Multi-scale Assessments On Ecological Quality Of Urban Forest In Shanghai

Urban forest,as an important green infrastructure in city,which is also an important part of urban ecosystem,plays important roles in urban ecological environment improvement,biodiversity protection,and city comprehensive competitiveness promotion.Under the background of the rapid urbanization,it is necessary to estimate the ecological quality of urban forest structure and function from multi-scale so that it will be beneficial for establishing urban forest with proper landscape layout,complex structure and high ecological service.In shanghai,landscape pattern of urban forests was analyzed and their species composition and community structure were surveyed in 201 sampling plots and 816 quadrats across the whole city.Material production functions including three-dimensional green volume,leaf area index?LAI?,biomass and annual biomass increasment were measured for most common species in the urban forests.Then,the current status of community structure and ecological function along urban-rural gradients were analyzed and their relationships with environmental factors were determined.Finally,the ecological quality of urban forests was evaluated from aspects of landscape structure,species composition,community structure and ecological function to determine theirs naturalness,based on which the specific restoration methods were suggested for forests with low ecological quality.The study could provide theoretical basis and practical reference for the construction and improvement of Shanghai urban forest.The main conclusions are as follows:?1?Landscape pattern of urban forests in ShanghaiThe patch information and landscape pattern of urban forests were interpreted and analyzed based on the remote sensing images in August 2015 and the distribution map of forest of Shanghai in 2014.The results showed that giant patches and large patches primarily dominated the urban forests in Shanghai.On urban-rural gradients,the order of total forest area was:the outer suburbs>the suburbs>the city center.But the city forest coverage of the suburbs was superior to the outer suburbs and the city center.It indicated that urban forests of Shanghai were in high landscape fragmentation with low landscape connectivity,neatly patch boundary and poor functional connectivity.?2?Species composition and community types of urban forests in ShanghaiThere were 126 families of vascular plants?405 genera and 567 species?including 246 native species?43.4%?and 22 alien species were recorded in 816 quadrats,in which 250 species was herbaceous plants and evergreen broad-leaved trees take only 5.8%.However,the cumulative frequency of evergreen broad-leaved trees was up to 14.4%,especially for Cinnamomum camphora and Ligustrum lucidum.According to the origin of the forest and the human disturbance degree,urban forest in Shanghai can be classified into natural secondary forest,near natural forest,artificial secondary forest and artificial forest.The main vegetation types were evergreen broad-leaved forest,evergreen and deciduous broad-leaved mixed forest and deciduous broad-leaved forest.Although evergreen broad-leaved forest has less community types,they widely distributed in Shanghai such as Cinnamomum camphora community.The common community types in natural secondary forests such as Cyclobalanopsis glauca community were scarce in artificial forest.?3?Community structure of urban forests in ShanghaiAt plot level,urban forest of Shanghai had high species richness.Plots with 1-2 species and 3-5 species accounted for 10%,Plot with more than 6 species accounted for 80%.The native species richness was higher than cultivated and naturalized cultivation species which higher than alien species.The species richness of tree layer,shrub layer,large small shrub layer and herb layer were 2.49,3.46,3.66 and 7.64 respectively.The richness of near natural forest was the highest following by artificial secondary forest and natural secondary forest,and artificial forest was the lowest.On urban-rural gradients,native species richness increased gradually from the city center to the outer suburb.The vertical structure of urban forests was mainly with stratified structure.The proportion of herb and shrub in suburbs,artificial forests and protection green land was significantly lower than that of other forest types.In total,the diameters at the height?DBH?of woody plants were mainly between 7cm and 25 cm,which indicated most of urban forests were middle or young forest stands.The variation coefficient of DBH and height of the suburb,plantation and protective green space were significantly lower than those of other forest types.Nearly 22%of artificial forests had higher individual density,while other forest types were at an appropriate density.Totally,71.7%plots had recruitments of woody plants in regeneration layer.The recruitments belonged to 54 families?102 genera,124 species?,accounting for 40.3%of the total species in upper layer of forest.Species of recruitment in mixed forest with complex vertical structure were more abundant than other forests.The commom recruitment species in the natural secondary forest,such as Aphananthe aspera,Cyclobalanopsis glauca and Machilus thunbergii,were rare in the artificial forests.This indicated that species source limit had great impact on urban forest regeneration.In addition,CCA ordination showed that soil pH,soil bulk density and soil organic matter content with canopy cover were main environmental factors influencing the species composition of recruitment.?4?Material production of urban forest in ShanghaiUnit three-dimensional green volume of urban forest was 5.792m3/m2.Average LAI was 35.89,and biomass per unit area and net increase of biomass per unit area of urban forest were 13.02kg/m2 and 1.75 kg/12/a respectively.The four indexes of the forests with complex vertical structure were higher than that of the forests with simple vertical structure.The index of LAI,biomass per unit area and net biomass per unit area were the highest in the artificial secondary forest,and the green volume per unit area was the highest in natural secondary forest.On urban-rural gradients,the order of the four indicators was the city center>the outer suburbs>suburbs,which reflects the join influence of urban environment and community structure on the urban forest production.There were interspecific differences in the material production function of common species in urban forest of Shanghai,and the order of the four indexes of different species was arbor>shrub>herb,conifer>broadleaf and alien species>native species.In addition,there were also intraspecific difference in the material production function,which was mainly affected by the local community structure and environmental factors.Among that,LAI and net biomass were negatively correlated with individual density and canopy closure.Deciduous trees are more sensitive to environment variation compared to evergreen tree.For deciduous trees,the LAI and annual biomass increasment were related to some air pollution indexes such as PM_2.5,SO₂ and NO₂,and had positive and negative correlation with soil bulk density and soil organic matter respectively.?5?The ecological quality of urban forest in ShanghaiUsing objective and subjective weighting method,ecological quality evaluation system of urban forest of Shanghai was established,including two object levels.A total of 5 criteria layers were designed including 35 specific indicators in naturalness object level,and a total of 2 criteria layers were designed including 8 specific indicators in landscape structure object level.The results showed that the ecological quality of Shanghai urban forest is not good based on landscape-community scale,and it was worse in Huangpu district and better in Chongming district and Songjiang district.The naturalness evaluation value ranged from 0.092 to 0.616 with an average of 0.374,which was in moderate low level of naturalness.The value of the mixed forest with complex vertical structure was significantly higher than other forests.On urban-rural gradients,the values of forests in suburban were significantly lower than that in outer suburban and urban center.The values of artificial forests were significantly lower than those of natural secondary forests,near natural forests and artificial secondary forests.According to the classification standard of comprehensive evaluation of naturalness value,the natural secondary forest in the V-grade?good?was as high as 87.5%,which suggested the natural secondary forest could be regarded as a reference for near-natural restoration of urban forest with low ecological quality in Shanghai.?6?Rules and methods of ecological restoration for low quality urban forest:Based on the results of comprehensive evaluation of naturalness of urban forest in Shanghai,we put forward the ways and countermeasures for ecological restoration of the low quality urban forest.First,we divided several ecological restoration units in Shanghai by considering ecotone region,forest ecological sensitivity,dominant ecological services and environmental quality,and then determined the restoration target based on the grade of naturalness.Second,restoration and reconstruction for low quality urban forest should take into account of the "community with suitable habitat" and "community with suitable function",and chose suitable plants and community types for different ecological restoration units.In addition,restoration and reconstruction should be combined with a variety of techniques,including soil remediation technology,replanting and renovation.