Spectral invariants and scattering across multiple scales from within-leaf to canopy

Abstract

Spectral invariants can be considered fundamental descriptors of the impact of canopy structure on canopy scattering where the scattering objects are large compared with the wavelength of radiation. The paper uses the concept of canopy spectral invariants to explore scaling relationships within canopy scattering. A new approximation to the leaf-level PROSPECT scattering model of Jacquemoud and Baret [Jacquemoud, S., & Baret, F. (1990). PROSPECT: A model of leaf optical properties spectra. Remote Sensing of Environment, 34, 75–91.] is developed by applying the spectral invariant approach to leaf internal scattering, in a similar manner to that which has been used to describe canopy-level scattering. We show that it is possible to express both the canopy- and leaf-level single scattering albedo as a function of canopy spectral invariants. This approach provides a framework through which structural information can be maintained in a self-consistent manner across multiple scales from leaf- to canopy-level scattering, at least for the simple canopy architectures considered. It is demonstrated that the nesting of scales described in these relationships implies limits to the retrieval of absolute concentrations of any biochemical constituents or absolute quantities of the amount of scattering material from hyperspectral observations of total scattering. The implication is that in general it may not be possible to separate the components of structural and biochemical influences on measured total scattering signals.