Regional variation drives differences in microbial communities associated with sugar maple across a latitudinal range.

Climate change is prompting plants to migrate and establish novel interactions in new habitats. Because of the pivotal roles that microbes have on plant health and function, it is important to understand the ecological consequences of these shifts in host-microbe interactions with range expansion. Here we examine how the diversity of plant-associated microbes varies along the host's current range and extended range according with climate change predictions, and assess the relative influence of host genotype (seed provenance) and environment in structuring the host microbiome. We collected sugar maple seeds from across the species current range, then planted them in temperate and mixedwood/transitional forests (current range) and in the boreal region (beyond range but predicted future range in response to climate change). We used amplicon sequencing to quantify bacterial, fungal, and mycorrhizal communities from seedling leaves and roots. Variation among sites and regions were the main drivers of the differences in host microbial communities, whereas seed provenance did not play a large role. No unifying pattern was observed for microbial community richness, diversity, or specialization, demonstrating the complexity of responses of different taxa on above- and belowground plant compartments. Along the latitudinal gradient, we (1) observed reductions in mycorrhizal diversity that can negatively impact maple establishment; (2) and revealed reductions in fungal leaf pathogens that can have opposite effects. Our results highlight the need for an integrated approach including the examination of various microbial taxa on different plant compartments to improve our understanding of plant range shifts and plant-microbe interactions.

Maple and hickory leaf litter fungal communities reflect pre-senescent leaf communities.

Fungal communities are known to contribute to the functioning of living plant microbiomes as well as to the decay of dead plant material and affect vital ecosystem services, such as pathogen resistance and nutrient cycling. Yet, factors that drive structure and function of phyllosphere mycobiomes and their fate in leaf litter are often ignored. We sought to determine the factors contributing to the composition of communities in temperate forest substrates, with culture-independent amplicon sequencing of fungal communities of pre-senescent leaf surfaces, internal tissues, leaf litter, underlying humus soil of co-occurring red maple (Acer rubrum) and shagbark hickory (Carya ovata). Paired samples were taken at five sites within a temperate forest in southern Michigan, USA. Fungal communities were differentiable based on substrate, host species, and site, as well as all two-way and three-way interactions of these variables. PERMANOVA analyses and co-occurrence of taxa indicate that soil communities are unique from both phyllosphere and leaf litter communities. Correspondence of endophyte, epiphyte, and litter communities suggests dispersal plays an important role in structuring fungal communities. Future work will be needed to assess how this dispersal changes microbial community functioning in these niches.

Virulence of Fungi Isolated from Ambrosia Beetles to Acer amoenum Branches.

In Japan, no association between the ambrosia beetle and their fungal symbionts causing branch dieback or tree mortality on maple, Acer amoenum, has been reported. However, we identified dieback of several branches and numerous holes created by three species of ambrosia beetles, Euwallacea fornicatus, Euwallacea interjectus, and Platypus calamus, on Acer amoenum trees at the University of Tokyo Tanashi Forest, Tokyo Metropolis, Japan, in 2016. The high attack density of the beetles was observed on the weakened trees; however, the contribution of the associated fungi to the branch dieback was still unknown. We isolated fungi carried by these three beetles and inoculated them to Acer amoenum cut main trunks and sapling branches to determine whether the associated fungi caused the branch dieback. Fusarium euwallaceae was isolated from all Euwallacea fornicatus and Euwallacea interjectus, whereas Arthrinium phaeospermum, Raffaelea cyclorhipidia, and Epicoccum nigrum were isolated from P. calamus, with 35, 15, and 5% isolation frequencies, respectively. Inoculation with F. euwallaceae and R. cyclorhipidia induced statistically significantly wider sapwood discoloration (six and four times wider for F. euwallaceae and R. cyclorhipidia, respectively) than the controls, and larger water-conductance loss (2 and 1.7 times larger for F. euwallaceae and R. cyclorhipidia, respectively) than the controls. However, the observed lesions were not large enough to cause discoloration, and symptoms of dieback were not observed, even 13 months after the inoculation. Therefore, we concluded that the virulence of the four investigated fungi to Acer amoenum was very low and that these fungi were likely not the primary cause of the branch dieback.

