Despite covering vast areas of boreal North America, the ecological factors structuring mycorrhizal fungal communities in peatland forests are relatively poorly understood. To assess how these communities vary by age (younger vs. mature), habitat (fen vs. bog), and host (conifer trees vs. ericaceous shrub), we sampled the roots of two canopy trees (Larix laricina and Picea mariana) and an ericaceous shrub (Ledum groenlandicum) at four sites in northern Minnesota, USA. To characterize the specific influence of host co-occurrence on mycorrhizal fungal community structure, we also conducted a greenhouse bioassay using the same three hosts. Root samples were assessed using Illumina-based high-throughput sequencing (HTS) of the ITS1 rRNA gene region. As expected, we found that the relative abundance of ectomycorrhizal fungi was high on both Larix and Picea, whereas ericoid mycorrhizal fungi had high relative abundance only on Ledum. Ericoid mycorrhizal fungal richness was significantly higher in mature forests, in bogs, and on Ledum hosts, while ectomycorrhizal fungal richness did not differ significantly across any of these three variables. In terms of community composition, ericoid mycorrhizal fungi were more strongly influenced by host while ectomycorrhizal fungi were more influenced by habitat. In the greenhouse bioassay, the presence of Ledum had consistently stronger effects on the composition of ectomycorrhizal, ericoid, and ericoid-ectomycorrhizal fungal communities than either Larix or Picea. Collectively, these results suggest that partitioning HTS-based datasets by mycorrhizal type in boreal peatland forests is important, as their responses to rapidly changing environmental conditions are not likely to be uniform.

1. Boreal forests often have a dense understorey of ericaceous dwarf shrubs with ecological adaptations that contrast those of the canopy-forming trees. It is therefore important to quantify contributions by understorey shrubs to ecosystem processes and disentangle shrub-and tree-driven responses to climatic factors.2. We quantified soil respiration driven by the pine canopy and the ericaceous shrub understorey over 3 years, using a factorial pine root exclusion and shrub removal experiment in a mature Pinus sylvestris forest. Soil temperature and moisturerelated responses of respiration attributed to autotrophs (shrubs, pine roots) and heterotrophs were compared. Additionally, we assessed effects of interactions between these functional groups on soil nitrogen availability and respiration.3. Understorey shrubs accounted for 22% ± 10% of total autotrophic respiration, reflecting the ericaceous proportion of fine root production in the ecosystem.Heterotrophic respiration constituted about half of total soil respiration. Shrubdriven respiration was more susceptible to drought than heterotrophic-and pine-driven respiration. While the respiration attributed to canopy and understorey remained additive, indicating no competitive release, the plant guilds competed for soil N. Synthesis.Ericaceous understorey shrubs accounted for a small, yet significant, share of total growing season soil respiration. Overlooking understorey respiration may lead to erroneous partitioning and modelling of soil respiration mediated by functional guilds with contrasting responses to soil temperature and moisture. A larger contribution by heterotrophs and pine root-associated organisms to soil respiration under drought conditions could have important implications for soil organic matter accumulation and decomposition as the climate changes in boreal forests.

<p>Boreal forest soils are dominated by three fungal guilds; ectomycorrhizal fungi associated with canopy-forming trees, ericoid mycorrhizal fungi associated with understory shrubs and free-living saprotrophic fungi. We followed decomposition of pine needle litter and mor-layer humus in a factorial pine root exclusion and shrub removal experiment in a mature pine forest over three years to evaluate fungal guild effects on mass loss. Litter mass loss was 10% faster when ectomycorrhizal fungi were excluded, however this ‘Gadgil effect’ was only found in one of two litter sets, and it was independent of shrub presence. In contrast, humus mass loss was hampered by shrub presence and promoted by ectomycorrhizal fungi, although presence of both guilds resulted in the largest humus mass remaining. This suggests that saprotrophic-ectomycorrhizal interactions are of little significance for early-stage litter decomposition, while ericoid and ectomycorrhizal guilds interact to determine late-stage organic matter balance in boreal forest soils.</p>

The immense diversity and biomass of ericoid-, ectomycorrhizal, and saprotrophic fungal guilds in boreal forest soils make them vital components of conservation and ecosystem processes, and in particular, many ectomycorrhizal fungi are considered species of conservation concern. However, amalgamated information on the functions and relationships of soil fungi to perceived forest conservation values, and how inter and intra-guild interactions affect the accretion and decomposition of soil organic matter is lacking. In a long-term factorial shrub removal and pine root exclusion experiment, I assessed guild contributions to soil respiration and decomposition of organic substrates guided by ecological theory. Then in the northern and southern boreal forest, I evaluated whether forest conservation values are aligned with the diversity of ectomycorrhizal fungi. Overall, the ericoid guild makes a significant contribution to total soil respiration (11 ± 9%), and ericoid activities appeared to be more sensitive to periods of drought compared to ectomycorrhizal (43 ± 1%) and saprotrophic (53 ± 5%) guilds. Saprotrophic-ectomycorrhizal interactions during decomposition led to a modest, yet inconsistent Gadgil effect (10%) for early-stage litter decomposition. Ericoid and ectomycorrhizal guilds interactions were determined to be more important for late-stage organic matter balance in boreal forest soils. Ectomycorrhizal species richness was significantly higher in the southern boreal forest compared to the north. Furthermore, forest conservation values across the boreal forest were not adequately related to ectomycorrhizal diversity through DNA-metabarcoding. Instead, soil fertility, corresponding to tree species basal area, was the clearest indicator of ectomycorrhizal diversity and composition in both regions. Mycorrhizal guilds may be underappreciated and understudied in terms of conservation, but their functional roles in the accumulation and decomposition of organic matter in long-term soil carbon pools emphasizes the importance of evaluating the many dimensions of fungal conservation in boreal forests.

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