Relationships between spectral reflectance and pigment concentrations in stacks of deciduous broadleaves

Abstract

This article presents the findings of a laboratory experiment using stacks of leaves obtained from four species of deciduous tree at various stages of senescence. This methodology generated a wide variation in pigment concentrations (per unit leaf area and per unit ground area) and leaf area index, as may be expected at the plant canopy scale, and provided independent variation between the chlorophylls and carotenoids and leaf area index. Spectral reflectance was measured for each leaf increment of each stack using a spectroradiometer and these data were used to evaluate the nature and strength of relationships between a number of existing and novel spectral transformations and the concentrations of pigments (per unit ground area) within the leaf stacks. Reflectance in narrow wavebands within the visible region was moderately related to chlorophyll concentrations, while ratio indices, employing wavebands in the near-infrared and visible, particularly the green, were more successful. While the wavelength position of the red edge was related to chlorophyll concentration, characteristics of the amplitude of the first and second derivatives of reflectance and pseudo absorbance were more strongly correlated with chlorophyll. No transformation of spectral reflectance that was tested was strongly related to the concentration of carotenoids but two ratio indices were highly correlated with the ratio of carotenoids to chlorophyll a. Interestingly, leaf area index (LAI) was unrelated to all spectral ratio indices including the broad-band normalized-difference and the simple ratio vegetation indices. This indicates that spectral indices may have limited applicability for estimating the LAI of vegetation canopies when the leaves that comprise these canopies have differing chlorophyll concentrations (per unit leaf area).