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Plant trait-based analysis reveals greater focus needed for mid-channel bar downstream from the Three Gorges Dam of the Yangtze River
Ecological Indicators 111 (2020) 105950
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Plant trait-based analysis reveals greater focus needed for mid-channel bar downstream from the Three Gorges Dam of the Yangtze River
T
Xuemei Yia, Yuanyang Huanga, Maohua Maa, Zhaofei Wena, Jilong Chena, Chundi Chenb, ⁎ Shengjun Wua, a b
Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, China Tongji University, Shanghai, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Dam Riparian zone Plant community Species richness Functional diversity
As a transitional zone between terrestrial and aquatic ecosystems, riparian ecosystems are critical in maintaining the health of river-ways and watersheds, and particularly sensitive to changes in hydrological regimes. Effects of dams on riparian zone are widely addressed, but less attention focus on riparian plant community of bar habitats. The Jingjiang reach downstream of the Three Gorges Dam has undergone significant changes of water and sediment condition and local base-level due to the Three Gorges Dam. To address responses of riparian plant communities to dams, the riparian plant communities of the mid-channel bars and the point bars in the Jingjiang reach were examined with a trait-based analysis. The results showed a lower species richness and a higher risk of invading by invasive species for mid-channel bars comparing with point bars. Moreover, the species richness of both the mid-channel bars and the point bars were observed increasing with the distance increasing from the dam, but the increasing rate of mid-channel bars was much smaller than that of point bar. Furthermore, functional redundancy of the mid-channel bar represented by the difference between functional diversity and species richness was marked lower than that of the point bars. Thus, we conclude that affected by the regulation of the dam, the mid-channel bars seem to be more hydrologically susceptible and harsher habitats for the assembly of plant community comparing with point bars. Therefore, more research of mid-channel bar downstream the dam were suggest and different emphasis of research and protecting management are needed for different bar types.
1. Introduction Affected by seed bank, damming and geomorphology (Hillebrand and Matthiessen, 2009; Kominoski et al., 2013), the riparian vegetation play key roles in functioning of freshwater ecosystems (Hladyz et al., 2011), delivering provisioning service and supporting service (Woodward et al., 2012). The interactions between river’s hydrological processes and ecological processes of surrounding riparian vegetation provide crucial information for environmental and management implications (Naiman and Decamps, 1997; Rood, 2006). As ecotones between aquatic and land systems (Perucca et al., 2007), the riparian ecosystems, consist of diverse, dynamic and complex habitats, are facing frequent shift of climate change, human activities and reservoir regulation. The response of plant communities to environment changes (Suding et al., 2008; Asner et al., 2014) and habitat gradients (Biswas and Mallik, 2010) have been investigated via a series of functional diversity indices which showed that the species composition and the
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functional diversity of plant communities are rapidly changing in responding to the changing factors. Due to dam operation, the ecosystems behind the dam became the most sensitive area against hydraulic changes (Majerova et al., 2015). Dramatic changes caused by river flow regulation have strong effects on biota (Bejarano and Nilsson, 2018), which is an important driver in shaping riparian communities in downstream ecosystems. Such changes in flow regime may lead to fragmentation of riparian forests, decline of native vegetation, substitution by exotic species and vegetation encroachment (Bejarano et al., 2011). Plant development is promoted or disfavoured, some species invade and others go extinct due to differing abilities of species to tolerate hydrological alterations, which in turn leads to a non-random sorting of species along hydrological gradients (Mouillot et al., 2013). The serial discontinuity concept (SDC) was proposed to describe the natural continuum disrupting of river by dams (Ward and Stanford, 1983). Few studies have examined the recovery gradients which indicate reduced species richness downstream the dam
Corresponding author at: Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China. E-mail address: [email protected] (S. Wu).
https://doi.org/10.1016/j.ecolind.2019.105950 Received 29 March 2019; Received in revised form 13 November 2019; Accepted 18 November 2019 1470-160X/ © 2019 Elsevier Ltd. All rights reserved.
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(Ellis and Jones, 2013; Saulino et al., 2017; Guzy et al., 2018). But the riparian zone plants have not been well tested (Ward and Stanford, 1995; Kingsford, 2000). Remarkable channel degradation has occurred in the Jingjiang Reach, caused by significant changes of water and sediment conditions and local base-level since the operation of the Three Gorges Dam (Xia et al., 2017a,b; Li et al., 2018). This has both direct and indirect effects on bar habitats which are highly sensitive to the changes in water and sediment regime (Chen and Chau, 2016). Bars are typical landforms constructed by fluvial process as riparian zone along low-gradient accretional reaches (Hupp and Osterkamp, 1996; Steiger and Gurnell, 2003; Dong et al., 2012). They are depositional features that form in river channels and are important in the morpho-dynamics of the river (Hauer et al., 2015). In the middle reach of Yangtze River, two common types of bars are point bars and mid-channel bar. The most distinguishing feature between them is their relative location to channel. The point bars are on the edge of the channels, and the mid-channel bar are in the middle of the channels. However, few studies have been conducted to examine downstream bar habitats downstream the dam, considering the importance of this process. Aim to understand riparian ecosystems downstream from the dam, we focused on the point bars and the mid-channel bars distributed along Jingjiang Reach downstream the Three Gorges Dam. We analyzed the taxonomic indices and the functional diversity of the riparian plant communities surveyed on the two types of bars. By comparing the differences of riparian plant communities between the mid-channel bars and the point bars, we explored the riparian plant community response to the downstream habitat changes caused by the Three Gorges Dam.
