Leaf area index

Leaf area index (LAI) or leaf area ratio is a dimensionless quantity that characterizes plant canopies. It is defined as the ratio of leaf area (LA) and ground area, in SI units of square meters per square meter (m2/m2). In broadleaf canopies, the leaf area is defined as the one-sided green part of leaves. In conifers, three definitions for LA have been used for the needles: total surface area, half of the total needle surface area, and projected area.

The definition “half the total leaf area” pertains to biological processes, such as gas exchange, whereas the definition “projected leaf area” is rarely used, because the projection of a given area in one direction may differ in another direction when leaves are not flat, thick, or 3D-shaped. Moreover, “ground surface area” is specifically defined as “horizontal ground surface area” to clarify LAI on a sloping surface. The definition “half the total leaf area per unit horizontal ground surface area” is suitable for all kinds of leaves and flat or sloping surfaces.

LAI is commonly used as an indicator of the growth rate of a plant. LAI is a complex variable that relates not only to the size of the canopy, but also to its density, and the angle at which leaves are oriented in relation to one another and to light sources. In addition, LAI varies with seasonal changes in plant activity, and is typically highest in the spring when new leaves are being produced and lowest in late summer or early fall when leaves senesce (and may be shed). The study of LAI is called "phyllometry."

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