| Urban forests are complex ecosystems created by the interaction of anthropogenic and natural processes. Structure and function are the first features of an urban forest to be studied. One key to better management of urban forest ecosystem is to understand urban forest structure and its relationship to forest functions, and to coordinate the multiple functions of urban forest.As an important part of urban forests in Shanghai, "The Shanghai Green Belt" was selected as an object in this study. In this paper, the researches were mainly developed from the following three aspects, community types and structural characteristics, ecological benefits and aesthetic benefits, based on survey of plant communities and measurement of typical communities' functions. The purpose of this research was to clarify the structural characteristics and health status of the Shanghai Green Belt, to explore the impacts of community structure on ecological benefits and aesthetic benefits, and to reveal the major structural factors. This study could provide theoretical basis for the Shanghai Green Belt structure optimization and ecosystem services improvement. Meanwhile, it also could provide models for the construction and improvement of urban forests. The main conclusions were as follows:The flora of vascular plant in the study area consisted of 226 species and varieties,184 genera and 87 families. But a few species dominated the whole belt, the ratio of native trees only 25%. There were 67 community types, which were divided into 7 vegetation types, most of them were deciduous broad-leaved forest and evergreen broad-leaved forest, however, few of them were zonal vegetation types. The species diversity of these communities was low. The result showed that the average number of species in the tree layer was 1.68, and in the shrub layer was 0.92. The community structure was excessively simple, featured as the dumbbell that was sparse in the middle and dense on both sides, most of which was the model of "tree with natural herb". The population structure of edificators was mainly the unimodal regeneration type, only a few species were sporadic type. Most populations could not recruit well, and lots of trees are in poor growth, causing declining in structure developing. The high stand density had limited the growth. The results indicated that, diameter at breast height (DBH) and crown diameter would decrease with the increased density, and it was inferred that the optimum density for the growth of the current community was about 1,000 individuals/hm2.Because of the inappropriate planting pattern and management regime, the species diversity in various communities was low. As the introduced exotic species received more attention than native species, and more application of the monoculture pattern, the community structure was unreasonable. Dense planting pattern intensified competition and further resulted in poor growth. At the same time, high intensity of management removed seedlings and litter, causing the decreased viability of community regeneration and poor ability of nutrient return. In addition, the problems were deteriorated due to the saline and cohesive soil, low terrain, high groundwater level and homogeneous landform in Shanghai.On the basis of community type classification, typical communities were selected for ecological benefits measurement, which was evaluated from the aspects of temperature and humidity amelioration, negative-ion supply, antibacterial function, noise reduction, air pollution reduction and soil improvement. Combined with community structural parameters, the key structural characteristics that influenced the ecological benefits were explored.In summer, deciduous coniferous forest Taxodium distichum var. imbricarium Comm. performed well on these ecological functions except for antibacterial function, while deciduous broad-leaved forest provide the fewest ecological benefits. In winter, evergreen broad-leaved forest Ligustrum lucidum Comm. provided the most ecological benefits in every aspect. The next one was evergreen and deciduous broad-leaved forest, and the performances of deciduous broad-leaved forest, broad-leaved and coniferous mixed forest and deciduous coniferous forest were not that good. The soil improvement effect of plant communities mainly arised in the surface of soil (0-20cm), where deciduous broad-leaved forest Platanus x hispanica Comm. and deciduous coniferous forest Taxodium distichum var. imbricarium Comm. deserved better assessment.In summer, except for soil improvement, other benefits, such as temperature and humidity amelioration, negative-ion supply and noise reduction, were mainly controlled by leaf area index (LAI), canopy closure and stand density. LAI and the above four benefits were significantly positively correlated. While the canopy closure and stand density just showed positive correlation with some of the benefits. In winter, except for soil improvement, other ecological benefits were mainly associated with LAI, canopy closure and canopy porosity. Hence, the plant community with higher LAI, greater canopy closure and lower canopy porosity will perform greater ecological benefits.To some extent, LAI and canopy