崇明岛南岸不同植物配置模式护岸能力及优化策略研究
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摘要
海岸带侵蚀是全球沿海城市普遍面临的一大难题。为此,如何减少海岸侵蚀保护海岸带一直是全球广泛关注的热点问题。
崇明岛南岸正是面临同样问题的地区之一。为减少海岸侵蚀保护海岸带,早在上世纪50年代,县水利局就利用滩涂湿地植物——芦苇(Phragmites australis),构建成了芦苇群落护岸带。在过去的几十年中,芦苇群落对崇明南岸岸滩的稳定发挥了关键作用。然而,它正面临着新的挑战。近年南岸深水航道建设及青草沙水库的建设势必将加剧对南岸岸滩的冲刷和侵蚀,使南岸坍势更为严重。同时也将对原有芦苇群落的护岸能力提出更高要求。为提升原有芦苇群落的护岸能力,在2002和2006年,崇明县水利局在考虑树种耐湿性、耐盐碱性等特性后,从针叶乔木与阔叶乔木,垂直根型与散生根型乔木中筛选出池杉(Taxodium ascendens)、垂柳(Salix babylonica)两种乔木,在主要防护岸段沿240cm-370cm潮位线将其分别以2×2m种植密度增配于原有芦苇群落中,分别构建成池杉—芦苇群落和垂柳—芦苇群落。目前南岸植物护岸模式为芦苇群落、4龄垂柳—芦苇群落、8龄垂柳—芦苇群落、4龄池杉—芦苇群落和8龄池杉—芦苇群落。对此,究竟哪种植物配置模式的护岸能力最佳一直是崇明南岸护岸林带建设与管理中普遍关注的问题。
要回答好这一关键问题,则主要取决于下述三个问题的回答：各季节各植物群落的护岸特点如何,及各植物群落护岸能力的季节变化规律如何?垂柳—芦苇群落和池杉—芦苇群落的护岸能力随乔木树龄的增加产生怎样的变化?研究区域岸滩侵蚀过程及侵蚀季节性变化规律如何?
本研究利用阻力系数比、土壤水稳性指数和根长密度三项指标对低潮位(240-270cm潮位线与侵蚀陡坎间岸滩)、中潮位(240-270cm与300-320cm潮位线间岸滩)、高潮位(300-320cm与350-370cm潮位线间岸滩),春季、夏季与秋冬季芦苇群落、4龄垂柳群落、8龄垂柳群落、4龄池杉群落和8龄池杉群落的地上部分与地下部分护岸能力进行了分析,总结出了各潮位不同植物群落护岸能力的季节变化规律,并结合研究区域岸滩侵蚀过程与侵蚀季节性变化规律,指出了崇明南岸护岸林带建设与管理现存的主要问题,并提出了相应的优化建议,期望能为未来南岸护岸林带的建设与管理提供科学依据。主要研究结果与结论如下：
1)芦苇群落在各潮位各季节对水流的阻力作用最大,在春夏两季对中、高潮位岸滩表土层的保护能力最强
从植物地上部分的阻力系数比分析,在春季、夏季、秋冬季,芦苇群落的阻力系数比均高于四种乔草群落,表明芦苇群落对水流的阻力作用最大,这与芦苇群落内芦苇密度最大有关。从土壤水稳性指数分析,在春夏两季的中、高潮位,0-20cm表土层中,芦苇群落的土壤水稳性指数和根长密度均显著性的高于乔草群落(P<0.05)。如与芦苇群落土壤水稳性指数相比,在春季中潮位4龄垂柳群落、8龄垂柳群落、4龄池杉群落、8龄池杉群落分别减少了13.2%(P<0.05)、42.8% (P<0.05)、16.2% (P<0.05)、11.7% (P<0.05),高潮位它们分别小13.7%(P<0.05)、42.3% (P<0.05)、11.1% (P<0.05)、10.0% (P<0.05)；夏季中潮位它们也分别减少了29.1% (P<0.05)、40.4% (P<0.05)、31.0% (P<0.05)、21.6%(P<0.05),高潮位分别低了8.2% (P<0.05)、47.9% (P<0.05)、21.5% (P<0.05)、12.0% (P<0.05)。土壤水稳性指数(y)主要受根长密度(x)的影响,二者的关系可用指数函数Y=aeb/x表示。
