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\begin{document}

\title{Historical and contemporary climate jointly determines seed plant
diversity patterns in eastern Eurasia}



\author[1]{WenQi Song}%
\author[1]{Yichao Li}%
\author[1]{Ao Luo}%
\author[1]{Xiangyan Su}%
\author[2]{Qinggang Wang}%
\author[1]{Yunpeng Liu}%
\author[1]{Tong Lyu}%
\author[1]{Yongsheng Chen}%
\author[1]{Shijia  Peng}%
\author[3]{Denis Sandanov}%
\author[4]{Zhiheng Wang}%
\affil[1]{Peking University}%
\affil[2]{China Agricultural University}%
\affil[3]{Institut obsej i eksperimental'noj biologii}%
\affil[4]{Center of Macroecology, Evolution and Climate}%


\vspace{-1em}



  
  \date{May 21, 2024}


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\selectlanguage{english}
\begin{abstract}
Large-scale spatial patterns of species richness are one of the central
issues in ecology. Although hypotheses based on effects of contemporary
climate, evolutionary history and historical climate change have been
employed to explain the mechanisms underlying species richness patterns,
variation in relative contribution of different hypothesis remains a
major challenge. Here, using newly-compiled distribution data with a
spatial resolution of 100 * 100 km2 for 43,023 seed plant species in
eastern Eurasia, we mapped species richness patterns for plants with
different growth forms (i.e., woody vs. herbaceous) and range sizes
(i.e., wide-ranged vs. narrow-ranged species), and compared the
contribution of four hypotheses on these patterns, i.e., freezing
tolerance hypothesis, historical climate change hypothesis, Janzen
hypothesis and diversification rate hypothesis. We found that species
richness of all seed plants presented a clear latitudinal gradient and
was the highest in southwestern China and Central Asian mountains.
Notably, species richness patterns and their dominant hypotheses
differed between species groups. Historical climate change dominated
richness patterns of overall, herbaceous and wide-ranged herbaceous
species. The freezing tolerance hypothesis dominated those for all woody
and wide-ranged woody species, while the Janzen hypothesis dominated
narrow-ranged woody and herbaceous species. Our study suggests that
different hypotheses contribute to large-scale species richness patterns
via their dominant effects in different species groups. Interestingly,
our study did not support the diversification rate hypothesis, but
demonstrated high contribution of historical climate change to plant
diversity, providing new perspectives on the mechanisms of plant
diversity patterns in eastern Eurasia.%
\end{abstract}%



\sloppy


\textbf{INTRODUCTION}

Global biodiversity is facing a rapid loss due to anthropogenic threats,
and effective conservation of biodiversity has become a global
challenge. Revealing large-scale patterns of species richness and the
underlying mechanisms provides data and scientific support for the
spatial prioritization of biodiversity conservation, and hence has
increasingly intrigued ecologists over the past decades (Wang et al.,
2011; Pimm et al., 2014; Mccauley et al., 2015). Previous studies had
widely investigated global and regional patterns in species richness of
terrestrial vertebrates, especially amphibians (Buckley \& Jetz, 2007;
Jenkins et al., 2013), birds (Davies et al., 2007; Hawkins et al., 2007;
Jetz et al., 2012) and mammals (Schipper et al., 2008; Karanth et al.,
2009; Buckley et al., 2010). However, despite the important role in
terrestrial ecosystems, the patterns of species richness of seed plants
remain much less studied due to the scarcity of high-resolution
distribution data across large spatial scales compared to terrestrial
vertebrates (Kreft \& Jetz, 2007; Jenkins et al., 2013).

