1. INTRODUCTION
Montane ecosystems are seriously threatened by global warming (Dullinger et al., 2012; Engler et al., 2011; Nogués-Bravo, Araújo, Errea, & Martínez-Rica, 2007), which has caused an upward shift in the distribution of warm-adapted plants and animals and has decreased the abundance of cold-adapted ones. In the mountainous regions of Europe, 36–55% of the alpine plant species, 31–51% of the subalpine species, and 19–46% of the montane species are expected to lose more than 80% of their suitable habitat by 2070–2100 due to climate change (Engler et al., 2011). Owing to the reduced opportunity for species upward migration, these predictions suggest a more pronounced influence of global warming on biota at higher elevations (Engler et al., 2011).
Elevation is the major factor that determines plant diversity in mountainous areas (Bhattarai & Vetaas, 2003; Grytnes, Heegaard, & Ihlen, 2006; Jiang, Ma, Liu, & Tang, 2018; Körner, 2007; Lee, Chun, Song, & Cho, 2013; Lee & La Roi, 1979; Sánchez‐González & López‐Mata, 2005; Zhou et al., 2019). These patterns are influenced by multiple factors including climate, local environments, spatial aspects, evolutionary processes, and biotic interactions (Culmsee & Leuschner, 2013; Grytnes, Heegaard, & Romdal, 2008; Lomolino, 2001; Song et al., 2020; Sun et al., 2013; Trigas, Panitsa, & Tsiftsis, 2013).
Plant responses to these factors are often explained by several hypotheses such as the mass effect, mid-domain effect, and Rapoport’s elevational rule. According to the mass effect hypothesis, species that cannot maintain viable populations in sink areas populate those adjacent to source areas occupied by larger populations (Grytnes, Heegaard, & Ihlen, 2006; Shmida & Wilson, 1985). These populations increase species richness, even to its maximum values around ecotones, where two different communities meet. Unlike this foregoing theory, the mid-domain effect hypothesis assumes a random distribution of species within a geometrically constrained domain such as that between the summit and base of a mountain. Consequently, species richness peaks in the middle of the domain because of the increasing overlap of species distributions towards the centre (Colwell & Lees, 2000; Colwell, Rahbek, & Gotelli, 2004). Rapoport’s elevational rule postulates that species at higher elevations can withstand a broad range of climatic conditions occurring across a high range of elevations (Stevens, 1992).
However, plant groups show disparate responses to elevation, and even plants within a single group have dissimilar elevational distributions across different areas (Bhattarai & Vetaas, 2003; Bruun et al., 2006; Grau, Grytnes, & Birks, 2007; Grytnes, Heegaard, & Ihlen, 2006; Lee et al., 2013; Lee & La Roi, 1979; Miyajima, Sato, & Takahashi, 2007; Sánchez‐González & López‐Mata, 2005; Zhou et al., 2019). These diverse responses indicate that the applicability of the aforementioned hypotheses may depend on the plant group and growth area. This highlights the importance of examining the elevational distribution of multi-plant groups in areas with erratic climate and diverse vegetation in understanding the drivers of plant diversity.
The mountainous areas of Japan experience strong winds and heavy snowfall due to the influence of jet streams and winter monsoons (Manabe, 1957; Riehl, 1962; Ueda, Kibe, Saitoh, & Inoue, 2015). This climate allows a single Japanese stone pine tree species (Pinus pumila Regel) to become dominant, owing to its resistance to wind and snow, at the subalpine-alpine transition zone (Okitsu, 1984). According to the mass effect hypothesis, this is a considerable change from the expected plant distribution, because the subalpine-alpine transition zones often harbour high species richness consisting of co-existing alpine and subalpine plants, resulting in hump-shaped patterns of species richness along these elevations (Grytnes, 2003; Grytnes, Heegaard, & Ihlen, 2006). However, this hump-shaped pattern has become suppressed due to the dominance of stone pine trees shading out other plants. These patterns further question the applicability of the hypotheses for predicting the elevational patterns of plants.
In this study, we investigated the elevational distribution of multi-plant groups in the Yatsugatake Mountains of Japan to determine whether the pertinent elevation hypotheses mentioned (i.e. mass effect, mid-domain effect, and Rapoport’s elevational rule) could substantiate the existing elevational patterns of the plant groups occurring there. The applicability of these hypotheses was examined based on the comparison between the actual elevational patterns of the plant groups and those expected by the following hypotheses:
Hypothesis 1: If the elevational patterns of plants follow the mass effect hypothesis, the highest species richness will be located around ecotones such as the intermediate zone between the subalpine and alpine zones.
Hypothesis 2: If the mid-domain effect hypothesis is applied to the elevational patterns of the plants, the distribution patterns will correspond to those expected by the random distribution of species between the summit and base of a mountain.
Hypothesis 3: If the distribution of plant species can be explained by Rapoport’s elevational rule, alpine species will exhibit larger elevational ranges than species at lower elevations.
Hypothesis 4: If the three elevational hypotheses do not apply to the observed patterns of plant distribution, other factors such as climate will largely determine their distribution.