2 | MATERIALS AND METHODS
| Study area
The
Kunlun Mountains are an independent physical geographic unit, located in
northwest China on the northern edge of the QTP. Geographically, they
border the Pamirs plateau to the west, southeast Qinghai to the east,
the Qaidam and Tarim Basins to the north, and northwest Tibet Autonomous
Region to the south. The Kunlun Mountains range is oriented east-west,
located between 34°N–40°N and 75°E–100°E, with an average altitude
>4,000 meters. The range extends for a total length of
~2,500 km and width of 130–200 km. The mountain range
is narrower in the west compared to the east and covers a total area of
more than 500,000 km2(Zheng, 1999; Wu, 2012–2015;
Figure 1).
The elevation of the mountain range
increases from the east to the
west, and ranges between 3,000 m and 7,719 m. The area has an annual
precipitation that varies from ~100 to 500 mm and an
average annual temperature of < 0℃. The annual precipitation
and temperatures show an obvious decrease from the east to the west. The
climate on the slopes of the mountain range varies greatly and the steep
climate gradient results in a dramatic change in vegetation cover. From
east to west, the vegetation types are alpine scrub, alpine meadow, and
alpine steppe. In addition, there are a few coniferous forests in the
east and west of the Kunlun Mountains (Zheng, 1999; Wu,
2012–2015).
The formation of the Kunlun Mountains coincided with the Himalayan
movement. According to numerous studies, extensive uplifts of the QTP
occurred ca. 15–13 Ma and ca. 8–7 Ma, and the last abrupt and rapid
uplift took place 3.6–1.2 Ma
(Harrison, Copeland, Kidd, & Yin, 1992; Li, Shi, & Li, 1995; Shi, Li,
& Li, 1998; Sun & Zheng, 1998; Spicer
et al., 2003). Recent studies have
demonstrated that the current QTP ecosystem began in the early Miocene
(Deng, Wu, Wang, Su, & Zhou,
2019), and that the Kunlun Mountains reached their present height over
the last 17 million
years (Pan, 2000; Sun
et al., 2015).
After the early Miocene,
the Kunlun Mountains experienced
the Kunhuang movement (Cui et al., 1998) and
numerous glacial events (Su,
1998).
To accurately reveal the current plant community, the study region was
divided into 28 county-level geographical units. Geographically, the
Kunlun Mountains are divided into three parts: east, west, and middle.
The western part consists of 6 counties; the middle part consists of 14
counties, with 6 counties on the southern slope and 8 counties on the
northern slope; and the eastern part consists of 8 counties.
| Distribution data
The basic distribution data were obtained fromFlora
Kunlunica published in four volumes by Wu and his
colleagues (Wu,
2012–2015), with reference to
published monographs and other literature, includingFlora of Xinjiang (Shen,
1993–2011),Flora
of Qinghai (Liu, 1996–1999),Flora of Tibet Autonomous
Region (Wu, 1983–1987), andThe Vascular Plants and Their Eco-geographical Distribution of the
Qinghai-Tibet Plateau (Wu, 2008). Based on
these sources, and using the order
of families from the Angiosperm Phylogeny Group
Ⅳ (APGⅣ, 2016), species were
classified into genera and families
according to A Dictionary of
the Families and Genera of Chinese Vascular Plants(Li, Chen, Wang, & Lu, 2018),http://www.catalogueoflife.org/annual-checklist/2019/,
andhttp://www.theplantlist.org.
Genera and species that were not native to the Kunlun Mountains were
excluded, and the infraspecific taxa were preserved.
The information presented a
comprehensive checklist of the
seed plant species in the Kunlun
Mountains. To analyze the spatial patterns, these species were divided
into 28 county-level geographical units based on
species distribution data.
| Similarity of
taxa
To determine the origin of the Kunlun Mountains flora (KMF), the species
were compared with the flora in
nearby biodiversity hotspots.
Three biodiversity hotspots exist around the Kunlun Mountains, including
the Mountains of Central Asia, the Eastern Himalayas, and the Mountains
of Southwest China, which are located to the north, south, and southeast
of the Kunlun Mountains, respectively (Myers, Mittermeier, Mittermeier,
da Fonseca, & Kent, 2000; Zachos & Habel, 2011). The Mountains of
Southwest China host the highest number of plant species among these
three biodiversity hotspots, and the Hengduan Mountains are
representative of the Mountains of Southwest China. Therefore, the
Hengduan Mountains form an important biodiversity hotspot, and harbor
one of the richest temperate floras in the world, with
~12,000 vascular plants (Boufford, 2014). We evaluated
the species-level, genus-level,
and family-level similarities between the KMF and the
Hengduan Mountains flora (HMF).
