Plant Restoration Potential And Environmental Influencing Factors In The Dongting Lake Under Changing River-lake Relationship Shift

Dongting Lake is the second largest freshwater lake in China,so maintaining its key ecological features is crucial for both the conservation of the Yangtze River and the global biodiversity.However,the planting of poplar forests in Dongting Lake around year 2000 and the commissioning of the Three Gorges Dam have resulted in strong anthropogenic interference with two key wetland ecological features which include the topography of the lake basin and the hydrological rhythm.In order to protect and restore the wetland,the relevant authorities launched a programme to return forests to wetlands in the Dongting Lake region in 2014,and implemented the return of fishing to wetlands on the farmed sub-lakes in 2018.However,after the recovery of the wetland and sub-lake areas which are strongly disturbed by humans,and the continuing changes in river and lake relations,following further deterioration of the hydrological situation in the future,how to enhance the quality of restoration through the dominant wetland plants has become an urgent issue to be addressed at present.(1)There were significant differences in species richness and diversity of surface plant communities at different stages during the wetland restoration from poplar forests.At the same time,the similarity between surface vegetation and soil seed bank germination results was lower than 0.2 for the four types of restoration stages,so it was difficult to achieve the effect of restoring the vegetation on the wetland by using the soil seed bank alone,and key species such as Phragmites communis,Triarrhena lutarioriparia,and submerged plants,which mainly reproduce asexually,needed to be supplemented artificially.(2)The main environmental factors affecting the distribution of submerged plants include water depth gradient,water transparency,PH value,and total phosphorus in sediments.At the same time,the type of lake also has a significant impact on the distribution of submerged plants,and the semi-barrier lakes and free connected backwater bays with slow flow are more suitable for submerged plants to grow.(3)Vallisneria spinulosa grown from seeds or propagules have different propagation propensities.The seed reproduction ones are more inclined to produce seeds to continue the population,while the clonal propagation plants tend to produce more stolons and overwintering spores to expand the population,but water level stress will affect the output of both the two reproduction methods.(4)With the addition of nutrient and changing of water depth,the growth and reproduction of submerged plants are affected.Biomass of submerged plants under competitive conditions is lower than that in non-competitive conditions.Also,the interaction between water depth gradients and interspecific competition has a significant effect on the growth of Potamogeton wrightii,Hydrilla verticillata,and V.spinulosa,while H.verticillata and V.spinulosa are more susceptible to interspecific competition than P.wrightii under the four experiment water depth conditions.Furthermore,in the non-nutrient-added environment,H.verticillata is more competitive at 30 cm,80cm,and 130 cm water depths,while P.wrightii was the most competitive at 180 cm water depth;but in the nutrient-added environment,P.wrightii is the most competitive at 30 cm and 80 cm water depths,while interspecific competitiveness of V.spinulosa was the lowest at both 130 cm and180cm water depths.In summary,(1)This study found that species diversity decreased with increasing restoration time after the forest was returned to the wetlands,indicating that anthropogenic disturbance had a negative impact on the diversity of the Dongting Lake wetland plant community,and that restoration of the continental beach vegetation using the current Dongting Lake soil seed bank is feasible,but still requires anthropogenic supplementation of key species such as P.communis,T.lutarioriparia,and submerged plants that reproduce mainly asexually;(2)Submerged plants in Dongting Lake are mainly found in the semi-barrier sub-lake habitat zoning with slower currents,higher transparency and seasonal connectivity,and too low and too high water depths have an inhibitory effect on submerged plants;(3)Different submerged plants respond differently to nutrient and water level stresses in the water column,and inter-species competitiveness also differs significantly,and factors such as regional water depth and plant configuration should be taken into account for submerged plant restoration.In this paper,the soil seed bank germination method was used to evaluate the feasibility and restoration potential of the use of the soil seed bank for the restoration of the Dongting Lake wetland,revealing the influence of hydrological regulation on the distribution of submerged plants and the responding mechanism of submerged plants with two reproductive modes,which are seed and clone,in response to hydrological stress.It is suggested that key species should be added to the beaches after returning to wetlands to speed up the restoration of vegetation.The reproductive allocation patterns of different sources of V.spinulosa at different water depth gradients were revealed,and the results showed that shallow water levels produce a large number of overwintering shoots,while deeper water levels produce more seeds.It is recommended that submerged plant restoration should be carried out in a way that not only considers the role of submerged plants for overwintering birds,but also considers excessive asexual reproduction leading to population homogeneity,so that an optimal balance between the two approaches can be achieved.Future experiments still need further research on the seasonal alternation mechanisms of mesophytic,wet and aquatic plants in the seed bank of the continental beaches,as well as suitable areas in the middle and lower reaches of the Yangtze River where restoration of continental vegetation and submerged plants can be carried out,and increased water depth and nutrient level gradients for controlled experiments to provide data support and theoretical guidance for wetland conservation and restoration in the Yangtze River's grand conservation.