Introduction

The riparian zone is a transitional area between the terrestrial and aquatic environments, which is crital for local and regional biodiversity conservation (Sargac et al. 2021). It is characterized by its complex and diverse habitat structures, playing a crucial role in the flow of nutrients and energy. Vegetation in the riparian zone is an important component of the riparian ecosystem, providing basal energy for the river ecosystem and playing a key role in the maintenance of riverine ecological function (Cornejo-Denman et al. 2018; Guo et al. 2021). The diversity of riparian vegetation is the basis for riparian zone ecosystem health and ecosystem services as it helps to regulate the local microclimate (Riis et al. 2020), to maintain the flow regime of the river (del Tanago et al. 2021), and to provide habitats for other biota (Epaphras et al. 2007; Bennett et al. 2014).

Previous studies have demonstrated that flooding regime, elevation gradient, soil organic matter, and nutrients all affect the composition and spatial distribution of riparian vegetation, determining the composition of plant communities, species diversity, and functional traits (Dwire et al. 2006; Sirolli et al. 2022). Among these, hydrology, including the frequency, depth and duration of inundation, is a vital factor in determining the characteristics of plant communities by directly influencing the growth, reproduction, distribution, and dispersal of plants (Zhu et al. 2017). However, due to rapid economic and social development in many catchments, industrial and domestic water demands have increased rapidly, leading to flow deficits and the associated impacts on riparian vegetation communities (Ellery Mayence et al. 2010; Chen et al. 2020; Shaeri Karimi et al. 2021). River drying can lead to decreases in the richness and abundance of helophytes and submerged plants, and increases in drought-resistant plants (Datry et al. 2014; Wassens et al. 2017). It also affects the dispersal and germination efficiency of plant reproductive bodies (Kong et al. 2021). Meanwhile, drying also affects the chemical properties of sediment and regional groundwater, which can further impact the riparian vegetation community (Yang et al. 2020; Ma and Ma 2022).

Environmental flow refers to the quantity, timing of river flow required to sustain aquatic ecosystems, with the aim of maintaining ecosystem integrity (Arthington et al. 2018; Mohan et al. 2022). Previous studies found that environmental flow can effectively improve plant diversity (Ahmadi et al. 2019), increase landscape heterogeneity and ecosystem complexity (Cao et al. 2023). It can also increase inundation extent, raise groundwater level, therefore, improve the soil water conditions for plant growth (Zhang et al. 2017; Chen et al. 2020). At the same time, environmental flow accelerates the exchange rate of water between atmosphere and sediment, improving the chemical properties of sediment, increasing plant height, coverage (Zhang et al. 2022) and biomass (Wang et al. 2018). For example, Gomez-Sapiens et al. (2020) found that the vegetation coverage in the riparian zone increased by 14% after environmental flow application (Gomez-Sapiens et al. 2020). Environmental flow can also decrease the concentration of pollutants and improve local water quality (Yang et al. 2022; Chen et al. 2023). Therefore, environmental flow is an effective way to restore riparian vegetation (Xie and Li 2022). However, environmental water allocation reduces the water availability of other users, it might also promote the dispersion of invasive species (Perkins et al. 2016), such as Juncus ingens (Vivian et al. 2014), Pacifastacus leniusculus (Mathers et al. 2020), Cyprinus carpio (Koehn et al. 2018), leading to changes in landscape patterns.

Therefore, this research aimed to address the following key questions: (i) how the vegetation community along the Yongding River changed after environmental flow application; and (ii) which environmental factors were associated with the changes in vegetation community structure before and after environmental flow application? The study could provide a scientific basis for improvements in Yongding River riparian vegetation management and urban watercourse management more broadly.

