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.