Ecological Study On The Mechanism Of Remnant Myricaria Laxiflora Population Regeneration And Plant Growth Restriction

Myricaria laxiflora has lost its habitats in the Three Gorges Reservoilr area and become an rare and endangered species because of the construction of the Three Gorges Dam.A few remnant populations survive on the beach of main stream of Yangtze River downstream of Three Gorges Dam and Xiangjia Dam.The remnant populations are suffering regeneration limitation which cause seedling dificit.The water level fluctuation pattern on the remnant habitats has changed obviously: the water level withdraws quickly in Autumn and drops seriously in Winter dry season which cause the declines of soil water level and soil wate content.This study reseached the ecological characteristics of soil seed bank vigor,seed germination,seedling growth,photosynthetic physiology,branch and leaf functional features and allometric growth,and their relationship with water ecological environment.The impacts of water ecological environment alteration on remant population regenation and plant growth were analyzed based on the changed hydrological pattern in the habitats of remnant populations to reveal the limitation machnism.The results show that:(1)The seed vigor of M.laxiflora seed bank decreased significantly with the extension of storage time.The germination rate of seeds at 15 days storage was only 10.9%,which was 79.8% lower than the germination rate at 0 days storage.Soil moisture has a significant effect on the durability of the seed bank,and the seed vigor of the seed bank decreases with increasing soil moisture.The germination rate of 0% treatment level of soil moisture was 105.3% higher than that of 10% treatment level.(2)The soil ground water level has a significant effect on the seed germination and seedling growth of M.laxiflora.The soil groundwater level that is more suitable for seed germination and seedling growth of M.laxiflora is-10 cm to-15 cm.The seed germination rate,seedling survival rate and seedling aboveground,The underground biomass is the highest when the soil water table is-10 cm or-15 cm.Among them,the seed germination rate of soil water level-10 cm treatment level was 16.8% and 166.7% higher than 0cm and-20 cm respectively,the seedling survival rate was 14.1% and 9.2% higher,and the above-ground biomass was 116.7% and 82.0% higher,respectively.The main root length is the highest at-15 cm treatment level,which is 49.8% higher than the minimum value(0cm treatment level).(3)The photosynthetic physiological activities of the remnant M.laxiflora plants have a rapid growth process during the recovery stage after water flooding,and then gradually decline.The diminishing gradient also had a significant effect on the photosynthetic physiological activities of the remaining populations.The photosynthetic physiological activities of plants distributed at high altitudes were significantly higher than those of plants distributed at low and medium altitudes.The changes of photosynthetic pigment content in the remnant population during the growth recovery stage and the entire growth period showed the same trend as the photosynthetic physiological activities of the plants.Correlation analysis showed that most of the photosynthetic physiological parameters and photosynthetic pigment content of the plants were significantly correlated with changes in the water level of the Yangtze River,exposure time,and altitude gradient of the plant distribution.(4)The functional traits of the branches and leaves of M.laxiflora trees show different patterns with the growth and recovery time.The length of the longest secondary branches increases rapidly with the growth time and then tends to be gentle.The plant height,the number of secondary branches and the water content of the branches and leaves all follow The length of recovery growth time increased first and then decreased,but there was no significant difference in each recovery growth time.Due to the artificial adjustment of the fluctuation pattern of water level,the branch and leaf traits have obvious differences between the fluctuation gradients.Correlation analysis shows that there is a significant correlation between the growth status of branches and leaves and the gradient of water level fluctuation in the habitat,the soil water content caused by the change of the pattern of water level fluctuation and the initial recovery growth time.(5)The number of branches and leaves of M.laxiflora plants and the volume of branches and leaves are in a rapid growth stage in the first 30 days of recovery.The formation and development of branches and leaves are the focus of plant growth and investment at this stage.After that,the increase in the number of branches and leaves gradually slowed down,and the volume of the branches and leaves still maintained a steady growth,which showed an isometric growth relationship with the weight of the branches and leaves.The investment strategy of the plant changes from "fast return" to "slow return" as the growth process resumes.The growth relationship of branches and leaves is obviously different between different rising and falling gradients.The investment strategy of the plant changes from the volume of branches and leaves and the water content of branches and leaves to the number of leaves,and the change from "slow return" to "fast return" as the decreasing gradient decreases.In summary,the persistence of M.laxiflora seed bank and seed germination and seedling growth are closely related to the ground water level,and the photosynthetic physiology of the remnant population plants,the formation of seedling components and growth are significantly related to the regulation of the Yangtze River water level.The regulation of rivers by large-scale water conservancy and hydropower projects has changed the pattern of water level fluctuation and soil water content in the habitats of the remnant M.laxiflora populations,and thus further affected the population regeneration and plant growth of the species.