| The typical plant communities in different succession phases were analysised in Volcanics Mountains of Xuyi, Jiangsu. The 2 years old young trees of 5 species (Pistacia chinensis Dalbergia hupeana,Celtis sinesis,Quercus acutissima. Pinus elliotii) were chosen as the experimental materials in order to reveal the drought tolerance mechanism.5 conclusions were drawn as follow:1,The species diversity of mixed forest was rich, there were 14 stocks in tree layer, the DBH structure of community was continuous, and the vertical structure of stand might be divided into 3 layers. The communities were in the highly mixed statement under intensity distribution, and the growth space for the trees was insufficient slightly. The dominant tree species was 45.4% in total. The a diversity index of the tree layer steady uptrend, and was higher than the shrub and herb layers in the Celtis sinesis Pers.+ Biota orientalis communities. And the top value appeared in the Celtis sinesis Pers.+ Pistacia chinensis Bunge communies. The succession underwent 4 steps:coniferous forest→mixed broadleaf-conifer forest (dominated by coniferous forest)→mixed broadleaf-conifer forest→mixed broadleaf forest.2,The species diversity of mixed forest was rich, there were 14 stocks in tree layer, the DBH structure of community was continuous, and the vertical structure of stand might be divided into 3 layers. The average mingling of the stand was 0.516, showed that the mixed degree of the forest is high. The angle index was 0.465, indicated that the tree position in this stand was uniform distribution. The neighborhood comparison was 0.454, and there were 45.4% trees in the stand being dominant. The average opening degree was 0.296, displayed the growth space of most trees in the forest was slightly less than sufficient. Q.variabilis, Q.acutissima and P.orientalis population are dominant now, and the community is being in a relatively stable state. But P.orientalis lack of updated individual, so when all of P.orientalis died, the stand will become deciduous broad leaved mixed forest.3,In drought stress, stomatal conductance and variation in content Chla shows significantly trend; from the whole can be seen that the praline and soluble sugar in the tree species of strong drought resistance shows ascendant trend, and most species in severe and moderate stress were present at the end of a significantly trend; SOD of strong drought resistance tree species in initial stress shows significant uptrend. With prolonged drought stress, weaker species first revealed a downward trend.4,Five species of the ultrastructure of mesophyll cells also had change:in normal moisture conditions, the mesophyll cell organelles structure was complete. Under slight drought stress, Pinus elliotii leaves ultrastructure were not hurt. There was no obvious change of quercus acutissima Carruth. mitochondrial, and expand phenomenon to the chloroplast. Degradation phenomenons were observed at the Pistacia chinensis Bunge and Dalbergia hupeana Hance mitochondrial membrane and chloroplast were swelling. Celtis sinesis Pers. mitochondria and chloroplasts were damaged obviously. Under severe stress, there were internal possibilities of degradation of Quercus acutissima Carruth. and Pinus elliotii mitochondrial, and chloroplast were damaged. Pistacia chinensis Bunge and Dalbergia hupeana Hance appeard qualitative wall separation, and chloroplast and mitochondrial badly damaged. The Celtis sinesis Pers. cells. were damaged most seriously.5. Compositing five species survival rate and external shape, with the method of multiple subordinate function of each species evaluation indexes of drought resistance and evaluate the physiological adaptive drought resistance, the sequence of the 5 trees from high to low in the resistance of drought stress was Pinus elliotii>Quercus acutissima Carruth.>Pistacia chinensis Bunge>Dalbergia hupeana Hance>Celtis sinesis Pers. |
