LAI2200 Protocol

This outlines the process and provides some general guidelines for indirect measurements of ground based LAI estimation using the LAI-2200 Plant Canopy Analyzer. The information is largely derived from the LAI-2200 field manual (LI-COR 2011). It is to be used at the TERN AusCover CALVAL sites as a field protocol (in conjunction with the LAI 2200 manual, the AusPlots-Rangelands Survey Protocols Manual, and the SLATS Star Transect Protocol). It is proposed that these measurements be collected in tandem with SLATS Star Transect FPC measurements.

It is important to note that any variability present in a site (different land covers or vegetation associations such as extensive areas of bare ground, shrubland, open woodland, closed forest, grassland) should be identified and used to inform the sampling scheme employed. The variability of the site could be estimated using maps (land use, soils, vegetation) and recently acquired aerial photographs and satellite imagery. Because each site has different vegetative and land cover characteristics, it may be necessary to modify the proposed sampling scheme accordingly.

 Important Note: The LAI-2200 captures LAI measurements in a range of canopies using one or two sensors attached to a single data logger (LI-COR 1990). As with hemispherical photography, LAI-2200 measurements are to be collected in dimly lit conditions, when the sky is uniformly grey (e.g. around dawn and dusk when the sun is below the horizon or during cloudy/overcast days). The quality of these readings for analysis purposes is compromised if taken during other times (e.g., direct sunlight will result in an underestimation of LAI). If this consideration cannot be met and readings can only be taken during direct sunlight conditions, it may be possible to mask out the influence of the sun (LI-COR 2009: 4-11). Nevertheless, it is recommended that collection is carried out around dawn or dusk or during uniform overcast days. For more information refer to chapter 4 of the LAI 2200 manual.

 Site Details

Plot Selection Process

A standard AusCover 5 x 5 kilometre site is to be divided into 1 km² tiles (the flux tower, if present, would fall in the middle). Two sampling units which consist of a 100 x 100 m Ausplot grid and star transects are to be employed within the 25 km² site (Figure 1).

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Figure 1: Overview of sample design for Chowilla, in which two sampling strategies are to be implemented. 

The majority of the LAI measurements are to be clustered around the flux tower. Within the 1 km² centre cell, two star transects are to be undertaken. Also in this cell (and not overlapping with the star transects) is to be a 100 x 100 m Ausplot grid with 10 internal transects.

Within the 1 km² cells surrounding the flux tower, 3 star transects (located in different cells) are to be undertaken (the location is to be randomly selected, following the guidelines specified in the SLATS Star Transect Protocol). Two additional star transects are to be completed along the outermost 1 km² cells. To allow for an area that may not be homogenous or of interest, a cell has been excluded from the site grid (for instance, in Chowilla, this may occur in the southwestern corner, which appears to have much lower vegetation than in surrounding areas).

The location of the sample plots should be selected in the manner outlined in the SLATS Star Transects Protocol (bearing in mind the same considerations). Furthermore, these should not overlap.

 The LAI-2200 collection process assumes that the measurements are taken at intervals along a predefined sampling strategy.

Description of Plot Layout

Star transect plots are 100 m in length (50m radius). LAI measurements are recorded every 1m throughout the length of the transect plots.

Ausplot grid plot consists of 10 internal transects within 100 m x 100 m (following guidelines specified by the Ausplots-Rangeland Survey Manual). Measurements (FPC and LAI-2200) are taken across the entire grid plot at 1 m intervals (total 1000 measurements).

Basic Data Required

The following data must be recorded for data management purposes:

• Geographic coordinates (easting, northing, zone) at plot centre

• Operators: who collected the data

• Date: consistent format (dd/mm/yyyy)

• Time: time of first and last measurement for each transect (check it is consistent with instrument recording) 

• Plot name: name of specific plot

• Transect/run name: record the number/name of each run or transect

• Weather conditions as precise as possible

• View caps used (if any) and direction the measurement was taken

Data Collection Process

LAI measurements should be carried out during uniform overcast sky conditions (without direct sunlight) or during dawn and dusk (if sunny conditions).

Field Equipment Checklist

• 3 Tape measures (50 to 100 m)

• LAI-2200 - preferably tall canopy package with dual sensors

• Tripod or monopod (if available)

• GPS – preferably differential (DGPS)

• Field sheets

• Compass

Process

1. Layout Plot

   • This protocol is designed for data to be collected in conjunction with SLATS star transects. As such the plot should already exist.

