| Scholars are concerned with the question that Whether the Sonneratia apetala plantation will turn into a healthy composition of forest stand system with the top layer of exotic species, i.e. Sonneratia apetala, middle layer of native arbors, and the bottom layer of native shrubs. This paper compared the community features of the Sonneratia apetala plantation and different forest stands on Qi’ao Island, hoping to find tree species suitable for human-inducing, and also to analyze its mechanism to light stress. This thesis selected7different stands of mangrove sample plot, altogether42quadrats in Qi’ao Island mangrove area, investigated the characteristics of its community and soil sediment. The stand reformation experiment interplanted Bruguiear gymnorrhiza, Kandelia obovata, Aegiceras corniculatum, Acanthus ilicifolius and Rhizophora stylosa (Interplanted in the previous period) in9-year Sonneratia apetala plantation, meanwhile, used the reformed stands of the prophase as supplementary research, continuously monitored them for a year, and in a glasshouse, simulated the light stress of understory shrubs in tidal flooding situation. It also used indigenous superior shrubs of A. ilicifolius and A. corniculatum, disposed with full exposure and75%,60%,45%,20%,15%and5%of full exposure. For every kinds of light intensity, there were three repetitions, with16hypocotyls (seeds) in one repetition. The main conclusions of the research were as follows:(1) The main natural regeneration in S. apetala plantation was A. ilicifolius and Acrostichum aureum. In the9-year S. apetala plantation and the12-year S. apetala plantation, there both appeared a large amount of A. ilicifolius, and a few A. aureum in the higher terrain area. In the9-year S. apetala plantation, the density of A. ilicifolius was89,375plant/hm2, the height were all1.48m, with a high coverage of93%; the density of A. aureum was10,425plant/hm2, the height were all1.41m, with a coverage of6%. But those figures of A. ilicifolius in the12-year S. apetala plantation were50,000plant/hm2,1.34m, and76%; of A. aureum were14,433plant/hm2,1.7m, and a higher coverage of23%. In the meantime, there appeared a few A. corniculatum, the density was267plant/hm2,, the height were all1.5m, the coverage was only1%. Though there arose high density and high coverage of undershrubs A. ilicifolius and A. aureum in S. apetala plantation, their height were no more than2m, in addition, there were no established arbor species.(2) Stand reformation could enrich the stand tree species in S. apetala plantation, especially promoted the growth of indigenous arbor species in the plantation. The experiment under S. apetala plantation indicated that:one year after the stand reformation, the preserving rate of B. gymnorrhiza, A. corniculatum, K. obovata, A. ilicifolius were75%、80.1%、51%,74.3%. The annual increment of diameter at breast height (DBH) were0.36、0.37、0.58,0.4cm respectively. The result showed that it was more suitable to conduct stand reformation with these four species, but K. obovata had a little bit worse performance. The comprehensive comparison of this experiment and the previous reformed stands:In the previous period (10years ago), about30acres of R. stylosa seedlings were constructed in the S. apetala plantation of Qi’ao Island; the current preserving rate was0. And for the B. gymnorrhiza, which were also planted at that time, the preserving rate was58.3%, their height were4.63m, also, they formed closing crown canopy. The5-year K. obovata had a low preserving rate, and it didn’t form crown canopy. As a whole, in the experiment of5semi-mangroves and5true mangroves, except R. stylosa, other species had ideal effects. Among all, B. gymnorrhiza and A. corniculatum had the best performance, which could be the main species of the stand reformation. The stand reformation didn’t apparently improve the soil sediment.(3) Under the situation of lower light (<60%), the stem height, stem diameter, the fresh and dry weight of different organs, the ratio of leaf length to width, the leaf area per plant, the chlorophyll a and b, also, the apparent quantum efficiency of A. ilicifolius and A. corniculatum all tended to rise. As for45%unit A. ilicifolius, the stem height, stem diameter, the fresh weight and dry weight of each plant, the ratio of leaf length to width were23cm,0.56cm,4227.9mg,895.4mg,2.73; as for45%unit A. corniculatum, the fresh weight and dry weight of each plant, also the ratio of leaf length to width were1781.8mg,397.6mg,1.68. For60% unit A. corniculatum, the stem height, stem diameter and the leaf area per plant were24.17cm、0.53cm,16.00cm2. For20%unit A. ilicifolius, the leaf area per plant, the chlorophyll a and b were28.27cm2,1.58、2.03mg·g-1; for15%unit A. corniculatum, the chlorophyll a and b were1.47,1.65mg·g-1; for45%unit A ilicifolius, the root activity was17.28ug·g-h-1. For5%unit A. ilicifolius and A. corniculatum, the apparent quantum efficiency were0.080,0.059molCO2·mol-1respectively. For5%unit A. ilicifolius and A. corniculatum, the light compensation point were10.8,3.1μmol·n-2·s-1, which was significantly lower than other groups. Even under the condition of the lowest light, the preserving rate of A. ilicifolius exceeded60%, and that of A. corniculatum was more than40%.The above showed that:The seedlings of A. ilicifolius and A. corniculatum had a strong ability of adapting to the low light, under the situation of mid-light and low light,5%-60%of them could also grow normally.(4) Under the situation of mid-light and low light (5%,20%,45%), the net photosynthetic rate of A. ilicifolius first rose then fell according to the time extension of waterlogging, however, the trend of A. corniculatum’s net photosynthetic rate fell first then rose again, and the more intense of the light, the more obvious change of net photosynthetic rate. With different light intensity, the stomatal conductance of two tree species both fell first then rose, finally they stabilized at a point below the initial value. The research showed that:under the simulation of tidal flooding situation, the experimented mangrove seedlings’utilization rate of light would rise with the longer time of waterlogging, but the reaction time and the responding effect varied between two tree species.(5) The result of this experiment didn’t accord with Chapman’s hypothesis. There exist a lot of A. ilicifolius and A. aureum in the S. apetala plantation of Qi’ao Island. Because Qi’ao Island locates in an estuary area of low salinity, this situation was the same with Lugo’s opinion that low salinity promoted the settlement of understory plants, and it was totally different from Jazen’s opinion that understory plants in the mangrove community were sparse. It could be inferred that:In the fast growing mangrove plantation, the understory community may have a higher volatility of characteristics than the natural mangrove and indigenous species plantation within a short period, in the meantime, forest self-thinning and soil salinity may be two important factors.From the above, the common characteristics of natural regeneration under S. apetala plantation was that there were ample undershrubs A. ilicifolius, but there weren’t arbor tree species, for that reason, through artificially introducing some arbor or shrub mangrove plants to enrich the tree species in the artificial plantation, the result showed that:except that R. stylosa couldn’t grow normally, other trees could grow, and B. gymnorrhiza grew best, A. corniculatum remained to be observe. In the meantime, from the experiment of indoor light control, it further illustrated that A. ilicifolius and A. corniculatum had a strong ability of adapting to overcast situation. It revealed the reason why there existed a large amount of A. ilicifolius and A. corniculatum in the12-year S. apetala plantation from the light aspect. Siddiqi artificially introduced arbors to conduct reformation on the Char Kukri island of Bengal in S. apetala plantation, with a result of high preserving rate. It also revealed the validity of this result. |
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