Leaf area index

Leaf Area Index (LAI) The Global Climate Observing System defines LAI as "one half the total green leaf area per unit ground surface area. LAI is expressed in terms of square meters of leaf (half surface area) per square meter of ground" (Law et al., 2010). On sloping surfaces, the LAI should be projected to the normal to the slope.

LAI ranges from 0 (bare ground) to over 10 (dense conifer forests).

LAI in silviculture

Forestry scientists often define Leaf Area Index as the one-sided green leaf area per unit ground surface 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 horizontal ground area (Chen and Black, 1992)
• Projected needle area per unit ground area

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Interpretation and application of LAI

LAI is used to predict photosynthetic primary production, evapotranspiration 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:

$P=P_{\max }\left(1-e^{-c\cdot LAI}\right)$

where P_max designates the maximum primary production and $c$ designates a crop-specific growth coefficient. This inverse exponential function is called the primary production function.

Determining LAI

LAI is determined directly by taking a statistically significant sample of foliage from a plant canopy, measuring the leaf area per sample plot and dividing it by the plot land surface area. Indirect methods measure canopy geometry or light extinction and relate it to LAI.

Direct methods

Direct methods require stripping and measuring the foliage of plant canopy samples. LAI for clip plots or individual plants is measured by hand or by using an LAI meter. Traditional LAI 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.

Indirect methods

A hemispherical photograph of forest canopy. The ratio of the area of canopy to sky is used to approximate LAI.

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 quadrats; and (2) indirect non-contact measurements. Due to the subjectivity and labor involved with the first method, indirect non-contact measurements are typically preferred. Non-contact LAI tools, such as hemispherical photography, the LAI-2200 Plant Canopy Analyzer [2] from LI-COR Biosciences and the LP-80 LAI ceptometer [3] from Decagon Devices, measure LAI in a non-destructive way. Hemispherical photography methods estimate LAI and other canopy structure attributes from analyzing upward-looking fisheye photographs taken beneath the plant canopy. The LAI-2200 calculates LAI and other canopy structure attributes from solar radiation measurements made with a wide-angle optical sensor. Measurements made above and below the canopy are used to determine canopy light interception at five angles, from which LAI is computed using a model of radiative transfer in vegetative canopies. The LP-80 calculates LAI by means of measuring the difference between light levels above the canopy and at ground level, and factoring in the leaf angle distribution, solar zenith angle, and plant extinction coefficient. Such indirect methods, where LAI is calculated based upon observations of other variables (canopy geometry, light interception, leaf length and width, etc.) are generally faster, amenable to automation, and thereby allow for a larger number of spatial samples to be obtained. For reasons of convenience when compared to the direct (destructive) methods, these tools are becoming more and more important.

Disadvantages of methods

The disadvantage of the direct method is that it is destructive, time consuming and expensive, especially if the study area is very large.

The disadvantage of the indirect method is that in some cases it can underestimate the value of LAI in very dense canopies, as it does not account for leaves that lie on each other, and essentially act as one leaf according to the theoretical LAI models.

See also

• Canopy (ecology)
• Hemispherical photography
• Normalized Difference Vegetation Index
• Specific leaf area

References

• FAO: calculation of primary production
• Non-Existence of an Optimum Leaf Area Index for the Production Rate of White Clover Grown Under Constant Conditions
• Chen, J.M., and Black, T.A. (1992): Defining leaf area index for non-flat leaves. Agricultural and Forest Meteorology 57: 1–12.
• Law, B.E., T. Arkebauer, J.L. Campbell, J. Chen, O. Sun, M. Schwartz, C. van Ingen, S. Verma. 2008. Terrestrial Carbon Observations: Protocols for Vegetation Sampling and Data Submission. Report 55, Global Terrestrial Observing System. FAO, Rome. 87 pp.
• W.W. Wilhelm, K. Ruwe, M.R. Schlemmer (2000)“Comparisons of three Leaf Area Index Meters in a Corn Canopy” Crop Science 40: 1179-1183
• LAI Definition of University of Giessen, Germany