Road system is the infrastructure in the construction of modern society, and the selection and application of street trees have positive environmental and ecological effects on improving road service system and quality.
The main cultivation places of street trees are sidewalk green belt, dividing line green island, citizen square path, riverside avenue and urban and rural roads. The ideal selection standard of street tree species should be strong adaptability from the requirements of maintenance and management, less pests and diseases, and dry, beautiful and lasting from the requirements of landscape effect.
As the network and skeleton of urban green space system, road greening has become the main component of system continuity and plays an outstanding role in directly reflecting urban landscape. In the selection and application of street trees, urban roads are dominated by deciduous broad-leaved trees, which are shaded by green trees and have beautiful shapes. Suburbs and general grade highways pay more attention to maintenance cost factors such as rapid growth, pollution resistance, barren resistance and easy management. The selection and application of street trees in memorial places such as tunnels and tombs are mainly evergreen conifers, such as juniper, cypress, cedar and masson pine. Deciduous trees include willow, locust and elm. In addition, palm trees are often selected. In recent years, with the improvement of urban environmental construction standards and the implementation of greening, purification, beautification and aromatization indicators, the selection and application of evergreen broad-leaved trees and colored fragrant flower trees have made great progress and are on the rise. At present, the commonly used street trees are Platanus acerifolia, Tilia amurensis, Ulmus pumila, Aesculus aesculus, Maple, Camptotheca acuminata, Ginkgo biloba, Liriodendron hybrid, Cinnamomum camphora, Magnolia grandiflora, Michelia Lechang, Ligustrum lucidum, Tung Tree, Poplar, Willow, Sophora japonica, Taxodium ascendens and Metasequoia.
The practical application of street trees should be based on the road construction standards and the specific circumstances of the surrounding environment, determine the appropriate tree species and varieties, and choose the appropriate tree body and tree shape. If there are restrictions such as power and communication lines above the planned planting belt, trees whose final growth height is lower than the elevation of overhead lines should be selected to save the cost of regular pruning. If there are enough funds, equipment and pruning technicians to maintain a high level of tree conservation, we can choose tall trees, adopt correct pruning methods and avoid the restrictions of overhead lines, so as to obtain satisfactory use results. In addition, when planting plastic, the branches of trees should be high enough so as not to hinder the normal walking of passers-by and vehicles, and not to block the sight of pedestrians and drivers, so as to avoid accidents. Especially in areas with small turning radius and poor line of sight when turning, more attention should be paid. The selection of tree size should be appropriate and should not exceed the requirements of the proportion with the architectural landscape on both sides of the street. Its size and growth form should meet the specific requirements of design and stand the test of time.
The crown shape of street trees is determined by the environment where they are planted. On the main road where overhead lines pass, regular crown shapes can be adopted to trim trees into cups or open hearts. General highway trunk roads or narrow carriageways can be dominated by natural crowns. Where there are poplar, mandarin jacket, Metasequoia glyptostroboides, Taxodium ascendens and other central leading tree species, the height of lateral branches should be more than 2.5 to 3 meters, and the lower skirt branches should be thinned according to the situation, especially at bends and intersections with poor traffic sight, and more attention should be paid to broadening the horizons to avoid traffic accidents. Tree species without central leadership, such as willow, elm, locust tree, camphor tree, privet, camphor tree, etc. The height of the branch point should be controlled at 2 to 3 meters, and the crown naturally forms a sphere. Cup-trimmed street trees have a typical crown shape of "3 strands and 6 branches 12 branches", and the height of branch points of trunk should be below the overhead line, which does not hinder the passage of pedestrians and vehicles, generally 2 to 3 meters. Every year, we should adjust the growth direction of the lateral branches of the crown in time, keep the crown shape uniform and tidy, eliminate the interference to overhead lines, and take care of the safety and lighting of adjacent buildings. Open-heart pruning evolved from cup-shaped pruning, which is mostly used for self-pruning trees without spindle or terminal buds. During pruning, 3 ~ 5 main branches with different directions and uniform distribution are selected at the branch point, and 2 ~ 3 lateral branches are selected on each main branch to make the crown develop naturally, but the inner cavity is empty.
The height of street trees on the same main road should be relatively consistent, and it should be more than 3 to 3.5 meters in the area where the road surface is narrow or large vehicles pass. In a wide road or pedestrian street, it can be reduced to 2.5 to 3.0 meters, and the tree species with small branching angle can be appropriately reduced, but not less than 2 meters. The size of the tree should be as neat and uniform as possible to avoid the aesthetic effect and management inconvenience caused by uneven height and different sizes and thicknesses.