Table 1 Sites properties. Site name
Bar types
YZB GZ SZK DSZ ZJK HJZ TPK SBT XTZ WXZ NXZ YJC
Mid-channel Mid-channel Point bar Mid-channel Mid-channel Mid-channel Point bar Point bar Point bar Point bar Mid-channel Point bar
bar bar bar bar bar
bar
The distance to dam (km)
Texture type
50.22 108.55 118.74 129.19 143.86 157.89 177.91 183.73 186.46 203.31 211.24 229.99
Gravel Sand Clay Gravel Sand Sand Gravel Clay Gravel Clay Sand Clay
between the central and north subtropical zones with a subtropical humid climate. The annual temperature in this area is between 16.5 and 19 °C and the annual precipitation is about 1100 mm. About 80% of the annual precipitation take place in summer months between April to October, with the daily temperature between 28 and 30 °C. In the winter months, the daily temperature is between 3.4 and 7.2 °C, and only 20% of annual precipitation occur. 2.2. Study design Field work was done in July 2015, November 2015, June 2016 and October 2016. We collected data from the riparian zone with elevation below the bankfull-stage and experienced inundation in summer every year in the downstream area of the Three Gorges Dam. A total of 12 sites (Fig. 1) and 67 quadrats of the riparian plants were investigated. For each site, Two types of habitats were identified by the geomorphic trait, namely point bars located at river banks (6 sites, black dot on the map) and mid-channel bars located in the middle of the River (6 sites, red dot on the map). Distance from the Three Gorges Dam of the sites were list in Table 1 with the texture type. At each sampling site, three
2. Material and methods 2.1. Study area The sampling area is located in Jingjiang reach, downstream of the Three Gorges Dam on the Yangtze River (Fig. 1). This area locates
Fig. 1. Location of the study sites. 2
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quadrates (1 m × 1 m) were surveyed in perpendicular. All plant species were inventoried except for moss and lichen which were scarce in each quadrat. The plant species were classified into four types with the distribution and the function to the ecosystem, which include native species, alien species, invasive species and endemic species. The area cover, height and the number of each plant species in each quadrat were recorded. Plant species richness with different life histories (perennial species and annual or biennial species) of bars were used to testify the recovery rate of the bars downstream the dam.
Table 3 Plant species in two types of bar. habitat type
Species numbers
Alien species
Invasive species
Endemic species
Point bar Mid-channel bar
144 71
24 13
5 4
– 3
Gramineae, Labiateae and Polygonace were the four most abundant plant families, contributing 43.3% in the plant species. 24 non-native species with 5 invasive non-native species were found on the point bars, while on the mid-channel bars the numbers are 13 and 4 respectively. Three species on the mid-channel bars were not observed on the point bars, including Salix variegata Franch., Myricaria laxiflora (Franch.) P. Y. Zhang et Y. J. and Ranunculus sceleratus Linn.
2.3. Data analysis Species richness which have been used as the main measure in most studies (Petchey and Gaston, 2002; Petchey et al., 2007; Grossman et al., 2017) linking species diversity to functional diversity and Pielou’s evenness (Smith and Wilson, 1996) ranges from zero to one with low values indicating more skewed abundance distributions were used to measure the community diversity. To assess the relative significance of variation in species richness and variation in species evenness for functional diversity within the concept of functional redundancy, Shannon-wiener index (Shannon and Weaver, 1950) and Simpson’s diversity index (Magurran, 2004) were also used. To assess the integrated function of plant communities, traits considered to determine the role in ecosystems, recognized as key supporter for ecosystem functions and associated ecosystem services (DÍAz et al., 2006) were selected in this study. These included traits such as dispersal type, growth form, life cycle, shoot height and flowering phenology (Table 2). The traits were obtained from literatures which applied in our study area or where the climate is subtropical. Functional diversity summarizes the variation in trait values of an organism that potentially affects its performance, fitness and ecological function within a community (Violle et al., 2007). We calculated Rao's quadratic diversity index Q (Rao, 1982; Botta-Dukát, 2005) for functional α diversity, which takes the differences between species pairs into account. For understanding the consequences of disturbance on the functioning of a given ecosystem (Sasaki et al., 2009), the relationship between community diversity and functional diversity was tested by three models, namely linearly, exponential and sigmoid logistic models (Sasaki et al., 2009). Akaike’s information criterion (AIC) was used to rank models in terms of their support by the data. The model with the lowest AIC (Walker, 1992; Naeem, 1998; Johnson and Omland, 2004; Micheli and Halpern, 2010) was selected.
3.2. Plant community diversity index Differences in taxonomic indices and functional diversity were detected in the different two habitat types (Fig. 2). Species richness of the point bars was 15.5% higher than that of the mid-channel bar, and having a wider variation range. Shannon-wiener index and Simpson diversity in point bars were higher than that of the mid-channel bars, but with smaller standard deviation. Species composition in the midchannel bars varied more significantly and had a relatively higher evenness than point bars but a lower functional diversity. 3.3. Species richness downstream from the dam Several linear relationships between the species richness and the distance from the dam of the bars were observed. One positive linear relationship is observed when considering the species richness of all the species on all the bars in the two bar types (Fig. 3a). Another positive linear relationship is observed when only considering the species richness of all annual or biennial plants (Fig. 3c). Furthermore, the positive linear relationship was found when considering the species richness of all the species on the point bars (Fig. 3b). As the distance increasing, species richness of mid-channel bar showed a linear decreasing but not significantly trend with the highest value observed at Guanzhou (Fig. 3d). Species richness of two types of bars showed a similar increasing trend but a sharper slope increasing was observed in the point bars than that of mid-channel bars. 3.4. Relationship between diversity index and functional diversity
3. Results The functional diversities of the two bar groups were detected to be linear or exponential related with the taxonomic diversities. When indexing the taxonomic diversities by species richness, the relationship was linear on the mid-channel bars while exponential on the point bars (Fig. 4a, b). When indexing by Shannon-winner index and Simpson
3.1. Plant species A total plant species of 144 were recorded in the point bar group and of 71 in the mid-channel bar group (Table 3). Compositeae, Table 2 Plant functional traits and ecosystem processes and service. Trait Reproductive traits Dispersal type Vegetative traits Growth form Life cycle Max. height (m) Phonological traits Flowering phenology
description
Measurement scale
Ecosystem services
Three types according to van der Pijl (1982): 1: anemochory; 2:endozoochory & exozoochory; 3: ballistochory; myrmecochory; unassisted.
