A Device for Continuous, in Situ Measurement of Leaf Area Index.

The attenuation of direct and diffuse radiation by a plant canopy provides valuable information about canopy development and structure, but it can be difficult to measure. Our goal was to build an inexpensive, automated shadow band apparatus that would allow us to use a standard line quantum sensor to measure spatially averaged direct and diffuse radiation beneath a crop canopy. The key components of the system are a line quantum sensor, a thin metal shadow band, and a small DC servo.  The latter is an inexpensive device (< $20) made for use in radio-controlled model cars and model airplanes.  When assembled, the shadow band can be rotated around an axis that passes through the quantum sensor, with a radius of 70 mm. Each position on the arc corresponds to a specific servo pulse width, and the system is controlled and interrogated with a data logger capable of pulse width modulation. During a measurement the shadow band is rotated through an arc of 180 degrees, centered on zenith and starting on one horizon. Sensor output is monitored throughout, allowing subsequent calculation of the direct and diffuse components of total PAR.  A similar sensor is positioned above the canopy and operated in similar fashion.  These four values can then be used, along with zenith angle, to estimate leaf area index and gap fraction.  Comparison of continuous, in situ estimates of corn canopy LAI with bi-weekly destructive measurements showed good agreement and no apparent bias.