In order to comprehend the mechanisms that drive the spatial patterns of
species richness, several theoretical hypotheses based on the effects of
contemporary environment, diversification rates and historical climate
change have been proposed and widely discussed (Jetz \& Rahbek, 2002;
Willig et al., 2003; Currie et al., 2004; Ricklefs, 2006; Svenning \&
Skov, 2007; Wang et al., 2012). Many studies have identified
contemporary climate as a crucial driver of patterns in species
richness. One of the contemporary climate hypotheses, i.e. the freezing
tolerance hypothesis, has been recognized as a key mechanism for
explaining the effect of contemporary climate on latitudinal gradients
in plant species richness across northern temperate regions (Lathern \&
Ricklefs, 1993; Wang et al., 2011; Xu et al., 2016; Su et al., 2020).
The prerequisite assumption under this hypothesis is evolutionary niche
conservatism (Latham \& Ricklefs, 1993; Zanne et al., 2014). This
hypothesis suggests that most clades originated in tropical climates,
and could hardly adapt to and survive the cold climate at high latitudes
due to evolutionary niche conservatism, which will lead to the decrease
in species richness from the tropics to temperate regions. Based on this
hypothesis, winter coldness is suggested to be the dominant factor for
the geographic patterns in species richness. This hypothesis has been
employed to explain woody species richness in eastern Eurasia (Wang et
al., 2011; Su et al., 2020). For example, mean temperature of the
coldest quarter was found to be the most influential factor for patterns
of woody plants richness in China and of woody families with the
tropical affinity in eastern Eurasia (Wang et al. 2011; Su et al.,
2020).

Another widely-discussed hypothesis concerning the influence of
contemporary climate on patterns of species richness is the Janzen
hypothesis (Ghalambor et al., 2006; Gill et al., 2016; Liu et al., 2017;
Lyu et al., 2018; Smith, 2018). Janzen (1976) found that tropical
mountains have very low temperature seasonality, while temperate
mountains have large temperature seasonality. The large temperature
seasonality in temperate mountains requires that temperate organisms
have to evolve broad ranges of temperature tolerances to adapt to large
climate seasonal changes. In contrast, tropical organisms could evolve
narrow temperature tolerances due to the less seasonal nature of the
tropical climate. Therefore, Janzen (1967) postulated that tropical
mountain passes pose a more formidable obstacle to species dispersal
than temperate mountain passes with the same elevations, which leads to
higher speciation rate and species richness in tropical than in
temperate mountains. Several recent studies provided support for the
predictions of the Janzen hypothesis. For instance, Polato et al. (2018)
reveled that tropical insect species have narrower temperature
tolerance, larger genetic differences and higher speciation rate than
temperate ones. The Janzen hypothesis also explains well the
Rhododendron species richness pattern in China (Shrestha et al. 2018).

In addition to the effects of contemporary environment, tecent studies
suggest that evolutionary history also exerts a crucial role in shaping
geographical species richness patterns (Stevens, 2006, 2011).
Specifically, the diversification rate hypothesis argues that high
species richness in certain regions is a result of high diversification
rate (Mittelbach et al., 2007; Svenning et al., 2008; Tietje et al.,
2022). Previous studies have offered evidential support for the
diversification rate hypothesis. For example, researches have shown that
diversification rate serves as the most influential predictor of the
latitudinal gradient of species richness for amphibians (Pyron and Wiens
2013), mammals (Moritz et al., 2014), and flowering plants (Jansson and
Davies, 2008).

Recent studies have found that modern species richness may have also
been influenced by historical climate change. Since the Last Glacial
Maximum (LGM, ca. 21 000 -- 18 000 years ago), high latitudes of the
northern Hemisphere were covered by massive glaciers. The presence of
glaciers had dramatic impacts on plant distributions (Svenning and Skov,
2005; Davies et al., 2009; Sandel et al., 2011), resulting in the
migration of a large number of plant species to the south or extinction
during the LGM. After the glaciers retreated, species re-colonized the
northern latitudes. However, many species didn't spread northward to
areas where climate is suitable due to dispersal constraints (Svenning
\& Skov, 2005; Normand et al., 2011). For example, recent studies showed
that climate change since the LGM was the dominant factor of species
richness for trees, amphibians, reptiles and dung beetles in Europe
(Svenning \& Skov, 2005; Ara\selectlanguage{ngerman}újo et al. 2008; Hortal et al. 2011; Feng et
al., 2019), for woody plants in Central Asia (Liu et al., 2019) and
trees in the northern Hemisphere (Feng et al. 2019), and played an
important role in shaping mammals, birds, and amphibians richness
patterns in Africa (Pinkert et al. 2020).

Although these hypotheses have been supported in different studies,
their relative contributions on species richness patterns remains highly
controversial, which has hindered the development of general theories on
mechanisms of species richness patterns. Moreover, previous studies
suggested that eastern Eurasia has been much less affected by climate
change since the LGM than Europe and North America (Qian, 2002; Qian \&
Ricklefs, 2004). Whether climate change since the LGM is a weaker
predictor of plant richness patterns in eastern Eurasia than
contemporary climate and diversification rates remains to be tested.

