Comparative Study on Flora Characteristics and Species Diversity on Dam Slopes for Sustainable Ecological Management: Cases of Eight Dams in Korea

Abstract

Dams are gray infrastructure, providing various benefits such as flood control, water supply, and power generation. In order to create the next generation of infrastructure that explores how nature can act as infrastructure to meet development and ecological sustainability, artificial plantings have been attempted on dam slopes in Korea since 2000. As the planted trees are now stabilized to form a forest, it is time to study the floral characteristics and functions for effective ecological management and the safety of the dams. In this study, we investigated and analyzed flora in the slopes of eight dams in Korea. The comparative study of the whole flora in both the planted zones of the slopes of dams and left and right forests of dams revealed that the number of plant species was higher in the planted zones than in the left and right forests of the same size area. The plant family containing the greatest number of species in the slopes was Asteraceae, followed by Poaceae, Fabaceae, and Rosaceae. Currently, the community structures and families in the slopes of dams exhibit the characteristics of habitats in the initial stage of vegetation succession. Our investigation of planted species and immigration species in the slopes revealed that the latter comprised 89.9%. An average of 34.4% of species were interacting with the dam slope and the left and right forests. The species diversity index on dam slopes showed a tendency to be higher as the number of planted species increased and the period time increased. Average growth heights of planted trees were identified as 0.5–1.6 m for the shrubs layer, 3.5–4.5 m for the small trees layer, and 6.0–7.2 m for the trees layer. The heights of major trees, including Pinus densiflora, Quercus spp., Prunus sargentii, Styrax japonicus, and Cornus controversa, were similar to or higher than those of their counterparts in natural forests. As a result, dam slopes were similar to natural forests, having potential as habitats for various flora. To harmoniously maintain the ecological health and safety of water resource facilities of the slopes of dams, however, it is necessary to conduct periodic and various investigations on changes of the flora and growth of trees, and actively manage them.

1. Introduction

Globally, there are over 45,000 large dams and approximately 800,000 small dams [1,2]. Traditionally, dam construction has been promoted as part of water resource development, as dams can secure a stable supply of water for irrigation. By reducing flood peaks downstream, dams have contributed greatly to the prevention of flood damage in downstream urban regions [3]. However, dam construction has also caused vertical and horizontal disturbances of stream ecologies, and major environmental changes [4,5]. Most dams in Korea were constructed prior to the 1980s, when environmental impact assessments were adopted. As a consequence, they were constructed without great concern for their environmental impacts [6].

Hydraulic structures, such as dams and banks, constitute artificial materials and structure-centered hard engineering [7,8], and typical gray infrastructures constructed by blocking a certain section of upper stream with rock and concrete [9]. While such large-scale gray infrastructures led by public authorities to prevent disasters are effective in managing the causes of disasters, they tend to raise certain issues, including that they are not eco-friendly facilities, and the forms and functions of such facilities are commonly uniform and monotonous.

The United Nations Conference on Environment and Development, also known as the “Earth Summit”, was held in Rio de Janeiro, Brazil, in 1992. The conference adopted “environmentally sound and sustainable development (ESSD)” as a practical strategy of Agenda 21 [10]. ESSD produced a paradigm shift in dam construction and management, and the concept of environmentally friendly dams emerged [9,11]. After this paradigm shift in dam construction and management, countries, including Korea, prioritized efforts towards minimizing the effects of dam construction and operation on the environment, and various technologies have been used to recover ecological systems [6]. Indeed, the environmental impacts of construction dams can be markedly pronounced, including landscape damage resulting from the huge artificial structure, aggravation of buffer functions of water zones and land zones, and, crucially, the blocking of movement of aquatic animals and aggravation of water quality [12].

In the 2000s, the Korean government established guidelines on designing environmentally friendly dams. Those guidelines have stimulated the development of technologies to construct eco-friendly slopes of dams, and the adoption of forest vegetation using native plants on slopes [13]. A dam is a typical artificial structure to prevent floods [14,15,16,17], and specific artificial foundation planting was adopted, in which various vegetation was planted to harmonize the dam with the surrounding forests after the embankment is completed. Such a process can be described as efforts to overcome limits of dams as gray infrastructures, and to change them into green infrastructures [9] for the ecological system, which comprises the optimal integration of water, land, and vegetation. The first planting on the slope of dams in Korea was performed on the Boryung dam in 2000, followed by eight other dams over time: the Daegok dam, Gampo dam, Jangheung dam, Pyungrim dam, Gunwi dam, Kimcheon Buhang dam, and Yeongju dam [18].

In recent decades, the effects of dams on the environment have been researched worldwide, primarily focusing on the effects of dams on changes in aquatic plants and animals, on downstream hydrology, and on damage to large-scale habitats [19,20,21,22,23]. In contrast, there is a paucity of literature investigating the ecological characteristics of planted zones on the slopes of dams, adopted to alleviate some of the effects of dams on their surrounding environment. In some countries, studies have been performed to determine the effect of trees growing on the slopes of old dams on the safety of dams, mainly to obtain guidelines concerning how to prevent accidents [24]. Major undesirable effects of such trees on the safety of dams include the following: holes and other damage left behind after trees die; penetration of tree roots into dams; intervention of filter zones by erosion; the additional weight of grown trees on dam slopes; damage to the land cover caused by tree shade; the difficulty in maintaining and repairing dams; the increasing costs of repairing dams; difficulty of checking and managing dams in extreme floods; damage of trees to the upper part and other structures of dams; and provision of habitats to underground animals [25,26,27,28,29].

