Dufrêne Eric, Bréda Nathalie
Laboratoire d'Ecologie Végétale, CNRS URA 1492 Bât. 362, Université Paris Sud-Orsay, F-91405, Orsay Cedex, France.
Laboratoire de Bioclimatologie et Ecophysiologie, Equipe Phytoécologie Forestière, INRA, Champenoux, F-54280, Seichamps, France.
Oecologia. 1995 Oct;104(2):156-162. doi: 10.1007/BF00328580.
This study evaluated one semi-direct and three indirect methods for estimating leaf area index (LAI) by comparing these estimates with direct estimates derived from litter collection. The semi-direct method uses a thin metallic needle to count a number of contacts across fresh litter layers. One indirect method is based on the penetration of diffuse global radiation measured over the course of a day. The second indirect method uses the LAI-2000 plant canopy analyser (PCA) which measures diffuse light penetration from five different sky sectors simultaneously. The third indirect method uses the "Demon" portable light sensor to measure the penetration of direct beam sunlight at different zenith angles over the course of half a day. The Poisson model of gap frequency was applied to estimate plant area index (PAI) from observed transmittances using the second and third methods. Litter collection from 11 temperate decidous forests gave values of LAI ranging from 1.7 to 7.5. Estimates based on the needle method showed a significant linear relationship with LAI values obtained from litter collections but were systematically lower (by 6-37%). PAI estimates using all three indirect techniques (fixed light sensor system, LAI-2000 and Demon) showed a strong linear relationship with LAI derived from litter collection. Differences, averaged over all forest stands, between PAI estimates from each of the three indirect methods and LAI from litter collections were below 2%. If we consider that LAI=PAI-WAI (wood area index) then, all three indirect methods underestimated LAI by an additional factor close to the value of WAI. One reason could be a local clumping of architectural canopy components: in particular, the spatial dispositions of branchlets and leaves are not independent, leading to a non-random relationship between the distributions of these two canopy components.
本研究通过将这些估算值与从凋落物收集得出的直接估算值进行比较，评估了一种半直接方法和三种间接方法来估算叶面积指数（LAI）。半直接方法使用一根细金属针来计算穿过新鲜凋落物层的接触次数。一种间接方法基于一天中测量的漫射全球辐射的穿透情况。第二种间接方法使用LAI - 2000植物冠层分析仪（PCA），它能同时测量来自五个不同天空区域的漫射光穿透情况。第三种间接方法使用“恶魔”便携式光传感器，在半天时间内测量不同天顶角下直射阳光的穿透情况。间隙频率的泊松模型被应用于使用第二种和第三种方法从观测透过率估算植物面积指数（PAI）。从11个温带落叶林收集的凋落物得出的LAI值范围为1.7至7.5。基于针方法的估算值与从凋落物收集获得的LAI值呈现出显著的线性关系，但系统性地偏低（低6 - 37%）。使用所有三种间接技术（固定光传感器系统、LAI - 2000和恶魔）估算的PAI与从凋落物收集得出的LAI呈现出很强的线性关系。在所有林分中，三种间接方法各自估算的PAI与凋落物收集得出的LAI之间的差异平均低于2%。如果我们认为LAI = PAI - WAI（木材面积指数），那么，所有三种间接方法都将LAI低估了一个接近WAI值的额外因子。一个原因可能是树冠结构成分的局部聚集：特别是，小枝和树叶的空间布局并非相互独立，导致这两个树冠成分的分布之间存在非随机关系。
