Experimental Study on Afforestation in the Engineering Treatment Areas in the Hydro-fluctuation Zone of the Three Gorges Reservoir:

Zhengchun WANG, Xiang FU, Haiyan JIANG, Qiang XIE, Hejun ZHANG

A Case Study on the Engineering Treatment Area in the Wuxi Section of the Daning River

Abstract Through the Betula nigra afforestation test in the engineering treatment area of the Wuxi Section of the Daning River in Chongqing， the feasibility of arbor vegetation restoration in the comprehensive management of the engineering treatment areas of the Three Gorges Reservoir was explored. The results showed that： ① before the flooding in the project area， the survival rate of B. nigra seedlings was high， over 95%， and ② in 2021， after a water storage period， affected by factors such as seedling specifications， flood and flooding time， the survival rate of the seedlings planted in 2020 was low， and the one-year-old seedlings were greatly competed by weeds and grew less. Two-year-old B. nigra seedlings were replanted in 2021， and the survival rate of the B. nigra seedlings was 100% in 2022. Two-year-old B. nigra seedlings have strong adaptability and have little impact on the stability of the engineering treatment area， and can be used as afforestation tree species in engineering treatment areas of hydro-fluctuation zones.

Key words Hydro-fluctuation zone; Engineering treatment area; Vegetation restoration; Water birch

Received： April 2， 2022  Accepted： June 5， 2022

Supported by Chongqing Technology Innovation and Application Development General Project （cstc2019jscx-msxmX0335）.

Zhengchun WANG （1981-）， male， P. R. China， senior engineer， master， devoted to research about wetland ecology， under-forest economy， scientific and technological assistance.

*Corresponding author.

The Three Gorges Reservoir is an important part of the Yangtze River ecosystem. Since the implementation of the follow-up work plan of the Three Gorges， a lot of work has been done in the ecological restoration area of the hydro-fluctuation zone， and the research on vegetation restoration techniques has been carried out. At present， relatively significant results have been achieved[1-6]， and a variety of flood-resistant plants have been screened. According to the survey， there are 411 sections in the area that need engineering treatment or have been treated by engineering， and the area of the hydro-fluctuation zone is 66.0 km2[7]. Existing engineering governance measures overemphasize safety and stability， weakening the quality of ecology and landscape. Our project selected a typical engineering treatment area of the hydro-fluctuation zone， conducted experimental research on Betula nigra afforestation， and improved the biodiversity and landscape in the engineering treatment area of the hydro-fluctuation zone， hoping to provide a reference for the planting of trees and shrubs in the engineering treatment areas of hydro-fluctuation zones.

B. nigra is an excellent tree species approved by Chongqing， and is suitable for planting in the hydro-fluctuation zone 165 m above sea level in the Three Gorges Reservoir area and in the wetland environments such as rivers and ponds within Chongqing. The Chongqing local standard "Technical Regulations for Cultivation of Betula nigra linnaeus" （DB 50/T673-2016） was issued and implemented in 2016.

Overview of the Project Area

The Daning River is a first-class tributary of the Yangtze River in Wuxi and Wushan Counties in the Three Gorges Reservoir Area of Chongqing City， located at 108°44′-109°59′ east longitude and 30°14′-31°44′ north latitude. The Daning River has a total length of 162 km and a drainage area of 4 181 km2. The main river in the Wuxi Section is 106 km long， and has a drainage area of 2 546 km2， and the average slope of the drainage basin is 8.34%. And the river valleys of the upstream tributaries are narrow， and there are many beaches and rapid currents， and the average slope of the river channel is more than 10%.

The project area was the engineering treatment area of the hydro-fluctuation zone in Huatai Township， Wuxi Section of the Daning River Basin in Chongqing （Fig. 1）. The ground elevation is 165-175 m （Wusong elevation）， and there are engineering treatment measures such as gravel slope protection and hexagonal block slope protection. After several seasonal fluctuations， the gravel slope protection area had accumulated some silt， forming a soil layer， which was distributed with a herb layer. The hexagonal block slope protection area was distributed with herbs and a small amount of shrubs.

Experimental Methods

Tree species configuration

After the water level drops every year， herbaceous plants in the project area begin to grow. After comprehensive consideration， B. nigra species with developed root system and resistance to flooding was selected for afforestation. Table 1 shows the specifications and configuration of tree species.

