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\begin{document}

\title{Redefining floristic zones on the Korean Peninsula using high-resolution
georeferenced specimen data and self-organizing maps (SOMs)}



\author[1]{Song Hie  Jung}%
\author[1]{Yong-chan Cho}%
\affil[1]{Korea National Arboretum}%


\vspace{-1em}



  
  \date{May 05, 2024}


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\selectlanguage{english}
\begin{abstract}
The use of biota to analyze the spatial range and distribution of
biogeographic regions is essential to gain a better understanding of the
ecological processes that cause biotic differentiation and biodiversity
at multiple spatiotemporal scales. Recently, the collection of
high-resolution biological distribution data (e.g., specimens) and
advances in analytical theory have led to their quantitative analysis
and more refined spatial delineation. This study was conducted to
redefine floristic zones in the southern part of the Korean Peninsula
and to better understand the eco-evolutionary significance of the
spatial distribution patterns. Based on the distribution data of 309,333
vascular plant species in the Korean Peninsula, we derived floristic
zones using self-organizing maps. We compared the characteristics of the
derived regions with those of historical floristic zones and
ecologically important environmental factors (climate, geology, and
geography). In a clustering analysis of the floristic assemblages, four
distinct regions were identified, namely, the cold floristic zone (Zone
I) in high-altitude regions at the center of the Korean Peninsula, cool
floristic zone (Zone II) in high-altitude regions in the south of the
Korean Peninsula, warm floristic zone (Zone III) in low-altitude regions
in the central and southern parts of the Korean Peninsula, and maritime
warm floristic zone (Zone IV) including the volcanic islands of Jejudo
and Ulleungdo. A total of 1,099 taxa were common to the four floristic
zones. Zone IV had the highest abundance of specific plants (those found
in only one zone), with 404 taxa. This study improves floristic zone
definitions using high-resolution regional biological distribution data.
It will help better understand and re-establish regional species
diversity. In addition, our study provides key data for hotspot analysis
techniques required for the conservation of plant diversity.%
\end{abstract}%



\sloppy


\section*{Introduction}

{\label{introduction}}

Using biota to investigate biogeographic regions, including their
spatial range and distribution patterns, is key to providing a better
understanding of the ecological processes that create biotic
differentiation and biodiversity at multiple spatiotemporal scales
(Ricklefs, 2004). In particular, as an important step in understanding
the spatial structure of biodiversity, delineating the spatial range of
these regions has been a foundational part of basic and applied research
in biogeography, ecology, earth science, and conservation ecology (Brum
et al., 2017; Graham and Hijmans, 2006; Ibanez-Erquiaga et al., 2018;
Kreft and Jetz, 2010; Lenormand et al., 2019; Olson et al., 2001;
Ricklefs, 2004; Sun et al., 2008). Until the late
20\textsuperscript{th}century, the proposed biological regions were
based on limited data, convenience, and the opinions of experts (White,
1983). Without distinct criteria, floristic zones were also suggested
based on delineating the distribution of endemic plants (Takhtajan,
1986), or accumulating floristic checklist data and applying spatial
statistics (McLaughlin, 1989). Relatively recently, there have been many
proposals for defining geographical spaces based on plants
(Gonzalez-Orozco et al., 2014; Kreft and Jetz, 2010; Lenormand et al.,
2019; Vilhena and Antonelli, 2015). Difficulties exist with regard to
producing precise mapping and understanding the regional patterns of
biodiversity because of issues surrounding the fidelity and reliability
of floristic surveys, and because flora is closely related to
environmental gradients (development and climate) and their complexity
(Gonzalez-Orozco et al., 2014). If these challenges can be overcome, the
delineation of biological spaces based on high-fidelity, reliable data
will provide new metrics and perspectives through biogeographic
regionalization (Lenormand et al., 2019).

The first attempt at biogeographic regionalization of the Korean
Peninsula, made around 100 years ago, proposed convenient northern,
central, and southern divisions based on descriptions of plant and
vegetation characteristics (Nakai, 1919). Subsequently, more finely
differentiated floristic zones were defined by redistributing the
divisions of Nakai (1919) (Lee and Yim, 1978). Other authors proposed
pseudo-floristic zones from the perspective of vegetation and climate
(Yim and Kira, 1975). All these previous floristic zones for the Korean
Peninsula (including pseudo-floristic zones), which were mostly
developed from expert opinion or for convenience, have been structured
around homogeneous bands based on the relationship between latitude and
mean annual air temperature. In the neighboring country China,
large-scale banded or planar vegetation-climate zones have been
delineated using a wetness index (Sun et al., 2008). However, regions
with a complex mountainous structure show marked changes in elevation
and topography over short distances and, when this is combined with
human influence, it results in especially complicated
spatiotemporally-driven biogeographic regions (see the map of floristic
zones on the Korean Peninsula in Appendix A1) (Lenormand et al., 2019).