Woody and herbaceous growth forms have different responses to climate
changes owing to their different life history strategies (Steinberg et
al., 1990; Pretzsch, 2002; Massad, 2013; Luo et al., 2022). Moreover,
species with different range sizes (i.e., narrow-ranged vs. wide-ranged
species) may also respond differently to climate changes (Svenning \&
Skov, 2007). Previous studies suggest that narrow-ranged species tend to
possess constrained dispersal abilities and cannot spread to all
suitable habitats, hence their species richness patterns are more
susceptible to local evolutionary processes (Dynesius \& Jansson, 2000;
Jansson, 2003; Liu et al., 2017). On the contrary, wide-ranged species
are more able to cope with climate variations, and their richness
patterns tend to be more strongly associated with environmental factors
(Cauwer et al., 2016). Therefore, understanding variations in drivers
behind species richness patterns among woody and herbaceous growth
forms, as well as among species with different range sizes would improve
our capability to predict the impacts of global changes on communities
dominated by different species (Zanne et al., 2018; Luo et al., 2022).
Previous studies on plant richness patterns have been strongly biased to
woody plants, while species richness patterns of herbaceous plants are
much less studied due to lack of data. Freezing tolerance has been
frequently considered to be an important factor influencing woody plant
richness patterns (Oommen \& Shanker, 2005; Wang et al., 2011; Su et
al., 2020). Compared with woody species, herbaceous species are less
sensitive to freezing due to their shorter lifespan, higher rate of
niche evolution, and weaker niche conservation (Smith \& Beaulieu, 2009;
Zanne et al., 2018). However, whether woody and herbaceous species
richness is similarly dominated by the freezing tolerance hypotheses
remains to be tested.

Eastern Eurasia has high plant species richness and preserves high
proportions of endemic and relict plant taxa, especially in the
mountains in Southwest China and Central Asia. In addition, eastern
Eurasia contains several global biodiversity hotspots (i.e., Central
Asian mountains, Himalayas, Southwest China mountains) (Myers et al.,
2000). Previous studies indicated that plant species richness in eastern
Eurasia is significantly greater than that observed at equivalent
latitudes in Europe and North America due to evolutionary and ecological
mechanisms (Milne \& Abbott 2002; Qian et al., 2003; Ding et al., 2020).
Therefore, understanding species richness patterns in this region is
important not only for gaining insights into the evolutionary
development of floras in the Northern Hemisphere (Xu et al., 2013; Chen
et al., 2018), but also biodiversity conservation planning. However, the
paucity of data on plant distributions in eastern Eurasia compared to
Europe and North America severely limits our understanding of plant
richness patterns and their genesis in this region. In earlier
investigations, Wang et al. (2011) and Su et al. (2020) established a
database on the distribution of woody plants across China and eastern
Eurasia, providing a foundational resource for exploring plant richness
in the region. However, data on the distribution of herbaceous plants in
this region are still very scarce. Here, we compiled the first more
comprehensive database of seed plant distributions in eastern Eurasia
and mapped the species richness patterns for all species, different
species groups (i.e. woody, herbaceous, wide-ranged and narrow-ranged
species). Then we explored the determinants of species richness patterns
for different species groups. Our objective is to examine the validity
of the following three hypotheses . 1) Climate change since the LGM has
lower effect on patterns of species richness in eastern Eurasia than
contemporary climate and diversification rate. 2) The dominant
hypothesis for species richness patterns differs between different
growth forms and different range size. Specifically, we expect that
woody and herbaceous species richness is dominated by the cold tolerance
hypothesis and climate change since the LGM respectively. Wide-ranged
woody and herbaceous richness is dominated by the contemporary climate,
while narrow-ranged woody and herbaceous richness is dominated by
climate change since the LGM .

