A “Matchstick Model” of microwave backscatter from a forest

Given that the majority of biomass is contained in the stems of trees within forests (as much as 90%), a new radar modelling approach is proposed here wherein the stems are the major biomass contributor in the context of Synthetic Aperture Radar backscatter sensitivity to forest biomass. The new model regards stems are as “matchsticks” consisting of constant radius, constant density, and constant dielectric properties. Furthermore, by considering only the larger constituents of the forest a clearer understanding of the correlation of biomass with backscatter from P and VHF Band SAR can be obtained.Using backscatter data, and specifically the trends, produced from the RT2 radiative transfer model this paper considers the effects of SAR scattering when interacting with forest stands consisting of identical vertical stems, or matchsticks. SAR frequencies of 50 MHz (VHF) and 429 MHz (P Band) are used to generate a comparative radar interaction data. These not only allow a comparison of the scattering of SAR signals of different wavelengths but also of how interactions with stems can reveal novel clues as to the source of the backscatter saturation phenomenon of forests. Removing the random volume scattering aspect of a forest, the canopy, would be expected to eliminate the saturation behaviour which occurs at increasing levels of biomass density, an effect believed to be due to extinction effects, but is shown here to not be the case. Results reveal that saturation behaviour can occur even for the low number density case where increasing the size of stems through the radii associated with Rayleigh, Mie and Optical scattering will result in backscatter saturation as the biomass density is increased. Consistent with this finding, a forest of solely vertical stems will exhibit saturation behaviour at lower biomass density values when lower planting densities of similar stems are used. In this study the backscatter model considers the ground contribution to be negligible but does include the double bounce as a result of interactions between the stems and ground.Also considered are Mie backscatter oscillations which are shown to average out when using both non-vertical stems and random radius values about a mean, both independently and collectively. These “Mie” effects can also be removed by altering the incident SAR angle. These findings allow a reduction of the backscattering scenario of forests to a two-region problem involving solely Rayleigh and Optical scattering. This serves to further provide an explanation as to the origin of saturation, whereby the balance between the Optical scattering increase and the matchstick number density decrease cancel each other out.The important conclusion is that backscatter does not measure biomass, but measures structural trends that are correlated with biomass in different ways, driven by stand level competition, resource use, etc. As a result different forest growth regimes should expect different backscatter–biomass trends.

► Simplified forest model proposed consisting of solely vertical stems.
► Size dependent scattering analysed using Rayleigh and Optical scattering.
► Influence of forest number and size variations explored using modelled SAR data.
► Forest thinning affects SAR saturation due to influence on forest basal area.
► Non uniqueness of forest backscatter to biomass relationship emphasised.