Effects Of Periodic Submergence On Growth,Photosynthesis And Nutrient Element Content Of Taxodium Ascendens And Taxodium Distichum In The Hydro-fluctuation Zone Of The Three Gorges Reservoir

After the completion of the Three Gorges Dam Project,the water level of the Three Gorges Reservoir Area?TGRA?fluctuates between 145 m and 175 m each year,forming a fluctuating zone with a vertical drop of 30 m and an area of nearly 349 km².This artificial water level regime bring many negative effects to the ecosystems of the reservoir,such as vegetation richness and diversity decrease,severe soil erosion,etc.These negative impacts damage the ecological structure and function of the hydro-fluctuation zone of the TGRA.Therefore,it is imperative to restore the vegetation of the hydro-fluctuation zone of the TGRA.In order to better restore the vegetation of the hydro-fluctuation zone,three submergence treatments were imposed in the hydro-fluctuation zone of the TGRA,including shallow submergence?SS,175 m,the control?,moderate submergence?MS,170 m?and deep submergence?DS,165 m?.The growth,photosynthesis and nutrient element contents of two water-tolerant species,Taxodium ascendens and Taxodium distichum,were tested in situ during the exposure period after they experiencing 4to 5 periodic submergence in the hydro-fluctuation zone of the TGRA.The test time included two phases,mid-July to early August of 2016 and 2017.The results were as follows:1.The results showed that different submergence treatments had significant effects on the growth of T.ascendens and T.distichum.?1?In 2016,there were significant differences among the three treatments of T.ascendens and T.distichum.The diameter at breast height?DBH?,canopy,and tree height of the two species under the MS and DS treatments were significantly inhibited when compared to that under the SS treatment.The changes of the DBH,canopy,and tree height of the two species in each treatment in 2017 were similar to those in 2016.The DBH,canopy,and tree height of the two species in all treatments in 2017 were significantly higher than those of the two species in all treatments in 2016.?2?The canopy of the two species under the SS treatment was significantly different.In the MS treatment,the canopy of T.distichum was significantly higher than that of T.ascendens.In the DS treatment,the DBH,canopy,and tree height of T.distichum were significantly higher than that of T.ascendens.2.The results indicated that submergence affected the photosynthetic physiology of the two species.?1?The fitting results of light/CO₂ response curves among different models was significantly different.Comprehensive analysis showed that the rectangular hyperbola modified model was the best model for the light/CO₂ response curve,which could better reflect the physiology of the two species.?2?In 2016,the maximum photosynthetic rate(P_nmax)of T.ascendens in the MS and DS treatments were higher than that in the SS treatment.Besides,the light compensation point?LCP?and dark respiration rate?Rd?in the DS treatment of T.ascendens were increased compared to the SS and MS treatments.The trends of P_nmax,LCP,and Rd in each treatment in 2017 was similar to those in 2016.The P_nmax of T.distichum in each treatment was similar to that of T.ascendens in the two growing seasons.The P_nmax of T.distichum also increased along with an increasing intensity of submergence.The LCP and Rd of T.distichum in the MS and DS treatments were higher than in the SS treatment,but had no significant difference.?3?Along with an increase in the intensity of submergence,the P_nmax and carboxylation rate?CE?of T.ascendens also increased in the two growing seasons.The photorespiration rate?Rp?of T.ascendens in the MS and DS treatments was significantly higher than that in the SS treatment in2017.In the two growing seasons,the P_nmax of T.distichum in the SS treatment was significantly different from that in the MS treatment.The Rp of the MS and DS treatments was significantly lower than that of the SS treatment in 2016.There was no significant difference in other response parameters.?4?Different photosynthetic characteristics of the two species were observed in different treatments.T.ascendens showed higher photosynthetic activity as its net photosynthetic rate?P_n?,P_nmax,apparent quantum yield???,CE,light saturation point?LSP?,and CO₂ saturation point?CSP?,LCP,CO₂ compensation point?CCP?,Rd,and Rp values are higher than T.distichum.?5?The P_n,light use efficiency?LUE?and CO₂ use efficiency?CUE?of the two species in the MS and DS treatments were significantly higher than that in the SS treatment.However,the P_n,stomatal conductance?Gs?,transpiration rate?Tr?,and LUE of T.ascendens were significantly higher than that of T.distichum in the same treatment conditions.In MS treatment,chlorophyII a?chla?,chlorophyII b?chlb?,total chlorophyII?chls?,and carotene?car?contents were significantly higher for T.ascendens than T.distichum.In the DS treatment,the chla,chlb,chls,car,and total chlorophyII to carotene?chls/car?contents were significantly higher for T.ascendens than T.distichum.3.The results showed that different submergence treatments had significant effects on the nutrient element contents of the two species.?1?In 2016,Mg content of T.ascendens in the MS and DS treatments was significantly higher than in the SS treatment,while N,K,Fe and Zn contents were significantly lower than in the SS treatment.In 2017,Fe and Mn in MS and DS treatments were significantly lower than in the SS treatment.There were no significant differences in N,K,Ca,Mg,Cu and Zn contents in different treatments.Fe,Cu and Zn contents in each treatment of T.ascendens showed a significant increase from 2016 to 2017.Mg content in DS and Mn content in the MS treatment in 2017 were significantly lower than those in 2016.For T.distichum,N and K contents in the MS and DS treatments were significantly higher than in the SS treatment in 2016,while Ca,Fe and Mn contents were significantly lower than that in the SS treatment.Mg content in the MS and DS treatments in 2017 was significantly higher than that in the SS treatment.Ca and Fe contents in each treatment were similar to that in 2016.In 2017,N,Ca,Cu and Zn in SS treatment,N,Ca and Zn in the MS treatment,Mn and Zn in DS treatment were significantly higher than that in different submergence treatments in 2017.?2?K,Ca,Mn,Zn contents of the two species in the SS treatment were significantly different.N contents of T.ascendens were significantly higher than that of T.distichum in the MS treatment.Mg of T.ascendens was significantly higher than that of T.distichum in the DS treatment.In summary,T.ascendens and T.distichum can adapt to the submergence-exposure stress of the hydro-fluctuation zone of the TGRA by increasing the utilization efficiency of light and CO₂and by adjusting the ratio of photosynthetic pigment distribution and elemental contents.The two species not only have strong submergence resilience,but also play an important role in maintaining the stability of the fragile ecosystem in the hydro-fluctuation zone of the TGRA and in restoring and reconstructing the damaged ecosystem.Considering the performance above,T.ascendens has a greater resistance to submergence and recovery than T.distichum in vegetation restoration of hydro-fluctuation zone.