| The construction of the Three Gorges Reservoir(TGR)forming the water-level fluctuation zone with a drop in water level of nearly 30 m and an area of 300 km2.The water level of water-level fluctuation zone fluctuated from 145 m in summer to 175 m in winter.This new hydrologic regime is the inverse of the natural hydrologic regime of Yangtze River before the Three Gorges Reservoir construction when the peak flows occurred in summer and low flows occurred in winter.With the rise of the water level in autumn,plants living within the belt would face flooding stress,and some flood intolerant plants gradually died off,exacerbating vegetation habitat fragmentation and the degradation of the water-fluctuation zone,and deleteriously affecting biological diversity,ecosystem structure,and ecosystem function.Therefore,a pressing scientific challenge is to recover and restore the vegetation there and ensure proper ecological function.Previous studies have shown that ecological restoration practice is the key to solve these problems.Therefore,species selection is critical for ecological restoration.However,the results of plant adaptation capability to the water-level fluctuation zone by yearly community observations in the past few years did not match those obtained from flooding simulation experiments,nor are they consistent with the short-term in situ planting experiments.One of the reasons leading to such inconsistency is that the practical water-level scheduling is different from the scheme adopted in the flooding simulation experiments with regards to the flooding rhythm,duration and depth.In the hydro-fluctuation zone,annual submergence stress can last as long as 182 days at 165 m a.s.l.and 364 days at 146 m a.s.l.However,the implications of such hydrological operations for the survival and growth of woody plants growing in the upper elevations of the hydro-fluctuation zone of the TGR are not well known.Therefore,there is thus a clear need to carry out in situ research to understand the physiological ecology adaptive mechanism of plants in the water-level fluctuation belt and screen suitable tree species for vegetative restoration of the water-level fluctuation zone of the TGR.In order to discuss the influence of large scale and anti-season flooding on nutrients absorption and carbohydrates reserve of Taxodium distichum and Salix matsudana,which planted in the vegetation restoration demonstrative bases in the water-level fluctuation zone of Three Gorges Reservoir,nutrient elements and carbohydrates contents of root,branch and leaf samples were determined.Samples were collected from three elevations,165 m,170 m and 175 m,which stand for the deep submergence,moderate submergence and shallow submergence(the control group)in the tow growing season of 2015 and 2016.1.Effects of flooding on nutrient elements contents of Taxodium distichum and Salix matsudanaThe results showed that:(1)Concentrations of N,P,K,Ca,and Zn in leaves and roots of T.distichum under flooding significantly decreased compared to SS(p < 0.05),whereas Fe and Mn contents significantly increased(p < 0.05);concentrations of N,P,Ca,Mg,Cu and Zn in leaves and roots of T.distichum under flooding significantly decreased(p < 0.05),whereas Fe and Mn contents significantly increased(p < 0.05).(2)Some nutrient elements concentrations of both spices under flooding decreased,but still maintained their normal performance.(3)Correlation analysis showed that plant heights of T.distichum were positively correlated with the concentrations of N,P,K,and Mg;plant heights were negatively correlated with the concentrations of Fe,Cu,and Mn;canopies of plants were positively correlated with the concentrations of N,P,K,and Mg,but negatively correlated with Fe and Mn.Plant heights of S.matsudana were positively correlated with the concentrations of N,P,K,and Mg;plant heights were negatively correlated with the concentrations of Fe;canopies of plants were positively correlated with the concentrations of N,P,K,Ca and Mg,but negatively correlated with Fe.This investigation indicates that the large scale and anti-season flooding had significantly influenced the nutritional characteristics of T.distichum and S.matsudana in water-level-fluctuating zone of the TGR.2.The dynamic change of nutrient elements contents in root,branch and leaves of Taxodiumdistichum and Salix matsudanaThe results showed that T.distichum and S.matsudana produced adaptive response to the special eco-environment and produced a stable mechanism of nutrient uptake.(1)During the two growing seasons of 2015 and 2016,the contents of nutrient elements in waterlogging treatments plant tissues were changed compared with the control group.The contents of N,P,K,Ca,Cu in roots,the contents of Ca in branch and P,K,Ca,Cu,Zn in leaf of T.distichum decreased,while the contents of Fe and Mn in root,the contents of Mg and Mn in branch increased.The contents of N,P,Mg and Cu in roots,the contents of P and Ca in leaf of S.matsudana decreased,while the contents of Mn in root increased.This situation exists throughout the study period.(2)Previous studies have shown that the contents of nutrient elements in plants will change regularly in different growth stage,but the results were different in our study.Furthermore,the nutrient element contents change trend of the same treatment in the two growing season were different,even the opposite.(3)There was no regular change in the content of elements between deep submergence and moderate submergence treatments.3.Effects of flooding on NSC,N and P contents of Taxodium distichum and Salix matsudanaThe results indicated that the contents of non-structural carbohydrates,N and P were affected by flooding.(1)The deep submergence and moderate submergence treatments T.distichum and the moderate submergence treatments S.matsudana maintained a high level of NSC reserves.Water flooding promoted the synthesis of NSC,the contents of non-structural carbohydrates in both tree plants species increased.(2)With the increase of water depth and flooding time,the proportion of soluble sugar increased in NSC.Soluble sugar is closely related to the physiological activities of plants,the results showed that the water resistance of two tree species increased.(3)The absorption of N and P were inhibited while the N/P remained stable.The N/P of both tree species were less than 14,plant growth may be limited by N.(4)With the increase of water depth and flooding time,the ratio of NSC to N and P increased,NSC output per unit mass N and P increased.In summary,Taxodium distichum and Salix matsudana have some adaptability to the large scale and anti-season flooding after 4 years since been planted in the water-level fluctuation zone of Three Gorges Reservoir.The nutrient elements absorption of T.distichum and S.matsudana were inhibited and these two species have made a positive physiological response,resulting in a stable nutrient absorption mechanism.Meanwhile,under the condition of limited absorption of N,P and other key elements,these tow tree species have the ability to balance the photosynthesis products in growth and store enough carbohydrates before the next flooding cycle.It can provide sufficient material reserves for the winter flood season and the recovery period. |
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