A global genetic analysis of herbarium specimens reveals the invasion dynamics of an introduced plant pathogen.

The introduction, spread, and impact of fungal plant pathogens is a critical concern in ecological systems. In this study, we were motivated by the rather sudden appearance of Acermacrophyllum heavily infected with powdery mildew. We used morphological and genetic analyses to confirm the pathogen causing the epidemic was Sawadaea bicornis. In subsequent field studies, this pathogen was found in several locations in western North America, and in greenhouse studies, A. macrophyllum was found to be significantly more susceptible to S. bicornis than nine other Acer species tested. A genetic analysis of 178 specimens of powdery mildew from freshly collected and old herbarium specimens from 15 countries revealed seven different haplotypes. The high diversity of haplotypes found in Europe coupled with sequence results from a specimen from 1864 provides evidence that S. bicornis has a European origin. Furthermore, sequence data from a specimen from 1938 in Canada show that the pathogen has been present in North America for at least 82 years revealing a considerable lag time between the introduction and current epidemic. This study used old herbarium specimens to genetically hypothesize the origin, the native host, and the invasion time of a detrimental fungal plant pathogen.

The hierarchy of root branching order determines bacterial composition, microbial carrying capacity and microbial filtering.

Fine roots vary dramatically in their functions, which range from resource absorption to within-plant resource transport. These differences should alter resource availability to root-associated microorganisms, yet most root microbiome studies involve fine root homogenization. We hypothesized that microbial filtering would be greatest in the most distal roots. To test this, we sampled roots of six temperate tree species from a 23-year-old common garden planting, separating by branching order. Rhizoplane bacterial composition was characterized with 16S rRNA gene sequencing, while bacterial abundance was determined on a subset of trees through flow cytometry. Root order strongly impacted composition across tree species, with absorptive lower order roots exerting the greatest selective pressure. Microbial carrying capacity was higher in absorptive roots in two of three tested tree species. This study indicates lower order roots as the main point of microbial interaction with fine roots, suggesting that root homogenization could mask microbial recruitment signatures.

Duganella aceris sp. nov., isolated from tree sap and proposal to transfer of Rugamonas aquatica and Rugamonas rivuli to the genus Duganella as Duganella aquatica comb. nov., with the emended description of the genus Rugamonas.

A Gram-reaction-negative, strictly aerobic, betaproteobacterial strain, designated SAP-35T, was isolated from sap extracted from Acer pictum in Mt. Halla in Jeju, Republic of Korea, and its taxonomic status was examined by a polyphasic approach. Cells of the organism were non-sporulating, motile rods and grew at 4-30 °C, pH 6-7 and in the absence of NaCl. 16S rRNA gene- and whole genome-based phylogenetic analyses showed that strain SAP-35T belonged to the family Oxalobacteraceae and was closely related to Rugamonas rivuli (98.9% 16S rRNA gene sequence similarity) and Rugamonas aquatica (98.4%). The phylogenomic clustering and average amino acid identity values supported that strain SAP-35T belonged to the genus Duganella and two Rugamonas species should be transferred to the genus Duganella. The major isoprenoid quinone of the isolate was Q-8. The major polar lipids were phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol and an unidentified aminophospholipid. The predominant fatty acids were summed feature 3, C16:0 and C17:0 cyclo. The G + C content of genome was 64.9%. The average nucleotide identity and dDDH values between strain SAP-35T and the members of the genera Rugamonas and Duganella were < 85.1% and < 49%, respectively. Based on the combined data presented here, strain SAP-35T (= KCTC 72227T = NBRC 113903T) represents a novel species of the genus Duganella, for which the name Duganella aceris sp. nov. is proposed. Also, Rugamonas aquatica Lu et al. (Int J Syst Evol Microbiol 70: 3328-3334, 2020) and Rugamonas aquatica Lu et al. 2020 are reclassified as Duganella aquatica comb. nov., with the emended description of the genus Rugamonas.