In 2015, the United Nations adopted the Sustainable Development Goals (SDGs) with 17 goals to be achieved. Goal 15 of these is “protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, and halt and reverse land degradation and halt biodiversity loss” [30,31,32]. The planting of dam slopes attempted in Korea can also be viewed as an ecological engineering technology that contributes to the protection and promotion of the species diversity of terrestrial ecosystems and alleviates the unavoidable impact on the ecosystem caused by dam construction. The introduction of ecosystem services evaluation to ecological engineering could secure sustainable policy formulation [33]. In order to understand dams in terms of sustainability and evaluate the value of ecosystem services, research should be conducted not only on the vegetation of reservoirs and waterside, but also on the newly planted dam slopes. In particular, for the evaluation of the supporting ecosystem service, it is necessary to measure the primary production quantity and the provision of habitat [34,35,36] on the basis of vegetation condition and diversity. By closely investigating the growth of artificially planted species, the situation of naturally immigrated species, and the emergence of ecologically invasive species, it will be possible to elucidate the value of planted zones as the ecological base and predict the changes of vegetation.

Considering important functions of dams, such as the prevention of floods and protecting water supplies, it is critical to properly manage vegetation on the slope of dams so as not to threaten safety and dam maintenance. There are approximately 11,600 dams built before the 1960s in Korea, and over 65% of the country’s dams are over 50 years old. Considering that the usual life span of a dam is 50–100 years, this aging of an increasing number of dams constitutes a dire problem [37]. In particular, as most of the planted zones on the slopes of dams are now over 10 years old, now is the appropriate time to identify their characteristics and functions as ecological bases and propose effective ways to manage them. In order to secure the safety of slopes of dams and use reservoir water, research is urgently needed on the effects of vegetation that artificially and naturally settled down on the slopes of dams. For the optimal management of slopes of dams with the dual purpose of ecological functions of the planted zones and dam safety, it is necessary to examine the characteristics and changes of the flora.

For the first time, we investigated the flora on the slopes of eight dams that have been artificially planted for more than 5 years. In this study, we aim to analyze the whole flora of the dam slopes and surrounding natural forests, and assess the characteristics of planted and immigrant species, species diversity, and growth of planted trees. Through this, the potential of dam slope planting sites as an artificial eco-based environment and lessons for sustainable and eco-friendly management can be obtained. We expect the study results can help in the management provision of integrating gray and green infrastructure to meet development and ecological sustainability. In addition, it is expected to be used as a practical measure to alleviate the environmental impact caused by dams and the limitations of gray infrastructure, and to operate dams as a green infrastructure where humans and nature harmonize in the long term.

2. Materials and Methods

2.1. Study Scope and Area

The first plantings on the slopes of dams began in 2000 and continued until 2016. Our field investigations and analyses were conducted from April to September 2020, and we compiled a list of plants that appeared in the entire area of the dam slopes and adjacent forests. The targets of the field investigation were eight dams in Korea, for which more than five years have passed since planting on their slopes. To reveal the characteristics of the flora in the planted zones on the slopes of the dams, and to elucidate the mutual relationships and effects between the planted zones and surrounding natural forests, this research conducted on-the-spot investigations on the forests on the left and right sides of the dams (Figure 1). Considering the distribution scope of forest vegetation and dispersion methods of emerging plants, this research performed on-the-spot investigations of the area approximately 1.5 times of the eco-friendly vegetation zone on both sides of the dams from the ridge of the mountains surrounding the dams.

Planting on the slopes of dams began on the Boryung dam in 2000, followed by eight other dams over time: the Daegok dam, Gampo dam, Jangheung dam, Pyungrim dam, Gunwi dam, Kimcheon Buhang dam, and Yeongju dam. The total planted zones total 204,033 m² on the eight dams (

Table 1. Status of study area for both the planted zone and natural zone, including completion year, size, height, and depth for each dam.

Dam	Completion Year	Dam Type	Dam Size (Height × Width, m)	Fill Height (m)	Soil Depth (m)	Study Area
						Dam Slope (Planted Zone) (m2)	Left and Right Forest (Natural Zone) (m2)
BR	2000	ECRD	50.0 × 291.0	34	1–12	29,500	44,250
DG	2005	CFRD	52.0 × 192.0	22	1.5–8	5100	7650
GP	2006	ECRD	35.0 × 108.0	20	2.4	2685	4028
JH	2006	CFRD	53.0 × 403.0	44	2–23	25,908	38,862
PR	2007	ECRD	37.3 × 390.5	30	1–10	18,000	27,000
GW	2011	CFRD	45.0 × 390.0	45	1–28	29,840	44,760
KB	2014	CFRD	64.0 × 472.0	47	1–17	55,000	82,500
YJ	2016	CFRD	55.5 × 390.0	45	1–20	38,000	57,000

BR: Boryung dam, DG: Daegok dam, GP: Gampo dam, JH: Jangheung dam, PR: Pyungrim dam, GW: Gunwi dam, KB: Kimcheon Buhang dam, YJ: Yeongju dam, ECRD: Earth Core Rock Fill dam, CFRD: Concrete Face Rockfill dam.

