Water Use Strategy Of Carex Cinerascens And Its Response To Water Condition Changes In Poyang Lake Wetland

Water is a key factor in maintaining plant growth and determining plant species distribution.Wetland plants,as the product of the interaction of hydrology and other environmental factors,are the most sensitive indicators of wetland hydrological regime changes.Their growth process and distribution pattern are affected by hydrological factors（water level,submergence depth,submergence frequency,soil water content,etc.）.When the water environment changes,wetland plants can adjust their physiological and ecological characteristics to adopt appropriate water use strategies to meet their own water needs.In recent years,extreme hydrological events in Poyang Lake have occurred frequently and become increasingly serious.The most significant manifestation was that the low water level had been further refreshed,and the dry season had been advanced and extended.This had a certain negative impact on the growth and development of wetland plants,leading to the development of wetland vegetation succession in the direction of uncertainty and complexity.Therefore,revealing the water use strategies and response processes of wetland vegetation under different hydrological conditions has gradually become a hot issue worthy of attention.In this paper,Carex cinerascens（autumn grass）,the dominant plant in Poyang Lake wetland,was selected as the research object to study the water use strategies of plants.Using stable isotope techniques,we quantitatively studied the water source,water use efficiency（WUE）,water niche breadth of C.cinerascens by combining the Mix SIAR model,quantitative water use efficiency methods,and niche index models（Levins and Shannon-Wiener index models）.The water use strategy of C.cinerascens in Poyang Lake wetland and its response mechanism to changes in water conditions were clarified.The research results can provide a reference basis for hydrological regulation and vegetation restoration in Poyang Lake wetland.The conclusions of this study are as follows:（1）During the observation period,the water level of the Poyang Lake was in the recession period,ranging from 7.55 to 12.29 m,showing a downward trend overall.Therefore,except for the C.cinerascens in 12 m elevation zone,which was submerged by the lake water at the early stage of growth,the plants were basically not subjected to flooding stress.Lakeshore was widely exposed above the lake,and the lake water was absorbed and utilized by the C.cinerascens through lateral infiltration.The water content of0–20 cm soil was slightly higher than that of 20–40 cm soil during the growth and maturation stages,while it was slightly lower than that of 20–40 cm soil during the wilting stage.On the whole,the water content of0–20 cm soil continuously decreased over time,while the water content of 20–40 cm soil presented a trend of increasing over time.（2）The main water source of C.cinerascens gradually changed from lake water to 0–20 cm soil water.In the 12,13,and 14 m elevation zones,the contribution rates of lake water to C.cinerascens decreased by 51.6%,23.4%,and 39.9%,respectively,while the contribution rates of 0–20 cm soil water to C.cinerascens increased significantly by 31.4%,11.0%,and 21.1%,respectively.The contribution rate of20–40 cm soil water to C.cinerascens also increased significantly,but at the same period,its contribution rate was relatively lower than that of 0–20 cm soil water.Rainwater is also one of the water sources for C.cinerascens,which was indirectly absorbed and utilized by the root system.The contribution rate of rainwater to the C.cinerascens of maturation stage on the 13 m elevation zone was high,while the contribution rate to the C.cinerascens in other situations was very small.（3）During the growing season,the variation range ofδ¹³C value in the aboveground parts of the C.cinerascens was-32.65‰to-29.88‰,with an average value of-31.24‰.The variation range of WUE was 29.33μmol·mol^-1to 60.15μmol·mol^-1,with an average value of 45.01μmol·mol^-1.In the 12 m and 13m elevation zones,the WUE in the aboveground parts of the C.cinerascens was the highest during the growth stage,followed by the maturation stage,and the lowest during the wilting stage.The decrease in WUE from the growth stage to the maturation stage was lower than that from the maturation stage to the wilting stage.In the 14 m elevation zone,the variation characteristic of WUE in the aboveground parts was maturation stage>wilting stage>growth stage.The variation range ofδ¹³C value in the belowground parts was-31.61‰to-28.72‰,with an average value of-30.41‰.It had similar variation characteristics in different elevation zones,reaching the highest value during the maturation stage and the lowest value during the growth stage.This indicated that the WUE of the belowground parts was the highest during the maturation stage,while the WUE of the growing stage is the lowest.（4）In the 12 m and 13 m elevation zones,the highest values of the water niche breadth of C. cinerascens appeared in the maturation stage,while the lowest values appeared in the growth stage and the wilting stage,respectively.The C.cinerascens in the 14 m elevation zone had a relatively wide niche breadth during both the maturation and wilting stages,while the water niche breadth during the growth stage was relatively narrow.（5）C.cinerascens had strong adaptability and adopted different water use strategies under different water conditions.When water was excessive,the respiration and photosynthesis of C.cinerascens were inhibited,and the growth rate slowed down.Under waterlogging condition,C.cinerascens adopted dormancy or tolerance strategies,while under half-waterlogging conditions,it adopted escape strategies such as elongation and growth of branches and leaves to cope with water stress.When water was sufficient and appropriate,C.cinerascens selected lake water as the main water source,with a narrow water niche breadth.The aboveground part consumed a large amount of water for photosynthesis to complete biomass accumulation,with a high WUE,while the belowground part maintained a low WUE due to its relatively easy absorption of water.The decrease in lake water level increased the difficulty of root absorption and utilization of water,causing C.cinerascens to suffer from a certain degree of drought stress.When water is insufficient,the plant used 0–20 cm of soil water that was more easily utilized as the main water source.At the same time,water storage strategies were adopted to deal with water shortage conditions,and the stem and leaf parts close stomata to reduce water loss.The WUE of the aboveground and belowground parts increased significantly,and the water niche breadth also increased significantly.