Forest list archive: msg00080
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Re: PAR light meter
At 16:26 7/06/98 -0400, you wrote:
>my lab is looking to purchase a light meter to measure PAR above
the
>herbacious understory in regenerating red spruce clearcuts in
southwestern
>Nova Scotia. I have used the Decagon "wand" PAR meter in
northwestern
>Ontario and liked it. Does anyone have experience with other PAR
meters
>available.
>
>thanks
>
>faizal moola
>PhD. candidate
>Dalhousie University
>Nova Scotia, Canada
>
I reported on "Light transmission and pasture growth in a
subtropical silvopastoral trial in south east Queensland" in
1995
as part of studies primarily aimed at determining the factors influencing
and affecting the growth of trees and pastures. The ultimate aim may be
to elucidate optimal conditions for the concurrent growth of woody
perennials and pastures and crops.
The influence of trees on the light environment of a silvopastoral trial,
and consequently on the growth and composition of the pasture was
assessed.
Measurement of photosynthetically active radiation ( PAR ) beneath
trees
Newman ( 1989 ) reviewed methods for sensing and integrating the
transmissivity of agroforestry canopies to PAR. PAR was measured with a “
PAR M94 Integrating Meter ” designed and constructed by Graham Kerven,
Department of Agriculture, University of Queensland. The meter comprises
a single cosine corrected photodiode sensor that responds to the PAR
component ( 400 - 700 nm waveband ) of the incoming solar radiation. The
photodiode output current is scaled by a current to voltage amplifier and
converted into a frequency by a voltage to frequency converter based on
the circuit design of Muchow and Kerven ( 1977 ). The output pulses are
counted by a crystal controlled timing circuit and the result displayed
on a LCD display. In the integrate mode the pulses are divided down and
accumulated for the set timing period. At the end of the integration
period the display is updated and holds the current reading.
PAR transmission beneath the tree canopy required the synchronised use of
two meters. One meter was secured horizontally on a tripod in full sun to
measure the reference input PAR value while the second meter was
hand-held and taken through a pre-determined transect within the tree
canopy to spatially average out the light intensity level. The sensor was
held just above the pasture level. The transect consisted, for each plot,
of walking from the lower side of the plot, perpendicular to the slope up
the plot to the top at approximately 1 m spacings. At the end of
the integration period the two readings were recorded. In the 5 minute
integration mode final PAR was measured as mMol m-2. Percent light
transmission for a 5 minute integration was calculated thus :
% transmission = under canopy PAR ( mMol
m-2 ) x 100
reference PAR ( mMol m-2 )
Muchow, R. W. and Kerven, G.L. ( 1977 ). A Low Cost Instrument for
Measurement of Photosynthetically Active Radiation in Field Canopies.
Agric. Meteorol., 18:187-195.
Newman, S.M. ( 1988 ). Inexpensive Instrumentation for monitoring PAR in
agroforestry. Meteorology and Agroforestry ( Proceedings of an
international workshop on the application of meteorology to agroforestry
), Vol. 23, pp. 297-304.
______________________________________________
Matt Kovacs
M.For.Sc. Scholar
Forest Technology Program
CSIRO Division of Forestry and Forest Products
Banks St, Yarralumla ACT 2600
PO Box E4008, Kingston, ACT 2604
Matt.Kovacs@ffp.csiro.au
Tel: 02 62818237 Fx: 02 62818239
Int.Tel.:+61 2 62818237 Fx: +61 2 6281 8239
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