A MODELING APPROACH FOR STUDYING FOREST CHLOROPHYLL CONTENT IN RELATION TO CANOPY COMPOSITION J. Verrelst a, *, M.E. Schaepman a , J.G.P.W. Clevers a 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 Cab-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. * Corresponding author. 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, ‘pure’ 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 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 25