User manual

42 •
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• Measurement options Document code: SS1-UM-1.05
Canopy type and BFS practicalities
Canopy type is the next variable to be considered. As a general guide, the above-
canopy reference measurements should be made close to, or above, the position of
the SunScan probe. Tall canopies will undoubtedly exercise your ingenuity in
achieving this! If you want to use the BFS for LAI readings you must be able to see
the top of it to set the shade ring correctly.
Canopy
type
Options Comments
Low BFS connected, with extension
cables, if necessary.
Best, but long cables need
management
Low No BFS Slower. Needs slow-changing light
conditions
High Devise a portable BFS mount. Use
extension cables.
Good when possible. Checking BFS
shade ring difficult.
High Use clearings to get out from under
the canopy. (No BFS required)
Needs steady light conditions.
Clearing light may be partly shaded.
High Use independent sensor for above
canopy PAR.
Needs slow-changing light, and
possibly time average readings. LAI
readings not available.
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 Comments
Low, uniform (e.g. cereal crops, trial
plots)
Good for LAI
Low, regular but not uniform (e.g.
row crops)
Absolute LAI dubious. May show valid trends. PAR
mapping
Isolated trees or bushes (e.g.
orchard trees)
PAR mapping only
sparse vegetation (e.g. scrub) PAR mapping only
high, uniform , not clumped (e.g.
some timber plantations)
In principle good for LAI, but practical difficulties for
above-canopy reference
high, clumped vegetation (e.g.
natural woodland)
PAR mapping only
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 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.