| After the impoundment of the Three Gorges Project(TGP),the Three Gorges Reservoir(TGR)was affected by the periodic water level regulation,which formed a large hydro-fluctuation zone with periodic exposure and submergence.Many native plants of this region gradually died due to sensitivity to the huge habitat change caused by great anti-seasonal water level change.This leads to many serious eco-environmental problems and threatens the safe and sustainable operation of the TGP and there is an urgent need for ecological restoration.The man-made forest construction can restore the forest of the upper portion of the hydro-fluctuation zone of the TGR quickly.This will stabilize the soil of the hydro-fluctuation zone of the TGR and enhance the buffer function of the last ecological barrier in the hydro-fluctuation zone.Thus,the man-made forest construction is an effective measure to solve the eco-environmental problems in the hydro-fluctuation zone of the TGR.Trees can absorb nutrient elements from the runoff and soil during the exposed period.They can also decompose and release the nutrients again during submergence period,which influence the water environment of the TGR.Therefore,it is of great significance to study the growth,leaf decomposition and appropriate management measures after the man-made forest construction.In this study,three deciduous tree species with high submergence tolerance selected by former studies,Taxodium distichum(Linn.)Rich.,Taxodium ascendens Brongn.and Salix matsudana Koidz.,were used to construct a man-made forest at the upper portion of165 m a.s.l.to 175 m a.s.l.in the hydro-fluctuation zone of the TGR.Their growth,especially the leaf production after experiencing multi-cycle submergence,the nutrient loading produced by leaf decomposition during the submergence period,the factorsaffecting leaf decomposition(e.g.submergence depth,water temperature,leaf quality),and the method to reduce the total leaf biomass of the trees before submergence were tested.The main results were listed as follows:(1)In July 2015,after the trees experienced three cycles of water level changes,the net photosynthetic rates of Taxodium distichum under moderate submergence(170 m a.s.l.)and deep submergence(165 m a.s.l.)treatment were not recovered to the control level(175 m a.s.l.).Compared to the control treatment,they decreased significantly by39% and 25%,respectively.The lower net photosynthetic rates of this species under moderate submergence treatment was caused by lower stomata conductance and pigment content.In addition,the shorter recovery time was another main reason for the decrease of the net photosynthetic rate.The lower net photosynthetic rate of this species under deep submergence treatment was caused by non-stomatal limitation.Unlike the changes of T.distichum,the net photosynthetic rate and pigment content of Taxodium ascendens under moderate submergence and deep submergence treatment recovered to the control level.In the leaves of Salix matsudana,the net photosynthetic rate and pigment content under moderate submergence treatment recovered to the control level.However,in deep submergence treatment,although its pigment content recovered to the control level,its net photosynthetic rate was not recovered to the control level.The main reason for its lower net photosynthetic rate was the effect of non-stomata limitation.The diameter at breast height(DBH)and tree height(TH)of T.distichum and T.ascendens under moderate submergence and deep submergence treatments deceased as the elevation decreased.Their branch numbers were also significantly lower than the control.The DBH and TH of S.matsudana under moderate submergence and deep submergence treatment were significantly lower than the control.Submergence caused the relative growth rates of DBH and TH of the three tree species to decrease significantly.However,their grown rates values were positive.The DBH,TH,and canopy area of T.distichum and S.matsudana had a significant correlation with their net photosynthetic rates.While the DBH,TH,canopy area and branch number of T.ascendens had no significant correlation with its net photosynthetic rate.Leaf production of T.distichum and S.matsudana under the control treatment were significantly higher than that under moderate submergence and deep submergence treatments.While the leaf production of T.ascendens decreased as elevation decreased.In the same treatment,leaf production of T.distichum and T.ascendens were significantly higher than that of S.matsudana.(2)The initial macroelement contents of the leaves of the three main tree species varied among different elevations due to different submergence stresses in the hydro-fluctuation zone of the TGR.In the first batch of samples(after 179 days of incubation commenced September 20),all foliar mass loss rates of the three tree species at 165 m a.s.l.were significantly higher than that at 175 m a.s.l.