nominal
Regulating service
Two classes: Graminoids and Herbs Annual; Biennial and Perennial Five classes modified from Hodgson et al. (1995): 1: < 0.1 m; 2: 0.11–0.29 m; 3: 0.30–0.59 m; 4: 0.600.99 m; 5: 1.0–3.0 m
Nominal Nominal Ordinal
Supporting service Supporting service Supporting service
Five classes according to Mabry et al. (2000): 1: spring flowering; 2: mid-summer flowering; 3: spring-midsummer flowering; 4: fall flowering; 5: mid-summer-fall flowering
Nominal
Regulating service
3
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Fig. 2. Biodiversity index of two types of bar.
Fig. 3. Species richness of bars with the distance downstream from dam. 4
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Fig. 4. The relationships between diversity index and functional diversity.
The annual and biennial species were easily affected by the erosion regime caused by the dam (Gilvear and Willby, 2006). Therefore the recovery trends of species richness of annual and biennial species was observed as prediction. We infer that the annual or biennial species an important indicator for riparian zone plants community response to dam construction. The higher recovery rate of point bar between species richness and downstream distance to dam indicate that the point bar benefited more from the distance increasing to the Dam. Comparing with the annual or biennial species, perennial plants have more resistance to the erosion regime caused by the dam. Two shrub plants, i.e., S. variegata, M. laxiflora, were only observed on the mid-channel bars as main species, especially on the bars in the upstream reach. Therefore, when considering all species, the recovery trend of the species richness was not found on the mid-channel bars. The river regulation significantly affected the viability of S. variegata and M. laxiflora which caused low seedling production and population growth (Bao et al., 2010; Ayi et al., 2016; Chen et al., 2019b). As time goes by, with the shrub species’ declining, more attention is needed for the plant communities of the mid-channel bars.
diversity, the relationship was positive linear on both types of bars (Fig. 4c–f). When indexing by evenness, the relationship was negative linear on both types of bars (Fig. 4g, h). 4. Discussion Downstream from the Three Gorges Dam, riparian plant communities of the two habitat types, point bar and mid-channel bar, are most directly influenced by hydrological regimes triggered by the dam operation. 4.1. Plant species and bar types Edge effects apply to the plant community of both types of bar habitats adjacent to the watercourse. The point bars neighbored river banks and terrestrial lands, while the mid-channel bars neighbored high lands and disconnected to terrestrial lands. The species pool of neighboring terrestrial lands can be an important source to the point bars. As the transition zone, the seed bank and the edge-effect allow the point bars to maintain higher richness by the habitat’s nutrient heterogeneity, while evenness was restricted (Biswas and Mallik, 2010). Due to connectivity to mainland, anthropological disturbance from complex nearby land-use could be another key factor in shaping plant community in the point bar habitats. Since disconnected to terrestrial lands, flood events and fluvial maintenance operations homogenized initial conditions of mid-channel bar in terms of topography and grain-size distribution (Wintenberger et al., 2015), which made the mid-channel bars a lower species richness but a higher evenness. Riparian zones are well known to be highly invaded by non-native plant species and may facilitate the spread of non-native species across the landscape (Chen et al., 2019a). A positive correlation was reported between native species richness and non-native species richness (Cleland et al., 2004; Xiong et al., 2018). This phenomenon has been identified in our study area. The non-native species have similar reachability on two types of habitats by water flow and wind, especially for the seed plants. Stresses and human disturbances changed the habitats and inter- and intra-specific interaction of plants on the bar, make it easier for the non-native species establishing and spreading. Stressed system reduce the biodiversity, increase smaller, short-lived opportunistic species but also increase the dominance of exotic species (Naiman et al., 2005). Located in the middle of the river, a lower species richness and a higher ratio of non-native invasive species to total species indicate that mid-channel bars had been affected more from the stresses and will be more affected by invasive species.
4.3. Mid-channel bars: harsher habitats for plants Through the relationships between the functional diversity and taxonomic indices, the mid-channel bars would be harsher habitats for plant community comparing with the point bars. The functional diversity was found exponentially related to species richness on the point bars, while linearly on the mid-channel bars. It can be inferred that with the increasing of species richness, the functional redundancy increased on the point bars but kept consistent on the mid-channel bars. In other word, the ecosystem functioning were relatively robust with respect to changes in taxonomic diversity (Mayfield et al., 2005; Sasaki et al., 2009) on the point bars, but a relatively high turnover rate, low recovery rate and low stabilization to changes in taxonomic diversity on the mid-channel bars. The functional diversity was found linearly and negatively related to the evenness on the bars, and the functional diversity decreased faster with the increasing of evenness on the mid-channel bars comparing with that on the point bars. As an important component of community response to environment changes (Kardol et al., 2010), a higher evenness was linked to high-stress (Drobner et al., 1998), because uniformly barren landscape reduced the heterogeneity (Tilman, 1982; Levy et al., 2019). A negative relationship between evenness and functional diversity was suspected in high-stress ecosystem (Sasaki et al., 2009). The functional diversity kept being lower on the mid-channel bars with higher evenness and a steeper relationship between evenness and functional diversity because of the habitat homogenization induced by the erosion regime caused by the dam. Therefore, the mid-channel bars were indicated as a higher stressed habitat for riparian plant communities comparing with the point bars.
