Pathways and determinants of early spontaneous vegetation succession in degraded lowland of South China Wen-Jun DUAN1, 2, Hai REN1*, Sheng-Lei FU1, Qin-Feng GUO3, Jun WANG1,2 (1. Heshan Hilly Land Interdisciplinary Experimental Station, South China Botanical Garden, the Chinese Academy of Sciences, Guangzhou 510650, China; 2. Graduate School of the Chinese Academy of Sciences, Beijing 100039, China; 3. USDA – Southern Research Station, Asheville, NC 28804, USA)) Tel 020-37252916; Fax: 020-37252916; Email:renhai@scib.ac.cn Handling editor: Jian-Xin Sun Received : 12 Sept. 2006 Accepted: 20 Mar. 2007 Category: Ecology 1 Abstract: Continuous and prolonged human disturbances have caused severe degradation of a large portion of lowland in South China and how to restore such degraded ecosystems becomes an increasing concern. The process and mechanisms of the spontaneous succession, which plays an important role in vegetation restoration, have not been adequately examined. To identify the pathways of early spontaneous vegetation succession, 41 plots representing plant communities abandoned over different times were established and investigated. The communities and indicator species of the vegetation were classified by analyzing the important values of plant species using multivariate analyses. The results indicated that the plant species could be classified into 9 plant communities representing 6 succession stages. The pathway and species composition also changed in the process of succession. We also measured 13 environmental variables of microtopography, soil structure and soil nutrition in each plot to examine the driving forces of succession and the vegetation-environment relationships. Our results showed that the environmental variables changed in diverse directions, and the soil bulk density, soil water capacity and soil acidity were considered to be the most important factors. Keywords: Degraded lowland; ecological restoration; soil; spontaneous succession Supported by the National Natural Science Foundation of China (No. 30200035) and Field Station Project of the Chinese Academy of Sciences (CAS) to Heshan Hilly land Interdisciplinary Field Station, CAS *Author for correspondence. Tel +86 (0)20 37252916; Fax: +86 (0)20 37252916; Email :< renhai@scib.ac.cn>. 2 Ecological restoration is intended to initiate or accelerate the recovery of an ecosystem in terms of its health, integrity and sustainability (SER 2004). Restoration is often closely related to the reconstruction of the structure, biological interactions, and related processes of the ecosystem (Hobbs & Norton 1996; Reay & Norton 1999; Ren & Peng 2001; Ren 2004). The traditional approach to reclaim degraded lowland in South China has been to plant fast-growing trees to minimize financial and human resource expenditures (Yu & Peng 1996). Since plantations could achieve short-term socioeconomic goals by protecting the soil surface from erosion, by facilitating native species, and by accelerating the recovery of genetic diversity. However, spontaneous vegetation succession or natural recovery as an alternative approach to restoration has gained increasing attention (Bradshaw 1997; Pensa et al.2004). Plant community succession is becoming one of the important aspects of restoration ecology and forms a large body of ecological research (Zhang 2005). Most studies on terrestrial vegetation succession were focused on the restoration of abandoned mine land (Martinez 2005; Dutta 2005; Pensa 2004), degraded grassland (Peintinger 2006; Bornkamm 2006; Scholes 1997; Callaway 1993), forest land after deforestation or fire disturbance (Choung 2006; Karlo 2006; Shimizu 2005; Hooper 2005; Calvo 2005; Turner 1997). However, the pathways and determining factors of vegetation succession in degraded lowland are unclear, and little information is available. Environmental degradation is especially serious in China. It is estimated that the total area under soil erosion is about 3.56×106 km2, accounting for 37.1% of the national land (SEPA, 2006). As a traditional approach, plantation may sometimes quickly increase forest coverage and rehabilitate degraded ecosystem. Most previous studies of ecological restoration focused on the selection of the pioneer species and the development of engineering technology. However, few studies have examined the feasibility of restoration through spontaneous vegetation succession (Qian 1994; Ren & Peng 2001; Peng 2003). With rapid economic development, many farmers are 3 abandoning the agricultural fields and shifting to manufacturing. Consequently,many