Distribution, Effect, and Control of Exotic Plants in Republic of Korea

Abstract

Simple Summary

This study was carried out to clarify the spatial distribution of exotic plants at national, regional, and local levels, as well as their ecological impacts, and to prepare a strategy to reduce the impacts in Republic of Korea. A review of the biological characteristics of invasive plants showed that therophytes, annual plants, plants that disperse seeds by gravity (D4), erect form (E), and nonclonal growth form (R5) occupied the highest proportion. Exotic plants usually preferred disturbed areas such as lowlands, roadsides, and bare ground. At the national level, the distribution of exotic plants tended to be dominated by topographic conditions and increased around urbanized areas, agricultural fields, and coastal areas. At the regional level, they appeared in artificial plantations, vegetation due to disturbance, and vegetation established on lower slopes compared with upper slopes. At the local level, exotic plants appeared abundantly in the introduced vegetation, whereas they were rare in the native vegetation. Restorative treatments recovered species composition close to the reference vegetation and species diversity reduced by invasive species.

Abstract

This study was carried out to clarify the spatial distribution of exotic plants at national, regional, and local levels, as well as their ecological impacts, and to prepare a strategy to reduce the impacts in Republic of Korea. This study was attempted at the national, regional, and local levels throughout Republic of Korea. Compositae occupied the highest percentage among invading exotic plants in Republic of Korea. A review of the biological attributes of exotic plants based on the dormancy form, longevity, disseminule form, growth form, and radicoid form showed that therophytes, annual plants, plants that disperse seeds by gravity (D₄), erect form (E), and nonclonal growth form (R₅) occupied the highest proportion. At the national level, the spatial distribution of exotic plants tended to depend on topographic conditions such as elevation and slope degree, and to increase around urbanized areas, agricultural fields, and coastal areas. The habitat types that exotic plants established were similar in their native habitat and in Korea, where they invaded. They preferred disturbed land such as roadsides, bare ground, agricultural fields, and so on. The spatial distribution of vegetation types dominated by exotic plants was restricted in the lowland. The proportion of the exotic/native plants tended to proportionate reversely to the vegetation type richness (the number of vegetation types); that is, the ecological diversity. The proportion of the exotic plants was higher in artificial plantations, vegetation due to disturbance, and vegetation established on lower slopes compared with upper slopes. Even at the local level, the exotic plants appeared abundantly in the introduced vegetation, while they were rare in the native ones. In the vegetation infected by exotic species, not only the species composition changed significantly, but the species diversity also decreased. Restorative treatment by introducing mantle vegetation around the hiking trail inhibited the establishment of exotic plants. Further, the restoration practice recovered the similarity of the species composition compared to the reference vegetation and increased the species diversity.

1. Introduction

The geographical distribution ranges of many species are restricted by the dispersal barriers of major environmental factors, including climate. As a result of geographical isolation, evolution proceeded in different patterns in each major area of the world [1,2,3]. Humans have greatly changed this pattern by transporting species throughout the world. Before industrialization, people moved cultivated plants and domestic animals from place to place when they established new agricultural fields and colonies. In modern times, a wide variety of species have been introduced, intentionally and accidentally, into places other than their native habitat [4,5,6,7,8,9,10].

Such exotic plants usually expand their distribution range beyond the place of initial establishment by leveraging their advantageous life-history strategies. In particular, the disturbed land provides microsites advantageous for exotic species equipped with opportunistic or ruderal life-history strategies [7,8,11,12,13]. Exotic plants with favorable life-history strategies in disturbed environments have reduced or replaced native species and changed ecosystem functions, raising concerns about the conservation of native ecosystems [7,8,13,14,15,16,17,18,19,20].

One of the major threats to biodiversity in the world is the direct destruction of their habitats by people through inadequate resource use or pollution. Another serious, but underestimated, problem is the threat to natural and seminatural habitats through the invasion of exotic species, which is a potentially lasting and pervasive threat [9,21,22,23,24]. When exploitation or pollution stops, the ecosystem often begins to recover; therefore, it is not a lasting threat. However, even if the introduction of alien organisms stops, the existing exotic species do not disappear; rather, they sometimes continue to spread and consolidate, and thus they are judged as a more serious threat [6,25,26,27].

Exotic species have the potential to invade other ecosystems and directly or indirectly affect native biota. They have, in fact, invaded all types of ecosystems on the planet, affecting their native biota. These species have been involved in many hundreds of extinctions, especially under islands conditions, whether it is on real islands or ecological islands. The environmental costs include the irreversible loss of native species and ecosystems [7,28,29,30].

Exotic plants can cause various problems in plant communities composed of native species: excluding native species; altering the habitat, hydrology, and nutrient cycle; significantly impacting biodiversity [8,15,31,32,33]. Exotic species can transform the structure and function of ecosystems by suppressing or excluding native species, either directly by competing for resources or indirectly by changing the way nutrients cycle through the system [28,33]. Increasing global domination by a few invasive species risks producing a homogenous world rather than a world characterized by great biological diversity and local distinctiveness [7,8,33].

The rate at which new invasions are detected is increasing over time across many ecosystems and regions. The observed rate is often increasing exponentially. Efforts to search for exotic species have increased in recent years, possibly contributing to the observed increase in invasions within some taxonomic groups. However, the overall pattern is consistent, arising from conspicuous and well-known taxa and in well-studied systems, indicating a dramatic increase in the invasion rates in the last half of the twentieth century [34,35,36,37,38].

Along with an apparent increase in the rate of invasions, the strong impacts of invasions have drawn widespread public and scientific attention, propelling policymaking and management actions. Therefore, many countries now have policies to prevent the risk of future invasions and to control the impact and spread of established non-native species [39,40,41].

The prevention of new invasions has a clear priority in new policies. Management measures against established invasions can also have the advantage of providing post-settlement options, but such efforts are often specific to the particular species and are potentially costly and long-term proposals [42,43,44]. The successful eradication of existing invasions is an impossible outcome in many instances, despite the significant improvements in the ability to detect new invasions early. Furthermore, a separate effort may be required for each invasion case, whether an eradication program or an effort to control the spread and abundance, even if it is the same species. In contrast, strategies for preventing new invasions target major transfer mechanisms, or vectors. These vector managements can be used to interfere with the transfer of a particular target species; at the same time, they are designed to prevent the wholesale transfer of diverse assemblages, including both target and non-target species, providing a robust and efficient management approach [7,28,36,45,46,47].

Invasive plants are typically controlled by applying physical or chemical methods [48,49], but they require repeated application [48]; despite that, the effect is not significant [50]. Despite the considerable time and resources required to develop and implement secure and effective eradication or control agents, these programs are often unsuccessful. Furthermore, without careful selection and execution, many chemical, mechanical, and biological control methods can be detrimental to non-target species and ecosystem health, which can facilitate the process of invasion. However, implementing restoration after the invasive species are eradicated or restoring ecosystems to their full potential are useful as a strategy to control invasive species [28,29,42,47,50,51,52,53,54,55].

Once the exotic species are established, the cost of management is dramatically increased and it is almost impossible to eradicate species that have begun to spread. Moreover, eradication can also create a bare ground, and thus promote reinvasion [56,57]. Therefore, management strategies should be prioritized over eradication measures. Restoration strategies reflecting ecological theory are based on limiting the assembly and invasion of exotic species [58,59,60,61].

The objective of this paper is to answer the following big questions of invasion ecology based on the results obtained from Republic of Korea: (1) Which taxa invade? (2) What types of ecosystems are vulnerable to invasive species and their impacts? (3) What is the ecological impact of their invasion? (4) How can we manage invasions?

