Vegetation Changes and Management Plans for the Godeok Riverside Restoration Area along the Hangang River in Seoul

Study area

The study site was located in the 396th district of Godeok-dong, Gangdong-gu (Amsa Intake to Gangil I.C.), bordering the Hangang River to the north and Olympic Boulevard to the south, covering an area of 79,607 m² identified in a 2021 survey (Fig. 1). The sandbanks along the riverside have expanded since the early years, and have been used as a field post-embankment in the 1970s. The area’s average elevation exceeds 14 meters above the Hangang River surface, providing a significant elevation advantage. In 2001, monitoring was conducted to ecologically restore the site, leading to the implementation of a restoration plan based on these results. Subsequent restoration and construction efforts from 2002–2003 aimed to reestablish vegetation spatially. Planting for vegetation restoration was done by planting native plants in areas where naturalized plants dominate, such as existing bare fields and fallow fields, making it difficult to transition to native species. However, following the restoration in 2003 and 2004, the absence of proper management led to extensive shrub and herb mortality. Moreover, ongoing issues included persistent problems from prior cultivation activities and the proliferation of invasive dryland species such as Humulus japonicus and Aster pilosus. To address these challenges, the Seoul Metropolitan Government initiated private management of the ecological restoration site in 2005.

Fig. 1

Study site location map.

Methods

Identification of plants on the flora was carried out locally whenever possible, and plants that could not be identified were identified indoors after taking photographs and referring to the literature. Flora identification took place in 2003, 2005, 2010, 2015, and 2021, aligning with Lee’s studies (2003). Plants were organized using the National Standard Plant List of the Korea National Arboretum (2023) and Engler’s classification system (Melchior 1964), focusing on 360 Korean specialty taxa (Chung et al. 2017). Life history analysis was performed using Raunkiaer’s method (1934). The existing vegetation was also surveyed at the same intervals, plotting it on a 1:1,000 scale topographic map of Seoul according to the vegetation association of the dominant layers, whether woody or herbaceous. Existing vegetation maps and area ratios were prepared using ArcGIS Pro.

Flora

The trend of flora change was observed. The total number of taxa was found to have declined steadily, from 289 in 2003 to 163 in 2021. In detail, the following decline in taxa numbers was observed: from 71 species, 269 varieties, and 18 variants in 2003, to 57 species, 154 varieties, and 7 variants in 2021. Despite the overall decrease, the number of naturalized species remained relatively stable after a slight decrease in 2003. Four to six species continued to disrupt the ecosystem, while the number of protected species such as specialty plants fluctuated between 3–6 species. After an initial stabilization of vegetation in 2005, species composition remained relatively unchanged. In the past, vegetation restoration was performed by selecting plants suitable for riverside wetlands first, and carrying out vegetation restoration by green space (falling field, bare field, periphery of the levee, buffer vegetation). Vegetation restoration was carried out in the fallow land using shrub vegetation, in the bare land with shrubs and herbs suitable for flooded areas, and in the periphery of the levee using plants with strong adaptability to dry environments. Prior to restoration, disturbances from fallow land and soil excavation were prevalent. However, species diversity increased immediately after restoration, driven by the introduction of plants and the emergence of native and naturalized herbs from seed banks in the soil. Notably, therophytes such as Asteraceae Bercht. and J. Presl, Polygonaceae Juss, and Fabaceae Lindl initially surged following restoration but gradually declined thereafter (Fig. 2).

Fig. 2

Changes in flora in the ecological restoration area of the Godeok Riverside Ecological Restoration Area (2003–2021). M: Microphanerophyte; N: Nanophanerophyte; Ch: Chamaephyte; H: Hemicryptophyte; G: Geophyte; HH: Hydatophyte; Th: Therophyte.

In 2021, the flora survey revealed that Salix pierotii, a wetland tree, predominantly occupied the Hangang River Riverside area, while Miscanthus sinensis primarily covered the herbaceous layer. In several regions, species such as Humulus japonicus and Sicyos angulatus were observed. Along the riverbank, which is less influenced by the Hangang River water system, Salix pierotii, Rosa multiflora Thunb, and Spiraea prunifolia var. simpliciflora were predominant in the herbaceous layer, while species like Setaria viridis, Chelidonium majus var. asiaticum, and Galium spurium were also present. The count of naturalized species rose sharply to 45 in 2003 following ecological restoration, and has since shown a declining trend. Post-2010, the overall species count and the number of invasive species have largely stabilized due to the prevalence of environmentally adapted species. Species such as Sicyos angulatus, Ageratina altissima, Ambrosia artemisiifolia, Ambrosia trifida, Lactuca scariola, Aster pilosus, and Humulus japonicus are classified as ecosystem disruptors by the Ministry of Environment and require ongoing management to prevent unchecked growth.