croplands are left untended. Moreover, large forestlands are degraded into grasslands because of intensive logging and poor management. In South China, more than 3.74 ×106ha (Zhang, 1994) lowlands were degraded. In order to increase forest coverage and mitigate soil erosion, the national and local governments have devoted considerable amount of human and financial resource to extend the plantation. The fast growing species such as Eucalypt (Eucalyptus citriodora; Eucalyptus exserta; Eucalyptus robusta) and pines (Pinus elliotii; Pinus massoniana) or nitrogen fixing species such as Acacia auriculaeformis have been widely used as pioneer species. However, negative effects such as biodiversity loss (Rawinder 1990; Lisanework 1993), soil nutrient depletion (Madeira 1989), and soil acidification also occur but are largely overlooked. To avoid these problems, we must understand the processes and mechanisms of vegetation succession in degraded lowland of South China. The major objective of this study is to identify the pathways and mechanisms of vegetation succession associated with restoration processes in South China. Specifically, we address the following issues: (1) what are the early succession series of plant communities in South China? (2) Which environmental factor are the key ‘filters’ to determine the vegetation composition and structure in succession; and (3) what are the most effective approaches to accelerate the vegetation succession and to reach the ‘futuristic’ restoration goal? Results Classification of succession stages Based on TWINSPAN analysis, 60 species were classified into 9 groups, representing 6 succession stages (Figure1). Stage I (0-3 years after abandonment): This stage was the primary period of recovery. The plant coverage was low (10%) and the community composition was very simple. Plant species was exiguous (less than 5) and mainly comprised decumbent and profuse root system species. Stage II (3-5 years): This stage included two different community types (II-I: 4 Imperata cylindrica and II-II: Lycopodium cernuum). Most of the plant species were gramineous species, which could tolerate the drought and infertile soils. Some leguminous species also appeared. Stage III (5-12 years): This stage had two different community types (III-I: Glochidion eriocarpum+Panax stipuleanatus and III-II: Neyraudia neyraudiana+Miscanthus.sinensi). Most of the species were annual herbs. Many shrub species also appeared in the communities. The vegetation was generally dwarfish. This was the transition stage when many grasses species were gradually replaced by shrub species. Stage IV (12-17 years): This stage can be divided into two community types. IV-I: Eurya chinensis+Rhaphiolepis indica At the beginning of this stage, the dominant species was Dicranopteris linearis. Some shrub species such as Gerdenia sootepensis and Rhaphiolepis indica were scattered in the community. However, at the end of this stage, the community structure changed dramatically and some shrub species, such as sychotria rubra, Eurya chinensis, Litsea rotundifolia. oblongifolia colonized the community. IV-II: Litsea cubeba+Litsea gluinosa In contrast to IV-I, some gramineous species also appeared in this community. At the end of this stage, many broad-leaved species emerged. Stage V (17-23 years): The dominance of shrubs over grasses became evident and many herbaceous species disappeared. Moreover, the coverage of vegetation greatly increased (about 80%). Stage VI (23-30 years): The total vegetation cover reached 95%, and the herb species were highly restricted and their coverage was less than 20% due to the limitation of light. Both tree and shrub species were dominant. The community structure of this stage was relatively steady which could resist some extents of disturbance, reassembling “climax” community under natural conditions. TWINSPAN analyses showed that, on the same substrate, there was a combination-separation-combination process of vegetation succession in degraded lowland, indicating two parallel paths in stage II, III, IV finally combined in stage V 5 (Figure 1). Ordination analysis of succession In the two-dimensional DCA scatter diagram (Figure 2), the axis 1 most likely represented the time course. From right to left, the succession periods became longer and the community composition and structure vary greatly. In the stages I and II, there were few species and most of them rarely appeared at the adjacent areas, the community boundary was clear. However, from stages III to IV, with the improvement of environment, species number