Leaf Area Density

Leaf Area Index (LAI) is the ratio of the foliage area to the ground area. The measurement of LAI is of fundamental importance in agricultural and ecological research because LAI is a measure of plant growth; it directly affects the interception and absorption of light by the canopy, and it influences the heat balance and evaporation from the landscape.

Methodology 1: Measurements

From ground and from space the Leaf Area Index of the vegetation can be obtained using optical methods. While this methods is relatively simple and fast, it does not provide information about the vertical distribution of the leaf area. To obtain this information empirically, the optical sensor must be placed in different levels inside the vegetation stand. Another method is to pick the leafs from the tree and measure the leaf area based on the collected material. There is an obvious drawback on the second approach related to the tree of interest, but it still is an option.

Methodology 2: Analytical Approaches

Analytical approaches can help in obtaining the LAD distribution especially if the LAI is known. There are a few papers worth while reading on that aspect:

Meir et al. (2000) provide some ideas how the LAD profile for a tropical rain forest might look like. Their paper is basically focusing on the measurement of LAD/LAI using a photographic method, but it is also useful for getting some ideas on LAD for tropical situations.
Attention: The profiles shown in their figures are normalized with the LAI. Before using them as profiles in ENVI-met, you have to re-calculate the absolute values.

Ross et al. (2000) present an empirical model which allows to calculate the distribution of LAD and LAI based on different probability functions. First they calculate the stem height of a plant and then the correlation of the stem height with the stem leaf area. Finally, the stem leaf area is distributed over the stem height and the LAD profile is calculated. However, this method requires some input data, namely the distribution coefficients for the leaf area to be known. This approach is especially useful if the effects of the growing period should be included in the model.

Stadt and Lieffers (2000) show in their paper how they get the plant characteristics for light transmission model for forest stands (MIXLIGHT). Especially Tab. 1 is very useful as it provides values for the LAD statistical distribution of different species.

Finally, a recent and very useful paper is presented by Lalic and Mihailivic (2004), which fits well with the Stadt and Liefers (2000) paper. Lalic and Mihailivic present a very simple and very general method to obtain an LAD profile from very few parameters: type, height and max LAD (which could be extracted for example from the Stadt and Liefers paper)

References:

Meir, P., Grace, J. and Miranda, A. C. (2000): Photographic method to measure the vertical distribution of leaf area density in forests, Agri.Forrest Met., 102, 105-111

Lalic, B. and Mihailovic, D. T. (2004): An empirical relation describing leaf-area density inside the forest for environmental modelling, J.Appl. Met. 43(4) 641-645

Stadt, K. J. and Lieffers, V.J (2000): MIXLIGHT: a flexible light transmission model for mixed-species forest stands, Agri. Forrest Met., 102, 235-252

Ross, J., Ross, V. and Koppel, A. (2000): Estimation of leaf area and its vertical distribution during growth period, Agri. Forrest Met. 101, 237-246