ext_co

Light extinction coefficient

This coefficient is used to calculate the amount of intercepted photosynthetically active radiation.

Differences in canopy structure for a species are described by the number of leaves present (leaf area index) and the leaf orientation. Leaf orientation has a significant impact on light interception and consequently on radiation-use efficiency. More erect leaf types spread the incoming light over a greater leaf area, decreasing the average light intensity intercepted by individual leaves (see figure below). A reduction in light intensity interception by an individual leaf favors a more complete conversion of total canopy-intercepted light energy into biomass.

Using the light extinction coefficient value (kℓ) in the Beer-Lambert formula to quantify efficiency of light interception per unit leaf area index, more erect leaf types have a smaller kℓ.

To calculate the light extinction coefficient, the amount of photosynthetically active radiation (PAR) intercepted and the mass of aboveground biomass (LAI) is measured several times throughout a plant’s growing season using the methodology described in the previous sections. The light extinction coefficient is then calculated using the Beer-Lambert equation:

$\frac{TPAR}{PAR}=(1-exp⁡(-k_l⋅LAI))$

or

$k_l=-ln⁡(\frac{TPAR}{PAR}) * \frac{1}{LAI}$

where TPAR is the transmitted photosynthetically active radiation, and PAR is the incoming photosynthetically active radiation.