Leaf area index (LAI) is ratio of the total area of all leaves on a plant to the area of ground covered by the plant. This means that if a plant had only one layer of leaves all placed next to each other, it would have a leaf area index of exactly 1.0, because the leaf area would equal the ground area covered. For most plants leaf area index increases with age (up to the beginning of senescence) and reaches a maximum of 2.0 to 5.0.