Figure 1

Figure 1. The TRAC is a hand-handled instrument that can be operated by one person.


Figure 2

Figure 2. An example of TRAC measurements in a mature jack pine stand near Candle Lake, Saskatchewan. The measured photosynthetic photon flux density (PPFD) along a 20 m transect (a small portion of the original 200 m record) shows large flat-topped spikes corresponding to large canopy gaps between tree crowns and small spikes resulting from small gaps within tree crowns. The baseline is the diffuse irradiance under the canopy measured using the shaded sensor.

  Figure 3
Larger image (21kb jpg)

Figure 3. The original measurements shown in Fig. 2 are converted into this canopy gap accumulation curve (Fm) where the gap fraction is accumulated from the largest gap (about 1.8 m in this case) to the smallest gap. The accumulated gap fraction at canopy gap size of zero is the total canopy gap fraction as measured from the total radiation transmittance. After using a gap removal approach, the measured gap size distribution (Fm) becomes (Fmr) and is brought to the closest agreement with the distribution (Fr) predicted by the random theory (Miller and Norman, 1971). In this case Fmr and Fr agree very closely. The difference between Fm and Fmr on the ordinate determines the clumping index and the Fmr determines LAI.

Tracing Radiation and Architecture of Canopies (TRAC)

- What is TRAC? - Gap size measurement - Gap size distribution - Clumping effect
- Validation - Other uses - Commercially availability - References

What is TRAC?

TRAC is a new optical instrument for measuring the Leaf Area Index (LAI) and the Fraction of Photosynthetically Active Radiation absorbed by plant canopies (FPAR). TRAC measures canopy 'gap size' distribution in addition to canopy 'gap fraction'. Gap fraction is the percentage of gaps in the canopy at a given solar zenith angle. It is usually obtained from radiation transmittance. Gap size is the physical dimension of a gap in the canopy. For the same gap fraction, gap size distributions can be quite different.
TRAC manual for download

Why do we measure gap size?

Plant canopies, especially forests, have distinct architectural elements such as tree crowns, whorls, branches, shoots, etc. Since these structures dictate the spatial distribution of leaves, this distribution cannot be assumed to be random. Previous commercial instruments have been based on the gap fraction principle. Because of foliage clumping in structured canopies, those instruments often considerably underestimate LAI. A canopy gap size distribution contains information of canopy architecture and can be used to quantify the effect of foliage clumping on indirect (i.e., non-destructive) measurements of LAI.

How is the gap size distribution measured?

TRAC (including the recording and data analysis components) is hand-carried by a person walking at a steady pace (about 0.3 meter per second). Using the solar beam as a probe, TRAC records the transmitted direct light at a high frequency. Figure 1 shows an example of such measurements where each spike, large or small, in the time trace represents a gap in the canopy in the sun's direction. These individual spikes are then converted into gap size values to obtain a gap size distribution shown in Figure 2. The curve in Figure 3 is an accumulated gap fraction from the largest to the smallest gap. The total accumulated gap fraction on the ordinate (at gap size of zero) is the gap fraction that is usually measured from the radiation transmittance. A gap size distribution curve like this reveals the composition of the gap fraction and contains much more information than the conventional gap fraction measurements.

Can we quantify the clumping effect from a gap size distribution?

Yes. A gap size distribution, contains many gaps which result from non-randomness of the canopy, such as the gaps between tree crowns and branches. Since we know the distribution for a random canopy, Fr in Figure 3 (Miller and Norman, 1991), the gaps resulting from non-randomness can be identified and excluded from the total gap fraction accumulation using a gap removal method (Chen and Cihlar, 1995a). The difference between the measured gap fraction and the gap fraction after the gap removal can then be used quantify the clumping effect.

Has this new method been validated?

TRAC technology has been validated in several studies (Chen and Cihlar, 1995a; Chen, 1996a, Chen et al., 1997; Kucharik et al., 1997). These studies showed that instruments based on gap fraction, such as LI-COR LAI-2000, measure the effective LAI under the assumption of random leaf spatial distribution. In forests, the effective LAI is generally only 30% to 70% of the true LAI because of foliage clumping. The clumping index obtained from TRAC can be used to convert effective LAI to LAI. When TRAC is used for half a clear day, an accurate LAI value for a stand can also be obtained using TRAC alone. It is recommended (Chen et al., 1997) that TRAC be used to investigate the foliage spatial distribution pattern while LAI-2000 is useful to study foliage angular distribution pattern. The combined use of TRAC and LAI-2000 allows quick and accurate LAI assessment of a canopy.

Are there any other uses of TRAC?

The following uses of TRAC have been demonstrated:
  1. the measurement of photosynthetic flux density (PPFD) along a transect is the best way to obtain the mean value of the transmitted light through the canopy, and it has been successfully used to quantify the fraction of photosynthetically active radiation (FPAR) absorbed by the canopy (Chen, 1996b);
  2. the gap size distribution can be used to estimate several canopy architectural parameters including foliage clump size and area, and foliage element size (Chen and Cihlar, 1995b);
  3. the gap size distribution has been used to model the hotspot and the bi-directional reflectance distribution function (BRDF) of the optical remote sensing signal from plant canopies (Chen and Leblanc, 1997). We believe you can find new uses of this new instrument.

Is TRAC commercially available?

Yes. The instrument is commercially available. For more information, please contact:

3rd Wave Engineering
14 Aleutian Road
Nepean, Ontario
Canada, K2H 7C8
Attention: Mr. Mike Kwong
Tel: (613)828-2195
Fax: (613)828-9498


Chen, J. M., P. M. Rich, T. S. Gower, J. M. Norman, S. Plummer, 1997. "Leaf area index of boreal forests: theory, techniques and measurements". Journal of Geophysical Research, 102(D24):29,429-29,444.

Chen, J. M., 1996a. "Optically-based methods for measuring seasonal variation in leaf area index of boreal conifer forests". Agricultural and Forest Meteorology, 80:135-163.

Chen, J. M., 1996b. "Canopy architecture and remote sensing of the fraction of photosynthetically active radiation in boreal conifer stands". IEEE Transactions on Geoscience and Remote Sensing, 34:1353-1368.

Chen, J. M. and J. Cihlar, 1995a. "Quantifying the effect of canopy architecture on optical measurements of leaf area index using two gap size analysis methods". IEEE Transactions on Geoscience and Remote Sensing, 33:777-787.

Chen, J. M. and J. Cihlar, 1995b. "Plant canopy gap size analysis theory for improving optical measurements of leaf area index". Applied Optics, 34:6211-6222.

Chen, J. M. and S. Leblanc, 1997. "A 4-scale bidirectional reflection model based on canopy architecture". IEEE Transactions on Geoscience and Remote Sensing, 35:1316-1337.

Kucharik, C. J., J. M. Norman, L. M. Murdock and S. T. Gower, 1997. "Characterizing canopy nonrandomness with a Multiband Vegetation Imager (MVI)". Journal of Geophysical Research, 102(D24): 29455-29473.

Miller, E. E. and J. M. Norman, 1971. "A sunfleck theory for plant canopies. I length of sunlit segments along a transect". Agronomy Journal, 63:735-738.

© Revised: Mar., 2005