Vegetation pattern in Shell Ridge Island in China’s Yellow River Delta

Abstract

In general, coastal habitat conditions are extremely harsh, with the ecological equilibrium inextricably related to the plant community. Understanding the natural vegetation features of a coastal zone with little human disturbance could provide a reference for future vegetation restoration and ecosystem maintenance services. In this study, the vegetation patterns of Wangzi Shell Ridge Island in the Yellow River Delta were investigated. A total of 35 taxa of vascular plants were documented, representing 15 families and 33 genera (of which most were mono-specific). Surveys identified only one to eight taxa in each plot. From sea to land, the vegetation showed a typical zonal distribution pattern. There was a correlation between the landform and important factors that influenced the plants including soil factors and distance from the sea. Thus, the taxa distribution and vegetation had a significant correlation with landform. The dune crest, backdune and interdune lowlands were areas with weak storm surges and were the important locations for the taxa to be become established. Plants along the high-tide line formed important defenses from large waves and high winds. The significant protection provided a suitable living environment for many organisms with high medicinal value. Special attention and protection could be provided to this area by reducing the use of the beach road and enclosing the complete section from sea to land with a protective fence. In addition, vegetation protection and restoration on Shell Ridge Island would aid in the formulation and implementation of reintroduction strategies for similar vegetation in similar habitats.

Graphical abstract

Keywords

wetland / vegetation diversity / landform / zonal distribution / conservation value

1 Introduction

The ecological impacts of global climate change have been a hot academic topic in recent years. Studies of coastal shell ridge dunes demonstrate the complexity of sea-land dynamics and ecological processes (Maun, 2009; Psuty and Silveira, 2010). Shell ridges are a specific type of costal dune primarily composed of the shells of dead marine creatures as well as detritus deposited at the high-tide line. The distribution and sedimentary features of shell dunes indicate changes in the coastline, sea levels, and ancient climate (Otvos, 2000). Shell dunes and their coastal wetland ecosystems also play a key role in offering protection against the wind and waves. They also provide habitats that are valuable as resting grounds for some migratory species and as breeding grounds for others, and hence are important areas for ecological conservation (Wang and van Strydonck, 1997).

Shell ridge wetlands are found in flood plains on the coast of North America (Gosselink, 1979; Gabrey and Afton, 2001; Draut et al., 2005), the Suriname coastal plains (Augustinus, 1989), the deltas of the Thames in New Zealand (Dougherty and Dickson, 2012), and the Mekong River in Vietnam (Ha et al., 2012). Shell ridge wetland ecosystems also exist on the silty and muddy coastal plains of North and East China(Wang and van Strydonck, 1997). The largest chain of shell ridge islands in China is found in the Yellow River Delta on the southwest coast of Bohai Bay, and was formed during the Late Miocene. Collectively, this formation, the ancient shell ridge of Louisiana in the United States, and that of Suriname (South America), are referred to as the three major ancient shell ridges in the world (Pan et al., 2001).

Most research to date has focused on the geology of Yellow River Delta shell islands, including their origin (Jia, 1996; Saito and Xue, 2001), age (Wang et al., 2000), morphology (Du et al., 2009), and composition (Wang et al., 2007). In addition to geology, vegetation plays an important role in maintaining ecological balance and stability (Grace and Pugesek, 1997; Stanley et al., 2005; Wang et al., 2015). Thoroughly understanding the plant taxa distribution patterns of a well-preserved coastal dune will provide an important foundation and reference point for follow-up restoration work on the habitat and vegetation of coastal dunes (Acosta et al., 2009). However, less research of this type could be found.

In the last 20 years, human activities such as land reclamation and construction activities have led to a large reduction in the size of the shell ridge islands, and the continued destruction of the habitat has resulted in a rapid loss of plant taxa (Tian et al., 2011). As a result, there has been increased attention on ways that the taxa can be maintained while enhancing ecological protection and restoration (Tian et al., 2009).

The study described in this article focuses on Wangzi Island in the Yellow River Delta. This area was selected because its shell ridges were well-preserved with little human disturbance. We investigated the change in plant taxa diversity and vegetation coverage of plant communities. The main objectives of the study were: 1) to determine the plant taxa composition in this shell ridge; 2) to examine the distribution pattern of plant taxa from sea to land; and 3) to verify whether landform characteristics of the area affect the distribution of plant taxa. Based on the results, we propose a vegetation restoration plan based on the natural distribution patterns of the taxa.

