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.