林地植被水土保持结构化叶面积指数(FSL): 多角度和垂向集成的分层遥感估算及应用研究

Forestation is a key measure to control soil and water loss, making the forest cover rate increased to over 20 percent in China. Emerging soil and water loss under forest canopy, however, prompts us to take further insight into the forest-erosion coupling mechanism as well as its monitoring and evaluation system. This project will carry out in Changting County, southern water-eroded area of China, using forest structured vegetation leaf area index for soil and water conservation (FSL) which computed from forest layered leaf area index (LAI) and their contribution to soil and water conservation, to investigate soil and water conservation mechanism of forest canopy and under-canopy vegetation, and to explore remote sensing strategies and their applications with integrated multi-angular and vertically observed imageries. With the observation data of water-eroded forest stands and soil-erosion experimental plots, we analyze the properties of weighted contribution of forest LAIs of canopy and under-canopy vegetation to soil and water conservation, and then compute effective FSL values for soil and water conservation; We explore the estimation method of forest layered LAI and FSL with integrated multi-angular (MODIS BRDF) and vertically observed (SPOT) imageries; Based on the ratios of remotely estimated multi-temporal FSLs to the effective FSL of the study area, we trace the spatiotemporal evolvement of the restoration level of the regional FSL, and its social driving mechanism and environmental response characteristics.

植树造林作为治理水土流失的关键措施，已使得我国森林覆盖率持续增长至20%以上。然而日渐凸显的林下水土流失现象促使人们深入思考森林-侵蚀耦合机理及其监测评估体系。本项目以南方红壤水蚀区福建省长汀县为研究区，基于林地植被分层叶面积指数（LAI）及其水土保持贡献构建水土保持结构化叶面积指数（FSL），藉以进行林地植被水土保持作用机理观测，并集成多角度和传统垂向遥感技术开展FSL的分层遥感估算与应用研究。根据不同土壤侵蚀强度的林地调查和土壤侵蚀试验小区观测数据，分析林冠和林下植被水土保持作用权重特征并基于林冠和林下植被LAI计算水土保持有效FSL值；探讨多时相、多角度遥感影像(MODIS BRDF)和传统垂向观测（SPOT）集成的林地分层植被LAI和FSL的遥感估算方法；基于遥感估算的多时相FSL与研究区有效FSL的比值，跟踪分析研究区FSL恢复水平的时空演变及其社会驱动机制与环境响应特征。

选择合理的植被结构指标并进行遥感反演，是提升水土保持治理水平的迫切需要。本项目以南方红壤水蚀区福建省长汀县为研究区，分析林地垂直层次植被水土保持机理，研究林地叶面积指数（LAI）分层遥感反演方法，并基于结构化LAI探讨生态环境的时空变化特征。在林地植被水土保持机理方面：次降雨和月尺度较有利于研究林地植被水土保持效应类型，而从模型精度和节约成本的角度考虑，则以季节尺度观测较为适宜；草地植被的保水效应多受制于降雨因素, 而保土效应则多受制于植被因素；水土保持权重系数的大小顺序为：草本 > 林冠 > 林草交互作用，保土权重系数分别为0.40、0.35和0.25，保水权重系数分别为0.76、0.14和0.10。林地植被遥感反演研究表明：研究区周围LAI较大，而中部盆地LAI较小，尤其是林下灌草LAI，不同土壤侵蚀等级的结构化LAI与地表温度呈负相关，反映了水蚀区独特的土壤-植被-大气系统能量交换过程；林地植被演替不同阶段的LiDAR有效波形和归一化累积能量曲线均存在显著差异，显示大光斑LiDAR系统可响应林地植被垂直结构的变化；结构化LAI较之传统的总体LAI对地表温度的影响更加显著。林地生态环境变化研究表明：长汀县及所辖乡镇的生境质量指数多呈略微下降趋势，外围乡镇高于中心地区，西部地区高于东部地区；近十几年来长汀县及所辖大部分乡镇的水土保持和水源涵养生态功能指数正逐步降低，各乡镇生态功能指数存在显著差异，表明植被结构和城镇化进程对生态环境的影响。项目还就土壤深度、土壤有机碳和养分与环境变量的关系以及地表反照率的影响因素进行了研究。项目成果为南方严重水蚀区的水土保持和生态恢复进一步丰富了理论依据和技术支撑。