2 Material and methods

2.1 Study area

The study region is located in a national nature reserve composed of the shell ridge and its wetland ecosystem, in the Yellow River Delta within Wudi County, Shandong Province, China. It lies in the East Asia Monsoon Continental Sub-humid Climate Zone in the Warm Temperate Zone (geographic coordinates 37°54'30" to 38°19'10" N, 117°45'08" to 118°05'37" E), which is characterized by a mild climate with distinct wet and dry seasons (mean temperature= 13.6°C and annual precipitation= 500 to 900 mm). The soil matrix consists of shells, sand, and mud, with a bulk density of 1.2 g·cm^‒3, and it is characterized by a coarse particle diameter, large porosity, weak water retention and fertilizer conservation, rapid increases in daytime temperature, rapid decreases in nighttime temperature, and large differences in daily temperature. Recognizing the importance of the shell ridge for studies of marine geology, paleo-climate, and for the protection of biodiversity, in 2006 the State Council approved the establishment of national nature reserves within the shell ridge wetland ecosystem. These areas are enclosed by fences for their protection.

Wangzi Island is located in the core of the nature reserve (Fig. 1). It measures about 15 km in length along an east-west axis and about 100 to 300 m in width perpendicular to the coastline. Due to its position within the national nature reserve, it has been protected from human activities and has good vegetation development. There is only a single village (an area of 0.01 km²) with about 20 people. A beach road, about 1 m wide, located between the high-tide line flat ground (HTL) and the shell ridge foredune (FD), allows people and mini-motor vehicles to travel along the coast and access fishing spots (Fig. 2). To prevent human-caused degradation of the embankment and its vegetation, the area between the foredune and interdunal depression was fenced for protection in 2011. This protection enabled the shrub-grass vegetation to become well-established (Fig. 2).

2.2 Experimental design and plant sampling

Field sampling took place from June to July in 2013. Seven transects (15 m in width) at intervals of 300 m perpendicular to the coastline were set up on Wangzi Island to verify the effects of the dunes’ topography on plant taxa. It should be noted that there was an interval of 500 m between the third (III) and fourth (IV) transects due to the presence of a watchtower and monitoring station. The locations of transects are listed in Table 1. Sampling started at the point where the vegetation was first encountered and measured using transects and quadrat survey methods (Fang et al., 2009). Depending on the length of the sandy shell deposit within each transect, quadrats were laid at intervals of 5 m (quadrat area was 15 m×5 m, so that they could include sparse shrubs for which the largest crown width was 4‒5 m, and then divided into three 5 m×5 m subplots (Shen et al., 2011)). For each quadrat, the distance from the coast, community coverage, and geographical location of HTL, foredune (FD), dune crest (DC), backdune (BD), and interdune lowland (IDL) were recorded based on the literature (Monserrat et al., 2012). Plant taxa names, number of taxa, average plant height, and spatial coverage in the subplots were also recorded. Identification and classification of plant taxa was based on the Flora Reipublicae Popularis Sinicae (http://frps.eflora.cn/) and Flora of Shandong (Chen et al., 1992, 1997). The number of plant taxa was simply counted. Vegetation coverage was determined using the visual estimation method, and expressed as a percentage (Sutherland, 1999). A tape measure was used to measure the plant heights.

In addition to the quadrat survey, soil profiles from 0‒30 cm were excavated in the center of each plot. The soil was collected and sealed into plastic bags. This process was repeated five times, and all samples were analyzed in the laboratory for physical and chemical properties.

The soil samples were passed through 2 mm sieve. The water content was measured using the oven drying method (Gardner, 1996). The soil organic carbon content (SOC) was determined using the K₂CrO₇ plus H₂SO₄ oxidation method (Bao, 2000). Soil total nitrogen (TN) was determined using a flow injection analyzer (SEAL-AA3, German), total phosphorus (TP) was determined colorimetrically (Kuo, 1996), and total potassium content (TK) was measured using an atomic absorption spectrometer (Xie et al., 2012).

