| It is estimated that dams have been built across about two-thirds of the major natural rivers at present, China has the largest number of reservoirs in the world and there are 98002 dams with water storage capacity of over 100000 cubic meters, but still cannot meet the needs for water and hydropower resources for the economic development, so numeral large and medium-sized reservoirs are under construction. Construction of large and medium-sized reservoirs have brought huge economic and social benefits nevertheless also bring ecological environment problems in water level fluctuation zone(WLFZ) of the reservoir area, especially making enormous damages to drawdown area ecosystems. Studies have shown that: in the early operation of large reservoirs, riparian zone was dominated by native terrestrial vegetation——trees, shrubs and perennial herbaceous, however, were quickly destroyed under the impact of reservoir flooding, and replaced by annual herbaceous plants. But which kinds of annuals can substantially exist in WLFZ of reservoirs is rare reported.In regulated reservoirs, impoundment season, depth, duration, frequency and outcrop season of inundation are main factors affecting annual seed germination, plant growth and reproduce in WLFZ, and season timing of impoundment and outcrop are reported to be the most effective factors. Depending on the characteristics of seed germination and annuals life history, the nature of the annual life cycle can be divided into two different types, namely summer annuals(seeds germinate in the spring, and complete the formation of being mature in summer or autumn), and winter annuals(seeds germinate in autumn and winter, complete the formation of being mature in spring or early summer). Annuals use seeds to suffer the submergence, as a result, their submergence tolerance in flooding and germinability after flooding is critical in the process of estimating that whether a plant can adapt to the appropriate environment and be long-term survival. Investigating annuals seed flooding tolerance and germinability in a large drawdown area are essential for understanding annual species distribution pattern, predicting the early evolution of vegetation dynamics and helping vegetation restoration in the WLFZ.The Three Gorges Dam(TGR) is the worlds’ largest newly built running hydropower projects. Vegetation changes in that area were tracked and widely reported ex or after its construction, which can provide a basis for verification: using flooding tolerance of different life-form annuals seeds to predict vegetation evolution in large-scale drawdown area. This study collected 21 species winter annuals and 23 species summer annuals seeds, then evaluated their seeds persistence(intactness ratio) in water and germination capacity(germination percentage and max seedling ratio) when they outcrops from elevations 170 m, 165 m, 160 m, 155 m in WLFZ of the TGR, 25% of intactness ratio and max seedling ratio is considered to be the boundary of whether a specie can tolerance the submergence of WFLZ. The germination behaviors of these two life history types annuals in native terrestrial environment are used to be control group, we compared seedling capacity and germination dynamics of the two life-history annuals at different elevations in the Three Gorges reservoir(TGR) drawdown zone, the results showed that:(1) The seed density and species richness of seed bank will decrease significantly after flooding at all elevations in WFLZ of TGR. The decay of two lifecycle annuals seeds caused the decreased seed density of seed bank,while only the increasing species disappear due to the intolerance of submergence within the decreased elevations. The seed intactness ratio of winter annuals at elevation 170 m, 165 m, 160 m, 155 m are 62.39%, 47.81%, 41.11%, 48.78%, respectively, while intactness ratio of summer annuals are 56.9%, 83.8%, 78.43%, 79.99% at the same elevation. The portion of species with lower intactness ration than 25% in winter annuals at 170 m, 165 m, 160 m, 155 m are 9.52%, 28.57%, 38.1%, 33.33%, respectively; and in summer annuals, there almost none except 13.04% at elevation 170 m in WLFZ of TGR.(2) The flood at all elevations in WLZF of TGR not only reduces the coverage of winter annuals, but also reduces their species richness with the increasing depth of flooding. The submergence at all elevations in WLZF of TGR significantly reduced the seedling sprouting abilities of winter annuals. After the first flooding, the seedling ratio of winter annuals seeds at elevation 170 m, 165 m, 160 m, 155 m were 28.3%, 23.28%, 15.22% and 19.66%, respectively, while 47.08% in terrestrial natural environment, and corresponding elevations, the proportion species that seedling ratio accounted less than 25% in all winter annuals were 47.62%, 52.38%, 80.95%, 71.43%. The submergence at all elevations in WLZF of TGR had no significant effect on seedling ability of summer annuals, the seedling ratio of summer annuals at all elevations tested in this study were maintained at around 50%, the same as seedling ratio in terrestrial natural conditions. Although summer annuals seeds had a certain degree of death during the experiment, the increasing germination rate caused by submergence have a positive effect on improving and ensuring their seedling capacity in WLZF of TGR.(3) Because of the reservoir impoundment, the limited spatial and temporal resources of WLFZ will result in competition between winter annuals and summer annuals, and it is critically important for plants to survive in WLFZ as seed germination establishes a competitive advantage at the early period of outcropping. At the elevation of 170 m and 165 m, winter annuals germinated faster than summer annuals, which established a certain advantage in seedling at the beginning of the exposing, making winter annuals occupy a certain advantage in vegetation of these region, but with the extension of exposed time, summer annuals replaced winter annuals completely. While at 160 m and 155m(elevation), summer annuals owning an absolutely advantage at numeral than winter annuals from the outset of expose will dominate in these areas, and few winter annuals can survive.In the new ’winter storage summer row’ operating mode large hydropower dam, winter annuals can sprout and have certain advantages in those areas outcropping in winter or early spring, but as the flooding depth increasing, species richness and coverage will be substantially reduced. While summer annuals can sprout and dominate absolutely at all area in WLFZ. |
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