). In most cases, planting bases were prepared by adding earth and sand on the slope after the fill dam was constructed. Fill dams, such as the Earth Cored Rockfill dam and the Concrete Faced Rockfill dam, provide pores and friction on the rock foundation of the slope of the dam, which makes it possible to prevent the flow of earth and sand, and to perform soil compaction. Most commonly, herbaceous plants were planted on the upper part of the slope from the middle line of the embankment, where it was difficult to secure sufficient earth and sand. However, various trees were planted on the lower part of the slope, where sufficient soil depth could be secured. The embankment was built to deepen the soil as it extended into the ground which helped to achieve and maintain a gentle slope. In the cases of the Boryung dam, Jangheung dam, Gunwi dam, Kimcheon Buhang dam, and Yeongju dam, the slope was gentle, with a ratio of approximately 1:2.5 from the top, and 24–47 m in height and 1–28 m in soil depth. In the cases of the Daegok dam, Gampo dam, and Pyungrim dam, the partial embankment was established to make it 20–30 m high from the middle line and 2.4–10 m in soil depth (Table 1).

Design documents and as-built drawings of dams revealed that, in most cases, planting on the dam slopes was carried out with module-based techniques with native trees growing near the dam, such as coniferous tree colonies and deciduous tree colonies. Trees or stumps were transplanted from areas which would be submerged under dam water to the dam slope as much as possible [18].

2.2. Research Method

2.2.1. Research Framework

We aimed to investigate the characteristics of the flora, species diversity, and the situation of planted species and immigration species as ecological bases on the slopes of dams, and to suggest one or more optimal management methods. First, to analyze the characteristics of the flora, and species diversity on the slopes of dams, we carried out a literature review and field investigation. Second, to determine the growth of planted trees, we measured their heights and diameters of breast height (DBH). Finally, based on the above examination and analysis, we suggest a potential of dam slope as an artificial eco-based environment and lessons for the sustainable and eco-friendly management of dams (Figure 2).

2.2.2. Field Investigation

The investigation was conducted according to two methods. If the slope of the dam and left and right forests were accessible on foot, we examined the planted zone and left and right forests on foot. In order to avoid double counting of plant populations and omission of the list, about 1200 samples of 20 m × 20 m quadrats were set up according to the size of the survey area. If they were inaccessible and had too-steep slopes, we observed trees with binoculars. We compiled a list of plants using binoculars only on the inaccessible slopes of Kimcheon Buhang dam and Daegok dam (ca. 300 m²). Investigation of the flora was performed from June to September, when the species diversity of plants is the most abundant. To list all flora, we recorded all of the plants that we encountered on the paths of investigation. If it was not possible to identify a species, we collected it, and entered it on the list after identifying it later.

To determine how the trees have grown in the planted zones, we compared as-built drawings made when the dam was constructed with the current situation of the trees. Using a measuring rod, measuring STA, caliper, and a tape measure, we measured the height and diameter of breast height (DBH) of the trees.

2.2.3. Analytic Method

The classification of the flora list in this research followed the KPNIC (Korean Plant Names Index Committee) and IPNI (International Plant Names Index). Each plant was morphologically classified according to KPNIC [38]. We first listed scientific names, then synonyms and name corrections of the first-categorized scientific names were processed by IPNI [39]. In the process, we re-verified morphological characteristics and finalized the categorization. The botanical names under the same genus were arranged in alphabetical order. Naturalized plants and alien plants were analyzed on the basis of the Checklist of Alien Plants in Korea [40], and they were classified into the categories of Arc (archaeophyte), PIP (potentially invasive plant), and IAP (invasive alien plant). Ferns were classified according to the system of Christenhusz (2011). The life cycle was analyzed using Raunkiaer life forms (1934) [41,42]. The biodiversity index was calculated for trees only as the removal of herbaceous species and planting management have been carried out on dam slopes of the study area. The Shannon–Wiener Diversity Index was used to analyze the species diversity for each dam slope [43].

Comparing the as-built drawings when the dam was constructed and the current list of the flora, this research classified planted species and immigration species, and examined the characteristics of immigration species. By identifying botanical species discovered both in the planted zone on the slopes of dams and in the left and right forests, we attempted to predict plant species expected to immigrate to the planted zones. To determine the growth level of the vegetation in the planted zones, we measured tree height and DBH for some species of plants and compared the obtained values with the corresponding values. It was difficult to analyze the growth of planted trees on the slopes of dams because the measurement indicators on height, DBH, and root diameter were various. Accordingly, this study compared the average heights and DBHs of frequently planted trees on the slopes of dams, such as Pinus densiflora, Quercus spp., Prunus sargentii Rehder, Styrax japonicus Siebold & Zucc., and Cornus controversa Hemsl., with the average heights and DBHs of trees suggested in the 5th National Forest Resources Survey (2011) [44]. On the slopes of the Daegok dam, as there were no data in the above-mentioned inventory for heights and DBHs of Pinus strobus and Acer palmatum Thunb., we used corresponding data for planted trees over 10 years old studied in Lee and Lee (1999) [45].

3. Results

3.1. Flora

3.1.1. The Whole Flora

Investigation of all the flora in the planted zones of the slopes and in the left and right forests of eight dams proved that the flora could be classified into 397 total taxa: 376 species, 11 varieties, 2 cultivars, and 8 subspecies (belonging to 95 families and 267 genera). On the slopes of eight dams, there were 273 total taxa: 258 species, 9 varieties, 1 cultivar, and 5 subspecies (belonging to 81 families and 198 genera). In the left forests, there were 274 classified groups in total; in the right forests, there were 251 classified groups in total. There was no substantial difference in the number of classified groups between dam slopes and left and right forests (

Table 2. Number of vascular plants.

