## Introduction

Leaf area index (LAI) is defined as the total one-sided green leaf area per unit of ground surface. When considering the array of global vegetation, there is an infinite variety of plant canopy shapes, sizes, and attributes. LAI indicates the area of ground occupied by plants and is an important structural property of vegetation. Because these leaf surfaces are the primary sites of energy and mass exchange, important processes such as canopy interception, évapotranspiration, and gross photosynthesis are directly proportional to LAI.

Along with the green LAI, there are several subtly different definitions of LAI itself. The total LAI (or all-sided LAI) is based on the total outside area of the leaves, taking leaf shape into account, per unit ground area. The effective LAI (Le) is calculated from the canopy gap fraction assuming the foliage spatial distribution is random:

Jo where p(9) is the proportion of gaps in the foliage in the view direction with the zenith angle 9.

For needle leaf forest, the formula for calculating green LAI (or needle area index, L) is expressed as

7e is the needle-to-shoot area ratio quantifying the effect of foliage clumping within shoots, is the element clumping index quantifying the effect offoliage clumping at scales larger than shoots, and a is the woody-to-total area ratio used to remove the contribution ofthe supporting woody material to the total area including foliage, branches, and tree trunks affecting ground-based optical measurements.

The subsequent sections first discuss LAI ground measurement methods and remote-sensing estimation methods. Following this, some notable LAI products provided by remote-sensing systems, as well as their spatial and temporal characteristics are introduced. Finally, how LAI is applied in various ecological models is discussed.

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