To arrive at the goal, first of all we analyzed the family composition and the spatial distribution of the exotic plants at the national level. Secondly, we carried out a vegetation survey in eight major mountains of Seoul, and we analyzed the relationship between the occupancy ratio of the exotic plants and the landscape structure at the regional level based on the data. Thirdly, we analyzed the relationship between the performance of an exotic plant and the landscape structure on the mountain to clarify the environmental factors and the influences on the establishment and expansion of the exotic plant at the local level. Fourthly, we investigated the effects that the exotic plant caused based on species composition and diversity in a forest ecosystem and a riparian ecosystem. Finally, we evaluated the effect of restoration as a control measure of the exotic plant in a forest ecosystem and a riparian ecosystem.

2. Materials and Methods

2.1. Study Area

Study areas were selected at the national, regional, and local levels. The whole national territory was regarded as the study area at the national level. Seoul and Mt. Mido, which is located in the urban center of Seoul, were selected as the study areas at the regional and local levels, respectively. Korea is located in the middle latitudes of the Northern Hemisphere, Temperate Zone, and thus has four distinct seasons (between 38°21′ N, 126°11′ E and 33°12′ N, 129°52′ E; Figure 1). Annual mean temperatures in Korea range between 10 °C and 16 °C, except in the high mountain areas. The monthly mean temperatures range from 20 °C to 26 °C in the month of August, and from −5 °C to 5 °C in January. Annual precipitation is recorded as about 1500 mm and 1300 mm in the southern and the central parts of Korea, respectively. Winter precipitation is less than 10% of the total precipitation. Humidity reaches 70–80% nationwide in July, which is the highest. In contrast, monthly mean humidity remains at its lowest level in January and April, at 30–40% levels [62].

Figure 1.

Maps showing the study areas at the national (upper left), regional (lower left and upper right), and local (lower right) levels. A map showing a study area at the local level represents the spatial distribution of vegetation and the cover of Ageratina altissima in the study area, Mt. Mido located in southern Seoul. A, B, C, D, and E indicate the entrances of the urban park and numbers in the quadrangle show the section number of the hiking trail.

The Korean peninsula has experienced frequent human interferences as a corridor, which links between continent and sea. Moreover, Korea experienced the most rapid economic growth in the world and depends highly on trade. Those societal factors urge human disturbance as well. In addition, Korea also experienced severe artificial disturbance through wars, including the Second World War and the Korean War. All of those factors have contributed to the invasion and expansion of exotic species.

Seoul, the capital of Republic of Korea, is located in the central part of the Korean Peninsula and covers 605 km² of land (126°46′15″ to 127°11′15″ E longitude, 37°25′50″ to 37°41′45″ N latitude; Figure 1). The mountainous vegetation of Seoul consists of five major plant communities distributed along an elevational gradient, with the Pinus densiflora community in the mountain peaks, the Quercus mongolica community in the upper slopes, the Q. aliena community in the lower slopes, and the Zelkova serrata and Carpinus laxiflora communities in the mountain valleys. Alnus japonica stands remain in the plains and valleys of the lowland that have escaped development, such as the cultural heritage areas [63,64,65]. Much of the natural landscape in the Seoul metropolitan area has disappeared due to extensive deforestation for fuel, building material, and other purposes during the 20th century [66]. The population of Seoul has increased from 2.4 million in 1960 to 9.8 million as of 2010 [67]. During this period, the proportion of green space has decreased from 70% in 1960 to 28% in 2017, mostly for housing [63,66,68]. The central government of Republic of Korea has designated most of the forested mountains located in the suburbs as green belts to prevent further loss of green space. Under the Seoul Metropolitan Government’s current Green Belt Ordinance, commercial, industrial, and urban development is not allowed in these forests [68].

Mt. Mido is located in Southern Seoul (Figure 1). Mt. Mido is an ecological island enclosed by a residential area (Banpo apartment complex), educational (Catholic University) and public facilities (National Library), a public facility (Court of Law), and a transportation facility (Gangnam Express Bus Terminal) in the east, west, south, and north, respectively. The vegetation of this site is composed of a plantation and secondary forest (Figure 1; [64]). The plantation is composed of Robinia pseudoacacia, Populus tomentoglandulosa, Z. serrata, and Pinus koraiensis plantations. The secondary forest is composed of Q. acutissima, and Q. mongolica communities. The vegetation of this site, especially the site facing the hiking trail, is severely affected by frequent visits by nearby residents, the indiscriminate introduction of sports and recreational facilities, and the indiscreet introduction of non-native plants for gardening and landscaping.

The effects of the exotic plants on the forest vegetation and the restorative treatment as a control plan of the exotic plants were investigated in Mt. Mido (Figure 1). The effect of the exotic plants on the riparian vegetation was investigated in the Han River watershed (Figure 1).

The effect of the restorative treatment as a control measure of the exotic plants was investigated in the Dongmun stream, which is located in Munsan-eub, Paju-si, Gyunggi province in central–western Korea. “Eub” and “Si” are the administrative units, which correspond to “town” and “city”, respectively. Although the riparian zone of the Dongmun stream was covered with natural vegetation, which were dominated by Salix pierotii (Korean willow), S. gracilistyla, and Phragmites communis, exotic plants, including Ambrosia trifida and Sicyos angulatus, are emerging as the new dominant species, as they have expanded their range rapidly in recent years [69,70]. The Suip stream, which is located in Bangsan-myun, Yanggu-gun, Gangwon province in eastern Korea (where “Myun” and “gun” are the administrative units, which correspond to “town” and “county”, respectively), was selected as the reference stream, as it was left to its natural process for more than 70 years after the Korean War.

2.2. Survey on Forest Vegetation

Distribution of exotic plants was investigated at three spatial scales, at the national (Republic of Korea), regional (Seoul, the capital of Republic of Korea), and local (Mt. Mido) levels. Ecological information for analysis on the nationwide distributional characteristics of the exotic plant species was obtained from the Ecobank of the National Institute of Ecology of Korea (http://www.nie-ecobank.kr, accessed on 30 May 2023). A field survey on distribution of exotic plant species was carried out based on the guideline for monitoring wild plants that disturb ecosystems [71]. Survey sites were designated at regular intervals throughout the whole national territory in Republic of Korea (refer to Figure 1).

The habitat types of each exotic plant were classified into the mountainous area, riparian zone, agricultural field, roadside, bare ground, and so on. Species nomenclature followed the Korean Plant Names Index [72]. The percentage of exotic plant species to native plant species was obtained based on flora survey data in each site. The field survey was conducted by recording the distribution area and coverage of the plants appearing in those quadrats after installing three quadrats (5 × 5 m²) at the locations selected in each grid. The ratio of exotic plants was calculated as the ratio of the number of exotic plants to the total number of plant species appearing in the target area.

We divided life-forms based on dormancy form, longevity, disseminule form, growth form, and radicoid form [73]. The dormancy form was divided into therophyte, wintering therophyte, hemicryptophyte, geophyte, chamaephyte, hydrophyte, and phanerophyte. The disseminule form was divided into disseminated widely by wind or water (D1), disseminated by attaching to or eaten by animals and man (D2), disseminated by mechanical propulsion of the dehiscence of fruits (D3), and having no special modification for dissemination (D4). The life-form based on longevity was divided into annual, wintering annual, biennial, perennial, and woody plants. The growth form was divided into the tussock form (T), branched form (B), procumbent form (P), erect form (E), partial-rosette form (Pr), and pseudo-rosette form (Ps). The radicoid form was divided into moderate extent of rhizomatous growth (R2), narrowest extent of rhizomatous growth (R3), clonal growth by stolons and struck roots (R4), and nonclonal growth (R5). An ecological map obtained from Seoul City (www.seoul.go.kr, accessed on 30 May 2023) was used to identify vegetation types and landscape boundaries in Seoul. Landscape ecological analyses of the maps were determined with the ArcView GIS software [74].