(except that of S.matsudana at 165 m a.s.l.).In the second batch of samples(after 138 days of incubation commenced October 5),foliar mass loss rates of the three tree species at 170 m a.s.l.were significantly higher than that at 175 m a.s.l.Moreover,the leaf mass loss rates of T.distichum and T.ascendens were significantly lower than that of S.matsudana when compared at the same elevation of the same incubation period.In addition,foliar release rates of C,N and Ca in T.distichum,N and Ca in T.ascendens as well as Ca in S.matsudana at 165 m a.s.l.of the first batch and at 170 m a.s.l.of the second batch were significantly higher than that at 175 m a.s.l.,respectively.The leaf mass loss rates of T.distichum and S.matsudana had a significant correlation with the initial foliar ratios of C/P.In contrast,the leaf mass loss rate of T.ascendens had a significant correlation with its initial foliar K and Mg content.The foliar macroelement release rates of the three tree species were significantly correlated with their leaf mass loss rates(except for K release rate of S.matsudana).The foliar release rates of C,N and P of the three tree species had significant correlations with the initial foliar P content and the ratio of C/P and N/P.In foliar nutrient release load,due to the effect of leaf biomass production and nutrient release rate,leaf C and K load of the three tree species at 175 m a.s.l.of the first and second batch were significantly higher than that at other elevations.While,leaf N load of T.ascendens and Mg load of S.matsudana at 165 m a.s.l.of the first batch and leaf N,P and Ca load of T.distichum and N load of T.ascendens at 170 m a.s.l.of the second batch were significantly higher than that at 175 m a.s.l.C load caused by leaf decomposition of the three tree species was the maximum nutrient load.(3)The different water submergence depth significantly affected the temperature,pH value,dissolved oxygen content and conductivity of the water during leaf decomposition.When compared the leaf chemical content of T.distichum and T.ascendens,the leaves of S.matsudana had higher initial N content and lower C and lignin content and leaves ratios of C/N and lignin/N.Water depth had significantimpacts on leaf decomposition rate and nutrient release rate of the three tree species.The leaf decomposition rate and release rates of C,N and P of T.distichum,T.ascendens and S.matsudana under 0.2 m and 2 m submergence group were higher than that under control group(CK,non-submergence).Besides,the leaf decomposition rate and release rates of C,N and P of S.matsudana(broad leaf tree species)under the same submergence depth were higher than that of T.distichum and T.ascendens(coniferous species).Release amounts of C,N and P of the three tree species under 0.2 m and 2 m submergence group were higher than that under the control group.Under the same submergence depth,release amounts of N and K of T.distichum and T.ascendens were significantly lower than that of S.matsudana.The leaf mass remaining rate and N,Ca and Mg content were largely influenced by submergence depth.(4)The water temperature at different elevations at the initial stage of submergence had significant effects on leaf decomposition rate and nutrient release rate of T.distichum,T.ascendens and S.matsudana.Initial leaf quality of S.matsudana was significantly higher than that of T.distichum and T.ascendens,which showed higher N content and lower C and lignin content and ratios of C/N and lignin/N.The leaf decomposition rate and C release rate of T.distichum,T.ascendens and S.matsudana under 20 ℃ and 24 ℃ treatment were higher than that under 17 ℃ treatment.Moreover,the leaf decomposition rate and C release rate of S.matsudana(broad leaf tree species)under the same water temperature were higher than T.distichum and T.ascendens(coniferous species).The C and K(except for T.ascendens)release amounts of T.distichum,T.ascendens and S.matsudana under 20 ℃ and 24 ℃ treatment were significantly higher than that under 17 ℃ treatment.Under the same temperature,release amounts of N and K of T.distichum and T.ascendens were significantly lower than that of S.matsudana.(5)The different pruning intensity had no significant effect on Pn of the three main tree species of the man-made forest in the hydro-fluctuation zone of the TGR in the recovery stage after submergence,but it had a significant effect on their specific leaf area and growth.Compared to the control(CK),pruning treatment could increase the specific leaf area(SLA)of T.distichum(only light pruning intensity,T1),T.ascendens and S.matsudana at 175 m a.s.l.Unlike the changes of trees at 175 a.s.l.,light pruning intensity(T1)had no significant effect on SLA of T.ascendens at 170 m a.s.l.However,moderate pruning intensity(T2)increased its SLA significantly.Pruning treatment hadno significant effect on SLA of T.distichum and S.matsudana at 170 and 165 m a.s.l.Unlike the changes of SLA,pruning intensity had no significant effect on base diameter,diameter at breast height and crown area of T.distichum,T.ascendens and S.matsudana at 175 m a.s.l.Light pruning intensity also had no significant effect on base diameter,diameter at breast height,crown area and branch number of T.distichum,T.ascendens and S.matsudana at 170 m a.s.l.and 165 m a.s.l.However,the base diameter of S.matsudana at 170 m a.s.l.under moderate pruning intensity(T2)was significantly lower than light pruning intensity(T1).In conclusion,long-term periodic deep submergence significantly inhibited the photosynthesis(except for T.ascendens)and growth of the three tree species.However,all of the species could sustainably grow at the upper portion of the hydro-fluctuation zone and produce large amounts of leaves.During the submergence period,the water environment of different elevations relatively promoted leaf decomposition and some nutrient release of the three tree species.The C load,as the main nutrient load,caused by leaf decomposition of the three tree species at high elevation was higher than that at low elevation.Deeper water submergence,higher water temperature and leaf quality could promote leaf decomposition and some nutrient release of the three tree species.In order to reduce the effect of leaves on water quality,the three tree species at 175 m a.s.l.could be pruned at high intensity.However,T.ascendens and S.matsudana at 170 m a.s.l.and 165 m a.s.l.could be pruned at light intensity while T.distichum could be pruned at moderate intensity to reduce the total leaf biomass. |
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