Methods

Study Area

Yongding River (Fig. 1) is about 865 km long, draining an area of 76,700 km2. The basin belongs to a temperate sub-humid and semi-arid continental monsoon climate, with an annual average precipitation is approximately 450 mm (Du et al. 2023). The length of the Beijing section is about 170 km, mainly flowing through the southwest and southern parts before entering Baiyangdian in Hebei Province. The section from Guangting Reservoir to Sanjiadian Dam, termed the mountain section, has a narrow river channel, and is differentiated from the section extending from South Six Ring Road to Qujiadian Reservoir, termed the plain section. Yongding River is an important ecological asset of Beijing. Due to the increased demand of water for production and domestic consumption, the Yongding River diversion canal was built in the mid-1950s, diverting almost all the water from the Yongding River above Sanjiadian into Beijing, as a result, the riverbed below Sanjiadian was dry for most time of the year. Consequently, the prevalence of riparian plant species along the Yongding River is gradually replaced by xerophytes, resulting in a reduction in groundwater levels, decrease of landscape heterogeneity, and degradation of the river ecosystem. In the downstream area, lack of aquatic plants, severe soil erosion, and increased sedimentation promote the infilling of the river. At the same time, riparian vegetation degradation weakens the water conservation capacity, resulting in increased risks of serious floods and droughts (Ren et al. 2021). To restore the damaged ecosystem, Beijing government initiated an environmental water project using reservoir water and recycled water to assist the recovery of the riverine ecosystem (Bai et al. 2023). In September 2021, the 865 km long river channel of the Yongding River achieved bank-full flow for the first time since the cutoff in 1996, and the full flow was maintained in spring and autumn in 2022. This study conducted riparian vegetation surveys at six sites in the mountain section (S1-S6) and seven sites in the plain section (P1-P7) before (from April 28th to May 2nd) and after (From June 28th to July 1st) spring replenishment of river channels (From May 1st to June 15th) in 2022 (Fig. 1; Table 1).

Fig. 1
figure 1

The distribution of 13 riparian vegetation community survey sites in Yongding River (Beijing section). S1-S6 are mountain sites and P1-P7 are plain sites

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Table 1 Riparian vegetation community survey sample point information
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Field Survey

Small-scale environmental flow application testing in the Yongding River proved that the approach effectively alleviated the water stress and improved the local hydrological conditions (Luo et al. 2019; Qin et al. 2019). The goal for the mountain section is to maintain a persistent river flow. In the plain section, the goal is to maintain the presentence of surface water for more than 200 days in a year with a combination of reservoir and local recycled water. As the targets and durations of environmental flow in the mountain and plain sections of the Yongding River varied, this study stratified sampling between these river segments. Our field surveys were used to estimate riparian vegetation community characteristics including species composition, abundance, and dominant species variation before and after an environmental flow release.

Because the environment is highly modified in Yongding River, we chose the display of transects and quadrats to make sure it covers the widest riparian area. As the width of riparian zone varies so we chose 2–3 transects at each site, and each transect has 3–6 quadrats. The transects with various lengths were set perpendicular to the riverbank (Table 1). The transects were at least 10 m apart. From the riverine area to the edge of the river, along these transects, 3–6 1 m × 1 m sample quadrats were set along the transect line. A GPS was used to record the latitude, longitude, and altitude of the center point of each quadrat. In total, across the 13 sites, 98 vegetation quadrats were surveyed before the environmental flow application and 110 quadrats were surveyed after environmental flow application due to differences in vegetation distribution along the Yongding River.

The species identity and abundance, total vegetation coverage, and mean height for each species in each quadrat were recorded. The above-ground parts of the harvested plants were brought back to the laboratory, oven-dried at 70℃, and weighed to calculate standing biomass. Each plant species in every quadrat was individually weighed for measurement. Soil moisture and redox potential were measured using a Delta-T Theta Probe sensor (UK) by inserting a probe into the surface to a depth of 10 cm. The surface layer soil (0–20 cm) was collected from each quadrat, air-dried in the laboratory, and then crushed and sieved. Total nitrogen (TN) and soil organic matter (SOM) of the soil were measured using an automatic soil carbon and nitrogen analyzer (Flash Smart, Germany). Total phosphorus (TP) of the soil was measured using the alkali-melting method and molybdenum-antimony spectrophotometry.

Data Analysis

The importance value of each species in the quadrat was calculated as follows:

Importance value = (relative height + relative frequency + relative coverage) / 3.

The plant community matrix was composed of species with importance values greater than 5% in the quadrat. Species with less than 5% importance value deemed to be of small ecological contribution (Dai et al. 2006; Rolecek et al. 2009).

Two-way indicator species analysis (TWINSPAN) was used to classify the samples and plant species. According to the principles of vegetation classification and field investigation, the vegetation types of indicative and dominant species were analyzed.

Species richness (\( S\)) is the sum of the number of species in the community.