increased. Accordingly, the community boundary became unclear. This was also embodied in the diagrams. Compared with stage I and II, the shapes of stages III, IV, V were abnormal. Moreover, there were many transitional species which distributed in the middle position of these 3 stages. In the end, an obvious change of plant diversity was observed in the Figure 2. From stage I to stage VI, there was an increasing-decreasing process of the plant diversity. This diagram shows plots of the same succession stages with similar ordination scores. Compare with Figure 3, we found the ordination of quadrats at the same stages had similar position in these two diagrams. It indicated that the environmental factors of the community changed significantly from one stage to the next, and the species composition varied with the environment changes. Changes of species number and composition During succession, the number of species and species composition changed over time (Figure 5). Since the succession stages were the inchoate phases in the whole process of vegetation restoration, the fluctuation of species number was obvious. Moreover, during all 6 succession stages, the species number of different life forms changed in different ways. Overall, the temporal pattern of herb and shrub species was parallel to that of total species number. However, the change of shrubs lagged behind the herbs. In stage V and IV, the herbs decreased rapidly but shrubs were still increasing. The species number of lianes changed randomly at all 6 succession stages, probably because the lianes could better adapt to the environment, and species 6 number was small. The tree species increased evidently at the last three stages. In general, pioneer species at the study site were heliophytes which could endure drought and leanness. During the first two stages, pioneer species dominated the community. However, as succession progressed, the microenvironment was meliorated and the proportion of the pioneer species decreased. This trend was more obvious in the last two stages. In these two stages, trees and tall shrubs dominated the communities while many heliophytes, especially herbs and shrubs were shaded out and a few mesophytes and shade-tolerant species colonized understory. In the first two stages, most species had very small but large number of seeds mainly dispersed by wind. In the late stages, however, more species had larger seeds that were mainly dispersed by animals. Changes of environmental factors There was a steady-going degressive direction of soil bulk density during the 6 succession stages, suggesting that the soil structure would be meliorated during succession process. Moreover, the degressive direction was more obvious in the first 4 stages. It was because that the vegetation coverage increased dramatically in these stages and it played an important role in water and soil conservation. The changing pattern of soil saturation moisture (SSM) and soil capillary moisture (SCM) was completely contrary to soil bulk density (SBD). This indicated that SBD played an important role in the increase of soil moisture. The pH decreased slowly during the succession possibly because the acidity of laterite substrate was relatively high. At the beginning of succession, surface soil was eroded and the soil substrate became exposed because of low vegetation cover. The pH would decrease due to the effects of following pedogenic process. Soil organic matter greatly increased during the succession, especially in the late stages probably due to the increase of plant biomass and litter. At the first 3 stages, there was little hydrolyzed nitrogen accumulated in the soil, however, this situation changed at the late stages. Available phosphorus and exchangeable kalium in the soil showed an increase at the beginning and a decrease afterwards during the succession, which was possibly resulted from the complex 7 interactions between the changes in pH and soil organic matter along the successional process (Fig. 7). Analysis of determining environmental factor The length of lines in the CCA ordination diagram indicates the strength of the correlation between plant community and the environment, and the angle between two lines represents their relativity. Figure 8 shows that soil bulk density, soil moisture, N, pH were the most important factors in vegetation restoration. The soil of our sites was laterite, acidic, with the pH ranging from 3.6 to 5. Soil pH can affect vegetation succession in several ways.. First, it could filter out certain species thus change the community composition.. Second, many