Delta-T Devices SunScan SS1 User Manual page 35

Canopy type and LAI estimates
Some types of canopy do not conform well to the assumptions about
canopy structure used by the SunScan in calculating LAI. The following
table will give you an initial idea of whether it is applicable to your canopy.
You should read the chapter on the LAI theory for a fuller appreciation of
the subject. Some guidance on specifying values for the typical leaf angle
(the ELADP parameter) and leaf absorptivity of your canopy comes later
in this section.
Canopy type
Low, uniform (e.g. cereal
crops, trial plots)
Low, regular but not uniform
(e.g. row crops)
Isolated trees or bushes (e.g.
orchard trees)
sparse vegetation (e.g. scrub)
high, uniform , not clumped
(e.g. some timber plantations)
high, clumped vegetation (e.g.
natural woodland)
Canopy sampling volume
It is useful to be aware of the volume of the canopy that the SunScan is
"seeing" when calculating LAI, so you can take this into account when
planning your sampling scheme.
With the Direct beam, the SunScan only sees the canopy elements along
a thin 1 metre wide sheet between the probe and the sun. With Diffuse
light, the SunScan sees a much larger volume, covering a region centred
on the probe, extending out approximately as far as the canopy is high,
but with the canopy above the probe making the greatest contribution.
These two very different sampling volumes are measured in the same
proportions as the incident Direct and Diffuse light.
This means that in strong sun (high Beam Fraction) the canopy volume
sampled is fairly small and well defined. As the Beam Fraction decreases,
the volume sampled increases, and has less well defined limits.
SS1 User Manual v3.3
Comments
Good for LAI
Absolute LAI dubious. May show valid
trends. PAR mapping
PAR mapping only
PAR mapping only
In principle good for LAI, but practical
difficulties for above-canopy reference
PAR mapping only
What to Measure and How  35