Given the lack of regionalization based on biological distribution data
(e.g., specimens), pseudo-biogeographical divisions have also been
developed for conservation at large spatial scales (Olson et al., 2001;
Sun et al., 2008). When delineating biogeographical zones, it is
necessary to maximize differences between zones while also maximizing
the homogeneity of the taxonomic assemblages within them (Stoddart,
1992). Improving the analytical accuracy through the quantitative
accumulation of organism distribution data, informatization of
geography, and large-scale distribution data---which has previously been
a challenge for the extraction of floristic and other biological
zones---enables quantitative and rigorous regionalization (Linder et
al., 2012). In recent studies, floristic zones have been delineated
using accumulated data to incorporate information about plant
distribution (Gonzalez-Orozco et al., 2014). The reliability of point
data for the distribution of organisms can only be ensured by using
specimen data. Because of the use of global positioning systems (GPS),
plant specimens that include spatial information are being collected.
From the late 20\textsuperscript{th} century, accurate and extensive
plant catalogs and data on the distribution of specimens have been
collected (e.g., Korea National Arboretum, 2016), and floristic regions
are being defined at the regional and national level using plant
location data (Gonzalez-Orozco et al., 2014; Korea National Arboretum,
2016; Lenormand et al., 2019). It is, therefore, possible to develop
approaches to delineate floristic zones using these data, reliably and
accurately. In particular, in the southern part of the Korean Peninsula,
the plant specimens that have been collected and the distribution maps
that have been composed since 2000 can be evaluated with reliable
large-scale data.

To analyze accumulated species distribution data, artificial neural
networks (ANNs) are increasingly used as an alternative to traditional
statistics for the analysis of multi-dimensional data (Chon, 2011;
Cottrell et al., 2018; Snedden, 2019). Specifically, self-organizing
maps (SOMs), an ANN-based technique using unsupervised learning, are
suggested as an alternative to conventional primary component analysis
(Ahn et al., 2018; Chon, 2011). Essentially, SOMs are classified as a
non-linear sequence analysis method, since the training data set is
non-linearly projected onto a lesser dimensional space (generally two
dimensions) approximating the probability density function (Kohonen,
1995; Snedden, 2019). Unlike statistical approaches using mediator
variables, SOMs do not make assumptions related to the correlations
between variables or the distribution of variables (Chon, 2011; Giraudel
and Lek, 2001; Snedden, 2019), and so are suitable for use with species
presence or absence data (C\selectlanguage{ngerman}éréghino et al., 2005). Efforts to derive and
visualize zones from plant species distribution data using conventional
univariate statistical analyses have generally assumed the response of
species data to environmental gradients, in accordance with Eigen-based
analytical approaches, but these analyses are limited in cases when the
shape of the species-abundance response (e.g., linear or unimodal) is
not clear (Ahn et al., 2018; Liu et al., 2006; Snedden, 2019). Recently,
numerous collections of distribution point data have been used for
biological regionalization, but it is not possible to ascertain the
relationships between species as variables. SOMs reduce multidimensional
data to two or three dimensions, making them useful for typification
analyses using distribution points for a large number of plant species
(including regionalization).

Quaternary glacial--interglacial oscillations have been an important
mechanism in shaping the current distribution of plants (Ricklefs,
1987). The Korean Peninsula, which is topographically composed of around
80\% mountains, is characterized by a backbone mountain range (the
Baekdudaegan Mountains) running north to south with sub-ranges branching
off. The major mountains (\selectlanguage{english}[?]1000 m above sea level) are considered to be
a single glacial refugium based on altitude rather than latitude, and
they have a mixture of boreal and temperate flora (Chung et al., 2017b;
Chung et al., 2018; Kim et al., 2014). The botanical importance of
peninsulas and mountainous regions is well established because of their
topographic characteristics (Medail and Diadema, 2009). The mountains
that form the core topography of the Korean Peninsula possess a
floristic composition that has been affected by latitude and
spatiotemporal gradients, and this might be significant for its mutually
distinct functions and evolutionary spaces. The flora of the Korean
Peninsula is fundamentally controlled by a mixture of boreal and
temperate abiotic conditions, and is affected, like other regions, by
the agricultural and urbanization activities of humans.

The accumulation of a large volume of recent distribution point data and
the application of analytical methods suited to the nature of the data
have resolved the previous difficulties of floristic regionalization,
making it possible to propose rigorous floristic zones according to the
actual distribution of species. To date, however, it has been difficult
to find case studies of accumulated, high-resolution, georeferenced
specimen data for plants, or to find studies related to geographic
regionalization using a SOM. The present study was conducted with the
purpose of redefining the floristic zones in the southern part of the
Korean Peninsula with SOMs and understanding the eco-evolutionary
significance of the spatial distribution patterns. We used point
distribution data for vascular plants collected at high resolution in
the southern part of the Korean Peninsula between 2003 and 2015. We
aimed to (1) derive floristic delimitations, (2) identify the
correlations with ecologically important environmental factors, and (3)
discuss the eco-evolutionary significance of the derived regions for
floristic assemblages.

\section*{Materials and methods}

{\label{materials-and-methods}}

\subsection*{Study region}

{\label{study-region}}

Our study was conducted in the southern part of the Korean Peninsula,
which is located the East Asia region (33deg--38deg N, 125deg--131deg E; Fig.
1). The total area of the study region is 100,033 km\textsuperscript{2},
and the human population is 51 million (Ministry of Land Infrastructure
and Transport, 2016). The mean annual air temperature range ranges from
10--15 degC, and the range of the mean annual precipitation is 1000--1900
mm (Korean Meteorological Administration, 2020).

The Korean Peninsula is situated adjacent to the west Pacific Ocean and
is surrounded by water in three directions---east, west, and south. It
is a temperate region with four distinct seasons associated with the
East Asian monsoon that occurs on the far eastern side of the Asian
continent (Yi, 2011). Winter (December--February) is cold and dry
because of the formation of the strong Siberian anticyclone from the
Tibetan Plateau, while summer (June--August) is hot and humid, with
around 70\% of the annual precipitation focused in this period (Korean
Meteorological Administration, 2020) (Appendix A2). Although the Korean
Peninsula is located at the eastern edge of the Eurasian continent, the
humid air supplied from the Yellow Sea to the west affects the diversity
and distribution of local plants.

