| Coastal erosion is a problem currently facing many coastal cities world-wide. In recent decades in particular, increasing intensity of human activities within river catchments and along coasts have aggravated ongoing coastal erosion problems in many locations. These changes will reduce coastal city land resources for reclamation and could threaten coastal defense efforts. Therefore, improving shore stabilization abilities and protecting shorelines is a topic of global concern.One site that faces such erosion problems and the associated questions is the southern coast of Chongming Island, Shanghai. To protect the shoreline in this region, coastal shelterbelts covered by a reed (Phragmites australis) community were established by the local Water Resources Bureau in the early 1950s. In the past several decades, this vegetation has played a crucial role in increasing the beach stability and protecting the coastline. However, the situation has recently begun to change. In 2004, the Chinese central government approved the Master Plan for Development of the Chongming Three Islands. One of its main objectives was to develop the southern coast as the economic lifeline of Chongming and Shanghai for future development. To achieve this objective, two unprecedented engineering projects, including deepwater navigation channels and the Qincaosha Reservoir, are currently underway. These two engineering projects will inevitably alter the sedimentary and hydro-dynamic properties of the region, accelerating coastal erosion of the southern coast and creating a greater demand for the shore stability capacity of the coastal shelterbelt. Recent research has shown that the rate of coastal beach siltation along the southern coast has been slowing since 2002. Not only has coastal erosion accelerated in recent years, but the sedimentation processes of some shores have actually transformed from undergoing net deposition to erosion (Cao 2008). These situations raise a serious challenge to the local Water Resources Bureau:how can the structure of the current coastal shelterbelt be optimized and its ability to stabilize the shore improved?One potential method for improving shore stability is to replant woody tree species into the current reed community. In 2002, the Water Resources Bureau of Chongming County selected two tree species, Salix babylonica and Taxodium ascendens, because of their tolerance to hydric and saline conditions. In 2002 and 2006,2-yr-old S. babylonica and T. ascendens saplings were transplanted separately into reed communities along an intertidal gradient. This has resulted in the formation of four mixed-species communities:a 4-yr-old S. babylonica-P. australis community (4Sb), an 8-yr-old S. babylonica-P. australis community (8Sb), a 4-yr-old T. ascendens-P. australis community (4Ta), and an 8-yr-old T. ascendens-P. australis community (8Ta). A question concerining these efforts is whether S. babylonica and T. ascendens can enhance the shore stabilizing capacity of the reed vegetation?Before we can answer this question, we need to understand the characteristics of soil susceptibility and soil stability in response to water erosion under these vegetation communities and the erosion processes and their period of occurrence in the study area because the soil susceptibility and stability are closely related to shore failure, and identifying coastal erosion processes and their period of occurrence is essential for analysis of the merits of each vegetation protection measure.In this study, the protection ability for shore under reed community,4Sb,8Sb,4Ta and 8Ta within lower intertidal zone (LIT), middle intertidal zone (MIT) and higher intertidal zone (HIT) in spring, summer and autumn-winter was evaluated by resistance coefficient ratio and soil stability index. And then combining with erosion processes and their period of occurrence in the study area, this paper pointed out some disadvantages to current coastal shelter management and establishment in Chongming Island, and also presented some suggestions for solving these existing problems. The results and conclusions were as follows:1) The reed community can best reduce tidal current effects on shore, and can also provide the best protection for mitigating shore downward erosion in spring and summer within MIT and HIT.As for resistance coefficient ratio, the reed community had higher ratio than other vegetation of addition of S. babylonica and T. ascendens in spring, summer and autumn-winter. It is probable that the highest reed density of reed community among five vegetation conditions resulted in this result. On the other hand, The reed community had the greatest soil stability index and RLD among all vegetation communities in the 0-20cm upper soil in spring and summer within the MIT and HIT. It had the greatest stability index and its value of stability index was significantly higher than 8Ta,4Sb,4Ta and 8Sb (P<0.05). For example, in spring, the stability index value of 4Sb,8Sb,4Ta and 8Ta decreased by 13.2%(P<0.05),42.8%(P<0.05), 16.2%(P<0.05),11.7%(P<0.05) within MIT, respectively, corresponding by 13.7%(P<0.05),42.3%(P<0.05), 11.1%(P<0.05),10.0%(P<0.05) within HIT, respectively, compared with the value under reed community. In summer, the stability index of 4Sb,8Sb,4Ta and 8Ta decreased by 29.1%(P<0.05),40.4%(P<0.05), 31.0%(P<0.05),21.6%(P<0.05) within MIT, respectively, corresponding to by 8.2%(P<0.05),47.9%(P<0.05),21.5%(P<0.05),12.0%(P<0.05) within HIT, respectively compared with reed community. The effect of root length density (x) on soil stability index (y) could be expressed by the equation y=aeb/x.2) 4Sb may the best vegetation type to reinforce the 20-50-cm basal soil layer of the