| There is a reservoir in the upstream of Three Gorges Dam (TGD) after impounding which the length is more than660km. The water level is fluctuated by human regulating. Currently, the impoundment begins in September each year, the water level reach to the maximum (175m elevation) in mid-October, and begin to decline gradually in January the next year, finally dropped to the lowest water level (145m elevation) in May. The impoundment make the drawdown zone between them can get to30m vertically. The original water-level fluctuation rhythm of Yangtze River is changed dramatically because of the instruction of the Three Gorges Dam. It also leads to the formation of the Three Gorges Reservoir drawdown zone with a vertical drop of30metres.Drawdown zones exposes in every spring and summer, and the exposed time is different in different elevations. Large area of bare lands is presented after impoundment every year, but before the advent of next impoundment, it will develop lush vegetation which annual plants as dominant species. Vegetation is important for the drawdown zones of the Three Gorges Reservoir. The seedling bank characteristics, seedling emrgence patterns and seasonal dynamics all affect the stability of the plant community in this region. Therefore, we selected Wu Shan, Yun Yang, Wan Zhou, Feng Du as sample sites in Three Gorges Reservoir drawdown zones and they were surveyed once a month from April to August through fixed quadrats in different transects. The results showed that:(1) The investigation of annual seedling bank characteristics showed that, the species richness, diversity and seedling density were all showed a decline tendency with the elevation drop. Because of the shorter flooding duration of the172m,168m and164m elevations than160m,156m and 152m elevations, more seeds were dead during the flooding of the160m,156m and152m, so the species richness, seedling density and diversity of the seedling bank was lower than172m,168m and164m significantly. The summer floodling also disturbs the seedling bank characteristics of the lower elevations which made the richness and diversity even lower. The composition of the seedling bank is not completely same in different elevations, which Erigeron canadensis-Gnaphalium affine-Setaria viridis-Oxalis corniculata-Trigonotis peduncularis-Digitaria sanguinalis dominant the vegetation of the172m elevation, Setaria viridis-Digitaria sanguinalis-Xanthium sibiricum-Bidens tripartita-Oxalis corniculata-Solanum nigrum dominant the vegetation of the168m elevation, the Digitaria sanguinalis-Setaria viridis-Polygonum lapathifolium-Gnaphalium affine-Echinochloa crusgalli-Bidens tripartita dominant the vegetation of the164m elevation, Digitaria sanguinalis-Setaria viridis-Echinochloa crusgalli-Eclipta prostrata-Bidens tripartite dominant the vegetation of the160m elevation, the Setaria viridis-Digitaria sanguinalis-Bidens tripartita-Eclipta prostrata-Echinochloa crusgalli dominant the vegetation of the156m elevation, the Setaria viridis-Digitaria sanguinalis-Bidens tripartita-Eclipta prostrata-Youngia japonica ominant the152m elevation. In general, the dominant species of each elevation were these that can finish their life history, produce more seeds before floodling and the flooding tolerence of seed is more obvious.(2) Through the investigation of the seedling emergence rule of the annual species in the Three Gorges Reservoir drawdown zone, we found that the number of species and the seedling emergence ratio displayed a decrease tendency with time goes on. Although the background discrepancy made the seedling composition between sampling sites different, the common species were similar which the Setaria viridis-Echinochloa crusgalli-Digitaria sanguinalis-Erigeron canadensis-Bidens tripartita-Polygonum lapathifolium-Eriochloa villosa were common species. These common species mianly displayed two emergence patterns: early germinators and continuous germinators. Early germinators were those species that geiminated rapidly after expoisure to air with low germination during the remainder of time periods. Continuous germinators were those species that with even germination rates throughout exposure period. Early germination increased the fitness of seedlings in virtually, because geminate early can help the seedling individuals obtain the limited resources and to occupy the space, for increasing the chances of survival. Germinate early can may positively effect the growth, reproduction and so on. Judging from the survival respect, large number of seedlings germinate in a limited time can provide enough individuals for the environment selection. Thus, early germination was the dominant strategy in the predictable environment. However, continuous germination was a kind of risk sharing strategy, so germination throughout the period of suitable environmental conditions was the dominant strategy for persistence in the unpredictable environment. Examination of seedling emergence from seed banks throughout a growing season indicates that the floristic community structure in the Three Gorges Reservoir drawdown zone would be dynamic in the short term even if the environment is stabilized. However, stabilized conditions will eventually result in a decrease in species diversity as seed banks of species requiring differing environmental conditions are unable to replenish themselves.(3) The investigation of the annual vegetation seasonal dynamics showed that the number of species changed with time in different way in different elevations. It decreased with time in172m and168m elevation. It was found that first increased and then decreased in164m and160m elevation. However, in156m and152m, due to the fierce impact of summer flooding, the number of species show a sharp decline in August compared with June. But no significant difference (P>0.05) was found between time in all elevations. Seasonal changes of vegetation composition and species dominance were also different in different elevations. In higher elevations (172m and168m), it perform a significant seasonal dynamics which winter-germinated annuals (such as Trigonotis peduncular is, Gnaphalium affine, Mazus japonicas, Alopecurus aequalis et al) and spring-germinated annuals (such as Erigeron canadensis, Oenanthe linearis, Chenopodium ambrosioides, Melilotus officinalis) dominant the vegetation collectively in April and May, but spring-germinated annuals dominant the vegetation in June, July and August, because of the winter-germinated annuals completed their life cycles in spring and spring-germinated annuals’fast growth. And as time goes on, the advantage of the dominant species become more and more obviously. In intermediate elevation (164m and160m elevation), the vegetation is more stable and the dominant species is more fixed compared with higher elevations. The dominant species combination were Polygonum lapathifolium-Echinochloa crusgalli-Setaria viridis-Bidens tripartita-Digitaria sanguinalis and Echinochloa crusgalli-Polygonum lapathifolium-Digitaria sanguinalis-Bidens tripartita-Ageratum conyzoides, respectively. However, in lower elevations (156m and152m), due to the effect of summer flooding in July, vegetation was in an unstable state, annuals is difficult to complete life cycle and also hard to survive in drawdown zones. In conclusion, the higher in elevation, the more stable of the vegetation seasonal dynamics. So this research is of great significance to understand the species composition and vegetation dynamic of the Three Gorges Reservoir drawdown zones, and it may provide theoretical basis for vegetation restoration of the Three Gorges Reservoir drawdown zones. |
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