Pielou’s index (\( J\)) indicates the evenness of the distribution of different species in the community, measured using biomass, coverage, or other indicators. It is an important indicator for measuring the evenness of the distribution of different species.

$$ J=\frac{H}{\text{l}\text{n}\left(S\right)}$$

\( H\) is the Shannon-Wiener diversity index.

The hazard level of invasive plants was divided into 7 levels (Level 1: malignant invasion; Level 2: severe invasion; Level 3: partial invasion; Level 4: general invasion; Level 5: to be observed; Level 6: recommended exclusion; Level 7: China domestic species) based on the Chinese Invasive Species Information System (http://www.iplant.cn/ias/protlist) (Hao and Ma 2023).

Based on the Bray-Curtis dissimilarity measure, the SIMPER (Similarity Percentage) analysis was used to identify riparian vegetation contributions to community dissimilarity before and after environmental flow application in Yongding River.

Canonical correspondence analysis (CCA) was used to analyze the relationship between plant abundance and environmental factors (soil moisture content, soil redox potential, soil total nitrogen, soil total phosphorus, soil organic matter). The main environmental factors affecting the spatial distribution of the community were determined by the Monte Carlo test (significance level 0.05), correlation coefficient and explanatory rate. Non-metric multidimensional scaling (NMDS) was used to analyze the differences in plant community composition before and after environmental flow application in mountain and plain sections of the Yongding River. Analyses were performed using R4.2.2 (https://www.r-project.org/) and Origin (https://www.originlab.com/).

Two-way indicator species analysis (TWINSPAN) was used to help identify and group riparian vegetation communities before and after environmental flow application in Yongding River.

Results

Riparian Vegetation Community Composition Variation Before And After Yongding River Environmental Flow Application

A total of 123 species of herbaceous plants were found in the riparian zone of the Yongding River before and after the environmental flow application, belonging to 43 families and 92 genera. Before the environmental flow application, 43 species of herbaceous plants were sampled in the mountain section of Yongding River, mainly composed of Asteraceae (10 species, 23.26%), Poaceae (3 species, 6.98%), and Potamogetonaceae (3 species, 6.98%). In the plain section before environmental flow application, plants such as Asteraceae (13 species, 28.26%).

SIMPER analysis showed the top 10 species contributing to the dissimilarity between mountain and plain section riparian vegetation communities were mainly hygrophytic plants and introduced Xanthium italicum, while after environmental flow application four submerged plants appeared to determine community dissimilarity (Table 2).

Table 2 Top ten contributors of individual species to Yongding River riparian vegetation communities’ dissimilarity before and after environmental flow application
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Poaceae (6 species, 13.04%), and Fabaceae (5 species, 10.87%) were predominant. After the environmental flow application, Poaceae, Asteraceae, Cyperaceae, and Potamogetonaceae were the main families found in both the plain and mountain sections.

In addition, 17 species of introduced invasive plants were found, belonging to 8 families and 14 genera (Appendix S1). Among them, 8 introduced invasive species were found before the environmental flow application, and 14 introduced invasive species were found after environmental flow application. Among all the invasive plants, 53% belonged to Asteraceae, 11% belonged to Poaceae. 29% were classified as level 1 malignant invasive, and 24% were classified as level 2 serious invasive. The level 1 malignant invasive species Ambrosia trifida and Conyza canadensis were found before and after the environmental flow application.

Riparian Vegetation Community Diversity Variation Before And After Yongding River Environmental Flow Application

Plant species richness in Yongding River slightly increased after environmental flow application, but the difference was not significant. The homogeneity of the mountain section was significantly increased after environmental flow application (Fig. 2a and b). In the plain section, the richness was significantly higher after environmental flow application, but the inter-site homogeneity did not show significant changes (Fig. 2c and d).

Fig. 2
figure 2

Richness variations (a) and Pielou variations (b) of riparian vegetation communities in the Yongding River mountain sections before and after environmental flow application. Richness variations (c) and Pielou variations (d) of riparian vegetation communities in the Yongding River plain sections before and after environmental flow application. Asterisk indicates significant differences among riparian vegetation communities

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Factors Determined Riparian Vegetation Communities Before And After Yongding River Environmental Flow Application

CCA results showed that the first axis described 30.25% of mountain section plant community structure variations before and after environmental flow application, which was mainly related to soil TP, soil TN, and SOM. The second axis explained 24.62% of the changes, which was mainly related to soil redox potential and soil moisture content (Fig. 3a). For the plain section, the first axis described 30.95% of the plant community structure changes, which was mainly related to soil redox potential and soil moisture content. The second axis explained 22.30% of the changes, which was mainly related to SOM and TP (Fig. 3b).