Canopy Position Has a Stronger Effect than Tree Species Identity on Phyllosphere Bacterial Diversity in a Floodplain Hardwood Forest.

The phyllosphere is a challenging microbial habitat in which microorganisms can flourish on organic carbon released by plant leaves but are also exposed to harsh environmental conditions. Here, we assessed the relative importance of canopy position-top, mid, and bottom at a height between 31 and 20 m-and tree species identity for shaping the phyllosphere microbiome in a floodplain hardwood forest. Leaf material was sampled from three tree species-maple (Acer pseudoplatanus L.), oak (Quercus robur L.), and linden (Tilia cordata MILL.)-at the Leipzig canopy crane facility (Germany). Estimated bacterial species richness (Chao1) and bacterial abundances approximated by quantitative PCR of 16S rRNA genes exhibited clear vertical trends with a strong increase from the top to the mid and bottom position of the canopy. Thirty operational taxonomic units (OTUs) formed the core microbiome, which accounted for 77% of all sequence reads. These core OTUs showed contrasting trends in their vertical distribution within the canopy, pointing to different ecological preferences and tolerance to presumably more extreme conditions at the top position of the canopy. Co-occurrence analysis revealed distinct tree species-specific OTU networks, and 55-57% of the OTUs were unique to each tree species. Overall, the phyllosphere microbiome harbored surprisingly high fractions of Actinobacteria of up to 66%. Our results clearly demonstrate strong effects of the position in the canopy on phyllosphere bacterial communities in a floodplain hardwood forest and-in contrast to other temperate or tropical forests-a strong predominance of Actinobacteria.

Rahnella aceris sp. nov., isolated from sap drawn from Acer pictum.

A Gram-reaction-negative, facultatively anaerobic bacterium, designated SAP-19T, was isolated from sap extracted from Acer pictum in Mt. Halla in Jeju, Republic of Korea and its taxonomic statue was investigated by a polyphasic approach including genome- and 16S rRNA gene-based phylogenetic analyses. Cells were motile, short rods and showed growth at 20-30 °C, pH 4-9 and 0-6% (w/v) NaCl. The whole genome- and 16S rRNA gene-based phylogenetic analyses exhibited that strain SAP-19T belongs to the genus Rahnella and forms a tight cluster with Rahnella aquatilis. The isolate shared average nucleotide identity of 92.7% and 16S rRNA gene sequence similarity of 99.6% with the type strain of Rahnella aquatilis. The polar lipids contained phosphatidylethanolamine, an unidentified aminophospholipid and an unidentified lipid. The major isoprenoid quinone was Q-8. The predominant fatty acids were C16:0 and C17:0cyclo. The G + C content of the genome was 52.3%. The low average nucleotide identity (92.7%) and digital DNA relatedness (48.6%) values between the isolate and the most closely related strain showed that the isolate can be considered a different genospecies. On the basis of combined data obtained in this study, strain SAP-19T (= KACC 21744T = NBRC 114407T) represents a novel species of the genus Rahnella, for which the name Rahnella aceris sp. nov. is proposed.

Genome comparison and transcriptome analysis of the invasive brown root rot pathogen, Phellinus noxius, from different geographic regions reveals potential enzymes associated with degradation of different wood substrates.

Phellinus noxius is a root-decay pathogen with a pan-tropical/subtropical distribution that attacks a wide range of tree hosts. For this study, genomic sequencing was conducted on P. noxius isolate P919-02W.7 from Federated States of Micronesia (Pohnpei), and its gene expression profile was analyzed using different host wood (Acer, Pinus, Prunus, and Salix) substrates. The assembled genome was 33.92 Mbp with 2954 contigs and 9389 predicted genes. Only small differences were observed in size and gene content in comparison with two other P. noxius genome assemblies (isolates OVT-YTM/97 from Hong Kong, China and FFPRI411160 from Japan, respectively). Genome analysis of P. noxius isolate P919-02W.7 revealed 488 genes encoding proteins related to carbohydrate and lignin metabolism, many of these enzymes are associated with degradation of plant cell wall components. Most of the transcripts expressed by P. noxius isolate P919-02W.7 were similar regardless of wood substrates. This study highlights the vast suite of decomposing enzymes produced by P. noxius, which suggests potential for degrading diverse wood substrates, even from temperate host trees. This information contributes to our understanding of pathogen ecology, mechanisms of wood decomposition, and pathogenic/saprophytic lifestyle.