4.2. Serial discontinuity concept confirmed Dam disrupt the natural continuum in uninterrupted river systems from headwaters to downstream (Guzy et al., 2018). The serial discontinuity concept predicts a downstream recovery gradient of biodiversity with the distance from dams (Ward and Stanford, 1983), because of the downstream recovery trend of habitat from the dam disruption (Merritt and Wohl, 2006; Ellis and Jones, 2013; Guzy et al., 2018). In this study, the recovery trends of species richness of annual and biennial species were found on both the point bars and mid-channel bars but with different increasing rate. The recovery trend of the species richness of all species was also found on the point bars, but not found the mid-channel bars. Instead, a slightly degradation trend of species richness of all species exhibited on the mid-channel bars. The Three Gorges Dam induced a dramatic decrease in the sediment supply, making the downstream Jingjiang reach an erosion reach (Hu et al., 2009). A relatively weak riverbed erosion was observed on the downward reach comparing with the upward reach (Luo et al., 2012).
5. Conclusions Downstream riparian ecosystems from dams became the most sensitive area against the water and sediment regime regulated by dams (Bejarano et al., 2018). Remarkable channel erosion has occurred in the Jingjiang reach downstream from the Three Gorges Dam on Yangtze River. Mid-channel bars and point bars are two most sensitive geomorphology unit to the channel erosion. How the riparian plant communities on the bars respond to the erosion regime induced by the Three Gorges Dam is the key for the protection of downstream geomorphology and riparian ecosystem. This study designed a field work from 2015 to 2016, consisting of four plant community survey on 12 bars, including 6 mid-channel bars 6
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and 6 point bars. The taxonomic diversity and functional diversity of the plant community of the bars were further analyzed. Through the analysis, it is found that the riparian habitats were stressed by erosion regime on the both types of bars, with a higher stress on the bars in upstream reach and lower stress on the bars in downstream reach. It is also found that the mid-channel bars has lower plant diversity comparing with the point bars because of the disconnecting with species pool on the terrestrial lands. Additionally, two perennial plants only observed on the mid-channel bars, i.e., S. variegata, M. laxiflora enhanced the habitat resistance to the erosion regime, but the declining of the two plants may further decrease the habitat resistance. Therefore, a long-term study of the riparian plant community on the bars to clarify the assembly mechanisms affected by the Three Gorges Dam is encouraged for further study. Especially, as a harsher and declining habitat for riparian plant community, the mid-channel bars worth an urgent and greater focus for the protection of downstream geomorphology and riparian ecosystem.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Xuemei Yi: Conceptualization, Methodology, Writing - review & editing. Yuanyang Huang: Visualization, Software, Data curation, Writing - original draft. Maohua Ma: Writing - original draft. Zhaofei Wen: Data curation, Formal analysis. Jilong Chen: Data curation, Writing - original draft. Chundi Chen: Data curation, Writing - original draft. Shengjun Wu: Supervision, Writing - review & editing. Acknowledgment We thank Xiaoxiao Wang, Yi Jiang, Qiao Chen, Yu Wang, Xiaoxiao Tong, Hui Cheng and Enpei He for field assistance. This study was supported by the National Natural Science Foundation of China [51709250; 51779241; 41601042]. Data accessibility Data are available as the electronic Supplementary material. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ecolind.2019.105950. References Asner, G.P., Martin, R.E., Tupayachi, R., Anderson, C.B., Sinca, F., Carranza-Jiménez, L., Martinez, P., 2014. Amazonian functional diversity from forest canopy chemical assembly. Proc. Natl. Acad. Sci. Ayi, Q., Zeng, B., Liu, J., Shi, S., Niu, H., Lin, F., Zhang, Y., 2016. Seed sojourn and fast viability loss constrain seedling production of a prominent riparian protection plant Salix variegata Franch. Sci. Rep. 6, 37312. Bao, D.C., Zhi-Jun, L.U., Jiang, M.X., Shao-Dong, X.U., Yao, Q., Liu, Q.F., Wang, Q., 2010. Population structure and dynamics of Remanent Myricaria laxiflora downstream from the Three Gorges Dam. J. Wuhan Bot. Res. 28, 711–717. Bejarano, M.D., Nilsson, C., 2018. Riparian plant guilds become simpler and most likely fewer following flow regulation. J. Appl. Ecol. 55. Bejarano, M.D., Nilsson, C., Tánago, M.G.D., Marchamalo, M., 2011. Responses of riparian trees and shrubs to flow regulation along a boreal stream in northern Sweden. Freshw. Biol. 56, 853–866. Bejarano, M.D., Nilsson, C., Aguiar, F.C., 2018. Riparian plant guilds become simpler and most likely fewer following flow regulation. J. Appl. Ecol. 55, 365–376. Biswas, S.R., Mallik, A.U., 2010. Disturbance effects on species diversity and functional diversity in riparian and upland plant communities. Ecology 91, 28–35. Botta-Dukát, Z., 2005. Rao's quadratic entropy as a measure of functional diversity based
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Plant trait-based analysis reveals greater focus needed for mid-channel bar downstream from the Three Gorges Dam of the Yangtze River
T
Xuemei Yia, Yuanyang Huanga, Maohua Maa, Zhaofei Wena, Jilong Chena, Chundi Chenb, ⁎ Shengjun Wua, a b
Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, China Tongji University, Shanghai, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Dam Riparian zone Plant community Species richness Functional diversity