2)4龄垂柳群落在各潮位各季节对岸滩20-50cm土层的保护能力最强；8龄池杉群落在低潮位各季节及中、高潮位秋冬季对岸滩滩面的保护能力最强
从阻力系数比分析,所有乔草群落的阻力系数比均低于芦苇群落,这与乔草群落中芦苇密度低于芦苇群落内芦苇密度有关。从土壤水稳性指数分析,在0-20cm表土层中,与其它四种群落相比,8龄池杉群落在低潮位各季节及中、高潮位秋冬季具有最大的土壤水稳性指数和根长密度。如在秋冬季,与8龄池杉群落相比,中潮位芦苇群落、4龄垂柳群落、8龄垂柳群落和4龄池杉群落下的土壤水稳性指数均分别少7.2%、6.6%、32.7%、2.8%；高潮位它们的土壤水稳性指数则分别减少了19.6%、14.9%、42.9%(P<0.05)、21.8%。在20-50cm土层,在各潮位各季节,五种植物群落中4龄垂柳群落的土壤水稳性指数与根长密度最高。如在低潮位,4龄垂柳群落之后,春季的土壤水稳性指数大小依次为8龄池杉群落、4龄池杉群落、芦苇群落、8龄垂柳群落,它们分别比4龄垂柳群落低34.5%(P<0.05)、39.8%(P<0.05)、46.5%(P<0.05)、51.8%(P<0.05)；夏季则分别是8龄池杉群落>4龄池杉群落>8龄垂柳群落>芦苇群落,它们与4龄垂柳群落相比,分别降低了0.04%、19.6% (P<0.05)、37.4% (P<0.05)、44.9% (P<0.05)；秋冬季则分别是8龄池杉群落>芦苇群落>4龄池杉群落>8龄垂柳群落,与4龄垂柳群落相比,它们均分别下降了10.1%、46.0%(P<0.05)、49.9%(P<0.05)和50.0%(P<0.05)。土壤水稳性指数主要受根长密度影响,二者的关系为指数函数Y=aeb/x。
3)各潮位各植物群落地上部分阻力系数比的季节变化规律是由春季向夏季和秋冬季增加,地下根系固岸能力的季节变化规律则是由春夏两季向秋冬季减小
各植物群落地上部分阻力系数比的变化规律均是春季低,夏季与秋冬季较高,由春季向夏季、秋冬季增加。阻力系数比的变化规律与群落内芦苇密度紧密相关。春季正处于芦苇萌芽期,及受上年冬末芦苇收割的影响,因而此时芦苇密度较低阻力系数比也低；夏季进入芦苇旺盛生长期,群落内芦苇密度较高阻力系数比大；进入秋冬季虽然进入芦苇开花期,但群落内芦苇密度依然较大阻力系数比也高。各植物群落内表土层和深土层土壤水稳性指数的季节变化规律,则正好与阻力系数比的变化相反,表现为秋冬季最低,春夏两季高,由春夏两季向秋冬季减小。这与群落根长密度的季节变化相关。春季是植物萌芽阶段,夏季是植物旺盛生长阶段,这两季地下根系生长的速度超过根系衰退的速度,所以春夏两阶段的根系量大,根长密度长,土壤水稳性指数较高。秋冬季进入植物生长末期,根系生长速度比春夏逐渐放慢,所以根长密度小,土壤水稳性指数也就低。
4)4龄与8龄乔草群落相比,垂柳群落根系固岸能力随垂柳树龄增加而减小,池杉群落地下部分固岸能力随池杉树龄增加而增大