essential elements such as P, Ca and Mg are fixed. For example, Figures 8 showed that these elements were positively correlated with pH, suggesting that pH could also affect vegetation restoration by restricting the effectiveness of some soil elements. Third, pH can affect the quantity and activity of soil microorganisms which play important roles in vegetation succession. The two CCA ordination diagrams also indicated soil bulk density and soil moisture were two important factors. At the beginning of the succession, the soil was poor with high soil bulk density and low soil organic matter content. Accordingly, the soil water holding capacity was low, leading to greater soil erosion. The diagrams also showed soil capillary moisture and soil saturated moisture were negatively related to soil bulk density, indicating that meliorating soil structure was propitious to vegetation restoration. At the same time, slope and aspect of a catchment could affect the spatial-temporal distribution of water and heat. Generally, soil erosion was more serious in steep lowland. The accumulation of soil organic matter and minerals was relatively low in these areas. This effect was more pronounced in monsoon area. N and soil organic matter were easy to decompose and relatively little was accumulated in the soil because of the high temperature and intense dry-rewetting process. However, at the latter stages, the microenvironment was improved; the accumulation of soil organic matter and minerals increased. Therefore N and SOM became more important in late succession. 8 Discussion Controlling factors of vegetation succession The vegetation succession and ecological restoration represent processes in community assembly that involves a series of filters and thresholds sifting species out of the regional pool (Temperton et al., 2004; Whisenant, 1999; Díaz et al.1998). It has been recognized that the importance and action of different filters will vary from place to place and over time through restoration (Temperton et al., 2004; Woodward and Diament, 1991). In extremely degraded sites, environmental stresses act as abiotic filter and only some pioneer species may be able to tolerate the severe conditions. South China is in the lower subtropical monsoon region. The annual precipitation is about 1700mm and the rainfall is generally intense and easy to cause surface runoff, which usually peels off the fertile topsoil, especially in hilly land. Consequently, the soil becomes extremely lean, acidic and compact thus functions as an abiotic filter. In these severely degraded areas, some pioneer gramineous species have extensive root systems and can tolerate strong acidity and drought. As pioneer species, these species can reduce surface runoff, accumulate soil organic matter, and improve soil structure, thus act as nurse plants to facilitate the colonization of later species. They are also keystone species in transition of threshold controlled by abiotic limitations. In the first four stages, abiotic filters determined the fate of vegetation restoration and the species composition was very simple, the vegetation cover was low, and thus the competition was weak. However, as succession progressed, the substrate was improved, more species emerged and plant cover increased, therefore, competition became stronger. For example, in late succession, many species were shaded out and the total species richness declined. In general, the importance of biotic filters increases along with succession. There are also five different kinds of plantations in the study site, which are of 23 years old. The understory vegetation was compared with 23 years natural restoration.. Most heliophytes such as Rhodomyrtus tomentosa, Baeckea frutescens were scanty under all plantations. However, shade-tolerant species such as Psychotria rubra and 9 Circaea mollis became most important components of understory community. This indicated the importance of light in late succession. When forest canopy closed, understory vegetation would shift from heliophilous to mesophytes and shade-tolerant species. A common observation in both plantations and grassland was that few native tree saplings were established.. The comparison of microclimate, soil mineral, and soil structure with native forest showed that these environmental factors may be less important because native saplings could survive in native forests with harsh environments (Peng 2003). However, seed sources and seed dispersal may be the most important biotic factors in late succession. Few seeds of native species were found