	Family	Genus	Species	Variety	Form	Subspecies	Total
Dam slope	81	198	258	9	1	5	273
Left forest	84	198	262	7	2	3	274
Right forest	77	181	235	9	1	6	251
Total	95	267	376	11	2	8	397

).

The lowest and highest numbers of classified species observed on the planted zone on the slopes and surrounding forests were found at Gampo (107 taxa) and Jangheung (209 taxa) dams, respectively. The lowest and highest numbers of classified species observed on the planted zone on the slopes were found at Yeongju (67 taxa) and Jangheung (121 taxa) dams, respectively. Overall, we found that the larger the planted zone was, the higher the number of species (Figure 3).

Five taxa—Equisetum arvense L., Quercus aliena Blume, Humulus scandens, Clematis apiifolia DC., and Miscanthus sinensis Andersson—were observed in all of the planted zones on the slopes of the eight dams. Thirty-five taxa, including Albizia julibrissin Durazz., Amphicarpaea edgeworthii Benth., Lactuca indica L., and Persicaria senticosa (Meisn.) H.Gross ex Naka, were found in the planted zones of more than six dams. In the whole flora, there were as many as 45 taxa climbing plants, such as Gynostemma pentaphyllum (Thunb.) Makino, Hedera rhombea (Miq.) Siebold & Zucc. ex Bean, Cynanchum rostellatum (Turcz.) Liede & Khanum, Pueraria montana var. lobata (Willd.) Maesen & S.M.Almeida ex Sanjappa & Predeep, Humulus scandens (Lour.) Merr., etc. Most species, excluding planted species, were found to be pioneer species, mantle vegetation species, and secondary herb vegetation species. The full species list is presented in Appendix A.

3.1.2. Flora per Family

We determined that among the whole flora, the family under which the greatest number of species was observed in the planted zone and in the natural forests was the chrysanthemum family (Asteraceae) (44 taxa, 11.1%), followed by the rice family (Poaceae) (34 taxa, 8.6%), the bean family (Fabaceae) (29 taxa, 7.3%), and the rose family (Rosaceae) (22 taxa, 5.5%) (Figure 4).

The numbers of Rosaceae (16 Taxa, 5.9%) and Fabaceae (22 taxa, 8.1%) on the dam slope were similar to those of the adjacent natural forests, but Asteraceae (37 taxa, 13.6%) and Poaceae (27 taxa, 9.9%) were relatively higher on dam slopes (

Table 3. Number of plant families by each survey area.

Family	Dam Slope		Left Forest		Right Forest
	N	Ratio (%)	N	Ratio (%)	N	Ratio (%)
ROSACEAE	16	5.9%	16	5.8%	16	6.4%
FABACEAE	22	8.1%	20	7.3%	22	8.8%
ASTERACEAE	37	13.6%	28	10.2%	27	10.8%
POACEAE	27	9.9%	20	7.3%	23	9.2%
Subtotal	102	37.4%	84	30.7%	88	35.1%
Other Families	171	62.6%	190	69.3%	163	64.9%
Total	273	100.0%	274	100.0%	251	100%

).

3.1.3. Life Cycle

Investigation of the life cycles of plants distributed on the dam slopes and left and right forests of the eight dams showed that the average proportions of the number of perennial tree species were 40.1% on dam slopes, 52.8% in left forests, and 47.5% in right forests. The number of perennial tree species on dam slopes was the highest at the Jangheung dam (52 taxa), and lowest at the Yeongju dam (21 taxa). Overall, older dams tended to have higher numbers of perennial tree species.

The average proportion of the number of perennial herb species was 37.1% on dam slopes, 33.2% in left forests, and 33.6% in right forests. As in the case of perennial trees, older dams tended to have higher numbers of herb species. The highest number of herb species (47 taxa) was on the slopes of the Kimcheon Buhang dam with the largest planting area.

The average proportion of the number of annual herb species was 22.8% on dam slopes, 14.0% in left forests, and 18.9% in right forests. Their relative number was the highest (31 taxa) on the slope of the Jangheung dam, and the lowest (six taxa) on that of the Daegok dam. The relative number of annual herb species on the slopes of dams was higher than that of left forests and right forests. In the case of the Yeongju dam, the youngest of the eight dams, the proportion of annual herb species on dam slopes was the largest (28.3%) (Figure 5).

3.1.4. Alien Plants and Ecosystem Disturbance Species

According to the Checklist of Alien Plants in Korea [40], there were 49 taxa alien species in total on the dam slopes and left and right forests: 9 taxa under the Arc category, including Morus alba L., Houttuynia cordata Thunb., and Brassica napus L.; 40 taxa under the IAP category, including Rumex crispus L., Phytolacca americana L., and Chenopodium album L.; and 11 species under the PIP category, including Larix kaempferi (Lamb.) Carrière, Pinus rigida Mill., and Chamaecyparis obtusa (Siebold & Zucc.) Endl. The Kimcheon Buhang dam contained the highest number of alien species (35 taxa), and the Daegok dam contained the lowest (eight taxa).