As a transitional zone between terrestrial and aquatic ecosystems, riparian ecosystems are critical in maintaining the health of river-ways and watersheds, and particularly sensitive to changes in hydrological regimes. Effects of dams on riparian zone are widely addressed, but less attention focus on riparian plant community of bar habitats. The Jingjiang reach downstream of the Three Gorges Dam has undergone significant changes of water and sediment condition and local base-level due to the Three Gorges Dam. To address responses of riparian plant communities to dams, the riparian plant communities of the mid-channel bars and the point bars in the Jingjiang reach were examined with a trait-based analysis. The results showed a lower species richness and a higher risk of invading by invasive species for mid-channel bars comparing with point bars. Moreover, the species richness of both the mid-channel bars and the point bars were observed increasing with the distance increasing from the dam, but the increasing rate of mid-channel bars was much smaller than that of point bar. Furthermore, functional redundancy of the mid-channel bar represented by the difference between functional diversity and species richness was marked lower than that of the point bars. Thus, we conclude that affected by the regulation of the dam, the mid-channel bars seem to be more hydrologically susceptible and harsher habitats for the assembly of plant community comparing with point bars. Therefore, more research of mid-channel bar downstream the dam were suggest and different emphasis of research and protecting management are needed for different bar types.
1. Introduction Affected by seed bank, damming and geomorphology (Hillebrand and Matthiessen, 2009; Kominoski et al., 2013), the riparian vegetation play key roles in functioning of freshwater ecosystems (Hladyz et al., 2011), delivering provisioning service and supporting service (Woodward et al., 2012). The interactions between river’s hydrological processes and ecological processes of surrounding riparian vegetation provide crucial information for environmental and management implications (Naiman and Decamps, 1997; Rood, 2006). As ecotones between aquatic and land systems (Perucca et al., 2007), the riparian ecosystems, consist of diverse, dynamic and complex habitats, are facing frequent shift of climate change, human activities and reservoir regulation. The response of plant communities to environment changes (Suding et al., 2008; Asner et al., 2014) and habitat gradients (Biswas and Mallik, 2010) have been investigated via a series of functional diversity indices which showed that the species composition and the
⁎
functional diversity of plant communities are rapidly changing in responding to the changing factors. Due to dam operation, the ecosystems behind the dam became the most sensitive area against hydraulic changes (Majerova et al., 2015). Dramatic changes caused by river flow regulation have strong effects on biota (Bejarano and Nilsson, 2018), which is an important driver in shaping riparian communities in downstream ecosystems. Such changes in flow regime may lead to fragmentation of riparian forests, decline of native vegetation, substitution by exotic species and vegetation encroachment (Bejarano et al., 2011). Plant development is promoted or disfavoured, some species invade and others go extinct due to differing abilities of species to tolerate hydrological alterations, which in turn leads to a non-random sorting of species along hydrological gradients (Mouillot et al., 2013). The serial discontinuity concept (SDC) was proposed to describe the natural continuum disrupting of river by dams (Ward and Stanford, 1983). Few studies have examined the recovery gradients which indicate reduced species richness downstream the dam
Corresponding author at: Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China. E-mail address: [email protected] (S. Wu).
https://doi.org/10.1016/j.ecolind.2019.105950 Received 29 March 2019; Received in revised form 13 November 2019; Accepted 18 November 2019 1470-160X/ © 2019 Elsevier Ltd. All rights reserved.
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(Ellis and Jones, 2013; Saulino et al., 2017; Guzy et al., 2018). But the riparian zone plants have not been well tested (Ward and Stanford, 1995; Kingsford, 2000). Remarkable channel degradation has occurred in the Jingjiang Reach, caused by significant changes of water and sediment conditions and local base-level since the operation of the Three Gorges Dam (Xia et al., 2017a,b; Li et al., 2018). This has both direct and indirect effects on bar habitats which are highly sensitive to the changes in water and sediment regime (Chen and Chau, 2016). Bars are typical landforms constructed by fluvial process as riparian zone along low-gradient accretional reaches (Hupp and Osterkamp, 1996; Steiger and Gurnell, 2003; Dong et al., 2012). They are depositional features that form in river channels and are important in the morpho-dynamics of the river (Hauer et al., 2015). In the middle reach of Yangtze River, two common types of bars are point bars and mid-channel bar. The most distinguishing feature between them is their relative location to channel. The point bars are on the edge of the channels, and the mid-channel bar are in the middle of the channels. However, few studies have been conducted to examine downstream bar habitats downstream the dam, considering the importance of this process. Aim to understand riparian ecosystems downstream from the dam, we focused on the point bars and the mid-channel bars distributed along Jingjiang Reach downstream the Three Gorges Dam. We analyzed the taxonomic indices and the functional diversity of the riparian plant communities surveyed on the two types of bars. By comparing the differences of riparian plant communities between the mid-channel bars and the point bars, we explored the riparian plant community response to the downstream habitat changes caused by the Three Gorges Dam.