研究区域岸滩侵蚀驱动力主要为潮流,侵蚀过程包括滩面侵蚀和侵蚀陡坎蚀退两类,岸滩稳定更为依靠地下根系作用。4龄垂柳与8龄垂柳群落,各潮位各季节各土层,8龄垂柳群落的土壤水稳性指数均显著性低于4龄垂柳群落(P<0.05),表明垂柳群落的固岸能力并没有随垂柳树龄的增大而增强,而是随之减弱。其原因可能在于在2×2m的种植密度下,随着树龄增加,具有伞形树冠的垂柳对其林内的“遮光效应”也在不断增加,下层光线的减弱影响了群落内喜光植物芦苇生长,造成8龄垂柳群落内芦苇密度极显著的低于4龄垂柳群落(P<0.01),显著的降低了群落的根长密度(P<0.05),从而表现出8龄垂柳群落的土壤水稳性指数与根长密度皆低于4龄垂柳群落。相反,对于4龄与8龄池杉群落,虽然也同为2×2m的种植密度,但8龄池杉群落的土壤水稳性指数和根长密度在各潮位各季节各土层均高于4龄池杉群落,池杉群落的固岸能力随池杉树龄增加而增强。原因可能是池杉属慢生树种,树冠为狭圆锥形,对林内芦苇的遮光影响不如快生伞形树冠的垂柳。
5)崇明南岸护岸林带建设与管理中存在：侵蚀陡坎区域缺乏有效防护、护岸植物群落结构配置不合理、群落内芦苇地上部分冬季收割高度过低等问题
目前南岸侵蚀陡坎上仍多为芦苇群落。依据前述研究结果,仅在中高潮位春夏两季对表土层有较强护岸能力的芦苇群落将很难满足减缓或阻止陡坎后退的需要。因此对侵蚀关键区域——侵蚀陡坎缺乏有效防护是当前南岸护岸林带建设与管理中存在的最大问题。
护岸植物群落结构配置的不合理主要体现在植物护岸模式的选择及建设缺乏与岸滩侵蚀过程、季节性变化规律相结合及垂柳种植密度和池杉树龄选择欠合理等方面。首先,最佳植物护岸模式的选择应结合岸滩的侵蚀过程和“春夏储秋冬输”或“春夏输秋冬储”的季节性变化规律,依据各配置模式在各季节的护岸特点,从低潮位至高潮位分梯级的选择最优植物护岸模式。然而目前南岸护岸林带为单一连续式,即采用同一植物配置模式从低潮位至高潮位连续呈片种植。其次,在护岸林带日常管理中,缺乏对8龄垂柳群落中垂柳2×2m种植密度的调整,导致垂柳下芦苇密度锐减,使8龄垂柳群落的护岸能力低于4龄垂柳群落,影响其护岸能力。另外,对于池杉群落,与其它植物配置模式相比,4龄池杉群落的护岸能力并不理想,这可能与池杉属慢生树种有关。
每年12月至来年1月对芦苇地上部分进行齐地收割,以保证第二年萌芽期芦苇能有充足光照是崇南海岸带管理中的一项重要管理措施。然而,应当指出的是,齐地收割的方式将使岸滩滩面直接暴露于潮流的冲刷中,加速地表凋落物随潮流流失的速度,加快滩面沉积物的流失速度。
6)根据研究区域岸滩的侵蚀过程,侵蚀季节性变化在“春夏储秋冬输”和“春夏输秋冬储”情景模式下,岸滩需采用不同植物配置方式
研究区域岸滩侵蚀过程包括侵蚀陡坎蚀退和滩面侵蚀两种类型。本研究假定研究区域内岸滩侵蚀季节变化规律存在“春夏储秋冬输”“春夏输秋冬储”两种可能情景。若侵蚀季节性变化属“春夏储秋冬输”,植物根系固岸作用的低峰期恰与侵蚀高峰期相符,结合前面研究结果,在陡坎区域,要减缓或防止陡坎后退,可选择对表土和深土层护岸能力高的垂柳—芦苇的配置方式,对此还需考虑增加灌木树种,提高表土的稳定性。而侵蚀陡坎下区域可选择灌木—镳草配置模式。同时,对于侵蚀陡坎位于低潮位岸滩(此类岸滩属侵蚀程度较轻),除了陡坎及其之下区域外,中潮位和高潮位应选择高龄池杉—芦苇的植物配置方式,因其在秋冬季表土层的根长密度最高,对表土层的保护效果最好；对于侵蚀陡坎位于中潮位岸滩(此类岸滩属侵蚀程度较高),除了陡坎及其之下区域外,高潮位应选择高龄池杉—芦苇模式；若侵蚀陡坎位于高潮位(此类岸滩属侵蚀严重岸滩),陡坎及以上区域应选择高龄池杉—芦苇或垂柳—灌木—芦苇的配置方式,而侵蚀陡坎下区域可选择灌木—镳草配置模式。
