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ig 2008 
A MODELING APPROACH FOR STUDYING FOREST CHLOROPHYLL CONTENT IN 
RELATION TO CANOPY COMPOSITION 
J. Verrelst 3 ’*, M.E. Schaepman 3 , J.G.P.W. Clevers 3 
a Centre for Geo-Information, Wageningen UR, Wageningen, The Netherlands - (Jochem.Verrelst, Michael.Schaepman, 
Jan.Clevers)@wur.nl 
Commission VII, WG VII/1 
KEY WORDS: Chlorophyll Content, Non-Photosynthetic Vegetation, PROSPECT, FLIGHT, Canopy Structure, Woody Elements 
ABSTRACT: 
Foliar concentration of the main photosynthetic pigments chlorophyll a and b (Cab) is widely regarded as a generic bioindicator of 
the actual plant status. However, when the scale moves up to stand level, relationships between the spectral response and leaf 
chemistry tend to break down due to confounding factors such as canopy structure, woody elements and background contributions. 
Especially in old-growth forests large numbers of standing and fallen dead wood are generated. We questioned the role of woody 
elements in the retrieval of Cab content on the basis of synthetic reflectance data through coupling of leaf-level (PROSPECT) and 
canopy-level (FLIGHT) radiative transfer models. For a wide range of forest stands the Ca6-induced dispersion (Coefficient of 
Variation: CV Cab ) and total spread (Standard Deviation: SD) was calculated. The magnitude of CV Cab and SD provides information 
about the Cab-related spectral spread and can therefore be regarded as stand-specific indicators of the theoretical Cab detectability. 
Results demonstrate that in dense canopies woody elements are key players in suppressing the Cab-related spectral spread. Apart 
from composition canopy structure also exerts influence: e.g. an overstory with crown coverage (CC) of 60% and a crown LAI of 1.5 
propagated greatest spectral spread. In sparse stands (e.g. CC<40%) the background contribution is the dominant confounding factor. 
The impact that woody elements exert in the theoretical retrieval of Cab content was quantified for four distinct real-world 
coniferous forest types. 
1. INTRODUCTION 
Foliar concentration of the main photosynthetic pigments 
chlorophyll a and b (Cab) is widely regarded as a generic 
bioindicator of the actual plant status, such as stress condition 
(Lichtenthaler et al., 1996) and vegetation gross primary 
productivity (Gitelson et al., 2006). Various leaf and canopy 
experiments have indicated that imaging spectroscopy is a 
potentially powerful tool for assessing variation in chlorophyll 
content of trees (Ustin et al., 2004). However, when the scale 
moves up to stand or landscape level, relationships between the 
reflected electromagnetic radiation and leaf chemistry tend to 
break down (e.g. Trotter et al., 2002). Then the subtle scattering 
and absorption properties of the foliar chemistry are 
confounded by whole-tree features such as the foliage structural 
arrangement, woody elements and background reflectances (e.g. 
Asner, 1998). 
At landscape level, a common approach to deal with sub-pixel 
complexity is to unmix a pixel into its most distinct, ‘p ure ’ 
endmembers. For instance, a vegetated surface might be 
decomposed into fractions of photosynthetic vegetation (PV), 
non-photosynthetic vegetation (NPV) and bare soil. Although 
pixel unmixing into NPV-PV-bare soil endmembers facilitated 
to study ecosystem dynamics (e.g. Asner et al., 2004; Jia et al., 
2006), it does not provide understanding of the true complexity 
between the interaction of phytoelements and radiant energy. 
Alternatively, leaf chemistry estimates retrieved from optical 
remote sensing can be investigated using radiative transfer 
models, which describe the transfer and interaction of solar 
radiation inside the canopy based on physical laws and thus 
* Corresponding author. 
provide an explicit connection between the phytoelements and 
the canopy reflectance. While much work exists in the realm of 
radiate transfer modeling, the relative importance of woody 
elements (NPV) in the context of quantifying canopy 
chlorophyll content however has not adequately been evaluated. 
Only recently the influence of the 3D structure of trunks and 
branches in a coniferous canopy on reflectance has been 
explicitly quantified (Malenovsky et al., 2008). Yet this work 
comprised a young production forest (e.g. <30 year), where 
woody elements are exclusively part of the standing trees and 
concentrated in the lower part of the canopy. By contrast, in 
old-growth forests a surplus of woody parts in the form of lying 
and standing deadwood are scattered within the canopy layer 
and on the forest floor and can encompass 18-40% of total 
woody biomass (Siitonen 2001). In these older forests woody 
parts play a significant role in determining canopy reflectance 
(Asner, 1998), as they are an important photon absorbing and 
scattering component. Apart from changing canopy 
composition, the forest also increases in heterogeneity during 
aging (Franklin et al., 2002); making structural attributes 
equally important drivers of the canopy spectral response (Song 
et al., 2002). This paper reports on the propagation of canopy 
compositional and structural effects when inferring chlorophyll 
content assessments on the basis of synthetic reflectance data. 
We used the leaf-level model PROSPECT (Jacquemoud et al., 
1996) coupled with the canopy-level ray-tracing model 
FLIGHT (North, 1996). The coupled models allow controlling 
simultaneously foliar chemistry and biophysical variables. The 
objective of the present study was to assess the influence of 
NPV and forest structure on the determination of leaf 
chlorophyll concentration from synthetic reflectance