Abstract
Synthetic Aperture Radar (SAR) signals respond to the interactions of microwaves with vegetation canopy scatterers that collectively characterise forest structure. The sensitivity of S-band (7.5–15 cm) backscatter to the different forest types (broadleaved, needleleaved) with varying aboveground biomass (AGB) across temperate (mixed, needleleaved) and tropical (broadleaved, woody savanna, secondary) forests is less well understood. In this study, Michigan Microwave Canopy Scattering (MIMICS-I) radiative transfer model simulations showed strong volume scattering returns from S-band SAR for broadleaved canopies caused by ground/trunk interactions. A general relationship between AirSAR S-band measurements and MIMICS-I simulated radar backscatter with forest AGB up to nearly 100 t/ha in broadleaved forest in the UK was found. Simulated S-band backscatter-biomass relationships suggest increasing backscatter sensitivity to forest biomass with a saturation level close to 100 t/ha and errors between 37 t/ha and 44 t/ha for HV and VV polarisations for tropical ecosystems. In the near future, satellite SAR-derived forest biomass from P-band BIOMASS mission and L-band ALOS-2 PALSAR-2 in combination with S-band UK NovaSAR-S and the joint NASA-ISRO NISAR sensors will provide better quantification of large-scale forest AGB at varying sensitivity levels across primary and secondary forests and woody savannas.
Highlights
Satellite remote sensing technology has shown promising capabilities for consistent and systematic observations of the dynamics of large-scale forest ecosystems [1]
The results suggest a greater sensitivity of the S-band backscatter signal to forest aboveground biomass (AGB) for broadleaved species up to 100 t/ha
Simulated S-band backscatter suggests that S-band Synthetic Aperture Radar (SAR) may be able to provide information on forest biomass in low-biomass forests up to 100 t/ha with the smallest achievable errors between 37 t/ha and 44 t/ha for HV and VV polarisations in different forest types, broadleaved forest in temperate and tropical forest
Summary
Satellite remote sensing technology has shown promising capabilities for consistent and systematic observations of the dynamics of large-scale forest ecosystems [1]. Electromagnetic radiation in the microwave domain is related to forest properties due to its high sensitivity to canopy structure and moisture content. Forest structure information such as tree density, average stand height and diameter at breast height based on sample plots is necessary [2] to calibrate with microwave backscatter signals and for predictions of forest biophysical parameters such as aboveground biomass (AGB) [3]. The sensitivity of radar backscatter to forest structure depends on the canopy type (intact to open savanna, homogeneity to complex density) and environmental conditions such as moisture content [5]. Forest biomass is largely related to the cross-polarised channels (HV—horizontal transmit vertical return) which have comparatively lower returns but generally increase asymptotically to the amount of woody biomass up to a saturation level [6,7]
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