上海城市森林植被固碳功能及其抵消能源碳排放效果评估

Abstract

植被固碳功能是城市森林的重要生态服务之一，但是定量揭示城市森林植被固碳功能空间格局及其抵消碳排放的研究较少。基于光合速率法和生物量法测算了上海城市森林植被的固定CO₂功能，并结合其空间分布格局与区域CO₂排放状况对比分析了抵消能源CO₂排放的成效。结果表明，2017年上海城市森林植被可固定CO₂ 135.57万t，约合单位面积固碳17.02 t/hm²。上海城市森林植被的固定CO₂功能呈现出"中间低、四周高"的空间格局，崇明区和浦东新区的城市森林植被提供了近52.8%的固定CO₂效益。2017年上海市能源消费的CO₂排放量高达2.73亿t，且呈现出"中间高、四周低"空间分布特征，中心城区能源CO₂排放约占11.49%。整体来看，上海城市森林植被可抵消0.50%的能源CO₂排放，但植被固定CO₂功能与能源CO₂排放的空间匹配较差，中心城区城市森林植被的CO₂抵消率仅为0.08%。因此，重点提升中心城区森林植被的固碳功能并加强海洋碳吸收功能的监测与利用尤其重要。;Accompanied by the expansion of cities worldwide and the massive use of fossil fuels, ever-increasing carbon dioxide (CO₂) emissions in the urban areas contributes to global climate change. Studies from urban forest have reported that quantification of forest biomass and photosynthetic rate are of great significance to forest carbon cycle studies and climate-change mitigation. However, most former studies have focused on calculating CO₂ sequestration of urban forests at regional and urban scale, and few reports are available in the spatial differences of carbon fixation and the efficacy on offsetting energy CO₂ emissions in urban areas at finer scale. Therefore, based on field survey data and urban forests data derived from high-resolution Gaofen No.2 images, this paper modeled the spatial changes of CO₂ fixation by using assimilation method and biomass method in Shanghai, the most prosperous metropolitan area in China. We then mapped CO₂ emission in pixel size based on yearbook data, population distribution and NPP-VⅡRS nighttime light data. The results showed that the urban forest in Shanghai could annually absorb 1.36 million tons of CO₂, and the average CO₂ fixation capacity reached 17.02 tons per hectare. The CO₂ fixation capacity varied among different urban forest types. The broadleaved forest ranked first (18.22 t/hm²) while the conifer forest manifested the lowest CO₂ fixation capacity (7.11 t/hm²). The spatial distribution of CO₂ fixation presented a significantly uneven overall spatial distribution: low in the middle and high on all sides, and the forest in Chongming District and Pudong New District jointly contributed 52.8% of the total sequestrated CO₂. In addition, the results indicated that the amount of CO₂ emission from annual energy consumption reached 0.27 billion tons in 2017 and the amount of CO₂ emission from center area occupied 11.49% of the total CO₂ emission, being spatially high at center and low at fringe. Collectively, the annual CO₂ sequestration of urban forest could only offset 0.5% of the annual energy CO₂ emissions in Shanghai. Furthermore, the CO₂ sequestration spatially mismatched the CO₂ emission. Only 0.08% of energy CO₂ emission in the center area could be offset by its urban forest. Therefore, these results can be used to help assess the actual role of urban forests in reducing atmospheric CO₂ in Shanghai, and we suggested Shanghai should promote the carbon dioxide fixation capacity of urban forests in the center area, and strengthen the observation and utilization of carbon dioxide sequestration in the marine ecosystem.