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The Li-Cor Plant Canopy Analyzer sensing head has five
concentric silicon ring detectors beneath a nearly hemispherical
(150? field of view) lens. A blocking filter restricts radiation
of less than 490 nm, which minimizes the effect of light
scattered by foliage (Welles, 1990 ). The five rings provide
data at five zenith angles for use in the inversion model. A
separate dedicated controller-data logger is attached to the
sensing head and records light readings for each ring. Readings
of above- and below-canopy light conditions are necessary for
the calculation of LAI and average leaf inclination angle by the
built-in program. Below-canopy data were taken at eight compass
directions and the mean of these values was stored in the
instrument. In the orchard, above-canopy data were taken by
walking to an adjacent open field immediately (less than 2 min)
after recording each below-canopy reading. In the forest,
above-canopy readings were acquired by another Plant Canopy
Analyzer unit atop a nearby butte, which was about 1 km south
and at 50 m higher elevation. Data were recorded every 5 min
during the period in which below-canopy readings were
made. Above-canopy values were then linearly interpolated to
provide an appropriate value for each below-canopy reading.

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