What is leaf area index (LAI)?

The answer is a bit more involved and complex than it might seem. Leaf area index is a two-dimensional quantity that characterizes plant canopies, and the basic definition is the ratio of total one-sided green leaf area per unit ground area. (That might seem a bit unreal, seeing as leaves are three, not two, dimensional, but the measurements taken to determine LAI are in square inches and not cubic inches.) Mathematically, LAI is determined by measuring the surface of all the leaves of a plant, and then dividing the sum of those surfaces by the surface of soil that a plant occupies. The final LAI indicates the number of times the plant’s leaves would cover the soil. For example, if a plant has a LAI of 3.0, its leaves would cover the soil beneath the plant three times over. The more leaf area a plant has in comparison to the soil area beneath it, the higher the LAI. To attain an accurate portrayal of a plant’s (or crop’s) LAI, one needs to take into consideration the vertical area as well as the horizontal area. The height of the canopy plays just as significant a role as does the horizontal area of that canopy.

Direct measurement is the most accurate way to attain LAI, but it’s also the most exhausting and usually requires some destruction. To directly measure a deciduous plant’s LAI, one would collect the leaves during leaf fall. Then, one would calculate the combined area by measuring a small sample of the leaves, figuring out the average area, and then drying and weighing this sample. Next, one would dry and weigh the entire collection of leaves and interpolate the average leaf area using the ratio obtained from the total versus sample gross weight. This same, complicated process could be used to calculate the LAI of an evergreen plant, but one would have to destroy the tree to attain the leaves. So, though considerably accurate, this method is not very practical.

Use Indirect Measurement to Calculate LAI

Indirect measurement is the typically preferred way to compute LAI. Not only is it much easier and quicker, but non-contact measurement tools such as plant canopy analyzers and hemispherical photography calculate LAI in non-destructive ways. No matter if they’re measuring a plant, a crop, or a section of a forest, plant canopy imagers quickly provide digital information indicating the LAI with good practical accuracy. Light meters can also be useful for measuring the LAI of individual plants or small crops. (The amount of light that is intercepted by plant foliage before it reaches the ground is also at the heart of LAI, though it isn’t the only consideration.) The meters are placed at strategic locations, and their measurements are compared against known available light measurements. This technique is basically measuring intercepted light.

How LAI Helps Keep Tabs on Plant Health

So, now that we know what LAI is and how to calculate it, the next question is: why would we want to measure it and what can it do for our crop production or plant health? Leaf area index is used to predict photosynthetic production and evapotranspiration, estimate a crop’s water demand, calculate a plant’s estimated carbon absorption, and evaluate theoretical production ecology. It is also a way to analyze crop growth and the success of a growing strategy, especially in comparison to previous seasons. For example, if a crop is a quarter way into its growth cycle and its LAI is notably lower than the same crop was at the same time in a previous season, this indicates something needs correcting. Anything—available light, soil moisture, nutrition, pests, disease, and even greenhouse layout—could be the cause of these growth rate changes. So, the discovery of a decrease like this should trigger the search for the cause. If found, a remedy or solution can be applied to stay on track for maximum yields. The earlier in the seasonal growth cycle an adjustment or treatment is made, the more effective it can be.

LAI Important for Fruiting Crops

Obviously, when growing a crop that produces leaves for consumption (spinach, for example), maximum LAI is the goal. However, LAI is also important for fruiting crops as leaf density is often the goal during their early stages of growth (later in that plant’s crop cycle, some amount of thinning can enhance the size and amount of fruit produced).

As mentioned, LAI can also relate to photosynthesis. Remember photosynthesis is the power within a plant to grow and produce, and this process is driven by light application. The more light that falls on something other than foliage, the less efficient the use of that light source (and vice versa). So, there is a direct relationship between LAI and light interception; as LAI increases, the percent of light intercepted by (and applied to) a plant’s leaves increases.

In indoor grow rooms, the lights are often stationary. As such, the garden needs to be carefully planned out to take full advantage of the non-moving light source. One benefit of using sunlight in a greenhouse, on the other hand, is that the light’s angle of application moves throughout the day. Foliage that might be in shade at one hour is later in sunlight, so more advantage is taken of all the existing foliage. There are lighting systems and strategies for indoor grows that also utilize this same principle of angular light movement. Sometimes leaves and even branches need to be removed to allow more light into the center of a plant. Though this reduces the LAI to some degree, this type of pruning is desirable for fruiting crops as it encourages a more thorough production of fruit.

To be a successful grower, one needs to use science and proven methods to their benefit. Taking notes on important aspects and observations for each crop, including the LAI at various points in the cycle and recording them for reference in the future can be the difference between adequate and maximum yields. For additional information on LAI, discussion of how various meters can be used to measure it, and to review the reference information for this article, scan the QR code.