Vegetation data for study at the regional level were collected in Mts. Acha, Bukak, Buram, Cheonggye, Daemo, Gwanak, Inwang, and Surak, with five plots at each site in Seoul (Figure 1). A vegetation survey was conducted in forty plots, with five plots in each site [75].

The field survey on Mt. Mido selected for study at the local level was carried out from May to September 2002 and a resurvey was done from May to September 2017. A vegetation map (scale of 1:5000) of the study site was constructed with the GIS (Geographic Information System) program supported by ArcView [74] based on an urban ecological map [64] and field surveys. The distribution map of Ageratina altissima was prepared by representing the cover class of Braun–Blanquet [76], evaluated by the field survey on the maps at 1:5000 scales. Sections of the hiking trail were divided by expressing the homogeneous range of the cover class of A. altissima. The vegetation survey was conducted at 6, 21, and 10 plots in the A. altissima, Q. mongolica, and Q. acutissima stands, respectively, for a total of 37 plots.

The plot size was 20 m × 20 m in the studies of both regional and local levels. The vegetation survey was conducted by applying the phytosociological procedure of Braun–Blanquet [76]. Dominance of each species in each plot was evaluated on an ordinal scale, and each ordinal scale was converted to the median value of the percent of the cover range in each cover class. Relative coverage was considered as the importance value of each species. Relative percentage in percent was calculated by dividing the cover fraction of each species by the summed cover of all species in each plot, and then multiplying the value by 100. A matrix of importance values for all species in all plots was prepared and used as data for ordination using detrended correspondence analysis (DCA) [77]. To describe and compare species diversity and dominance among sites, rank–abundance curves [65,78,79] were prepared.

Restorative treatment was practiced by introducing Rhododendron yedoense var. poukhanense with a one m breadth beside the hiking trail to prevent the expansion of artificial disturbance on the trail.

2.3. Survey on the Riparian Vegetation

The cover class of all plant species in all plots installed randomly was recorded [76,80]. Plot sizes were 1 m × 1 m in the riparian zones dominated by herbaceous vegetation immediately adjacent to stream channels, 5 m × 5 m in the shrub lands, and 10 m × 10 m in the forests distant from the stream channels. Nomenclature followed the KNA (Korea National Arboretum) [72]. The cover class was estimated with the ordinal scale (from 1 for <1% to 5 for >75%) of Braun–Blanquet [76].

Experimental restoration was practiced by introducing S. pierotii and S. gracilistyla on the bank covered with A. trifida and S. angulatus with a 20 cm interval.

2.4. Statistics Analysis

We obtained Pearson’s correlation coefficient to deduce the relationship between the percentage of the exotic plants and environmental factors (α = 0.05). One-way analysis of variance (ANOVA) was used to compare the difference in the percentage of the exotic plants among the plant communities different in backgrounds and topographic locations (α = 0.05). The difference in values among the sites was tested by Scheffe’s test.

Data were analyzed using SPSS (version 24, SPSS Inc., Chicago, IL, USA) and R software version 4.2.3 (R Project for Statistical Computing).

3. Results

3.1. A Comparison among Taxa of Exotic Plants

Compositae occupied the highest percentage among exotic plants at 20.4% and followed in order by Gramineae (19.9%), Cruciferae (9.0%), Leguminosae (7.0%), Solanaceae (4.2%), Caryophyllaceae (3.4%), Amaranthaceae (3.1%), Polygonaceae (2.8%), Scrophulariaceae (2.8%), Malvaceae (2.5%), Convolvulaceae (2.5%), Chenopodiaceae (2.0%), Umbelliferae (2.0%), and so on (Figure 2, Appendix A).

Figure 2.

Proportion of exotic plants by family found in Republic of Korea.

3.2. Life-Form Composition of Exotic Plants

In the dormancy form composition of the invading exotic plants in Korea, therophytes occupied the highest percentage, followed by wintering therophytes, hemicryptophytes, geophytes, chamaephytes, hydrophytes, and phanerophytes (Figure 3). Based on the longevity, annual plants occupied more than half, followed by perennial, biennial, wintering annual, and woody plants (Figure 3). The percentage of the disseminule form of the invading exotic plants in Korea was higher in the order of dispersal by gravity (D4), dispersal by wind and water (D1), dispersal by animal (D2), and dispersal by elasticity (D3) (Figure 3). The percentage of the growth form was higher in the order of the erect form (E), procumbent form (P), branched form (B), tussock form (T), partial-rosette form (Pr), and pseudo-rosette form (Ps). The radicoid form of the invading exotic plants in Korea was higher in the order of the R5, plants isolated without any connecting organ, R3 with a narrow rhizome, R4 with a stolon, and R2 with a relatively wide rhizome (Figure 3).

Figure 3.

Dormancy form composition of the invading exotic plants in Korea. D1: disseminated widely by wind or water; D2: disseminated attaching to or eaten by animals and man; D3: disseminated by the mechanical propulsion of the dehiscence of fruits; D4: having no special modification for dissemination. T: tussock form; B: branched form; P: procumbent form; E: erect form; Pr; partial-rosette form; Ps: pseudo-rosette form; R2: moderate extent of rhizomatous growth; R3: narrowest extent of rhizomatous growth; R4: clonal growth by stolons and struck roots; R5: nonclonal growth.

3.3. Habitat Types of Exotic Plants

Habitat types in the original habitats of exotic plants that established in Korea showed a higher frequency in roadsides, bare ground, agricultural fields, pastures, riversides, polluted soils, wetlands, and landfill areas (Figure 4). Habitat types invaded in Korea showed a trend similar to that in their original habitats, as the percentage followed the order of roadsides, bare ground, seashores, landfill areas, pastures, wetlands, agricultural fields, forest edges, and riversides (Figure 4).

Figure 4.

Habitat type composition of the invading exotic plants in Korea in their original place (upper) and Korea (lower).

3.4. Spatial Distribution of Exotic Plants at the National Level

Spatial distribution of exotic plants, elevation, slope degree, percentage of urbanized land, and percentage of agricultural fields throughout the whole national territory of Republic of Korea is depicted in the grid maps (Figure 5). The occupied rate of the exotic plants tended to depend on the elevation and slope degree, and to increase around urbanized areas, agricultural fields, and coastal areas (Figure 5).

Figure 5.

Spatial distribution of percentage of the exotic plant species (upper left), elevation (upper center), slope degree (upper right), urbanized rate (lower left), and percentage of agricultural fields (lower right) throughout the whole national territory of Republic of Korea. Korean letters (“-군”, “-시”) on maps are the administrative units, which correspond to “county” and “city”, respectively.

As a result of the correlation analysis, the percentage of exotic plants showed significant correlation with the elevation (negative logarithmic), slope degree (negative linear), the percentage of urbanized land (second function), and the percentage of agricultural fields (linear) (Figure 6).

Figure 6.

Relationships between elevation, slope, percentage of urbanized land, and percentage of agricultural fields and percentage of exotic plants (α = 0.05).

3.5. Spatial Distribution of Exotic Plants at the Regional Level

The spatial distribution of plant communities dominated by exotic plants at the regional level showed that their distribution was restricted to lowlands compared to uplands (Figure 7).

Figure 7.

Spatial distribution of plant communities dominated by exotic plants (upper) and elevation (lower) in the Seoul Metropolitan Area, central–western Korea. Their distribution is restricted to the lowlands.

The percentage of exotic to native plants investigated in eight major mountains tended to proportionate reversely to the vegetation type richness (Figure 8).

Figure 8.

Relationship between percentage of exotic plants and the number of vegetation types in eight major mountains of Seoul.

