INTRODUCTION
One of the major interests of ecologists is to understand changing processes that plant communities and societies exhibit by interacting with environmental factors (Clements, 1916;Gleason, 1926;Whittaker, 1953;Connell and Slatyer, 1977;Grubb, 1977;Pickett and Thompson, 1978). As an analysis method based on such an interest, the changing process in which a plant community regenerates, grows, reaching climax, and becomes extinct as a single organism has been proposed (Clements, 1916). As another analysis method, it has been also asserted that the species that makes up a plant community continuously shows changes because the environment changes as the community regenerates and develops and because different species individually regenerate and grow in each changing process (Gleason, 1917;1926). In addition, in order to analyze ecological succession which is the process of community development, a gradient analysis method and diverse analysis tools including mathematical analysis have been developed (Whittaker, 1953). With the focus on Automatic succession in which the direction of succession is determined by the species consisting the community, Facilitation model, Tolerance model, and Inhibition model have been proposed as three types of succession models (Connell and Slatyer, 1977). In addition, since the proposal that communities can reach an equilibrium state that is the final stage of ecological succession is in question, the concept of non-equilibrium succession has been proposed and utilized as a tool to explain diversity changes of plant species that make up communities (Pickett and Thompson, 1978). Although mathematical tools have been proposed along with various theories and concepts like this to explain changes in plant communities, there are difficulties in analyzing and predicting the results of complex interactions (Austin, 1976;Sprugel, 1991).
Plant species possess different ecological niches (Hutchinson, 1957;Pearman et al., 2007). Individuals of a species that possesses the same ecological niche come together to form a population (Crawley and Ross, 1990;Harper, 1977;Wiens and Graham, 2005). In addition, each population is not independently distributed and interacts with other species to form a community (Harper, 1977;Whittaker, 1953;1972). At this time, the dominant species that represents the community leads to an ecological process including changes in the species that make up the community (Whittaker, 1972;Connell and Slatyer, 1977). Accordingly, studies on plant communities are paying attention to evaluating and analyzing the environmental factors that affect organizing species and the number of species, focusing on the species that represent the community (Whittaker, 1972;Smith and Smith, 2011). By the way, the results of plant species’ interactions with environmental factors are often difficult to analyze simply with one or two theories, hypotheses, or models (Austin, 1976;Sprugel, 1991). In other words, not only are the same environmental conditions not given every year, but also the result brought by a change in a specific environmental factor is often unpredictable (Sprugel, 1991;Nakamura et al., 1997;Ferreira and Stohlgren, 1999). For this reason, an expansion is required which explains and predicts temporal/spatial changing processes based on the information collected in the current plant community (Shafroth et al., 2002).
A stream is a dynamic space. The cycle of natural disturbance in a stream is short period of times, differently from mountain forest vegetation (Casanova and Brock, 2000;Park and Kim, 2020). In addition, there are differences in the cycle, intensity, frequency, and duration of the disturbances given laterally and longitudinally (Ferreira and Stohlgren, 1999;Wilkinson, 1999;Casanova and Brock, 2000;Park and Kim, 2020). For this reason, the plant community occupying a stream sides is closely related to the disturbance regimes of the stream (Nakamura et al., 1997;Casanova and Brock, 2000;Chung et al., 2003;Park and Kim, 2020). For such a reason, most studies on plant communities formed in a stream pay attention to the spatial location according to the disturbance regimes of the stream (Chung et al., 2003;Park and Kim, 2020). Moreover, attention is also paid to the adaptation strategy of plant species to disturbance regimes (Naiman and Decamps, 1997;Cho and Cho, 2005). However, the plant community formed in a current stream is the result of a combination of periodic disturbances that have occurred at a specific time in the past and the characteristics of the species consisting each community (Nakamura et al., 1997;Casanova and Brock, 2000;Park and Kim, 2020). Since a spatial difference in a stream is related to the difference in frequency, intensity, and duration of disturbances, plant communities that appear as a result of interactions with the causes of disturbances show significant differences in the downstream, midstream, and upstream regions (Shafroth et al., 2002;Park and Kim, 2020). Nevertheless, few studies have paid attention to the process through which a plant community has come to its present form and how will the future of the present plant community be changed.
