The LAI of a plant canopy is defined as its leaf area per unit of ground area. In other words, it tells us how many layers of leaves would be on the ground if they would all fall down and be arranged exactly side by side. In the canopy, however, the leaves are not arranged side by side but more or less randomly. Therefore, even with a LAI greater than 1 there is some light reaching the ground.
The leaf area index (LAI) is an important parameter in plant ecology. Because it tells how much foliage there is, it is a measure of the photosynthetic active area, and at the same time of the area subjected to transpiration. It is also the area which becomes in contact with air pollutants. The LAI is further an indication of how much light is coming through the canopy; in the case of a multi-layer canopy, the LAI of an upper layer (a tree layer for example) is important for the light received by a lower layer (a ground vegetation).
Direct measurements: in a canopy like a crop or a pasture, it
is possible to harvest all the plants on a small plot and to measure the total area
of their leaves. Depending on the number of leaves and on the ease to keep them
flat for the measurement, this destructive method can be quite time-consuming.
In the case of forests, it is usually not possible to harvest a whole plot and measures all the leaves. In the autumn, however, the leaves of deciduous trees fall and can be collected in litter traps (collectors). It is then possible to measure the area of representative samples.
Indirect measurements: it is also possible to estimate the LAI
by measuring the light beneath the plant canopy as compared to the light of the
open sky. Such indirect methods are non-destructive, but they also measure the area
of plants other than leaves (stems, branches, fruits etc.). The result of an
indirect measurement should thus more properly be termed plant area index.
There are two main categories of indirect measurements: light measurements in the field and canopy photographs. Light measurement in the field requires specific sensors to record the light on points or transects in the canopy. Photographs record the apparent structure of a canopy. Hemispherical photographs taken with fish-eye lenses record the complete image of the canopy from any point on the ground, showing the zenith in the centre and the horizon as a circle. Photographs, however, require a more complex processing to yield LAI estimates. This is the goal of the Hemisfer software.