Long‐term nitrogen addition does not sustain host tree stem radial growth but doubles the abundance of high‐biomass ectomycorrhizal fungi

Global change has altered nitrogen availability in boreal forest soils. As ectomycorrhizal fungi play critical ecological functions, shifts in their abundance and community composition must be considered in the response of forests to changes in nitrogen availability. Furthermore, ectomycorrhizas are symbiotic, so the response of ectomycorrhizal fungi to nitrogen cannot be understood in isolation of their plant partners. Most previous studies, however, neglect to measure the response of host trees to nitrogen addition simultaneously with that of fungal communities. In addition to being one‐sided, most of these studies have also been conducted in coniferous forests. Deciduous and “dual‐mycorrhizal” tree species, namely those that form ecto‐ and arbuscular mycorrhizas, have received little attention despite being widespread in the boreal forest. We applied nitrogen (30 kg ha−1 year−1) for 13 years to stands dominated by aspen (Populus tremuloides Michx.) and hypothesized that tree stem radial growth would increase, ectomycorrhizal fungal biomass would decrease, ectomycorrhizal fungal community composition would shift, and the abundance of arbuscular mycorrhizal (AM) fungi would increase. Nitrogen addition initially increased stem radial growth of aspen, but it was not sustained at the time we characterized their mycorrhizas. After 13 years, the abundance of fungi possessing extramatrical hyphae, or “high‐biomass” ectomycorrhizas, doubled. No changes occurred in ectomycorrhizal and AM fungal community composition, or in ecto‐ and AM abundance measured as root colonization. This dual‐mycorrhizal tree species did not shift away from ectomycorrhizal fungal dominance with long‐term nitrogen input. The unexpected increase in high‐biomass ectomycorrhizal fungi with nitrogen addition may be due to increased carbon allocation to their fungal partners by growth‐limited trees. Given the focus on conifers in past studies, reconciling results of plant–mycorrhizal fungal relationships in stands of deciduous trees may demand a broader view on the impacts of nitrogen addition on the structure and function of boreal forests. We applied nitrogen (30 kg ha−1 year−1) for 13 years (2006–2019) to a boreal forest dominated by aspen (Populus tremuloides). Stem radial growth initially increased with nitrogen addition but then decreased with time. After 13 years, the abundance of fungi with extramatrical hyphae, or ‘high‐biomass’ ectomycorrhizas, doubled with nitrogen addition. The un