Photograph of (A) TRAC Instrument (length ~ 80 cm) and (B) PAR detectors (close-up).
Position along transect (m)
TRAC transect in loblolly pine plantation (site: Hertford). Peaks (black spikes) are canopy gaps.
Computed parameters for this transect were gap fraction = 9%; clumping index (
) = 0.94; PAI =
= 4.4 (assuming
= 0.1, and mean element width = 50 mm).
canopy gap size (physical dimension of a canopy gap), (2) canopy gap fraction (percentage of
), (4) plant area index (LAI, which includes
both foliage and woody material), and (5) LAI with clumping index (
foliage element clumping index,
) incorporated. Note that
in each case the parameters are for the particular solar zenith angle
at the time of data acquisition,
defining an inclined plane slicing the canopy between the moving instrument and the sun.
Parameters entered into the TRACWin software to invert measured PPFD to the derived output
parameters include the mean element width (the mean size of shadows cast by the canopy), the
needle-to-shoot area ratio (
) (within-shoot clumping index), woody-to-total area ratio (
tude/longitude, and time.
Potential uncertainties were inherent in the first three parameters and will
be assessed in future computational error analyses.
Solar zenith and azimuth influence data quality. Optimal results are achieved with a solar zenith
> 60˚), the relationship between
LAI and light extinction becomes increasingly nonlinear. Similarly, best results are attained when
TRAC sampling is conducted with a solar azimuth perpendicular to the transect azimuth. Sky
condition is a significant factor for TRAC measurements. Clear, blue sky with unobstructed sun is
optimal. Overcast conditions are unsuitable; the methodology requires distinct sunflecks and shadows.
between 30 and 60 degrees. As
approaches the horizon (