Plant-fungal associations can strongly influence forest carbon and nitrogen cycling. The prevailing framework for understanding these relationships in forests is through the relative abundance of arbuscular (AM) versus ectomycorrhizal (EcM) trees. Ericoid mycorrhizal (ErM) shrubs are also common in the understory of forests, and interactions between co-occurring ErM shrubs and AM and/or EcM trees could shift soil biogeochemical responses. Our study tested hypotheses regarding the effects of understory ErM shrubs on soil carbon and nitrogen concentrations across a gradient of AM versus EcM tree relative basal area. We predicted that the effects of ErM shrubs would either extend or be redundant with those of EcM trees. To test these hypotheses, we used surface soil data from 414 plots within a temperate forest in Connecticut, USA to analyze relationships between ErM plant cover, relative EcM tree basal area, and soil carbon and nitrogen concentrations while accounting for other biogeochemical controls, such as soil moisture and pH. We then used published ErM plant biomass data to analyze the abundance and distribution of ErM plants in boreal, temperate, and tropical forests and in AM- versus EcM-dominated forests to assess the co-occurrence of ErM shrubs in different forest types at the global scale.
Within the temperate forest site, ErM plants strongly modulated tree mycorrhizal dominance effects. The relative basal area of EcM trees was negatively associated with soil carbon and nitrogen, but these relationships were weak to negligible in the absence of ErM plants. Both EcM tree relative basal area and understory ErM plant cover were positively associated with the soil carbon-to-nitrogen ratio. However, this relationship was driven by relatively lower nitrogen for EcM trees and higher carbon for ErM plants. As such, the functional effects of ErM plants neither extended nor were redundant with those of EcM trees. Instead, EcM and ErM plant effects were functionally distinct. At the global scale, ErM plants occurred in 96%, 69%, and 29% of boreal, temperate, and tropical forests, respectively. Most ErM plant biomass was in EcM-dominated forests, but in tropical forests, the majority of ErM plant biomass was in AM-dominated stands. Our findings suggest that ErM shrubs could confound interpretation of AM versus EcM tree effects in forests where they co-occur and that this could have implications for predicting forest biogeochemical processes at the global scale. Overall, however, these results bolster growing calls to consider mycorrhizal functional types as variables that strongly influence forest biogeochemistry.