during our vegetation and seed bank survey. In general, the species have large numbers of small seeds at the early stages and most of these seeds can be dispersed by wind in great distances with little dispersal barrier. Nevertheless, most of the native tree species such as Castanopsis chinensis, Cryptocarya chinensis, Aporosa yunnanesis, and Castanopsis fissa had larger seeds and only appeared in late stages. If the distance between the native forests and degraded land is far, seed dispersal of heavy seeds is more difficult. Dispersal barrier may be the most important biotic filter in the late succession stages. It is always difficult to comprehensively analyze many environmental factors in a single study. Here we identified and analyzed 13 environmental factors and concluded that pH, SBD and soil moisture were the most important environmental factors in vegetation restoration in the studies area. However, soil organic matter and nitrogen were considered to be the dominant environment factors in many previous studies. This could be mainly due to the mineralization of nitrogen, and the decomposition of organic matter was comparatively faster at our study sites. In addition, some essential minerals were swept away by surface runoff due to heavy rainfall in the region. Consequently, the nitrogen and organic matter content were similar in the first 4 stages and could not be the limiting factors. However, the soils of our study sites are acidic and the availability of P, K, Ca, and Mg were closely correlated with pH. Therefore, soil acidity could be a determinant of vegetation succession. Although the laterite soil has strong capillary attraction and is difficult for rain water to infiltrate, 10 the evaporation is high and precipitation varies greatly in different seasons. Thus, soil moisture is also an important factor affecting vegetation succession. It is noteworthy to point out, however, that these observations may only be the case in early succession stages and the situation may be different in later succession stages. The community assembly models There are three models of community assembly, i.e., deterministic, stochastic, and alternative stable states (ASS). Most ecologists tend to agree that the ASS model of species assembly is likely to be the one that better reflects reality in nature (Temperton et al., 2004). ASS asserts that communities are structured and restricted to a certain extent but can be developed into numerous states because an element of randomness can be inherent in all ecosystems (Sutherland, 1974; Gleick, 1987). In our study, the vegetation succession did not follow one fixed pathway; instead, it was diverted into two parallel directions during early stages and then incorporated in the late stages. This conclusion is different from early studies of spontaneous succession (Bornkamm 2006; Karlo 2006; Calvo 2005; Scholes 1997) but seems to support the ASS theory. Many factors can result in the formation of different communities and the succession pathways. Soil seed banks vary greatly across space partly due to their different distances to seed sources; therefore, seed dispersers can affect the colonization of certain species. Consequently, the sequences of species settlement can be quite different. For example, early settled species always affect the colonization of latter species. In addition, different environmental conditions gave rise to different communities in sites with identical species pools, as clearly illustrated at stage IV (Figures 4-7). At this stage, some plots with better soil structure and high N content were mainly occupied by Dicranopteris linearis. The coverage was relatively high. However, in some plots with relative poorer soil structure and nutrient condition, some pioneer species such as Miscanthus sinensis were abundant. Conclusions 11 Our results clearly show that, in order to accelerate the restoration process, some immediate new strategies need to be planned and adopted.. On the one hand, a more flexible restoration plan should be in place. The climax communities may be different because vegetation succession and ecological restoration are determined by somewhat different and uncertain factors. Moreover, because of the effect of microtopography, microclimate, and substrates, vegetation succession may be diverted into different trajectories and various patterns can be observed in the same succession stage. Therefore, to establish various self-sustaining ecosystem with abundant species and integrated function should be our goal of restoration. On the other hand, different