| The natural environmental characteristics of Three Gorges Reservoir are changing andcomplex. The alternate flooding and drying environment induced by the typical hydro-ecologicalprocesses of winter impoundment and summer flooding，the widespread concern has been causedamong the environmental and ecological researchers. At present, the water-level-fluctuating zone(WLFZ) vegetation restoration and reconstruction becomes one of the major tasks of the ThreeGorges Reservoir ecological environmental management under the context of resources andenvironment eco-friendly utilized. However, large differences concerning about criterions exist inhow to evaluate pioneer suitable plants. Furthermore, little is known about the physiological andecological responses of the pioneer plants to the alternating wet and dry habitats. In this study,fluctuating zone plants Salix rosthornii was selected as the researched object. A field plotinvestigation was conducted to determine its plant communities, niche characteristics andresponse mechanism of the physiological resistance in natural habitats. Parallel pot experimentswere carried out under laboratory conditions, which simulated drought and flooding stressconditions, in order to analyze physiological characteristics of resistance and photosyntheticcharacteristics of Salix rosthornii. This research tries to provide a theoretical basis for suitableplant selection in the reservoir restoration and reconstruction, particularly for indigenous plantspecies applied in ecological environment fluctuating zone.By a field quadrat survey, dominances of the dominant species, niche characteristics wereinvestigated of Shimen fluctuating zone vegetation restoration demonstration area of main cityChongqing. The results indicated that:1) The dominant plants in Shimen fluctuating zonedemonstration area were: Salix rosthornii, Salix variegate, Distylium chinense, Saccharumspontaneum, Phragmites karka, Deyeuxia Arundinacea, Hemarthria compressa, Veronicaundulate. And their niche breadth occupies a dominant position in tree, shrub, and herb layer,respectively;2) Various levels niche overlap performance was herb layer> tree layer> shrub layer,indicating that plant niche overlap in the region, the differentiation is not obvious, and theinterspecific competition is fierce. The niche breadth and niche overlap of Salix rosthornii werecompared to other39main plant species in the demonstration area, which indicated that Salixrosthornii had absolute dominance in the tree layer. This survey proved that Salix rosthornii had astrong resistance to flooding; it could be used at different elevations of the fluctuating zone andform dominant community with other shrubs and herbaceous plants.Based on the hydrological process of Three Gorges Reservoir, plant coverage, basal diameter and plant height of Salix rosthornii at different elevations were measured. The effects of seasonalflooding on biological characteristics and species diversity were also analyzed. The resultsshowed that: winter flooding significantly increases the average coverage and plant height at172m altitude and the average plant height at175m altitude (P <0.05). However, plant basaldiameter had no difference at either elevation (P>0.05). After summer flooding, the averagecoverage reduced significantly at172m altitude (P <0.05), while the basal diameter and plantheight were less affected (P>0.05); At175m altitude, none of the biological characteristicsreached significance level (P>0.05). After the winter and summer flooding, the172m elevationspecies Patrick richness index and Shannon-Wiener diversity index first increases and thendecreases, while the175m showed the opposite trends; At172m elevation, the Simpsondominance index significantly increased (P <0.05), while at175m it only increased after winterflooding (P <0.05). For the short time biological characteristics after flooding, Salix rosthorniican be effectively used as the Three Gorges fluctuating zone vegetation restoration species. Inorder to reduce the physical stress caused by sediment pile pressure after summer flooding, itrequires proper maintenance and protection.In field conditions, the effects of summer flooding on physiological responses of Salixrosthornii were investigated. The flooding duration, flooding frequency and depth of flooding atdifferent elevation were measured. Two typical alternating wet and dry processes were observedin the2012summer flood season. The initial flooding lasted35days, and the second processlasted only5days. The results showed that, RWC and EL of Salix rosthornii at172m altitudeincreaseed rapidly after the initial flooding and then decreased, while the175m and the controlgroup showed a decrease trend. The MDA content of treatment group increased and thendecreased at both172m and175m elevation, while the control gradually increased. The prolinecontent had the same trends as the MDA. The activities of three antioxidant enzymes trends werenot synchronized, which might due to the variety of antioxidant enzymes, plant feedbackregulation mechanism, and the high temperature and drought conditions of the fluctuating zone.Overall, the summer flooding had a significant impact on Salix rosthornii at lower elevation, butnot so obvious at middle or upper part of the fluctuating zone. In addition, the reservoir area hightemperature and drought stress in summer flooding season also had great influences on the plantsat upper elevation, which suggests that physiological responses of Salix rosthornii were thecombine effects of the alternating wet and dry conditions and high temperature and droughtstress.In order to ascertain the drought tolerant characteristics of fluctuating zone plants Salixrosthornii, and provide theoretical basis for fluctuating zone population structure and plantselection, a pot experiment was conducted on Salix rosthornii1-year-old seedlings under watercontrol drought stress. Leaf relative water content (RWC), electrolyte leakage (EL),malondialdehyde (MDA) and a series of physiological indicators related to plant droughttolerance and resistance to oxidation activity changes were measured. The results indicated that:1) Salix rosthornii seedling leaves had strong moisture retention capacity under drought stress and relatively fast recovery after re-watering;2)10days and20days of drought stress had nosignificant effects on EL and MDA. With the continued drought stress, EL and MDA increasedsignificantly;3) the overall trend of superoxide dismutase (SOD), peroxidase (POD) and catalase(CAT) activity under drought stress was first increased and then decreased. CAT was the mostsensitive to drought stress, while POD was relatively insensitive. After re-watering20days, allthree enzyme activities started to recovery, but not to the level of the control group. Thesimulated study also investigated the impact of the Three Gorges Reservoir water changecharacteristics on the Salix rosthornii seedlings photosynthesis characteristics. The resultsshowed that drought stress had a significant negative effect on net photosynthetic rate. Due to thedifferences of stress disturbance intensity, however, net photosynthetic rate might not be the bestindicator to depict the size of the light synthetic response capacity among treatments. Variousfactors could affect net photosynthetic rate. The photosynthetic gas exchange parameters and theefficiency of energy use are closely related to the net photosynthetic response capabilities underdifferent drought stress. This study found that net photosynthetic rate was positively related totranspiration rate, CO2utilization efficiency and relative leaf water content (P <0.05). It wassignificantly related to stomatal conductance (P <0.05); however, intercellular CO2concentrationwas negatively related (P <0.05), and was not significantly related to water use efficiency (P>0.05).A30-day indoor simulated waterlogging experiment was conducted to investigatewaterlogging resistance of Salix rosthornii. The results showed that different waterlogging depthhad an impact on the relative leaf water content when waterlogging duration reaches a certainthreshold. The leaf electrolyte leakage (EL) was more sensitive to70cm waterlogging than40cm.During the waterlogging cycle, the highest EL was the30-days. The MDA content underwaterlogging had wide ecological amplitude, the greater the depth of waterlogging, the higher thecontent of MDA. In20days of duration, proline content was higher under waterlogging than thecontrol. Leaf antioxidant enzyme activities did not reach to its peak value during the experiment,and this study found that physiological and biochemical regulation through antioxidant enzymesystem to accommodate waterlogging stress only take effects when waterlogging depth wasrelatively higher. |
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