| The operation mode of "storing clean water and discharging muddy water" of the Three Gorges Reservoir resulted in the formation of a water-level fluctuation zone of 30 m along the Yangtze River and its tributaries.Due to the seasonal water level fluctuation,the ecological environment in the littoral zone of Three Gorges Reservoir is obviously sensitive and fragile,and the research on the evolution and regulation mechanism of the ecosystem has received extensive attention.The formation of the littoral zone led to the fundamental change of the original riparian vegetation and the formation of a plant community dominated by a few suitable herbaceous plants.At the same time,the formation of littoral zone of Three Gorges Reservoir resulted in the emergence of different waterlogging depth and duration of stress habitats in the riparian zone,which promoted the formation of different adaptation strategies of plants,accelerated the divergent variation of plant species in the regional scope,and then changed the structure and function of the ecosystem.The variation of plant functional traits and stoichiometric characteristics not only reflect the adaptive differentiation law of plants to the environmental gradient,but also reveal the evolutionary process of plants along the environmental gradient and the response strategies of plants to the external environment.In order to explore the adaptive structure and function response law of littoral zone to suitable herbaceous plants in a small scale,in this study Baijiaxi,a small watershed at the backwater end of the Three Gorges Reservoir Area,was taken as the research area,and Cynodon dactylon,Bidens tripartita,Xanthium strumarium and Polygonum hydropiper,which are the most widely distributed herbaceous plants in the littoral zone,were selected as the research subjects.A multi-dimensional systematic survey of plant communities,typical plant functional traits,and ecological stoichiometric characteristics was carried out along the(longitudinal)habitat gradients of natural riparian zones outside the backwater zone to the littoral zone,as well as the(transverse)habitat gradients of water-level-fluctuation depth inside the littoral zone.The spatial difference and correlation between functional traits and ecological stoichiometry of suitable plants were analyzed in various directions of habitat gradient change,and the variation mechanism was further analyzed in terms of habitat factors such as soil,community structure and waterlogging characteristics.The main conclusions are as follows:(1)The fresh and dry weight,chlorophyll and net photosynthetic rate of rhizome and leaf of Cynodon dactylon,Bidens tripartita,Xanthium sibiricum and Polygonum hydropiper displayed an upward trend from natural riparian zone to littoral zone;water content of rhizome and leaf changed stably from natural riparian zone to littoral zone,while fresh and dry weight of rhizome and leaf remained stable.The results showed that the water content of roots,stems and leaves was stabilized by coordinating the growth of fresh weight and dry weight of them at the same speed,so as to maintain the water demand of plants for habitat changes.The relationship between plant functional traits and environmental factors was analyzed by RDA,and the results showed that the flooding time,flooding depth,soil p H and soil temperature were the main driving factors leading to plant functional traits in natural riparian and water-level habitats,which further explained the consumption.The formation of littoral zone accelerated the spatial differentiation of plant functional traits in the watershed.(2)The leaf length,leaf thickness and leaf area of Cynodon dactylon,Xanthium sibiricum and Polygonum hydropiper all increased along the water level gradient and increased first,then decreased.The low water level and high water level areas were lower than the central littoral zone;the plants in the low water level areas were affected by deep flooding and summer flood interference.Adequate water supply led to reducing leaf thickness and was beneficial to the enhancement of photosynthetic capacity;although the winter flooding time in high water level areas was short,but summer was severely stressed by drought,plants shrink their leaves to reduce transpiration loss;4plant photosynthetic traits are along the water level.The increase and decrease of the gradient indicated that although the low-water level plants had a long flooding time,they had sufficient water supply in summer,which was conducive to water replenishment and the photosynthesis capacity's enhancement.At the same time,the increase in plant roots,stems and leaves water content along the water level gradient remained relatively stable.It showed that plants kept their own water content through changes in morphological characteristics so as to adapt to different habitat changes.Through correlation analysis and RDA analysis,it was concluded that there were significant interspecific differences in the association of plant functional traits,and the responses to environmental factors were more sensitive,which indicated that the distribution of plants along the water level gradient was affected by habitat changes,and had different response strategies.