closure indicated the amount of leaves, which indirectly associated with the intensity of photosynthesis and transpiration. These two factors showed significant positive correlation with ecological benefits. Therefore, LAI and canopy closure were the important structural parameters that indicated the ecological benefits of plant community.On the basis of community type classification, typical communities were selected for aesthetic assessment of each seasonal landscape. Combined with community structural parameters, the key structural characteristics that impact the aesthetic benefits were explored.The scenic beauty estimation (SBE) values of deciduous coniferous forest were the highest in all four seasons among different vegetation types, and SBE values of bamboo forest were the lowest. Among the coniferous forests, the SBE values of evergreen coniferous forest were lower than deciduous coniferous forest in all four seasons. Among the broad-leaved forests, the SBE values of deciduous broad-leaved forest were higher than evergreen deciduous forest in spring and summer, but reversely in autumn and winter. Among mixed forests, except for autumn, the SBE values of broad-leaved and coniferous mixed forest were higher than evergreen and deciduous broad-leaved mixed forest. The average SBE values of in-forest and outer-forest landscape were both the highest in summer, and the lowest in winter.The structural factors that impacted in-forest landscape of the Shanghai Green Belt were mainly average DBH and variation coefficient of diameter, canopy closure and canopy porosity. In spring, the communities with greater DBH had higher SBE values. In summer, the high level of canopy closure could improve the SBE values of plant community. In autumn, the communities with lower variation coefficient of diameter had higher SBE values. In winter, canopy porosity had greatest effect on SBE values of in-forest landscape.In addition to structural characteristics, the seasonal characteristics of plant community were also important factors that impacted the community SBE values. The seasonal characteristics influencing the community SBE values were diverse in different seasons. The communities marked with high brightness color such as yellow or purple and moderate amount of bloom, had the higher SBE values in spring. In summer, the communities, which shown better growth vigor, smaller changes in forest canopy and clearer trunk, had higher SBE values. Flowering trees could significantly improve the SBE values of community in summer. Color variation of the colorful leaves could impact the SBE values of community in autumn, and the purer the color was, the higher SBE values would be. In winter, the community with dark bark had the highest SBE values.Physical characteristics of the community had a significant impact on the outer-forest landscape. Forest canopy line provided the bigger contribution to the SBE values, followed by the forest edge line. Undulating canopy line and complex structural layers would increase the outer-forest SBE values, and community with natural forest edge shape had the highest outer-forest SBE values. In summer, except for antibacterial function, deciduous coniferous forest Taxodium distichum var. imbricarium Comm. provided the best ecological benefits and aesthetic benefits. The ecological benefits and aesthetic benefits of evergreen broad-leaved forest, broad-leaved and coniferous mixed forest, and evergreen and deciduous broad-leaved mixed forest were lower than deciduous coniferous forest. The SBE values of deciduous broad-leaved forest was higher, but ecological benefits were lowest.In winter, evergreen broad-leaved forest Ligustrum lucidum Comm. performed well in ecological benefits and aesthetic benefits. The ecological benefits and aesthetic benefits of deciduous broad-leaved forest, broad-leaved and coniferous mixed forest, and evergreen and deciduous broad-leaved mixed forest were lower than evergreen broad-leaved forest. The ecological benefits of deciduous coniferous forest were lower, but the SBE value was the highest.The relationship between the community structure and the ecological benefits and aesthetic benefits, shown us that canopy closure was positively related with both two benefits in summer, which means that by increasing the canopy closure of community, the ecological benefits and aesthetic benefits can both be optimized.In winter, the main structural parameter related with these two benefits was canopy porosity. The compacter the community was, the greater the ecological benefits would be provided. The sparser the community was, the higher the aesthetic benefits were. Canopy porosity did not made accordant effect on these two benefits. Canopy closure showed a significant negative correlation with canopy porosity. In consequence, by increasing the canopy closure of plant community, the ecological benefits can also be raised in winter. As structural characteristics and seasonal characteristics could impact aesthetic benefits, the trees with dark bark could be applied to increase SBE values in winter and to guarantee the aesthetic benefits as well as ecological benefits.
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