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.

Ectomycorrhizal fungal communities in two North American oak forests respond to nitrogen addition

How nitrogen (N) deposition impacts ectomycorrhizal (EM) fungal communities has been little studied in deciduous forests or across spatial scales. Here, it was tested whether N addition decreases species richness and shifts species composition across spatial scales in temperate deciduous oak forests. Combined molecular (terminal restriction fragment length polymorphism (T-RFLP), sequencing) and morphological approaches were used to measure EM fungal operational taxon unit (OTU) richness, community structure and composition at the spatial scale of the root, soil core and forest during a 3-yr N fertilization experiment in Quercus-dominated forests near Chicago, IL, USA. In N treatments, significantly lower OTU richness at the largest but not smaller spatial scales and a different community structure were detected. The effects of N appeared to be immediate, not cumulative. Ordination indicated the composition of EM fungal communities was determined by forest site and N fertilization. The EM fungi responded to a N increase that was low compared with other fertilization studies, suggesting that moderate increases in N deposition can affect EM fungal communities at larger spatial scales in temperate deciduous ecosystems. While responses at large spatial scales indicate that environmental factors can drive changes in these communities, untangling the impacts of abiotic from biotic factors remain limited by detection issues.

Evidence of species interactions within an ectomycorrhizal fungal community

Ectomycorrhizal fungal communities can be structured by abiotic and biotic factors. Here, we present evidence for community structuring by species interactions. We sampled ectomycorrhizas and forest floor seven times during a 13-month period. The presence of various ectomycorrhizal fungal species was determined for each sample, and species co-occurrence analyses were performed. For both ectomycorrhizas and forest floor samples there was significantly less co-occurrence among species within the community than expected by chance, mostly because of negative associations involving Cenococcum geophilum or Clavulina cinerea. For some species pairs, there was significantly more co-occurrence than expected by chance. Both nitrogen and tannin additions to the forest floor altered some interactions among species. The causes of these nonrandom distributions are currently unknown. Future investigations on competition, antibiosis, parasitism and facilitation among ectomycorrhizal fungal species appear to be warranted.

Long-term increase in nitrogen supply alters above- and below-ground ectomycorrhizal communities and increases the dominance of Russula spp. in a temperate oak savanna

•  Here we examine the effects of increased nitrogen (N) supply on the ectomycorrhizal fungal communities of a temperate oak savanna. •  In a 16-yr N-addition experiment in which replicate 1000 m² plots received 0, 5.4 or 17 g N m^-2  yr^-1 , ectomycorrhizal sporocarp production was measured in the 14th, 15th and 16th year of fertilization. Ectomycorrhizal fungi (EMF) colonizing roots were examined by morphotyping-PCR-RFLP and sequence analysis in the 14th and 15th year of fertilization. •  Total sporocarp richness was reduced by > 50% in both fertilization treatments in all 3 yrs, whereas Russula spp. produced approx. five times more sporocarps with 17 g N m^-2  yr^-1 . Below-ground, treatment-scale species richness and species area curves were lower with 17 g N m^-2  yr^-1 but richness, diversity indices and evenness at smaller spatial scales were not. Dominant fungi colonizing roots included Cenococcum geophilum, common in all treatments, Cortinarius spp., dominant in unfertilized plots, and Russula spp., dominant with 17 g N m^-2  yr^-1 . •  Communities of EMF in this temperate deciduous ecosystem responded to N addition similarly to those of coniferous ecosystems in that increased N supply altered EMF diversity and community composition but differently in that dominance of Russula spp. increased.

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

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