4.2 Functional types and plant–plant interactions
The elevational patterns of plant diversity are further discussed in
relation to plant functional types. Evergreen shrub species (P.
pumila ) began to dominate at the subalpine–alpine ecotone (ca.
2600–2800 m) instead of evergreen conifer trees. These changes are
displayed by the increase in the dominance of evergreen shrubs
accompanied by the decrease in that of evergreen trees (Figure 3a, 3b).
In accordance with these changes, the dominance of graminoids increased,
whereas that of liverworts decreased at these elevations (Figure 3c,
3d). These associations can be attributed to the improved light
availability and reduced humidity owing to the change from a close to an
open canopy. Graminoids dominate open forests (Roberts & Zhu, 2002;
Thomas, Halpern, Falk, Liguori, & Austin, 1999) and are more resistant
to drought stress than forbs (Rosbakh et al., 2017), whereas liverworts
are more vulnerable to drought stress (Grau Grytnes, & Birks, 2007; Jan
& Wolf, 1993) and are more shaded-adapted (Marschall & Proctor, 2004)
than mosses. Given the characteristics of these functional types, the
reduced canopy caused the increase in light availability (except for
under the P. pumila cover) and drought stress at the
subalpine–alpine ecotone, leading to the changes in the functional
types of herbs and bryophytes. These changes in plant functional types
support the increase in the beta diversity of herbs and bryophytes at
2600–2800 m (Figure 2d, 2f).
4.3 Mass effect on elevational patterns
Among all the plant groups, only the shrubs clearly showed the highest
alpha diversity around the subalpine–alpine transition zone (Figure 1),
which contradicts the expected patterns based on mass effect (Hypothesis
1). These results may be explained by the combined influence of
plant-plant interactions and climatic factors, as the dominance of stone
pine trees in the transition zone inhibits the establishment of small
plant species by shading and curbs the increase in the species richness,
as discussed in section 4.1.