2.3 Data analysis

No significant differences were detected among the three subplots located at each sample location on the transect, so they were combined to form 66 quadrats in total. TWINSPAN classification was performed for the 66 quadrats (Hill and Šmilauer, 2005). We used the progressive detrending and nonlinear regression method for the detrended correspondence analysis (DCA) to examine changes in taxa composition (Shen et al., 2011; Diekmann et al., 2014). Statistical indices for the quadrats employed the importance value of each taxon in the community; the formula used the mean values of relative abundance (proportion of individual taxa relative to the total plants in a quadrat), relative coverage, and relative height for each taxon. In the TWINSPAN classification and DCA ordination, plant taxa were represented by the first English letter of the genus and first two letters of the taxon.

Plant taxa abundance was expressed as the number of taxa per quadrat. Simpson’s index and Evenness were calculated as follows (Mahdavi et al., 2013):

\begin{array}{l}H=-{\displaystyle \sum _{i=1}^s{P}_1\ln P_i}\hfill \\ E=H/\ln S\hfill \end{array}

where H and E are the Shannon-Wiener diversity and Pielou evenness index, respectively. S is the total number of taxa in a quadrat, and P_i is the proportion of individual taxa i relative to the total plants in a quadrat. S, H, and E include the amount of equitability of the taxa distribution, and represent the α-diversity level of diversity. The higher H and E values indicate richer diversity of taxa and larger community uniformity (Xu et al., 2001).

The landforms were represented by numbers (HTL=1, FD=2, DC=3, BD=4, IDL=5) to enable correlations with the vegetation distribution to be tested. T tests and Pearson correlations between the landform and SOC, TN, TP, and TK were studied (p<0.05). The taxa were assigned representative numbers from 1 to 35.

The software Excel (version 2003) was used to calculate importance values, Shannon-Wiener diversity index, and evenness of taxa in a quadrat. PC-ORD 5.0 was used for the TWINSPAN classification and DCA ordination (http://www.planta.cn/forum/viewtopic.php?p=101211, Mefford, 1999). The T test and Pearson correlation were calculated using SPSS 11.0 (Zhang, 2002).

Results

2.4 Plant taxa in Shell Ridge Island, Yellow River Delta

Field sampling identified a total of 35 taxa of vascular plants in all quadrats. The plants represented 15 families and 33 genera, mainly mono-specific. Among them, Gramineae was the most common family (even taxa, accounting for 20% of the total plant taxa), followed in succession by the Chenopodiaceae family (six spp/17%) and the Compositae family (five spp/14%). In addition, the number of taxa in each plot appeared small (usually one to three taxa in each quadrat with the most being eight taxa, Table S1, Appendix A, B).

2.5 Plant community composition

TWINSPAN classification revealed that the vegetation of the shell ridge wetland could be divided into two groups, each of which had different taxa compositions and habitat requirements. In Fig. 3 the groups are indicated by the numbers 1 and 2. Group 1 was sparsely distributed with low taxa diversity, while in group 2 the taxa were relatively abundant with an average of five taxa in each quadrat and vegetation coverage of 85.06%. The most common taxa in the quadrats were noted in the classification diagram, and could be described as follows.