\textbf{MATERIAL AND METHODS}

\textbf{Species distribution data}

The Database of Seed Plant Distribution in Eastern Eurasia was
newly-compiled, containing geographic distributions with a spatial
resolution of 100 × 100 km2 for 43,023 seed plant species (from 4,479
genera and 316 families). The species distributions were compiled from a
great number of published regional and local floras and checklists for
China, Central Asia (Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan,
Uzbekistan), Mongolia and Russia. Occurrence data for these countries
were also obtained from online databases, including the `National
specimen information infrastructure' (NSII,
\textless{}\selectlanguage{english} www.nsii.org.cn\selectlanguage{english} \textgreater{}, accessed in May 2019), `the
global biodiversity information facility' (GBIF,
\textless{}http://www.gbif.org/\textgreater{}), and the `Virtual guide
to the flora of Mongolia'
(\textless{}http://greif.uni-greifswald.de/floragreif/\textgreater{}).

The raw species distribution data in the data sources were normally
recorded in the following formats: 1) spatial polygons (e.g., species
distribution ranges), 2) spatial points (e.g., occurrences of specimen
collections), and 3) administrative (e.g., villages, towns, counties)
and geographic units (e.g., mountains and nature reserves) where species
occurrences were recorded. The distribution data in the first two
formats from published floras and checklists were manually digitized
using ArcGIS 10. In contrast, distribution data in the third format were
georeferenced using the following databases, including GeoNames
(containing over 25 million geographical
names,\emph{http://www.geonames.org} ), Global Administrative Areas
Database (\emph{https://gadm.org/} ) and World Geographical Scheme for
Recording Plant Distributions
(\emph{https://www.tdwg.org/standards/wgsrpd/} ). To enhance the
precision of species distributions, we only kept the records of
administrative/geographic units at the level of or smaller than counties
so that the records can be precisely transformed into 1-2 grid cells.
Specially, the coordinates of specimen occurrences available in the
online databases were downloaded.

Then the raw distribution data were transformed into gridded
distributions using a grid of 100 x 100 km2. Specifically, a grid cell
was considered as presence for a species when it contained at least one
point of specimen locations, or more than 50\% of its area was covered
the species' distribution range comprised of polygons and
administrative/geographic units. The nomenclature of species names from
different sources were merged following the `Catalogue of Life'
(\textless{}www.catalogueoflife.org \textgreater{}). After finishing the
general database, we carefully examined the species distribution data
manually to enhance the quality of the database. The records of
cultivated species and exotic species were removed and intraspecific
units (i.e., subspecies and varieties) were merged to species. It's
noteworthy that the completeness of our database varies across different
regions. Compared to China and Russia, the sampling effort in Mongolia
and Central Asian countries may be less sufficient. However, our
database has collected all available data that could be obtained, and it
is currently one of the most comprehensive plant species distribution
databases for East Asia to date.

\textbf{Environmental data}

To assess the impact of the freezing tolerance hypothesis on species
richness patterns, mean temperature of coldest quarter (MTCQ, degC. The
coldest quarter is defined as period from December through January to
February) was used following prior researches (Wang et al., 2011; Liu et
al., 2017; Su et al., 2020). We selected temperature seasonality (TSN,
standard deviation of monthly mean temperature within one year) and
elevational range within a grid cell (TOPO\_RANGE) to represent Janzen
hypothesis. MTCQ and TSN were obtained from the WorldClim database
(\emph{http://www.worldclim.org}, Ver. 2) with a spatial resolution of
30 arc seconds (\selectlanguage{english}~1 km at the equator). Then, we calculated the average
of MTCQ and TSN in each grid cell (100 * 100 km\textsuperscript{2}).
TOPO\_RANGE with each grid cell was determined by calculating the
difference between the maximum and minimum elevations, using GTOPO data
at a spatial resolution of 1 x 1 km (\emph{https://www.usgs.gov/}).

Temperature and precipitation changes since the LGM (MAT anomaly LGM and
MAP anomaly LGM) were computed as the absolute differences between the
LGM and contemporary temperature and precipitation values and were used
to assess the impacts of historical climate change on species richness
patterns. The temperature and precipitation data in the LGM were
reconstructed by MPI-ESM-P model (Watanabe et al. 2011) and were also
retrieved from the WorldClim website, with a spatial resolution of 1 arc
minute.

