摘要:
叶面积指数(leaf area index,LAI)是森林生态系统重要参数,如何以较小成本提升区域尺度的山地森林LAI的遥感估测精度,对于精确掌握森林LAI的情况和进一步了解森林生态系统有重要意义。本研究以星载激光雷达ICESAT-2/ATLAS为主要信息源,以西南山地香格里拉市为研究区,基于随机森林回归(random forest,RF)遥感估测模型,结合地面51块LAI实测样地数据,在前期进行RF超参数优化基础上,采用决定系数、均方根误差、绝对平均误差和中位数绝对误差作为模型精度评价指标,对估测效果进行分析。结果表明: 使用随机表面查找算法进行RF回归模型的超参数优化,能明显提升模型估测LAI精度。提取出的地面光斑特征参数在山地森林LAI估测中有较高的贡献度和极佳的效果,可用于区域尺度的山地森林物理结构参数LAI的估测。同时,利用随机表面查找算法优化后的RF回归模型,估测精度更高,估测结果与研究区森林分布现状吻合,具有一定普适性。最后,研究确定了使用ICESat-2/ATLAS数据产品估测LAI是可行的,能为星载激光雷达估测中大范围的LAI提供一定的参考。
Abstract:
The leaf area index (LAI) is a critical parameter for the forest ecosystem. Improving the remote sensing estimation accuracy of the regional LAI of mountain forests at a low cost is of great significance for accurately determining the LAIs of forests and for further understanding the forest ecosystem. With spaceborne LiDAR ICESat-2/ATLAS data as a primary information source, this study investigated Shangri-La City in mountainous areas in southwestern China. Based on the remote sensing estimation model using random forest (RF) regression, RF hyperparameter optimization, and the data of 51 measured sample plots of LAI, this study analyzed the estimation effects of the model using accuracy evaluation indicators such as coefficient of determination (R2), root mean square error (RMSE), mean absolute error (MAE), and median absolute error (MedAE). The results are as follows: The hyperparameter optimization of the RF regression model using a random surface search algorithm can significantly improve the estimation accuracy of LAI. The extracted characteristic parameters of ground spots showed high contribution and excellent effects in the LAI estimation of mountain forests. Therefore, they can be applied to the estimation of regional LAI of mountain forests. The RF regression model optimized using the random surface search algorithm yielded higher estimation accuracy. The estimation results were consistent with the forest distribution in the study area, indicating certain generality. Finally, this study determined that it is feasible to employ ICESat-2/ATLAS data products for LAI estimation, providing a reference for medium- to large-scale LAI estimation based on spaceborne LiDAR.
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