若侵蚀季节性变化规律为“春夏输秋冬储”,植物根系固岸作用的高峰期正与侵蚀高峰期相符。则在陡坎区域应选择垂柳—灌木—芦苇的配置方式,而在侵蚀陡坎之下区域可选择灌木—镳草配置模式。此外,对于侵蚀陡坎位于低潮位岸滩,除侵蚀陡坎及之下区域外,中潮位和高潮位则可保持芦苇群落的植物配置模式,因为它不仅对水流的阻力作用最大,而且在春夏两季表土层的土壤水稳性指数和根长密度均最高,更有利于保护岸滩滩面。对于侵蚀陡坎位于中潮位岸滩,除侵蚀陡坎及之下区域外,高潮位仍应保持芦苇群落。若侵蚀陡坎位于高潮位,则陡坎区域应选择高龄池杉—芦苇或垂柳—灌木—芦苇的配置方式,而侵蚀陡坎下区域可选择灌木—镳草配置模式。
以上需要增配灌木的植物配置模式,灌木可以考虑选择耐水湿、耐荫、浅根系的种类,如细叶水团花(Adina rubella)、赤杨(Alnus tinctoria)、花叶柳(Salix integra cv. Hakuro Nishiki)、海滨木槿(Hibiscus hamabo)。考虑到林带未来整体的景观效果,还可以选择与池杉、垂柳同功能群的乔木树种,增强林带的观赏价值。如选择落羽杉(Taxodium distichum)、水杉(Metasequoia glyptostroboides)、江南桤木(Alnus trabeculosa)、沼生栎(Quercus palustris)等树种。
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.
崇明岛南岸正是面临同样问题的地区之一。为减少海岸侵蚀保护海岸带,早在上世纪50年代,县水利局就利用滩涂湿地植物——芦苇(Phragmites australis),构建成了芦苇群落护岸带。在过去的几十年中,芦苇群落对崇明南岸岸滩的稳定发挥了关键作用。然而,它正面临着新的挑战。近年南岸深水航道建设及青草沙水库的建设势必将加剧对南岸岸滩的冲刷和侵蚀,使南岸坍势更为严重。同时也将对原有芦苇群落的护岸能力提出更高要求。为提升原有芦苇群落的护岸能力,在2002和2006年,崇明县水利局在考虑树种耐湿性、耐盐碱性等特性后,从针叶乔木与阔叶乔木,垂直根型与散生根型乔木中筛选出池杉(Taxodium ascendens)、垂柳(Salix babylonica)两种乔木,在主要防护岸段沿240cm-370cm潮位线将其分别以2×2m种植密度增配于原有芦苇群落中,分别构建成池杉—芦苇群落和垂柳—芦苇群落。目前南岸植物护岸模式为芦苇群落、4龄垂柳—芦苇群落、8龄垂柳—芦苇群落、4龄池杉—芦苇群落和8龄池杉—芦苇群落。对此,究竟哪种植物配置模式的护岸能力最佳一直是崇明南岸护岸林带建设与管理中普遍关注的问题。
要回答好这一关键问题,则主要取决于下述三个问题的回答：各季节各植物群落的护岸特点如何,及各植物群落护岸能力的季节变化规律如何?垂柳—芦苇群落和池杉—芦苇群落的护岸能力随乔木树龄的增加产生怎样的变化?研究区域岸滩侵蚀过程及侵蚀季节性变化规律如何?