Thus, the aim of the our study was to find an appropriate analysis method for the following four questions targeting U. pumila distributed at the edges of streams with communities: 1) What kind of space is required for regeneration?; 2) How does the species be changed making up a community representing different development stages?; 3) How the site changes as the community develops after regeneration?; and 4) Will U. pumila communities be sustainable under the current conditions?
MATERIALS AND METHODS
1. Species
Ulmus pumila L. is a deciduous tree that grows up to a height of about 25 m and a DBH(Diameter at Breath Height) of 1 m (Liguo et al., 2002). Its bark is dark gray and irregularly fissured in the vertical direction. Its young twigs are yellowish-brown without having any hair or with soft hairs and lenticels. Its winter buds vary in color ranging from dark brown to reddish brown. Its s are 4 to 10 mm long with hairs. Its leaves are ovate or elliptically lanceolate, 2 to 8 cm long, and 1.2 to 3.5 cm wide. Its young leaves have hairs on the back but they disappear gradually or rarely remain only on the veins and the bases of the veins. Its flowers hang cymosely on branches grown in the previous year around March before leaves come out. Its seeds are located at the center of the key fruit or a little above the center part. Its fruits ripen in May to June (Liguo et al., 2002; Figure 1A, B, C).
Globally, U. pumila is distributed over Central Asia, Eastern part of Siberia, Russian Far East, Mongolia, Tibet, Northern part of China, Northern Kashmir of India, and Korean Peninsula (Liguo et al., 2002). In the Korean Peninsula, it is distributed along edges of rivers and streams in the middle/northern region. It is distributed individually or in small groups. It rarely forms a community. Although it is a Least Concern (LC) species according to the red list evaluation criteria of IUCN, its distribution areas and the number of individuals are on a gradually decreasing trend (Barstow, 2018).
It flowers between late February and the beginning of April with fruits growing as soon as flowers fall, although its flowering time varies depending on the distribution area in the southern region of the Korean Peninsula. Grown fruits are green like leaves and they photosynthesize (Figure 1B). As leaves grow, fruits ripen to turn brown. They are dispersed mainly under the influence of wind (Figure 1C). Dispersed fruits immediately germinate and grow after rainfall or under the condition where appropriate moisture is supplied (Song et al., 2011;Hirsch et al., 2012; Figure 1D, E). Its s seedlings grow up to about 25 cm in height on average in the first year (Figure 1F).
2. Communities
U. pumila is distributed over Gyeongsangbuk-do, Gangwon-do, the northern region of Gyeonggi-do, and Pyeonganbuk-do, Hamgyeongnam-do, and Hamgyeongbuk-do in North Korea. In the southern part of the Korean Peninsula, regions where U. pumila is observed to form communities include Danyang-gun of Chungcheongbuk-do and Yeongwol-gun, Jeongseon-gun, and Pyeongchang-gun of Gangwon-do, which fall under the mid/upstream region of the Namhangang River. In addition, a small-scale community was observed in Gagokcheon of Samcheok-si, Gangwon-do (Author's observation, unpublished). Whole areas of Danyang-gun of Chungcheongbuk-do, Okdongcheon in Gimsatgat-myeon of Yeongwol-gun, Seokdongcheon in Yeongwol-eup of Yeongwol-gun, Pyeongchanggang River in Jucheon-myeon, Donggang River in Sindong-eup of Jeongseon-gun, Jijangcheon in Nam-myeon of Jeongseon-gun, Jijangcheon in Imgye-myeon of Jeongseon-gun, and Daehwacheon and Anmicheon in Daehwa-myeon of Pyeongchang-gun that fell under the upstream region of the Namhangang River where U. pumila communities were observed were surveyed in advance. In regions where the pre-survey was conducted, there were very few regions in which the original form of U. pumila community was maintained.
