|Other Abstract||The ecosystem services of coastal wetlands are degrading mainly due to climate change and human activities. Ecological restoration is an efficient method to reduce the degradation of the natural coastal wetland, which could promote the positive succession of vegetation communities by improving the conditions of soil and water. However, ecological restoration takes a long time and may show different trends in different restoration stages. Therefore, it is essential to evaluate the restoration effects for a long-term monitoring. However, long-term monitoring of the effects of coastal wetland restoration projects and their underlying mechanism are still poorly explored. The purpose of this study was to explore the effects of different restoration ages on plant diversity and community stability of wetlands in the Yellow River Delta. Using the in-situ observation method, we investigated the dynamics of soil indicators (pH, electric conductivity, available nutrients, total nutrients, ecological stoichiometric ratio of carbon, nitrogen and phosphorus) and vegetation indicators (basic quantitative characteristics, species diversity and community stability) during the early stages of restoration (1, 2, 3 a) in a typical ecological restoration area. Additionally, using the method of space-for-time substitution, the effects of restoration ages (0, 3, 7, 10, 19 a; 0 means without restoration, CK) on soil physiochemical properties, quantitative characteristics of plant community and plant community stability were explored in the Yellow River Delta. Meanwhile, the interaction mechanism between soil properties and plant communities was analyzed. Our main results are as follows:
(1) In the 3-year study, the soil environment gradually improved during the early stages of restoration, and some vegetation indicators also showed significant differences as the restoration proceeded. During the early stages of restoration, the soil electric conductivity (EC) decreased significantly. However, in addition to available nitrogen, the contents of available nutrients and total nutrients increased year by year. In addition, the C: N decreased continuously, while the N: P showed an increasing trend, indicating that the soil nitrogen availability was increasing. The coverage and height of plant community increased significantly as the restoration proceeded. However, there was no obvious change trend in plant species diversity and plant community stability index.
(2) On a long time-scale (nearly 20 years), the soil salinization has been decreased significantly as the restoration processed in the restored wetlands. The soil pH and EC of each restored wetland were lower than those of CK, and the soil EC of each soil layer decreased with the increase of the restoration age. As the restoration proceeded, the contents of available phosphorus and available potassium in 0-40 cm soil layer decreased first and then increased. The changing trend of soil organic carbon and total nitrogen content was similar, and there was a significant and positive correlation between them. The N꞉ P in the study area was low, and there may be nitrogen limitations. However, with the increase of restoration age, the N: P of 0-10 cm layer gradually increased, which indicated that the nitrogen limitation was alleviated.
(3) On a long time-scale (nearly 20 years), the community coverage and height increased significantly with the increase of the restoration age. The dominant species of plant community changed and their salt tolerance decreased gradually. In addition, Tamarix chinensis gradually occupied a dominant position in the community. As the restoration proceeded, the Shannon-Wiener index, Simpson index, Pielou evenness index and species richness index of plant community increased continuously. Furthermore, soil EC was the main environmental factor that contributed to the difference in plant species diversity at different restoration ages. In addition, the stability index of plant community also increased with increasing restoration age.
In summary, restoration age has an important effect on soil physiochemical properties and plant community characteristics of coastal wetland. The change of soil EC was the main environmental factor affecting plant species diversity. With the increase of restoration age, soil salinization of coastal wetland was gradually decreased, and the plant community coverage, species diversity and community stability index also increased on a long time-scale. However, the restoration disturbance during the early stages of restoration may have a negative effect on the vegetation restoration process. Therefore, the important role of restoration age in ecosystem recovery should be considered and long-term restoration should be carried out. Our result highlight that the sufficient time should be reserved for the restoration of degraded coastal wetland to achieve the best restoration effect.|