Comparing the percentages of the exotic plants among plant communities with different traits, the ratio tended to be higher following the order of artificial plantations (constructed by introducing Alnus hirsuta, Alnus hirsuta var. sibirica, R. pseudoacacia, P. tomentiglandulosa, Pinus rigida, P. koraiensis, Larix leptolepis, and Betula platyphylla var. japonica, respectively), vegetation due to disturbance (dominated by Sorbus alnifolia, Arudinella hirsuta, and Lespedeza cyrtobotrya, respectively), vegetation established in lower slopes (dominated by A. japonica, Prunus sargenntii, Q. acutissima, Fraxinus mandshurica, and P. densiflora, respectively), and natural vegetation established in upper slopes (dominated by Q. mongolica, Betula davurica, Q. variabilis, P. densiflora, Q. serrata, C. laxiflora, Q. aliena, and Z. serrata, respectively) (Figure 9).

Figure 9.

A comparison of the percentage of exotic plant species among plant communities with different traits in eight major mountains of Seoul. Each bar expresses the mean and standard deviation of the mean. An ANOVA test was conducted on the percentages of the exotic plants in the plant communities different in establishment backgrounds and topographic locations at α = 0.05; the means with the same alphabetical character (in superscript) for each parameter were not different from each other. Error bars indicate the standard deviation of the mean percentage of exotic plants for each vegetation type. Disturbance: plant communities due to disturbance; Lower slope: plant communities established in lower slopes; Upper slope: plant communities established in upper slopes.

3.6. Spatial Distribution of A. altissima as an Exotic Plant

In 18 sections (Figure 1), divided by a homogeneous range of cover classes, cover classes of A. altissima ranged from none to IV. The cover class of A. altissima was the highest in sections 1 and 14 as cover class IV, and sections 2 and 13 (cover class III), section 11 (cover class II), followed by sections 3, 10, 12, and 15 (cover class I). On the other hand, A. altissima did not appear in sections 5, 6, 7, 8, and 9, which are located around the summit, and sections 16 and 17, which are located amidst young Korean pine plantations afforested recently (Figure 10).

Figure 10.

Maps showing the cover class of an exotic plant, A. altissima, before (left) and after (right) a restorative treatment, which was implemented by introducing the Korean azalea beside the hiking trail. The section numbers of the hiking trail were shown in Figure 1. A, B, C, D, and E indicate the entrances of the urban park.

3.7. The Ecological Effects of Exotic Plant Infection

The effects of the exotic plant infection were investigated in terms of changes of species composition and species diversity. The effect in the forest ecosystem was investigated by comparing the species composition and diversity of a plant community dominated by A. altissima with three native plant communities dominated by Q. mongolica and Q. acutissima.

As a result of the DCA ordination based on the vegetation data, the species composition of stands infected with a tree-of-heaven showed a large difference from that of Mongolian oak stands, which was established on the upper slope. Although it was not a large difference, it also showed a difference in species composition from stands of Q. acutissima, which were established on the mid-slopes (Figure 11).

Figure 11.

DCA ordination of stands based on vegetation data collected from native oak (Q. acutissima and Q. mongolica) stands and stands infected with an exotic plant, tree-of-heaven, based on vegetation data.

The effects of the exotic plant infection were also investigated in terms of changes in species composition and diversity in the riparian ecosystem. The effect on the riparian ecosystem was investigated by comparing the species composition and diversity of two plant communities dominated by A. trifida and S. angulata with those of three native plant communities dominated by S. pierotii, S. gracilistyla, and Phragmites japonica, which represent the vegetation zones dominated by herbaceous plant, shrub, and tree in the riparian zone.

As the result of the DCA ordination based on the vegetation data, the species composition of the stands infected with two exotic plants was remarkably different from those of Phragmites japonica (herb), S. gracilistyla (shrub), and S. pierotii (tree), which dominate the native riparian vegetation in Republic of Korea (Figure 12).

Figure 12.

DCA ordination of stands based on vegetation data collected from three natural vegetation types dominated by S. pierotii, S. gracilistyla, and P. japonica and two vegetation types infected with two exotic plants, A. trifida and S. angulata.

Species diversity was compared by the species rank–abundance curve, and the richness of the stands infected with a tree-of-heaven was lower than those of the three oak communities, and the slope of the curve that the exotic plant community formed was steeper than those done by the native plant communities (Figure 13).

Figure 13.

A comparison of species rank–abundance curves of A. altissima, Q. acutissima, and Q. mongolica stands.

The richness of the stands infected with two exotic plants established in the riparian zone was lower than those of three native riparian vegetation stands, and the slope of the curves that the two exotic plants communities formed were steeper than those done by the native riparian plant communities (Figure 14).

Figure 14.

A comparison of species rank–abundance curves of two exotic species stands established in the riparian zone (A. trifida and S. angulata) and three native riparian vegetation stands (S. pierotii, S. gracilistyla, and P. japonica).

3.8. The Effects of Ecological Restoration for Control of Exotic Plant Species

Restorative treatment by introducing mantle vegetation around the hiking trail reduced the cover of A. altissima in sections 1, 2, 11, 13, and 14. However, the cover increased in sections 5, 7, 16, 17, and 18 despite the treatment. On the other hand, sections 3, 4, 6, 8, 9, 10, 12, and 15 did not show any change (Figure 10).

As the result of the DCA ordination based on the vegetation data obtained from the nonrestored stand, which were dominated by A. trifida and S. angulate, the sites were restored by imitating the natural river dominated by S. pierotii; in the reference river, the nonrestored stands showed a species composition remarkably different from that of the reference stands, while the restored sites showed a species composition similar to that of the reference stands (Figure 15).

Figure 15.

Stand ordination of the restored, non-restored, and reference stands. Non-restored stands were dominated by A. trifida or S. angulate, whereas the reference stands were dominated by S. pierotii and the restored sites were treated based on the reference information obtained from the natural river. Data of the reference stands were collected in the Suip stream, a natural stream which was left in its natural process for more than 70 years since the Korean War.

The richness of the restored sites was higher than that of the reference stands. The richness of the nonrestored stands was the same as that of the reference, but the slope of the curves that the former stand formed was steeper than that of the reference stand (Figure 16).

Figure 16.

A comparison of species rank–abundance curves among the sites restored by introducing S. pierotii (Sp) and S. gracilistyla (Sg), nonrestored sites dominated by A. trifida (At) and S. angulatus (Sa), and reference sites.

4. Discussion

Ecologists studying exotic species try to address the following basic questions: Which taxa often invade? How fast do they invade? What kinds of ecosystems are vulnerable to exotic taxa and their impacts? What are the ecological effects of their invasion? How can we contain, control, or eradicate harmful invaders? Our discussion was focused on those five questions.

4.1. Which Taxa Invade?

Among 357 exotic plants investigated in Korea, compositae occupied the highest percentage among exotic plants at 20.4%, followed in order by Gramineae (19.9%), Cruciferae (9.0%), Leguminosae (7.0%), Solanaceae (4.2%), Caryophyllaceae (3.4%), Amaranthaceae (3.1%), and so on (Figure 2, [81,82]). On the other hand, reviewing the biological attributes of exotic plants based on the dormancy form, longevity, disseminule form, growth form, and radicoid form showed that therophytes, annual plants, plants that disperse seeds by gravity (D4), erect forms (E), and R₅ forms occupied the highest proportion (Figure 3).

Taxa, which account for a higher percentage, usually have better dispersal agents, higher reproductive capacity, and shorter life cycles [83]. McNeeley [28] suggested the following five predictions regarding invasive species: Firstly, the probability of a species becoming invasive increases as the initial population size increases, so species that are introduced intentionally and kept under cultivated or maintained under animal husbandry over a long period are more likely to establish. Secondly, species with larger native geographic ranges are more likely to become invasive than those with smaller native ranges. Thirdly, species invading a country or region should be considered as a high risk of becoming invasive in an ecologically or climatically similar country or region. Fourthly, species with specialized pollinators are less likely to become invasive unless they are introduced together. Finally, successful invasions usually require that the new habitat conditions are similar to those at the origin, especially in terms of climate conditions. Another group of exotic species are those that have expanded their ranges within the continental areas because they fit the ways in which humans have altered the environment [84]. The other special class of exotic species includes those that have close relatives in the native biota. When exotic species hybridize with indigenous species and varieties, unique genotypes may be removed from local populations and taxonomic boundaries may become obscured [85].

