| Stellera chamaejasme is a perennial root-crown herbaceous plant of Thymelaceae, as one of major poisonous grassland plant species, distributed in Northeast, Northwest, and Southeast of China. The dissertation dealt with S.chamaejasme populations on the degraded steppe, distributed on the transitional region from low mountain or foot hill zone located on the southeast slope of Da Xing'an Ling mountains to the western Liaohe river. Under the background of the three grazing successional stages: HG (heavy grazing), OG(over gazing) and EG(extreme grazing), I investigated such many aspects of S.chamaejasme populations as seed dispersion, soil seed bank dynamics, seed germination, age structure, reproductive ecology, distribution patterns, interspecific association and life-history strategies. Through the field survey and indoor experiments during the two years' (2000 and 2001) period, I finally drew the following conclusions:(1) The seed populations of S.chamaejasme displayed random distribution before seed dispersion, but aggregated style just following seed fall. The factors including grazing disturbance could modify the pattern intensities even pattern styles of S.chamaejasme seed population.(2) S.chamaejasme seeds dispersed very randomly on .all the eight directions including eastward, southward, westward, northward, northeastward, southeastward, southwestward and northwestward, and showed no common dispersion regularity. The comprehensive analysis indicated that, S.chamaejasme populations had a seed dispersion mechanism as "dispersion distance close to parent plant", with the dispersion radius of 0-50cm. The seed amount dispersed on a direction wassignificantly positively correlated to the windward frequency, suggesting that wind was an important ecological factor that influence S.chamaejasme seed dispersion. The capacity of S.chamaejasme populations to extend their distribution area through seed dispesion was very limited.(3) The monthly dynamic changes of soil seed banks indicated that the stocks of the seed banks were highest at the seed fall period, i.e. seed rain fastigium, and thereafter would decrease gradually with the processing of time. Seed germination was not the main way for the output of S.chamaejasme seed banks, but ingestion by animals and insects probably was the main cause for the depletion of S.chamaejasme seed banks.(4) The S.chamaejasme seed germination was not sensitive to light, and the suitable germinating temperatures were 30癈 if constant or 10-30 *C if vibrant. The hardness of the seeds was mainly responsible for the seed dormancy and low germination rate. To rupture or to remove seed coats could significantly increase the germination rate. The germination capacities of S.chamaejasme seeds of the three successional stages showed up insignificant differences. The S.chamaejasme seeds in the soil seed banks were higher than those gathered in the immediate year in germination rate. July was a proper period for the germination of local S.chamaejasme seeds.(5) The branching mode of S.chamaejasme root-crown in the study site had obvious regularity, as I named "quasi-dichotomous branching". On the basis of that, a new basic method of "the times of quasi-dichotomous branching plus two" to estimate the real age of S.chamaejasme individual was put forward for the first time. Remnant stubbles, branch marks and annulate marks were important morphological features to help determine the times of quasi-dichotomous branching.(6) The age structures of S.chamaejasme populations at all the three grazing successional stages were declining types. There existed the phenomenon of "generation absence" in the age structure ofS.chamaejasme populations. The oldest S.chamaejasme individuals investigated were 19 years old. With the intensification of grazing disturbance and grassland degradation, the age structure of S.chamaejasme populations displayed an aging trend.(7) S.chamaejasme seeds did not germinate... |
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