| The water-level fluctuation in the Three Gorges Reservoir Region has changeddramatically as a result of the hydroelectric project for flood control and power generation. The riparian seasonal hydrological environment also has changed from summer flooding with winter drought to summer drought with winter flooding. Soil seed banks provide key propagule sources for vegetation regeneration and restoration. This disturbance of the reverse seasonal fluctuation regime is seriously affecting the formation of soil seed banks and the restoration of vegetation on the water-level-fluctuation zone. In order to evaluate the effects of restoration method on soil seed bank and its response to the change of water-level fluctuation in the drawdown zone in the Yangzi River, investigate the changes of riparian seed bank and vegetation to determine the effects of the reverse seasonal flooding on the composition and spatial distribution of riparian soil seed bank and the similarity of seed bank to standing vegetation on different restoration sites were studied by the field investigation of vegetation and soil, and the germination of seed bank samples. The sample plots were located in the hydro-fluctuation zones of the Shanmu River in Zigui County(natural restoration sites), the Shibaozhai in Zhong County(artificial and naturalrestoration sites), and the Xiangxi River in Xingshan County(artificial and naturalrestoration sites). The main results were as follows:1. In the Shanmu river, the mean overall seed density of the soil seed bank was 13475.30 ind/m2. Density and the number of seeds increased initially and then decreased withincreased altitude. Maximum seed density(22500.20 ind/m2) was found at 165 m above sea level in the intermediately flooded riverbank, with the seed number? accounting for 27.80% of the total soil seed bank. Average seed density declined significantly with soil depth. The soil seed bank was composed of 48 species from 22 families and 40 genera. Most species were annuals and perennials. Rumex dentatus was the dominant speciesaccounting for 27.00% of the total seeds. Diversity and composition of the seed bankchanged along an altitude gradient and soil depth. Maximum species richness was found in the top soil layer at 165 m and 175 m above sea level. The standing vegetation wascomposed of 63 species from 22 families and 55 genera on flooding zone, meanwhile 55 species from 48 genera of 29 families on the non-flooding zone. Most species on floodingzone were annuals and perennials with Asteraceae and Poaceae accounting for 22%and 15.87 of the total families. The highest Margalef index, Simpson index andShannon-wiener index occurred on the 180 m, while Pielou index occurred on 170 m of the flooding zone. Both in flooding zone and non-flooding zone annual herbaceous species has a large proportion.The similarity index on the non-flooding zone was slightly higher than that on the flooding zone. The reverse seasonal flooding due to the dam reshape the composition and spatial distribution of riparian soil seed bank and limit the vegetation to a grassland dominated by a few annuals and perennials in the Three Gorges Reservoir Region.2. The mean overall seed density of the soil seed bank was 6539.28ind/m2 on flooding zone of Shibaozhai, while 6082.80 ind/m2 on non-flooding zone. Density and thenumber was change as shape of “∧” with the altitude. Maximum seed density was found at 165 m above sea level in the intermediately flooded riverbank, but each altitude was no significant difference. Average seed density declined significantly with soil depth, but the middle and lower showed no significant difference. The soil seed bank was composed of 50 species from 29 families and 45 genera. Most species were annuals and perennials accounting for 40% of the total seeds. Diversity and composition of the seedbank changed along an altitude gradient and soil depth. TheMargalef index, Simpsonindex, Shannon-wiener index and Pielou index on the flooding zone was slightly higherthan that on the non-flooding zone. The standing vegetation was composed of 59 speciesfrom 28 families and 55 genera on flooding zone, meanwhile 37 species from 33 genera of 18 families on the non-flooding zone. Most species on flooding zone were annuals and perennials with Poaceae and Asteraceae accounting for 18.40% and 16.95% of thetotal families.The life form of the standing vegetation was similar to the soil seed banks,but the proportion of vines, shrubs and trees were increased. In flooded zone andnon-flooded zone, the co-occurring species in standing vegetation and the soil seed bankwere 30 and 18. The similarity coefficients of species composition between the soil seedbank and the standing vegetation is 0.550 and 0.537. The reverse seasonal flooding led tothe increase in the similarity of seed bank to standing vegetation as their composition both degraded, meanwhile the artificial vegetation restoration modle can promote the species diversity of soil seed bank in Shibaozhai water-level fluctuation zone.3. The restoration method influenced composition, biodiversity and distribution of soil seed bank. The soil seed bank on the artificial restoration site was composed of 44 species from 38 genera of 22 families meanwhile 50 species from 42 genera of 19 families on the natural restoration site. The species biodiversity on the artificial restoration site was slightly lower than that on the natural restoration site. The highest species biodiversity occurred on the upper part of the artificial restoration site and the middle part on the natural restoration site respectively. The species composition and biodiversity became simple, and the life form of soil seed bank was mainly composed of annuals under the influence of water-level fluctuation. Restoration method and water-level fluctuation also influenced the quantitative spatial distribution of soil seed bank, and the similarity between soil seed bank and standing vegetation. The average seed density of soil seed bank on the artificial restoration site was significantly lower than that on the natural restoration site. The quantitative spatial distribution of soil seed bank displayed“∧”pattern on both restoration sites, the maximum seed density occurred at the middle part of water-level fluctuation zone. The similarity index on the artificial restoration site was slightly higher that on the natural restoration site. It is concluded that present artificial vegetation restoration had limited effects on the restoration of soil seed bank. The artificial restoration site needs further monitoring and introducing more appropriate species. |
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