Preliminary Course.
Eco-Physiology
Light interception.
Light absorbption
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LAI
The LAI or Leaf Area Index is the ratio of total leaf area of vegetation divided by the surface area of the land on which the vegetation grows. LAI is a dimensionless value, typically ranging from 0 for bare ground to 9 for a dense forest.
Beer-Lambert Law
Detailed numerical simulation of radiation absorption has shown that the following approximation is excellent, and never deviates more than 1 or 2% from a detailed simulation with sunlit and shaded leaves:
IabsLAI = (1-p) I0 (1-e-k*LAI)
In this formula, called the Beer-Lambert Law,
k stands for the extinction coefficient,
IabsLAI stands for the radiation level at canopy depth LAI (expressed in overlying LAI).
I0 stands for the radiation level at depth 0 above the canopy.
p parameter stands for the value of the canopy reflection coefficient.
Typical values for k are in the range of 0.5 to 0.9
For a LAI above a value of 3, almost 90% of PAR is absorbed by the canopy.
White cover on the soil surface may reflect 50 to 80% of the PAR and increase crop photosynthesis by at least 7% for a LAI of 3 (as long as CO2 is not a limiting factor) (Gijzen, 1995a).
Influence of the Leaf Area Index (LAI) on the fraction of light intercepted by a tomato crop.
In this example, the extinction coefficient k is experimentally found from three sets of data respectively represented by the green and pink symbols.
(Graph: E. Heuvelink, WAGENINGEN UNIVERSITY)
As extinction depends on both the direction of radiation and the geometry of leaf position and orientation, more detailed models have been developed (De Wit, 1965; Ross, 1981).
For horizontal leaves, the fraction of radiation intercepted by any leaf will be proportional to the leaf area itself, independent of the radiation direction.
At leaf level, absorption of PAR by green leaves is about 80 to 85% (Moss and Loomis, 1952; Heuvelink, 1996b).
At canopy level, the resulting absorption is greater because radiation is scattered and leaves may have multiple opportunities to absorb.
Definition
LAI (Leaf Area Index)
Physiology. (abrev.). LAI is a dimensionless quantity that characterizes plant canopies, defined as the one-sided green leaf area per unit ground surface.Bibliography
Gijzen, H. 1995. CO2 uptake by the crop. Crop growth and development. In: J.C. Bakker, G. P. A. Bot, H. Challa, and Van de Braak, N.J. (eds), Greenhouse climate control-an integrated approach, Wageningen Pers, The Netherlands. pp. 16-35
Heuvelink E. 1996. Dry matter partitioning in tomato: validation of a dynamic simulation model. Annals of Botany - London 77, pp. 71-80 (access to pdf)
Moss, R.A, and Loomis, W.E. 1952. Absorption Spectra of Leaves. I. The Visible Spectrum. Plant Physiology. April 1952, 27, pp. 370-391. (pdf)
Ross, J. 1981. The radiation regime and architecture of plant Stands. Tasks for Vegetation sciences. 3, Dr. W Junk Publ., The Hague.
de Wit C.T. 1965. Photosynthesis of leaf canopies. Centre for Agricultural Publications and Documentation, Wageningen, 1965, 63 p. (pdf)