本研究利用阻力系数比、土壤水稳性指数和根长密度三项指标对低潮位(240-270cm潮位线与侵蚀陡坎间岸滩)、中潮位(240-270cm与300-320cm潮位线间岸滩)、高潮位(300-320cm与350-370cm潮位线间岸滩),春季、夏季与秋冬季芦苇群落、4龄垂柳群落、8龄垂柳群落、4龄池杉群落和8龄池杉群落的地上部分与地下部分护岸能力进行了分析,总结出了各潮位不同植物群落护岸能力的季节变化规律,并结合研究区域岸滩侵蚀过程与侵蚀季节性变化规律,指出了崇明南岸护岸林带建设与管理现存的主要问题,并提出了相应的优化建议,期望能为未来南岸护岸林带的建设与管理提供科学依据。主要研究结果与结论如下：
1)芦苇群落在各潮位各季节对水流的阻力作用最大,在春夏两季对中、高潮位岸滩表土层的保护能力最强
从植物地上部分的阻力系数比分析,在春季、夏季、秋冬季,芦苇群落的阻力系数比均高于四种乔草群落,表明芦苇群落对水流的阻力作用最大,这与芦苇群落内芦苇密度最大有关。从土壤水稳性指数分析,在春夏两季的中、高潮位,0-20cm表土层中,芦苇群落的土壤水稳性指数和根长密度均显著性的高于乔草群落(P<0.05)。如与芦苇群落土壤水稳性指数相比,在春季中潮位4龄垂柳群落、8龄垂柳群落、4龄池杉群落、8龄池杉群落分别减少了13.2%(P<0.05)、42.8% (P<0.05)、16.2% (P<0.05)、11.7% (P<0.05),高潮位它们分别小13.7%(P<0.05)、42.3% (P<0.05)、11.1% (P<0.05)、10.0% (P<0.05)；夏季中潮位它们也分别减少了29.1% (P<0.05)、40.4% (P<0.05)、31.0% (P<0.05)、21.6%(P<0.05),高潮位分别低了8.2% (P<0.05)、47.9% (P<0.05)、21.5% (P<0.05)、12.0% (P<0.05)。土壤水稳性指数(y)主要受根长密度(x)的影响,二者的关系可用指数函数Y=aeb/x表示。
2)4龄垂柳群落在各潮位各季节对岸滩20-50cm土层的保护能力最强；8龄池杉群落在低潮位各季节及中、高潮位秋冬季对岸滩滩面的保护能力最强
从阻力系数比分析,所有乔草群落的阻力系数比均低于芦苇群落,这与乔草群落中芦苇密度低于芦苇群落内芦苇密度有关。从土壤水稳性指数分析,在0-20cm表土层中,与其它四种群落相比,8龄池杉群落在低潮位各季节及中、高潮位秋冬季具有最大的土壤水稳性指数和根长密度。如在秋冬季,与8龄池杉群落相比,中潮位芦苇群落、4龄垂柳群落、8龄垂柳群落和4龄池杉群落下的土壤水稳性指数均分别少7.2%、6.6%、32.7%、2.8%；高潮位它们的土壤水稳性指数则分别减少了19.6%、14.9%、42.9%(P<0.05)、21.8%。在20-50cm土层,在各潮位各季节,五种植物群落中4龄垂柳群落的土壤水稳性指数与根长密度最高。如在低潮位,4龄垂柳群落之后,春季的土壤水稳性指数大小依次为8龄池杉群落、4龄池杉群落、芦苇群落、8龄垂柳群落,它们分别比4龄垂柳群落低34.5%(P<0.05)、39.8%(P<0.05)、46.5%(P<0.05)、51.8%(P<0.05)；夏季则分别是8龄池杉群落>4龄池杉群落>8龄垂柳群落>芦苇群落,它们与4龄垂柳群落相比,分别降低了0.04%、19.6% (P<0.05)、37.4% (P<0.05)、44.9% (P<0.05)；秋冬季则分别是8龄池杉群落>芦苇群落>4龄池杉群落>8龄垂柳群落,与4龄垂柳群落相比,它们均分别下降了10.1%、46.0%(P<0.05)、49.9%(P<0.05)和50.0%(P<0.05)。土壤水稳性指数主要受根长密度影响,二者的关系为指数函数Y=aeb/x。