The Korean Peninsula comprises a large number of mountains centered in
the Baekdudaegan Mountain Range, and only 22.5\% of the peninsula is
flat land (Appendix A3) (Ministry of Land Infrastructure and Transport,
2016). In addition, although the elevation is not generally high, the
region displays complex tectonic characteristics with a relatively
diverse topography. Because the altitude gradients are shallower than
those in other regions in East Asia, the borders between mountains and
plateaus are relatively indistinct, making the region well suited for
the spatiotemporal movement of plants (Ministry of Land Infrastructure
and Transport, 2016).

There are around 4,300 known species of vascular plant on the Korean
Peninsula (with approximately 3,000 species in the southern part),
including 280 species of pteridophyte, 53 species of gymnosperm, and
3,963 species of angiosperm. In terms of specialized
genera,\emph{Pentactina} , \emph{Echiosophora} , \emph{Abeliophyllum}
,\emph{Hanabusaya} , \emph{Mankyua} , and \emph{Megaleranthis} are
present. According to the Whittaker biome classification (Whittaker,
1962), the southern part of the Korean Peninsula is mostly occupied by
temperate seasonal forest biomes but may also contain some temperate
rain forest and woodland/shrubland biomes (Fig. 1). In terms of the
remnant vegetation landscape of the Korean Peninsula, strong policies to
promote agriculture throughout the Joseon Period (1392--1910) led to a
large decrease in forests and an increase in grassland and shrubland
habitats. Later, in the southern part of the Korean Peninsula, the South
Korean government pursued policies to promote forests from the 1970s,
resulting in most natural habitats being located in forests (Cho et al.,
2018). Currently, approximately 30.3\% of the southern part of the
Korean Peninsula is urbanized or used for agriculture and 63.8\% is
occupied by forests, with other land covers accounting for the remaining
5.9\% (Ministry of Land Infrastructure and Transport, 2016).

\subsection*{Plant distribution data}

{\label{plant-distribution-data}}

We used vascular plant distribution data based on specimen and
coordinate data for plants collected between 2003 and 2015 in the
southern part of the Korean Peninsula (Korea National Arboretum, 2016).
The vascular plant distribution maps contained coordinate data for
309,333 specimens, corresponding to 2,954 taxa in 175 families and 919
genera. For analysis, a grid system was overlaid on a national
topographic map to combine the taxonomic groups located in each cell of
the grid (cell size, 11.2 km x 13.9 km) with the location coordinates in
a single data set (Graham and Hijmans, 2006; Lenormand et al.,
2019)\textbf{.} All 771 grid cells were used in the analysis, but some
large urban regions were excluded from the floristic survey conducted by
the Korea National Arboretum, and so these were left as empty cells.

\subsection*{Analysis of floristic assemblage clusters and
characteristics}

{\label{analysis-of-floristic-assemblage-clusters-and-characteristics}}

Using distribution data for the 771 grid cells and 2,965 plant taxa, a
SOM training data set was constructed in the form of a presence-absence
matrix (771 rows x 2,954 columns) (Fig. 2). The `kohonen' R package was
used for the SOM algorithm (Wehrens and Kruisselbrink, 2018), and the
output layer was composed of 81 output nodes arranged in a square
lattice. To determine the types, hierarchical cluster analysis was
applied to the weight vectors of the SOM map units after conversion to
Euclidean distance metrics (via the function hclust in R using the
complete linkage method). The optimal number of types was calculated by
applying the silhouette coefficient to the range of 2--15 types
(Rousseeuw, 1987). In mapping the regionalization results, the grid
cells that were empty because of exclusion from the survey were filled
using the maximum frequency value from the surrounding eight cells.
Since some island regions (Ulleungdo and Dokdo) showed heterogeneous
values because of their distance from the adjacent grid cells, mapping
was performed using type values within the local range.

The correlations in species composition between the floristic zones were
analyzed using Venn diagrams (in the `VennDiagram' package) based on
lists of species in each zone (Chen and Boutros, 2011). After producing
species catalogs for each zone, the common taxa (those appearing in all
zones) and specific taxa (those appearing in only specific zones) were
distinguished. Then, floristic compositions were investigated by
analyzing the identification of specific taxa at the family level.

\subsection*{Environmental data and
analysis}

{\label{environmental-data-and-analysis}}

Geographic and climate factors were analyzed as macro-environmental
factors, using the defined floristic zones. For geographic factors, the
latitude and longitude were used, and for climate factors, air
temperature and precipitation data---provided by the Korean
Meteorological Administration (2020) and collected from 583 points
between 1970 and 2010---were used. In addition to the direct
environmental data, the warmth index (WI) and coldness index (CI) were
calculated and used as indirect climate data (Kira, 1945) (Eq. 1 and 2).
The values for these environmental factors were converted to values
covering the whole southern part of the Korean Peninsula by linear
interpolation, accounting for topography and altitude, with ArcGIS
program (ver. 10.0). The mean values for the environmental factors in
each grid cell were then calculated and used in the analysis.

\begin{equation}
\text{WI}=\sum_{1}^{n}{\left(t-5\right):t>5}\nonumber \\
 \end{equation}\begin{equation}
\text{CI}=-\sum_{1}^{n}{\left(t-5\right):t<5}\backslash n\nonumber \\
 \end{equation}

As physical factors affecting plant distribution, parent materials,
topography, effective soil depth, and soil texture for the southern part
of the Korean Peninsula were used (Rural Development Administration,
2010). Parent materials were categorized as acidic rock, metamorphic
rock, sedimentary rock, quaternary deposit, volcanic ash, and other;
topography was categorized into mountain, hill, pediment, interrill
area, fan, lava terrace, or other; effective soil depth was categorized
into four classes (\textless{}20 cm, 20--50 cm, 50--100 cm, and
\textgreater{}100 cm), and soil texture was categorized as sandy gravel,
silt and sandy loam, clay loam, and clay (Appendix A4).