Deciphering bacterial and fungal endophyte communities in leaves of two maple trees with green islands.

Green islands (the re-greening of senescent leaf tissues) are particularly evident on leaves infected with fungal pathogens. To date, there is only a limited number of studies investigating foliar endophytic microorganisms in phytopathogen-infected leaves. Here, we analysed bacterial and fungal endophyte communities in leaves without green islands (control leaves; CL), within green island areas (GLA) and the surrounding yellow leaf areas (YLA) of leaves with green islands of Acer campestre and A. platanoides. GLA samples of A. campestre and A. platanoides were dominated by Sawadaea polyfida and S. bicornis, respectively, suggesting that these fungi might be responsible for the green islands. We detected a higher fungal richness and diversity in CL compared to GLA samples of A. campestre. Leaf status (CL, GLA, YLA) significantly altered the composition of fungal communities of A. campestre. This was related to differences in fungal community composition between YLA and GLA samples. Site was the main driver of bacterial communities, suggesting that bacterial and fungal endophytes are shaped by different factors. Overall, we observed Acer species-specific responses of endophyte communities towards the presence of green islands and/or leaf type, which might be attributed to several fungi and bacteria specifically associated with one Acer species.

Effects of root and leaf litter identity and diversity on oribatid mite abundance, species richness and community composition.

Habitat heterogeneity is an important driver of aboveground species diversity but few studies have investigated effects on soil communities. Trees shape their surrounding by both leaf litter and roots generating small scale heterogeneity and potentially governing community patterns of soil organisms. To assess the role of vegetation for the soil fauna, we studied whether tree species (Fagus sylvatica L., Acer pseudoplatanus L., Fraxinus excelsior L., Tilia cordata Mill.), markedly differing in leaf litter quality and root associated mycorrhizal symbionts, affect oribatid mite communities by shaping below- and aboveground resources and habitat complexity and availability. Oribatid mite abundance, species richness, community structure and the proportion of litter living and parthenogenetic individuals were analyzed and related to microbial biomass and the amount of remaining litter mass. Although leaf litter species with higher nutritional values decomposed considerably faster, microbial biomass only slightly differed between leaf litter species. Neither root species nor leaf litter species affected abundance, species richness or community structure of oribatid mites. However, root species had an effect on the proportion of parthenogenetic individuals with increased proportions in the presence of beech roots. Overall, the results suggest that identity and diversity of vegetation via leaf litter or roots are of minor importance for structuring oribatid mite communities of a temperate forest ecosystem.

Bacterial and Fungal Midgut Community Dynamics and Transfer Between Mother and Brood in the Asian Longhorned Beetle (Anoplophora glabripennis), an Invasive Xylophage.

Microbial symbionts play pivotal roles in the ecology and physiology of insects feeding in woody plants. Both eukaryotic and bacterial members occur in these systems where they facilitate digestive and nutrient provisioning. The larval gut of the Asian longhorned beetle (Anoplophora glabripennis) is associated with a microbial consortium that fulfills these metabolic roles. While members of the community vary in presence and abundance among individuals from different hosts, A. glabripennis is consistently associated with a fungus in the Fusarium solani species complex (FSSC). We used amplicon sequencing, taxon-specific PCR, culturing, and imaging to determine how bacterial and fungal communities differ between life stages and possible modes of symbiont transfer. The bacterial and fungal communities of adult guts were more diverse than those from larvae and eggs. The communities of larvae and eggs were more similar to those from oviposition sites than from adult female guts. FSSC isolates were not detected in the reproductive tissues of adult females, but were consistently detected on egg surfaces after oviposition and in frass. These results demonstrate that frass can serve as a vehicle of transmission of a subset for the beetle gut microbiota. Vertically transmitted symbionts are often beneficial to their host, warranting subsequent functional studies.

