Does the Azimuth Direction of Observation of Trees influence Chlorophyll Content and Spectral Reflectance?
Elisabeth A. Addink, Steven M. de Jong & Wiebe Nijland
Department of Physical Geography, Faculty of Geographical Sciences, Utrecht University P.O. Box 80115, 3508 TC Utrecht, The Netherlands
Methods
Spectral reflectance (ASD FieldSpec with leaf clip) and Chlorophyll-ab (SPAD) content were measured around 13 individual free- standing Quercus ilex trees in 8 azimuth directions in steps of 45 °. Per direction 20 SPAD measurements and 6 spectra of different leaves were collected. SPAD measurements were converted to chlorophyll concentrations using relations described by Ribas et al. (2005). Directional dependence of reflectance and chlorophyll was investigated and the relation between chlorophyll content of leaves and leaf spectral properties such as red edge position, Carters stress indices and NDVI.
Study Area
The Peyne area is located 60 km west of Montpellier (France) and is covered by Mediter- ranean oak forest dominated by Quercus ilex and
Quercus pubescens, and ‘Garrigue’ shrub lands. The area is characterised by a complex of geological and lithological substrates resulting in a wide range of soil and moisture conditions and hence, growing conditions.
The area is an experimental study site for airborne imaging spectroscopy (DAIS7915, HyMap) since 1997.
Chart1
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Average Chlorophyll-ab all 13 trees
122.00 122.50 123.00 123.50 124.00 124.50 125.00 125.50 126.00 126.50 127.00
45 90 135 180 225 270 315 360
Direction (degrees)
Chlorophyll-ab
5.80 6.00 6.20 6.40 6.60 6.80 7.00 7.20 7.40 7.60 7.80 8.00
St.Dev. Chlorophyll-ab
Chl-ab StDev
Chart2
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Red Edge Position in 8 directions
718.200 718.400 718.600 718.800 719.000 719.200 719.400 719.600 719.800
45 90 135 180 225 270 315 360
Direction (degrees)
Red Edge Position (nm)
0.800 0.900 1.000 1.100 1.200 1.300 1.400 1.500 1.600
St.Dev. Red Edge Position
REP StDev REP
REP-Chl, 13 trees, all directions
y = 3.6989x - 2535.3 R2 = 0.4888
100.0000 105.0000 110.0000 115.0000 120.0000 125.0000 130.0000 135.0000 140.0000
715.0 716.0 717.0 718.0 719.0 720.0 721.0 722.0 723.0
Guyot-REP (nm)
Chl-ab ug/cm2
Chlab-Avg Linear (Chlab-Avg)
Results & Conclusions
Azimuth direction plays a small but noteworthy role in hyperspectral observations and chlorophyll measurements of Quercus ilex properties. Chlorophyll concentration is about 3 μg/cm2 higher at 270° than at 360°. The Red Edge Position shows also a small directional dependency of 1.5 to 2nm. REP is smallest for south-oriented leaves while suprisingly chlorophyll concentrations is highest for south oriented leaves. The Red edge Position gives an indication for the chlorophyll contents for the Quercus ilex leaves in the Peyne study area with the presented regression function with a r 2 of 0.49
▲ Detached Holm Oak tree (Quercus ilex).
Spectral measurements (leafclip) and Chlorophyll (SPAD) of leaves. ▼
► Average 8-directional relations (13 trees) for chlorophyll-ab content (upper) and the REP: Red Edge Position (centre), right axes show standard deviations.
The relation between average (n=2080) leaf chlorophyll concentration and the Red Edge Position ▼
Hyperspectral Images are widely used for quantitative mapping of vegetation parameters such as Leaf Area Index, aboveground biomass, canopy structure and canopy moisture content. Based on statistical relations between field observation and hyperspectral measurements, predictive models are built to map the vegetation parameters for larger areas. The effect of azimuth angle of observation of the trees however is unknown and never investigated. The objective of this study is to determine the effect of observation azimuth directions on leaf reflectance and chlorophyll contents of Quercus ilex trees.
It was anticipated that the south oriented parts of trees may yield higher chlorophyll contents and different reflectance properties.
Directional Chlorophyll-ab (green) and REP (red) for all trees
125.3
125.5 123.6
125.1
124.0
126.5
124.0 124.3
719.4
719.2
718.9
719.1 718.7
718.8
719.7
719.4 0
45
90
135
180 225
270
315
0
45
90
135
180 225
270
315
Red Edge Position (red) and Chlorophyll content (green) of all sampled trees in 8 directions. ▼
