Evaluation and correction of optically derived leaf area index in different temperate forests
Evaluation and correction of optically derived leaf area index in different temperate forests
复制标题
DOI：
10.3832/ifor1350-008
复制
发表时间：
2016-02-01
影响因子：
1.7
通讯作者：
Zhou, Ming
中科院分区：
文献类型：
--
作者：
Liu, Zhili;Jin, Guangze;Zhou, Ming
近年来发展了快速测量LAI的光学技术，但对落叶-常绿混交林LAI光学估算精度的评价研究较少。通过与4个阔叶与常绿混交林和1个落叶针叶林凋落物收集LAI（LAI(lit)）的比较，对数字半球摄影（DHP）和LAI-2000估算有效LAI（L-e）的准确性进行了评价。我们还评估了主要误差源对光学方法测定LAI的相对贡献，包括木材与总面积比（alpha）、元素结块指数（Omega(E)）和针尖与梢面积比（gamma(E)）。此外，不正确的自动摄影曝光也被认为是DHP。DHP L-e在不同林分平均低估LAI(lit) 44 ~ 70%， 5个林分经自动曝光、alpha、Omega(E)和gamma(E)校正后LAI（lit）与DHP L-e的差异在1% ~ 21%之间。LAI-2000的LAI值与直接凋落物收集LAI值更接近。LAI-2000 L-e平均低估了这些森林的LAI(lit) 13-40%，而在考虑alpha、Omega(E)和gamma(E)后，LAI-2000方法对LAI的最佳估计精度超过93%。在所有林分中，自动暴露对LAI-2000 L-e的误差均大于其他因素，其中gamma(E)是对LAI-2000 L-e的主要不确定因素。此外，光学LAI（DHP和LAI-2000）与LAI（lit）的相关性显著（P < 0.01），尤其是LAI-2000得到的校正LAI（R-2 = 0.83, RMSE = 1.04）。结果表明，上述因素对光学方法估算LAI有影响，为提高光学方法估算LAI的精度，应认真考虑林分的物种组成。
In recent years optical techniques for rapid LAI measurements have been developed, but few studies have been performed to evaluate the accuracy of optical estimation of LAI in mixed deciduous-evergreen forest stands. In this paper, we assessed the accuracy of digital hemispherical photography (DHP) and the LAI-2000 for the estimation of effective LAI (L-e) by comparison with litter collection LAI (LAI(lit)) in four mixed deciduous broadleaf and evergreen needleleaf forests and one deciduous needleleaf forest. We also evaluated the relative contribution of major error sources to the determination of LAI by optical methods, including the woody-to-total area ratio (alpha), the element clumping index (Omega(E)) and the needle-to-shoot area ratio (gamma(E)). Additionally, incorrect automatic photographic exposure has been considered for DHP. DHP L-e underestimated LAI(lit) by an average of 44-70% in different forests, and the difference between LAI(lit) and DHP L-e after correction for the automatic exposure, alpha, Omega(E) and gamma(E) ranged from 1% to 21% in five forest stands. In contrast, LAI values from LAI-2000 were more similar to the direct litter collection LAI. The LAI-2000 L-e underestimated LAI(lit) by an average of 13-40% in these forests, while the accuracy of the best estimates of LAI using LAI-2000 methods is over 93% after considering alpha, Omega(E) and gamma(E). The error caused by automatic exposure to DHP L-e is larger than other factors in all forest stands, and the gamma(E) was the main uncertainty to LAI-2000 L-e in most forest stands. Moreover, optical LAI (both DHP and LAI-2000) was significantly (P < 0.01) correlated with LAI(lit), especially the corrected LAI obtained by the LAI-2000 (R-2 = 0.83, RMSE = 1.04). Our results demonstrate that the above factors affect the estimation of LAI by optical methods, thus the species composition of a forest stand should be seriously considered in order to improve the accuracy of LAI by optical methods.