Erigeron canadensis L. and Erigeron annuus (L.) Pers. were discovered at all dams, and nine taxa, including Erigeron canadensis, Erigeron annuus, Stellaria media (L.) Vill., and Oenothera biennis L., were discovered at more than six dams (Figure 6). The full list of alien species is presented in Appendix A.

Among the ecosystem disturbance species [46] designated by the Ministry of Environment, four taxa—Humulus scandens, Solanum carolinense L., Ambrosia artemisiifolia L., and Hypochaeris radicata L.—were identified in and around some of the eight dams. All of the four species were identified at the Gampo dam and the Jangheung dam. At the Pyungrim dam and the Boryung dam, only Humulus scandens was identified. Humulus scandens and Ambrosia artemisiifolia appeared at the Yeongju dam, Gunwi dam, and Daegok dam, and three species including Solanum carolinense appeared at the Kimcheon Buhang dam. Hypochaeris radicata, a plant that typically lives in coastal areas, was discovered at the Gampo dam and the Jangheung dam, which are both located near the sea.

None of the Endangered Wild Plants [47] designated by the Ministry of Environment were observed on the slopes of dams. However, among the Least Concern (LC) species [48] designated by the Korea Forest Service, and on the list of IUCN, Berchemiella berchemiifolia (Makino) Nakai was found at the Gunwi dam, and Monotropa hypopitys L. was observed at the Daegok dam.

3.1.5. Analysis of Species Diversity

As a result of the Shannon–Wiener Diversity index analysis of eight dams, the Boryung dam showed the highest diversity value of 1.29, and the Yeongju dam had the lowest at 0.13. The species diversity index in dams showed positive relationships with the number of tree species planted; exceptionally, the Daegok dam showed high species diversity despite the low number of tree species planted. The Yeongju dam, built most recently, has low species diversity compared to other dams (

Table 4. The species diversity index of eight dams.

	GP	JH	PR	YJ	GW	DG	KB	BR
Completion year	2006	2006	2007	2016	2011	2005	2014	2000
Total number of tree species	16	21	7	3	19	17	12	23
H’(Shannon–Wiener index)	0.87	1.14	0.73	0.13	1.23	1.04	0.54	1.29

).

3.2. Analysis of Planted Species and Immigration Species

3.2.1. Planted Species and Immigration Species

The analysis of proportions of planted species and immigration species in the planted zones of the slope of the eight dams revealed that the average proportion of immigration species was 89.9%, which is quite high. The proportion was the highest in the Pyungrim dam (97.1%) and the lowest in the Gunwi dam (82.2%). In the case of the dams where the proportions of immigration species were relatively high, i.e., Pyungrim dam, Yeongju dam and Daegok dam, the relative numbers of planted species were low. In the case of dams where the proportions of immigration species were relatively low, climatic climax species, which are likely to appear when natural succession occurs, i.e., oak species, such as Quercus serrata Thunb. Ex Murray, Quercus aliena, and Quercus acutissima Carruth., and azalea species, such as Rhododendron mucronulatum Turcz. and Rhododendron yedoense Maxim. Ex Regel, were planted on the slopes of dams.

Exchange species are what appear on both the slopes of dams and in left or right forests, and non-exchange species are what appear only in one place, either on the slopes of dams, or in the left or right forests. The place where the largest proportion of exchange species appeared was the Boryung dam (72 taxa, 47.7%), and the place where the smallest proportion of them appeared was the Gunwi dam (45 taxa, 28.9%). At the Gunwi dam, the proportion of non-exchange species was 71.1%. The exchange of plants between the slopes of dams and left or right forests had not been active (

Table 5. Analysis of planted species and immigration species per dam.

	GP	JH	PR	YJ	GW	DG	KB	BR
Species of dam slope	77	121	70	67	73	80	106	109
- Planted	8	16	2	2	13	3	10	15
- Immigration	69	105	68	65	60	77	96	94
- Immigration ratio (%)	89.6	86.7	97.1	97.0	82.2	96.2	90.5	86.2
Total species	107	209	125	119	156	112	189	151
- Exchange (dam slope + left or right forests)	40	68	43	44	45	33	73	72
- Exchange ratio (%)	37.4	32.5	34.4	37.0	28.8	29.5	38.6	47.7
- Non exchange	67	141	82	75	111	79	116	79
- Non exchange ratio (%)	62.6	67.4	65.6	63.0	71.1	70.5	61.4	52.3

).

Among immigration species, Miscanthus sinensis, Clematis apiifolia, Humulus japonicus, and Equisetum arvense were observed at all of the dams, and all tended to appear in places where periodic and strong human interventions were made, such as vacant land, dumping grounds, waterfront areas of streams, secondary herb vegetation, etc. [49,50,51].

Other frequently observed species were those that appear in habitats where human intervention occurs, such as Lespedeza cuneata G.Don, Pueraria montana var. lobata, Oplismenus undulatifolius (Ard.) P.Beauv., Artemisia princeps Pamp., Erigeron canadensis, Erigeron annuus, Rosa multiflora Thunb., Stellaria media, etc.

3.2.2. Growth Level of Planted Trees on the Slopes of Dams

The overall growth level of the planted trees was analyzed by measuring the average height values of trees that dominated the vegetation structure for shrubs, small trees, and trees. There were no values on the growth of trees at the Pyungrim dam, Yeongju dam, Daegok dam, or Boryung dam, because there was either no life cycle, or there were no sufficient records on the original sizes of trees when they were planted (

Table 6. Analysis of average growing height of trees planted per dam.