In the region where U. pumila was distributed forming a community, each community was in the process of regenerating, growing, and developing in different periods (Figure 2; Figure 3). In particular, no seedling that newly regenerated in an old community was observed. Each individual making up a community had a relatively similar size. Paying attention to such a point, we selected communities of which the regeneration times were presumed to be different. However, since U. pumila communities were located in sedimentary terrains along the river, most of original forms of old communities were damaged by village resort spaces, and the installation of sports facilities and camping grounds. Communities that regenerated relatively recently were rare due to construction of levees and periodic execution of stream management. Based on the pre-survey of communities, we selected 15 communities at different regeneration stages taking into account the naturality of communities first and the accessibility next (Figure 2; Figure 3; Table 1).
3. Methods
The plots of 225 ㎡ (15m x 15m or 10m x 22.5m) were installed at points of target communities of this study where the characteristics of communities were judged to be well reflected. U. pumila communities were formed in the shape of a half-moon along edges of streams. At this time, a 15 m x 15 m plot was installed in communities if it could be installed. A 10 m x 22.5 m plot was installed if the width of the community did not exceed 20 m. As it was judged that riverside vegetation showed a big difference in constituent species depending on the survey period due to seasonal factors and the effect of flooding, the vegetation survey was conducted from May 12, 2021 to June 15, 2021 before the rainy season. In each plots, plants appearing in each layer were recorded in accordance with the phytosociological vegetation survey method. The coverage was evaluated using percentages and recorded (Braun-Blanquet, 1964). The D100Hs(Diameters at Height of 100 cm) of the stems of U. pumila appearing in each plots were measured. 15 individual plants distributed in each plots were selected in the order of height. Their heights were measured using a height meter (SUUNTO Height meter: PM5/1520PC). At this time, if 15 individual plants were not distributed in an polt, individual plants in the same community were additionally selected to secure 15 individual plants. Lastly, the height from the water surface of the stream was evaluated at a right angle to the central point of each plots. At this time, the communities located within 10m from water surfaces were evaluated using a tape measure and a leveler (SUUNTO Compass/Clinometer, Tandem 360PC/360R G, Japan). When the center of a community was 10 m or more from the water surface of the stream, the measurement was made down to 10 cm unit using a height meter (SUUNTO Height meter: PM5/1520PC).
In communities comprised of the seedlings newly regenerated in 2021, the densities of the seedlings were measured after installing a 1 ㎡ plots at the following three points respectively: the point where the density was the highest in the plot, the point where it was the lowest, and the point in the middle. Among seedlings distributed over a community, 30 individual seedlings were selected in the order of height and their heights were measured. In order to evaluate the growth state of initially regenerated seedlings, a total of 45 seedlings were selected in JD3 Community (Imgye-myeon, Jeongseon-gun, Gangwon-do). The lengths of the roots and the heights of the stems were then measured.
The species appearing in each plots were listed. The list was used to perform a DCA(Detrended Correspondence Analysis), an ordination method for evaluating differences between communities (Lepš and Šmilauer, 2007). A species composition table was prepared with the plants observed in the plots and the Importance Values (IVs) were calculated using Relative Coverage (RC) and Relative Frequency (RF) (Curtis and Mclntosh, 1951; Appendix 1). In addition, the Density (D), Richness (S), Dominance (C') (Simpson, 1949), Evenness (J') (Pielou, 1969), Diversity (H') (Shannon and Weaver, 1949), and Maximum diversity (H'max) were calculated.
