Detecting forest structure and biomass with C-band multipolarization radar: Physical model and field tests

Abstract

We tested the ability of C-band radar (4.75 GHz) to discriminate features of forest structure, including biomass. We used a truck-mounted scatterometer for field tests on a 1.5–3.0 m pygmy forest of cypress (Cupressus pygmaea) and pine (Pinus contorta ssp. Bolanderi) near Mendocino, CA. We quantified 31 structural variables of the forest at seven sites. We also measured backscatter from a life-sized physical model of the pygmy forest, composed of nine wooden trees with “leafy branches” of sponge-wrapped dowels. This model enabled independent testing of the effects of stem, branch, and leafy branch biomass, branch angle, and moisture content on radar backscatter. Field results suggested that surface area of leaves played a greater role in leaf scattering properties than leaf biomass per se. Tree leaf area index was strongly correlated with vertically polarized power backscatter ( r = 0.94; P < 0.01). Field results suggested that the scattering role of leaf water is enhanced as leaf surface area per unit leaf mass increases, i.e., as the moist scattering surfaces become more dispersed. Fog condensate caused a measurable rise in forest backscatter, both from surface and internal rises in water content. Tree branch mass per unit area was highly correlated with crosspolarized backscatter in the field ( r = 0.93; P < 0.01), a result also seen in the physical model. Emergent trees had the predominant effect on radar backscatter in the field, with understory shrubs playing an insignificant role, even in these open-canopied forests. In the physical model, the presence of wet stems and wet leafy branches and the angle of branches affected backscatter most, with lesser contributions by dry stems and branches and changes in branch thickness respectively.