Leaf litter species identity influences biochemical composition of ectomycorrhizal fungi.

In forest ecosystems, ectomycorrhizal (ECM) fungi are important for plant growth and soil biogeochemical processes. The biochemical composition of ECM mycelium is an important fungal effect trait with consequences for its decomposition rate, and consequently on soil carbon pools and plant nutrition. Although the link between ECM fungi and leaf litter-released nutrients is well known, the response of ECM fungal biochemical composition to different leaf litter species remains poorly understood. To determine how leaf litter quality influences ECM fungi's biochemical profiles, we planted young beech trees in an oak forest and replaced the natural leaf litter with that of European beech (Fagus sylvatica), ash (Fraxinus excelsior), maple (Acer pseudoplatanus), or lime (Tilia cordata). We assessed the biochemical profiles of ECM root tips colonized by common fungal taxa in temperate forests (i.e., Cenococcum geophilum, Inocybe sp., and Lactarius subdulcis), using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). ECM fungal biochemical composition changed with leaf litter species. Changes were apparent in the infrared absorption bands assigned to functional groups of lipids, amides, and carbohydrates. C. geophilum and L. subdulcis exhibited large spectral differences corresponding to the initial pattern of leaf litter chemical composition between samples collected in the beech and ash leaf litter treatments. In contrast, Inocybe sp. was influenced by lime, but with no differences between samples from ash or beech leaf litter treatments. Although the spectral bands affected by leaf litter type differed among ECM fungi, they were mainly related to amides, indicating a dynamic response of the fungal proteome to soil nutritional changes. Overall, the results indicate that the biochemical response of ECM fungi to leaf litter species varies among ECM fungal species and suggests that the biochemical composition of ECM mycelium is a fungal response trait, sensitive to environmental changes such as shifts in leaf litter species.

Pigmentiphaga aceris sp. nov., isolated from tree sap.

Two Gram-stain-negative bacterial strains, SAP-32T and SAP-36, were isolated from sap drawn from the Acer pictum from Mount Halla in Jeju, Republic of Korea. The organisms were strictly aerobic, non-sporulating, motile rods and showed growth at 10-30 °C, pH 7-8 and with 0-2 % NaCl. The major isoprenoid quinone was Q-8. The predominant fatty acids were C16 : 0, cyclo-C17 : 0, summed feature 3 and C18 : 0. The polar lipids contained phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, an unknown aminophosphoglycolipid, an unknown glycolipid, an unknown phospholipid and two unknown lipids. The DNA G+C content was 64.4 mol%. The results of phylogenetic analyses based on 16S rRNA gene sequences indicated that SAP-32T and SAP-36 formed a distinct cluster with members of the genus Pigmentiphaga within the family Alcaligenaceae. Both strains showed 16S rRNA gene sequence similarity of 100 % to each other. The closest relatives of the isolates were Pigmentiphaga daeguensis (97.08 % sequence similarity), Pigmentiphaga kullae (97.01 %) and Pigmentiphaga litoralis (96.73 %). On the basis of data from phenotypic, chemotaxonomic and phylogenetic analyses, SAP-32T (=KCTC 52619T=DSM 104039T) and SAP-36 (=KCTC 52620=DSM 104072) represent members of a novel species of the genus Pigmentiphaga, for which the name Pigmentiphaga aceris sp. nov. is proposed.

Genome sequence and physiological analysis of Yamadazyma laniorum f.a. sp. nov. and a reevaluation of the apocryphal xylose fermentation of its sister species, Candida tenuis.