erosion scarp, and 8Ta can provide the best protection for the beach face in all recorded periods in the LIT and in Nov-Dec in the MIT and HIT.8Ta had the greatest RLD and stability index in the 0-20-cm surface soil layer in all intertidal zones and observation periods, except in spring and summer in the MIT and HIT. For example, in autumn-winter, the stability index of reed,4Sb,4Ta and 8Sb decreased by 7.2%,6.6%,32.7% and 2.8% compared with 8Ta within MIT, respectively, corresponding to 19.6%,14.9%,42.9%(p<0.05) and 21.8% within HIT, respectively.4Sb had the greatest stability index and RLD among the five vegetation conditions in the 20-50-cm deep soil layer of all observation periods within each intertidal zone. For example, within LIT,4Sb had the greatest soil stability index among the five vegetation conditions, followed by 8Ta,4Ta, reed and 8Sb in spring, compared with 4Sb, their value of stability index decreased of 34.5%(P<0.05), 39.8%(P<0.05),46.5%(P<0.05) and 51.8%(P<0.05) in spring, respectively. In summer within LIT,4Sb was followed by 8Ta,4Ta,8Sb and reed. Compared with 4Sb, their value of stability index decreased of 0.04%,19.6%(P<0.05),37.4%(P<0.05) and 44.9%(P<0.05) respectively. In autumn-winter of LIT,4Sb was followed by 8Ta, reed community,4Ta and 8Sb, their value of stability index decreased of 10.1%, 46.0%(P<0.05),49.9%(P<0.05) and 50.0%(P<0.05), respectively. The effect of root length density (x) on soil stability (y) could be expressed by the equation y=aeb/x.3) The seasonal variation of resistance coefficient ratio under five vegetation conditions indicated that the value for ratio increased from spring to summer and autumn-winter. On the other hand, the seasonal variation of soil stability index showed that stability index value decreased from spring and summer to autumn-winter.The coefficient ratio under each individual vegetation condition is lower in spring than in summer and autumn-winter, and it increased from spring to summer and autumn-winter. These results are closely related to reed quantity or density variation in different growing period. Spring is the sprout period of reed, and thus the reed quantity is less in spring than in summer and autumn-winter. To summer and autumn-winteT, reed enters into vigorous period and blooming stage, respectively. Thus, the reed quantity is higher in summer and autumn-winter than in spring. Additionally, the soil stability index was higher value in spring and summer than in autumn-winter, and it showed the seasonal variation with decreasing from spring and summer to autumn-winter. Plant roots has directly influence on the variation of stability index. Spring and summer is sprout stage and vigorous period of reed, and the speed of root growth of reed surpass the speed of root death in these two period. To autumn-winter, reed begins to enter into the last growth stage, and its root death speed surpasses the root growth speed in this stage. Therefore, the soil stability index with lower root length density was less in autumn-winter than in spring and summer.4) The shore stability capacity under the S. babylonica community decreases with increasing tree age of S. babylonica. In comparison with the S. babylonica community, the stability index observed for the T. ascendens community was in the opposite direction, and the soil stability capacity under the T. ascendens community increases with increasing tree age of T. ascendens.With respect to the soil stability index of 4Sb and 8Sb at LIT, the results were more interesting. The stability index value of 4Sb was higher than 8Sb at all observed periods and soil depths, and their differences were statistically significant at the 0.05 level, except in the topsoil depth in Nov-Dec. This result means that the shore stability capacity under the S. babylonica community decreases with increasing tree age of S. babylonica. It is possible that a major decrease in the density of reeds under 8Sb compared to 4Sb can explain this result. To evaluate this possibility, the total number of reed culms was recorded in each of four randomly located 2 X 2-m plots in both 4Sb and 8Sb. The results of this experiment showed that the reed density under 8Sb was significantly lower than under 4Sb in all intertidal zones and recorded periods (p<0.01). The most likely reason for this decrease in the reed density is a result of the 2×2-m plant spacing of S. babylonica. This spacing was feasible for 4-yr-old S. babylonica; however, with increasing tree age and size, it became impractical for 8-yr-old S. babylonica due to the increasing coverage and shading of S. babylonica's vase-shaped crown. The shade effect of 8-yr-old S. babylonica causing reduction in solar radiation available for understory reed vegetation could result in the observed decrease in reed density. In contrast, the 2 X 2-m plant spacing had little influence on the reed density under 8Ta. Two potential reasons for this are that T. ascendens is a slow-growing tree and that its crown is narrow and conical-shaped and does not shade reeds nearly as much as S. babylonica's vase-shaped crown.5) Some disadvantages existed in coastal shelterbelt management and establishment along southern coast of Chongming Island including no protection for erosion scarp, lacking scientific consideration for vegetation configuration along intertidal zone, and the over lower height for reed reaping in winter.The erosion scarp failure and recession is the main reason for shoreline retreat. Due to the lack of root distribution below the 20-cm soil depth, blocks of soil can be removed from the basal area of the scarp by tidal current entrainment, leaving a block of unsupported material on the shore top. Unsupported