To test the effect of environmental factors on the floristic composition
and zonation, the geographic and climate (mean annual temperature,
annual precipitation, warmth index, and coldness index) data were
analyzed using the one-way analysis of variance (ANOVA) and Tukey's
tests (Zar, 1984). The categorical physical factors (parent materials,
topography, effective soil depth, and soil texture) were analyzed using
box plots for each zone. The ``ggplot2'' R package was used for data
visualization (Wickham, 2016). Statistical analyses were performed using
R (R Core Team, 2019).

\section*{Results}

{\label{results}}

\subsection*{Flora extraction and taxon
distribution}

{\label{flora-extraction-and-taxon-distribution}}

After the coordinates of the plant specimens were converted to floristic
composition data for the 771 grid cells, the phytogeographic structure
of the southern part of the Korean Peninsula was divided into four zones
(maximum silhouette coefficient: 0.7968, Table 1, Figs. 3 and 4). The
peninsula was divided into three inland zones---the cold floristic zone
(hereafter, Zone I), corresponding to the high-altitude regions in the
central part of the peninsula; the cool floristic zone (hereafter, Zone
II), corresponding to the high-altitude regions in the southern part of
the peninsula; and the warm floristic zone (hereafter, Zone III),
corresponding to the lowlands in the central and southern parts of the
peninsula---and the maritime warm floristic zone (hereafter, Zone IV),
including Jejudo and Ulleungdo.

Although these zones formed large patches and occupied large areas,
smaller patches were scattered within the other floristic zones (Fig.
4). This is because Zone III, which was connected to Zone I, contained
most of the major cities in a given region or broadscale production
(forestry and agriculture) regions. Our study region also included the
military demarcation line between North and South Korea. The region near
the military demarcation line (near the 38th parallel, length, 248 km;
area, 907 km\textsuperscript{2}) is characterized by a high level of
disruption and environmental management activities.

Zone III---broadly covering the lowlands of the southern part of the
Korean Peninsula---occupied the largest area (72.0\%), followed by Zones
I (12.3\%) and II (11.5\%) in regions with high-altitude mountains, and
Zone IV (4.2\%), which included coastal and island regions (Table 1,
Fig. 4). Unlike the area gradient, the order of species abundance (from
highest to lowest) was: Zone III (2,379 taxa), Zone IV (2,200 taxa),
Zone I (1,700 taxa), and Zone II (1,668 taxa). The species abundance per
classified grid cell was highest in Zone I (464.7 \selectlanguage{ngerman}± 9.9 taxa), followed
by Zone II (331.7 ± 11.5 taxa), Zone IV (298.6 ± 14.0 taxa), and finally
Zone III (79.3 ± 3.4 taxa), which contained a large number of developed
regions.

\subsection*{Geographic range and climatic environment in each floristic
zone}

{\label{geographic-range-and-climatic-environment-in-each-floristic-zone}}

All the floristic zones showed significant differences in the mean
longitude and latitude, mean annual temperature, annual precipitation,
WI, and CI per grid cell (\emph{p} \textless{} 0.001, Fig. 5a--f). While
Zone I was focused in the central part of the Korean Peninsula, Zone II
was mostly situated in the center of the southern part of the peninsula,
although there were scattered small patches within the central region
adjacent to Zone I or inside Zone III (Figs. 4 and 5a--b). Zone IV was
mostly located on the coast and islands in the west and south of the
peninsula, as well as some inland areas in the west. Zone III showed a
relatively broad longitudinal and latitudinal range, since it was
distributed throughout the study region across the whole southern part
of the Korean Peninsula.

When the climatic environments were compared between the floristic
zones, differences were observed in mean annual air temperature, mean
precipitation, WI, and CI (Fig. 5c--f). Mean annual air temperature
showed a gradually increasing trend from Zones I to IV. Annual
precipitation showed the opposite trend, but there was no clear
difference between Zones I and II. As indirect climate factors, mean WI
and CI also showed clear differences between zones. In accordance with
its wide geographic distribution, Zone III showed the largest ranges for
these climate factors.

\subsection*{The physical environment in each floristic
zone}

{\label{the-physical-environment-in-each-floristic-zone}}

The parent materials in the southern part of the Korean Peninsula are
predominantly acidic rocks (48.5\%) and metamorphic rocks (26.3\%), and
the ratio of plateaus is very low. The most common soil depth is 50--100
cm (39.1\%), and 93.7\% of the soil consists of silt and sandy loam
(51.6\%) or clay loam (42.1\%) (Ministry of Land Infrastructure and
Transport, 2016) (Appendix A4).

The parent materials across the four floristic zones consisted of over
70\% acidic and metamorphic rocks; however, Zone IV, which includes the
volcanic island of Jejudo and the oceanic island of Ulleungdo, showed an
especially high ratio of volcanic ash (23.7\%) among its parent
materials (Table 2). In addition, Zone IV showed a far higher proportion
of lava terrace as its terrain type and clay loam for its soil type, and
the soil depth classes were more evenly distributed compared to the
other zones (Fig. 6).

\subsection*{The relationships between species composition in each
floristic
zone}

{\label{the-relationships-between-species-composition-in-each-floristic-zone}}

A total of 1,099 common taxa was found in all zones (Fig. 7).
Interestingly, the zone with the most specific taxa was Zone IV (404
specific taxa, 18.4\%), which had the smallest area. This was followed
by Zone III (192, 8.1\%), Zone I (72, 4.2\%), and Zone II (25, 1.5\%).

