Spatio-Temporal Variations Of The Key Ecosystem Services Of Urban Forests In The Three Gorges Reservoir Area,China

Forest is the most important terrestrial ecosystem.It has irreplaceable ecosystem service functions in many aspects such as carbon fixation and oxygen release,water conservation,and soil conservation.Climate change and human activities have led to irreversible degradation of global forest ecosystems.Many countries have gradually begun to implement ecological restoration projects and policies.In recent years,China has made great efforts in forest restoration and protection.Forest,as the most important component of a city,directly determines the supply capacity of urban ecosystem services.However,forest degradation and forest restoration coexist in cities and urbanized areas,resulting in the complexity and complexity of changes in the structure and function of urban forest ecosystems.The Three Gorges Reservoir Area is a typical ecologically sensitive area in China and an ecological barrier area of important strategic significance in the Yangtze River Basin.Since the construction of the Three Gorges Project,the contradiction between protection and development has been very prominent.In order to reveal the variability and complexity of urban forest ecosystem services in the Three Gorges Reservoir area,this study takes six typical mountainous cities in the Three Gorges Reservoir area(Zigui,Zhongxian,Wanzhou,Kaixian,Yunyang,and the core urban area of Chongqing)as the research objects.,Studied the change characteristics of the forest landscape structure of each city from 2000 to 2020,and simulated and evaluated the forest ecology of each city and its different urban gradients(central city,suburban area,and outer suburban area)from 2000 to 2020 based on CASA model,In VEST model,and RUSLE models.The system’s carbon sequestration,water conservation and soil conservation and their temporal and spatial evolution trends,and the relationship between different urban forest ecosystems and social and economic development influencing factors are analyzed.The main results are as follows:(1)From 2000 to 2020,the forest area of the six cities showed a gradual increasing trend.The change rate of Yunyang was the largest(+ 1.56% / yr),and that of Kaixian was the smallest(+ 0.86% / yr).Generally speaking,small cities(Zigui,Zhongxian)and medium-sized cities(Wanzhou,Kaixian and Yunyang)are the main landscape types,accounting for a large proportion of land area,and the change rate of forest area is high;The growth of construction land area is the largest in large cities,followed by mediumsized cities.The expansion of construction land is more obvious along the Yangtze River and rivers;The reduction of agricultural land area in the six cities in the study area is the largest,of which the reduction of medium and large cities is 960.05 km2 and 853.47 km2respectively;This spatial-temporal evolution of landscape pattern can effectively alleviate the land conflict between ecological restoration and economic development.(2)Forest carbon sequestration in small and medium-sized cities showed an increasing trend,and there was no significant change in large cities.The change rate of Kaixian county is the largest(+ 2.24% / yr);Zhongxian took the second place(+ 1.85% /yr);The core urban area of Chongqing has the lowest change rate(-0.23% / yr).Compared with large cities,the carbon sequestration per unit area of small and mediumsized cities is higher as a whole,indicating that the carbon sequestration per unit area of urban forest vegetation is inversely proportional to the size of the city.From 2000 to 2020,the spatial distribution characteristics of forest carbon sequestration in cities in the study area have changed significantly.The areas with low carbon sequestration per unit area are mainly expanded along the Yangtze River and rivers,and the larger the urban scale,the larger the area with low carbon sequestration per unit area.Small cities have little change in water conservation per unit area,and other cities show an obvious downward trend.Large cities have the highest water conservation per unit area of forest,followed by small cities,and medium-sized cities have the lowest water conservation per unit area.From2000 to 2020,the water conservation per unit area of the six cities in the study area was low,mainly along the Yangtze River and along the river,especially in medium-sized and large cities.From 2000 to 2020,forest soil conservation services in the study area showed an increasing trend year by year.The change rate of Yunyang is the highest(+ 1.98% / yr),followed by Kaixian(+ 1.33% / yr),and the change rate of Chongqing core urban area is the lowest(+ 0.12% / yr).Small cities have the largest amount of soil conservation per unit area,followed by medium-sized cities,and large cities have the smallest,indicating that the service function of urban forest soil conservation is inversely proportional to the size of the city.(3)The carbon sequestration and soil conservation in central urban area,suburban area and outer suburban area showed a synergistic change trend.The improvement level of forest carbon sequestration and soil conservation service function in the near and far suburbs with relatively low urbanization intensity is better than that in the central urban area,and the synergistic inertia of carbon sequestration and water conservation in the near and far suburbs is more obvious.The complex impact of urbanization and forest restoration has brought more uncertainty to urban forest development.Forest carbon sequestration and soil conservation services in the study area mainly show a synergistic growth trend,while water conservation services and other two key ecosystem service indicators mainly show a trade-off relationship.From 2000 to 2020,the gradient carbon sequestration and soil conservation of three small cities showed a synergistic change trend,while the three key ecosystem service indicators of different urban gradients of mediumsized and large cities showed a phased trade-off and synergy,and the change was more complex.(4)Forest ecosystem services respond significantly to social and economic development.The correlation between forest carbon sequestration and the driving factors of social and economic development is the weakest,and the correlation coefficient is small;Forest water conservation services have a strong positive correlation with grain yield,and a strong negative correlation with other socio-economic factors such as urban population;Except that the forest soil conservation has a strong negative correlation with grain yield,it has a strong positive correlation with other socio-economic factors;The correlation between forest area and socio-economic development factors is similar to forest soil conservation services;However,the results show that there is a strong positive correlation between forest area,forest soil conservation services and forest carbon sequestration and urban grain yield in Zhongkai County,indicating that forest restoration can increase in synergy with multiple factors such as social economy.