The plant species richness in the
Kunlun Mountains was approximately one sixth of that in the Hengduan
Mountains. The taxa similarity
(TS) between the KMF and the HMF
was calculated as follows:
\begin{equation}
\text{TS}=\frac{\text{SR}}{\text{TR}}\times 100\%\nonumber \\
\end{equation}where SR represents the number of shared taxa between a KMF sample and
the HMF, TR represents the number
of taxa in the KMF sample (taxa represent families, genera, or species),
and TS is the similarity of taxa between the KMF sample and the HMF.
| Phylogenetic diversity and structure
We calculated the species level
phylogenetic diversity (PD),
standard effect size phylogenetic diversity (SES-PD), net relatedness
index (NRI), and nearest taxon index (NTI) in each county.
PD was the sum of the phylogenetic length of the communities in each
sample and was measured based on the approach developed by Faith (1992).
SES-PD was calculated by dividing
the difference between the observed (PDobserved) and
expected phylogenetic diversity (PDrandom) by the
standard deviation (s.d.) of the null distribution
(s.d.[PDrandom]) (Rodrigues et al., 2005), as
follows:
\begin{equation}
SES-PD=\frac{\text{PD}_{\text{observed}}-\text{PD}_{\text{random}}}{\text{s.d.}\left(\text{PD}_{\text{random}}\right)}\nonumber \\
\end{equation}The NRI and NTI were calculated to analyze community phylogenetic
structure (clustering or overdispersion), and to examine possible
ecological and evolutionary processes within communities (Webb, Ackerly,
McPeek, & Donoghue, 2002). The NRI was based on the mean phylogenetic
distance (MPD), which is an estimate of the average phylogenetic
relatedness between all possible pairs of taxa within a sample. The NTI
was based on the mean nearest taxon distance (MNTD), which is an
estimate of the mean phylogenetic relatedness between each pair of taxa
in a sample and its nearest relative in a phylogeny. The NRI and NTI
values were calculated as follows:
\begin{equation}
NRI=-1\times\frac{\text{MPD}_{\text{observed}}-\text{MPD}_{\text{random}}}{\text{s.d.}\left(\text{MPD}_{\text{random}}\right)}\nonumber \\
\end{equation}\begin{equation}
NTI=-1\times\frac{\text{MNTD}_{\text{observed}}-\text{MNTD}_{\text{random}}}{\text{s.d.}\left(\text{MNTD}_{\text{random}}\right)}\nonumber \\
\end{equation}where, respectively, MPDobserved and
MNTDobserved represent the observed MPD and MNTD values;
MPDrandom and MNTDrandom represent the
mean values of the expected MPD and MNTD of the randomized assemblages
(n = 999); s.d.(MPDrandom) and
s.d.(MNTDrandom) represent the standard deviations of
the MPDrandom and MNTDrandom values for
the randomized assemblages. The null distributions of MPD and MNTD were
created by randomly selecting the observed number of taxa in each sample
999 times, with all taxa in the phylogeny serving as the sampling pool.
The phylogenetic analyses require a
phylogenetic tree of seed plants,
the phylogenetic tree was constructed using Phylomatic
(http://phylodiversity.net/phylomatic/) with the stored tree in Zanne et
al. (2014). The ecological indexes were calculated using R version 3.3.3
(R Core Team, 2017) and picante packages (Kembel et al., 2010).
| Statistical analysis
To examine the community assembly
processes in the Kunlun Mountains, two key issues were studied: 1) the
patterns of species diversity in
the area, and 2) relevant mechanisms relating to community assembly.
The
HMF has been assembled through recent in situ diversification
during its rapid uplift over the last 8 Ma (Xing & Ree, 2017). In the
northern hemisphere, it is the largest refuge for species in the glacial
age (Liu, Luo, Li, & Gao, 2017), and a center for species
diversification for many current plants (Wen, Zhang, Nie, Zhong, & Sun,
2014; Chen, Deng, Zhou, & Sun, 2018), such as Saussurea ,Gentiana , Pedicularis , Salix , Primula ,Saxifraga , Ranunculus , and Corydalis . Endemic
species of seed plants account for 32.4% of the total species in the
Hengduan Mountains (Zhang, Boufford, Ree, & Sun, 2009). Therefore, the
results of the TS revealed the relationship of species migrations
between the Kunlun Mountains and the Hengduan Mountains. The analysis of
these results, the geological history, and rules of species migration
revealed the patterns of species diversity.
PD and SES-PD have been used to determine the floristic histories of
communities (Faith, 1992; Rodrigues et al., 2005). Negative SES-PD
values imply that the actual PD is lower than the predicted PD, and the
community consists of young flora with relatively close phylogenetic
relationships among species; whereas positive SES-PD values imply that
the actual PD is higher than the predicted PD, and the community
consists of ancient flora with more distant phylogenetic relationships
among species. The NRI primarily reflects the structure in deeper parts
of a phylogeny and is more appropriate for explaining evolutionary
processes; whereas the NTI reflects the structure in shallower parts of
a phylogeny, and is more appropriate for exploring ecological processes
(Webb, Ackerly, McPeek, & Donoghue, 2002). At the community level,
positive NRI and NTI values indicate phylogenetic clustering, whereas
negative values indicate phylogenetic dispersion. The analysis of NRIs,
NTIs, and SES-PDs were used to explain
the evolutionary and ecological
processes of community assembly.