When specific taxa were analyzed at the family level (Fig. 8), Zone
III---generally consisting of low-lying hilly terrain heavily affected
by human activity and development---showed higher diversity of Poaceae
(18 out of 192 taxa) compared to Zones I and II, while the marine and
coastal Zone IV was characterized by a high diversity of Orchidaceae (27
out of 404 taxa), Asteraceae, and Rosaceae. Among the common species
appearing in all zones, the families with the highest diversity, in
descending order, were Asteraceae (113 taxa), Poaceae (98 taxa),
Cyperaceae (68 taxa), Fabaceae (55 taxa), and Rosaceae (47 taxa) (Fig.
8).

\section*{Discussion}

{\label{discussion}}

Historically, biogeographic spaces have predominantly been delineated
using the distribution of organisms according to specific knowledge
(e.g., partially usable data, such as endemism or the distribution of
evergreen trees), or corresponding geographical and climatic factors,
rather than using actual recorded biota (Takhtajan, 1986; White, 1983).
Historical definitions of vegetation-climate and floristic zones on the
Korean Peninsula have used these methods (Lee and Yim, 1978; Yim and
Kira, 1975). Recently, there have been attempts to review bioregions
using quantitative analytical techniques, with some showing similar, and
some different, results to the historical regions (Gonzalez-Orozco et
al., 2014; Lenormand et al., 2019). The present study differs
fundamentally from previous approaches to phytogeographic
regionalization because we used high-resolution, georeferenced specimen
data for the southern part of the Korean Peninsula. Although restricted
to the southern regions, our results provide clearly improved
phytogeographic zones through our use of SOMs and georeferenced data for
recently collected plant specimens. The spatial distribution of
floristic zones and the basic taxonomic composition in the southern part
of the Korean Peninsula reflect a combination of repeated migrations,
expansions, and reductions of species associated with climatic
variation, physical and geographic factors, and human activity patterns,
including the political situation (Valladares et al., 2015). In
addition, we reconfirmed differences identified in historical studies in
geographic and climatic conditions, which are deterministic factors in
the spatial patterns of floristic assemblages (Lee and Yim, 1978; Yim
and Kira, 1975). We were also able to verify the correlations between
zones and the distribution of zones based on the characteristics of the
floristic assemblages and the physical conditions where they are found,
such as the topographic characteristics of the Korean Peninsula. In
particular, we verified the importance of the topography of the Korean
Peninsula (especially the connected mountainous regions and isolated
mountains), which has been emphasized in recent studies (Chung et al.,
2017b), and its contribution to shifts in distribution related to
climate change and the process of floristic reassembly. As well as the
deterministic factors for floristic assembly that have been used
historically (e.g., climatic factors), more diverse ecologically
important environmental factors will need to be considered at the same
spatial range as the southern part of the Korean Peninsula.

\subsection*{Spatial characteristics of the new floristic
zones}

{\label{spatial-characteristics-of-the-new-floristic-zones}}

The transitional changes in the flora of the southern Korean Peninsula
(from warm to cool), and the effects of physical and human activity were
investigated via the derived floristic zones. The historical floristic
zones of the Korean Peninsula were constructed to broadly reflect a
combination of climatic factors and plant distribution patterns, and had
a banded, near-planar shape (Appendix A1) (Lee and Yim, 1978). Our SOM
results, revealing four statistically significant spatial clusters
(Zones I--IV) representing distinct territories (detailed below), were
consistent with unification (or partial inclusion) or division of the
historical zones. The historical central and southern zones partially
reflected the distribution patterns of some tree ecotypes, including
evergreen broad-leaved trees (e.g., \emph{Citrus} and \emph{Ilex} ), but
there was limited consideration of important biogeographic factors in
small regions, such as complex mountain groups and the effects of
altitude. Rather than forming a broad banded pattern, recent studies
have revealed spatial patterns (at global and regional scales) that
include large numbers of small, heterogeneous patches within larger
biotic assemblage zones, arising from changes in the climatic and
physical environment, such as quaternary glacial-interglacial
oscillations, and the roles of factors such as topography (Kreft and
Jetz, 2010; Lenormand et al., 2019; Médail and Diadema, 2009; Silva and
Souza, 2018).

Zone I is focused around high-altitude mountains in the central Korean
Peninsula (maximum altitude: 1708 m, Mt. Seoraksan) and included some
high-altitude regions in the south. Among the historical zones, this
partially corresponds to the central and southern zones. Zone I reflects
floristic elements that moved south from the previous periglacial
environment and remained in the high-altitude mountain ranges in the
center of the Korean Peninsula after the periglacial environment
receded. In East Asia, this region represents the southern limit of the
ranges of the dwarf Siberian pine (\emph{Pinus pumila} ), Korean
arborvitae (\emph{Thuja koraiensis} ), and Khingan fir (\emph{Abies
nephrolepis} ) (Kong et al., 2019; Korea National Arboretum, 2015). It
is also an area with active cool-climate highland agriculture, and
timber production (\emph{P. koraiensis} and \emph{Larix kaempferi} ).

