1 | INTRODUCTION
Understanding biodiversity and community assembly has long been a major interest for ecologists (Grierson et al., 2011; Ma, 2017; Patino et al., 2017). Numerous hypotheses have been proposed to explain these phenomena. The biodiversity hypotheses can be divided into two key categories. Some researchers think that contemporary environmental factors, for instance, climate and habitat heterogeneity, dominate the current mechanisms mediating biodiversity (Kerr & Packer, 1997; Brown, Gillooly, Allen, Savage, & West, 2004; Currie et al., 2004; Wang, Brown, Tang, & Fang, 2009). Others hypothesize that historical processes, such as speciation, extinction, and dispersal, predominantly influence biodiversity (Zobel, 1997; Ricklefs, 2005; Mittelbach et al., 2007). To date, there is no universal theory that integrates the relative influences of contemporary environments and historical processes on biodiversity patterns, even though it is widely accepted that they jointly influence biodiversity (Hawkins & Porter, 2003; Svenning & Skov, 2005, 2007; Montoya, Rodríguez, Zavala, & Hawkins, 2007; Wang, Fang, Tang, & Lin, 2012). In addition, it is a challenge to distinguish the effects of historical processes from those of contemporary environments because of collinearity.
For more than a century, ecologists have proposed numerous hypotheses to explain community assembly. Currently, niche theory (Hutchinson, 1959; Vandermeer, 1972; Silvertown, 2004) and neutral theory (Bell, 2000; Hubbell, 2005) are widely recognized. The former theory argues that species have different niches, while the latter acknowledges that different species within an ecological community can have equivalent ecological functions. Habitat filtering and competition exclusion are principal community assembly rules in niche theory; these are opposing forces that jointly promote community assembly and maintain biodiversity (Webb, Ackerly, McPeek, & Donoghue, 2002). In the neutral theory, the process of community assembly is modeled as being random (Hubbell, 2001). In recent years, ecologists have increasingly recognized that niche and neutral processes are not diametrically opposed and that they are both involved in community assembly (Tilman, 2004; Chase, 2005; Gravel, Canham, Beaudet, & Messier, 2006; Leibold & McPeek, 2006). Rapid advancements in molecular biology, phylogeography, and phylogeny studies, may offer novel insights into community assembly processes. Therefore, ecologists are using these technologies and methods to explain the evolutionary and ecological processes of community assembly over time and at different spatial scales (Faith, 1992; Webb, Ackerly, McPeek, & Donoghue, 2002).
Mountains are topographically complex regions that affect biodiversity and neighboring lowland ecosystem processes by facilitating biotic interchange, influencing regional climate and nutrient runoff (Rahbek et al., 2019a). They offer natural laboratories for studying the mechanisms that govern biodiversity and community assembly. Mountains reportedly influence global terrestrial biodiversity disproportionately, particularly in the tropics, where they host hotspots with extraordinary levels of richness. In the arctic and temperate regions, however, mountain regions host few endemic species and typically have low levels of species diversity, which barely exceed those of the adjacent lowlands (Rahbek et al., 2019b).
The mountains in China are distributed mainly on the Qinghai-Tibetan Plateau (QTP) and in adjacent regions (Wang, Wang, & Fang, 2004). The QTP is the highest and most expansive plateau on the globe, occupying an area of 2.5 million km2 with an average elevation over 4,000 m (Zhang, Li, & Zheng, 2002). Extensive research has been conducted on the QTP and the accumulated datasets offer opportunities to investigate the relationship between biodiversity and community assembly in such regions. After the Pleistocene, the QTP experienced four major glacial events (Shi, Li, & Li, 1998; Zhang, Li, & Zheng, 2002; Yi, Cui, & Xiong, 2005), including the Largest Glaciation, which occurred 1.2–0.6 Ma (Liu, Duan, Hao, Ge, & Sun, 2014), when parts of the QTP were entirely covered by ice sheets (Shi, Li, & Li, 1998; Owen, Caffee, Finkel, & Seong, 2008). Such geological processes have driven radiation and species diversification in various groups of plants (Wen, Zhang, Nie, Zhong, & Sun, 2014). According to data from published monographs and literature, the QTP harbors ~10,000 species of vascular plants (Wu, 2008; APGⅣ, 2016), of which ~20% are endemic to the region (Wu, 2008; Yan, Yang, & Tang, 2013; Yu, Zhang, Liu, Chen, & Qi, 2018); the southern regions have especially high levels of species richness (Mao et al., 2013).
Several studies have determined the geographical distribution of species in the QTP (Wu, 2008; APGⅣ, 2016), but the complex environment suggests that species richness would vary considerably across the region (Tang et al., 2006; Yang, Ma, & Kreft, 2013). Due to major advancements in phylogeography studies and tools, the evolutionary histories and underlying adaptations of plants in the QTP have become increasingly clear (Liu, Duan, Hao, Ge, & Sun, 2014), includingLigularia -Cremanthodium -Parasenecio (Liu, Wang, Wang, Hideaki, & Abbott, 2006),Nannoglottis (Liu, Gao, Chen, & Lu, 2002), Saussurea(Wang, Brown, Tang, & Fang, 2009), Rheum (Sun, Wang, Wan, Wang, & Liu, 2012), Gentiana (Favre et al., 2016), and Rhodiola(Zhang, Meng, Allen, Wen, & Rao, 2014), and others (Qiu, Fu, & Comes, 2011; Liu, Luo, Li, & Gao, 2017). Chinese botanists have now completed the construction of the phylogenetic tree for Chinese vascular plants (Chen et al., 2016; Lu et al., 2018), and such datasets facilitate our understanding of plant community of the QTP. Rapid speciation and habitat filtering have been reported to dominate biodiversity and community assembly processes on the QTP, and the phylogenetic structure of vascular species is clustered in most regions of the QTP (Yan, Yang, & Tang, 2013). A study revealed that the current flora in the Hengduan Mountains exhibited rapid speciation (Xing & Ree, 2017). Therefore, datasets from different regions provide the opportunity to explore the biodiversity and community assembly in plant communities of different areas.
In the study, we hypothesized that the current plant community in temperate region mountains might be simultaneously drove by species dispersion and habitat filtering. In order to test the hypothesis, the Kunlun Mountains were used as the region of study. These mountains form an independent physical geographical unit with a relatively clear geological history, geographic range, and plant distribution data; however, they are not a biodiversity hotspot (Su, 1998; Zheng, 1999; Pan, 2000; Zachos & Habel, 2011; Wu, 2012–2015; Sun et al., 2015). Thus, they formed a relatively typical temperate mountain chain where we could study plant communities. We used datasets from the Kunlun Mountains region to explore the spatio-temporal patterns of current plant community assembly in this area to: 1) clarify how the plant diversity in the Kunlun Mountains emerged, 2) determine the driving forces of community assembly, and 3) examine the above-mentioned hypotheses with reference to the current plant community assembly in the Kunlun Mountains. An improved understanding of the plant diversity patterns between different regions will help develop better biodiversity conservation strategies.