| As an important part of urban ecosystem,urban forest is an indispensable important ecological infrastructure for cities with functions of maintaining the balance of atmospheric carbon and oxygen,regulating urban microclimate and guaranteeing urban ecological security.It is also an important sign to measuring urban sustainable development level and civilization degree.With the integrated development of Chengdu-Chongqing urban agglomeration,the urbanization of Chongqing is accelerating.This made the role of urban forests in the construction of urban ecological civilization and sustainable development in this region become more prominent.Scientific and reasonable urban forest ecological network is an important guarantee for the urban forest ecological function.Therefore,it is necessary to analyze and optimize the urban forest ecological network while protecting the existing forest vegetation as much as possible within the limited urban space,so as to promote the balance between ecological protection and economic development and realize human and natural sustainable development.This study took the Chongqing metropolitan area as the research object.Four time nodes in2009,2012,2015 and 2018 were selected,and 16 landscape indices(8 type levels and 8 landscape levels)from the regional scale(8570 km~2,including nine districts of Chongqing's main city,Bishan,Yongchuan and the north area of the Yangtze River in the Jiangjin districts)and the city scale(2737 km~2,including nine districts of Chongqing's main city)were used to analysis of urban forest landscape patterns.The minimum cumulative resistance model was used to estimate the potential ecological network.The ecological corridor comprehensive index was used to evaluate and optimize the ecological network.Among the source patches and important ecological nodes at the urban scale,Quadrat surveys were conducted in 2009 and 2018 on semi-natural forests and plantations with high anthropogenic disturbance intensity.Community changes of woody plants during 10 years were analyzed.Then,the construction of plant community of highway ecological corridor were taken as an example to analyze the impact of pioneer plant species on community structure and small environment.Mainly results obtained as follows:(1)At the regional scale,from 2009 to 2018,the urban forest area increased by 27640.08 ha.The area proportion of small patches,medium patches and large patches in urban forests decreased year by year,while the area proportion of giant patches increased year by year.At the landscape level,the number of patches,patch density,diversity index,evenness index and separation index decreased year by year,while the spread index,connectivity index and aggregation index increased year by year with relatively small change.There were fewer types of urban forest landscape,lower patch density and higher landscape patch aggregation and uniformity,while the connectivity was lower.The terrain gradient of urban forest patches was mainly distributed in the low-middle and middle-high terrain.In 2018,the urban forest comprehensive evaluation with a score more than 3generated 40 source patches,extracting 780 potential ecological corridors and 320 nodes.Merging and deleting redundant duplicate corridors,540 optimized ecological corridors and 224 ecological nodes(153 ecological breakpoints and 71 stepping stones)were obtained.The optimized ecological network with a mesh structure could effectively improve the urban forest landscape pattern at the regional scale.(2)At the urban scale,from 2009 to 2018,the urban forest area in the study area decreased by 1206.69 ha.Among them,ecological forests and river bank shelterbelts decreased year by year,while recreational forests,landscape beautification forests and road shelterbelts increased.During the decade,the area of natural forests and semi-natural forests decreased,while the area of planted forests increased year by year.The area of small patches,medium patches and large patches of urban forests decreased year by year,while the area of micro patches and giant patches increased year by year.At the landscape level,the number of patches,patch density,diversity index,evenness index and dispersion index showed an increasing trend,but the spreading,connectivity and aggregation index decreased to some extent.The terrain gradient of urban forest patches was mainly distributed in the low-middle terrain.In 2018,the urban forest comprehensive evaluation with a score more than3 generated 31 source patches,extracting 465 potential ecological corridors and 199 nodes.Merging and deleting redundant duplicate corridors,278 optimized ecological corridors and 124 ecological nodes(83 ecological breakpoints and 41 stepping stones)were obtained.The degree of fragmentation of the urban forest landscape in the study area were improved,and the urban forest landscape pattern were optimized.