Zone II is focused on mountainous land adjacent to Zone I, mostly
consisting of inland, high-altitude mountains further south in the
Korean Peninsula (1915 m, Mt. Jirisan and 1614 m, Mt. Deogyusan). Among
the historical zones, this partially corresponds to the same central and
southern floristic zones as Zone I. This region is the northern limit of
the range of the Korean fir (\emph{A. koreana} ), and the southern limit
in East Asia of the range of dark-bark spruce (\emph{Picea jezoensis} )
(Korea National Arboretum, 2015). Through the same historical
geographical processes as Zone I, this zone shows the remnants of the
influence of the periglacial environment in the southern Korean
Peninsula. In addition, trees that are mostly distributed in warmer or
maritime climates (e.g., \emph{Stewartia koreana} and \emph{Lindera
sericea} ) can be found growing in the medium and high altitude areas of
Zone II (Kim et al., 2014). There is almost no highland agriculture in
this zone, but some timber production occurs (mostly \emph{Larix
kaempferi} ), and the zone is adjacent to numerous large cities.

Zone III accounts for most of the southern Korean Peninsula. Although
this zone contains tall mountains and mountain ranges, it is also
subject to complex effects from agricultural activity and cities with a
relatively intensive level of land use. For example, the inclusion of
areas in Zone III with higher latitudes than Zone I is likely because of
the impact of long-term military activity in these areas. Notably, in
Zone III, the plants constituting the other floristic zones (Zones I,
II, and IV) remain in isolated islands. These heterogeneous patches
inside Zone III are a vestige of shifts in plant diversity patterns
driven by historical geographical changes, and thus are important in the
heterogeneous formation of the regional floristic composition (Laliberté
et al., 2014; Zobel, 1997). Recently, the function of the Korean
Peninsula as a shelter for biodiversity has been emphasized, because of
its topography that includes a number of core mountains (Chung et al.,
2018). In-depth studies need to be conducted on the biodiversity
conservation functions (e.g., provision of shelter) of these small,
heterogeneous patches and on their long-term changes.

Zone IV unifies the historical floristic zones of the southern coast,
Jejudo, and Ulleungdo, which have previously been more finely divided
(Appendix A1), and contains a high relative abundance of Orchidaceae,
Asteraceae, and Rosaceae as well as specific taxa that only appear in
this zone. Although Zone IV includes some inland areas, it mostly
consists of regions along the coast of the Korean Peninsula and islands
that are important for biodiversity, such as the oceanic island
Ulleungdo (Chang and Gil, 2014; Choi et al., 2019; Holman et al., 2017)
and the volcanic island Jejudo, which contains Hallasan Mountain (1950
m). This zone ranges from the temperate zone of evergreen broad-leaf
trees (e.g., \emph{Castanopsis sieboldii} ) to the cool zone of polar
trees (e.g., \emph{Diapensia lapponica} var. \emph{obovate} ).
Importantly, among the four identified floristic zones, Zone IV shows
relatively high diversity of specific taxa, and is a core part of the
range of Orchidaceae, making it an important zone from a conservation
and evolutionary perspective. Because this zone includes the volcanic
islands of Ulleungdo and Jejudo, the physical conditions differ
considerably from the other zones, including in parent materials
(volcanic ash) and terrain (lava terraces). The north of Jejudo has
acted as an agricultural and administrative center for longer than the
south, which could explain the differences in the flora.

\subsection*{Spatial clustering and separation of floristic
assemblages}

{\label{spatial-clustering-and-separation-of-floristic-assemblages}}

In addition to plant distribution, regionalization of floristic
assemblages is important for understanding the formation processes on
the Korean Peninsula, including floral changes. The regional species
pool is the result of numerous settlement processes caused by climatic
changes, which is restricted by ecological filtration, and ultimately
forms the local biotic assemblage (Ricklefs, 1987). The Korean Peninsula
is composed of a network of mountain ranges along a latitudinal gradient
and has witnessed interactions between the Manchurian flora region at
higher latitudes, the North Chinese flora region at lower latitudes, and
the Japan-Korean flora region (Takhtajan, 1986). This is the background
for the current plant diversity and species composition on the peninsula
(Appendix A2). Thus, through repeated historical geographical processes,
such as periglacial climates, the Korean Peninsula has acted as a
geographical and biological corridor, with a mixture of high- and
low-latitude plants, which has produced the present-day spatial
distribution of biodiversity (Chang et al., 2016; Chung et al., 2017a;
Chung et al., 2017b; Kim et al., 2014; Kim et al., 2005; Kong et al.,
2019).

Unlike the historical banded or homogeneous planar patterns, our revised
floristic zones in the southern part of the Korean Peninsula reveal a
new pattern, with Zones I and II forming central regions surrounded by a
background of Zone III, with small patches of Zones I and II present
within Zone III. At regional scales (e.g., the Korean Peninsula),
complex physical and topographic factors can affect biotic assemblages
(Lasmar et al., 2020; Tsiftsis et al., 2008; Xu et al., 2016). Across a
broader area, there are also effects of climate, such as latitude
(Sanders et al., 2007). The sequential settlement, expansion,
contraction, and maintenance of species with historical geographic
environmental oscillations are major processes involving interactions
between the biota and topographic locations in a given region, which
have been reported in other ecological regions (Lenormand et al., 2019;
Silva and Souza, 2018). The Mt. Mudeungsan (1187 m) and Mt. Gyeryongsan
(846 m) national parks in the center of the southern Korean Peninsula,
the high-altitude Mt. Palgonsan (1192 m) region in the southeast Korean
Peninsula, and the Youngnam Mountains (an aggregation of nine mountains
over 1000 m) are important locations containing local-scale plant
communities (e.g., \emph{A. koreana} , \emph{Primula farinose} ,
and\emph{Carex tenuiformis} in the Youngnam mountains), which are
distant from the heterogeneous flora and the central distributions of
Zone III (Kim et al., 2015; Korea National Arboretum, 2014). The
mountains in these regions are an important factor in the formation of
heterogeneous, small-scale biological interactions (Gentili et al.,
2015; Thomson, 2005) and the process of species differentiation.

