西双版纳地区森林变化碳效应与生态效益评估

Abstract

减少发展中国家因森林砍伐与森林退化导致的碳排放和保持碳储量（REDD+），不仅能减少因森林砍伐和森林退化造成的碳排放，而且还可以带来其它生态效益，如减缓森林破碎化、保护生物多样性和增强水土保持功能等。以中国的西双版纳地区为研究区域，以毁林最严重的1976-2007年为REDD+基线，基于卫星影像，并结合植被指数，提取了研究区的土地利用变化信息。基于IPCC温室气体清单方法，计算了研究区的森林碳储量变化。在此基础上，对REDD+的碳汇效益和生态效益进行了系统综合评估。结果显示：（1）1976-2007年间天然林碳储量从占总碳储量的78.24%减少至50.52%，这是造成西双版纳地区碳储量减少的主要原因。（2）1976-2007年，天然林的斑块数量和平均最近邻距离分别增加了120.00%和25.21%，平均斑块面积下降了71.98%，说明天然林的破碎化程度加剧。从研究区整体景观格局来看，斑块数量、Shannon多样性指数和Shannon均一性指数分别增加了8.16%、51.39%和34.07%；与此同时，平均斑块面积和景观内聚力指数分别下降了26.26%和2.13%，表明研究区整体景观格局朝破碎化方向发展，这和碳排放的增加相一致。（3）该区的土壤侵蚀程度逐年加剧，并与碳储量的减少密切相关。分析表明，土壤侵蚀剧烈的区域，其平均碳排放也相对较高。1999年到2007年间，剧烈侵蚀区域的平均碳排放是轻度侵蚀区域的6倍多。这些结果说明：如果REDD+政策得以实施，不仅能减少因毁林和森林退化导致的碳排放，而且能带来高的生态效益。从这个角度出发，REDD+的本质可以认为是以"增汇"或"减排"为主导功能，同时提升生态效益的一种管理方式。;The REDD+ partnership works to promote the reduction of greenhouse gas (GHG) emissions by reducing emissions from deforestation and forest degradation in the developing countries through positive incentives and conversation of forest carbon stocks. It is regarded as an essential component of the post-2012 climate regime to stabilize GHG emissions and engage the developing countries in worldwide mitigation endeavors. If cost-efficient carbon benefits can be achieved through REDD+, increases in atmospheric CO₂ concentrations could be slowed, effectively buying much needed time for countries to move to lower emissions technologies. We chose tropical forests in Xishuangbanna of China as our study area and took the most severe deforestation from 1976 to 2007 as the baseline of REDD+. Land use information was obtained through the combination of Landsat TM images and Normalized Difference Vegetation Index masks. Forest carbon storage change in the study area was firstly calculated based on IPCC methodology. Secondly ecosystem-based benefits from REDD+ were assessed systematically. Our results showed that from 1976 to 2007, the natural forest carbon storage decreased from 78.24% to 50.52% of total carbon storage, which was the main reason for the reduction of carbon storage in Xishuangbanna. At the meantime, significant degradation of ecosystem services emerged. This was demonstrated by two important indicators:forest fragmentation and soil erosion. For example, from 1976 to 2007 the percentage change in the carbon stocks was 1.4%, accompanying with increasing 8.16% in number of patches, 51.39% in Shannon's diversity index and 34.07% in shannon's evenness index. Meanwhile, mean path area and patch cohesion index decreased by 26.26% and 2.13%, respectively. It suggested that the forest ecosystem was changing in both carbon storage and forest landscape structure. The number of patches and average nearest neighbor distance of natural forest increased by 120.00% and 25.21%, respectively, and the average patch area decreased by 71.98%, indicating the degree of exacerbated fragmentation in natural forest. In this sense, carbon emission and forest fragmentation were well integrated. Furthermore, we investigated relationship between carbon emission and soil erosion and high correlation was found. For example, carbon emissions which accounted for 45% of the total located in the high erosion area, which is 18% of the whole study area. As a result, when forests that would have been lost or degraded are retained or restored through REDD+, they deliver ‘multiple benefits’ in addition to protecting or enhancing carbon stocks. These ecosystem-based benefits may include conservation of forest biodiversity, water regulation, soil conservation, timber, forest foods and other non-timber forest products, most of which have been demonstrated or proved in this study. By securing benefits beyond carbon, REDD+ has the potential to encourage countries to realize a broader range of values.