Table 1 Sites properties. Site name
Bar types
YZB GZ SZK DSZ ZJK HJZ TPK SBT XTZ WXZ NXZ YJC
Mid-channel Mid-channel Point bar Mid-channel Mid-channel Mid-channel Point bar Point bar Point bar Point bar Mid-channel Point bar
bar bar bar bar bar
bar
The distance to dam (km)
Texture type
50.22 108.55 118.74 129.19 143.86 157.89 177.91 183.73 186.46 203.31 211.24 229.99
Gravel Sand Clay Gravel Sand Sand Gravel Clay Gravel Clay Sand Clay
between the central and north subtropical zones with a subtropical humid climate. The annual temperature in this area is between 16.5 and 19 °C and the annual precipitation is about 1100 mm. About 80% of the annual precipitation take place in summer months between April to October, with the daily temperature between 28 and 30 °C. In the winter months, the daily temperature is between 3.4 and 7.2 °C, and only 20% of annual precipitation occur. 2.2. Study design Field work was done in July 2015, November 2015, June 2016 and October 2016. We collected data from the riparian zone with elevation below the bankfull-stage and experienced inundation in summer every year in the downstream area of the Three Gorges Dam. A total of 12 sites (Fig. 1) and 67 quadrats of the riparian plants were investigated. For each site, Two types of habitats were identified by the geomorphic trait, namely point bars located at river banks (6 sites, black dot on the map) and mid-channel bars located in the middle of the River (6 sites, red dot on the map). Distance from the Three Gorges Dam of the sites were list in Table 1 with the texture type. At each sampling site, three
2. Material and methods 2.1. Study area The sampling area is located in Jingjiang reach, downstream of the Three Gorges Dam on the Yangtze River (Fig. 1). This area locates
Fig. 1. Location of the study sites. 2
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quadrates (1 m × 1 m) were surveyed in perpendicular. All plant species were inventoried except for moss and lichen which were scarce in each quadrat. The plant species were classified into four types with the distribution and the function to the ecosystem, which include native species, alien species, invasive species and endemic species. The area cover, height and the number of each plant species in each quadrat were recorded. Plant species richness with different life histories (perennial species and annual or biennial species) of bars were used to testify the recovery rate of the bars downstream the dam.
Table 3 Plant species in two types of bar. habitat type
Species numbers
Alien species
Invasive species
Endemic species
Point bar Mid-channel bar
144 71
24 13
5 4
– 3
Gramineae, Labiateae and Polygonace were the four most abundant plant families, contributing 43.3% in the plant species. 24 non-native species with 5 invasive non-native species were found on the point bars, while on the mid-channel bars the numbers are 13 and 4 respectively. Three species on the mid-channel bars were not observed on the point bars, including Salix variegata Franch., Myricaria laxiflora (Franch.) P. Y. Zhang et Y. J. and Ranunculus sceleratus Linn.
2.3. Data analysis Species richness which have been used as the main measure in most studies (Petchey and Gaston, 2002; Petchey et al., 2007; Grossman et al., 2017) linking species diversity to functional diversity and Pielou’s evenness (Smith and Wilson, 1996) ranges from zero to one with low values indicating more skewed abundance distributions were used to measure the community diversity. To assess the relative significance of variation in species richness and variation in species evenness for functional diversity within the concept of functional redundancy, Shannon-wiener index (Shannon and Weaver, 1950) and Simpson’s diversity index (Magurran, 2004) were also used. To assess the integrated function of plant communities, traits considered to determine the role in ecosystems, recognized as key supporter for ecosystem functions and associated ecosystem services (DÍAz et al., 2006) were selected in this study. These included traits such as dispersal type, growth form, life cycle, shoot height and flowering phenology (Table 2). The traits were obtained from literatures which applied in our study area or where the climate is subtropical. Functional diversity summarizes the variation in trait values of an organism that potentially affects its performance, fitness and ecological function within a community (Violle et al., 2007). We calculated Rao's quadratic diversity index Q (Rao, 1982; Botta-Dukát, 2005) for functional α diversity, which takes the differences between species pairs into account. For understanding the consequences of disturbance on the functioning of a given ecosystem (Sasaki et al., 2009), the relationship between community diversity and functional diversity was tested by three models, namely linearly, exponential and sigmoid logistic models (Sasaki et al., 2009). Akaike’s information criterion (AIC) was used to rank models in terms of their support by the data. The model with the lowest AIC (Walker, 1992; Naeem, 1998; Johnson and Omland, 2004; Micheli and Halpern, 2010) was selected.
3.2. Plant community diversity index Differences in taxonomic indices and functional diversity were detected in the different two habitat types (Fig. 2). Species richness of the point bars was 15.5% higher than that of the mid-channel bar, and having a wider variation range. Shannon-wiener index and Simpson diversity in point bars were higher than that of the mid-channel bars, but with smaller standard deviation. Species composition in the midchannel bars varied more significantly and had a relatively higher evenness than point bars but a lower functional diversity. 3.3. Species richness downstream from the dam Several linear relationships between the species richness and the distance from the dam of the bars were observed. One positive linear relationship is observed when considering the species richness of all the species on all the bars in the two bar types (Fig. 3a). Another positive linear relationship is observed when only considering the species richness of all annual or biennial plants (Fig. 3c). Furthermore, the positive linear relationship was found when considering the species richness of all the species on the point bars (Fig. 3b). As the distance increasing, species richness of mid-channel bar showed a linear decreasing but not significantly trend with the highest value observed at Guanzhou (Fig. 3d). Species richness of two types of bars showed a similar increasing trend but a sharper slope increasing was observed in the point bars than that of mid-channel bars. 3.4. Relationship between diversity index and functional diversity
3. Results The functional diversities of the two bar groups were detected to be linear or exponential related with the taxonomic diversities. When indexing the taxonomic diversities by species richness, the relationship was linear on the mid-channel bars while exponential on the point bars (Fig. 4a, b). When indexing by Shannon-winner index and Simpson
3.1. Plant species A total plant species of 144 were recorded in the point bar group and of 71 in the mid-channel bar group (Table 3). Compositeae, Table 2 Plant functional traits and ecosystem processes and service. Trait Reproductive traits Dispersal type Vegetative traits Growth form Life cycle Max. height (m) Phonological traits Flowering phenology
description
Measurement scale
Ecosystem services
Three types according to van der Pijl (1982): 1: anemochory; 2:endozoochory & exozoochory; 3: ballistochory; myrmecochory; unassisted.
