A study of spectral-polarization characteristics of plant canopies using land-based remote sensing

Highlights

• The reflection and polarization properties of plant were studied.

• The compiled electronic database of the spectrophotometric information of plant.

• The study of influencing of viewing angle to SBC of plant canopies were carried out.

Abstract

The study addresses reflection and spectral-polarization characteristics of forest stands and farm crops obtained under field conditions. The study of the reflection properties of farm crops shows that during the summer plant growing season, the major factors influencing the plant canopy reflectance are morpho-physiological parameters, plant architectonics, solar elevation h₀, and viewing angle. The crop reflectance minimum was recorded at viewing angles 25–30° with respect to the nadir. Coniferous and broadleaf forest stands had similar reflectance spectra of polarized light. The polarized component was smaller for all coniferous stands than for broadleaf ones. For broad-leaved farm crops (wheat and corn), the polarized component of the spectral brightness coefficients had a greater influence on the plant reflectance in the red and near-infrared spectral regions, λ>720 nm.

Introduction

Satellite monitoring of terrestrial plant communities and land-based optical remote sensing are aimed at developing methods to identify farmlands, determine the plant species composition, and study morpho-physiological traits of plants [1], [2], [3], [4]. Advances in space monitoring of farm crops and coniferous and broadleaf forests enable more efficient data acquisition and better qualitative and quantitative interpretation of experimental material [1], [2].

The most convenient optical characteristics for identification of the optical properties of plant communities are their spectral brightness and reflectance. The spectral brightness coefficients of the study object (surface) ρ_λ (ϑ,φ) is defined as the ratio of the brightness of the study surface B_λ(ϑ,φ) within the wavelength range (λ,λ+dλ) in the direction of (ϑ,φ) to the brightness of the perfectly scattering orthotropic surface, whose reflectance factor, in accordance with Lambert's law, is equal to unity in the same direction B_o(ϑ,φ), under the same lighting conditions:

$${\rho }_{\lambda }(\vartheta ,\phi )=\frac{B_{\lambda }(\vartheta ,\phi )}{B_0(\vartheta ,\phi )},$$

where (ϑ,φ) are the azimuth and polar angles, respectively.

Plant canopy (PC) reflectance contains a considerable amount of information on the morpho-physiological state of the plants. Specular reflection of sunlight from plant canopies can be so strong that the canopies viewed sunwards look whitish rather than green [5]. Fresnel law states that the light reflected by the smooth leaf surfaces is polarized light.

The value of the polarized component is essentially determined by the incidence angle of light on the leaf surface, the indices of refraction of the leaf waxy coating, and the roughness of its surface [5], [6], [7], [8]. These data depend upon plant species and are potentially related to the morpho-physiological state and the stage of development of the plant canopy. Talmage and Curran discussed the possibilities of using polarization for remote sensing with sample data from soil and vegetation [8]. Shibayama and Watanabe measured further polarization from wheat canopies around specular direction noting some dependence on leaf inclination angle [6], [7]. Polarization observations are a potential source of useful information on the state of farm crops in satellite measurements. Such studies will provide a basis for determining interactions between spectrophotometric properties of plants and parameters characterizing their state, such as the morpho-physiological structure of crops in different phases of their growth.

The purpose of this study was to investigate spectral-polarization properties of forest communities and farm crops using land-based remote sensing and analyze the optical information carried in the reflected and polarized light simultaneously obtained in the field conditions.