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: CVCab) and total spread (Standard Deviation: SD) was calculated. The magnitude of CVCab 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.