Discussion
The range size of all groups of vascular plants shown uniform increasing
trends along the elevational gradient of the Gyirong Valley. As our
prediction, climate factors did play a greater role than other factors
in shaping above trends. Both of the climate variability hypothesis and
the mean climate condition hypothesis were supported to explain such
climate-range size relationship. Therefore, it is not surprising that
Rapoport’s rule was supported regardless of life form and
biogeographical affinities.
4.1 The influence of life form and biogeographical
affinities
Life form and biogeographical affinities have been considered to cause
differences in the response of species to environmental gradient, but
studies about how they influence the elevational variations in species
range size are relatively scarce (but see Feng et al., 2016 and Zhou et
al., 2019). In Mount Kenya, Zhou et al. (2018) observed a monotonic
increasing trend for herbaceous species but an obvious right skewed
unimodal trend for woody species, whereas in Nepal, Feng et al. (2016)
reported that the tropical species shown partial support for Rapoport’s
rule, while temperate species opposed the rule. However, in the Gyirong
Valley of the central Himalayas, range size of vascular plants across
different life form and biogeographical affinities showed uniform
increasing trends, though woody species and tropical species did have
relatively stronger range size-elevation relationship as they are
supposed to be more sensitive to environmental gradient. Zhou et al.
(2018) attributed the decrease of woody species range size at the higher
elevation of Mount Kenya to higher proportion of endemic species. But in
the Gyirong Valley, both richness and proportion of endemic species
shown a left skewed unimodal pattern (Fig. S2), higher elevation was
characterized by widely distributed nonendemic species such asSpiraea alpine , Potentilla parvifolia , Lonicera
spinosa , which might account for above difference in elevational trends
of woody species range size. On the other hand, it must be noted that
none of the climatic variability variables in Nepal shown increase
trends with elevation (Feng et al., 2016), whereas all climatic
variability variables in the Gyirong Valley increased monotonically
along elevational gradient. Given that the increasing climatic
variability gradient is indispensable for Rapoport’s rule, it’s thus the
rule got equivocal supported in Feng’s study but got strong supported in
our study. Taken together, we have reasons to contend that the influence
of life form and biogeographical affinities on the range size variation
may be context-dependent.
4.2 The role of different environmental
factors
Climate, especially contemporary climate, played a greater role than
other environmental factors in shaping the increasing trends of vascular
plants range size in the Gyirong Valley. This result echo the
predominance of climate in determining elevational gradient of plants
richness in the Himalayas (Bhattarai and Vetaas 2003; Manish et al.,
2017; Sun et al., 2020; Liang et al., 2020), which could be attribute to
the fact that the Himalayas have a more distinct and complete vertical
climatic gradient compare to that of most mountains at the same latitude
due to its unparalleled elevational range. For example, all contemporary
climate variables including MAT, MAP, TS, and MATR shown monotonic
trends along the elevational gradient in the Gyirong Valley. TS and MATR
are the most important variables for range size of all groups of
vascular plants and showed significant positive relationship with range
size. Higher elevations where climate is more variable did indeed harbor
more large range species, which provide a supporting evidence for the
climate variability hypothesis. In addition, the mean climate condition
hypothesis was also supported as MAT and MAP shown great importance and
significant negative relationship with range size, suggesting that the
impacts of climate variability and mean climate condition on range size
variation are inseparable.
Beyond the primary importance of contemporary climate, competition and
historical climate also play supplementary roles in shaping the
elevational trends of range size. Competition could constrain species
dispersal (Jiang & Ma, 2014). Species in communities with high species
richness tend to have narrow distribution, and vice versa (Jiang & Ma,
2014). Thus, it is not surprising that species range size was correlated
negatively with SR along the elevational gradient in the Gyirong Valley.
What’s surprising is that, species range size also showed negative
relationships with TC and PC. It is possible that historical climate
oscillations could promote speciation (Hewitt, 1996, 2004; Leprieur et
al., 2011; Zhao et al., 2016), resulting in higher proportion of
narrowly distributed endemic species at the lower elevations of the
Gyirong Valley.
Disturbance factors and MDE pose minimal contribution to the elevational
variation in range size. Since the Gyirong Valley is located within the
Mount Qomolangma National Nature Reserve (Fig. 1), human activities are
restricted in the vicinity of Gyirong town and Zongga town, and hence
has less impact on the distribution of vascular plants. The influence of
the MDE was affected by the range size of species, species with large
range are more sensitive to MDE (Colwell et al., 2004). In the Gyirong
Valley, over 90% of the vascular plant has small range less than 1800 m
(half of the sampling gradient), which could account for weak
explanation power of MDE.
4.3 The applicability of Rapoport’
rule
Since its formulation, the validity of Rapoport’s rule has been
controversial. The applicability of the rule varies greatly in different
region around the world. In general, the rule appears to be more
well-defined in Northern Hemisphere and higher latitude than in Southern
Hemisphere and lower latitude (Böhm et al., 2017). It is important to
remember that, when Stevens first introduced the Rapoport’s rule in
1989, he emphasized the rule should apply to species that live in
regions with conspicuous gradient of climate variability (Stevens 1989).
Further studies have been also confirmed the necessity of climate
variability to the validity of Rapoport’s rule. For example, Whitton
(2011) suggested that the primary importance of climate variability may
explain why Rapoport’s rule is largely restricted to northern latitudes,
as this is where temperature seasonality is most pronounced. Similarly,
Pintor et al. (2015) attributed the absence of Rapoport’s rule in
Australia to the complex climate pattern across the whole continent with
minimum and maximum temperatures varying considerably at any given
latitude. In our study, climate variability showed monotonical
increasing pattern along the elevational gradient in Gyirong Valley, and
was the most influential factor behind the elevational variation in
range size of all groups of vascular plants. Therefore, it’s not
surprising that Rapoport’s rule was supported regardless of life form
and biogeographical affinities. Our results challenge previous argument
that life form and biogeographical affinities may influence the
applicability of Rapoport’ rule, and support that climate variability
are the ultimate determinant for the validity of Rapoport’s rule.
4.4 Conservation
implication
Since climate plays a large role in determining species range, there is
urgent need to keep eyes on the impact of climate change. It has been
widely reported that climate change will force species to shift their
distribution upward in mountains (Feeley et al., 2010, 2011; Rehm,
2014), and lead to the movement of elevational biodiversity hotspot (Wu
et al., 2016). On the other hand, climate change has been also
implicated in species range contractions at many mountains. For example,
Engler et al. (2011) assess the impacts of climate change on 2632 plant
species across all major European mountain ranges, and found that
36-55% of alpine species, 31-51% of subalpine species and 19-46% of
montane species lose more than 80% of their suitable habitat by
2070-2100. Since the Himalayas are among the most sensitive region to
climate change (Xu et al., 2009), we have reasons for concerns about the
susceptibility and adaption of plants to the impact of climate change.
Specifically, given the extreme environmental condition and geographic
constraints at the high elevation of the Gyirong Valley, plants might
fail to expand their upper limit while their lower range limit rise with
their upward range shift under climate change, leading to range
contraction, and even extinction of narrow range species. Considering
the response to climate change are species-specific, therefore,
long-term monitoring is imperative for understanding the impact of
climate change on local biodiversity.