Introduction

The species range size is a fundamental unit in macroecology (Böhm et al., 2017). Understanding variation of species range size along spatial gradients is of primary importance in study of climate change, biodiversity pattern, gene flow and extinct mechanism. A well-known theory about spatial variation in species range size is the Rapoport’s rule, which proposed that species range size correlates positively with latitude and elevation (Stevens, 1989). However, despite of multiple early evidences from the Northern Hemisphere (e.g., Letcher & Harvey, 1994; Blackburn & Gaston, 1996; Arita et al., 2005), further study from other regions have found complex and partial support for the rule (e.g., Hawkins et al., 2006; Whitton et al., 2011; Feng et al., 2016), suggesting that the rule might be a regional phenomenon hinge on local environment (Whitton et al., 2011). Therefore, recent attention has shifted from simply documenting variation in range size to exploring drivers for the variation.
Various studies have been conducted to understand the association between variation in range size and environment factors such as climate (e.g., Whitton et al., 2011; Sheldon & Tewksbury, 2014), disturbance (e.g., Lozada et al., 2008; Borkowski et al., 2016), competition (e.g., Glazier & Eckert, 2002; Grigione et al., 2010), and the mid-domain effect (e.g., Luo et al., 2011). Climate factor is the most widely supported driver in terms of both latitudinal and elevational variation in range size. Several hypotheses have been proposed to explain the climate-range size relationship, among which the climate variability hypothesis is most commonly accepted (Whitton et al., 2011; Pintor et al., 2015). This hypothesis was first proposed by Stevens at 1989, and was believed to be the underlying mechanism of Rapoport’s rule (Stevens, 1989,1992). Stevens stated that climate specifically temperature is more variable at higher latitudes and elevations. Such greater climate variability favor species with wider tolerances and larger ranges, and thus leading to a positive relationship between range size and latitude and elevation. The mean climate condition hypothesis is another prominent explanation for climate-range size relationship, which are supposed to cooperate with climate variability to generate increasing trends of range size along latitude and elevation (Luo et al., 2011). The mean climate condition hypothesis proposes that species living at higher latitude or elevation are not only subject to greater climate variation but also lower mean climate condition, and thus tend to be geographically widely distributed (Luo et al., 2011; Jiang & Ma, 2014). Besides contemporary climate, the historical climate such as Quaternary climate has also been proposed as explanation for range size variations based on the premise that historical climate oscillations select for species with wider physiological tolerance and adaption (Jansson, 2003; Araújo et al., 2008).
Apart from climate factors, disturbance and competition are also considered to have impacts on species range size. The disturbance hypothesis proposes that anthropogenic threats might constrains species distribution due to the consequent population declines and extinctions (Whitton et al., 2011). The competition hypothesis proposes that species in rich communities would face intense competitive pressure which might limit their range size (Stevens, 1996; Gaston et al., 1998). In addition, patterns of range size variation might be subjected to the mid-domain effect (MDE), as large range species necessarily overlap in domain center due to the limit of geometric constraints on species geographical ranges, leading to a mid-domain peak in species range size regardless of ecological factors (Colwell et al., 2004; Moreno et al., 2008).
In addition to environment factors, variations in species range size might also be associated with life-form and biogeographical affinities. This is because, species’ life form and biogeographical affinities reflect their ecophysiological traits and evolutionary history, and hence affect their response to environmental variation. For example, compared with herbaceous plants, woody plants tend to have narrower adaptability for their longer reproductive cycles and slower accumulation rate of genetic changes (Smith & Beaulieu, 2009), and thus might more sensitive to the environmental gradient. Similarly, tropical taxa, which experienced more stable climatic environment in their evolutionary history, may be hence more susceptible to climate variation and were prone to increase their range size to adapt to increasing latitude and elevation (McCain, 2009). However, little work has been done to examine variations in species range size in terms of the influence of life-form and biogeographical affinities (but see Fend et al., 2016 and Zhou et al., 2019).
As one of the world’s 34 biodiversity hotspots, the Himalayas contain a diverse range of eco-climate zones, and have been receiving much attention from various ecological and biogeographical studies. Especially in the central Himalayas, where the towering mountains block the moisture from the Indian Ocean, a series of north-south valleys contain rich biodiversity and conspicuous elevational environmental gradient in small spatial scale, making them ideal place for uncovering the underlying mechanisms of the spatial variation in species range size and examining the validity of Rapoport’s rule. However, while a number of studies exploring the elevational variation and its drivers for species richness in the Himalayas (e.g., Acharya et al., 2011; Manish et al., 2017; Kluge et al., 2017; Yang et al., 2018; Sun et al., 2020), corresponding studies for species range size are limited. Since understanding range size variations are a prelude to effective biodiversity conservation (Luo et al., 2011), filling up such a research gap will not only help address the theoretical issue but also contribute to the conservation practice in this high-profile region.
Since long, vascular plants have been considered as an excellent matter of study for the spatial variation in range size, because of their wide distribution and easy observation. In this study, we aim to examine elevational variations in vascular plants range size for different life form and biogeographical affinities, and to explore the role of climate, disturbance, competition and the mid-domain effect on above variations, based on a detailed field survey in the Gyirong Valley, the longest valley in China’s central Himalayas. Since species range size is considered to be closely associated with species richness (Stevens, 1992), and climate has been found to be the primary determinant for species richness in the Himalayas (Bhattarai and Vetaas 2003; Manish et al., 2017; Sun et al., 2020; Liang et al., 2020), we expect that climate factors are also played a greater role than other factors in explaining the elevational variations in vascular plants range size in the Gyirong Valley. If that is the case, considering Rapoport’s rule gets supported in regions where the influence of climate is most pronounced (Pintor et al., 2015), we also expect the vascular plants range size increases with elevation as the rule predicted, especially for woody and tropical species which are supposed to be more sensitive to climate variation.