Indirect optical method

The indirect optical method calculates LAI through measurements of canopy transmittance. Canopy transmit-tance was converted into LAI using the Beer-Lambert law:

where Kis the light extinction coefficient and QJQg is the canopy transmittance, where Qi is below-canopy photo-synthetic active radiation (PAR), and Qg is average total incoming PAR. The Beer-Lambert law assumes that leaf inclination angles are spherically and randomly distributed and that the foliage is distributed randomly in space.

LAI-2000 is one of the commonly used instruments to measure LAI (Figure 2). The ratio of each ring's above-and below-canopy radiation value is referred to as the canopy gap fraction for each detector. The inversion of gap-fraction data to LAI values assumes that only skylight is seen by the sensor beneath the canopy. Thus, the measurements should be made on a cloudy day or in the twilight.

Hemispherical canopy photography acquires photographs through a hemispherical (fisheye) lens from beneath the canopy (oriented towards zenith) or placed above the canopy looking downward. Hemispherical photographs show a complete view of all sky directions, with the zenith in the center of the image and the horizons at the edges (Figure 3). Traditionally, analog hemispherical photography is used to determine LAI. Digital cameras are available now with a very large number of pixels and high radiometric image quality.

The Sunfleck Ceptometer (Decagon Devices Inc., Pullman, WA, USA; see Figure 4) is a model of line quantum sensor making use of 80 individual sensors on

Figure 2 LAI-2000 canopy analyzer. ©LI-COR Biosciences.
Figure 3 Hemispherical image. Reproduced from Jonckheere I, Fleck S, Nackaerts K, etal. (2004) Review of methods for in situ leaf area index determination. Part I: Theories, sensors and hemispherical photography. Agricultural and Forest Meteorology 121(1-2): 19-35, with permission from Elsevier.

a probe and a control unit. It strictly measures the sun fleck fraction or the quantity of PAR radiation by means of the probe under a canopy in an open field. Due to the large variability between the measurements, it is necessary to make enough measurements in order to get a reliable and representative result. Moreover, this technique is not suitable in coniferous forests, due to penumbral effects in the sun fleck fraction.

The tracing radiation and architecture of canopies (TRAC) instrument (Third Wave Engineering, Ontario, Canada; see Figure 5) accounts not only for canopy gap fraction but also for canopy gap size distribution. It is hand-carried by a person walking at a steady pace.

Figure 4 AccuPAR. Image courtesy of Decagon Devices Inc.

Figure 5 TRAC. Image courtesy of 3rd Wave Engineering Inc.

Using the solar beam as a probe, it records by means of three photosensitive sensors the transmitted direct light at high frequency. The clumping index obtained from TRAC can be used to convert the effective LAI to LAI.

Indirect method of estimating can be conducted with a standard strategy - multiple readings per plot placing the sensor at sites selected systematically on a transect. LAI can also be achieved with a nonstandard strategy, consisting of reading at a single point per plot, standardizing the distance and orientation from a subject tree to reduce variability.

The indirect method must take into account the influence of shoot structure, dead branches, and stems. One of the basic assumptions under the indirect method is that the foliage is black and randomly distributed. Actually, no real canopy conforms exactly to this assumption. Thus, all of the optical instruments that indirectly estimate LAI actually estimate effective LAI Le when foliage in the canopy is nonrandomly distributed, that is, clumped. In general, ground LAI measurements usually focus on a specific study site or small patches of vegetation.

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