Theory of Operation
Leaf Area Index calculations from the LAI-2200C are based on four assumptions about the plant canopy:
- The foliage absorbs all light that is incident upon it. It is assumed that the below-canopy readings do not include radiation that has been reflected or transmitted by foliage. Note: this assumption is removed when you apply scattering corrections, thus accounting for foliage reflectance and transmittance in post-processing.
- The foliage elements are small compared to the area of view of each ring. This is ensured when the distance from the optical sensor to the nearest foliage element, such as a leaf, is at least four times the element width.
- The foliage is randomly distributed within certain foliage containing envelopes. These envelopes might be parallel tubes (a row crop), a single ellipsoid (an isolated bush), an infinite box (turf grass), or a finite box with holes (deciduous forest with gaps).
- Foliage is azimuthally randomly oriented. That is, it does not matter how the foliage is inclined, but the leaves should be facing all compass directions.
No real canopy conforms exactly to these assumptions. Foliage is never random, but is clumped along stems and branches, and is not “black.” Many species exhibit some degree of heliotropism, which violates the azimuthal randomness assumption. However, many canopies can be considered random, and living foliage does have low transmittance and reflectance below 490 nm. Also, it is now possible to correct for errors caused by any transmittance or reflectance that does affect readings.
Offsetting errors are common, such as when leaves are grouped along stems (transmittance higher than the random model). In practice, most violations of the assumptions can be overcome with the proper measurement technique, and the model still works well even if all the assumptions are not met exactly.