红壤侵蚀恢复区植被垂直结构水土保持效应定量表征

Soil erosion is one of the most severe environmental problems in the red soil region of southern China. Vegetation fractional canopy (VFC) is widely used to access the water and soil loss in the area. However，problems were encountered when using VFC as the sole index to assess soil erosion. Even if the VFC has risen up to 60%, the soil erosion is still observed under forest canopy, due to the poor understory vegetation. Therefore, the characteristics of vegetation vertical structure are desiderated to quantify, and the vegetation vertical structure factor should be established accordingly, to precisely analysis the effect of soil and water conservation. The terrain in South China is dominated by mountains and hills, the measure of vegetation vertical structure via traditional method cannot be realized. In the present study, experimental simulation and field survey are employed, and the terrestrial three-dimensional laser scanner is used to obtain point cloud of the vegetations. Based on extracted indexes of vegetation vertical structure (e. g. tree high, VFC of differing heights and leaf area index), the vegetation vertical structure factor would be constructed. The factor would be then used to study the relationships between runoff and sediment production in the runoff experiment. On the other side, the factor would also be applied to study the effects on soil quality and soil erosion environment. Finally, the soil and water conservation function would be quantitatively expressed by vertical vegetation structure, which would remedy the limitation of soil erosion assessment under forest canopy by VFC. The results of the study can provide theoretical and technical support for ecological restoration and rehabilitation in South China.

土壤侵蚀是我国南方红壤地区最严重的生态问题之一，植被覆盖度被广泛用于该区域水土流失的评价。但即使植被覆盖率高达60%，由于林下植被稀疏，红壤地区仍存在林下侵蚀现象。单纯靠植被覆盖度评判土壤侵蚀已无法满足当前水土保持的要求，亟需量化植被垂直结构特征，构建植被垂直因子，精确分析植被的土保持效应。南方地形以丘陵为主，传统方法测量植被垂直信息较为困难。本研究采用实验模拟与野外实测相结合的方法，利用地面激光扫描仪获取植被点云信息，基于建模提取植被垂直结构指标（树高、不同冠层覆盖度及叶面积指数等），构建三维植被垂直结构因子，一方面基于径流小区试验模拟植被垂直结构因子与产流产沙的关系，一方面在自然环境下研究植被垂直结构因子与林下土壤质量及侵蚀环境间的交互作用，最终定量表征植被垂直结构的水土保持效应，以弥补二维植被指标（如植被覆盖度）评价土壤侵蚀的缺陷，为加速该地区生态恢复与重建提供理论依据和技术支持。

我国南方红壤地区曾遭受严重的土壤侵蚀，至20世纪50年代末，该区域原始植被基本消失。30多年来，大规模的植被恢复措施（如飞播马尾松苗、人工植树种草等）在该区域得以实施，植被覆盖率平均达60%以上，但由于林下植被稀疏，红壤地区仍存在林下侵蚀现象。植被垂直结构的定量研究是红壤侵蚀恢复区植被水土保持效应表征的重要途径。本研究采用实验模拟与野外实测相结合的方法，利用地面三维激光扫描技术反演植被分层信息，结合林下土壤样品的采集分析，分析南方红壤侵蚀区不同植被垂直结构下的土壤侵蚀特征。结果显示，成土母质是引起植被垂直结构差异的重要原因，不同成土母质红壤上植被垂直结构恢复度由高到低依次为：千枚岩林地 > 红砂岩林地 > 花岗岩林地。灌草层是红壤侵蚀的重要影响因素，树高1.8m情境下，林下植被覆盖度增加30-40%，年均减少径流和泥沙量分别为1.44cm/m2/次；当树高达3.0m，年均减少径流和泥沙量分别为3.33cm/m2/次和24.9g/m2/次。不同植被结构投影叶面积指数的增大有利于侵蚀区退化土壤质量的恢复，在投影叶面积指数大于 0.2 的情形下，土壤恢复特征随植被垂直层次的增多呈正向演变，植被投影叶面积指数与归一化土壤恢复指数呈显著正相关(r = 0.332*，P < 0.05)。本研究在植被覆盖度评价土壤侵蚀的基础上有所提升，为加速南方红壤侵蚀区生态恢复与重建提供了理论依据和技术支持。