The leaf area index measures the area of leaves and needles per unit of ground area. In deciduous trees, it can be estimated by analysing the litterfall, for example. Optical methods are also used to measure the leaf area index. These include, for example, photographic approaches from below through the canopy with subsequent image analysis.

The leaf area index is an important parameter for quantifying many biological and physical processes, such as primary production, plant respiration, transpiration, photosynthesis and nutrient cycles.