The results of this study, showing that compositae account for the highest proportion of invading taxa in Korea (Figure 2), with the life-form composition of therophytes, annual plants, plants that disperse seeds by gravity (D4), erect forms (E), and R₅ forms occupying the highest proportion (Figure 3), are well in line with the general characteristics of the exotic plants mentioned before.

4.2. How Fast Do They Invade?

The rate of spread is a function of both reproduction and dispersal; there are species that reproduce quickly and spread much faster [86]. In order to determine the rate of spread of plants, information on rare dispersal events that can send plants over an unusually long distance is required. While the rate of dispersal is critical, other factors, such as reproductive maturity age, disturbance frequency, intensity of habitat disturbance, and fecundity, are also important. Seeds can often be transported over long distances by agents such as water, wind, vehicles, or livestock at very high speeds [28].

Among the invading exotic plant species in Korea, therophytes and annular plants accounted for a high proportion, and the disseminule form of seeds was high in plants that disperse seeds by gravity (D4) (Figure 3). These biological attributes correspond to favorable conditions to reproduce quickly and spread more rapidly [86].

On the other hand, as the type of habitat where the exotic plant species established included roadsides, bare ground, and coastal areas, accounting for a high proportion, the result was similar to the origin of the habitat types of those plant species (Figure 4). These results mean that such exotic plants are properly finding their preferred places, which is interpreted as a result of active human and material exchanges around the world. In reality, the increased mobility of people and their goods increases the likelihood of moving species around the world [27,28].

4.3. What Types of Ecosystems Are Susceptible to Exotic Plant Species and Their Impacts?

One reason that exotic species can so easily invade and dominate new habitats and replace native species is that there are no natural predators, pests, and parasites in the new habitat. Human activity may create unusual environmental conditions, such as nutrient pulses, increased disturbances including fire, or increased light intensity, to which exotic species can adapt more easily than native species. The higher occurrence of exotics is often found in habitats that have been further altered by human interference. Fragmented forests, suburban developments, and easy access to landfills have allowed for an increase in the numbers and ranges of so-called wandering species [12,33,44,87].

All ecosystems, including those in well-protected national parks, could potentially be invaded, but some seem more vulnerable than others. Evolutionarily and geographically isolated ecosystems, especially oceanic islands, are particularly vulnerable. Urban–industrial areas, habitats suffering from periodic disturbance, ports, lagoons, estuaries, and water fronts, where the effects of natural and artificial disturbances often coexist, are also particularly vulnerable to invasions [88,89]. Virtually all ecological communities are vulnerable to invasion to some extent, and artificial disturbance increases the vulnerability of most ecosystems. Therefore, the continued expansion of human activity is likely to increase the vulnerability of ecological communities to invasion [28,44,90].

In the result of this study, exotic species tended to be distributed in places with low elevations and gentle slopes, around cities and agricultural fields, and along the coastal area at the nation level (Figure 5). Therefore, the percentage of exotic species tended to be proportionate to the elevation and slope degree (negatively), and the urbanization ratio and percentage of agricultural fields (positively) (Figure 6). The vegetation type dominated by exotic plant species investigated in Seoul was restricted to the lowlands, with frequent disturbances (Figure 7). The proportion of exotic plant species surveyed in eight mountains within Seoul city showed the opposite trend to the richness of vegetation types that those mountains possessed (Figure 8). The low richness of vegetation types means that the lowlands of the mountain have been transformed into urbanized areas, including residential areas. As a result of comparing the ratio of exotic plant species by vegetation type, exotic species showed a high rate in the artificial afforestation and the vegetation type caused by disturbance, whereas they appeared at a low rate in natural vegetation (Figure 9). In an isolated mountain surrounded by urbanized areas, the coverage of A. altissima surveyed along the hiking trail was high along the low-lying hiking trail close to the residential area, and it was low or did not appear around the mountain peak dominated by natural vegetation (Figure 10). These results are evidence to demonstrate that the invasion of exotic plants begins at sites disturbed by human influence, and that the spread is also due to the effects of such disturbances.

The forest edge, including the hiking trail, is vulnerable to the invasion of exotic plants due to physical disturbance and nutrient input, as a place where organisms, matter, and energy are exchanged between the two habitats [91,92,93,94]. Thus, exotic plants are abundant while species diversity is lower in the forest edge compared with the undisturbed forest interior [95,96,97,98]. In reality, experimental manipulation, which removed the forest canopy and undergrowth, facilitated the invasion of exotic plants [99,100]. This phenomenon suggests that the invasion and expansion of exotic plants are closely related to artificial disturbances. The results of this study resemble the facts commonly known regarding the invasion and expansion of the exotic plants.

A. altissima, as an exotic plant species, appears more abundantly at the lower elevations, where frequent artificial interferences are prevailing, than at the higher elevations, where such impacts are lessened (Figure 10). On the other hand, they appeared abundantly in the introduced vegetation, such as the black locust plantation, but they did not appear or were rare in the natural one, such as the oak forest (Figure 10). However, an exceptional phenomenon was found at an entrance C. The lack of A. altissima in the entrance was due to the dense coverage of the recently afforested Korean pine stand (Figure 10). In this respect, the light condition is also important [65].

4.4. What Is the Ecological Impact of Exotic Species Invasion?

Every exotic species alters the species composition of native biological communities in some way. Whether it becomes invasive, and thus harmful, depends on the specific nature of the exotic species, the vulnerability of the host ecosystem, and opportunity [101,102,103,104]. Changes in the ecosystems may be initiated by natural disturbances, such as storms, earthquakes, volcanic eruptions, fires, climate, or management regimes, but are reinforced or accelerated by the invasion of exotic species. Land transformation and invasions are interlinked, bringing more opportunities for invasion [27,33,105,106].

The species composition of an ecosystem at any given location and time is determined depending on the current environmental conditions, the levels and types of disturbance, the balance of loss and recruitment, and the composition of the regional species pool. Increasing levels of human-induced ecosystem transformation may accelerate environmental change, and the dramatic increase in the intentional and accidental biota transport across the world inevitably will increase the regional species pool, whereas it will perhaps also decrease native species and ultimately decrease the global species pool. The combination of these factors lays the base for a radical alteration of an ecosystem [12,28,107,108,109,110].

In a result of this study, the species composition of vegetation types infected with a tree-of-heaven showed a difference from that in the reference area, where the exotic species did not invade (Figure 11). The species composition of riparian vegetation infected with giant ragweed and S. angulatus also changed significantly compared with that in the reference area, where they did not invade (Figure 12). In both forest ecosystems and riverside ecosystems, species diversity of vegetation types in which exotic species invaded decreased (Figure 13 and Figure 14).

4.5. Restoration as a Tool to Inhibit Invasion and Expansion of Invasive Species

Invasive species provide a serious challenge to environmental managers because of their explosive growth [111,112]. It is known that disturbed ecosystems are usually more vulnerable to the infestation of exotic plants than undisturbed ecosystems [44,87,113]. The results of this study showed that the invasion and spread of exotic species were closely related to disturbance at national, regional, and local levels. In this respect, it is imperative to protect disturbed ecosystems from human interference, such as excessive use and management, to prevent the invasion and expansion of exotic species. It is imperative to foster closed undisturbed conditions to discourage disturbance-adapted exotic plants [44,114]. In fact, a comprehensive restoration of entire ecosystems may be necessary [115,116]. Artificial interference for forests is currently declining in rural areas in Korea, but interference due to forest management still remains in urban areas, which may cause further invasion of exotic plants. Therefore, we propose a management plan with ecological restoration principles reflected to address ecosystems infected with exotic species [30,117,118].