\textbf{Diversification rate}

Net diversification rate (DR) was used to evaluate the effect of
diversification rate hypothesis. DR was estimated following Jetz et al.
(2012). This method treated the inverse of species' mean equal splits
measure as the species-level lineage diversification rate for each
individual species. Here, we used the dated phylogeny containing 353,185
seed plant species generated by Smith and Brown (2018) (i.e., ALLOTB),
which has gained substantial traction in macroecological and
biogeographical studies (Sandel et al., 2019; Weigelt et al., 2019; Luo
et al., 2023). This phylogeny was constructed by placing the molecular
phylogenies (containing 79,855 species in total) for major clades (i.e.,
orders) in the backbone of the Open Tree of Life, and inserting species
without molecular data as polytomies in their families. To generate a
fully-resolved phylogeny, we conducted a polytomy resolving analysis
using a birth-death (Yule) bifurcation process (see Lu et al., 2018). To
evaluate whether polytomy resolving process may influence our results,
we generated 100 resolved phylogenies by repeating this process 100
times, and repeated our analysis using all these 100 phylogenies.

\textbf{Statistical Analyses}

Species richness was quantified as the cumulative count of species
within each grid cell. We draw species richness patterns in the whole
study area using ArcGIS 10. To explore the differences in species
richness patterns between growth forms, we divided all species into
woody and herbaceous growth forms. Woody species were defined following
Zanne et al. (2014) and included trees, shrubs and lianas (Luo et al.,
2023). Among all species, 15207 recorded as woody. Then, we also divided
all species into wide-ranged and narrow-ranged species. We ranked the
species in descending order of range size, and then categorized the
upper 25\% species as wide-ranged species and the lower 50\% as
narrow-ranged species following Araujo et al. (2008).

To compare the impacts of different hypotheses on species richness, we
first conducted univariate generalized linear models (GLMs) with
quasi-Poisson residuals between species richness and different
environmental variables, and the slopes and the explanatory power of
each variable (i.e., model pseudo-R\textsuperscript{2}) were extracted.
We also draw scatter plots between species richness and each variable.
Second, hierarchical partitioning analysis was applied to estimate the
independent and joint contributions of each hypothesis. We drew Venn
diagrams of the relative percentage contribution of each hypothesis. Due
to the significant negative correlation between species richness and DR,
which is in contrast to the prediction of the diversification rate
hypothesis, we did not include DR in the hierarchical partitioning
analysis. Positive spatial autocorrelations in model residuals can
inflate type-I errors when testing statistical hypothesis and lead to an
excess of significant results in model tests due to the inter-dependency
of residuals. Therefore, in all analyses, R2 instead of p values were
used to compare the performance of models.

All analyses were conducted in R 4.0.3 with ggplot2, picante and ape
packages (\emph{http://www.Rproject.org}).

\textbf{RESULTS}

\textbf{Species richness patterns in eastern Eurasia}

The overall species richness and species richness of the two growth
forms (i.e., woody and herbaceous species) and different range size
groups (i.e., wide-ranged and narrow-ranged) in eastern Eurasia
decreased with latitude (Figs 1,2). Species richness centers differ
between growth forms (Fig. 2a, b) and between wide-ranged and narrow
ranged species (Fig. 2c, d, e, f). Specifically, woody species peak in
the Southern China (e.g. Yunnan-Guizhou Plateau) (Fig. 2a), while
herbaceous species peak in the mountains of central Asia and Hengduan
Mountains (Fig. 2b). Most of narrow ranged woody and herbaceous species
concentrate in the Southwestern China (Fig. 2d), while wide-ranged woody
species peak in the Southern China (e.g. the border of Guangxi and
Guizhou, Qinba Mountain) and wide-ranged herbaceous species also peak in
the mountains of central Asia (Fig. 2c, e). The overall species richness
is similar to the wide-ranged species (Figs. 1,2c).

\textbf{Determinants of species richness patterns}

Univariate GLMs showed that richness patterns of overall species were
positively corrected with MTCQ and TOPO\_RANGE, but negatively corrected
with LGM anomaly MAT, LGM anomaly MAP, TSN and DR (Tab. 1; Fig. 3). In
other words, regions with high MTCQ and TOPO\_RANGE and low LGM anomaly
MAT, LGM anomaly MAP and TSN have high species richness. These results
are consistent with the freezing tolerance hypothesis, the hypothesis of
climate change since the LGM and Janzen hypothesis. Species richness was
significantly negatively correlated with DR, suggesting that regions
with high diversification rate do not have high species richness. This
result contradicts the prediction of diversification rate hypothesis.

