城市森林碳汇及其抵消能源碳排放效果——以广州为例

城市森林及其管理相关政策作为减少CO₂排放的有效策略得到了较为广泛的关注。采用材积源生物量方程与净初级生产力方法来定量分析了广州市城市森林碳储量和碳固定量,根据化石能源使用量及其碳排放因子核算了广州城市能源碳排放,最后评估了城市森林碳抵消效果。结果显示广州市城市森林碳储量为654.42×10⁴t,平均碳密度为28.81 t/hm²,而森林碳固定量为658732 t/a,平均固碳率为2.90 t·hm^-2·a^-1。2005-2010年广州市年均能源碳排放则达到2907.41×10⁴t。广州城市森林碳储量约为城市年均能源碳排放的22.51%,其通过碳固定年均能够抵消年均碳排放的2.27%,不过从城市森林综合效益来看其仍是城市低碳发展重要举措之一。分析了林型组成和林龄结构对于广州森林碳储量和碳固定量的影响,并从森林管理角度为城市森林碳汇提升提出建议。这些结果和讨论有助于评估城市森林碳汇在抵消碳排放中所起的效果。

The carbon sink of urban forests and efficacy on offsetting energy carbon emissions from city in Guangzhou

Urban forest and relevant management policies have been widely considered as a tool to mitigate carbon dioxide (CO₂) emissions. However, the urban forest structure, function and role in carbon(C) offsets of Chinese urban forests are little understood. The main objective of this study was to evaluate the C storage and sequestration by urban forests and C emissions from energy consumption by urban energy consumption in Guangzhou, and analyse the effect of urban forests offsetting the C emissions from energy use. This paper also discussed the relationship between urban low carbon development and forest C sink, and explored the management practices for improving Guangzhou''s urban forest function of offsetting C emissions. Guangzhou is located in north side of the Pearl River, along southeastern coast of China. The forests in Guangzhou includes five main types: Eucalyptus, Plus massoniana, Cunninghamia lanceolata, economic forest and evergreen broad-leaved forest. C storage and sequestration were quantified using urban forest inventory data and by applying volume-derived biomass equations and other models related to net primary productivity (NPP). C emissions from urban energy use were estimated by accounting for fossil fuel use and C emission factors. Total C storage by Guangzhou''s urban forests was estimated at 654.42×10⁴ t, and average C density was 28.81 t/hm². C storage in different districts ranged from 1.50×10⁴ t to 354.99×10⁴ t, as in the following order: Conghua>Zengcheng>Huadu>Baiyun>Luogang>Panyu>Tianhe>Huangpu. C storage of evergreen broad-leaved forest and young age class forest were respectively 530.67×10⁴ t and 271.86×10⁴ t, which had played a chief role in forest total C storage in terms of forest type and age class. C sequestration by urban forests was 658,732 t/a, with an average C sequestration rate was 2.90 t·hm^-2·a^-1. The spatial distribution of C sequestration in Guangzhou was imbalanced: most of C was sequestrated in exurb just like Conghua and Zengcheng. The C sequestration percentages of Eucalyptus, Plus massoniana, Cunninghamia lanceolata, economic forest and evergreen broad-leaved forest were 6.25%, 8.13%, 3.67%, 1.48% and 82.25% respectively. C sequestration rate decreased with increasing forest age. For Eucalyptus, C sequestration for young, middle-aged, premature, mature and post-mature forests decreased with age from 3.06 t·hm^-2·a^-1 to 2.35 t·hm^-2·a^-1. Results were similar for Plus massoniana which decreased from 1.99 t·hm^-2·a^-1 to 1.34 t·hm^-2·a^-1, for Cunninghamia lanceolata from 4.06 t·hm^-2·a^-1 to 2.61 t·hm^-2·a^-1, and for evergreen broad-leaved forest from 4.17 t·hm^-2·a^-1 to 3.68 t·hm^-2·a^-1. Average annual C emissions of urban energy use from 2005 to 2010 in Guangzhou were 2907.41×10⁴ t. Therefore, the C stored by urban forests equaled 22.51% of the average annual C emission from fossil fuel use, and C sequestration could offset 2.27% of the average annual C emission in Guangzhou. Urban forest''s C sequestration had a small contribution on urban carbon reduction, but it is still one of the important measures of low carbon development from its comprehensive benefits such as releasing Oxygen, air purification, VOC(volatile organic compounds)management, stormwater runoff capture, building energy saving, aesthetics and recreation. Furthermore, this paper also analyzed the impacts of forest composition and forest age structure on C storage and sequestration of urban forests in Guangzhou, and gave suggestions on improving forest C sink to enhance C offset effect through the forest management.