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.
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., Citrus and 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 (Pinus pumila ), Korean
arborvitae (Thuja koraiensis ), and Khingan fir (Abies
nephrolepis ) (Kong et al., 2019; Korea National Arboretum, 2015). It is
also an area with active cool-climate highland agriculture, and timber
production (P. koraiensis and 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 (A. koreana ), and the southern limit
in East Asia of the range of dark-bark spruce (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., Stewartia koreana and 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 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., Castanopsis sieboldii ) to the cool zone of polar
trees (e.g., Diapensia lapponica var. 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.
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., A. koreana , Primula farinose , andCarex 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., Habenaria
chejuensis —and Asteraceae (24.3%)—e.g., 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., 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).