According to the taxa composition and importance values in each quadrat, groups 1 and 2 could be further divided into five and six subgroups, from A‒E and F‒K, respectively (Fig. 3, Fig. 4, and Table S1). Subgroup A was a community of Tamarix chinensis, accompanied by Phragmites australis, Messerschmidia sibirica, Zoysia macrostachya, Scorzonera mongolica, and Cynanchum chinense. On average, there were two taxa in each quadrat, with a low Shannon-Wiener diversity and Pielou evenness index of 0.20 and 0.18 respectively. The community coverage was 44%. Subgroup B was a community of Z. macrostachya - P. australis, with an average of three to five taxa per quadrat. C. chinense, M. sibirica, and Limonium bicolor had alternating distributions with a Shannon-Wiener index of 0.92 and coverage of 59.8%. Subgroup C contained communities of Metaplexis japonica (Aeluropus sinensis, Z. macrostachya, or Melilotus officinalis) - M. sibirica. On average, four taxa per quadrat were found. Subgroup D was a community of P. australis-M. sibirica, accompanied by C. chinense, Othriochloa ischaemum, and Calystegia soldanella. The plant coverage varied from 35% to 85%. Subgroup E was a M. sibirica community, with single dominant taxa and only two plant taxa present on average. Subgroup F was the richest subgroup with 20 plant taxa distributed between the plots. Among them, a shrub (Z. jujube) or liana (C. japonica) - A. mongolica community was the main vegetation type with an average plant coverage of 78.09%. Sometimes, M. officinalis, M. japonica, and Periploca sepium were the co-dominant taxa in quadrats (quadrats 13, 14, 21, and 58). Subgroup G was a weed community of R. cordifolia, S. cuscutae, Xanthium sibiricum, Salsola collina, M. sibirica, and C. chinense, which had average community coverage of 97.67%. Subgroup H in group 2 was dominated by C. japonica and A. mongolica, with low taxa richness. Its diversity index was 0.57. The above-ground part of C. japonicae grew vigorously with greater community coverage of 93.57%, while occasionally Rubia cordifolia and also shade-requiring plants such as O. ischaemum, Chenopodium glaucum, and the small shrub Z. jujube were present. Subgroup I had a dominant community of A. Mongolica- Artemisia carvifolia. In some plots (18, 19, 31, 46, and 66) Z. jujube became the co-dominant taxa. Subgroup J was a Suaeda salsa (or Triarrhena sacchariflora) – M. japonica community. Kochia scoparia was the most common taxa. Subgroup K was a typical Deyeuxia arundinacea community. The taxa diversity index and Evenness index were 0.76 and 0.51, respectively. The community coverage was 88.13%.

2.6 Correlation between vegetation and landform

Due to the specific micro-topography present in dunes, various microhabitats are created with a range of major abiotic variables (Maun, 2009; Fenu et al., 2012). In the Yellow River Delta, the landform of the shell ridge coupled with the distance from the sea and altitude had a significant impact on the soil moisture, SOC, TN, TP, and TK levels (F = 21.404, 11.361, 24.033, 7.132, and 10.226, respectively, p<0.05, Table 2), which might be vital factors affecting plant growth.

Through the DCA ordination, a significant correlation between plant taxa and landform could be found (Fig. 5). The first two axes explain 8.16% of the data’s total variability, while the first and second axes explain 0.82 and 0.55 of the vegetation patterns, respectively. The hygrophyte plant D. arundinacea was near point zero on axis 1, indicating its location on the landward side of the dune lowland, followed by Asparagus dauricus and Glycine soja. Astragalus adsurgens, O. ischaemum, A. mongolica, C. japonicae, S. collina, R. cordifolia, Atriplex centralasiatica, Aster tataricus, Bothriochloa ischaemum, Z. jujube, Chenopodium glaucum, Artemisia carvifolia, Medicago sativa, C. chinense, P. sepium, and X. sibiricum were also present, which were mainly on the enclosed dune crest and backdune, and accounted for 57% of the total number of taxa (Table S1). The quadratsfrom the HTL and FD are positioned on axis 1 to the right side of point 100. T. chinensis was located along the edge of the coast, and going inland it was followed by S. mongolica, Z. macrostachya, and L. bicolor, whereas, M. sibirica, A. sinensis, S. seepweed, P. australis, and C. soldanella mainly occurred at the FD. Therefore, the plant shift from right to left on axis 1 represents the gradient change from the sea to the land. In addition, S. salsa,K. scoparia, and T. sacchariflora were found at the top end of the DCA ordination axis 2, and they were plants with typical salt and barren tolerance where indicated humid, barren, and saline-alkali conditions. The occurrence of C. chinense, X. sibiricum, R. cordifolia,S. collina, P. sepium, A. sinensis, and C. japonica, at the lower end of axis 2, near point 30, represents frequent interference, mosaic, and fertile soil conditions. In addition, the wide distribution of the vegetation that occurs on axis 2 indicates the complexity of the environmental factors represented by this axis.