Each community was rated using D100H of U. pumila measured in the plots (Table 2). The community of the seedlings regenerated in 2021 was evaluated to be Class 0 (Table 2). PH1 and PH2 of which the D100H-Max did not exceed 10 cm were included in Class 1 and DS of which the D100H-Max was less than 15 cm was included in Class 2 (Table 2). Those of which the D100H-Max did not exceed 30 cm and of which the D100H-Max Av. did not exceed 20 cm were classified as Class 3. Those of which the D100-Max Av. did not exceed 30 cm were classified as Class 4. Although there were old individuals of which the D100H was 51.50 cm in JN1, since JN1 was located close to JN2, they were evaluated to be of the same class. For Class 5, the D100H-Max Av. and the D100H-Av. Max were compared together, and the fact that, although the D100H-Max Av. of YJ1 did not exceed 30 cm, it was located in the same community as YJ2 and the D100H-Av. Max was a minimum 40 cm were taken into account (Table 2). Most individuals in communities falling under Class 6 were old and the D100Hs(Diameters at Height of 100 cm) were minimum of 50cm. The changes in the location difference, the vegetation structure difference, richness, dominance, and diversity depending on each class were then compared.
Observed plant species were identified using Lee (1980), Lee (1996a;1996b), Lee (2003a;2003b), and Lee (2006a;2006b) and, as for naturalized plants, Park (2009) was used. Scientific names and common names were used based on the National List of Species of Korea (Kim et al., 2019a). In addition, for species difficult to be identified with the above literature, the Illustrated Book of Korean Poaceae and Cyperaceae (Cho et al., 2016) and Korean Ferns (Korean Fern Society, 2005;Lee and Lee, 2018) were referred to.
4. Statistical Analysis
DCA ordination was carried out using the vegetation data collected from 15 communities at different regeneration stages (Canoco 4.53, Microcomputer Power, USA; Lepš and Šmilauer, 2007). The number of species of annual plants and biennial plants among the plants appearing in each plots and the percentage of annual plants and biennial plants among total emerging plant species were divided into Early Stage of Regeneration (ESR: class 0, 1 and 2 was included, height was not exceed 8m and D100H was below 20cm) and Late Stage of Regeneration (LSR: class 3, 4, 5 and 6 was included, height was exceed 8m and D100H was over 20cm). Differences in the numbers of species and the percentages of the annual plants and biennial plants between ESR and LSR were compared using a Mann-Whitney U Test (p＜0.05). For the statistical analysis, SYSTAT 12 (Systat software, Inc. USA) was used.
RESULTS AND DISCUSSION
1. Vegetation Change
Plants which appeared in the communities that were at different regeneration stages showed clear differences at both ESR and LSR (Table 3). Although there was no difference in the number of annual and biennial plant species appearing in the communities during the ESR (Mann-Whitney U Test; χ2=0.50, p=0.48), there was a clear difference in the percentage of annual and biennial plants among total emerging species (Mann-Whitney U Test; χ2=6.72, p=0.01)(Figure 4). As the communities entered the LSR, woody plants, perennial plants, and plants with nutritive reproduction potential increased among species consisting the communities (Appendix 1; Table 3). In addition, the seedlings and young individuals of U. pumila were observed only in communities that were at the ESR and did not appear in the communities that were at the LSR (Table 3).
Plants classifying the early regeneration stage of U. pumila community included Persicaria lapathifolia, Leonurus japonicus, Viola mandshurica, and Trifolium repens, besides U. pumila and plants classifying the late regeneration stage of U. pumila community included Acer tataricum subsp. ginnala (a woody plant species), Staphylea bumalda, Securinega suffruticosa, Lonicera maackii, Rosa multiflora var. multiflora, and Hemiptelea davidii (Table 3). As the plants appearing only in the communities that were at the LSR, Rubus oldhamii, Cardamine leucantha var. leucantha, Adoxa moschatellina, Celastrus flagellaris, Lilium lancifolium, Carex miyabei, and Rubia akane showed higher importance values in the order listed (Table 3).
