Introduction
The Seoul Metropolitan Government enacted the “Seoul Metropolitan City Natural Environment Conservation Ordinance” in 1999, designating and managing ecological and landscape conservation areas. Following the designation of Bamsom Ecological and Landscape Conservation Area as the first such site in 1999, Baeksasil Valley, Seongnaecheon Stream, and Gwanaksan Buxus sinica var. insularis colony were designated on November 26, 2009, totaling 17 sites with an area of 4,961,571 m2 as of January 2022. Godeok-dong Ecological and Landscape Conservation Area was established on October 20, 2004, and has been recognized for its value as a riverine wetland with well-developed waterside grassland and bottomland forest. In 2007, it was expanded to incorporate Godeok Riverside Ecological Restoration Area and Hanam-si Riverside Area, covering a total of 320,377 m2. This Ecological Restoration Area was established in 2002 based on the plan created for the Godeok Riverside Ecological Park in 2001.
Three essential elements define a wetland ecosystem: biology, hydrology, and pedology (Cowardin et al. 1979). Wetlands have been considered in ecology as an initial phase of land’s transition in which organic matter accumulates, raising the bottom until it is no longer submerged, and eventually developing into a forested peak (Wilson and Loomis 1967). Wetlands are crucial natural resources, providing ecosystem benefits like climate change mitigation, water purification, flood control, and carbon fixation (Laiho 2006), and are habitats for a diverse range of flora and fauna. Recently, despite the continuous damage to wetlands from conversion to agricultural uses and the development of various facilities, the restoration and creation of wetlands of varying functions and scales are being pursued, driven by an increased recognition of their value that has boosted societal interest (Hong et al. 2018). Wetland restoration aims to return the functions and structures of original wetlands in degraded areas to their natural states, closely approximating or significantly improving their functionality. At the ecosystem level, restoration involves various measures to restore, recover, or regenerate the structure and function of degraded ecosystems (Bradshaw 2000), which may include removing invasive species or restoring hydrological and geomorphic conditions (Cronk and Fennessy 2001).
Restored and created wetlands in urban areas face multiple threats of degradation, such as habitat loss, pollution, altered hydrological regimes, and the introduction of invasive species. They are also susceptible to structural changes within a city (Ghosh and Das 2019; Alikhani 2023). Moreover, wetland area and functionality are in continuous decline. Identifying the reasons for this decline is challenging due to insufficient post-construction monitoring and a lack of adaptive management (Race and Fonseca 1996; Zedler 2000). Long-term monitoring at a single site provides detailed insights on species persistence and dominance throughout the transition process (Drury and Nisbet 1973; Connell and Slatyer 1977). Consideration of changes in vegetation structure from the outset of wetland restoration is essential. A detailed maintenance plan for critical factors like water flow and vegetation should be established, based on mid- to long-term monitoring results (Ministry of Environment 2020; Paul 2013). Various studies have investigated vegetation changes post-wetland creation and restoration (An et al. 2016; Kim et al. 2018; Wanek et al. 2022), and during transitions in wetland vegetation (Brinkmann et al. 2020; Son et al. 2015). However, there is a shortage of studies that formulate effective management plans from long-term monitoring data. This study was able to suggest an effective planting species selection and management method for riverside urban wetland restoration through the long-term monitoring of vegetation changes in the same area. Since plant ecosystems are clustered with plant species with different life histories, the main factors can vary depending on the transition process and external factors (human interference, flooding, typhoons, etc.). Effective natural ecosystem management can thus be achieved when vegetation changes and external factors are observed through the long-term monitoring of plant ecosystems. This study aims to identify trends in riverine vegetation ecosystem changes by monitoring flora and vegetation in the Godeok Riverside Ecological Restoration Area from 2003 to 2021, and proposes management strategies to enhance biodiversity.
Research Methods
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.
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.
Results and Discussion
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).
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).
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).
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).
Conclusion
Godeok-dong Ecological and Landscape Conservation Area is a segment of the Hangang River in which a natural seawall is preserved. This riverine wetland ecosystem boasts high biodiversity. Ecological restoration was conducted from December 2001 to April 2003, following the basic plan for the Godeok Riverside Ecological Park established in 2001. This study monitored changes in the flora and existing vegetation from 2003 to 2021 in the riverside and waterside areas within the ecological restoration zone. Based on these observations, detailed management plans were formulated to enhance the biodiversity of the plant ecosystem in the restoration area.
A variety of species were initially observed due to the introduction of new plant species at the outset of the restoration. However, as time progressed, the total number of species declined due to the expansion of species adapted to the target environment, the decline of some other species, and the death of non-adapted species. The Godeok Riverside Ecological Restoration Site did not have its physical structure and water system restored at the time of the restoration. Continuous management is essential to ensure the stable establishment and maintenance of planted species. The vegetation type was initially simple post-restoration in 2003, but by 2005, some of the native dryland wood species had settled and formed a community, increasing the area of the Salix pierotii community. Since 2003, the native wetland herbaceous vegetation has shown an overall increasing trend despite fluctuations. The area of naturalized herbaceous vegetation expanded after the 2005 restoration but has since decreased in intensity with continued management. Initially, planted shrubs and forbs were dominant, but the soil from the former cropland was not removed during the restoration, allowing seeds in the soil to germinate. Various annual and naturalized herbaceous species became established and competed with the planted species. Over time, as the strength of the riverside Salix pierotii forest increased and the shrubs in the dry waterside area stabilized, the dominance of non-native species, annual herbs, and naturalized plants decreased.
The management plan for enhancing biodiversity proposes managing the riverside Salix pierotii community inside the restoration area according to the monitored Salix pierotii growth and changes in understory vegetation. The waterside planting area, which is subject to disturbance and transitional vegetation changes, requires the introduction of native species, and the maintenance and removal of vegetation. In the primary disturbed area, the focus should be on conserving native plant vegetation and securing a buffer zone.
A limitation of this research is that it does not examine the current status of the base environment, topography, water system, and pedology, which are essential to identifying factors affecting the river plant ecosystem. Through such an examination, a comprehensive ecosystem management plan could be established that includes vegetation management, soil cultivation and improvement, while managing factors of anthropogenic disturbance.
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