Raunkiaer’s dormancy analysis of 289 taxa in 2003 revealed that 112 taxa (38.8%) were hemicryptophytes (H), 97 taxa (33.6%) were therophytes (Th), and 33 taxa (11.4%) were woody plants (M). In 2021, an analysis of 163 taxa showed 57 taxa (35.5%) as hemicryptophytes (H), 52 taxa (31.9%) as therophytes (Th), and 17 taxa (10.4%) as nanophanerophytes (N). The number of tree/subtree micro-phanerophytes (M) declined after 2003, increasing to 41 species in 2015, then dropping significantly to 16 species in 2021, likely due to the introduction of shrubby landscape species within the restoration area. The number of hemicryptophytes (H) has remained relatively stable after dropping from 112 species in 2003 to 58 species in 2005. It was observed that the soil in the area where Godeok Riverside Ecological Restoration Area was landfilled was drier, leading to a dominance of certain species and a simplification of vegetation along the Hangang River. The number of geophytes (G) has been consistent after a notable decrease from 23 species in 2003 to 15 species in 2005. Similarly, the number of therophytes (Th) remained steady after decreasing from 97 species in 2003 to 62 species in 2005, due to the lack of soil restoration, which resulted in various annual plants emerging from seeds in the soil. However, the prevalence of Riverside Salix pierotii and the influence of adapted species among the planted trees reduced the number of therophytes over time. The counts of other nanophanerophytes (N), chamaephytes (Ch), and hydrophytes (HH) remained similar to initial levels at the onset of restoration.

Regarding floristic changes, a diverse range of species increased initially with the introduction of new plant species at the commencement of the restoration. However, over time, the total number of species declined due to the expansion of adapted species, the decline of some species, and the death of non-adapted species. These changes in flora were attributed to variations in the deposition of organic matter in the soil, seed dispersal from upstream during flooding, seed dispersal caused by human activity, and the timing and methods of surveys. The physical structure and water system of the Godeok Riverside Ecological Restoration Site were not restored at the time of restoration, which means that ongoing management is needed to ensure the stable establishment and maintenance of planted species (Table 1).

Table 1

Changes in plant life style at the Godeok riverside ecological restoration area

Existing vegetation

In 2003, the vegetation area was restored by implementing a plan that resulted in 66.5% coverage, including 7.5% native wetland woodland species, 1.4% native wetland herbs, and 6.5% naturalized herbs. By 2005, the variety of woody plants and herbaceous vegetation in the restoration area had increased, leading to the formation of new plant communities and the decline of others. The most prevalent species were native dryland shrubs such as Rosa multiflora Thunb (17.0%) and Spiraea prunifolia var. simpliciflora (5.3%), constituting 28.3% of the vegetation, followed by native wetland herbs like Equisetum arvense (4.6%) and Alopecurus aequalis (2.5%), and 5.6% native dryland herbs including Bromus japonicus (3.8%). The prevalence of native wetland herbs slightly increased to 10.0%, while native dryland herbs remained stable at 5.4%. Naturalized herbs saw a minor reduction from the previous year due to management activities. In 2010, the proportion of native dryland woody plants decreased to 28.1% following reductions in Rosa multiflora Thunb (12.8%) and Rhus javanica (3.9%).

Dominance of native wetland herbaceous vegetation increased from 14.4% to 9.2%, primarily due to the growth of Persicaria thunbergii, which thrives in eutrophic areas. The dominance of naturalized herbaceous vegetation rose from 11.0% to 9.2%, influenced by Aster pilosus, which has proliferated alongside the construction of bike paths near levees. In 2015, the coverage of Rosa multiflora Thunb and Spiraea prunifolia var. simpliciflora remained stable at 8.3% and 14.1%, respectively. Other native dryland trees such as Prunus mume S. et Z. (1,798 m², 1.5%) and Quercus acutissima (2,513 m², 2.1%) also showed an increase. The proportion of invasive dryland woody plants grew from 5.2%, while native wetland herbs decreased slightly to 13.7%. Phragmites australis (4.8%) and Equisetum arvense (5.2%) experienced growth, but Persicaria thunbergii (0.7%) decreased. The prevalence of naturalized herbs (2.5%), including Erigeron annuus, Aster pilosus, and Humulus japonicus, was also diminished. In 2021, Salix pierotii, a native wetland woody plant species, covered an extensive area of 26,493 m² (33.3%). The community of Salix pierotii has been strengthening since the vegetation’s restoration, primarily along the riverside adjacent to the Hangang River. Salix pierotii is a representative tree species that grows along the waterside, and has been continuously gaining strength, so it was judged to be a tree species with high conservation value in the riverside wetland category.

Native dryland woody plants included Rhus javanica, Rosa multiflora Thunb, Spiraea prunifolia var. simpliciflora, and Prunus padus, with planted Rosa multiflora Thunb dominating at 9,518 m² (11.96%). Spiraea prunifolia var. simpliciflora and Prunus padus had the same distribution areas of 1,340 m² (1.68%) and 546 m² (0.69%), respectively. Five invasive dryland woody plants were found, including Ginkgo biloba, Magnolia denudata, Platanus occidentalis, Robinia pseudoacacia, and Eucommia ulmoides, with acacia occupying 2,598 m² (3.26%). Native wetland herbs included Miscanthus sacchariflorus (2.10%) and Equisetum arvense at 1,396 m² (1.75%). Five native dryland herbs were distributed in small clumps, comprising Humulus japonicus, Pseudolysimachion rotundum var. subintegrum, Aster koraiensis, Artemisia indica, and Carex miyabei. The proportion of Humulus japonicus changed from 1.8% (2003) → 0.0% (2005) → 3.2% (2010) → 0.6% (2015) → 0.0% (2021). This variation was attributed to the continuous removal of invasive species by a contracted management organization (Fig. 3, Fig. 4).