Hierarchical partitioning analyses showed consistent results with those
of univariate GLMs. The relative contributions of different hypotheses
(the freezing tolerance hypothesis, climate change since the LGM and
Janzen hypothesis) on species richness patterns differ between growth
forms and range size groups. For overall, herbaceous, and wide-ranged
herbaceous species, the climate change since the LGM had the highest
independent contribution on species richness patterns (Figs. 4,5). In
contrast, the freezing tolerance hypothesis and Janzen hypothesis had
the highest joint and independent effects on woody and narrow-ranged
species richness respectively (Fig. 5).

\textbf{Discussion}

The patterns of species richness for all species and different species
groups in eastern Eurasia presented clear latitudinal gradients
(Fig.1,2), which was similar to the widely validated latitudinal
gradients in species richness of other groups and in other regions
(Hawkins et al., 2007; Buckley et al., 2010). However, there are still
differences in the spatial patterns in species richness between
different species groups, which is mainly due to differences in the
responses to the environment, evolution histories and life history
strategies among different species groups (Svenning \& Skov, 2007;
Normand et al., 2011; Zanne et al., 2018). Indeed, our results revealed
that the species richness patterns of different groups had different
responses to environmental factors. These differences suggest that
species richness patterns of different species groups are likely
dominated by different hypotheses.

\textbf{Climate change since the LGM dominated the patterns of seed
plant richness in eastern Eurasia}

Early studies generally suggested that eastern Eurasia had been less
affected by climate change since the LGM, especially compared to Europe
and North America (Qian, 2002; Qian \& Ricklefs, 2004). Therefore, there
is a general tendency to assume that the species richness patterns in
eastern Eurasia are only weakly influenced by climate change since the
LGM in comparison with the effects of contemporary climate and long-term
diversification rate. However, our results did not support this
traditional view. Our findings indicate that climate change since the
LGM dominated all plant species richness patterns in eastern Eurasia
(Fig. 3). Specifically, the richness of all species combined was
significantly negatively correlated with the climate change since the
LGM. In other words, species richness of plants was lower at sites with
stronger temperature and precipitation changes since the LGM but was
higher in regions with more stable climate. The impact of climate change
since the LGM on species richness patterns has also been demonstrated in
other taxa and other continents. For example, studies on species
richness of trees (Svenning and Skov 2007), reptiles and amphibians in
Europe (Araujo et al. 2008), consistently highlighted climate change
during the LGM as a significant driver shaping species richness
patterns.

Previous studies suggest that climate change since the LGM may influence
contemporary species richness patterns via its effects on species
dispersal. The strong temperature and precipitation changes since the
LGM may have significantly affected the northern regions, causing a
large number of species to migrate south and to mountainous regions.
After the LGM, many species did not spread north and colonize all the
areas with suitable climates (Sandel et al., 2011; Wang et al., 2012;
Liu et al., 2017). The Central Asian mountains and southern China had
experienced relatively weak climate change since the LGM and hence had
become refuges for species (Mittermeier et al., 2005; Lopez-Pujol et
al., 2011a, b; Sandel et al., 2011; Tang et al., 2018). Their
topographic diversity provided sufficient ecological stability to buffer
the effects of extreme climatic fluctuations (Sandel et al., 2011).

\textbf{The influence of diversification rate hypothesis on species
richness patterns}

Notably, our results reveled that species richness patterns were
negatively correlated with the geographic pattern of average
diversification rate, which does not support the diversification rate
hypothesis (Mittelbach et al., 2007; Svenning et al., 2008; Tietje et
al., 2022). In recent studies, the contribution of net diversification
rate to large-scale species richness patterns remains debated. Previous
studies on the species richness of some flower plant groups, such as
palms (Arecaceae) (Svenning et al., 2008), Zygophyllaceae (Wang et al.,
2018) and Rhododendrons (Shrestha et al., 2018), indicated that net
diversification rate was strongly positively correlated with species
richness patterns, and hence supported the diversification rate
hypothesis. In contrast, Jansson and Davies (2008) and Igea \& Tanentzap
(2020) found that higher species richness of angiosperms in the tropics
cannot be explained by higher diversification rates, which is consistent
to our findings. One explanation for the debate on diversification rate
as the dominant factor of species richness patterns is that temperate
lineages have fewer species and their unoccupied ecological niche space
may be large, thus increasing the chances of species divergence
(Simpson, 1953; Schluter, 2016). In addition, species richness may also
be influenced by the accumulation time of speciation (Mittelbach et al.,
2007; Etienne et al., 2019). Compared with the accumulation time of
temperate taxa in temperate regions, taxa with tropical origin have
occupied tropical regions for a much longer time, and had a longer time
to diversify there (Stephens and Wiens, 2003; Li and Wiens 2019).