How do we return a disturbed ecosystem to a stable ecosystem? Can we contain, control, or eradicate harmful invaders through such restoration? The goal of restoration ecology is to aid the recovery of an ecosystem that has been disturbed or damaged by external influences, such as fire, logging, mining, agriculture, or urban development. A major goal of restoration practitioners is to return a habitat to a more desirable condition, involving a particular species composition, community structure, and/or set of ecosystem functions [119,120,121].

Restoration is used to maintain the overall health and sustainability of an ecosystem, and thus can be the most effective way to increase the resilience and resistance of an ecosystem to invasion by exotic species. Although restoration is used for various purposes, it is particularly being used as a way to combat exotic species, especially under the stresses of climate change and the expanding influence of human activities. Optimal, balanced, and stable ecological conditions arising as a result of restoration can more effectively limit the spread and settlement of exotic species [50,122,123,124].

In a result of this study, the restorative treatment that introduced mantle vegetation around the hiking trail where A. altissima invaded reduced the coverage of the exotic plant (Figure 10). In addition, the riparian vegetation, where giant ragweed invaded, showed significantly different species composition from the reference vegetation and showed low species diversity (Figure 12 and Figure 14). However, the restoration implemented by introducing Korean willow restored species composition similar to the reference vegetation and increased species diversity (Figure 15 and Figure 16). This result suggests that plantings of willow could help restore riparian zones that have been overtaken by giant ragweed. In reality, willows can act as a nurse crop for other understory plants by ameliorating high light, temperature, and soil moisture [125,126]. In particular, the decline of giant ragweed dominance by willow shading might increase plant diversity in this restored riparian ecosystem. Based on the results, we suggest a comprehensive restoration plan which recovers the integrity of the ecosystems disturbed due to various human activities to inhibit the invasion and expansion of exotic species [29,95,127,128].

5. Conclusions

Compositae occupied the highest percentage among exotic plants, followed by Gramineae, Cruciferae, Leguminosae, and so on. At the national level, the spatial distribution of exotic plants tended to depend on topographic conditions, such as the elevation and slope degree, and to increase around urbanized areas, agricultural fields, and coastal areas. The habitat types that exotic plants established were similar in their native habitat and in Korea, where they invaded. They preferred disturbed lands such as roadsides, bare ground, agricultural fields, and so on. The spatial distribution of vegetation types dominated by exotic plants was restricted in the lowlands. The proportion of the exotic/native plants tended to proportionate reversely to the vegetation type richness; that is, ecological diversity. The proportion of the exotic plants was higher in the artificial plantations, vegetation due to disturbance, and vegetation established in lower slopes compared with the upper slopes. Even at the local level, exotic plants appeared abundantly in the introduced vegetation, whereas they were rare in the native ones. In the vegetation infected by exotic species, not only the species composition changed significantly, but also the species diversity decreased. Synthesizing these results, the invasion of exotic plants begins at sites disturbed by human influence, and the spread is also due to the effects of such disturbances. Restorative treatment by introducing mantle vegetation around the hiking trail inhibited the establishment of exotic plants. Further, the restoration practice recovered the similarity of the species composition compared to the reference vegetation and increased the species diversity. Based on the results, we suggest a comprehensive restoration plan which recovers the integrity of the ecosystems disturbed due to various human activities to inhibit the invasion and expansion of exotic species.