As a result of the DCA ordination conducted using the vegetation data collected in the 15 communities, the U. pumila communities were clearly classified into communities comprised of the seedlings that were at the ESR and communities which were at the intermediate stage or of which the ages are D100H-Max of 30 cm or older (Figure 5). In other words, there were differences between the plant species that appeared at the ESR, the plant species that appeared at the intermediate stage, and the plant species that appeared in old communities (Table 3). On the other hand, the difference was not big between old communities (Table 3).
2. Changes in Height, D100H, Number of Stems, and Density
When 15 communities at different stages were divided into 7 classes and compared, the height from the water surface of the stream to the center of the habitat increased as age increased after the regeneration of the communities (Figure 6A). The height of the communities comprised of the seedlings that were at the ESR was measured to be 39.00 (±5.57) cm on average from the water surface of the stream. The height of the distribution area from the water surface of the stream increased to a maximum of 490 cm (Class 6) as the community became old (Figure 6A). The height of U. pumila showed a trend of increasing and then decreasing with a lapse of time after regeneration (Figure 6B). The highest was 22.99 (±1.26) m at the stage of Class 4 and the lowest was 15.97 (±1.59) m at the stage of Class 6 (Figure 6B). When average value of D100H (D100H_Av.) and the maximum value of D100H (D100H_Max) measured for each individual were compared, the D100H increased as the age of U. pumila increased (Figure 6C, D). The average value of the maximum D100H measured for each individual was found to be 4.36 (±1.65) cm for Class 1 and 58.24 (±16.48) cm for Class 6 (Figure 6C, D).
The number of stems observed in each individual of U. pumila decreased with the lapse of time after regeneration. It finally became 1 (Figure 7A). While the number of stems was 4.23 (±3.03) in Class 1, it decreased to 1.07 (±0.25) in Class 5. Only one stem was observed in Class 6 (Figure 7A). In addition, the density also decreased as the tree age increased after regeneration. In Class 0, the regeneration stage of seedlings, an average of 69.33 (±34.96) individuals were observed in 1 ㎡ (i.e., 15,600.00 (±7,865.35) individuals when converted to area of the enumeration district). On the other hand, 68.50 (±31.82) individuals were distributed in Class 1. As the tree age increased, it rapidly decreased to 22 individuals at the stage of Class 2 and 17.00 (±1.41) individuals at the stage of Class 3 (Figure 7B). In the stage of Class 6, 7 individuals were distributed over an area of 225 ㎡ (Figure 7B).
3. Changes in Richness, Dominance, Evenness, and Diversity
The Richness (S) of plants appearing in the communities of U. pumila gradually increased with the lapse of time to reach the maximum value in Class 4. It then decreased (Figure 8A). In other words, it was the lowest in Class 0, showing a value of 19.00 (±5.00) species, whereas it was the highest in Class 4, showing a value of 80.67 (±14.50) species. It then decreased to 52 species in Class 6. The Dominance (C') was the highest in Class 0, showing a value of 0.40 (±0.22). It gradually decreased to reach the lowest value of 0.07 (±0.02) in Class 5. It then increased to 0.13 in Class 6 (Figure 8B). The Evenness (J') was the lowest in Class 0, showing a value of 0.52 (±0.19). It showed a trend of gradually increasing and then decreasing (Figure 8C). The Diversity (H') was the lowest in Class 0, showing a value of 1.52 (±0.59). It was the highest in Class 4, showing a value of 3.34 (±0.24). It then decreased to 2.74 in Class 6 (Figure 8D). The Maximum diversity (H'max) also showed a trend of gradually increasing with the lapse of time after regeneration and then decreasing again (Figure 8E). When values were calculated using only species that appeared in the herbaceous layer, changes in the Richness, Dominance, Evenness, Diversity, and the Maximum diversity with the lapse of time after regeneration showed similar trends (Table 4).
