This article aimed to conduct a study on the urban forest system in Miluo City, analyze the existing problems and provide theoretical references for the construction of the urban forest in Miluo City.

The sample plot method and sample line method were used to study the four main types of urban forests in Miluo City: parks, campuses, residential areas and roads. The species richness, Shannon index, Simpson index and Pielou evenness index were calculated, and the species composition characteristics, species diversity and overall growth characteristics of the four types of urban forests were analyzed.

1) The study showed that there were 44 families, 94 genera, and 120 species of angiosperm plants in the urban forest of Miluo City. The main species were Fagaceae, Rosaceae, and Magnoliaceae, accounting for 36.4%, 22.7%, and 11.2%, respectively. 2) The species diversity in Miluo City was not high, and the overall number of arboreal species was relatively scarce. Among the four types of urban forests, parks had a relatively high level of species diversity and a relatively uniform distribution of tree species. The species diversity level on roads was relatively low, which varied greatly. 3) The tree height distribution in the urban forest of Miluo City was concentrated in the range of 5-10 m, and the proportion of tall trees higher than 20 m was relatively small. The diameter distribution generally showed a trend of “less at both ends and more in the middle”, the diameter of the forest trees was concentrated between 10 cm and 20 cm, and the proportion of tall trees with a diameter greater than 30 cm was relatively small.

The urban forest vegetation of Miluo City is still dominated by a few species, especially the road forest belts in the urban and rural areas. The tree species are few with smaller specifications, which species diversity has great potential for improvement. It is suggested to plant local characteristic native species and strengthen the cultivation of urban forests. The overall diversity of urban landscapes is low, and the average crown diameter of road trees is Ⅳ (> 6 m), which accounts for 20% of the total and does not meet the national forest city evaluation index. It is necessary to pay attention to the application of tall trees and strengthening the cultivation of urban forests, to improve landscape quality and enhance the application of arbor species with colored leaves, flowers and fruits to improve the quality of urban forests.

The response mechanism of Sapium discolor seedlings under water stress was discussed, and the effects of different water gradients on the growth and physiological and biochemical characteristics of Sapium discolor seedlings were studied, in order to provide a theoretical basis for water control and cultivation of high-quality tree species at the seedling stage.

In this experiment, 50 cm high Sapium discolor seedlings were used as the experimental materials, and three soil moisture gradients were set up: normal water supply (CK) (70%±5% of field water capacity), drought stress (SD) (40%±5% of field water capacity), and flooding stress (SF) (water surface was 2 cm below the soil surface of potted seedlings). The growth indexes such as seedling height, ground diameter, leaf area and biomass, as well as physiological indexes such as contents of proline Pro, malondialdehyde MDA, peroxidase POD, superoxide dismutase SOD, abscisic acid ABA, auxin IAA, gibberellin GA₃ and zeatin riboside ZR were measured. The correlation, principal component and membership function analysis methods were used to comprehensively evaluate the drought and flood resistance of the test materials.

Water stress had a significant effect on the growth of height, ground diameter and leaf area of Sapium discolor seedlings. The increase of the growth indexes was SF > CK > SD, the root-shoot ratio was in the order of SF > SD > CK, and the aboveground biomass was greater than the underground biomass. In terms of physiology, after 21 days of stress, the scavenging ability of POD and SOD activities in the stress treatment group was reduced, which was insufficient to resist the damage caused by reactive oxygen species, accelerating the growth of MDA content, with a maximum changing range reaching 35.02%. After 28 days of stress, Pro in the SF group reached the strongest regulation ability. The total content of (IAA+GA₃+ZR) was less than that of ABA before and after the treatment. The ratios of IAA/ABA, GA₃/ABA and ZR/ABA in the stress treatment group were lower than those in the CK group, while the ratios of IAA/GA₃, IAA/ZR and GA₃/ZR were higher than those under the CK treatment. The balance and proportion distribution of endogenous hormones affected self-growth. Using correlation analysis, principal component analysis and membership function analysis and other comprehensive evaluation methods, it was found that the resistance of Sapium discolor seedlings to waterlogging was greater than their drought resistance.

In summary, Sapium discolor seedlings reduce the damage caused by water stress and maintain the normal growth and development of cells by regulating their own growth characteristics, osmotic adjustment substances, antioxidant enzyme systems and endogenous hormone distribution, indicating that Sapium discolor has certain drought resistance and waterlogging resistance.