Xylose fermentation is a rare trait that is immensely important to the cellulosic biofuel industry, and Candida tenuis is one of the few yeasts that has been reported with this trait. Here we report the isolation of two strains representing a candidate sister species to C. tenuis. Integrated analysis of genome sequence and physiology suggested the genetic basis of a number of traits, including variation between the novel species and C. tenuis in lactose metabolism due to the loss of genes encoding lactose permease and ß-galactosidase in the former. Surprisingly, physiological characterization revealed that neither the type strain of C. tenuis nor this novel species fermented xylose in traditional assays. We reexamined three xylose-fermenting strains previously identified as C. tenuis and found that these strains belong to the genus Scheffersomyces and are not C. tenuis. We propose Yamadazyma laniorum f.a. sp. nov. to accommodate our new strains and designate its type strain as yHMH7 (=CBS 14780 = NRRL Y-63967T). Furthermore, we propose the transfer of Candida tenuis to the genus Yamadazyma as Yamadazyma tenuis comb. nov. This approach provides a roadmap for how integrated genome sequence and physiological analysis can yield insight into the mechanisms that generate yeast biodiversity.

Paenibacillus aceris sp. nov., isolated from the rhizosphere of Acer okamotoanum, a plant native to Ulleungdo Island, Republic of Korea.

Strain KUDC4121T was isolated from the rhizosphere of Acer okamotoanum, a plant native to the Korean island of Ulleungdo. The strain was a Gram-stain-positive, non-spore-forming, non-motile, rod-shaped bacterium that can grow at 18-37 °C and pH 6.0-7.5, with optimum growth at 30 °C and pH 7.0. It grew on tryptic soy agar containing less than 0.5 % (w/v) NaCl and in R2A broth. Cell length ranged from 2.0 to 2.5 µm. Strain KUDC4121T was oxidase- and catalase-positive and did not hydrolyse starch or casein. The genomic G+C content was 48.8 mol%. The major fatty acids were anteiso-C15 : 0 and iso-C16 : 0. Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain KUDC4121T belongs to the genus Paenibacillus. The closest type strain was Paenibacillus chondroitinus DSM 5051T, with 97.8 % similarity, followed by Paenibacillus alginolyticus DSM 5050T (97.6 %), Paenibacillus ferrarius CY1T (97.5 %), Paenibacillus pocheonensis Gsoil 1138T (97.5 %), Paenibacillus frigoriresistens YIM 016T (97.5 %), Paenibacillus pectinilyticus RCB-08T (97.2 %) and Paenibacillus aestuarii CJ25T (96.9 %). Based on its phenotypic properties and phylogenetic and genetic data, strain KUDC4121T should be considered to represent a novel species of the genus Paenibacillus, for which the name Paenibacillus aceris sp. nov. is proposed. The type strain is KUDC4121T (=KCTC 13870T=DSM 24950T).

Reduced snow cover alters root-microbe interactions and decreases nitrification rates in a northern hardwood forest.

Snow cover is projected to decline during the next century in many ecosystems that currently experience a seasonal snowpack. Because snow insulates soils from frigid winter air temperatures, soils are expected to become colder and experience more winter soil freeze-thaw cycles as snow cover continues to decline. Tree roots are adversely affected by snowpack reduction, but whether loss of snow will affect root-microbe interactions remains largely unknown. The objective of this study was to distinguish and attribute direct (e.g., winter snow- and/or soil frost-mediated) vs. indirect (e.g., root-mediated) effects of winter climate change on microbial biomass, the potential activity of microbial exoenzymes, and net N mineralization and nitrification rates. Soil cores were incubated in situ in nylon mesh that either allowed roots to grow into the soil core (2 mm pore size) or excluded root ingrowth (50 µm pore size) for up to 29 months along a natural winter climate gradient at Hubbard Brook Experimental Forest, NH (USA). Microbial biomass did not differ among ingrowth or exclusion cores. Across sampling dates, the potential activities of cellobiohydrolase, phenol oxidase, and peroxidase, and net N mineralization rates were more strongly related to soil volumetric water content (P < 0.05; R2  = 0.25-0.46) than to root biomass, snow or soil frost, or winter soil temperature (R2  < 0.10). Root ingrowth was positively related to soil frost (P < 0.01; R2  = 0.28), suggesting that trees compensate for overwinter root mortality caused by soil freezing by re-allocating resources towards root production. At the sites with the deepest snow cover, root ingrowth reduced nitrification rates by 30% (P < 0.01), showing that tree roots exert significant influence over nitrification, which declines with reduced snow cover. If soil freezing intensifies over time, then greater compensatory root growth may reduce nitrification rates directly via plant-microbe N competition and indirectly through a negative feedback on soil moisture, resulting in lower N availability to trees in northern hardwood forests.

