The Water-level Fluctuation's Plant Functional Type And Effects Of Flooding Conditions On Cynodon's Growth

The plants growing in the water-level-fluctuating zone play a very important role in the ecological environment of reservoirs,and are critical to solution to the environmental problems in controlling the water pollution of reservoirs,protecting the water quality of reservoirs and preventing and controlling soil erosion.The water-level-fluctuating zone is extremely unstable due to the influence of reservoir water level variability.Under the frequent disturbance of reservoir water level,the environmental factors change drastically,resulting in the frequent succession of vegetation,which is manifested in the ecological fragility of the water-level-fluctuating zone ecosystem.The construction of artificial reservoirs changes the law of river water level and the law of hydrodynamics in the natural state,thus posing a great influence on the distribution,migration and settlement of plant seeds in the water-level-fluctuating zone ecosystem.Moreover,the frequent changes to the water level against the season also show a significant impact on growth and development of the plants in the water-level-fluctuating zone,thus affecting the water quality of and the contiguous operation ability of the reservoir.To reveal the inherent potential of natural reconstruction of plants in the water-level-fluctuating zone of Danjiangkou Reservoir,ten sampling areas and 57sampling points were selected according to different months,different soil layers and distribution of plant seeds on the water-level-fluctuating zone of Danjiangkou Reservoir and samples were collected in April and June respectively on the basis of changes of water level.With regard to different regional coverage of Danjiangkou Reservoir,the relationship between the distribution of plant seeds in different months and the environmental factors such as soil bulk density,soil water content,soil organic matter content and soil p H value were analyzed by using canonical correspondence analysis?CCA?.In order to exhibit the effects of hydrological stress of water-level-fluctuating zone on the growth and development of typical plant populations in the water-level-fluctuating zone,11 water stress experiments?flooding depth of 0–50cm,interval of 5cm,21 treatments per group,a total of 231 culture containers?were set up to observe the three physiological indexes,photosynthetic efficiency,chlorophyll content and specific leaf weight of typical herbaceous plants in water-level-fluctuating zone of Danjiangkou Reservoir,the regression equation was established by regression analysis,and the intrinsic relationship between physiological indexes and photosynthetic efficiency was created to achieve the purpose of determining the optimal value of the artificial water level of the artificial reservoir through the growth condition of the typical plant Cynodon under different flooding depths.The following conclusions are drawn from the experiments:?1?The relationship between plant seeds and environmental factors was analyzed by CCA two-dimensional ordination.The environmental factors with correlation coefficient greater than 0.5 were selected,in which the correlation coefficients between saturation moisture capacity and field capacity of soil layer a in April and axis 1 are-0.50 and-0.51respectively,and the correlation coefficients between sand content and axis 2 is-0.52;the correlation coefficient between capillary water holding capacity,saturation moisture capacity,and field capacity of soil layer b between axis 1 are 0.64,0.62 and 0.60respectively,the correlation coefficient between sand content and axis 2 is 0.51,the main environmental factor that restricts the performance of seeds in soil in April may be soil moisture content,at this moment,the water level of water-level-fluctuating zone just declines and water is still a major influencing factor affecting the environment.In soil layer a in June,the correlation coefficients between sand content,grain content and axis 1are-0.60 and 0.54,and in soil layer b,the correlation coefficients between saturation moisture capacity and organic matter of soil and axis 2 are 0.52 and 0.51 and in June,the main environmental factor that restricts plant seeds in the soil is the mechanical composition of soil.?2?The effect of flooding on the photosynthetic efficiency of Cynodon was studied.The results showed that the photosynthetic efficiency of Cynodon was 2.54?mol CO₂ m^-2s^-1 at the flooding depth of 0cm on day 4,3.5?mol CO₂ m^-2 s^-1 at the flooding depth of5cm,and 4.16?mol CO2 m^-2 s^-1 at the flooding depth of 10cm,an increase of 37.8%and18.7%respectively.The photosynthetic efficiency of Cynodon at the flooding depth of10cm was 63.8%higher than that at the flooding depth of 0cm;on day 8,the photosynthetic efficiency of Cynodon at the flooding depth of 0cm was 5.45?mol CO₂m^-2 s^-1,and 6.3?mol CO₂ m^-2 s^-1 at the flooding depth of 5cm,an increase of 15.6%,which indicated that in the initial stage of flooding,the appropriate increase of water content could promote the photosynthetic efficiency of Cynodon.?3?When the flooding time was less than 12 days,the change law arising from the photosynthetic efficiency of Cynodon in 77 samples of 11 treatment groups was the same,complying with the binomial distribution,with R² of 0.579,indicating that the short-term increase of flooding depth has less influence on photosynthetic efficiency of Cynodon.When the flooding time was 12–20 days,in terms of change law of the photosynthetic efficiency and the flooding time of the Cynodon,the treatment groups of flooding depth of 0 and 5cm had the same change trend,the treatment groups of flooding depth 15–45cm also showed the same change trend,and R² for changes in the photosynthetic efficiency and flooding time was 0.680 and 0.460 respectively.When the flooding time was 20–30 days,the change trend in the treatment group of flooding depth of 0–10cm,treatment group of flooding depth of 15–25cm,treatment group of flooding depth of 30–35cm and treatment group of flooding depth of 40–45cm were respectively the same,and R² for changes in the photosynthetic efficiency and flooding time of Cynodon was0.599,0.442,0.783 and 0.558 respectively.It is shown that under a certain range of flooding depth,the change to flooding depth does not affect the change trend of photosynthetic efficiency of Cynodon;with the increase of flooding time,the change trend of photosynthetic efficiency of Cynodon in different flooding depths varies more frequently.?4?In the course of the chlorophyll content of Cynodon changing with the flooding time,the chlorophyll content of Cynodon on day 25 of flooding was more than that on day 12,day 16,day 20 and day 30 of flooding,at the end of flooding,Cynodon maximized the maintenance of normal photosynthesis,the chlorophyll content increased,and the chlorophyll content increased with the increase of water flooding time on day 15and day 25.The chlorophyll content of Cynodon decreased on day 25 to day 30,this was consistent with the change trend of light efficiency of Cynodon,and the growth of Cynodon was inhibited seriously by water.?5?Under the flooded conditions,in terms of the correlation coefficient between the leaf weight and the flood depth of the Cynodon,p was 0.026<0.05 only on day 4 of flooding,and in the remaining time,the correlation coefficient between the leaf weight and the flood depth of the Cynodon was more than 0.05,with weak correlation;in the initial stage of flooding,to adapt to flooding environment,plant leaves grow to accommodate less underwater light,reduced plant chlorophyll content and other unfavorable factors to maintain normal photosynthetic efficiency,and with the increase of flooding depth,the specific leaf weight of Cynodon leaves also changed;when the flooding time was too long,the self-regulation ability of Cynodon was weakened,and it hypothesized that the resistance of Cynodon in flooding conditions was weakened.Studying the growth law of typical plant Cynodon in water-level-fluctuating zone under water stress provides some scientific theoretical basis for the restoration of ecological environment in the water-level-fluctuating zone.In the follow-up study,comparative experiments of growth condition under the water stress at a variety of community levels shall be carried out,the field fixed-point observation of hydrological flow law in water-level-fluctuating zone shall be established,sediment content and other indicators shall be determined to strive to clarify the change law of ecological environment in the water-level-fluctuating zone under the frequent interference of water on the more systematic and perfect basis.