Light Penetration Measurements Methods

From 1982 to 1988, light meter readings were taken using a Li-Cor, Inc. Integrating Quantum/Radiometer/Photometer, model LI-188B. Two people were needed to take light readings with this system. One would hold the control box and record data and the other would hold the light sensor. The batteries would be checked before starting each session. If the batteries were OK, they would proceed to take readings. The sensor was connected to the control box by a relatively short cord, so the two people taking readings were required to stay close together. When taking readings, it was necessary to get a range value for each light value entered. If the integrating time of 1 second was not sufficient, it was increased to 10 seconds. This was also recorded.

In 1989, two new light meters were acquired. These are SF Sunfleck Ceptometers, model SF-40 (40cm probe). They were purchased from Decagon Devices, Inc., P.O. Box 835, Pullman, WA 99163. One person can easily handle a ceptometer alone. This makes is possible for three people to get the readings done more quickly and easily. One person records data while the other two take readings from the plots simultaneously. No range values are needed. To take readings a person needs to select function #1 (PAR readings), position the probe (see below) and press ``A'' (read value). More than one reading can be taken and then averaged by pressing a certain sequence of letters (A, A, B, B, A).

Measurements are taken within a 4 hour period, 2 hours on either side of solar noon. (Solar noon is half way between sunrise and sunset; it is not 1200 hours). Solar noon is at 1315 hours, Central Daylight Time. Samples are taken between 1115 hours and 1515 hours. Measurements are not taken when the plot being sampled is shaded. Light readings are done when the sky is clear, whenever possible. If a cloud passes over the plot being sampled, assistants wait for the cloud to pass before taking the readings. If the sky is mostly cloudy, light meter readings are not taken.

Two measurements are taken in each plot. Each measurement consists of one reading above the vegetation and a second reading at ground level. Both values are taken to get the percent of sunlight above the vegetation that reaches ground level. In taking the above vegetation reading, the sensor must be kept level, held high above all vegetation, kept out of the shade (of plants and people) and it must be clean. When taking the below vegetation reading, at ground level, the sensor must be kept level, out of the soil and out of the shade created by people.

In 1991, light meter readings were only taken in E026 and E055. Light profiles were taken using an A-shaped frame made of aluminum. Wires were strung across the frame at 10cm intervals. The frame was placed over the subplot being metered. A reading was taken over the top of the frame, and then at each 10cm level, by placing the light meter across the wires, starting at 90cm above the ground. Readings were taken every 10cm down the frame and again at ground level.

Light Data Transformations:

Light readings are transformed to obtain percent light penetration which represents the percent of light above the vegetation that reaches the ground surface. In cases where the experiment involves shading, another variable is computed to reflect the percent of sun light that reaches above the canopy. This variable is called light available. In the case of absence of artificial shades, the latter is set to 1.

percent light penetration = ( Light below canopy / Light above canopy ).

percent light available = ( Light below shade / Light above shade ).

 

 

dman@lter.umn.edu