Host species identity, site and time drive temperate tree phyllosphere bacterial community structure.

BACKGROUND: The increasing awareness of the role of phyllosphere microbial communities in plant health calls for a greater understanding of their structure and dynamics in natural ecosystems. Since most knowledge of tree phyllosphere bacterial communities has been gathered in tropical forests, our goal was to characterize the community structure and assembly dynamics of phyllosphere epiphytic bacterial communities in temperate forests in Quebec, Canada. We targeted five dominant tree species: Acer saccharum, Acer rubrum, Betula papyrifera, Abies balsamea, and Picea glauca. We collected 180 samples of phyllosphere communities on these species at four natural forest sites, three times during the growing season. RESULTS: Host functional traits (i.e., wood density, leaf nitrogen content) and climate variables (summer mean temperature and precipitation) were strongly correlated with community structure. We highlight three key findings: (1) temperate tree species share a "core microbiome"; (2) significant evolutionary associations exist between groups of bacteria and host species; and (3) a greater part of the variation in phyllosphere bacterial community assembly is explained by host species identity (27 %) and species-site interaction (14 %), than by site (11 %) or time (1 %). CONCLUSIONS: We demonstrated that host species identity is a stronger driver of temperate tree phyllosphere bacterial communities than site or time. Our results suggest avenues for future studies on the influence of host functional traits on phyllosphere community functional biogeography across terrestrial biomes.

Citizen science identifies the effects of nitrogen dioxide and other environmental drivers on tar spot of sycamore.

Elevated sulphur dioxide (SO2) concentrations were the major cause of the absence of symptoms of tar spot (Rhytisma acerinum) of sycamore (Acer pseudoplatanus), in urban areas in the 1970s. The subsequent large decline in SO2 concentrations has not always been accompanied by increased tar spot symptoms, for reasons that have remained unresolved. We used a large citizen science survey, providing over 1000 records across England, to test two competing hypotheses proposed in earlier studies. We were able to demonstrate the validity of both hypotheses; tar spot symptoms were reduced where there were fewer fallen leaves as a source of inoculum, and elevated nitrogen dioxide concentrations reduced tar spot symptoms above a threshold concentration of about 20 µg m(-3). Symptom severity was also lower at sites with higher temperature and lower rainfall. Our findings demonstrate the power of citizen science to resolve competing hypotheses about the impacts of air pollution and other environmental drivers.

Assembly of Active Bacterial and Fungal Communities Along a Natural Environmental Gradient.

Dormancy is thought to promote biodiversity within microbial communities, but how assembly of the active community responds to changes in environmental conditions is unclear. To measure the active and dormant communities of bacteria and fungi colonizing decomposing litter in maple forests, we targeted ribosomal genes and transcripts across a natural environmental gradient. Within bacterial and fungal communities, the active and dormant communities were phylogenetically distinct, but patterns of phylogenetic clustering varied. For bacteria, active communities were significantly more clustered than dormant communities, while the reverse was found for fungi. The proportion of operational taxonomic units (OTUs) classified as active and the degree of phylogenetic clustering of the active bacterial communities declined with increasing pH and decreasing C/N. No significant correlations were found for the fungal community. The opposing pattern of phylogenetic clustering in dormant and active communities and the differential response of active communities to environmental gradients suggest that dormancy differentially structures bacterial and fungal communities.