Herbarium specimens reveal that mycorrhizal type does not mediate declining temperate tree nitrogen status over a century of environmental change.

Rising atmospheric carbon dioxide concentrations (CO) and atmospheric nitrogen (N) deposition have contrasting effects on ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) symbioses, potentially mediating forest responses to environmental change. In this study, we evaluated the cumulative effects of historical environmental change on N concentrations and δN values in AM plants, EM plants, EM fungi, and saprotrophic fungi using herbarium specimens collected in Minnesota, USA from 1871 to 2016. To better understand mycorrhizal mediation of foliar δN, we also analyzed a subset of previously published foliar δN values from across the United States to parse the effects of N deposition and CO rise. Over the last century in Minnesota, N concentrations declined among all groups except saprotrophic fungi. δN also declined among all groups of plants and fungi; however, foliar δN declined less in EM plants than in AM plants. In the analysis of previously published foliar δN values, this slope difference between EM and AM plants was better explained by nitrogen deposition than by CO rise. Mycorrhizal type did not explain trajectories of plant N concentrations. Instead, plants and EM fungi exhibited similar declines in N concentrations, consistent with declining forest N status despite moderate levels of N deposition.