material then slides or drops off and causes scarp recession. To discourage scarp recession requires root reinforcement in both the 0-to 20-cm upper and 20-to 50-cm basal soil layers, which will then behave as a composite block, resulting in additional beach strength and apparent cohesion via friction between the root surface and soil particles. For achieving this composite block, high root density needs to occur in not only the surface soil layer, but also in the 20-50-cm basal soil layer. Along the southern coast of Chongming Island, the reeds still bordered the erosion scarp, and erosion scarp lack the relevant trees to reinforce the scarp basal soil these mixed-species zones. Lacking scientific consideration for vegetation configuration included that the selection for coastal shelterbelt configuration pattern didn't combine with erosion process and occurrence period, and impertinence application for adding woody species into reed community. Firstly, the optimum selection for vegetation pattern for shore protection should combine with the erosion processes and erosion occurrence period along southern coast and depend on the characteristic of each individual vegetation condition. Secondly, as mentioned above, excessive shade from 8-yr-old S. babylonica can result in a drastic decrease in number of reeds, leading to increased erosion risk. Therefore, management of the coastal shelterbelt is necessary to maintain a vigorous cover of herbaceous vegetation for promoting shore stabilization. Thirdly, it was generally that the shoreline stabilization of 4Ta was the least among the five vegetation community. The reason is probably that T. ascendens is a slow-growing tree and it needs a much longer period to improve shore stabilization than S. babylonica.From December to January, reaping reed stem, for providing enough light for reed germination in subsequence growth years, is an important management measurement on coastal shelterbelt in Chongming Island. In process of harvesting, the reed stems are cut down to near ground level. However, it should recognize that this reaping method, cutting down reed stem to near ground level, will reduce or even disappear the cover of reed stem for beach surface, and therefore also will improve the loss of sediments from shore. Assumed the erosion periods happening from October to December, this reaping will reduce the resistance coefficient ratio of reed stem to zero. Meanwhile, from October to December, soil stability index and root length density also will reach to the least value. These two unfavorable factors will further improve the erosion rate of shoreline and erosion efficiency of tidal current.6) In this study area, there probably exist two different erosion occurrence periods including either occurrence from October to December (scenario 1) or from March to August (scenario 2). It should adopt different vegetation configuration pattern for shore protection under different scenario.We assumed that the erosion period happens from October to December. To discourage shore face downward erosion,8Ta is the most optimal of the all vegetation communities along the intertidal gradient because it had the greatest RLD and stability index in the 0-20-cm topsoil layer in autumn-winter within each intertidal zone. Under scenario 1, as mentioned above results, an S. babylonica community with interplanting of hydric-tolerant and shade-tolerant native shrub or T. ascendens community with 6- to 8-yr-old T. ascendens may be recommended to border the erosion scarp for reducing scarp retreat. Regardless of the scarp location, we may select a T. ascendens community along intertidal gradient for reducing beach face downward erosion.We assumed that the erosion period occurs from March to August. For reducing shore face downward erosion,4Sb and 8Ta were more suitable than the other three vegetation communities in the LIT because they had a higher RLD and stability index than the other vegetation conditions in the 0- to 20-cm upper soil in spring and summer within the LIT. However, in the MIT and HIT, the reed community will be able to provide the most protection for mitigating beach face downward erosion because it had a much greater RLD and stability index than all of the mixed-species communities in the 0-20-cm topsoil layer in spring and summer. Under scenario 2, according to our results, we recommend the most suitable vegetation community for retaining shore stability along an intertidal gradient on the southern coast of Chongming Island. Along the southern coast, there exist three erosion levels of shores that can be identified by the location of erosion scarp. The scarp located in the LIT exhibits slight erosion, and the scarps in the MIT and HIT exhibit an intermediate erosion hazard and serious erosion hazard, respectively. The suggested vegetation communities along the intertidal gradient are shown in Fig 4. Regardless of the erosion scarp location, an S. babylonica community may be utilized for reducing the scarp recession with interplanting of hydric-tolerant and shade-tolerant native shrubs to border the scarp. The buffer width of S. babylonica may be 15-20 m. For shores with slight erosion, if there was still a remnant tidal flat within the LIT, an S. babylonica or T. ascendens community might be selected to reduce beach face backward erosion, except for in the scarp zone with S. babylonica. In the MIT and HIT, the reed community may be retained because of its high RLD and stability index for the surface soil in spring and summer. For the shore exhibiting middle erosion and serious erosion levels, the reed community may be selected for protection beach face.
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