		GP	JH	PR	YJ	GW	DG	KB	BR
Planting year		2006	2006	2007	2016	2011	2005	2014	2000
Species of planted tree		8	16	4	2	17	4	12	18
Species of existing tree		8	16	2	2	13	3	10	15
Average growing height of existing tree (m)	Shrubs	1.1	1.6		0.7	1		0.5	0.6
	Small trees	4.5	3.5			3.5			4.0
	Trees	7.2	6.6	6.0		6.5		5.1

).

While all of the tree species planted at the Gampo dam (eight taxa), Jangheung dam (16 taxa), and Yeongju dam (two taxa) were found to have survived, two to four taxa out of all of the planted species were missing at the Pyungrim dam, Gunwi dam, Daegok dam, Kimcheon Buhang dam, and Boryung dam. Pinus strobus, Symplocos sawafutagi Nagam., Ulmus davidiana Planch., and Juglans regia L. planted on the slopes of the Pyungrim dam, Gunwi dam, Daegok dam, Kimcheon Buhang dam, and Boryung dam were not observed when we investigated these dams.

The average growth heights of shrubs, small trees, and trees on the slopes of the eight dams were 0.5–1.6 m, 3.5–4.5 m, and 6.0–7.2 m, respectively, compared with the original heights when they were planted. The slopes where trees grew the most were those of the Gampo dam. There, while Quercus mongolica Fisch. Ex Ledeb. and Quercus serrata 2 m high were planted, they did not grow sufficiently compared with other trees, such as Pinus steraceae Parl., which had grown to 7.5–8.0 m.

The growth level of each species was determined by Pinus densiflora, Quercus spp., Prunus sargentii Rehder, Styrax japonicus Siebold & Zucc., and Cornus controversa Hemsl., et al. (

Table 7. Comparison of growth levels of major planted species with National Forest Resources Survey data.

Site	Years after Planting (year)	Species	Planting Size		Survey Size		Sample Tree’s Size of the National Forest Resources Survey (2011)
			H (m)	R (cm)	H (m)	DBH (cm)	Age (year) (mean ± S.D)	H (m) (mean ± S.D)	DBH (cm) (mean ± S.D)
GP	15	Pinus steraceae	2.0–2.5		10	17.4	32 ± 9.9	10.9 ± 4	19 ± 8.7
		Prunus sargentii	2		8	11.8	30 ± 9.7	9.6 ± 2.9	15.1 ± 6.1
		Quercus mongolica	2		3	1	38 ± 14.8	10.8 ± 3.1	16.5 ± 6.8
		Quercus serrata	2		3	6.7	31 ± 10.4	10.2 ± 3.3	15 ± 6.8
		Styrax japonicus	1.5		6	7.1	28 ± 8.1	7.8 ± 2.1	11 ± 4.1
JH	15	Pinus densiflora		8–24	8	22.8	35 ± 9.8	10.6 ± 3.3	18.8 ± 7.6
		Quercus aliena	1.5		8	18.1	29 ± 7.4	10.6 ± 3.2	15.1 ± 6.1
		Quercus serrata	0.5–0.5		7	19	31 ± 10.4	10.3 ± 3.3	15 ± 6.8
		Quercus acutissima	1.5		9	26.2	30 ± 8.5	12.2 ± 3.7	18.3 ± 6.9
		Quercus variabilis	1.5		7.5	21.3	34 ± 10.7	11.9 ± 3.5	17.5 ± 6.5
PR	14	Quercus serrata	3		9	17	31 ± 10.4	10.3 ± 3.3	15 ± 6.8
		Quercus acutissima	3		9	18.1	30 ± 8.5	12.2 ± 3.7	18.3 ± 6.9
YJ	5	Pinus densiflora		15–30	9	24.3	35 ± 9.8	10.6 ± 3.3	18.8 ± 7.6
GW	10	Pinus densiflora	0.5	5–15	7	15.8	35 ± 9.8	10.6 ± 3.3	18.8 ± 7.6
		Quercus variabilis	0.5	5–15	7	19.3	34 ± 10.7	11.9 ± 3.5	17.5 ± 6.5
		Quercus acutissima	0.5	5–10	7.5	17.6	30 ± 8.5	12.2 ± 3.7	18.3 ± 6.9
		Quercus aliena	0.5	5	7	16.9	29 ± 7.4	10.6 ± 3.2	15.1 ± 6.1
		Quercus mongolica	0.5		7	15.5	38 ± 14.8	10.8 ± 3.1	16.5 ± 6.8
		Quercus serrata	0.5		7	5.3	31 ± 10.4	10.3 ± 3.3	15 ± 6.8
DG	16	Pinus strobus			9	25	10	5.6 ± 2	9.8 ± 3.1
		Acer palmatum			2	2	10	4.3 ± 1.2	9.4 ± 3.3
BH	7	Quercus acutissima	3	10–25	7	19.1	35 ± 8.5	10.6 ± 3.3	18.8 ± 7.6
		Quercus aliena	0.5		6	17.7	29 ± 7.4	10.6 ± 3.2	15.1 ± 6.1
		Quercus serrata	0.5		6	21	31 ± 10.4	10.3 ± 3.3	15 ± 6.8
		Quercus variabilis Blume	0.5		6	17.7	34 ± 10.7	11.9 ± 3.5	17.5 ± 6.5
BR	21	Pinus densiflora		5–45	9	27.8	35 ± 9.8	10.6 ± 3.3	18.8 ± 7.6
		Pinus koraiensis		4–17	8	22.5	28 ± 11.3	10.8 ± 3.9	17.8 ± 8.2
		Castanea crenata		8–23	8	26.5	27 ± 8.2	9.8 ± 3.2	16.7 ± 6.9
		Prunus sargentii		5–20	9	32.2	30 ± 9.7	9.6 ± 2.9	15.1 ± 6.1
		Styrax japonicus		12	2	4.4	28 ± 8.1	7.8 ± 2.1	11 ± 4.1
		Cornus controversa		7	9	24.3	35 ± 11.8	11.6 ± 3.2	19.5 ± 7.6

H: height, R: root-collar caliper, DBH: diameter at breast height.