The DCA ordination results of the two vegetation groups were similar to their TWINSPAN classification, which indicated significant differences in habitat and vegetation. Group 1 was mainly located between 6.00 and 53.00 m away from the coast, namely near the HTL and FD, which corresponded to a distribution mainly between 50 and 100 on axis 1 (Fig.5, Table S1). The T. chinensis community was at the very forefront of the coastal line (subgroup A, Fig.5). Subgroup B was the community of Z. macrostachya - P. australis, with significant intermixing with the T. chinensis community (subgroup A, Fig.5, Table S1). Subgroup E was between subgroups C and D on the DCA ordination axis 1, and was a community of M. Japonica; whereas, subgroups C and D were the A. sinensis, Z. macrostachya, M. officinalis- M. sibirica community and P. australis-M.sibirica community, respectively (Fig. 5,Table S1). Most quadrats in group 2 were either at the top of the dunes or in the lowland areas behind and between the dunes, between 12.00 m and 154.00 m away from the coast (Table S1, Fig.5). High similarities between the taxa that make up the communities are shown by the distribution on the DCA ordination axis. The vegetation communities in subgroup F were located on the dune crest after group 1. Subgroup K was typically found in the lowland areas among the dunes (Fig.5, Table S1).

3 Discussion

3.1 Plant taxa composition and vegetation pattern

Wetlands have important ecological functions, such as water conservation (Shao et al., 2014), filtering pollutants (Wu et al., 2012; Zeng et al., 2012; Wang et al., 2013), regulating climate (Cao et al., 2012; Chen et al., 2013; Saunders et al., 2014), protecting species diversity (Hughes, 2001; Froyd et al., 2014), and so on, most of which could not be achieved without vegetation (Grace and Pugesek, 1997; Pollock et al., 1998). Because of the naturally harsh habitat,the increasing pressures from human activities and global climate change,,wetland vegetation has been faced with degradation all over the world (Forey et al., 2008).The shell ridge along with the wetland ecosystems in the Yellow River Delta in Bohai Bay is a typical sandy coast with shell fragments as its main matrix. The sparse shrub-grass vegetation nurtured in these coastal dunes plays an extremely precious role in ecological functions, however, despite this, little previous research could be found in this area (Xie et al., 2012).

We assessed the distribution patterns of vegetation in several zones from the sea to land, to provide a theoretical foundation for the development of vegetation restoration and protection measures in the future. Similar to other coastal ecosystems, there was low plant taxa diversity with a total of only 35 taxa of vascular plants in the quadrats located on the shell ridge wetland. The small number of plant taxa per quadrat, ranging from one to eight, suggest that environmental conditions in the region are harsh. Compared with other coastal wetlands, the shell ridge has an even lower number of taxa, which might be explained by the coarser soil matrix, lower availability of fresh water and soil nutrient, narrower transverse north-south length, and the smaller island area (Ghazanfar et al., 2001; Fenu et al., 2013a; Xia et al., 2014).

Significant characteristic of coastal dune systems is the strong environmental gradients in the consistency of sediments, wind, marine aerosol, salinity, and presence of brackish water (Fenu et al., 2013a). Accordingly, it is well known that vegetation presents typical zonal distribution patterns along the coastline in many coastal sand dune habitats (Fenu et al., 2013b). Based on TWINSPAN classification, the vegetation communities in Wangzi Shell Island showed an obvious zonal distribution pattern from the marine to terrestrial environment, namely in the order of T. chinensis, Z. macrostachya-P. australis, P. australis-M. sibirica, M. sibirica, shrubs (Z. jujube) or liana (C. japonica)-A. mongolica, and A. mongolica-A. carvifolia, D. arundinacea communities.

3.2 Correlation between taxa, vegetation pattern and landform

Coastal dune environments are complex, vulnerable, and variable because of shifting substrate, burial by sand, varying underground water levels, bare areas, porous nature of sand, and little or no organic matterand hence they form complex habitatmosaics (Fenu et al., 2013a). This was shown in the DCA ordination that showed significant correlations between the landform and taxa distributions. T. chinensis, Z. macrostachya, and P. australis mainly occurred near the HTL; D. arundinacea was only found in the lowland areas with high soil moisture between the dunes; M. sibirica was mainly in the FD areas; and A. mongolica, C. chinense, Z. jujube, and C. japonicae were only distributed in patches on the DC and BD. Thus, T. chinensis, Z. macrostachya-P.australis, and M. sibirica communities were mainly in front of the dunes facing the frequent sea wind and waves, or near the HTL. The shrubs (Z. jujube) or liana (C. japonicae)-Artemisia mongolica, A. mongolica–A. annua, and D. arundinacea communities were found mainly at the bottom of the semi-fixed and fixed dunes, or where the wind was less and the vegetation was sheltered behind the shell ridge, indicating the major role of the landform on the distribution pattern of the vegetation.

