of the same or different mycorrhizal type. We sampled a tree diversity experiment with saplings (~7 years old), where tree species richness (monocultures, 2-species, and 4-species mixtures) and mycorrhizal type were manipulated. AMF and EMF richness significantly increased with increasing tree species richness. AMF richness of mixture plots resembled that of the sum of the respective monocultures, whereas EMF richness of mixture plots was lower compared to the sum of the respective monocultures. Specialisation scores revealed significantly more specialised AMF than EMF suggesting that, in contrast to previous studies, AMF were more specialised, whereas EMF were not. We further found that AMF communities were little driven by the surrounding trees, whereas EMF communities were. Our study revealed drivers of mycorrhizal fungal communities and further highlights the distinct strategies of AMF and EMF.

The size and the age of the metabolically active carbon in tree roots

Little is known about the sources and age of C respired by tree roots. Previous research in stems identified two functional pools of non-structural carbohydrates (NSC): an "active" pool supplied directly from canopy photo-assimilates supporting metabolism and a "stored" pool used when fresh C supplies are limited. We compared the C isotope composition of water-soluble NSC and respired CO₂ for aspen roots (Populus tremula hybrids) cut off from fresh C supply after stem-girdling or prolonged incubation of excised roots. We used bomb radiocarbon to estimate the time elapsed since C fixation for respired CO₂ , water-soluble NSC and structural α-cellulose. While freshly excised roots (mostly <2.9 mm in diameter) respired CO₂ fixed <1 year previously, the age increased to 1.6-2.9 year within a week after root excision. Freshly excised roots from trees girdled ~3 months ago had respiration rates and NSC stocks similar to un-girdled trees but respired older C (~1.2 year). We estimate that over 3 months NSC in girdled roots must be replaced 5-7 times by reserves remobilized from root-external sources. Using a mixing model and observed correlations between Δ¹⁴ C of water-soluble C and α-cellulose, we estimate ~30% of C is "active" (~5 mg C g^-1 ).

Management After Windstorm Affects the Composition of Ectomycorrhizal Symbionts of Regenerating Trees but Not Their Mycorrhizal Networks

Due to ongoing climate change, forests are expected to face significant disturbances more frequently than in the past. Appropriate management is intended to facilitate forest regeneration. Because European temperate forests mostly consist of trees associated with ectomycorrhizal (ECM) fungi, understanding their role in these disturbances is important to develop strategies to minimize their consequences and effectively restore forests. Our aim was to determine how traditional (EXT) and nonintervention (NEX) management in originally Norway spruce (Picea abies) forests with an admixture of European larch (Larix decidua) affect ECM fungal communities and the potential to interconnect different tree species via ECM networks 15 years after a windstorm. Ten plots in NEX and 10 plots in EXT with the co-occurrences of Norway spruce, European larch, and silver birch (Betula pendula) were selected, and a total of 57 ECM taxa were identified using ITS sequencing from ECM root tips. In both treatments, five ECM species associated with all the studied tree species dominated, with a total abundance of approximately 50% in the examined root samples. Because there were no significant differences between treatments in the number of ECM species associated with different tree species combinations in individual plots, we concluded that the management type did not have a significant effect on networking. However, management significantly affected the compositions of ECM symbionts of Norway spruce and European larch but not those of silver birch. Although this result is explained by the occurrence of seedlings and ECM propagules that were present in the original forest, the consequences are difficult to assess without knowledge of the ecology of different ECM symbionts.

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

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.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.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. 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.

Partner turnover and changes in ectomycorrhizal fungal communities during the early life stages of European beech (Fagus sylvatica L.)

The first life stages of a tree are subject to strong environmental stresses and competition, limiting their chances of survival. Establishing a mutualistic relationship with mycorrhizal fungi during early life stages may increase growth and survival rates of trees, but how mycorrhizal communities assemble during these stages remains unclear. Here, we studied variation in the ectomycorrhizal (EcM) fungal communities in the soil and roots of Fagus sylvatica seedlings and saplings. Fungal DNA was extracted from the soil and seedling and sapling roots collected in 156 plots across the beech-dominated Sonian forest (Belgium) and community composition was determined through metabarcoding. EcM fungal community composition significantly differed between soil, seedlings and saplings. Russula, Amanita and Inocybe were most abundant in soil, while Lactarius and Scleroderma were more abundant in seedling and sapling roots and Xerocomellus and Laccaria were most abundant in sapling roots. Our results provide evidence of partner turnover in EcM fungal community composition with increasing age in the early life stages of F. sylvatica.

The dominance of Suillus species in ectomycorrhizal fungal communities on Larix gmelinii in a post-fire forest in the Russian Far East

Wildfires can negatively affect ectomycorrhizal (EM) fungal communities. However, potential shifts in community structures due to wildfires have rarely been evaluated in the forests of eastern Eurasia, where surface fires are frequent. We investigated EM fungal communities in a Larix gmelinii-dominated forest that burned in 2003 in Zeya, in the Russian Far East. A total of 120 soil samples were collected from burned and adjacent unburned forest sites. The EM fungal root tips were morphotyped and internal transcribed spacer (ITS) sequences were obtained for fungal identification. We detected 147 EM fungal operational taxonomic units, and EM fungal richness was 25% lower at the burned site than at the unburned site. EM fungal composition was characterized by the occurrence of disturbance-adapted fungi (Amphinema and Wilcoxina) at the burned site and late-successional fungi (Lactarius, Russula and Cortinarius) at the unburned site. These findings suggest that the EM fungal communities did not recover to pre-fire levels 16 years after the fire. Suillus species were the dominant EM fungi on L. gmelinii, with greater richness and frequency at the burned site. Both Larix and Suillus exhibit adaptive traits to quickly colonize fire-disturbed habitats. Frequent surface fires common to eastern Eurasia are likely to play important roles in maintaining Larix forests, concomitantly with their closely associated EM fungi.

Mycorrhizas transfer carbon in a mature mixed forest

Mycorrhizal fungi transfer nutrients to plants in exchange for photosynthates. Plants allocate up to 20% of their carbon to mycorrhizal structures, mycelium and fruit bodies of their fungal partners. Individuals of mycorrhizal fungi may encompass hundreds of square metres of soil and defragmented litter, linking multiple plant individuals of different species and size (Figure 1). Using a free-air ¹³ CO₂ enrichment (web-FACE) technique in a mature forest, interspecific transfer accounted for 40% of fine root carbon after 5 years of back and forth transfer between trees. In this issue of Molecular Ecology, Rog, Rosenstock, Körner, and Klein (2020) show that closely related trees shared relatively more mycorrhizal fungi than distantly related trees in the same experimental site, which correlated to increased carbon sharing.