3)各潮位各植物群落地上部分阻力系数比的季节变化规律是由春季向夏季和秋冬季增加,地下根系固岸能力的季节变化规律则是由春夏两季向秋冬季减小
各植物群落地上部分阻力系数比的变化规律均是春季低,夏季与秋冬季较高,由春季向夏季、秋冬季增加。阻力系数比的变化规律与群落内芦苇密度紧密相关。春季正处于芦苇萌芽期,及受上年冬末芦苇收割的影响,因而此时芦苇密度较低阻力系数比也低；夏季进入芦苇旺盛生长期,群落内芦苇密度较高阻力系数比大；进入秋冬季虽然进入芦苇开花期,但群落内芦苇密度依然较大阻力系数比也高。各植物群落内表土层和深土层土壤水稳性指数的季节变化规律,则正好与阻力系数比的变化相反,表现为秋冬季最低,春夏两季高,由春夏两季向秋冬季减小。这与群落根长密度的季节变化相关。春季是植物萌芽阶段,夏季是植物旺盛生长阶段,这两季地下根系生长的速度超过根系衰退的速度,所以春夏两阶段的根系量大,根长密度长,土壤水稳性指数较高。秋冬季进入植物生长末期,根系生长速度比春夏逐渐放慢,所以根长密度小,土壤水稳性指数也就低。
4)4龄与8龄乔草群落相比,垂柳群落根系固岸能力随垂柳树龄增加而减小,池杉群落地下部分固岸能力随池杉树龄增加而增大
研究区域岸滩侵蚀驱动力主要为潮流,侵蚀过程包括滩面侵蚀和侵蚀陡坎蚀退两类,岸滩稳定更为依靠地下根系作用。4龄垂柳与8龄垂柳群落,各潮位各季节各土层,8龄垂柳群落的土壤水稳性指数均显著性低于4龄垂柳群落(P<0.05),表明垂柳群落的固岸能力并没有随垂柳树龄的增大而增强,而是随之减弱。其原因可能在于在2×2m的种植密度下,随着树龄增加,具有伞形树冠的垂柳对其林内的“遮光效应”也在不断增加,下层光线的减弱影响了群落内喜光植物芦苇生长,造成8龄垂柳群落内芦苇密度极显著的低于4龄垂柳群落(P<0.01),显著的降低了群落的根长密度(P<0.05),从而表现出8龄垂柳群落的土壤水稳性指数与根长密度皆低于4龄垂柳群落。相反,对于4龄与8龄池杉群落,虽然也同为2×2m的种植密度,但8龄池杉群落的土壤水稳性指数和根长密度在各潮位各季节各土层均高于4龄池杉群落,池杉群落的固岸能力随池杉树龄增加而增强。原因可能是池杉属慢生树种,树冠为狭圆锥形,对林内芦苇的遮光影响不如快生伞形树冠的垂柳。
5)崇明南岸护岸林带建设与管理中存在：侵蚀陡坎区域缺乏有效防护、护岸植物群落结构配置不合理、群落内芦苇地上部分冬季收割高度过低等问题
目前南岸侵蚀陡坎上仍多为芦苇群落。依据前述研究结果,仅在中高潮位春夏两季对表土层有较强护岸能力的芦苇群落将很难满足减缓或阻止陡坎后退的需要。因此对侵蚀关键区域——侵蚀陡坎缺乏有效防护是当前南岸护岸林带建设与管理中存在的最大问题。
护岸植物群落结构配置的不合理主要体现在植物护岸模式的选择及建设缺乏与岸滩侵蚀过程、季节性变化规律相结合及垂柳种植密度和池杉树龄选择欠合理等方面。首先,最佳植物护岸模式的选择应结合岸滩的侵蚀过程和“春夏储秋冬输”或“春夏输秋冬储”的季节性变化规律,依据各配置模式在各季节的护岸特点,从低潮位至高潮位分梯级的选择最优植物护岸模式。然而目前南岸护岸林带为单一连续式,即采用同一植物配置模式从低潮位至高潮位连续呈片种植。其次,在护岸林带日常管理中,缺乏对8龄垂柳群落中垂柳2×2m种植密度的调整,导致垂柳下芦苇密度锐减,使8龄垂柳群落的护岸能力低于4龄垂柳群落,影响其护岸能力。另外,对于池杉群落,与其它植物配置模式相比,4龄池杉群落的护岸能力并不理想,这可能与池杉属慢生树种有关。