steps should be taken to accelerate the transition of threshold at different restoration stages. The first step is to reduce water loss and soil erosion as soil erosion is one of the most serious problems in South China. We will then need to enhance soil pH and improve soil structure. In late stages of restoration, increasing seed supply may be helpful to facilitate seed dispersal and sapling establishment (Guo, 2007). Methods Study Site The study was conducted at Heshan Hilly Land Interdisciplinary Experimental Station (112 °54' E, 22 °41' N) located in Heshan County, Guangdong, China. The station was established in 1984 and it is located in a degraded costal ecosystem in the central Guangdong with a typical climate of subtropical monsoon. Its elevation varies from 0 to 90 m. The soil is laterite, a type of heavy acid soil with little soil organic matter (about 18.03 g/kg), N (total N is about 0.86g/kg) and available P (1.33mg/kg). The mean annual temperature is 21.7 ℃, the mean rainfall is 1,700 mm, and the mean evaporation is 1,600 mm. The region is a hilly agricultural zone with 78.6% of hilly land, 17.1% of farming land, and 4.3% of water body. The vegetation is evergreen broad-leaved forest of typical subtropics. The representative species of climax community are Cryptocarya concinna and Cryptocarya chinensis. Due to human disturbances, much of the current vegetation is degraded into hilly land grassland. 12 Field and Laboratory Methodology To study the composition and structure of various plant communities, quadrat samples were set up at different succession stages. Quadrats of 20×20, 5×5, and 1× 1 m were used for investigating the vegetation of arbor, shrub and grass species, respectively. Three small quadrats for grassland or shrub land were nested in the large ones for shrub land and tree land. In each quadrat, the height, density and basal diameter of various arbor and shrub species, as well as the height and coverage of different grass species were recorded. In order to get accurate coverage of grasses, 1 ×1m frame with small pane (5×5 cm) was used. The cover of grasses was determined by counting the number of pane covered by grasses. A total of 41 plots at different succession stages were established and investigated. Soil samples were randomly collected from 5 points in each quadrat using a 5 cm diameter soil corer to a depth of 20 cm after removing surface litter. A composite soil sample of about 1 kg from each point was collected, air-dried and sieved for analysis of soil chemical characteristics (Institute of soil science, CAS 1978; Liu 1996). To measure the soil physical characteristics, 9 intact soil cores were taken randomly in each quadrat using ring knifes after removing the litter and humus layer, and 3 of them were used to determine the soil bulk density, soil saturate moisture, and soil capillary moisture, respectively. Data analysis A total of 60 species were recorded in the 41 plots (Table 1). The species importance values (IV) in each sample were calculated using the following formula: IV tree and shrub= (Relative density + Relative frequency + Relative dominance)/3 IV herb= (Relative coverage + Relative height)/2 Frequency refers to the percentage of the quadrat containing a tree species over the total quadrat number at the same succession stage and stands. Dominance value refers to the sum of the basal areas of a tree or shrub species within a quadrat. We developed 13 a matrix of IVs of 60 species × 41 Plots and used it as the basis of the succession analysis. First, we used Two-way Indicator-Species Analysis (TWINSPAN; Hill 1979) to classify the succession stages of the communities. Second, we analyzed the relationships between the succession stages by De-trended Correspondence Analysis (DCA; Hill & Gauch 1980). Finally, the relationships between vegetation and environmental factors were analyzed using Canonical Correspondence Analysis (CCA; Braak 1986). Acknowledgements This research was funded by National Natural Science Foundation of China (No. 30200035) and Field Station Project of the Chinese Academy of Sciences (CAS) to Heshan Interdisciplinary Field Station, CAS. The authors are indebted to Dr Zhian Li, Jingping Zhang and Zuoyun Yin for helpful suggestions. The authors thank Bi Zou for soil analysis, Fuwu Xing for plant species identification, and Yongbiao Lin for field assistance. 