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Calculating Leaf Area Index (LAI) with the LAI-2200C requires certain assumptions about the plant canopy. This allows accurate measurements withoutt the need to destructively sample the canopy. The LAI-2200C relies upon four assumptions: all light is absorbed by the foliage, the foliage is randomly distributed, the foliage orientation is random, and the foliage elements are limited by the view ring.
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.
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.
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).
That is, it does not matter how the foliage is inclined, but the leaves should be facing all compass directions.
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.
Leaf Area Index (LAI) is the ratio of foliage area to ground area. The LAI‑2200C computes LAI from measurements made above the canopy and below the canopy, which are used to determine canopy light interception at 5 angles. These data are fit to a well-established model of radiative transfer inside vegetative canopies to compute leaf area index, mean tilt angle, and canopy gap fraction.
The optical sensor of the LAI‑2200C consists of a fisheye lens and an optical system. The fisheye lens "sees" a hemispherical image, which the optical system focuses onto the photodiode optical sensor, which is made up of five concentric rings.
The gap fraction technique is at present the most powerful and practical tool available for indirect sensing of canopy structure. It can be applied not only to continuous canopies, but also to discrete foliage-containing envelopes, such as row structure or individual trees.
Canopy structure information, including amount and orientation of foliage, can be estimated from measurements of gap fractions.
The gap fraction of a canopy is the fraction of view in some direction from beneath a canopy that is not blocked by foliage. The fractional sunfleck area is equivalent to the gap fraction in the solar direction.Download journal article
One of the traditional underlying assumptions of the LAI‑2000 and LAI‑2200 has been that foliage absorbs all the radiation in the blue waveband seen by the sensor (320-490 nm). This is usually a good assumption under diffuse light conditions such as uniform overcast, just before sunrise, or just after sunset. In direct sunlight, however, reflectance off foliage causes a much greater overestimation of the gap fraction and underestimation of leaf area index.