At plot or stand level, the most common method of estimating LAI and its seasonal variation is from measurements of the fraction of light transmitted through the canopy to the ground. Nondestructive optical methods have been developed to estimate LAI periodically. The simplest approach, using Beer's law inversion with an extinction coefficient depending on the crop or tree
properties, is useful and efficient for crops and broad-leaved forests. However, for many evergreen species, the procedure requires some corrections to take the clumping of needles and branches into account. In any case, accurate equipment and methods for ground estimates of LAI are now available. Optical methods of estimating LAI use the inversion of gap fraction data. One fruitful approach involves measurement of the gap fraction, the proportion of unobscured sky in a set of sky directions as seen from beneath a plant canopy. Recent advances in the theory make it possible to calculate a useful array of canopy properties from gap fraction measurements, including light extinction coefficients, LAI, and leaf angle distribution. A variety of techniques can be employed to obtain gap fraction measurements, such as linear arrays of light sensors (SunSCAN, Delta-T Devices Ltd., Cambridge, UK and AccuPAR, Decagon Devices, Pullman, USA). Two other devices measure gap fraction for different zenith angles. The LAI-2000 (Li-Cor, Lincoln, Nebraska, USA) measures 5 zenith angles simultaneously, through a fisheye light sensor, while the DEMON instrument (CSIRO, Canberra, Australia) measures direct beam radiation from the Sun through a directional narrow angle of view (0.302 sr). Measurements with the DEMON instrument have to be repeated several times from early morning until noon to collect data over a range of zenith angles. Hemispherical photography and imaging hemispherical sensors (e.g., the CI-100 Canopy Analyzer, CID, Vancouver, USA) are also widely used but these frequently underestimate LAI.
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