(3)In 2009 and 2018,quadrat surveys were conducted on semi-natural forests and planted forests with different interference intensities.100 quadrats of 5 plots were set for each type.In 2009,there were 2 groups of vegetation type,5 vegetation types,26 formations,41 families,55 genera,72species in semi-natural forest.In 2018,a total of 2 groups of vegetation type,5 vegetation types,25formations,42 families,56 genera,75 species were record.3 species in 1 family and 1 genera were increased.In 2009,a total of 1 group of vegetation type,3 vegetation types,24 formations,26families,38 genera,47 species were recorded in the plantation.1 species in 1 family and 1 genera were decreased.5 species were changed in trees,and 7 species were changed in shrubs.In the past10 years,the order of the important values of the top 5 arbor tree species in the semi-natural forest plots had little changed.Only Erythrina variegata in the Pingdingshan plot had fallen out of the top5 species,while Cinnamomum camphora reached to be one of the top 5 species.Tree species in the shrub layer had no changed,but the order of species in some plots changed.The species diversity of semi-natural forests had increased generally.The species diversity of woody plants and shrub layers decreased,while the tree layer increased slightly.The arbor-shrub ratio and evergreen-deciduous ratio of woody plants in semi-natural forest plots remained the same in category,but the number of shrubs increased.Evergreen-deciduous ratio of woody plant in tree layer and shrub layer did not change significantly during this period.The tree-shrub ratio of the woody plant in the plantation forest plot did not change in category,but the number of shrubs decreased.The DBH levels of semi-natural forest trees showed increasing from 2009 to 2018,with no decrease in the middle,indicating that the community regenerated very well.The average DBH of all plots decreased from12.3 cm to 11.6 cm in 10 years.The average DBH of the tree layer increased from 13.85 cm to 14.91cm in 10 years.The individuals with DBH larger than 1 cm showed a clear"decay",with individual distributions in 8 diameter levels,indicating that the community was relatively stable on the whole with the characteristics of aging community.The average biomass of semi-natural forest and plantation increased,showing that the semi-natural forests in the plot were in positive succession,while the plantation had a greater preference for man-made forests and remained relatively stable under human intervention.(4)Through artificial plant community construction and continuous observation and investigation,it showed that in the initial stage after planting,the number of species of the initial planting plant could directly affect the formation time of the vegetation coverage of the road ecological corridor and the accumulation of community biomass.The greater the number of initially planted species,the shorter the formation time of high vegetation coverage,and the more biomass accumulated in the community,which had a significant positive correlation.The number of initial plant species could indirectly positive affect the carbon sequestration and oxygen release capacity of vegetation communities in road ecological corridors,which was conducive to the ecological function of road corridor ecosystems in carbon sequestration and oxygen release.The species richness of the initial planting plants was closely related to the species diversity level in the structural characteristics of the community.In the same growing season,the more species of the initial planting plant,the greater potential for new species in the habitat and the number of species in the community and the community coverage,the higher the plant diversity level of the community.The level of species diversity in the community of the road ecological corridor was closely related to the ability of the vegetation to retain water and soil(slope protection performance),showing as the higher the level of plant diversity in the community,the more stable the ecosystem and the stronger the ability to resist interference.The greater the number of initial plant species,the higher the level of plant diversity in the community,the stronger the erosion resistance,water storage and soil conservation capacity,and the smaller the runoff coefficient and soil erosion modulus in the region.Moreover,this trend became more obvious with the extension of planting time.The number of initial plant species could affect the soil physical and chemical properties in the planting area.The greater the number of initial plant species,the more the physical and chemical indicators of soil in the planting area were improved.For example,soil pH,available N,P,K and total N,P and K indexes increased with the increase of species,and there was a significant positive correlation between them.Therefore,under the same planting conditions,increasing the species richness of the initial planting community could effectively improve the structural stability of the plant community and promote the ecological function of the plant community in the road ecological corridor.In summary,the urban forest area of Chongqing metropolitan area presents different trends under different research scales,showing as increasing year by year on the regional scale and decreasing year by year on the urban scale.The two-scale ecological network formed by the study could provide some guidance for the optimization of the urban forest landscape pattern in Chongqing metropolitan area.Semi-natural forests in urban forests showed an obvious succession characteristics of zonality,while the planted forests were affected by human preferences and had no obvious changes trend.The initial number of plant species directly affected the quality and development trend of the ecological corridor plant community. |
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