Leaf Area Index

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The Leaf Area Index or LAI is the ratio of total upper leaf surface of a crop divided by the surface area of the land on which the crop grows.

The LAI is a dimensionless number.

1 LAI in silviculture
2 Interpretation and application of the LAI
- 2.1 LAI as a crop management parameter
3 Determining LAI
- 3.1 Direct method
- 3.2 Indirect method
4 References

[edit] LAI in silviculture

Forestry scientists define Leaf Area Index as the one-sided green leaf area per unit ground area in broadleaf canopies. In conifers, three different definitions have been used:

Total needle surface area per unit ground area
Half of the total needle surface area per unit ground area
Projected needle area per unit ground area

[1]

[edit] Interpretation and application of the LAI

The LAI is used to predict the photosynthetical primary production and as a reference tool for crop growth. As such, LAI plays an essential role in theoretical production ecology. An inverse exponential relation between LAI and light interception, which is linearly proportional to the primary production rate has been established: $P = P m a x (1 - e - c * L A I)$ , in which $P m a x$ designates the maximum primary production and $c$ designates a crop-specific growth coefficient. This inverse exponential function is called the primary production function.

[edit] LAI as a crop management parameter

A higher LAI corresponds to a higher leaf mass and therefore a higher respiration rate of the crop. As follows from the primary production function, an increased LAI increases primary production by only a fraction. The optimum LAI is thus those in which any increase in primary production is exactly offset by the increase in respiration rate, which can be found by equalising the derivatives of the respiration function (linearly dependent on leaf biomass, hence on LAI) and the primary production function and solving for LAI. Plants have a crude but effective system to regulate their leaf mass. Mature leafs cannot act like assimilate sinks, but must be self-sufficient. As a result, all leafs which are net consumers of assimilates eventually die off [2] and the final crop LAI closely resembles optimum LAI. Typically, this optimum LAI equals to less than 3 for succulent leafs up to 5 or 6 for thin leafs.

[edit] Determining LAI

The LAI is determined directly by taking a statistically significant sample of plants from a crop, measuring the mean leaf area per plant and dividing it by the mean available land surface per plant. The indirect method measures light extinction and relates it to LAI.

[edit] Direct method

The mean leaf area per plant is measured by hand or by using a leaf area index meter. Traditional leaf area index meters require each plant leaf to be stripped and fed through the entrance of the machine, which can be likened to a kind of crude image scanner. This requires stripping of the foliage of the plants and is only admissible in large-scale experiments.

[edit] Indirect method

Newer types of LAI meters, such as the LAI-2000 [3] from LI-COR Biosciences [4], measure LAI in a non-destructive way by means of measuring the difference between light levels on top of the crop and at ground level and using the inverse exponential relation given by the primary production function between light absorption and LAI. Indirect methods, in which leaf area is inferred from observations of another variable, are generally faster, amendable to automation, and thereby allow for a larger number of spatial samples to be obtained. For reasons of convenience when compared to the direct methods, they are becoming more and more important. Indirect methods of estimating LAI in situ can be divided in two categories: (1) indirect contact LAI measurements such as plumb lines and inclined point quadrates (Gower et al., 1999; Scurlock et al., 2001; Jonckheere et al., 2004a); and (2) indirect non-contact measurements.