利用3种校正方案提高间接法测定兴安落叶松人工林叶面积指数的精度

木质部和集聚效应是影响间接法测定叶面积指数(LAI)精度的主要因素, 尤其是木质部的校正一直存在争议。针对这一问题, 该研究首先利用半球摄影法(DHP)和LAI-2000植物冠层分析仪法(LAI-2000法) 2种间接法测定了小兴安岭兴安落叶松(Larix gmelinii)人工林叶面积最大时期的有效LAI (L_e), 然后提出了A、B、C 3种校正方案来提高间接法的测定精度。同时, 利用凋落物法和异速生长方程法2种直接法测定LAI, 以凋落物法测定值为标准来评估3种校正方案的校正效果, 并检验天顶角范围对校正结果是否存在显著影响。结果表明: 在0-45° (1-3环)、0-60° (1-4环)、45°-60° (4环)及0-75° (1-5环) 4种不同天顶角范围内, DHP测定的L_e比凋落物法、异速生长方程法测定值分别低估19%-32%和8%-29%; 而LAI-2000法也得到相似的结论, 分别低估9%-30%和8%-28%。虽然校正方案A高估了木质部对LAI的贡献, 但在45º-60º天顶角范围内, 能有效地校正DHP测定的L_e, 在1-3环和1-4环天顶角范围内, 能有效地校正LAI-2000法测定的L_e。4种天顶角范围内, 校正方案B均能有效地校正DHP测定的L_e。整体来看, 4种天顶角范围内, 校正方案C对DHP和LAI-2000法测定值的校正效果均优于其他2种方案。研究结果表明除木质部和集聚效应外, 天顶角范围的选择也是决定间接法测定LAI精度的重要因素。

Improving the accuracy of indirect methods in estimating leaf area index using three correction schemes in a Larix gmelinii plantation

Aims Woody materials and clumping effects are key error sources in estimating leaf area index (LAI) by optical methods. However, how to correct the error caused by woody materials has not reached consensus. The aims of this study are (1) to evaluate the accuracy of optical methods for estimating effective LAI (L_e) in a deciduous needle leaf forest stand, and (2) to develop a practical correction scheme to improve the accuracy of optical methods in estimating LAI.Methods L_ewas estimated by two indirect methods (i.e., digital hemispherical photography (DHP) and LAI-2000 plant canopy analyzer method (LAI-2000 method) in an annual maximum leaf area period in a Larix gmelinii plantation. Then, we developed three correction schemes to improve the accuracy of indirect methods in estimating LAI. Meanwhile, two direct methods (i.e., litter collection and allometry methods) were used to estimate LAI. Taking LAI from litter collection as a reference, we evaluated the effectiveness of three correction schemes and tested the influence of zenith angle ranges on the correction results.Important findings With zenith angle ranges of 0-45° (rings 1-3), 0-60° (rings 1-4), 45°-60° (ring 4) and 0-75° (rings 1-5), L_eobtained from DHP underestimated LAI from both litter collection and allometry by 19%-32% and 18%-29%, respectively. L_efrom LAI-2000 method with four zenith angles also underestimated LAI from both litter collection and allometry by 9%-30% and 8%-28%, respectively. Although the contribution of woody materials to LAI was overestimated in correction scheme A, it was effective in correcting L_efrom DHP with zenith angles of 45º-60º (ring 4), and also effective for L_efrom LAI-2000 method with zenith angles of rings both 1-3 and 1-4. Correction scheme B was all effective in correcting L_efrom DHP with four zenith angle ranges. Generally, correction scheme C was more effective than other two schemes in correcting L_efrom both DHP and LAI-2000 method with four zenith angle ranges. These results indicate that the zenith angle range is a key factor for determining the accuracy of optical methods in estimating LAI besides woody materials and clumping effects.