Differences between arbuscular (AM) and ectomycorrhizal (EcM) trees strongly influence forest ecosystem processes, in part through their impact on saprotrophic fungal communities. Ericoid mycorrhizal (ErM) shrubs likely also impact saprotrophic communities given that they can shape nutrient cycling by slowing decomposition rates and intensifying nitrogen limitation. We investigated the depth distributions of saprotrophic and EcM fungal communities in paired subplots with and without a common understory ErM shrub, mountain laurel (Kalmia latifolia L.), across an AM to EcM tree dominance gradient in a temperate forest by analyzing soils from the organic, upper mineral (0–10 cm), and lower mineral (cumulative depth of 30 cm) horizons. The presence of K. latifolia was strongly associated with the taxonomic and functional composition of saprotrophic and EcM communities. Saprotrophic richness was consistently lower in the Oa horizon when this ErM shrub species was present. However, in AM tree-dominated plots, the presence of the ErM shrub was associated with a higher relative abundance of saprotrophs. Given that EcM trees suppress both the diversity and relative abundance of saprotrophic communities, our results suggest that separate consideration of ErM shrubs and EcM trees may be necessary when assessing the impacts of plant mycorrhizal associations on belowground communities.
We carried out this study in the northeastern United States at Yale-Myers Forest (41°57’ N, 72°07’ W), which is characterized as temperate deciduous forest with a mean annual precipitation of 133 cm, a mean January temperature of -4.6°C, and mean July temperature of 21.7°C. The three forest stands used in this study are situated on glacial origin inceptisols from the Nipmuck-Brookfield complex and Woodbridge series, which consist of generally fine sandy loam soils (National Resources Conservation Service [NRCS], 2023). Elevations within sites ranged from 180-290 m and the mean plot slope was 9 degrees (range 0.5-26; CT ECO, 2016). The mean organic soil pH was 4.28 (range 3.16 -5.34) and mean mineral soil was 4.43 (range 2.78-5.48). Overall, the forest understory at our study site had a patchy distribution of the ErM shrub *K. latifolia*, which is the most abundant understory plant species at the forest and accounts for about one-third of all understory vegetation cover (Ward et al. 2021). In the organic horizon, carbon (C), and nitrogen (N) stocks were generally higher in plots with *K. latifolia*. In the surface mineral horizon (0-15 cm), soil C and N stocks were negatively associated with the percentage of EcM trees relative to AM trees (Ward *et al.*, 2023). ErM plant species other than *K. latifolia *make up a small percentage of understory plant cover at our forest site (<2%; Ward et al. 2021), so we limited our study to *K. latifolia *since it was consistently present under both AM and EcM tree associations within each of the three stands.
We set up six plots within the three forest stands that each included paired 1-m radius subplots with and without *K. latifolia* (n= 36 total). Subplots were ~2 m apart with one under *K. latifolia* and the other in open understory habitat with no shrub layer. This orthogonal design ensured that there was no correlation between tree mycorrhizal association and the absence or presence of *K. latifolia*, permitting the ErM shrub effect to be disentangled from the effects of tree mycorrhizal associations. We identified and measured diameter at breast height (DBH; 1.37 m height) of all trees ≥20 cm DBH within 10 m of plot center, ≥5 cm DBH within 5 m of plot center, and 1-5 cm DBH within 1 m of each subplot center (i.e. the nested subplots with or without *K. latifolia*). We calculated the basal area of each tree species in m2 ha-1, assigned mycorrhizal associations to each tree genus based on the designations in Soudzilovskaia et al. (2020), and calculated canopy tree mycorrhizal dominance in each plot as the percentage of EcM tree basal area out of total basal area. This study design resulted in plots that ranged from 0 to 97% EcM tree basal area.
Soil sampling was carried out in June 2021. In each subplot (*n* = 36), we collected soil samples from three depths: the organic horizon (Oa; Fig. 1, left panel), surface mineral horizon (0-10 cm of mineral horizon; Fig. 1, top right panel), and subsurface mineral horizon (beginning from a depth of 10 cm in the mineral horizon to a cumulative depth, including the Oa, of 30 cm; Fig. 1, bottom left panel). We sampled the Oa horizon by first removing plant litter and then pooling two 25 x 25 cm areas of Oa. For the mineral horizons, we pooled two cores from each depth using a 5-cm diameter soil corer. Out of a total of 108 subplot and depth samples, two subplots did not have an Oa horizon, resulting in a total of 106 soil samples. Soils from each subplot at the three soil depths were passed through a 4-mm sieve. A 5-g subsample of soil was placed in a sterile Whirl-Pak bag which was frozen at -20°C until DNA extraction. Soil pH was measured on fresh soil using a 1:1 volumetric soil to deionized water ratio and a benchtop pH probe. Gravimetric soil moisture was measured from fresh soil by oven drying soils at 105°C for 24 h.