\textbf{Difference in patterns and determinants of woody and herbaceous
species richness}

Our results indicated that the dominant hypothesis for geographic
patterns of species richness significant differed between woody and
herbaceous growth forms. Specifically, the geographic pattern in species
richness of woody plants is dominated by the freezing tolerance
hypothesis, while that of herbaceous plants is dominated by the
hypothesis related to climate change since the LGM. These findings
provide more evidence supporting the importance of the freezing
tolerance hypothesis for diversity patterns in different groups and
different regions (Francis \& Currie, 1998; Wang et al., 2011; Zhao et
al. 2020). In China, mean temperature of the coldest quarter has been
found to be the dominant factor driving the pattern of woody plant
richness (Wang et al., 2011; Su et al., 2020), mainly because China's
woody flora contains more tropical and subtropical floristic components
(Fang \& Yoda, 1989), and most of these tropical species are very
sensitive to low temperature in winter and cannot widely spread to the
north (Wang et al., 2011). This finding on Chinese woody plants,
together with our results at a larger spatial scale, suggested that
freezing tolerance hypothesis dominates the species richness patterns of
woody plants in the entire eastern Eurasia.

Interestingly, our results indicated much weaker effects of the freezing
tolerance hypothesis on herbaceous species richness than on woody
species richness. That might be related to difference in functional
traits and divergent evolution histories between the two growth forms
(Hawkins et al., 2011; Engemann et al., 2016; Zanne et al., 2014, 2018).
Above-ground parts of herbaceous plants can die in winter with shoots
located above or below ground to avoid cold stress, while woody plants
with shoots and branches above ground cannot (Engemann et al., 2016;
Zanne et al., 2014, 2018). Woody species typically grow their stems and
renewed shoots taller than herbaceous species, exposing them to lower
temperatures. Moreover, herbaceous species tend to have shorter
generation time, higher evolutionary rates and weaker niche conservatism
(Smith \& Donoghue, 2008; Qi et al., 2014; Liu et al., 2018), and
consequently tend to be less sensitive to freezing than woody species
(Smith \& Beaulieu, 2009).

Climate change since the LGM has had a weaker impact on woody plant
species richness than it had on herbaceous species richness. Compared
with woody species, herbaceous species tend to have higher evolutionary
rates (Smith \& Donoghue, 2008; Qi et al., 2014), which may speed up
microevolutionary adaptation of herbaceous plants during environmental
changes (Hof et al., 2011; Liu et al., 2018). Therefore, herbaceous
species are likely able to track climate change more quickly than woody
plant as they could better adapt to environmental changes since the LGM
(Albuquerque et al., 2011; Liu et al., 2018). In addition, the
distribution of herbaceous plants is also higher in temperate regions,
which experienced greater climate change during LGM compared to the
southern mountainous regions. There findings may explain why the
geographic pattern of herbaceous species richness is more sensitive to
climate change during LGM.

\textbf{Difference in patterns and determinants of wide-ranged and
narrow-ranged species richness}

Studies had shown that wide-ranged species are important contributors to
the overall geographic pattern of species richness because the larger
amount of distribution records of wide-ranged species have a higher
contribution to the species richness counts (Lennon et al., 2003; Liu et
al., 2017). Our results found that the richness pattern of wide-ranged
woody and herbaceous species showed similar pattern to that of all woody
and herbaceous species in eastern Eurasia (Fig. 2a,c; b,e). Moreover,
the dominant hypotheses explaining the geographic patterns in
wide-ranged woody and herbaceous species were also consistent with those
for all woody and herbaceous species respectively. Specifically, the
richness pattern of wide-ranged woody species was dominated by the
freezing tolerance hypothesis, while that of wide-ranged herbaceous
species was dominated by climate change since the LGM (see the previous
section).