4. Discussion
It is very important to understand the species that make up a population and a community not only for research about a plant population, but also for research about a community (Dupré and Ehrlén, 2002;Pearman et al., 2007;Kim et al., 2018;Chae et al., 2021b). In particular, there might be obvious limitations and ambiguity in conducting research and analyzing the result without understanding the characteristics of the species consisting the community (Kim et al., 2016;Chae et al., 2019;Chae et al., 2021a). U. pumila is known to be a species that can grow even under a dry and barren environment (Dulamsuren et al., 2009). Its yearly life cycle possesses strategies for surviving in semi-deserts or steppe regions (Hirsch et al., 2012). For example, the point that plants bloom and produce fruits with the onset of spring before leaves grow and fruits function like leaves could be evaluated as a strategy to effectively produce seeds under the conditions of insufficient moisture and resources (Figure 1). It has been reported that its seeds ripen and spread in early summer. If appropriate moisture condition is provided, 80% or more of its seeds will germinate within 1.5 days at the earliest and 5 days at the latest (Song et al., 2011;Hirsch et al., 2012; Figure 1). On the other hand, the lifespan of its seeds has been reported to be short (Song et al., 2011). Such characteristics of its seeds are related to the fact that the produced seeds are supplied in the early summer entering the dry season in semi-deserts or steppe regions when moisture is sufficiently supplied along streams during this time as a result of icecaps and glaciers melting due to the rise of temperature, with flood occurring sometimes (Blom and Voesenek, 1996). In other words, this period is a time when the seeds of U. pumila disperse and there is a space with appropriate moisture is maintained, in which the seedlings of U. pumila can survive after being regenerated and continue to maintain a community (Grubb, 1977;Blom and Voesenek, 1996). It is presumed that such characteristics of U. pumila in combination with the climate cycle of the Korean Peninsula have provided suitable conditions for it to form and maintain a community. Since seeds can be quickly produced under a dry condition in spring and a point-bars or a sandbank formed in the stream as a result of periodic flood has environmental conditions similar to those of a semi-desert region, a space suitable for its regeneration and growth (Chen, 2005;Chung et al., 2003).
U. pumila communities are formed on point bars or sandbanks in the mid/upstream of Namhangang River (Chung et al., 2003). Chung et al.(2003) have classified U. pumila communities distributed over point bars or sandbanks formed on the riverside in the Dong-gang River Basin Ecological and Scenery Conservation Area into two types: a disturbance type and a non-disturbance type. Such classification is similar to the result of the DCA ordination conducted in the present study using the vegetation data collected from 15 communities with different regeneration periods, and the communities that fall under the ESR correspond to the Disturbance type and those in the LSR corresponds to the Non-disturbance type (Figure 4). In the present study, U. pumila communities were divided into 7 classes depending on the regeneration time. Heights of the community centers from the water surface of the stream were found to increase with the lapse of time after regeneration (Figure 5A). Communities that fall under the ESR are judged to have been easily affected by rainfall and flood as they are located in areas of low elevation from the water surface of the stream. In other words, such communities are frequently affected by natural disturbance. The intensity of the disturbance might also strongly affect them (Blom and Voesenek, 1996;Ferreira and Stohlgren, 1999;Casanova and Brock, 2000). In particular, it was presumed that communities comprised of initially regenerated seedlings could develop to the stage of Class 1 only when there was no high-intensity natural disturbance until they grew to reach 3 to 4 m (Figure 2A). In communities that grew to the stage of Class 1 as a result of success in primary regeneration, it was observed that a large number of stems were generated. The density was also high as a strategy to defend itself from natural disturbances (Naiman and Decamps, 1997). In the present study, the number of stems reached 4.23 (±3.03) and the density was as high as 68.50 (±31.82) individuals at the stage of Class 1. The number of stems and density decreased as age increased (Figure 6). Generating a large number of stems as well as having a high density at the ESR is judged to be a strategy to reduce the impact of flooding and to induce sedimentation rather than erosion to take place in the distribution area of the community (Naiman and Decamps, 1997). As the flow of water slows down, it not only induces the sedimentation of small size particles but also may induce the bodies of dead plants that can be used as nutriment to be deposited together (Naiman and Decamps, 1997). Such an ecological process promotes the growth of regenerated communities and induces locational changes where the locations of communities gradually get higher from the water surface of the stream. As U. pumila communities gradually get higher from the water surface of the stream, the frequency, intensity, and duration of disturbances decrease, which is the process of developing into stable communities (Connell and Slatyer, 1977;Crawley, 1986). The growth of U. pumila communities was accompanied by changes in the environment, which appeared as changes in the species distributed together (Kim et al., 2019b; Figure 5 - 7).
