forest site in järvselja, estonia: summary of ground-based lai measurements tiit nilson, tartu...
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Forest site in Järvselja, Estonia: Summary of ground-based LAI
measurements
Tiit Nilson, Tartu Observatory
SPOT4
26.06.2003 Coniferous forests appear in dark, deciduous forests in green tones, recent clear cuts and open areas in pink and violet
Ground-based measurements: LAI-2000 and digital hemispheric photos
• Mixed forest dominated by birch, spruce and pine• Managed forests, some forest have been cut, so
some changes take place from year to year• 50-60 sample plots measured• 3x3 km region measured in summer conditions
2001,2002 and 2003; in leafless broadleaves conditions November 2003. For some plots, LAI-2000 measurements are available from yr 2000
• Angular distribution of gap fraction used• Forest canopy LAI derived by Nilson’s algorithm,
uses ancillary data from the forestry database
LAI-2000 results, average over all sample plots. Summer conditions
LAI tree storey LAI ground
LAI total
Year Nilson’s algorithm
Forestry database
LAI-2000
LAI-2000
2001
3.24
3.71 0.84 4.12
2002
3.54
3.56 0.86 4.40
2003
3.62
3.54
0.80
4.43
Comparison of LAI-2000 and hemispheric photos
• CAN_EYE and GLA programs used to process the photos
• A tendency to overestimate gap fraction by the photographic method was noted, especially for broadleaves
• With automatic exposition control, typically the crowns and branches from the upper canopy are over-exposed in photos and could be treated as sky
• After the processing of photos is finished, a new round of the LAI estimation is foreseen
Agreement between LAI-2000 and photos can be rather good,
especially for conifersJarvselja 75-02, 2002 summer
0.0
0.1
0.2
0.3
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0 10 20 30 40 50 60 70 80 90
View Zenith Angle, Degrees
Gap
Fra
ctio
n
LAI-2000
CAN_EYE
GLA
A pine dominated stand
Agreement is typically worse for broadleaves
A dense birch-dominatedforest
In some cases, large differences are found
111_04
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0 20 40 60 80
View zenith angle, degrees
Ga
p f
rac
tio
n CAN_EYE 02
CAN_EYE 03
LAI-2000 02
LAI-2000 03
LAI-2000 estimates on the same plots. Temporal trajectories from year to year
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2000 2001 2002 2003 2004
Year
Can
op
y_L
AI
160-04159-04161-05162-07104-17134-01106-12107-13111-0479-16141-06111-11186-08186-07164-04165-02137-0875-0276-04188-07188-09166-06167-07134-07137-03108-07108-08137-06136-05136-07
=
Larger drops in LAIare related to documentedcuttings
LAI-2000 measurement error is quite large, partly because of non-favorableweather conditions during measurements
A candidate for the transfer function: Reduced Simple Ratio (RSR). An example from an atmospherically
corrected SPOT4 image of 2 July 2002. Total LAI from ground-based LAI-2000 measurements
2002 y = 0.6215x - 0.6094
R2 = 0.5146
0
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2
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2 4 6 8 10 12
Reduced Simple Ratio (RSR)
LA
I to
tal
RSR of two
adjacent years
Jarvselja, SPOT 2001&2002
y = 1.1071x + 1.355
R2 = 0.8402
0
2
4
6
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10
12
0 2 4 6 8 10
RSR 2001
RS
R 2
002