Appendix A. A List of Exotic Plants Investigated in Republic of Korea

Af: Africa, Am: America, As: Asia, Au: Australia, Eu: Europe, Med: Mediterranean

Family Name	Scientific Name	Origin
Amaranthaceae	Amaranthus albus	Am
Amaranthaceae	Amaranthus arenicola	Am
Amaranthaceae	Amaranthus blitum	Eu
Amaranthaceae	Amaranthus hybridus	Am
Amaranthaceae	Amaranthus palmeri	Am
Amaranthaceae	Amaranthus patulus	Am
Amaranthaceae	Amaranthus powellii	Am
Amaranthaceae	Amaranthus retroflexus	Am
Amaranthaceae	Amaranthus spinosus	Am
Amaranthaceae	Amaranthus viridis	Am
Amaranthaceae	Celosia argentea	Am
Amaryllidaceae	Zephyranthes candida	Am
Boraginaceae	Amsinckia lycopsoides	Am
Boraginaceae	Asperugo procumbens	Eu–As–Af
Boraginaceae	Symphytum officinale	Eu
Campanulaceae	Triodanis perfoliata	Am
Cannabaceae	Cannabis sativa	As
Caryophyllaceae	Cerastium glomeratum	Eu
Caryophyllaceae	Dianthus armeria	Eu
Caryophyllaceae	Holosteum umbellatum	Eu–As
Caryophyllaceae	Saponaria officinalis	Eu
Caryophyllaceae	Scleranthus annuus	Eu
Caryophyllaceae	Silene alba	Eu
Caryophyllaceae	Silene antirrhina	Am
Caryophyllaceae	Silene armeria	Eu
Caryophyllaceae	Silene gallica	Eu–As
Caryophyllaceae	Spergula arvensis	Eu
Caryophyllaceae	Spergularia rubra	Eu–As
Caryophyllaceae	Vaccaria vulgaris	Eu
Chenopodiaceae	Atriplex hastata	Eu
Chenopodiaceae	Chenopodium album	Eu–As
Chenopodiaceae	Chenopodium ambrosioides	Am
Chenopodiaceae	Chenopodium ficifolium	Eu
Chenopodiaceae	Chenopodium glaucum	Eu
Chenopodiaceae	Chenopodium hybridum	Eu–As
Chenopodiaceae	Chenopodium pumilio	Au
Commelinaceae	Commelina benghalensis	As–Af
Commelinaceae	Commelina diffusa	etc.
Commelinaceae	Tradescantia reflexa	Am
Compositae	Achillea millefolium	Eu
Compositae	Ageratum conyzoides	Am
Compositae	Ambrosia artemisiifolia	Am
Compositae	Ambrosia trifida f. integrifolia	Am
Compositae	Ambrosia trifida	Am
Compositae	Anthemis arvensis	Eu
Compositae	Anthemis cotula	Eu
Compositae	Aster novi-belgii	Am
Compositae	Aster pilosus	Am
Compositae	Aster subulatus	Am
Compositae	Aster subulatus var. sandwicensis	Am
Compositae	Bidens frondosa	Am
Compositae	Bidens pilosa	Am
Compositae	Bidens pilosa var. minor	Am
Compositae	Bidens polylepis	Am
Compositae	Bidens subalternans	Am
Compositae	Carduus crispus f. albus	Eu–As
Compositae	Carduus crispus	Eu–As
Compositae	Carduus nutans	Eu
Compositae	Centaurea cyanus	Eu
Compositae	Chrysanthemum leucanthemum	Eu
Compositae	Cirsium arvense	Eu
Compositae	Cirsium vulgare	Eu
Compositae	Conyza bonariensis	Am
Compositae	Conyza canadensis	Am
Compositae	Conyza parva	Am
Compositae	Conyza sumatrensis	Am
Compositae	Coreopsis lanceolata	Am
Compositae	Coreopsis tinctoria	Am
Compositae	Cosmos bipinnatus	Am
Compositae	Cosmos sulphureus	Am
Compositae	Crassocephalum crepidioides	Af
Compositae	Crepis tectorum	Eu
Compositae	Dracopis amplexicaulis	Am
Compositae	Eclipta alba var. erecta	Am
Compositae	Erechtites hieracifolia	Am
Compositae	Erigeron annuus	Am
Compositae	Erigeron philadelphicus	Am
Compositae	Erigeron strigosus	Eu
Compositae	Eupatorium rugosum	Am
Compositae	Euthamia graminifolia	Am
Compositae	Galinsoga ciliata	Am
Compositae	Galinsoga parviflora	Am
Compositae	Gamochaeta pensylvanica	Am
Compositae	Gnaphalium calviceps	Am
Compositae	Gnaphalium purpureum	Am
Compositae	Helianthus debilis	Am
Compositae	Helianthus tuberosus	Am
Compositae	Hieracium caespitosum	Eu
Compositae	Hypochaeris radicata	Eu
Compositae	Lactuca scariola	Eu
Compositae	Lapsana communis	Eu
Compositae	Matricaria inodora	Eu
Compositae	Matricaria matricariodes	As
Compositae	Parthenium hysterophorus	Am
Compositae	Rudbeckia bicolor	Am
Compositae	Rudbeckia hirta	Am
Compositae	Rudbeckia laciniata var. hortensis	Am
Compositae	Senecio inaequidens	Af
Compositae	Senecio scandens	As
Compositae	Senecio vulgaris	Eu
Compositae	Solidago altissima	Am
Compositae	Solidago serotina	Am
Compositae	Sonchus asper	Eu
Compositae	Sonchus oleraceus	Eu
Compositae	Tagetes minuta	Am
Compositae	Taraxacum laevigatum	Eu
Compositae	Taraxacum officinale	Eu
Compositae	Tragopogon dubius	Eu
Compositae	Verbesina alternifolia	Am
Compositae	Xanthium canadense	Am
Compositae	Xanthium italicum	Am
Compositae	Xanthium strumarium	As
Convolvulaceae	Convolvulus arvensis	Eu
Convolvulaceae	Cuscuta pentagona	Am
Convolvulaceae	Ipomoea hederacea	Am
Convolvulaceae	Ipomoea hederacea var. integriuscula	Am
Convolvulaceae	Ipomoea lacunosa	Am
Convolvulaceae	Ipomoea purpurea	Am
Convolvulaceae	Ipomoea triloba	Am
Convolvulaceae	Jacquemontia tamnifolia	Am
Convolvulaceae	Quamoclit coccinea	Am
Crassulaceae	Sedum mexicanum	Am
Cruciferae	Alliaria petiolata	Eu–As
Cruciferae	Barbarea verna	Eu
Cruciferae	Barbarea vulgaris	Eu
Cruciferae	Brassica juncea	As
Cruciferae	Cakile edentula	Am
Cruciferae	Camelina microcarpa	Eu
Cruciferae	Cardaria draba	Eu
Cruciferae	Chorispora tenella	Eu–As
Cruciferae	Coronopus didymus	Eu
Cruciferae	Descurainia pinnata	Am
Cruciferae	Diplotaxis muralis	Eu
Cruciferae	Erucastrum gallicum	Eu
Cruciferae	Lepidium apetalum	Am
Cruciferae	Lepidium bonariense	Am
Cruciferae	Lepidium campestre	Eu
Cruciferae	Lepidium latifolium	Eu–As
Cruciferae	Lepidium perfoliatum	Eu–As
Cruciferae	Lepidium ruderale	Eu
Cruciferae	Lepidium virginicum	Am
Cruciferae	Myagrum perfoliatum	Eu
Cruciferae	Nasturtium officinale	Eu
Cruciferae	Neslia paniculata	Eu
Cruciferae	Raphanus raphanistrum	Eu
Cruciferae	Rapistrum rugosum	Eu
Cruciferae	Rorippa sylvestris	Eu
Cruciferae	Sinapis arvensis	Eu
Cruciferae	Sinapis arvensis var. orientalis	Eu
Cruciferae	Sisymbrium altissimum	Eu
Cruciferae	Sisymbrium officinale	Eu
Cruciferae	Sisymbrium officinale var. leiocarpum	Eu
Cruciferae	Sisymbrium orientale	Eu
Cruciferae	Thlaspi arvense	Eu
Cucurbitaceae	Sicyos angulatus	Am
Cyperaceae	Carex scoparia	Am
Euphorbiaceae	Euphorbia dentata	Am
Euphorbiaceae	Euphorbia heterophylla	Am
Euphorbiaceae	Euphorbia hirta	Am
Euphorbiaceae	Euphorbia maculata	Am
Euphorbiaceae	Euphorbia prostrata	Am
Euphorbiaceae	Euphorbia supina	Am
Fumariaceae	Fumaria officinalis	Eu
Geraniaceae	Erodium cicutarium	Eu
Geraniaceae	Geranium carolinianum	Am
Geraniaceae	Geranium dissectum	Eu
Gramineae	Aegilops cylindrica	Eu
Gramineae	Agropyron repens	Eu
Gramineae	Agropyron repens f. aristatum	Eu
Gramineae	Aira caryophyllea	Eu
Gramineae	Alopecurus japonicus	As
Gramineae	Alopecurus myosuroides	Eu–As
Gramineae	Alopecurus pratensis	Eu–As
Gramineae	Andropogon virginicus	Am
Gramineae	Anthoxanthum odoratum	Eu–As
Gramineae	Arrhenatherum elatius	Eu
Gramineae	Arrhenatherum elatius var. bulbosum	Eu
Gramineae	Avena fatua	Eu–As
Gramineae	Avena sativa	Eu–As
Gramineae	Briza minor	Eu
Gramineae	Bromus carinatus	Eu–Am
Gramineae	Bromus inermis	Eu
Gramineae	Bromus mollis	Eu
Gramineae	Bromus racemosus	Eu
Gramineae	Bromus rigidus	Eu
Gramineae	Bromus secalinus	Eu
Gramineae	Bromus sterilis	Eu
Gramineae	Bromus tectorum	Eu