It is presumed that U. pumila communities regenerate, grow, and develop when a space suitable for regeneration is created. They then disappear after about 100 years (Figure 5;Dulamsuren et al., 2009;Lepesko and Rybashlykova, 2021). In other words, U. pumila communities are thought to undergo the processes of regeneration, growth, propagation, and extinction like an organism (Clements, 1916). On the other hand, the constituent species also change with the developmental stage of the U. pumila community, which is consistent with the process where an individual species suitable for a given environmental condition can regenerate, grow, and extinct (Gleason, 1917; 1926). As shown in the present study, U. pumila regenerates and develops on point-bars or sandbanks formed in a stream as a result of a flood. In this process, the density decreased while the D100H increased with the lapse of time (Figure 5; Figure 6). As communities develop, the Richness, Evenness, and Diversity gradually increased. When the most developed stage was reached, they showed a trend of decreasing again (Nakamura et al., 1997;Ferreira and Stohlgren, 1999). On the other hand, the Dominance showed a trend of decreasing and then increasing in a small scale. Such a trend follows the individualistic concept of plant association (Gleason, 1926). In conclusion, it is judged that U. pumila communities show different environmental changes at each stage of regeneration and growth, and reproduction of a community is possible only when a space required for regeneration is formed above all (Grubb, 1977;Pickett and Thompson, 1978;Chung et al., 2003). However, at this time, the regeneration of U. pumila communities takes place within the range of seed rain, because of which communities with different regeneration periods are observed in regions where naturality is maintained (Skoglund and Verwijst, 1989; Author's observation, unpublished).
The U. pumila communities in the mid/upstream of Namhangang River where the present study was conducted might have contributed to the dynamicity of the stream (Naiman and Decamps, 1997). An U. pumila community that grows after regeneration makes a micro-geographical change of the stream, which changes the flow of the stream to create a new space. The newly created space may serve as a habitat where regeneration is achieved by the seeds supplied from adjacent U. pumila communities. Such a process is judged to have been repeated in natural streams for a long period of time. However, as more people use shores of streams, embankments have been installed. Locations over which relatively old U. pumila communities are distributed are presumed to have changed to farming areas after logging. Embankments constructed to use the flood plains of streams reduce the dynamicity unique to streams (Shafroth et al., 2002). Not only do they reduce the creation of the spaces required for regeneration of U. pumila communities, but also the communities that have been newly created and succeeded in regeneration are removed to prevent flooding (Figure 8). Old U. pumila communities of which the naturality is maintained are quickly dying out due to the expansion of farming areas and residential areas (site preparation for pensions and campsites). Accordingly, with the exception of shores of which the naturality is maintained by being included in a protection area, U. pumila communities are expected to die out in the long term. Although they are not in a community unit, small numbers of U. pumila individuals are growing. If a space for regeneration is created in the vicinity, it is expected that U. pumila can succeed in regeneration and develop to communities due to seed productivity of U. pumila (Figure 1; Figure 9A). Accordingly, we propose that it is required to induce the naturality of streams to be restored in the regions where the necessity of an embankment is low. This is required for maintenance of U. pumila communities in the southern region of the Korean Peninsula in the future (Grubb, 1977;Pickett and Thompson, 1978).