Excluding the oceanic island Ulleungdo, the islands of the Korean
Peninsula were last connected to the mainland before the Early Holocene,
around 7,000 years ago (Kim et al., 2016). The islands have spent a long
time, in terms of evolutionary biology, separated from the East Asian
mainland. Among the zones defined in this study, the coastal regions and
islands that constitute Zone IV (the warmest zone but with the least
annual precipitation) showed especially high abundance of certain plant
species (but low overall mean abundance) and specific plants (e.g.,
Orchidaceae and Asteraceae) were diverse. As discussed above, these
results were likely because of the inclusion of the volcanic island
Jejudo (1950 m above sea level) with its uniquely warm and humid
maritime climate, and the oceanic island Ulleungdo (986.7 m above sea
level). The Orchidaceae (71.4\%)---e.g., \emph{Habenaria chejuensis}
---and Asteraceae (24.3\%)---e.g., \emph{Artemisia hallaisanensis}
---that are only present in Zone IV include species that only grow on
Jejudo or Ulleungdo. Orchidaceae are most abundant in warm and humid
regions and show a negative correlation with the latitudinal gradient
(Cribb et al., 2003; Myers et al., 2000). The family can be an index of
biodiversity, since they show specialized habitat preference (Cho et
al., 2019) and form associations between multiple species, including
pollinators and mycorrhizal fungi (Pemberton, 2010). Spatial separation
(as seen on isolated islands) is a core mechanism of species
differentiation, but concomitant ecologically important environmental
variables (e.g., climatic and physical conditions) also operate in
combination with spatial separation to create selection pressures
(Anacker and Strauss, 2014). Despite its narrow width, Zone IV is a key
region in terms of ecologically important environmental characteristics,
taxon diversity, and species differentiation, and thus, among the
floristic zones on the Korean Peninsula, it occupies an important
position from a conservation and evolutionary perspective.

Away from the coast, the mainland regions in Zone IV include Mt.
Bukhansan, which is a national park close to the western coast, and Mt.
Mudeungsan, which is in the southwest mainland. These regions are large
mountains within Zone III and are thought to be vestiges left after the
entry of coastal and island floristic features from a past environment.
The slopes of Mt. Mudeungsan have many warm wind holes (Park, 2017) and
contain numerous plants that can otherwise be found mostly along the
coasts and in islands (e.g., \emph{Cyrtosia septentrionalis} (Rchb. f.)
Garay and Polypodiales) (Hong et al., 2013). Some islands to the south
and west of the Korean Peninsula were categorized into Zone III., A
landscape that includes grazing and crop farming predominates the
islands in the south of the Korean Peninsula, and thus, there has been
an active introduction of plant species from the mainland, changing the
flora considerably over time (Kim et al., 2017). These ecological
landscape characteristics could form the background for the high
floristic connectivity of some islands within Zone III, which mostly has
flora in inland developed regions and warm temperate climates.

There are research limits to explaining in detail the processes and
significance of spatial clustering and separation of floristic
assemblages using regional patterns alone. Detailed descriptions of the
plant reassembly process after the last glacial maximum and epoch-scale
studies, such as pollen analysis, are needed (Yi, 2011). In addition, in
the derived floristic zones, a convergent approach to phylogenetic
history and diversity would be particularly useful for analyzing
historical incidents in the formation of current biodiversity patterns,
and for ascertaining the historical and evolutionary relationships
between zones. Nevertheless, deducing the spatial arrangements of
floristic assemblages is of great importance for understanding the
ecologically important environmental factors involved in forming
biogeographic regions. This study provides essential background
knowledge to develop precise strategies for conservation based on micro-
(Fenu et al., 2010) and nano-hotspots (Grant and Samways, 2011) at the
local level. Moreover, there is great potential to quantitatively
calculate the rarity, endemicity, and commonness of plant species, and
to improve the priority of conservation and research (Casazza et al.,
2005).

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\section*{Figure legends}

{\label{figure-legends}}

Fig. 1. Representativeness of study sites. (a) Distribution of study
sites across the southern part of the Korean peninsula. (b) Distribution
of study sites across the global vegetation biomes defined by the
Whittaker classification.

Fig. 2. Conceptual diagram of the analysis steps and data flow in
defining the floristic zonation (Kreft and Jetz, 2010).

Fig. 3. Results of the self-organizing map (SOM) analysis for 9 × 9 SOM
map. (a) The neighbor distance plot or U-Matrix indicating the distance
between each node and its neighbors. (b) The classification of the
training samples according to the SOMs.

Fig. 4. Four floristic zones mapped to the southern Korean Peninsula,
derived using plant distribution data. The straight and solid lines
represent the named historical floristic regions. The lines dividing the
historical floristic regions have previously been used to explain the
observed distribution of plants on the Korean Peninsula (e.g., north,
central, and southern Korean Peninsula).

Fig. 5. Comparison of geographic and climate factors in the derived
floristic zones on the southern Korean Peninsula. (a) latitude; (b)
longitude; (c) mean annual temperature; (d) annual precipitation; (e)
warmth index; and (f) coldness index. Median values are marked inside
the boxplots, errors bars indicates the 90th and 10th percentiles, and
the points outside of the error bars indicate outliers. A Tukey post-hoc
test revealed significance groups, represented by letters (p \textless{}
0.05).

Fig. 6. Analysis of composition ratios for physical factors in each
floristic zone on the Korean Peninsula. (a) Parent material; (b)
topography; (c) effective soil depth; and (d) soil texture.

