4.1 Elevational patterns of plants
Contrary to the expected elevational patterns by mass effect, the alpha diversity of all the plant groups, except that for the shrubs, did not clearly show the highest values at the subalpine–alpine ecotone (ca. 2600–2800 m). Instead, these patterns were largely explained by climatic factors. These associations were examined in relation to the ecology of each plant group.
The alpha diversity of trees decreased with elevation (Figure 1b), and this elevational pattern was explained by Tempann. These results are supported by a previous study that the elevational range of trees can be determined by temperature, which strongly affects their physiochemical activities (Körner, 2003). This obvious influence of Tempann on tree diversity may have led to the relatively constant changes in the beta diversities as compared to those of other plant groups (Figure 2), because Tempann linearly decreases with elevation.
Similar to the results for trees, the alpha diversity of ferns also decreased with elevation (Figure 1e), which was attributed to Tempsnow in the GLM. These results are consistent with previous reports that fern species richness decreased at higher elevation and lower temperature in central Japan (Tanaka & Sato, 2013, 2014).
In contrast to the results for the trees and ferns, the alpha diversity of the herbs was positively correlated with elevation (Figure 1d). Given that snow cover positively affected herb diversity in the GLM, the species richness of the herbs increased with environmental changes related to snowfall, such as soil moisture and canopy closure. It has been known that long periods of snow augment soil moisture (Hardy et al., 2001), which is essential for the proliferation of herbs in arctic-alpine habitats (Litaor, Williams, & Seastedt, 2008; Nabe‐Nielsen et al., 2017; Roux, Aalto, & Luoto, 2013). Snow cover may decrease canopy closure by suppressing tree canopy growth (Song, Hogan, Brown, Cao, & Yang, 2017), which increases light intensity and reduces the accumulation of litter on the forest floor. These changes enhance herb species richness by guaranteeing the photosynthesis and germination of herbs (Speziale, Ruggiero, & Ezcurra, 2010).
However, the positive effects of snow cover on herb richness may be reduced at subalpine–alpine transition zone where a shrub species (P. pumila ) densely grows near the ground, since they shade out herbs. This effect explains the lower species richness of herbs at one 2800-m plot where the stone pine trees cover almost the entire plot. In fact, herbs at this plot were recorded in small areas uncovered by the stone pine trees; as a result, the herb layer mainly consisted of graminoids (Table S3), which prefer to grow in open areas (Roberts & Zhu, 2002; Thomas, Halpern, Falk, Liguori, & Austin, 1999).
The beta diversity of the herbs presented the highest values around the subalpine–alpine ecotones (Figure 2d). This is because shade-tolerant species dominate below the forest boundary, whereas species that can tolerate low temperatures (Jiang, Ma, Liu, & Tang, 2018) and light stress (e.g., graminoids growing in open areas between shrubs) prevail above it.
The alpha diversity of the shrubs rapidly increased from the subalpine–alpine to the alpine zone (Figure 1c). This elevational pattern was explained by the RHgrow in the GLM. Considering that canopy closure diminishes shrub diversity (Speziale, Ruggiero, & Ezcurra, 2010) and intensifies air humidity (Jung et al., 2017), RHgrow may be selected in the GLM as a surrogate that represents the negative influence of canopy closure on shrub richness. Accordingly, this change in canopy closure from dense to open may be attributed to the rapid increase in the beta diversity of the shrubs around the subalpine–alpine transition zone (Figure 2c).
Only the bryophytes showed a hump-shaped change in alpha diversity with increasing elevation (Figure 1f). The GLM results demonstrated that this pattern could be attributed to RHgrow, which is consistent with the results of a previous report suggesting that water stress is a major determinant of bryophyte diversity (Grau, Grytnes, & Birks, 2007). Therefore, alpha diversity was the highest in the subalpine zone with the highest RH. The higher beta diversity of bryophytes around the subalpine–alpine ecotones (Figure 2f) is discussed with the changes in functional types in the following section.