However, the dominant hypothesis of the richness patterns of
narrow-ranged woody and herbaceous species differed from those for all
woody and herbaceous species. Compared with other hypotheses, the Janzen
hypothesis had the highest explanatory power for the richness pattern of
narrow-ranged woody and herbaceous species (Table 1, Fig.5). The
dominant impact of Janzen hypothesis on species richness patterns has
also been demonstrated in several taxa such as Gesneriaceae and
gymnosperms (Liu et al., 2017; Lyu et al., 2018). These studies
consistently suggested that areas with low temperature seasonality and
high topographic heterogeneity have higher narrow-ranged species
diversity. Climate stability preserves more genotypes adapting to
specific environments, and these genotypes often have low dispersal
ability and propensity, and hence are more likely to have small
distributions (Dynesius \& Jansson, 2000; Jansson, 2003; Liu et al.,
2017). In addition, regions with high topographic heterogeneity can
provide a richer variety of habitat types and more biologically
available resources, thus providing more ecological niches for more
organisms to survive (Currie, 1991). High topographic heterogeneity can
also provide a refuge for organisms during environmental changes, thus
preserving more organisms (Cun \& Wang, 2010; Fjeldsa et al, 2012).
Since the mid-Cenozoic, due to diverse habitats and climatic stability,
the mountains of southern China have been a refuge for narrow-ranged
species, preserving a large number of narrow-ranged species of plants
and vertebrates (Lopez-Pujol et al, 2011a; Sandel et al, 2011; Xing \&
Ree, 2017). However, the mountainous areas in southern China have
undergone rapid climate warming, surpassing the rate observed in lowland
regions (Pepin et al., 2015).Furthermore, future air temperatures are
anticipated to be even higher than current levels (Nogues-Bravo et al.,
2007, Rebetez and Reinhard, 2007). Strong climatic changes may have
aggravated the extinction risk of the narrow-ranged plant diversity in
these mountains (Moritz et al., 2008; Menendez et al., 2014; Rowe et
al., 2015).

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Table 1 Relationship between All, different growth forms (Woody, Herb)
and range size (Wide-ranged, Narrow-ranged) species richness and
environment factors in eastern Eurasia. + indicates a positive
correlation, - indicates a negative correlation.\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
R2 & MTCQ & LGM\_MATanomaly & LGM\_MAPanomaly & TSN & TOPO\_RANGE &
DR\tabularnewline
\midrule
\endhead
All & 0.53 & 0.59 & 0.12 & 0.54 & 0.20 & 0.56\tabularnewline
Woody & 0.71 & 0.66 & 0.14 & 0.70 & 0.14 & 0.39\tabularnewline
Herbaceous & 0.35 & 0.48 & 0.08 & 0.39 & 0.21 & 0.56\tabularnewline
Wide-ranged Woody & 0.65 & 0.62 & 0.13 & 0.58 & 0.11 &
0.37\tabularnewline
Narrow-ranged Woody & 0.64 & 0.53 & 0.03 & 0.77 & 0.17 &
0.54\tabularnewline
Wide-ranged Herbaceous & 0.27 & 0.38 & 0.07 & 0.25 & 0.12 &
0.54\tabularnewline
Narrow-ranged Herbaceous & 0.32 & 0.35 & 0.01 & 0.50 & 0.29 &
0.32\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Fig 1 Patterns of all species richness in eastern Eurasia.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Fig 2 Patterns of woody (a), herb (b), wide-ranged (c) and narrow-ranged
species (d) richness in eastern Eurasia.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

Fig 3 Scatterplot of species richness of all, woody, herb, wide-ranged
and narrow-ranged species in eastern Eurasia in relation to impact
factors.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

Fig 4 Relative contributions of different hypotheses (percentage) to the
woody (a), herb (b), wide-ranged (c) and narrow-ranged (d) species
richness pattern. Blue represents the freezing tolerance hypothesis, Red
represents the climate change since LGM, Green represents the Janzen
hypothesis.

\selectlanguage{english}
\FloatBarrier
\end{document}