每年12月至来年1月对芦苇地上部分进行齐地收割,以保证第二年萌芽期芦苇能有充足光照是崇南海岸带管理中的一项重要管理措施。然而,应当指出的是,齐地收割的方式将使岸滩滩面直接暴露于潮流的冲刷中,加速地表凋落物随潮流流失的速度,加快滩面沉积物的流失速度。
6)根据研究区域岸滩的侵蚀过程,侵蚀季节性变化在“春夏储秋冬输”和“春夏输秋冬储”情景模式下,岸滩需采用不同植物配置方式
研究区域岸滩侵蚀过程包括侵蚀陡坎蚀退和滩面侵蚀两种类型。本研究假定研究区域内岸滩侵蚀季节变化规律存在“春夏储秋冬输”“春夏输秋冬储”两种可能情景。若侵蚀季节性变化属“春夏储秋冬输”,植物根系固岸作用的低峰期恰与侵蚀高峰期相符,结合前面研究结果,在陡坎区域,要减缓或防止陡坎后退,可选择对表土和深土层护岸能力高的垂柳—芦苇的配置方式,对此还需考虑增加灌木树种,提高表土的稳定性。而侵蚀陡坎下区域可选择灌木—镳草配置模式。同时,对于侵蚀陡坎位于低潮位岸滩(此类岸滩属侵蚀程度较轻),除了陡坎及其之下区域外,中潮位和高潮位应选择高龄池杉—芦苇的植物配置方式,因其在秋冬季表土层的根长密度最高,对表土层的保护效果最好；对于侵蚀陡坎位于中潮位岸滩(此类岸滩属侵蚀程度较高),除了陡坎及其之下区域外,高潮位应选择高龄池杉—芦苇模式；若侵蚀陡坎位于高潮位(此类岸滩属侵蚀严重岸滩),陡坎及以上区域应选择高龄池杉—芦苇或垂柳—灌木—芦苇的配置方式,而侵蚀陡坎下区域可选择灌木—镳草配置模式。
若侵蚀季节性变化规律为“春夏输秋冬储”,植物根系固岸作用的高峰期正与侵蚀高峰期相符。则在陡坎区域应选择垂柳—灌木—芦苇的配置方式,而在侵蚀陡坎之下区域可选择灌木—镳草配置模式。此外,对于侵蚀陡坎位于低潮位岸滩,除侵蚀陡坎及之下区域外,中潮位和高潮位则可保持芦苇群落的植物配置模式,因为它不仅对水流的阻力作用最大,而且在春夏两季表土层的土壤水稳性指数和根长密度均最高,更有利于保护岸滩滩面。对于侵蚀陡坎位于中潮位岸滩,除侵蚀陡坎及之下区域外,高潮位仍应保持芦苇群落。若侵蚀陡坎位于高潮位,则陡坎区域应选择高龄池杉—芦苇或垂柳—灌木—芦苇的配置方式,而侵蚀陡坎下区域可选择灌木—镳草配置模式。
以上需要增配灌木的植物配置模式,灌木可以考虑选择耐水湿、耐荫、浅根系的种类,如细叶水团花(Adina rubella)、赤杨(Alnus tinctoria)、花叶柳(Salix integra cv. Hakuro Nishiki)、海滨木槿(Hibiscus hamabo)。考虑到林带未来整体的景观效果,还可以选择与池杉、垂柳同功能群的乔木树种,增强林带的观赏价值。如选择落羽杉(Taxodium distichum)、水杉(Metasequoia glyptostroboides)、江南桤木(Alnus trabeculosa)、沼生栎(Quercus palustris)等树种。
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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