3.3 Conservation values and management guidelines

The environmental conditions of the Yellow River Delta are suitable for the growth of plants with high medicinal and conservation value as well as high resistance to harsh habitat (Pan et al., 2001; Tian et al., 2011). The BD and IDL are exposed to progressively less harsh conditions than those of the FD and HTL (Wiedemann and Pickart, 2004; Acosta et al., 2009). According to the national key protected wild plants list (Chinese State Report on Biodiversity Editorial Committee, 1998), six rare and endangered medicinal plants grow in this location. Among them, G. soja, Ephedra sinica and Glycyrrhiza uralensisare officially under protection, as they were listed as having critically endangered, vulnerable, and critically endangered status in China (Gu, 1997; Wang et al., 2006). During the field experiments, small populations of G. soja could be found in the quadrats; while the other species were rarely present with only a few on the BD or in the IDL. In addition, common populations of Nitraria schoberi, A. adsurgens, and A. dauricus were found in Shell Ridge Island, which in China are listed as being critically endangered, lower risk, and critically endangered, respectively (Chinese State Report on Biodiversity Editorial Committee, 1998; Wang et al., 2006). With the increase in human disturbance and global climate change, introduction of invasive species in coastal dunes has become an ecological problem (Carboni et al., 2010). In the shell ridge in Wangzi, the mosaic distribution of the invasive alien taxa Datura stramonium, Spartina anglica, Geranium carolinianum,Erodium stephanianum, C. chinensis, and M. officinalis were also found, and present a potential threat to the region’s taxa diversity and uniqueness.

An integrated strategy via in situ and ex situ processes for conservation and management was effective during the course of vegetation protection and restoration (Cogoni et al., 2013). The reduction in human trampling and enclosure maintenance are important components of the in situ conservation strategies (Fenu et al., 2013b; Deng et al., 2014). In the Wangzi Shell Islands, where fences have been erected from the FD to the landside, populations of T. chinensis and Z. macrostachya were rarely found on the FD and farther inland. Part of the reason was the frequent use of the beach road, which led to less species exchange between the FD and the HTL, vegetation fragmentation, and taxa loss. In addition, human activities often have a delayed effect; the disappearance of plant taxa and the introduction of invasive species need some time to be observed (Fenu et al., 2013b). This should be a consideration in future management. It is necessary to maintain intact sections from the sea to the land to promote vegetation succession by reducing the usage of the beach road and expanding the enclosed area (including the high-tide line as well as the tidal flat). On the other hand, a variety of reintroduction measures for species conservation through the establishment of gene and seed banks, a better understanding of taxa ecological adaptability, plant taxa ex situ protection, and in situ recovery, are essential (Cogoni et al., 2013; Ren et al., 2014). Recent studies in the Yellow River Delta about the physiological and ecological adaptability of suitable plant species to sand burial, and variation in underground water level, salinity, and fresh water have identified possible directions for taxa reintroduction (Cui et al., 2010; Yu et al., 2012; Guan et al., 2013; Sun et al., 2014; Xia et al., 2014), which could provide technical support for restoration of vegetation on the Shell Ridge Islands.

4 Conclusions

In this study, 35 taxa of vascular plants were documented in a quadrat survey, representing 15 families and 33 genera (of which most were mono-specific) on Wangzi Shell Ridge Island.The vegetation showed a typical zonal distribution pattern from sea to land. A significant correlation between taxa distribution and landform could be found. The DC, BD and IDL were important locations for reproduction. Plants at the high-tide line were important defenses against wave and wind attacks. Special attention and protection should be provided by reducing the use of the beach road and enclosing the complete section from the sea to the land. In addition, the formulation and implementation of reintroduction strategies would enhance vegetation protection and restoration on Shell Ridge Islands.

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