| Summary: | Societal Impact Statement Mycorrhizal fungi enable plants to thrive in the cold, waterlogged, organic soils of boreal peatlands and, with saprotrophic fungi, largely contribute to the sequestration of atmospheric carbon in peat. Hence, fungi support the contribution of peatlands to global climate regulation, on which society depends. Here we used high‐resolution minirhizotrons for an unprecedented glimpse of the belowground world of a forested bog and highlighted linkages between environmental change and the abundance, dynamics, and morphology of vascular plant fine roots and fungal mycelium. These changes may have implications for peat carbon accumulation on the boreal landscape. Summary Minirhizotron technology has rarely been deployed in peatlands which has limited our understanding of root‐fungal dynamics in one of planet's most carbon‐dense ecosystems. We used novel, high‐resolution minirhizotrons in a forested bog to explore temporal variation in the abundance and growth of plant fine roots and fungal mycelium with changes in peat temperature and moisture. We utilized the framework of the Spruce and Peatland Responses Under Changing Environments experiment and focused on two minirhizotron tubes installed at the coldest (+0, elevated CO2) and warmest (+9°C, elevated CO2) ends of the experimental temperature gradient, respectively. We found that in warmer and drier peat, ericaceous shrub roots and ectomycorrhizal fungal rhizomorphs were more abundant, and the growth of rhizomorphs and sporocarps was greater. In turn, fine roots of trees, ectomycorrhizas, and dark‐colored fungal hyphae were more abundant in colder, wetter peat. Ultimately, the belowground active season for both plant roots and fungi was extended by 62 days at the warmest compared to the coldest end of the gradient, with implications for belowground carbon, water, and nutrient fluxes. High‐resolution minirhizotrons in peatlands provided an unprecedented view of ericaceous shrub and tree fine roots and their mycorrhizal fungal partners in situ. Therefore, this technology advanced our understanding of linkages between environmental change and the abundance, morphology, and dynamics of vascular plant fine roots and fungal mycelium.
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