14 References Bornkamm R (2006). Fifty years vegetation development of a xerothermic calcareous grassland in Central Europe after heavy disturbance. Flora201 (4): 249-267 Braak T, CJF (1986). Canonical correspondence analysis: A new eigenvector method for multivariate direct gradient analysis. Ecology67:1167-1197 Bradshaw A (1997). Restoration of mined lands- using natural processes. Ecol Engin8:255–269. Callaway RM, Davis FW (1993). Vegetation dynamics, fire, and the physical environment in coastal central California. 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The S + Arabic numbers and P + Arabic numbers represent species and quadrats, respectively. Figure3. DCA ordination of the 41 quadrats in 6 succession stages based on species composition Figure 4. DCA ordination of the 41 quadrats in 6 successional stages based on their environment conditions Figure 5. Changes of species number in different life forms during the six successional stages Figure 6. Changes of species composition during the six successional stages Figure 7. Changes of dominant environmental factors along the six successional stages pH,(×10); SOM, soil organic matter (×10g/kg); SBD, soil bulk density (g/cm3); SSM, soil saturation moisture (×100%); SCM, soil capillary moisture(× 100%); N, hydrolyzed nitrogen (×103mg/kg); P, available phosphorus (mg/kg); K, exchanged kalium (×102mg/kg) Figure 8. The CCA ordination diagram of the 60 species and 13 environmental factors from 41 quadrats 18 Table 1.The species and codes recorded in sampling quadrats Code S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22 S23 S24 S25 S26 S27 S28 S29 S30 Name Dicranopteris linearis Cynodon dactylon Imperata cylindrica Circaea mollis Polygonum chiensis Melastoma dodecandrum Miscanthus.sinensis Blechnum orientale Rubus alceaefolius Paederia scandens Mussaenda pubescens Ischaemum indicum Lycopodium cernuum Schizoloma heterophyllum Adiantum capillus-veneris Adiantum flabellulatum Lophatherum gracile Dianella ensifolia Dianella ensifolia Neyraudia neyraudiana Fimbristylis dichotoma Abrus precatorius Desmodium triflorum Eupatorium catarium Panicum repens Urena lobata Murdannia triquetra Desmodium heterocarpon Hedyotis diffusa Cassytha filiformis Code S31 S32 S33 S34 S35 S36 S37 S38 S39 S40 S41 S42 S43 S44 S45 S46 S47 S48 S49 S50 S51 S52 S53 S54 S55 S56 S57 S58 S59 S60 19 Name Smilax china Embelia laeta Microlepia hancei Lygodium japonicum Clerodendron fortunatum Psychotria rubra Eurya chinensis Melastoma candidum Wikstroemia indica Rhaphiolepis indica Ficus hirta Glochidion eriocarpum Panax stipuleanatus Mallotus apelta Litsea cubeba Litsea rotundifolia Gardenia sootepensis Rhodomyrtus tomentosa Baeckea frutescens Evodia lepta Litsea gluinosa Trema cannabina Schefflera octophylla Ilex asprella Pinus massoniana Breynia fruticosa Mimosa sepiaria Setaria viridis Pseudosasa hindsii Ficus variolosa II-I III-I IV-I Stage V Stage I Stage II Stage III Stage IV IV-II II-II Stage VI III-II Figure1. The restoration model of plant communities in degraded lowland in South China Figure2. DCA ordination of the 60 species at six successional stages. The S + Arabic numbers and P + Arabic numbers represent species and quadrats, respectively. 20 Figure3. DCA ordination of the 41 quadrats in 6 succession stages based on species composition 21 Figure 4. DCA ordination of the 41 quadrats in 6 successional stages based on their environment conditions herbs shrubs trees lianes total number of species 25 15 10 5 succession stages 22 VI V IV -I I IV -I II III II II II -I I II -I 0 I number of species 20 Figure 5. Changes of species number in different life forms during the six successional stages drought-resisted species heliophytes leanness-resisted species 110 100 90 % 80 70 60 50 40 30 I II-I II-II III-I III-II IV-I IV-II V VI succession stages Figure 6. Changes of species composition during the six successional stages 2.5 pH SOM SBD SSM SCM N P K 2 1.5 1 0.5 VI V IV -I IV -I I II II II III II -I II -I I I 0 succession stages Figure 7. Changes of dominant environmental factors along the six successional stages pH,(×10); SOM, soil organic matter (×10g/kg); SBD, soil bulk density (g/cm3); SSM, soil saturation moisture (×100%); SCM, soil capillary moisture(× 100%); N, hydrolyzed nitrogen (×103mg/kg); P, available phosphorus (mg/kg); K, exchanged kalium (×102mg/kg) 23 Axis 2 2.0 s36 s14 s57 s40 s4 s50 s25 s29 s60 s17 s46 s43 s59 s28 1.0 s16 s27 s37 s56 s47 s13 s3 Na SLO s2 s23 KASP s60 s10 N SSM s12 s26 P s24 -2.0 s15 Mg SCM Axis 1 s35 Ca -3.0 SCM s1 ORG 0.0 s48 s20 -1.0 0.0 s38 s11 s39 s42 1.0 s58 s18s5 s6 SBD s34 PH s9 s8 s21 s41 s32 s54 s49 s45 s7 -1.0 s33 s44 s55 s51 s31s61 s30 s19 s22 -2.0 Figure 8. The CCA ordination diagram of the 60 species and 13 environmental factors from 41 quadrats 24