Gramineae	Bromus tectorum var. glabratus	Eu–As
Gramineae	Bromus unioloides	Am
Gramineae	Catapodium rigidum	Eu
Gramineae	Cenchrus longispinus	Am
Gramineae	Chloris virgata	Am
Gramineae	Coix lacryma-jobi	As
Gramineae	Dactylis glomerata	Eu–As
Gramineae	Dactyloctenium aegyptium	As
Gramineae	Dichanthelium acuminatum	Am
Gramineae	Eragrostis curvula	Af
Gramineae	Eremochloa ophiuroides	As
Gramineae	Festuca arundinacea	Eu
Gramineae	Festuca megalura	Am
Gramineae	Festuca myuros	Eu
Gramineae	Festuca pratensis	Eu
Gramineae	Glyceria declinata	Eu
Gramineae	Holcus lanatus	Eu
Gramineae	Hordeum jubatum	Eu
Gramineae	Hordeum murinum	Eu–As
Gramineae	Hordeum pusillum	Am
Gramineae	Leptochloa fusca	As
Gramineae	Lolium multiflorum	Eu
Gramineae	Lolium multiflorum var. ramosum	Eu
Gramineae	Lolium perenne	Eu
Gramineae	Lolium rigidum	Med
Gramineae	Lolium temulentum	Eu
Gramineae	Panicum dichotomiflorum	Am
Gramineae	Panicum miliaceum	As
Gramineae	Panicum virgatum	Am
Gramineae	Parapholis incurva	Eu
Gramineae	Paspalum dilatatum	Am
Gramineae	Paspalum distichum	As
Gramineae	Paspalum distichum var. indutum	Am
Gramineae	Paspalum notatum	Am
Gramineae	Paspalum urvillei	Am
Gramineae	Pennisetum flaccidum	As
Gramineae	Phalaris canariensis	Eu–As
Gramineae	Phalaris minor	Eu–As
Gramineae	Phleum paniculatum	Eu
Gramineae	Phleum pratense	Eu
Gramineae	Poa bulbosa var. vivipara	Eu
Gramineae	Poa compressa	Eu
Gramineae	Poa pratensis	Eu
Gramineae	Puccinellia distans	Eu
Gramineae	Rottboellia cochinchinensis	etc.
Gramineae	Saccharum arundinaceum	As
Gramineae	Sorghum halepense	Eu
Gramineae	Sorghum halepense f. muticum	Eu
Gramineae	Spartina anglica	Eu
Guttiferae	Hypericum perforatum	Eu
Iridaceae	Sisyrinchium angustifolium	Am
Iridaceae	Tritonia crocosmaeflora	Eu
Juglandaceae	Pterocarya stenoptera	As
Labiatae	Lamium purpureum	Eu–As
Labiatae	Lamium purpureum var. hybridum	Eu
Labiatae	Scutellaria baicalensis	As
Leguminosae	Amorpha fruticosa	Am
Leguminosae	Astragalus sinicus	As
Leguminosae	Lespedeza davidii	As
Leguminosae	Lespedeza floribunda	As
Leguminosae	Lespedeza lichiyuniae	As
Leguminosae	Lotus corniculatus	Eu
Leguminosae	Lotus uliginosus	Eu–Af
Leguminosae	Lupinus angustifolius	Eu
Leguminosae	Medicago lupulina	Eu
Leguminosae	Medicago minima	Eu
Leguminosae	Medicago polymorpha	Eu
Leguminosae	Medicago sativa	Med
Leguminosae	Melilotus alba	As
Leguminosae	Melilotus suaveolens	As
Leguminosae	Robinia pseudoacacia	Am
Leguminosae	Securigera varia	Eu–As
Leguminosae	Trifolium campestre	Eu
Leguminosae	Trifolium dubium	Eu–As
Leguminosae	Trifolium hybridum	Eu–As
Leguminosae	Trifolium incarnatum	Eu
Leguminosae	Trifolium pratense	Eu
Leguminosae	Trifolium repens	Eu–Af
Leguminosae	Trifolium resupinatum	Eu–As
Leguminosae	Vicia dasycarpa	Eu
Leguminosae	Vicia villosa	Eu
Lythraceae	Ammannia coccinea	Am
Magnoliaceae	Magnolia obovata	As
Malvaceae	Abutilon theophrasti	As
Malvaceae	Hibiscus trionum	Eu
Malvaceae	Malva neglecta	Eu–As
Malvaceae	Malva parviflora	Eu
Malvaceae	Malva pusilla	Eu
Malvaceae	Malva sylvestris var. mauritiana	Eu
Malvaceae	Modiola caroliniana	Am
Malvaceae	Sida rhombifolia	Am
Malvaceae	Sida spinosa	Am
Molluginaceae	Mollugo verticillata	Am
Onagraceae	Oenothera biennis	Am
Onagraceae	Oenothera erythrosepala	Am
Onagraceae	Oenothera laciniata	Am
Onagraceae	Oenothera rosea	Am
Onagraceae	Oenothera striata	Am
Oxalidaceae	Oxalis articulata	Am
Oxalidaceae	Oxalis corymbosa	Am
Papaveraceae	Papaver dubium	Eu
Papaveraceae	Papaver hybridum	Eu
Papaveraceae	Papaver rhoeas	Eu
Phytolaccaceae	Phytolacca americana	Am
Phytolaccaceae	Phytolacca esculenta	As
Plantaginaceae	Nuttallanthus canadensis	Am
Plantaginaceae	Plantago aristata	Am
Plantaginaceae	Plantago lanceolata	Eu
Plantaginaceae	Plantago virginica	Am
Polygonaceae	Fallopia convolvulus	Eu–As
Polygonaceae	Fallopia dentatoalata	Eu
Polygonaceae	Fallopia dumetorum	Eu
Polygonaceae	Persicaria capitata	As
Polygonaceae	Persicaria orientalis	As
Polygonaceae	Persicaria wallichii	As
Polygonaceae	Rumex acetosella	Eu
Polygonaceae	Rumex crispus	Eu
Polygonaceae	Rumex nipponicus	As
Polygonaceae	Rumex obtusifolius	Eu–As
Ranunculaceae	Ranunculus arvensis	Eu
Ranunculaceae	Ranunculus muricatus	Eu
Rosaceae	Potentilla amurensis	Eu
Rosaceae	Potentilla supina	Eu
Rosaceae	Rubus fruticosus	Eu
Rosaceae	Sanguisorba minor	Eu
Rubiaceae	Diodia teres var. hirsutior	Am
Rubiaceae	Diodia teres	Am
Rubiaceae	Diodia virginiana	Am
Rubiaceae	Oldenlandia corymbosa	As–Af
Rubiaceae	Sherardia arvensis	Eu
Saururaceae	Houttuynia cordata	As
Scrophulariaceae	Cymbalaria muralis	Eu
Scrophulariaceae	Gratiola officinalis	Eu
Scrophulariaceae	Lindernia anagallidea	Am
Scrophulariaceae	Lindernia dubia	Am
Scrophulariaceae	Verbascum thapsus	Eu
Scrophulariaceae	Veronica americana	Am
Scrophulariaceae	Veronica arvensis	Eu–As
Scrophulariaceae	Veronica hederaefolia	Eu
Scrophulariaceae	Veronica persica	Eu–As
Scrophulariaceae	Veronica serpyllifolia	Eu
Simaroubaceae	Ailanthus altissima	As
Solanaceae	Datura meteloides	Am
Solanaceae	Datura stramonium	As
Solanaceae	Datura stramonium var. chalybea	Am
Solanaceae	Nicandra physalodes	Am
Solanaceae	Physalis angulata	Am
Solanaceae	Physalis wrightii	Am
Solanaceae	Solanum americanum	Am
Solanaceae	Solanum carolinense	Am
Solanaceae	Solanum elaeagnifolium	Am
Solanaceae	Solanum nigrum var. humile	Eu
Solanaceae	Solanum photeinocarpum	Am
Solanaceae	Solanum rostratum	Am
Solanaceae	Solanum sarachoides	Am
Solanaceae	Solanum sisymbriifolium	Am
Solanaceae	Solanum viarum	Am
Umbelliferae	Anthriscus caucalis	Eu
Umbelliferae	Apium leptophyllum	Am
Umbelliferae	Bifora radians	Med
Umbelliferae	Chaerophyllum tainturieri	Am
Umbelliferae	Conium maculatum	Eu
Umbelliferae	Foeniculum vulgare	Eu
Umbelliferae	Lisaea heterocarpa	As
Valerianaceae	Valerianella olitoria	Eu
Verbenaceae	Verbena bonariensis	Am
Verbenaceae	Verbena brasiliensis	Am
Violaceae	Viola arvensis	Eu
Violaceae	Viola papilionacea	Am

Author Contributions

Conceptualization, B.S.L. and C.S.L.; methodology, B.S.L. and J.E.S.; validation, J.E.S. and B.S.L.; formal analysis, B.S.L. and C.H.L.; investigation, G.S.K., H.C.S., C.S.L., B.S.L. and C.H.L.; data curation, B.S.L.; writing—original draft preparation, C.S.L. and B.S.L.; writing—review and editing, G.S.K., H.C.S. and C.S.L.; visualization, J.E.S. and B.S.L.; supervision, C.S.L.; project administration, C.S.L. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Raw exotic plant data is available on IKAS (https://kias.nie.re.kr/home/main/main.do accessed on 3 June 2023).

Conflicts of Interest

The authors declare no conflict of interest.

Funding Statement

This work was supported by the Korea Environment Industry & Technology Institute (KEITI) through the Wetland Ecosystem Value Evaluation and Carbon Absorption Value Promotion Technology Development Project, funded by the Korea Ministry of Environment (2022003630002).