nominal
Regulating service
Two classes: Graminoids and Herbs Annual; Biennial and Perennial Five classes modified from Hodgson et al. (1995): 1: < 0.1 m; 2: 0.11–0.29 m; 3: 0.30–0.59 m; 4: 0.600.99 m; 5: 1.0–3.0 m
Nominal Nominal Ordinal
Supporting service Supporting service Supporting service
Five classes according to Mabry et al. (2000): 1: spring flowering; 2: mid-summer flowering; 3: spring-midsummer flowering; 4: fall flowering; 5: mid-summer-fall flowering
Nominal
Regulating service
3
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Fig. 2. Biodiversity index of two types of bar.
Fig. 3. Species richness of bars with the distance downstream from dam. 4
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Fig. 4. The relationships between diversity index and functional diversity.
The annual and biennial species were easily affected by the erosion regime caused by the dam (Gilvear and Willby, 2006). Therefore the recovery trends of species richness of annual and biennial species was observed as prediction. We infer that the annual or biennial species an important indicator for riparian zone plants community response to dam construction. The higher recovery rate of point bar between species richness and downstream distance to dam indicate that the point bar benefited more from the distance increasing to the Dam. Comparing with the annual or biennial species, perennial plants have more resistance to the erosion regime caused by the dam. Two shrub plants, i.e., S. variegata, M. laxiflora, were only observed on the mid-channel bars as main species, especially on the bars in the upstream reach. Therefore, when considering all species, the recovery trend of the species richness was not found on the mid-channel bars. The river regulation significantly affected the viability of S. variegata and M. laxiflora which caused low seedling production and population growth (Bao et al., 2010; Ayi et al., 2016; Chen et al., 2019b). As time goes by, with the shrub species’ declining, more attention is needed for the plant communities of the mid-channel bars.
diversity, the relationship was positive linear on both types of bars (Fig. 4c–f). When indexing by evenness, the relationship was negative linear on both types of bars (Fig. 4g, h). 4. Discussion Downstream from the Three Gorges Dam, riparian plant communities of the two habitat types, point bar and mid-channel bar, are most directly influenced by hydrological regimes triggered by the dam operation. 4.1. Plant species and bar types Edge effects apply to the plant community of both types of bar habitats adjacent to the watercourse. The point bars neighbored river banks and terrestrial lands, while the mid-channel bars neighbored high lands and disconnected to terrestrial lands. The species pool of neighboring terrestrial lands can be an important source to the point bars. As the transition zone, the seed bank and the edge-effect allow the point bars to maintain higher richness by the habitat’s nutrient heterogeneity, while evenness was restricted (Biswas and Mallik, 2010). Due to connectivity to mainland, anthropological disturbance from complex nearby land-use could be another key factor in shaping plant community in the point bar habitats. Since disconnected to terrestrial lands, flood events and fluvial maintenance operations homogenized initial conditions of mid-channel bar in terms of topography and grain-size distribution (Wintenberger et al., 2015), which made the mid-channel bars a lower species richness but a higher evenness. Riparian zones are well known to be highly invaded by non-native plant species and may facilitate the spread of non-native species across the landscape (Chen et al., 2019a). A positive correlation was reported between native species richness and non-native species richness (Cleland et al., 2004; Xiong et al., 2018). This phenomenon has been identified in our study area. The non-native species have similar reachability on two types of habitats by water flow and wind, especially for the seed plants. Stresses and human disturbances changed the habitats and inter- and intra-specific interaction of plants on the bar, make it easier for the non-native species establishing and spreading. Stressed system reduce the biodiversity, increase smaller, short-lived opportunistic species but also increase the dominance of exotic species (Naiman et al., 2005). Located in the middle of the river, a lower species richness and a higher ratio of non-native invasive species to total species indicate that mid-channel bars had been affected more from the stresses and will be more affected by invasive species.
4.3. Mid-channel bars: harsher habitats for plants Through the relationships between the functional diversity and taxonomic indices, the mid-channel bars would be harsher habitats for plant community comparing with the point bars. The functional diversity was found exponentially related to species richness on the point bars, while linearly on the mid-channel bars. It can be inferred that with the increasing of species richness, the functional redundancy increased on the point bars but kept consistent on the mid-channel bars. In other word, the ecosystem functioning were relatively robust with respect to changes in taxonomic diversity (Mayfield et al., 2005; Sasaki et al., 2009) on the point bars, but a relatively high turnover rate, low recovery rate and low stabilization to changes in taxonomic diversity on the mid-channel bars. The functional diversity was found linearly and negatively related to the evenness on the bars, and the functional diversity decreased faster with the increasing of evenness on the mid-channel bars comparing with that on the point bars. As an important component of community response to environment changes (Kardol et al., 2010), a higher evenness was linked to high-stress (Drobner et al., 1998), because uniformly barren landscape reduced the heterogeneity (Tilman, 1982; Levy et al., 2019). A negative relationship between evenness and functional diversity was suspected in high-stress ecosystem (Sasaki et al., 2009). The functional diversity kept being lower on the mid-channel bars with higher evenness and a steeper relationship between evenness and functional diversity because of the habitat homogenization induced by the erosion regime caused by the dam. Therefore, the mid-channel bars were indicated as a higher stressed habitat for riparian plant communities comparing with the point bars.