Experimental design

For different engineering measure areas， B. nigra seedlings of different specifications were used to conduct afforestation test research.

Gravel slope protection area： The seedlings were planted after the water level dropped for 15-30 d. Hole-like soil preparation was adopted， and the holes had a size of 50 cm×50 cm×60 cm. During planting， 20-30 cm of planting soil was filled back， and the plant-row spacing was 3 m×4 m.

Hexagonal block slope protection area： The seedlings were planted after the water level dropped for 7 d. First， hexagonal blocks were taken out， and the muck underneath was cleaned up to a depth of 60 cm. During planting， the planting soil was filled back， and the plant spacing was 3-4 m. After the planting of seedlings was completed， the seedlings were pressed with stones surrounding the soil.

The survival rate of seedlings was counted 1 month after planting， and the survival and growth of seedlings were investigated in September of that year. After the water level dropped in the second year （in March of the following year）， the survival rate of seedlings was counted， and replanting was carried out.

Results and Analysis

Water level fluctuations

The Wuxi Section of the Daning River Basin is affected by the backwater effect of the Three Gorges Reservoir area， and the water level is between 165-173 cm from October to February of the following year， and the highest water level is from November to January of the following year.  During the impoundment period of the Three Gorges Reservoir， the seedlings in the gravel slope protection area were flooded for about 85 d， and the seedlings in the hexagonal block slope protection area were flooded for about 60 d.

Situation of project area

In July 2020， due to factors such as heavy rainfall， increased water volume in upstream rivers， and flood discharge from the reservoir， the Wuxi Section of the Daning River Basin experienced several large water surges， and part of the gravel slope protection areas in the project area were washed away， resulted in the condition that 30 % seedlings were washed away. In March 2021， replanting was carried out. During the survey in March 2022， the project area went through periods of water storage and release， and the stability of the project area was not affected by the growth of seedlings.

Survival and growth of seedlings

It can be seen from Table 2 that before the water storage period， the B. nigra seedlings were basically not affected by the age of the seedlings and engineering measures， and the survival rate in the engineering treatment area was high. The survival rates of seedlings were both above 95% in the two times， but the 1-year-old seedlings suffered from great competition from weeds， and the growth was a little slower. In July 2020， due to floods， 30% of the seedlings in the gravel slope protection area were washed away， and the growth of the seedlings was slow， with an average annual growth of 42.5 cm; and due to engineering measures， 30% of the seedlings in the hexagonal block slope protection area showed symptoms of nutritional deficiency. After one water storage period， affected by the size of seedlings， weeds and inundation time and other factors， the preservation rate of seedlings in the project area was not ideal， and it was only 50% in the gravel slope protection area; and the survival rate and preservation rate of 2-year-old birch seedlings in the project treatment area were high， and the growth of seedlings was less affected by environmental factors， which was reflected in an average annual growth of seedling height of 76 cm， so the growth rate was much higher than that of 1-year-old seedlings.

Discussion

B. nigra is one of the plants selected in the vegetation restoration of the hydro-fluctuation zone of the Three Gorges Reservoir area. Two-year-old B. nigra seedlings had a high survival rate and preservation rate in the engineering treatment area of the Wuxi Section， so they can be used for afforestation in the engineering treatment area， which is consistent with the research conclusions of Gao Lan， Ou Yang and other scholars[8-9]. However， under the combined stress of various environmental factors such as off-season water level， flood， drought， weeds， and periodic water storage， long-term flooding has a certain impact on the growth of B. nigra seedlings. In hydro-fluctuation zones， due to the flooding and scouring of water， tree seedlings can be supplemented by mechanical measures or microbial calcification measures at the beginning of afforestation to fix the soil and assist the growth of seedlings.

This project tested the possibility of arbor tree planting in the engineering treatment area of the hydro-fluctuation zone. However， due to the short time of this study， it is necessary to continue to monitor the growth of arbor trees and the stability of the slope and bank in the engineering treatment area. It is also possible to continue to carry out comprehensive experimental research on the restoration of trees， shrubs and grasses in the project management area.

Zhengchun WANG et al. Experimental Study on Afforestation in the Engineering Treatment Areas in the Hydro-fluctuation Zone of the Three Gorges Reservoir

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