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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.