Radiometric Signature And Spatial Variability Of The Vegetation Coverage of A Boreal Forest

摘要

A judicious combination of spectral and spatial surface information can improve the understanding of the vegetation optical variability and typological differentiation. The objective of this study is to evaluate the potential of airborne spectral radiation and digital imagery data for vegetation canopy classification and the impact of canopy texture on the vegetation-solar radiation interaction. To conduct the study, two multispectral radiometers with wavelengths ranging from 350 to 1050nm and a fine pixel digital camera are used. One of the radiometers is positioned close to the digital camera, and, both instruments are carried by a radio-controlled helicopter flying above the canopy of a boreal forest of the northern Japanese island of Hokkaido. Analyses of the canopy reflectance signature show a clear species differentiation in the vegetation of the area and give an evaluation of the canopy radiation capacities. The bamboo grass species have the highest reflectance and the needle-leaf species the lowest. To understand the physical factors associated with the reflectance-species typological relationship, textural features are extracted from digital images, by using colour discrimination techniques. The features estimated are the brightness intensity of the canopy, the amounts of gaps and shadows, the degree of heterogeneity of light scattering, and the green vegetation fraction. Then, the relationship between these individual properties and reflectance is examined. The results obtained show that reflectance decreases with increasing amount of gaps and shadows and, increases with the brightness intensity and more importantly with light scattering heterogeneity of the canopy. This heterogeneity effect, derived from the vegetation luminance distribution is examined through three methods. The most elaborate among these methods is the semivariogram analysis. Results of this analysis show that the range of the semivarioragram reflects well enough the average size of the plants (short range for the bamboo grass and large range for the needle-leaf species). The needle-leaf species have the highest variability, i.e. are the most heterogeneous light scatterers, while the bamboo grass species are the least variable. The scale of variability of the distribution of luminance differs according to the species: it is dominated by macrovariability in the needle-leaf, and microvariability in the bamboo grass and the broadleaf. The needle-leaf species' high spatial heterogeneity of light scattering would reduce the measured canopy bi-directional reflected radiation and enhance the transmission of this radiation towards lower vegetation levels through a multiscattering radiation process.