Leaf Area Index

The Leaf Area Index or LAI is the ratio of total upper leaf surface of vegetation divided by the surface area of the land on which the vegetation grows. The LAI is a dimensionless value, typically ranging from 0 for bare ground to 6 for a dense forest.

LAI in silviculture

Forestry scientists 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 ground area
*Projected needle area per unit ground area [http://www.uni-giessen.de/~gh1461/plapada/lai/lai.html]

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:Fact|date=November 2007:$P=P\_mathrm\{max\}left(1-e^\{-ccdot\; LAI\}\; ight)$where $P\_mathrm\{max\}$ 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

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.

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.

Indirect method

Newer types of LAI meters, such as the LAI-2000 [http://www.licor.com/env/Products/AreaMeters/lai2000/2000_intro.jsp] from LI-COR Biosciences, 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, like leaf length and width, [cite journal |last=Blanco |first=F.F. |coauthors=Folegatti, M.V. |year=2003 |title=A new method for estimating the leaf area index of cucumber and tomato plants |journal=Horticultura Brasileira |volume=21 |issue=4 |pages=666–669 |doi=10.1590/S0102-05362003000400019 ] are generally faster, amendable to automation, and thereby allow for a larger number ofspatial samples to be obtained. For reasons of convenience when compared to the directmethods, they are becoming more and more important. Indirect methods of estimatingLAI in situ can be divided in two categories: (1) indirect contact LAI measurements suchas plumb lines and inclined point quadratesFact|date=November 2007 ; and (2) indirect non-contact measurements.

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 it underestimates the value of LAI.Fact|date=April 2008

ee also

*Normalized Difference Vegetation Index

References

* [http://www.fao.org/landandwater/agll/gaez/app/a6.htm FAO: calculation of primary production]
* [http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=550583&tools=bot 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.
* [http://www.uni-giessen.de/~gh1461/plapada/lai/lai.html LAI Definition of University of Giessen, Germany]