   • If data is being collected independently, then layout plot in a manner consistent with a predefined sampling strategy (i.e., 100 m x 100 m grid). Lay out the centre plot grid in accordance with the AusPlots rangelands Survey Protocol Manual guidelines. Try to avoid tracks and other anthropogenic land covers (should be as homogenous and representative of the dominant vegetation as possible).

2. Take GPS reading.

   • For the SLATS transect, take GPS reading at centre of plot. For the 100 m x 100 m grid, take a GPS reading in the four corners and the centre.

   • It is best to use survey-grade differentially corrected GPS if possible because it is important that the gaps within and between trees line up with the fractional cover estimated from LiDAR. A distance of 2 to 10m (taken with a handheld GPS) can substantially offset the validation of the LiDAR. If ground control points are available that will be identifiable in both the field and the imagery, these can be used to improve the integration of field and image data through a distance and bearing measurement between the GCP and the centre of the plot.

3. Record site details on field sheet

   • Record the following details on the field sheet:

     • Geographic coordinates

     • Plot name

     • Field operator

     • Date

     • Transect/run name

     • Time of fist and last measurement of each transect/run

     • Weather conditions, as precisely as possible.

     • Veiw caps used (if any) and direction the measurement was taken.

4. Take measurements

The guidelines specified below are for the LAI 2200 Tall Canopy package with dual sensors. Please refer to chapters 2 and 3 of the manual (LI-COR 2009) to become familiar with the instrument.

Before taking measurements, please refer to chapters 2 and 3 of the manual (LI-COR 2009) to become familiar with the instrument. Some guidelines for using this instrument when two sensors are utilised are as follows:

1. Follow the steps outlined in section 'Operation: Two Sensors, One Control Unit' 3-21 of the LAI 2200 manual.

2. Choose one sensor to take the above (A) canopy readings. This will be the reference sensor and will remain in the same location. The other sensor will take the below (B) canopy readings and will be carried around in the field.

3. Mount the 'A' sensor onto the tripod in the middle of the clearing (refer to the Measurement Guidelines sub-heading below).

4. Synchronise the clocks of both sensors (section 3-3 of the LAI 2200 manual).

5. Attach the same type of view cap to both sensors with the openings facing away from the handles. Orient the view caps so each sensor will view the same part of the sky. Record the view cap type and bearing of the  'A' sensor on the field sheet.

6. Match the two sensors by following the procedure described in section 3-10 of the LAI 2200 manual. Be sure that (at all times) both sensors have the same view caps, that the view caps are oriented identically on the lenses, view caps must cover the same portion of the lens of both sensors, that the sensors are oriented in the same compass direction, and that they are level (the float should be in the centre ring of the bubble level).

7. Configure the 'A' sensor to record data automatically at the desired interval. The recommendation here is that the interval be 5 seconds, which roughly matches taking a reading in the field every metre. Alternatively, have the recording mode set to manual click.

8. Disconnect the 'A' sensor from the console and leave it on the tripod to automatically record measurements until the 'B' readings of the sampling strategy are completed.

9. Either check the results in the field (see 'Importing A Readings from Separate Files (Console)' section of the LAI manual), or check the readings by post processing on a computer with the LAI 2200 software (section 6-14 of the manual).

Measurement Guidelines

• Use the 270° view cap on both instruments if the sky conditions are ideal.

• Remember that when using two sensors, at all times the view cap must be the same size, the sensors must be level, and the optical sensor must view the same part of the sky (the bearing of both sensors must match for every measurement).

• The operator must always stand behind the 90° mask section of the view cap.

• Set up the 'A' reference sensor as close to the 'B' readings as possible.

• The 'A' sensor is to take measurements around waist height (or what is comfortable), masking out the operator for all recordings.

• As a general rule, the distance between the B sensor and the nearest leaf above it should be at least four times the width of the leaf.

• The reference ‘A’ sensor should be placed in a clearing (see Figure 2 below). The sensor should be in the middle of a clearing with a diameter at least 7 times the height of the trees above the sensor. Remember to factor in the height of the tripod the sensor rests on.

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 Figure 2: Diagram showing the minimum clearing size for a 270° view cap

• If taking measurements on a significant slope, the sensor must be tilted so that it is parallel to the ground.

 Data Recording and Storage

Recording of data in field

The LAI measurements in the field will automatically be stored in the memory of the device. Measurement recordings can then be downloaded to a computer after the field work has been completed for post processing. Information from field sheets will be entered into an Excel template for upload to the Auscover spatial database.