Fig. 3

Changes in existing vegetation at the Godeok Riverside Ecological Restoration Area. M: Microphanerophyte; N: Nanophanerophyte; Ch: Chamaephyte; H: Hemicryptophyte; G: Geophyte; HH: Hydatophyte; Th: Therophyte.

Fig. 4

Changes in existing vegetation at the Godeok Riverside Ecological Restoration Area (2003–2021).

The Godeok Riverside Ecological Restoration Area initially exhibited a simple vegetation type following its restoration in 2003. By 2005, however, some of the planted native dryland woody plants had taken root and established a community, leading to an expansion of the Salix pierotii community. Native wetland herbs have shown fluctuations since 2003, with an overall increasing trend. Naturalized herbs expanded in area in 2005 after the restoration, but subsequently decreased due to ongoing management. Naturalized plants affect the fragmentation of biological phases and the qualitative decline of biodiversity with their growth characteristics (Zerbe et al. 2004) that adapt to areas where superior fecundity and artificial disturbance continue to appear in poor environments (NIER 2000). Analysis of the dominant species in the existing vegetation revealed that planted shrubs and herbs were initially prevalent. The soil from the previous cropland was not removed during the restoration, allowing seed germination from the existing soil. Consequently, various annual and naturalized herbs emerged and competed with the planted species. Over time, the dominance of non-native species, annual herbs, and naturalized plants diminished as the Riverside Salix pierotii forest expanded and shrubs in the dry waterside area stabilized.

Plant ecosystem management

In 2002, the initial restoration goals for the Godeok Riverside Ecological Restoration Area aimed to enhance biodiversity through the restoration of vegetation and to offer a venue for ecological education to the public. The detailed plan involved preserving and restoring the wetland ecosystem, managing vegetation based on land use, dividing the preservation and restoration spaces, transforming the restoration area into a natural learning center, and implementing long-term monitoring. However, the plan had limitations, most notably the absence of designated wetland target species. Moreover, the restoration strategy retained the pre-existing embanked topography and water system, resulting in a limited implementation through vegetation restoration, pond creation, and the establishment of ecological experience facilities rather than through comprehensive structural restoration.

Main threats to the ecological restoration site along the Godeok Riverside Ecological Restoration Area included anthropogenic disturbances from the proliferation of invasive plants and increased recreational use. The Riverside area is vulnerable to the effects of the Hangang River’s rising water levels, the expansion of Salix pierotii, and colonization by Phragmites australis, Miscanthus sacchariflorus, Equisetum arvense, and other marshy native herbs. The waterside area has been impacted by the construction of bike paths, trails, and roads. The expansion of Salix pierotii is progressing. In some areas, horticultural species such as herbaceous plantings are maintained within the restoration area to manage the emergence of naturally occurring herbs. Native wetland vegetation around waterholes has developed, serving as producers within the wetland ecosystem. However, invasive species such as Ambrosia artemisiifolia, Ambrosia artemisiifolia, and Erigeron annuus have been introduced and require continuous removal. Additionally, seeds from former Ginkgo biloba and Eucommia ulmoides nurseries are spreading both within and outside the ecological restoration area.

The Salix pierotii vegetation area (zone I) was closed to minimize human interference, and a proposal was made to stabilize the Salix pierotii community in the undisturbed area through tillage and dimensional planting. The open space vegetation restoration areas on both the left and right sides of the seedbed were designated as a Salix pierotii induction area (zone II), in which maintenance of Salix pierotii was proposed to foster growth and protect the habitat of wild birds. The bare sites’ restoration area on the southern side was established as a Mugwort Induction Area (zone III) to promote mugwort and sagebrush communities through the intensive management of invasive species such as Glycine soja, Humulus japonicus, and Aster pilosus. The southern cultivated land restoration area was designated as an intensive shrub planting management area (zone IV), as invasive species consistently overwhelmed native plants. A management plan for invasive species was developed to preserve the planted shrub community. By dividing the management area into an intensive management area for managed plants (zone V), an induction area for native plants (zone VI), and a sedimentation area along the Hangang River (zone VII), we planned to promote wet vegetation communities around the embankment, Miscanthus sinensis communities along Godeok Stream, and wet vegetation communities along the Hangang River. Around the bicycle path, managed plants were periodically removed to prevent their spread and to create buffer vegetation zones such as Phragmites australis and Miscanthus sinensis (Fig. 5).

Fig. 5

Vegetation management plan for the Godeok Riverside Efcological Restoration Area.