| Casuarina equisetifolia was the important protective tree on sandy coast in China, played an important role in remedying the environmental deterioration, the ecological vulnerability and resisting the natural disaster of coastal areas. Because of the long-term suffering of pure forest management and serious human disturbance, frequent problems such as woodland crushing, soil nutrient disordering, diseases and pests of forest gradually appeared. The stability of protection forest was affected seriously. Constructing an near-nature coastal protection forest system can keep higher species diversity, community stability and more harmonious relationship between species, which was helpful in building the stable and efficient coastal protection forest system. Therefore, this paper took 4 communities (A: secondary forest, B: planting Casuarina after selective cutting the secondary forest, C: planting Casuarina after clear cutting the secondary forest then protect to now, D: planting Casuarina after clear cutting the secondary forest then disturb the undergrowth frequently)under different patterns of disturbance as material, studied the species composition and diversity of each community, the population dynamics of keystone plants and the main population niche, revealed the response of plants to different patterns of disturbance and the natural succession law of vegetation in coastal sandy environment, provided basic materials for building a neat-nature coastal protection forest system, the results showed that:1. The number of family and genera in community increased at first and then downward with the increase of intensity and frequency of disturbance, which means moderate disturbance can increase diversity, supporting intermediate disturbance hypothesis. Different ways of disturbance made communities in different stages of natural recovery, which means the succession will further from climax if the intensity of disturbance was greater. With time passed, the recovery towards a direction that the compose of families and genera in community become more abundant and complex. The species composition and importance value in each community were different under different patterns of disturbance, Litsea glutinosa was the first dominant specie in both tree layer and shrub layer of natural secondary forest, the importance value were 98 and 137, the community structure was stable. In selective cutting forest, Litsea glutinosa became the third dominant specie in tree layer which the importance value was 33 but still kept the first dominant position in shrub layer which the importance value was 128. However, the dominant position of Litsea glutinosa gradually decreased in clear cutting forest. There was no Litsea glutinosa existed in tree layer. The community structure was simple and the stability was weak. Litsea glutinosa represented strong ability of natural recovery in succession, which can be taken as the keystone species in the near-nature construction of protection forest2. The age structure of Litsea glutinosa under different patterns of disturbance appeared to be inversed J shape. Quantitative analysis showed that: the V1—V6 of Litsea glutinosa in sample A were all > 0, V7 = V8 = 0, the whole dynamic index of quantity change of age structure Vpi = 73.78%, when considering external disturbance Vpi = 8.18%> 0; the V1—V5 of Litsea glutinosa in sample B were all > 0, V6= V7=0, the whole dynamic index of quantity change of age structure Vpi=86.93%, when considering external disturbance Vpi=10.87%>0; the V1—V2 of Litsea glutinosa in sample C were all > 0, V3=0, which indicated the population increased at the early stage, but kept stable at the late stage, the population showed a growth and stable trend.The survival curve of Litsea glutinosa in sample A(NX=N0e-0.7994x,R=0.952,NX=N0x-3.3025,R=0.945)belonged DeeveyⅡ, The survival curve of Litsea glutinosa in sample B(NX=N0e-0.851x,R=0.8812,NX=N0x-3.1489,R=0.9854)and sample C(NX=N0e-1.798x,R=0.8995,NX=N0x-3.9257,R=0.9295)belonged DeeveyⅢ, 4 functions all indicated the population increased at the early stage, but kept stable at the late stage, agreed the analysis of survival curve, date rate and lose rate in life table.3. With the increase of intensity and frequency of disturbance, the species diversity and evenness in tree layer gradually reduced, and the dominance rise gradually, the diversity of understorey in sample B were higher than sample A, which means selective cutting can increase the species diversity, the diversity of shrub and herb layer were highest in sample C, which means diversity was greatly reduced because of clear cutting in tree layer, but gradually recovered in shrub and herb layer when after protection, succession of community was in progress, it also said that, in the initial period of natural recovery after disturbance, the competition of resource utilize was more relax, more species can coexist in community. Species diversity in every layers were all low in sample D, Casuarina was the dominant species, controls properties and structure of community, the community was more different with natural vegetation.4. With the increase of intensity and frequency of disturbance, the speed of species replacement among communities had an increasing trend from low to high, the changes in species were greater and the common species decreased.βdiversity(CJ and Cs)trend from high to low, values were all bellow 50%. The natural degree of vegetation trend from high to low with the increase of intensity and frequency of disturbance, and became further from the initial vegetation.βdiversity(βWS,βC,βR)showed that the dissimilarly value between sample B and sample C was larger than others, which means the speed of species replacement among communities was largest under clear cutting disturbance, CN and CMH value downward at first and then increased with the increase of intensity and frequency of disturbance, the species composition and individual number in pure forest were most similar, the value was largest.5. According to the dynamics of niche breadth of each population, the main populations could be divided into 4 groups,niche invasive population,niche declining population,niche stable population and niche sensitive population;The dominant species of nature forest were wide distributing in shrub layer of Casuarina equisetifolia forest,and with wide niche breadth,but the niche differentiation of population were not sound,intersprcific competitions were severe,and community structure was not stable;Niche breadth of most native population in nature forest were wider than that of Casuarina equisetifolia forest,but the niche overlap of most population were lower except Litsea glutinosa,Sageretia thea and some niche sensitive population,which indicated that the niche differentiation of nature forest was greater,and the competitions among populations were not strong;On the whole,niche disposition in nature forest which composed by native species was more rational than that in Casuarina equisetifolia forest,and this was more meaningful on maintain the stability of community,so there was some reference value on the construction of stable coastal protection forest system if we can realize the rational disposition of native species and Casuarina equisetifolia.6. The density of soil in secondary forest was lower than other communities, but the porosity was higher, which was significant for the infiltration of rainfall and the reduce of surface water runoff,that can reduce the soil erosion with strong ability of soil and water conservation. With the increase of the intensity and frequency of disturbance, the function of water and soil conservation of community declined gradually, the porosity of soil in pure forest was lowest, and lacking in the function of water and soil conservation. PH value in secondary forest in higher than other communities, the nutrient was also higher except total N, so there was enlightening significance in reliving nutrient shortage, realizing nutrient balance and improving the site condition if we construct an near-nature coastal protection forest system. |
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