| Myricaria laxiflora, an endangered species, distributed in the Three Gorges Reservoir area and lost its habitats due to the construction of Three Gorges Project. In order to preserve its population, biological characters including seed development and dispersal, seed germination and establishment, seedling growth dynamic and their relationship with environment, especially the effects of flooding and physiological and ecological response, were studied in this thesis. The reintroduction of population was also discussed. Myricaria laxiflora reproduced a lot of flower branches, anthotaxies and seeds. Biomass of anthotaxies was significantly related with the biomass of stems and leaves except roots. Its seeds were dispersed by wind and river water, a direct dispersal approach and an indirect dispersal approach. Seeds dispersed by wind mainly distributed in 20 m from their parent plants according to the investigation of seedling distribution in fields. Individuals of Myricaria laxiflora clumped in populations. Seeds dispersed by river water usually landed and established on the strands of firth where the velocity of river flow was relatively slow. Seedlings germinated from the seeds dispersed by water usually occurred along the water fluctuating line on bottomland. The seed life-span of Myricaria laxiflora was about 7 days under natural condition. Most of seeds could germinate in 24 hours when they adequately absorbed water. Factor limiting Myricaria laxiflora distribution on the riverside was soil water content. Experiments shown that seeds could not germinate when soil water content was under 10% on sand support and 17% on sandy soil support, and that the optimal germination occurred on the soil with saturated water. As the water content of sandy soil on riverside was lower than 10% in autumn and winter, dry seasons in Three Gorges Area, seed only germinated and established on the sand beach where could be fluctuated by river water. According to our field investigation and other research reports, one-year-old seedlings took up the most of the community with a high density of population. The seedling survival rate, seedling underground biomass and up-ground biomass, seedling total biomass, the number of branch and length of the first grade branch all decreased remarkably with the increase of seedling density. It suggested that the density-dependence affected seedling establishment and growth of Myricaria laxiflora significantly. The relationship of total biomass and underground biomass with other factors were also regressed by us to show the effects of density on the growth of different parts of seedlings and to find proper density for population reconstruction and management. Two hundred and fifty square miters was thought as the limited density for population reconstruction. Myricaria laxiflora suffers a long time flooding in summer every year. Seedlings stopped growth (in the state of rest) and lost its biomass during summer flooding in our experiments no matter it was submerged. Both the number and length of the branch of plants declined significantly during experiment, but the biomass between up-ground and under ground, number and length of branch and ratio of root with crown of plants did not differentiate significantly among the flooded time treatments and flooded degree treatments. Seedling surviving rate did not differentiate significantly among the flooding treatments. It suggested that Myricaria laxiflora adapted flood by rest (dormancy) and biomass loss. Up-ground biomass, total biomass and number of branch increased significantly when submersion stress was relieved three months, but there were not significant differences in biomass increase among flooded treatments. All of these implied that flood did not stress seedling establishment and growth but releasing seedling from drought stress during summer. The changes of transpiration, photosynthesis and water potential in a day all displayed "single curve" pattern. Transpiration and photosynthesis could reach the maximum of a day near noon whereas water potential fall to the minimum. Transpiration changed distinctly with the fluctuation of photosynthesis.Photosynthesis and transpiration of plants on May were higher than that on October due to the plants in October just recovering from the state of rest in flooding season. Submerged plants in summer could recover in autumn better than that no submerged, and transpiration and photosynthesis of plants increased with the increase of submerged time. All those suggested in physiological ecology that Myricaria laxiflora had evolved a mechanism to adapt submersion. Submersion, as matter of fact, could favor plants to live through the dry season. During growing season, transpiration and photosynthesis of plants were primarily restrained by submersion, but recover late remarkably as plants adapted flooding. Plants on submerged sandy soil showing low Ψ100 , Ψ0 , AWC and high ROWC and RDW implied that submersion could promote plants to develop a good ability to resist dry stress. Submersion would also bring response of Myricaria laxiflora in biochemistry. The total soluble sugar and saccharose increased during summer no mater the plants were submerged, and malondialdehyde (MDA) content, peroxidase (POD) activity, Polyphenol oxidase (PPO) activity did not differentiate among flooded treatments in summer. It shown that submersion did not affect plants in physiology, and that these changes, as matter of fact, dominated by the rest state of plants in summer. Plants of all treatments recovered in growth and physiology after submersion, the total soluble sugar and saccharose, POD activity, PPO activity decreased, and MDA content,root activity increased. Submersion in summer did not affect the restoration progress. In growing season, flooding would primely increase MDA content, POD activity, PPO activity, but reduced root activity, total soluble sugar and saccharose at the same time. After long time submersion, plants adapt flooding, and all of the physiological changes disappeared gradually. Myricaria laxiflora was distributed in the flood zone from 70 m to 155 m above sea level along the Yangtze River in Three Gorges Reservoir area. Its natural distribution area was very small and narrow. All natural habitats of this species would be submerged when the water level was raised to 175m. The water level of future flood zone of the Three Gorges Reservoir area will lower to 145 m in winter and raise to 175 m in summer when the dam finished, which made in-situ conservation on thefuture flood zone impossible. Population ex situ and reintroduction was the only approach to preserve the species. The flood zone above 175 m in branches of the Yangtze River which has the similar ecological landscape and environment with its original habitats were the suitable sites for population reintroduction even though their community compositions and structures there were somewhat different from that of its original habitats. The Committee of Three Gorges Construction funded a program to save this species. According to the program, five populations were reintroduced into four branches of the Yangtze River in Three Gorges area in 2002. The population of Myricaria laxifloram was a metapopulation. Three to nine year-old plants from different populations in fields were transplanted to reconstructed populations in order to conserve genetic biodiversity and keep population viability. The sizes of all reconstructed populations were bigger than 1000 considering the need of minimum population and seedlings loss from flood and drought. Vegetations on the sites for population introduction were partially cut and cleared before transplantation in order to reduce species competition. Seedlings were planted on the middle flood zone to meet seedling needs for water and avoid flood washing. The reintroduced populations were irrigated after transplantation and in drought to facilitate seedling establishment and growth. The transplanted populations established there successfully. Germplasm exchange artificially among the reintroduced populations in different branch rivers and the establishment of reproducing system by reintroducing several populations along a branch river were necessary in future to facilitate genetic exchange and resolve the problem of population isolation.
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