水位节律差异对三峡水库消落区不同物候类型一年生植物构成的影响

Abstract

三峡水库蓄水后改变了原有长江自然的水位波动节律,并在三峡库区内形成了两种类型的消落区:蓄水前未经历过水淹的完全水库消落区,其水位节律夏低冬高;以及蓄水前属于自然消落区目前受蓄水和洪汛双重影响的消落区,其水位在冬季和夏季都存在高水位.这与自然消落区冬低夏高的水位节律均存在差别.1年生植物根据萌发结实特性可分为春萌秋实、冬萌夏实型植物和广适性植物3类.在不同消落区选取5个样地划分样带设置固定样方,涨水前和退水后调查发现:各消落区均存在3种1年生植物,但3种类型物种的比例和和优势度存在显著差异.只受夏季洪汛影响的自然消落区共发现1年生植物73种,冬萌夏实型植物以45种占优;而水位节律与之相反的完全水库消落区,1年生植物物种数为85种,其中春萌秋实型植物以45种以及较大的优势度成为该区域的优势1年生物种;双重影响消落区,1年生植物物种总数未明显下降,但是在蓄水和洪汛的双重影响下其种群大小相对较低.水位节律的巨大变化会引起1年生植物优势类型的显著改变,适合生长的1年生植物主要是因为其生长周期与淹没期不完全重叠而成为优势物种类型.;After the Three Gorges Dam impoundment, the original water-level fluctuation rhythm of Yangtze River change dramatically. As a result, two new kinds of drawdown zones appear in Three Gorges Reservoir (TGR): the preupland drawdown zone (PU-DZ) which never submerged before impoundment and the preriparian drawdown zone (PR-DZ) which is the former riparian zone of Yangtze River. The water-level fluctuation rhythm of PU-DZ is high in winter because of impoundment, and low in the summer because of flood discharged. Meanwhile, the PR-DZ is affected by both the impoundment in winter and the flood in summer, so the water-level can raise high in winter and summer. Apparently,they both have differences with the water-level fluctuation rhythm of natural riparian zone (NRZ), which is only high in summer for the flood. Annuals can be divided into three types according to their germination and fructified characteristics: annuals germinating in winter fruitifying in summer(AGWFS), annuals germinating in spring fruitifying in autunm (AGSFA) and the common annuals. In this study we chose five sample sites in each kind of habitat and survey them before the freshet and after the impoundment subsiding by fixed quadrats in different transects. All these three types annuals can be discovered in each drawdown zone, however, the proportion of the number of different type's species and their population size are significantly different. The major reason is discrepancy of occurred submergence time in different zones. There are 73 kinds of annuals found in NRZ. The number of AGWFS is 45, its proportion of species number are 52.26-67.46 as the elevations decrease, which is obviously dominant compared with the other two types, because they can avoid the disturbance of summer flood. The total number of annual in PU-DZ is 85, wherein the number of type of AGSFA is 45. The proportions of this type in the total species number are 59.42-82.44 as the elevations decrease. They are dominance in PU-DZ, for the water-level is high in winter from November to the next year's January has little effect on them. However, AGWFS can hardly germinate in this stressful environment. AGWFS are more easily seen in high elevation because this area can end of the impoundment. In PR-DZ, through the total number of annuals is 62, which is not significantly lower than the other two regions. Summer flood and the winter impoundment disturb so frequently that any type of annual cannot get dominance in this environment, the proportions of AGWFS in each elevation are 30-52.57. Meanwhile the proportions of AGSFA in each elevation are 13.33-48.32. Neither of them can get dominant in this region. Moreover, class ephemeral annuals may survive as the dominant type as the stress increasing. The ecological dominance is almost the same as the proportion of species number. This study show if the water-level fluctuation rhythm changed, the composition of annual will change as the response rapidly in the drawdown zone. The original riparian annual plants don't have any advantage in the competitions for survival and spaces in drawdown zone. The adaptive annuals mainly because of that their phenology do not compound with the submergence occurred time then survive as dominant species.