Before environmental watering soil moisture was associated with higher plant richness, but after watering this was a less important predictor of plant species richness, which was more strongly associated with nutrient level (TN and TP). The Monte Carlo test found that the dominant factors affecting the distribution of plant communities before and after environmental flow application in the mountain section were soil TN and SOM. For the plain section, environmental factors affecting plant communities before and after watering included soil redox potential, TN, and TP, but the effect was not significant (Table 3).

Fig. 3
figure 3

Canonical Correspondence Analysis of environmental factor and their relationship with richness variations of riparian vegetation communities in the Yongding River mountain section (a) and plain section (b) SMC - Soil moisture content, Eh - Soil redox potential, TN - Soil total nitrogen, TP - Soil total phosphorus, SOM- Soil organic matter

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Table 3 Monte Carlo test of environmental factors differentiating plant communities in the Yongding River mountain section and plain section before and after environmental flow application
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NMDS Analysis Of The Riparian Vegetation Communities Before And After Yongding River Environmental Flow Application

Based on the Bray-Curtis similarity coefficient, riparian vegetation communities NMDS analysis found there was little overlap in species prior to the environmental flow application and overlap increased after the environmental flow application indicating that the plant communities in the mountain and the plain sections tended toward homogeneity after environmental flow (Fig. 4a and b).

Fig. 4
figure 4

NMDS ordination plot showing the riparian vegetation community structure of the Beijing section of the Yongding River before and after environmental flow application. (a) Differences in riparian vegetation communities between the mountain section and the plain section before environmental flow application, and (b) differences in riparian vegetation communities between the mountain section and the plain section after environmental flow application. Points represent sample quadrats with the color indicating the type of each section

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TWINSPAN Classification of the Riparian Vegetation Communities Before And After Environmental Flow Application

In order to understand the assemblage structure of riparian vegetation communities before and after Yongding River environmental flow application 13 sample sites were analyzed using TWINSPAN classification (Fig. 5).

Fig. 5
figure 5

Results of TWINSPAN classification of recorded riparian vegetation communities on the Yongding River before (a) and after (b) environmental flow application

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The riparian vegetation communities before environmental flow application were divided into 5 groups by the hierarchical classification, with each group being named using the association method. The specific details are as follows (Table 4).

Table 4 Results of TWINSPAN analysis before environmental flow application
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After environmental flow application, the riparian vegetation communities could be classified into 7 groups, which are detailed below (Table 5).

Table 5 Results of TWINSPAN analysis after environmental flow application
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Discussion

There is growing recognition of the urgent need to restore aquatic ecosystems in the face of urbanization and environmental degradation (Geist and Hawkins 2016; Piczak et al. 2023; Stoffers et al. 2024). By assessing the effects of environmental flow application in a heavily affected river system, valuable insights can be gained not only for the restoration of Yongding River but also for informing future efforts to rehabilitate other rivers associated with large cities worldwide.

Impact of Environmental Flow Application On The Riparian Vegetation Community Compositions In The Yongding River

We found the dominant riparian vegetation in the Yongding River to belong to the families Asteraceae and Poaceae. Asteraceae, the largest family of angiosperms (Wang et al. 2022a), has adapted to riparian habitats through reproductive advantages and stress resistance (Liu et al. 2021; Zheng et al. 2021b). In Beijing wetlands, Asteraceae is widely represented and among the most common species (Li et al. 2022). The Poaceae family has a pervasive global distribution and holds a dominant position in several herbaceous vegetation communities worldwide (Moges et al. 2017; Zhong et al. 2021; Folega et al. 2023), including in hygrophytes communities (Li et al. 2006, 2019).

