*3.1. Arbuscular Mycorrhizal Colonization in Plant Roots at Different AltitudesSubsection*

The colonization of AM varied from 0 to 100%, with an average of 55.03% from 663 m to 3511 m (Figure 1a). AM colonization showed a quadratic function trend with *R*<sup>2</sup> was 53.46% and *P* was less than 0.01. It suggested that altitude has a significant effect on AM colonization. The highest colonization of AM occurred at 1170 m and 1450 m. Meanwhile, the highest colonization density also occurred at 1170 m with 41.09% (Figure 1b). And the colonization density changed from 0 to 41.09%, with an average of 10.39% in Mt. Taibai. Besides, the colonization density formed the quadratic function with *R*<sup>2</sup> was 34.18%. Elevation had a significant effect on AM colonization density.

**Figure 1.** Change of arbuscular mycorrhizal colonization (**a**) and colonization density (**b**) in plant roots among different altitudes. Note: There are overlapping data points in (**a**,**b**).

### *3.2. AMF Community Composition and Distribution at Different Altitudes*

A total of 287 OTUs belong to 62 species belonging to 8 identified and 1 unclassified genus representing four identified and one unidentified order (Table S2). Among them, *Glomus* was the dominant genus with the largest number of species and OTUs with 39 and 104, respectively. However, only 1 OTU was identified in the genus of *Pacispora*. In addition, the maximum number of species and OTU occurred at the altitude of 2190 m, with 38 and 166, respectively. While the minimum number of species occurred at 3097 m, and the minimum number of OTU was 25 and occurred at 2828 m.

The highest relative abundance of *Glomus* was 99.14%, which occurred at 1800 m (Figure 2). In addition, the relative abundance of *Glomus* exceeded 90% at the altitudes of 663 m, 1170 m, 1450 m, 1800 m, and 2070 m. At the higher altitudes of 2828 and 3250 m, the relative abundance of *Acaulospora* was higher with 49.14% and 56.52%. Besides, the genus of *Pacispora* only appeared above 2000 m and the relative abundance was the highest with 17.85% at the highest altitude of 3511 m.

**Figure 2.** Relative abundance of genera identified based on different altitudes at the genus level. Note: The genus, whose abundance was less than 1%, were collectively classified as others.

### *3.3. Diversity of AMF in Mt. Taibai*

AMF alpha diversity is expressed by the Sobs and Shannon indices (Figure 3). Both Sobs and Shannon indices showed the trends of increased first and then decreased with the increase of altitude based on the species and OTU level. Whether at species or OTU level, the highest Sobs and Shannon index both occurred at 2460 m. The highest Sobs index were 26 and 49 on the level of species and OTU, respectively (Figure 3a). At the same time, the highest Shannon indices were 2.68 and 2.08 at the level of species and OTU (Figure 3b). And the lowest alpha diversity indices both appeared at higher altitudes. With the increase in altitude, the changing trend of Sobs and Shannon indices could be simulated with quadratic function at the species and OTU level. What's more, elevation had a significant effect on alpha diversity indices.

AMF beta diversity was revealed by nonmetric multidimensional scaling analysis (NMDS) (Figure 4). The NMDS ordination resulted in a final stress value of 0.14 and 0.18 on species and OTU level, respectively. The results indicated that beta diversity also differed among the altitudes based on species and OTU level in Mt. Taibai.

**Figure 3.** The variation of AMF is based on the species and OTU level among different elevations with the Sobs index of species (**a**) and the Shannon index of species (**b**).