). In the case of Pinus densiflora, the average height and DBH of planted trees on the slopes of the Boryung dam, constructed 21 years ago, were 9 m and 27.8 cm, respectively. Those of the planted trees in the Jangheung dam, constructed 15 years ago, were 8 m and 22.8 cm, respectively. The corresponding values in the Yeongju dam, constructed five years ago, were 9 m and 24.3 cm, respectively. This was because relatively larger trees were originally planted on the slope. In contrast, on the slopes of the Gunwi dam, the average height and DBH of trees were the smallest of the eight dams, at 7 m and 15.8 cm, respectively, because smaller ones were originally planted on the slopes of this dam. Those trees can be regarded as growing well, compared with the average height of 10.6 m and DBH of 18.8 cm of 35-year-old standard pine trees in the National Forest Resources Survey [44].

Quercus spp. is a tree family that is commonly planted on the slopes of dams. In the case of Quercus mongolica, while it failed to grow well in the Gampo dam, it grew to 7 m in height and 15.5 cm in DBH 10 years after it was planted. Its growth was strong, considering that the average height and DBH of 38-year-old Quercus mongolica are 10.8 m and 16.45 cm, respectively, in the National Forest Resources Survey [44]. In the case of Quercus serrata, those at the Jangheung dam, Pyungrim dam, and Kimcheon Buhang dam grew to an extent that was similar to or exceeded the average height and DBH of 10.2 m and 15 cm, respectively, of 31-year-old Quercus in the National Forest Resources Survey [44]. Those at the Gampo dam and Gunwi dam did not grow much because those of smaller sizes were originally planted, and the environmental conditions there were not favorable. DBHs of the trees of Quercus aliena at the Jangheung dam, Gunwi dam, and Kimcheon Buhang dam were higher than the average DBH of 29-year-old Quercus aliena Blume in the National Forest Resources Survey [44]. Quercus variabilis Blume and Quercus acutissima also grew well.

The average DBHs of Pinus koraiensis Siebold & Zucc., Castanea crenata Siebold & Zucc., Prunus sargentii, Styrax japonicus, and Cornus controversa at the Jangheung dam all exceeded the DBH of standard trees in the National Forest Resources Survey [44]. At the Daegok dam, the average height and DBH of Pinus strobus were 9 m and 25 cm, respectively, both greater than the corresponding values of 5.6 m and 9.8 cm reported by Lee and Lee [45] for Pinus strobus 10 years after planting. However, Acer palmatum did not grow well, with an average height and DBH of 2 m and 2 cm, respectively.

4. Discussion

The number of plant species on the slopes of eight dams in Korea was comparatively higher than the corresponding numbers in the left and right natural forests with the same size area. The findings came from the fact that, while left and right forests usually tend to form forest communities resulting from the dominance of a single species, the slopes of dams can exhibit a variety of plant species for several reasons, including the increased variety of plant species under the tree canopy by the introduction of various planted species forming the tree canopy, increased opportunities for immigration species due to the wider area of the open canopy, the influx of unintentional species by the periodic intervention of humans, and so on [52,53].

Except for planted species, the most commonly discovered plant species on the slopes of dams were pioneer species, mantle vegetation species, and secondary herb vegetation species. The highest number of species discovered on the slope belonged to Asteraceae, which appear in barren environments, such as flood plains [49,54]. Over 30% of the species were of plant families discovered in cultivated lands, such as rice paddies, dry fields, and orchards, including Poaceae (34 taxa, 8.6%), Fabaceae (29 taxa, 7.3%), and Rosaceae (22 taxa, 5.5%). These species may have recently moved into the planted zone, been opportunistically germinated from buried seeds of embankment soil transported from nearby rice paddies and dry fields [49,54,55,56], or succeeded from continuous exchanges with nearby forests.

As in the embankment and artificial ground studies, as the slope ages and the area increases, the number of perennial tree species and perennial herb species tend to increase [54,57]. The number of annual herb species on the dam slopes was relatively higher than that in left and right forests. These results may be ascribed to the fact that the open canopy on the slope could constitute a conducive environment for herb species to take root and grow, as described in the study of Lee and Kim (1995) [52].