Before the environmental flow application, the dominant families in the mountain section were Asteraceae, Poaceae and Potamogeton. After the environmental flow application, Cyperaceae emerged as a co-dominant. The dominant families in the plain section before application were Asteraceae, Poaceae and Fabaceae, and after environmental flow application, Cyperaceae replaced Fabaceae as a co-dominant. Cyperaceae are the primary builders of wetland ecosystems, aquatic plants in lakes and rivers are predominantly classified under Cyperaceae (Ekoko et al. 2021). The family exhibits a high degree of diversity in reproduction and seed dispersal mechanisms. For instance, Cyperaceae species utilize various methods such as dispersal by birds, insects, and wind (Larridon et al. 2021). This diversity in reproductive strategies contributes to the widespread distribution of Cyperaceae following environmental flow application. Seasonal fluctuations in water levels facilitate the spread of Cyperaceae species (Wang et al. 2023). In our study, environmental flow provided suitable conditions for the growth and development of the Cyperaceae vegetation, for example allowing Cyperus glomeratus and Cyperus microiria to become dominant species in P5 and P6. The moisture in soil after environmental flow makes other species lose the competitive strength with Fabaceae (Gris et al. 2024). After environmental flow application, representation of submerged plants increased in the plain section of the river, including Potamogeton pusillus, Hydrilla verticillate and Myriophyllum spicatum. In addition to providing sufficient water resources for the plain section of the river, the environmental flow application also promoted the growth of aquatic plants by increasing the width and depth of the river (Ecke et al. 2016). Before the environmental flow application, there were 21 hygrophytes, and the number increased to 36 after environmental flow application. The number of aquatic plants increased from 9 species to 11 species.

The surveys identified a total of 17 invasive species, primarily belonging to the Asteraceae family, known for their high reproductive rates and extensive distribution, thus posing a significant threat to the ecosystem. Among these species, five are classified as malignant invaders. Bidens frondosa, a member of the Asteraceae family, exhibited the highest frequency in the surveys. With its large, lightweight seeds adorned with spiny tips, Bidens frondosa can rapidly disperse through water flow, wind, or attachment to animals and humans, maintaining germination viability for up to two years (Zhou et al. 2015). Similarly, Conyza canadensis, characterized by its small and abundant seeds, relies on wind dispersal for rapid expansion (Shah et al. 2014). Ambrosia trifida seeds primarily spread through road construction and vehicular traffic (Son et al. 2024), posing a significant threat to the density and diversity of the local seed bank (Wang et al. 2022b). Meanwhile, Cenchrus longispinus, highly tolerant to soil conditions, establishes dominant monospecific populations through seed reproduction (Wu 2023), whereas Ambrosia artemisiifolia, producing copious amounts of harmful pollen, quickly adapts to invaded habitats (Zhao et al. 2022).

The most widely distributed invasive species was the second-level invasive species Xanthium italicum, which also belongs to Asteraceae. It grows quickly, uses spiny seeds to spread by attaching them to human luggage, clothing, and animal fur, rapidly spreading to farther places as these carriers move and have strong adaptability, often forming a single plant community and out-competing local plants (Tai et al. 2016; Zhang et al. 2021). In the Yongding River, the lasting hydrological disconnection caused by urbanization and associated water abstraction may provide favorable conditions for invasive species (Wang et al. 2011; Xiu et al. 2016), though local environmental factors may also play a role. In the plain section of Yongding River, the riparian zone is close to a road with a large number of residents. Moreover, the river section is also associated with intensive construction projects and garbage dumping. These factors were conducive to the spread of introduced invasive plant seeds aided by human and vehicle traffic. These invasive plants can quickly occupy the habitat through rapid growth and reproduction, which has a negative impact on the stability of the habitat and the balance of the ecosystem (Rood et al. 2020; Zheng et al. 2021a). Therefore, it is necessary to consider the timing of environmental flow to prevent the reproduction and spread of invasive plants. In addition, water level fluctuations associated with the environmental flow may have introduced seeds of invasive species. This may be the reason why Xanthium italicum was widely distributed in the plain section after environmental flow application, and Bidens frondosa also appears in the mountain sections S1 and S2, where there were multiple interferences such as rubble piles, buildings, drainage pipes, and crops. Opportunities for the invasion of alien species are presented during the initial years of environmental flow application due to available niche space in the Yongding river riparian ecosystem, and therefore continuous monitoring is required.

