Ectomycorrhizal-dominated stands had significantly lower Hill numbers for bacterial/ archaeal communities at all orders of q, with 27% fewer effective species at q = 0, 40% fewer effective species at q = 1, and 46% fewer effective species at q = 2 (p < 0.05, Fig. 1). In contrast, we found no significant difference between EM-dominated and mixed AM-EM stands for fungal Hill numbers at any order of q (Fig. 1).
The relative abundances of fungal functional guilds differed significantly between stand mycorrhizal types. The relative abundance of EM fungi was significantly greater in EM-dominated stands (average 20%) relative to mixed EM-AM stands (average 0.3%; F1,18 = 11.75, p = 0.002; Fig. 2). On average, saprotrophic fungal relative abundance was 56% lower in EM-dominated than mixed AM-EM stands (F1,18 = 6.33, p = 0.02), and pathogenic fungal relative abundance was 50% lower in EM-dominated stands than AM-EM mixed stands (F1,18 = 10.2, p = 0.004; Fig. 2). We did not detect significant correlations among the relative abundances of saprotrophic, pathogenic, and EM functional guilds (p > 0.05). We also did not find a significant difference between stand mycorrhizal types in the relative abundance of ASVs not assigned to saprotrophic, pathogenic, or EM functional guilds (average 68.2%, p > 0.05).
Fungal and bacterial/ archaeal communities both differed significantly between stand mycorrhizal types, but the ways in which these communities differed, and the potential drivers of these differences were not the same for fungi and prokaryotes. Stand mycorrhizal type explained 7.71% of the variation in fungal community composition among samples (dbRDA: F1,18 = 2.81, p = 0.001), while site explained 13.1% of variation. Fungal communities in EM-dominated stands having significantly greater dispersion than those in mixed AM-EM stands (betadisper: F1,21 = 5.28, p = 0.03; Fig. 3a). Stand mycorrhizal type explained 15.3% of the variation in bacterial/ archaeal community composition (F1,18 = 5.02, p = 0.001), while site explained 14.1% of variation. We did not detect significant stand mycorrhizal type differences in community homogeneity (Fig. 3b).
There was a significant Procrustean rotational similarity correlation between fungal and bacterial/ archaeal ordinations (R2 = 0.78, p = 0.001). However, Procrustean residuals (PAM) were significantly lower (i.e., greater resemblance) in EM-dominated than mixed AM-EM stands (Fig. 3c). Further, there was a significant positive relationship between PAM and fungal pathogen relative abundance (R2 = 0.16, p = 0.03; Fig. 3d). We did not detect a similar relationship between PAM and the relative abundance of saprotrophic or EM fungi (p > 0.05).
The dominance of EM-associated trees was significantly correlated with the abundance fungal and bacterial/ archaeal groups at various phylogenetic levels, with some taxonomic groups exhibiting increased and others decreased relative abundance in mixed ECM-AM compared to ECM dominated stands. We found that three fungal phyla (Mortierellomycota, Kickxellomycota, and Chytridiomycota) were less abundant in EM-dominated than mixed AM-EM stands (Table S2). For fungi overall, five classes, six orders, 12 families, and 18 genera were less abundant in EM-dominated stands, while three classes, four orders, eight families, and nine genera were more abundant in these stands (Table S2). For bacteria/ archaea, six out of the ten phyla that responded to stand mycorrhizal type were less abundant in EM-dominated stands (i.e., Firmicutes, Gemmatimonades, Nitrospirae, WS3, GN02, and Bacteroidete); Proteobacteria, Acidobacteria, Planctomycetes, and WPS-2 were more abundant in EM-dominated stands (Table S3, Fig. 4). For prokaryotes overall, 31 classes, 73 orders, 120 families, and 115 genera were less abundant in EM-dominated stands (Table S3). Comparatively, far fewer prokaryotic groups were more abundant in EM-dominated stands, with five classes, nine orders, 14 families, and 18 genera falling into this category (Table S3). However, these groups often represented the most abundant bacterial/ archaeal groups overall, as shown in the graphical depictions of the relative abundance of the top 10 bacterial groups at these phylogenetic levels (Figures S1–S4).