[[alternative]]Studies on pupulation ecology and genetic variability of Cycas taitungensis

[[abstract]]Cycas taitungensis is an endemic species which is restricted to the riverside along Lu-Yei River and the mountainous area at the eastern coastal range in Taiwan. The thesis composed of two parts: the first included ecological study of C. taitungensis and the second genetic variability of C. taitungensis population. In ecological study, the age structure of the population habituated in the disturbed area is unequilibriated. The genetic study showed that the nucleotide diversity within populations is high. The enumeration measurment included (1) the amount of seed production per year, (2) one-year survival rate of seedling, (3) sex ratio and age structure and (4) the annual foliage leaf production. Three study plots and one 200-meter transect were arbitrary established in the natural reserve area. The results indicated that the field sex ratio was 1.7:1, the seed reproduction rate c.a. 45%, and one-year seedling survival rate in the range of 2-17%. Also, the numbers of foliage leaves were found positively correlated with the size of the plants. In the disturbed area, the age structure shows that the numbers of mid-age cycads were reduced, that implied the plants have been unlawfully deforested. In the genetic variability study, internal transcribed spacer （ITS）region of ribosomal DNA from 50 individuals were amplified, cloned and 77 sequences were obtained. Based on nucleic acid information, the population phylogeny trees were constructed. An unexpected high nucleotide diversity (0.17) among the 77 sequences was detected. However, no genetic differentiation between subpopulations（FST=0.002-0.006）was found, which is in agreement with allozyme analysis. A high genetic polymorphism can be accounted for the intramolecular recombination. The seed dispersal of C. taitungensis is known by gravity and possibly by insect pollination. Since the gene flow between the plants is weak, the poor differentiation between the subpopulation may be habitat fragmentation. Besides, the effect of environment factors was more important in the ecology of C. taitungensis than in genetic variability.