Theory

(from pages 1-5 and 1-6 of LICOR, 2012):

The amount of foliage in a vegetative canopy can be deduced from measurement of how quickly radiation is attenuated as it passes through the canopy. By measuring this attenuation at several angles from the zenith, foliage orientation information can be obtained. The LAI2000 measures the attenuation of diffuse sky radiation at five zenith angles simultaneously. The LAI-2050 optical sensor projects the image of nearly hemisphere view onto five detectors arranged in concentric rings. Thus if the sensor is level and viewing the sky, detector 1 will measure the brightness straight overhead, while detector 5 will measure the brightness of a ring centered at 68^o zenith angle (22^o above the horizon),subtending 13^o.

A measurement with the LAI-2000 consists of a minimum of ten numbers: five of the numbers are signals from the five detectors when the optical sensor was above the vegetation, and the remaining five are the readings made by the sensor below the vegetation. For both the readings, the sensor is looking up at the sky. Five values of canopy transmittance are calculated from these readings by dividing corresponding pairs. For example if the detector reads 50 (units not important) above the canopy and 5 below, then the transmittance at that angle (centered at 70 ) is 5/50=0.10.From transmittance at all five zenith angles, the LAI-2000 calculates foliage amount (leaf area index, LAI) and foliage orientation (mean foliage tilt angle, MTA).In practice, a number of below-canopy readings are usually made to improve the spatial average.

Sampling procedure

Measurements are made at paired locations; for the BCSE they occur on each of the four sides of the plots in Block 1. Sampling points are designated by treatment (e.g., G1), compass direction facing from the plot looking outward (i.e., N, E, S or W), and point (plant) location (1, 2, 3, etc. as needed). As an example, “G7 NP1” is point 1 on the north side of treatment G7.

A flag is placed at each measurement point at the beginning of the growing season. Measurements are made close to the flag throughout the growing season at two-week intervals, starting as soon as there is sufficient biomass that attenuates light. Where crops are in rows (corn, soybean) or grow in clusters (Miscanthus, switchgrass), measurements are taken near the base of the plant. In the case of the poplars, where the sun may shine in from the side, measurements are made in the shadows of tree trunks.

Measurements are made as early in the day as possible (i.e., dawn) so that light is diffuse, however by the time they are completed sunlight may strike the plots.

Components of LAI2000

1. LAI-2070 Control Unit contains the necessary electronics to measure, record and compute final values. It is powered by batteries and contains 64K of file storage space for recording measurements taken in the field. There are two BNC connectors to attach optional LICOPR light sensors, and two LAI 2050 optical sensors. An RS-232 interface transfers recorded data and calculations to a computer or printer. The control unit is powered by 6 D cells, which should provide ~270 hours of continuous use (over 1 month at 8 hours per day). When battery life drops to 15%, a low battery indicator is shown on the display.
2. Carrying pack allows the operator to wear the control unit suspended from the shoulder/neck strap. The control unit is upside down in the pack, with the connectors accessible through the open end.
3. LAI-2050 Optical Sensor contains a filter and a number of lenses, and should be protected against sudden impacts and intense vibrations. It is important that the outer lens be kept clean and unscratched.
4. View caps are used to limit the sensor’s azimuthal fields of view. The solid view cap to protect the sensor when not in use. The lens brush, cleaning solution and lens cleaning paper should be used to keep the lens clean and dry.
5. Two bubble levels are used in leveling the sensor in circumstances requiring the sensor to be above the operator’s head.
6. Two Rs-232 cables for connecting to control box to a DTE device such as computer and other cable for connecting the control box to a DCE device such as another LAI-2070 control box.
7. The 1000-90 Utilities Software contains a program (COMM) for downloading data from LAI2000 to computers.

Steps for taking observations

1. Connect an LAI-2050 sensor to the X connector. The X connector is on your left as you view the front panel keypad and display; the Y connector is on your right.
2. Try to avoid direct sun and take measurements at dawn or dusk or during a cloudy day.
3. Take observations on 3 randomly selected plants or spots per plot and flag them. The term “plants” is used for observations on individual plants like switchgrass (G5), miscanthus (G6), poplars (G8) etc., while “spots” is used for vegetation mixtures such as the native grasses (G7), early successional (G9) and restored prairies (G10) BCSE treatments.
4. Switch on the control unit and press log on button to create a new file. It will show the file reference number and ask for WHAT and WHERE. For example for taking observations in block one in GLBRC fields it will look like:
   

   
   
   FILE:1
   
   Date: Time of observation
   
   Where: G1R1 (Means Treatment & Replication number) 
   
   WHAT: P1 (Means Plant/spot number)
   
   

5. Take one above canopy and 3 below canopy observations at one spot. Use separate data sheet to record date, location, starting time, file number and sunlight conditions for your own reference.
6. View results of the measurement in the field by pressing function FCT 27, which prompts for the number of file to be viewed. View mode shows the file in five parts or lists:
   

   
   
   FILE:1
   
   Date: Time of observation
   
   Where: G1R1 (Means Treatment & Replication number) 
   
   WHAT: P1 (Means Plant/spot number)
   
   LAI= 2.59(Leaf area index)
   
   SEL=0.13 (Standard error of LAI)
   
   DIFN=0.15 (Fraction of sky visible)
   
   MTA=61 (Mean tilt angle)
   
   SEM=5 (Standard error of MTA)
   
   SMP=4 (Number of samples pairs used)
   
   2*(S+1) =2.62 (Alternative LAI calculation)
   
   

Transferring data to computer

1. Download FV2000 software from LICOR website.
2. Click the “Acquire” button in file menu or in the tool bar.
3. Define the destination where you want to store the data. Make sure “convert to tab delimited” format is selected.
4. Connect the LAI2000 control unit to your PC. Use the 2000-02 RS-23 cable to connect the 25 pin male connector on the LAI console to a 9 pin serial to USB.
5. Check the “Comm port” in system hardware and change the “Comm Port” at the top in the Acquire screen.
6. Configure the LAI2000 by pressing “FCT 31” on the console and configure it for “BAUD=4800” “DATA BITS=8”, PARITY=NONE”, and “XON/XOFF=NO”. Press “FCT 33 on the console, and configure it for “FORMAT: Standard”, and PRINT OBS:NO”
7. Begin receiving data from control unit to PC by clicking the “DOWNLOAD” button in the acquire dialog. The status line will show “Waiting for data”.
8. Start sending files by clicking “FCT 32” and enter range of file numbers you want to download. Now it will show “Printing file” with a number on the LAI200 console screen. To check the total number of files in the console press “FCT 21”. When it is finished click “Stop Comm and close FV 2000. If it may give you an error instead of “Printing file” as “DTR NOT TRUE”. It means it cannot connect so check the “Comm port”.
9. To verify the file download, open the destination file through notepad and review the observations.

For more information visit LICOR website

References

Li-Cor, Inc. 2012. LAI-2200 Plant Canopy Analyzer: Instruction manual. Lincoln, NE. Available at https://www.licor.com/documents/6n3conpja6uj9aq1ruyn