Bréda Nathalie J J
INRA, Forest Ecology and Ecophysiology Unit, Phytoecology Team, F-54 280 Champenoux, France.
J Exp Bot. 2003 Nov;54(392):2403-17. doi: 10.1093/jxb/erg263.
Leaf area index (LAI) is the total one-sided area of leaf tissue per unit ground surface area. It is a key parameter in ecophysiology, especially for scaling up the gas exchange from leaf to canopy level. It characterizes the canopy-atmosphere interface, where most of the energy fluxes exchange. It is also one of the most difficult to quantify properly, owing to large spatial and temporal variability. Many methods have been developed to quantify LAI from the ground and some of them are also suitable for describing other structural parameters of the canopy. This paper reviews the direct and indirect methods, the required instruments, their advantages, disadvantages and accuracy of the results. Analysis of the literature shows that most cross-validations between direct and indirect methods have pointed to a significant underestimation of LAI with the latter techniques, especially in forest stands. The two main causes for the discrepancy, clumping and contribution of stem and branches, are discussed and some recent theoretical or technical solutions are presented as potential improvements to reduce bias or discrepancies. The accuracy, sampling strategy and spatial validity of the LAI measurements have to be assessed for quality assurance of both the measurement and the modelling purposes of all the LAI-dependent ecophysiological and biophysical processes of canopies.
叶面积指数（LAI）是指单位地面面积上叶组织的总单面面积。它是生态生理学中的一个关键参数，特别是在将叶片水平的气体交换扩展到冠层水平方面。它表征了冠层 - 大气界面，大部分能量通量在此处交换。由于存在较大的空间和时间变异性，它也是最难准确量化的参数之一。已经开发了许多从地面量化叶面积指数的方法，其中一些方法也适用于描述冠层的其他结构参数。本文综述了直接和间接方法、所需仪器、它们的优缺点以及结果的准确性。文献分析表明，直接法和间接法之间的大多数交叉验证都表明，后一种技术会显著低估叶面积指数，尤其是在林分中。讨论了造成差异的两个主要原因，即聚集以及茎和枝的贡献，并提出了一些近期的理论或技术解决方案，作为减少偏差或差异的潜在改进措施。为了确保所有依赖叶面积指数的冠层生态生理和生物物理过程的测量和建模目的的质量，必须评估叶面积指数测量的准确性、采样策略和空间有效性。