Fig. 7. A Venn diagram showing the relationships between plant
distributions in each of the four floristic zones. Numbers represent
individual taxonomic groups.

Fig. 8. Comparison of the composition of specific plant families in each
floristic zone. As shown in Fig. 7, there were 1,099 taxa that appeared
in all 4 zones. The specific plant species in Zones I, II, III, and IV
numbered 72, 25, 192, and 404 taxa, respectively.

\section*{Data Accessibility
Statement}

{\label{data-accessibility-statement}}

The datasets generated and/or analyzed in the present study are
available in Dryad at DOI: https://doi.org/10.5061/dryad.3tx95x6cr
(temporary address:
https://datadryad.org/stash/share/z49UBwqmjjaPN59vNsg6AU24mqc8yuxc3juB3Lw3EC0)

\section*{Competing Interests
Statement}

{\label{competing-interests-statement}}

The authors declare that they have no competing interests.

\section*{Author Contributions}

{\label{author-contributions}}

Song Hie Jung: Writing---original draft (lead); formal analysis (lead);
methodology (equal). Yong-chan Cho: Conceptualization (lead);
methodology (equal); writing---review and editing (lead).

\section*{Acknowledgements}

{\label{acknowledgements}}

The study was financially supported by the Korea National Arboretum
(grant number KNA1-2-26, 16-4).

Tables

Table 1. The geographical and biological statistics used to delineate
the four floristic zones of the southern Korean Peninsula.\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
\begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
Floristic zones\strut
\end{minipage} & \begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
Area (km\textsuperscript{2}) (\%)\strut
\end{minipage} & \begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
Species richness\strut
\end{minipage} & \begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
Mean species richness (per grid) (\selectlanguage{ngerman}± S.E.)\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
Zone I (Cold floristic zone in central high elevation areas)\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
12,419 (12.3)\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
1,700\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
464.7 \selectlanguage{ngerman}± 9.9\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
Zone II (Cool floristic zone in central and southern high elevation
areas)\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
11,619 (11.5)\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
1,668\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
331.7 \selectlanguage{ngerman}± 11.5\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
Zone III (Warm floristic zone in central and southern hilly areas)\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
72,367 (72.0)\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
2,379\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
79.3 \selectlanguage{ngerman}± 3.4\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
Zone IV (Maritime floristic zone near the coast and islands)\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
4,265 (4.2)\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
2,200\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
298.6 \selectlanguage{ngerman}± 14.0\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Total\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
100,378 (100.0)\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
2,954\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
162.6 \selectlanguage{ngerman}± 5.8\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{ngerman}

Table 2. Analysis of the composition ratios (\%) of physical
environmental factors in the research area and derived floristic
assemblage zones, including topography, soil parent materials, soil
depth, and soil type.\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
Parameters & Zone I & Zone II & Zone III & Zone IV & Total land
area\tabularnewline
\midrule
\endhead
Topological classes & & & & &\tabularnewline
Mountain & 44.6 & 69.2 & 36.8 & 35.5 & 46.9\tabularnewline
Hill & 21.4 & 8.8 & 23.1 & 12.2 & 18.2\tabularnewline
Pediment & 14.7 & 7.6 & 15.8 & 7.0 & 12.8\tabularnewline
Interrill area & 7.6 & 9.0 & 7.4 & 11.3 & 8.1\tabularnewline
Fan & 2.5 & 2.1 & 2.4 & 2.7 & 2.3\tabularnewline
Lava terrace & - & 0.2 & 0.5 & 20.1 & 1.5\tabularnewline
Others & 9.0 & 3.1 & 14.0 & 11.2 & 10.1\tabularnewline
Sum & 100.0 & 100.0 & 100.0 & 100.0 & 100.0\tabularnewline
Parent material types & & & & &\tabularnewline
Acidic rocks & 50.1 & 46.3 & 48.2 & 56.9 & 48.5\tabularnewline
Metamorphic rocks & 26.2 & 38.2 & 22.4 & 5.0 & 26.3\tabularnewline
Sedimentary rocks & 13.5 & 11.5 & 13.8 & 0.7 & 12.3\tabularnewline
Quaternary deposits & 9.8 & 3.7 & 14.6 & 10.2 & 10.6\tabularnewline
Volcanic ash & - & - & 0.4 & 23.7 & 1.7\tabularnewline
Others & 0.3 & 0.3 & 0.6 & 3.4 & 0.6\tabularnewline
Sum & 100.0 & 100.0 & 100.0 & 100.0 & 100.0\tabularnewline
Soil depth classes (cm) & & & & &\tabularnewline
\textless{}20 & 19.0 & 20.9 & 17.7 & 26.8 & 19.3\tabularnewline
20--50 & 21.6 & 27.4 & 20.2 & 25.4 & 22.7\tabularnewline
50--100 & 40.5 & 42.9 & 38.4 & 24.9 & 39.1\tabularnewline
\textgreater{}100 & 18.9 & 8.9 & 23.7 & 22.9 & 18.8\tabularnewline
Sum & 100.0 & 100.0 & 100.0 & 100.0 & 100.0\tabularnewline
Soil texture types & & & & &\tabularnewline
Sandy gravel & 2.2 & 1.4 & 2.5 & 4.4 & 2.3\tabularnewline
Silt sandy loam & 52.6 & 69.4 & 44.5 & 27.8 & 51.6\tabularnewline
Clay loam & 41.5 & 27.1 & 47.9 & 63.1 & 42.1\tabularnewline
Clay & 3.7 & 2.2 & 5.2 & 4.7 & 4.1\tabularnewline
Sum & 100.0 & 100.0 & 100.0 & 100.0 & 100.0\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
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