The total number of alien plant species on the slopes of eight dams in Korea is 60 taxa. Most of them are species that are strong in adjusting to environments, dominating in garbage fields and barren lands [58]. We found that the wider the area was, the more alien species there were. The introduction of alien plants to dam slopes can be affected by the frequency and extent of human intervention and by the proportion of canopy cover over the slope [59,60,61,62]. The number of alien plants can vary depending on when the dam was built and how it has been managed. In fact, the slope can be changed into an area that alien plants can easily invade. With rapid growth and strong physiological resistance, alien plants can endanger ecological statuses of native plants, alter the structure and the number of existing plant species [63,64], and reduce the biodiversity of species [65]. Relatively tall plants, by blocking sunlight from reaching the Earth’s surface, impede the growth of small herb plants, thus reducing biodiversity [53]. The cover proportion and height of alien plants constitute decisive elements to reduce equality and diversity of plant species [66]. Accordingly, to maintain species diversity and protect native species on the slopes of dams, it is necessary to both periodically and continuously monitor the plant situations of the slope. In particular, it is necessary to periodically manage naturalized plants and vines, and to form mantel communities on the border of the forest. In particular, a lot of climbing plants were found in eight dams. They grow rapidly and cover the surrounding vegetation, inhibiting the growth of other species and forming monotonous vegetation communities, thereby weakening biodiversity in the long term [67]. Therefore, it is suggested that physical, chemical, and biological removal of climbing plants is absolutely necessary.

The species diversity of dam slopes showed a tendency to be higher as the number of planted species increased and more time elapsed. As such, it will be effective to plant a variety of tree species at the initial stage in order to enhance the species diversity. In addition, trees being planted at an appropriate density to form a high-coverage canopy layer and a forest gorge forming on the edge of the dam slope can be advantageous in preventing invasion of alien species and maintaining ecological soundness.

We found that, among planted species and immigration plant species in the planted zone of the dams’ slopes, the average proportion of the immigration plant species was 89.9%, which is markedly high. The slopes with a relatively higher proportion of immigration plant species had fewer planted species than did other slopes. When climatic plant species were planted that might appear if natural succession occurred, the proportion of immigration species became lower. Immigration plant species that were frequently observed on the slopes of dams were Miscanthus sinensis, Clematis apiifolia, Humulus scandens, and Equisetum arvense. Most of these species primarily appear in habitats that are strongly influenced by periodic human intervention, such as vacant land, garbage fields, stream banks, and secondary grasslands, as shown in studies on rivers and fields [54,55,56].

When the growth levels of major trees planted on the slopes of dams—namely, Pinus densiflora, Quercus spp., Prunus sargentii, Styrax japonicus, and Cornus controversa—are compared with corresponding values of height and DBH of trees described in the National Forest Resources Survey [44,45], they are higher or similar, even if there are some species that did not grow well. This means that the planted zone of dams is a rich artificial ecological basis. The current level of tree growth does not affect the safety of the dam, but it is expected that trees will grow more vigorously in the future. This means that it is necessary to continuously check the growth and roots development of trees, as described in studies related to the safety management of dams [24,25,26,27,28,29].

However, the planted zones of dam slopes are artificially planted areas that are physically isolated from surrounding forests. As it is difficult to import outside materials, and the plants in the zone depend greatly on vegetation and soil in the zone, they are in danger of soil acidification and nutrition imbalance in the long term [18]. As soil acidification and nutrition imbalance can affect the growth and health of plants [68,69], it is necessary to periodically monitor and manage the soil environment as the basis of planting. In addition, to evaluate the sustainable functions of the planted zones of the slopes of dams as a habitat basis of life, we suggest that thorough research be carried out on the habitation and movement of wild animals on the slopes. Furthermore, there may be limits to continuous on-site investigations, because the planted zones of slopes of dams are located in various regions of the country and the area is extensive. Therefore, in order to collect and analyze data continuously on changes in flora characteristics, tree growth, and the impact on dam safety, it will be useful to adopt advanced monitoring technology such as forest landscape succession modeling, remote sensing, big data, edge computing, etc. [70,71].

5. Conclusions

In this study, we investigated the flora and species diversity of the planted zones on the slopes of eight dams in Korea, where various kinds of vegetation were introduced to harmonize the dams with their surrounding forests, and we analyzed the occurrence of alien species and the growth of planted species.

First, it seemed that on the dam slopes, various plant species have emerged through species exchange with forests on the left and right sides of the dams. Furthermore, buried seeds germinated from the embanked soil and plant succession occurred, increasing the species diversity. These findings indicate that the introduction of vegetation at dam sites, which are part of gray infrastructure, satisfies the intention to alleviate the anti-ecological effects of dams and secure their functions as green infrastructure. However, many invasive alien species are disturbing the ecological system, even if there are differences in the proportions of such species, depending on when dams were constructed and how they have been managed. As alien species can affect the vegetation structure of the planted zones, thereby weakening species diversity, it is necessary to monitor and manage plant species on the dam slopes.

Second, growth levels of trees on the slopes of dams were found to be similar to or higher than corresponding growth levels of the general forest environment, proving that the slopes of dams are suitable as the basis for the growth of plants. However, as the continuous growth of trees on the slopes can affect the safety and management of dams, it is necessary to balance ecological functions of the planted zones and safety of dams by performing appropriate pruning, thinning, and supplementary planting. Moreover, considering the physical environment of the planted zones of the slopes of dams, it is critical to consider the provision of soil nutrition and prevention of soil acidification on the condition of minimizing the effect of such measures on water quality.

These measures will add the function of green infrastructure to dams, gray infrastructure, and contribute to water resource management in which ecology and safety become harmonious. Based on scientific monitoring and effect tests, the findings of this study will help us improve eco-friendly management of existing dams. Furthermore, a new dam is needed, the findings of this study can provide evidence-based guidance for its eco-friendly planning and design.