| The impoundment of the Three Gorges Reservoir (TGR) constructed a large area of drawdown zone. According to the difference of hydrology regime, the drawdown zone could be divided into two types, preupland drawdown zone (PU-DZ),which is the main body of the drawdown zone, and preriparian drawdown zone (PR-DZ).The PU-DZ,ranging from the reservoir region of Fuling City to the dam, was mainly af-fected by the impoundment of the reservoir, but the PR-DZ, locating between the reser-voir region of Fuling City and that of Jiangjin City, overlapping with the impoundment drawdown zone, was affected by both the impoundment of the reservoir and summer flooding. For the overwhelming majority of vegetation in the drawdown zone could not survive through the extreme submergence, a large area of naked land emergence after the impoundment each year. With the exposed time go on, mass of vegetation, dominat-ing by annuals, established on the naked land gradually. Riparian vegetation roled as an important structural and functional component of the drawdown, its self-sustaining abil-ity was concerned by many ecologist. Soil seed bank (SSB), acting as a template for vegetation regenerating, harbors a potential community. The size and dynamics of the SSB was a good indicator of the aboveground vegetation.To determine the self-sustaining ability of riparian vegetation of the TGR.Firstly, we studied the SSB character among different drawdown zone types and different flooding intensity at the last period of exposed season, the time seed rain had fallen. Then, we compared the SSB character of the PU-DZ at the last period of exposed sea-son with that at the initial period of the next exposed season, and studied the factors might account for the discrepancy, thirdly, to determine the potential contribution of SSB to establish standing vegetation in the PU-DZ, we compared the number of seedl-ings germinated from SSB during exposed season with that of standing plants in PU-DZ. The main results listed as following: 1. The SSB character under different types of drawdown zone and different flooding intensityThough the size and species abundance of the SSB in riparian zone was lower than that in upland significantly, there have a large number of germinable seeds in the SSB under each drawdown zone type and under each flooding gradient; the SSB might make a great contribution to establishing standing vegetation during the next exposed season. The size and species abundance of the SSB was correlated with flooding intensity nega-tively in the PU-DZ and in natural riparian zone (NRZ), the similarity of SSB among different flooding intensity was low, but for the PR-DZ, characters of SSB from differ-ent flooding were quite similar. This suggests that flooding intensity have a great influ-ence on the size and species abundance of SSB, but not a deterministic factor. Charac-ters of SSB among different types of drawdown zone varied significantly, and the varia-tion trend of the SSB is consistent with that of flooding intensity among different drawdown zone types.Drawdown zones with similar background vegetation, in spite of a great difference in flooding intensity, have a similar SSB. This suggests that back-ground vegetation might play a more important role than flooding intensity in structur-ing SSB to some degree.2. Variations of the SSB character before and after the impoundment and main factors should be attributed to in the PU-DZThe size and species abundance of SSB after the impoundment were significantly lower than that of SSB before the impoundment. Effects of physiological elementssuch as seeds depletion due to its poor anti-flooding ability during the impounding period could reduce SSB density and species abundance significantly and its intensity was correlated with flooding intensity positively. In terms of the whole study area, effects of physiological elements, such as seeds depletion due to its poor anti-flooding ability during the impounding period, was the dominating factor leading to a decline in the size and species abundance of SSB after the impoundment of the TGR. Both the effects of physiological elements and deposition made a great contribution to the variations of the SSB before and after the impoundment, however, effects of scour seems made a little contribution to that. For the intensity of flooding, scour, and deposition varied with ele-vations, the main factors leading to the variations of the SSB before and after the im-poundment varied with elevations. Effects of physiological elements were the dominat-ing factor in area between170m above sea level (ASL) and175m ASL, where deposi-tion is scarce. For area between162m ASL and170m ASL, deposition was quite com-mon, in deposition area, sediment covered previous SSB often result in the variation between previous SSB and current SSB, in erode area, effects of physiological elements were the dominating factor. Deposition not only changed the component of the SSB, but also increased the density of SSB in area between162m ASL and166m ASL.3. The potential contribution of SSB might make to the establishment of above-ground vegetation in the PU-DZThe seedling number was higher than the plant significantly throughout the ripa-rian zone, and in each study site, species whose ratio of the number of seedlings germi-nated from soil sample during exposed season to that of aboveground plants (SPR) was greater than one accounting for more than88%of the total standing plants. For species recorded both in SSB and standing vegetation, its total seedling number was higher than that of standing plants significantly,and species whose SPR was greater than one ac-counting for more than75%of standing plants in each transects. The number of seedl-ings was correlated with the flooding intensity soil sample suffered during impounding period negatively, but plant number had no significant correlation with that. The ratio of the total seedlings of species recorded both in SSB and standing vegetation to the total standing plants number of those species has a significant positive correlation with flooding intensity soil sample suffered during impounding period. In addition, the SPR value was greater than one for each dominating species in the standing vegetation. The result implies that, it is possible to establish standing vegetation rely on the SSB in ripa-rian zone of TGR, and for areas located in low elevation, though high flooding intensity might reduce its SSB density, most of standing vegetation could be establish by SSB survived through the flooding also, thus, the vegetation in the riparian zone of TGR would likely to be maintained by SSB in a short term. |
PDF Full Text Request