The relationship between dominant forest mycorrhizal type and the belowground mycobiome of the sugar maple and eastern hemlock

Climate change is imposing significant pressures on forest ecosystems, with many tree species projected to migrate towards the poles in response to changing temperature and precipitation regimes. Those who do not migrate will have to contend with these altered regimes, as well as changing surrounding forest communities. However, in both instances, biotic (e.g., mycorrhizal regimes and fungal communities) and abiotic conditions (e.g., soil chemistry) may act as important filters that potentially impede these migration and adaptation scenarios. Given that dominant forest mycorrhizal types are known to have strong impacts on forests’ belowground biogeochemistry and non-mycorrhizal microbial communities, their roles in this process should be more explicitly considered. The objectives of this project were therefore to determine the impact of dominant forest mycorrhizal types on soil and root (1) fungal diversity and community composition, (2) taxonomic and functional groupings, as well as (3) arbuscular mycorrhizal colonization along four gradients of forest mycorrhizal types. To do this, 435 seedlings of the sugar maple (Acer saccharum) and eastern hemlock (Tsuga canadensis) were identified, measured and harvested across four sites, Mont-Écho, Mégantic National Park, Lac-des-Plages, and the Station de biology des Laurentides. I characterized the root and soil fungal communities of maple and hemlock via amplicon sequencing, as well as root fungal colonization via microscopy. In addition, I characterized the edaphic parameters and the surrounding understory plant communities of seedlings. The results showed that forest mycorrhizal type had important associations with root fungal colonization by both arbuscular mycorrhizal and dark septate endophyte fungi, but I could not detect a significant impact on fungal diversity and community composition. However, I found evidence of dual-mycorrhization in the otherwise ectomycorrhizal eastern hemlock, which had only been previously reported in greenhouse studies. Conversely, dominant mycorrhizal type was found to strongly associate with the relative abundances of saprotrophic and ectomycorrhizal functional groups but showed no discernible association with fungal pathogens. Soil pH and calcium were significantly associated with dominant mycorrhizal types, while other soil variables showed no association. Taken together, these results suggest that dominant mycorrhizal types can have strong associations with certain fungal functional groups, important aspects of soil chemistry, and root colonization in both the eastern hemlock and sugar maple, which will be important factors to consider in the management of these species as they migrate into new forest regimes or see the composition of their resident forests change around them.