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.