Impact of Environmental Flow Application On The Vegetation Diversity Of Riparian Ecosystems In Yongding River

Biodiversity is an important factor that affects the functioning and dynamic balance of communities and ecosystems (Tilman et al. 2014). The richness directly reflects the number of species, and plant species richness was highly correlated with density (Wang et al. 2012). Homogeneity can also reflect the stability of the community succession process (Ding et al. 2018). After the environmental flow application, the inter-site homogeneity of plant communities in the mountain sections significantly increased. Environmental flow promotes hydrological connectivity, therefore riparian plant seeds can spread and diffuse through surface water flow (Jansson et al. 2005; Garssen et al. 2017, Zhang et al. 2023), more species spread out the riparian zone and increase plant diversity resulting the homogeneity of the whole riparian zone. At the same time, the increase in water flow in the plain sections after environmental flow application could promote the replenishment of the plant seed bank in plain area riparian soil, which may be beneficial to the growth and reproduction of local riparian vegetation (Pereira et al. 2021; Gaudichet et al. 2022). As a result, the species number after environmental flow application will be higher than before (Aronson et al. 2017). Periodic flooding is essential to maintain the stability and persistence of riparian vegetation communities (Richards et al. 2020; Fischer et al. 2021). However, due to the short duration of flooding in this study, the impact to plant community was limited, suggesting that moderate inundation is needed for successful restoration (Song et al. 2024).

Environmental Factors Influence On Riparian Vegetation In The Yongding River

Soil moisture content in the mountain section was higher than in the plain section at both surveys. After the environmental flow application, plant richness was significantly increased in the plain section. Compared to continuous environmental flow in mountain sections of Yongding River, intermittent water replenishment and lack of water may still be the main determinants of riparian plant diversity in the Plain section. Altitude is also an important factor in plant growth and distribution, the growth of wetland plants at low altitudes is limited by flood hazards, while the growth of plants at high altitudes may be limited by soil nutrients (Lan et al. 2021). The averaged soil TN, SOM were slightly higher in the mountain section than the plain section, while TP content in soil was on the contrary. Soil nutrients in the mountain section showed no significant difference, while P5 and P7 had the highest TN content, triple that of the P1, P2, P3 TN content. P3 had the highest TP content, which may influence hierarchical classification of plant community in Yongding River. The CCA results indicated that the main factors affecting the plant diversity in the mountain sections before and after environmental flow application are total nitrogen (TN) and soil organic matter (SOM). SOM is regarded as an important indicator for judging soil fertility, as it reflects the availability of plant nutrients that are essential for promoting plant growth and development. Highly fertile soils stimulate plant development but also increase plant biomass (Welch et al. 2006; Charles et al. 2023). The vegetation in mountainous areas is mainly dominated by TN and SOM. However, the standard deviation of environmental factors in plain section is smaller compared to mountainous section, thus the environmental factors we measured cannot explain the differences in communities. Different species respond differently to changes in soil environment, for example, Nymphaea. spp. is most influenced by soil moisture content and soil redox potential, while Potamogeton pusillus increases with the increase in soil total nitrogen and soil organic matter content. In the mountain area, topographic variation is higher, and the variation of TN and SOM is also higher than in the plain area. These may cause the increase of the coverage of Potamogeton pusillus.

NMDS results showed that the similarity of riparian vegetation communities between the mountain and plain sections increased after environmental flow application. Similarity increase is caused by environmental flow application dispersal of increased plant diversity and promote germinations from seed bank (Pereira et al. 2021), Providing environmental flow could enhance diversity. Hydrological connectivity made plant communities in the mountain and plain sections become more similar which may be caused by river flow bringing plant seeds and propagules from upstream to downstream, enriching the plant species in the plain, promoting vegetation restoration, and benefiting river restoration (O’Donnell et al. 2016; Richards et al. 2020; Zheng et al. 2021a). Similar species before and after environmental flow application include Xanthium italicum Moretti, Potamogeton pusillus, Imperata cylindrica, Phragmites australis, Humulus scandens.

Conclusions

Future management strategies should prioritize efforts to control invasive species, acknowledging the role of environmental flow in facilitating their spread. The timing of environmental flow should consider the growth and reproduction time of invasive species to prevent their widespread dissemination, while also paying attention to the rate of environmental flow to provide suitable environmental conditions for the seed bank of riparian vegetation. Long-term monitoring later would be more conducive to a comprehensive understanding of the impact of environmental flow on vegetation in the Yongding River.