4.2. Serial discontinuity concept confirmed Dam disrupt the natural continuum in uninterrupted river systems from headwaters to downstream (Guzy et al., 2018). The serial discontinuity concept predicts a downstream recovery gradient of biodiversity with the distance from dams (Ward and Stanford, 1983), because of the downstream recovery trend of habitat from the dam disruption (Merritt and Wohl, 2006; Ellis and Jones, 2013; Guzy et al., 2018). In this study, the recovery trends of species richness of annual and biennial species were found on both the point bars and mid-channel bars but with different increasing rate. The recovery trend of the species richness of all species was also found on the point bars, but not found the mid-channel bars. Instead, a slightly degradation trend of species richness of all species exhibited on the mid-channel bars. The Three Gorges Dam induced a dramatic decrease in the sediment supply, making the downstream Jingjiang reach an erosion reach (Hu et al., 2009). A relatively weak riverbed erosion was observed on the downward reach comparing with the upward reach (Luo et al., 2012).
5. Conclusions Downstream riparian ecosystems from dams became the most sensitive area against the water and sediment regime regulated by dams (Bejarano et al., 2018). Remarkable channel erosion has occurred in the Jingjiang reach downstream from the Three Gorges Dam on Yangtze River. Mid-channel bars and point bars are two most sensitive geomorphology unit to the channel erosion. How the riparian plant communities on the bars respond to the erosion regime induced by the Three Gorges Dam is the key for the protection of downstream geomorphology and riparian ecosystem. This study designed a field work from 2015 to 2016, consisting of four plant community survey on 12 bars, including 6 mid-channel bars 6
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and 6 point bars. The taxonomic diversity and functional diversity of the plant community of the bars were further analyzed. Through the analysis, it is found that the riparian habitats were stressed by erosion regime on the both types of bars, with a higher stress on the bars in upstream reach and lower stress on the bars in downstream reach. It is also found that the mid-channel bars has lower plant diversity comparing with the point bars because of the disconnecting with species pool on the terrestrial lands. Additionally, two perennial plants only observed on the mid-channel bars, i.e., S. variegata, M. laxiflora enhanced the habitat resistance to the erosion regime, but the declining of the two plants may further decrease the habitat resistance. Therefore, a long-term study of the riparian plant community on the bars to clarify the assembly mechanisms affected by the Three Gorges Dam is encouraged for further study. Especially, as a harsher and declining habitat for riparian plant community, the mid-channel bars worth an urgent and greater focus for the protection of downstream geomorphology and riparian ecosystem.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Xuemei Yi: Conceptualization, Methodology, Writing - review & editing. Yuanyang Huang: Visualization, Software, Data curation, Writing - original draft. Maohua Ma: Writing - original draft. Zhaofei Wen: Data curation, Formal analysis. Jilong Chen: Data curation, Writing - original draft. Chundi Chen: Data curation, Writing - original draft. Shengjun Wu: Supervision, Writing - review & editing. Acknowledgment We thank Xiaoxiao Wang, Yi Jiang, Qiao Chen, Yu Wang, Xiaoxiao Tong, Hui Cheng and Enpei He for field assistance. This study was supported by the National Natural Science Foundation of China [51709250; 51779241; 41601042]. Data accessibility Data are available as the electronic Supplementary material. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ecolind.2019.105950. References Asner, G.P., Martin, R.E., Tupayachi, R., Anderson, C.B., Sinca, F., Carranza-Jiménez, L., Martinez, P., 2014. Amazonian functional diversity from forest canopy chemical assembly. Proc. Natl. Acad. Sci. Ayi, Q., Zeng, B., Liu, J., Shi, S., Niu, H., Lin, F., Zhang, Y., 2016. Seed sojourn and fast viability loss constrain seedling production of a prominent riparian protection plant Salix variegata Franch. Sci. Rep. 6, 37312. Bao, D.C., Zhi-Jun, L.U., Jiang, M.X., Shao-Dong, X.U., Yao, Q., Liu, Q.F., Wang, Q., 2010. Population structure and dynamics of Remanent Myricaria laxiflora downstream from the Three Gorges Dam. J. Wuhan Bot. Res. 28, 711–717. Bejarano, M.D., Nilsson, C., 2018. Riparian plant guilds become simpler and most likely fewer following flow regulation. J. Appl. Ecol. 55. Bejarano, M.D., Nilsson, C., Tánago, M.G.D., Marchamalo, M., 2011. Responses of riparian trees and shrubs to flow regulation along a boreal stream in northern Sweden. Freshw. Biol. 56, 853–866. Bejarano, M.D., Nilsson, C., Aguiar, F.C., 2018. Riparian plant guilds become simpler and most likely fewer following flow regulation. J. Appl. Ecol. 55, 365–376. Biswas, S.R., Mallik, A.U., 2010. Disturbance effects on species diversity and functional diversity in riparian and upland plant communities. Ecology 91, 28–35. Botta-Dukát, Z., 2005. Rao's quadratic entropy as a measure of functional diversity based
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