Long Wavelength SAR Backscatter Modelling Trends as a Consequence of the Emergent Properties of Tree Populations

Matthew Brolly,Iain Woodhouse

doi:10.3390/rs6087081

Abstract

This study describes the novel use of a macroecological plant and forest structure model in conjunction with a Radiative Transfer (RT) model to better understand interactions between microwaves and forest canopies. Trends predicted by the RT model, resulting from interactions with mixed age, mono and multi species forests, are analysed in comparison to those predicted using a simplistic structure based scattering model. This model relates backscatter to scatterer cross sectional or volume specifications, dependent on the size. The Spatially Explicit Reiterative Algorithm (SERA) model is used to provide a widely varied tree size distribution while maintaining allometric consistency to produce a natural-like forest representation. The RT model is parameterised using structural information from SERA and microwave backscatter simulations are used to analyse the impact of changes to the forest stand. Results show that the slope of the saturation curve observed in the Synthetic Aperture Radar (SAR) backscatter-biomass relationship is sensitive to thinning and therefore forest basal area. Due to similarities displayed between the results of the RT and simplistic model, it is determined that forest SAR backscatter behaviour at long microwave wavelengths may be described generally using equations related to total stem volume and basal area. The nature of these equations is such that they describe saturating behaviour of forests in the absence of attenuation in comparable fashion to the trends exhibited using the RT model. Both modelled backscatter trends predict a relationship to forest basal area from an early age when forest volume is increasing. When this is not the case, it is assumed to be a result of attenuation of the dominant stem-ground interaction due to the presence of excessive numbers of stems. This work shows how forest growth models can be successfully incorporated into existing independent scattering models and reveals, through the RT comparison with simplistic backscatter calculations, that saturation need not solely be a direct result of attenuation.

Highlights

The effective exploitation of airborne and spaceborne Synthetic Aperture Radar (SAR) for modern forestry applications relies on sound theoretical understanding of the relationship between backscatter and generalised forest parameters such as volume and/or biomass
The constants associated with Equations (1) and (2) can be obtained and used to predict the theoretical backscatter values for the entire volume based on the combined results of the simple equations within the simplistic backscatter model. The results of this process indicate whether the backscatter and saturation within a forest can be considered a simple combination of two scattering types through comparison with the trends exhibited by the RT2 modelled backscatter data
The simplistic model, similar to the Matchstick Model, was applied to demonstrate that backscatter trends can be explained to some extent by the transition from Rayleigh to Optical scattering signified by the change in size of the stems within a multi-age forest

Summary

Introduction

The effective exploitation of airborne and spaceborne Synthetic Aperture Radar (SAR) for modern forestry applications relies on sound theoretical understanding of the relationship between backscatter and generalised forest parameters such as volume and/or biomass. A consequence of both models is that the saturation of backscatter with increasing forest biomass can only be explained as a result of the increasing attenuating properties of forest canopies; i.e., saturation occurs when the forest layer effectively becomes opaque. There is both theoretical [3] and empirical [4,5] evidence showing that in certain conditions saturation can occur at lower biomass than expected and sometimes saturation does not occur at all, suggesting that the WCM and RVoG models may not be as generally applicable as is widely thought. Saturation may be caused by other factors, such as the ratio of dominant stem radius to wavelength [6,7,8]

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