(3)The results showed that the typical herbaceous plants Cynodon dactylon,Bidens tripartita,and Xanthium sibiricum had similar nutrient allocation strategies,that is,the content of C in roots and stems was higher than that in leaves,while the content of N and P was lower than that in leaves.The nutrient content of Cynodon dactylon and Bidens tripartita in roots and stems was lower than that in Xanthium sibiricum,while the content of C in leaves was higher than that in leaves,which showed a high response to stress environment resilience.The results showed that the leaf nutrient contents of the three plants showed a "?" pattern with the increase of water level gradient,while the nutrient stoichiometry characteristics were relatively stable.The dual effects of flooding stress and summer drought led to intraspecific differentiation of plant nutrient uptake,however the plants in the littoral zone could maintain the stability of nutrient stoichiometry by adjusting the uptake of different nutrients,which indicated that they had a good ability to adapt to the water littoral zone stress environment;Further analysis of the relationship between the variation of plant leaf nutrient measurement characteristics along the water level gradient and environmental factors showed the changes between the variation of plant leaf nutrient measurement characteristics along the water level gradient and soil physical properties,flooding time and depth was weak,and it indicated that the variation of plant leaf nutrient measurement characteristics along the water level gradient was not influenced by environmental factors.(4)Leaf nutrient measurement characteristics of typical herbs Cynodon dactylon,Bidens tripartita,and Xanthium sibiricum showed inconsistent variation from natural riparian zone to littoral zone.Leaf C content was relatively stable,which indicates that the carbon fixation capacity of typical plants from upstream to downstream in Baijiaxi Stream was not affected by environmental changes,while leaf N,P,K content and N/P,K/P ratio were all along natural riparian zone to littoral zone.The results showed that the littoral zone was affected by the water level fluctuation,and the soil nutrient loss restricts the obtaining of plant nutrient content;the leaf C/N and C/K of the three plants was slightly increased along the vertical space of the basin from the natural river bank to the littoral zone,and the soil nutrient diversion in the lower reaches of the littoral zone was more serious,and the nutrient content of plant leaves was lower,which improved the nutrient utilization efficiency;the RDA analysis showed that the soil nutrient diversion in the lower reaches of the littoral zone was higher than that in the lower reaches of the littoral zone.There was a strong correlation between plant leaf N content and environmental factors,which indicates that the vertical spatial habitat change had a greater impact on the N content of different plants,and the coefficience of variation of K content was the highest,which indicated that the K content of plants in Baijiaxi Stream had a strong sensitivity to habitat change,so the plant growth in Baijiaxi Stream might be restricted by environmental N and K elements.(5)There were also inter-specific differences in the relationship between plant functional traits and leaf nutrient measurement characteristics,with different resource trade-off strategies.The relationship between leaf C content and root length was prominent,leaf P content and C/N were significantly positively correlated with leaf thickness and leaf dry weight respectively,leaf N content,N/P and N/K were significantly negatively correlated with leaf dry weight;Through increasing the root absorption of water and nutrients,Cynodon dactylon enhanced the carbon assimilation capacity of plant leaves,and increased the utilization of N in the N limited environment.With the increase of carbon fixation ability of leaves,the leaf thickness and dry weight were increased as follows,the P content was accumulated in leaves,while the water loss in plants was reduced.P content in leaves was positively correlated with leaf thickness and net photosynthetic rate,while C/P,N/P and K/P were significantly negatively correlated with net photosynthetic rate,root length,root fresh weight and leaf water content;The high content of nutrient element in per unit dry matter,strong photosynthetic capacity of leaves,increased leaf thickness,and lower economic input in leaf area made the plant have higher growth rate to adapt to the change of habitat.There was a significantly positive correlation between N content,N/P and leaf fresh weight,leaf water content,and a significant negative correlation between C/N and leaf water content,which indicated that Xanthium sibiricum also maintained a higher N content in leaves,which increased the growth rate of leaves,increased leaf thickness,and improved the preservation of leaf water content.There is a significant correlation between plant functional traits and nutrient traits,which indicated that the formation of the littoral zone may cause typical herb plants to maintain synergistic changes in the growth and nutrient balances,thus improving their own adaptability. |
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