L-Band SAR Backscatter Related to Forest Cover, Height and Aboveground Biomass at Multiple Spatial Scales across Denmark

Abstract

Mapping forest aboveground biomass (AGB) using satellite data is an important task, particularly for reporting of carbon stocks and changes under climate change legislation. It is known that AGB can be mapped using synthetic aperture radar (SAR), but relationships between AGB and radar backscatter may be confounded by variations in biophysical forest structure (density, height or cover fraction) and differences in the resolution of satellite and ground data. Here, we attempt to quantify the effect of these factors by relating L-band ALOS PALSAR HV backscatter and unique country-wide LiDAR-derived maps of vegetation penetrability, height and AGB over Denmark at different spatial scales (50 m to 500 m). Trends in the relations indicate that, first, AGB retrieval accuracy from SAR improves most in mapping at 100-m scale instead of 50 m, and improvements are negligible beyond 250 m. Relative errors (bias and root mean squared error) decrease particularly for high AGB values (>110 Mg ha −1 ) at coarse scales, and hence, coarse-scale mapping (≥150 m) may be most suited for areas with high AGB. Second, SAR backscatter and a LiDAR-derived measure of fractional forest cover were found to have a Remote Sens. 2015, 7 4443 strong linear relation (R 2 = 0.79 at 250-m scale). In areas of high fractional forest cover, there is a slight decline in backscatter as AGB increases, indicating signal attenuation. The two results demonstrate that accounting for spatial scale and variations in forest structure, such as cover fraction, will greatly benefit establishing adequate plot-sizes for SAR calibration and the accuracy of derived AGB maps.Keywords: ALOS PALSAR; airborne LiDAR; canopy density; aboveground biomass; mapping scale; non-linear modeling; vegetation interception ratio forests, the use of LiDAR may be hindered by cloud cover, edge effects or overhanging neighboring tree canopy [13,18]. Further, airborne LiDAR surveys are expensive, making them a relatively inefficient tool to map forest biomass periodically over large areas (although, see Mascaro et al. [22]). Spaceborne LiDAR offers an alternative, though it cannot achieve quasi-full coverage, like airborne systems, due to beam dispersal and power constraints, restricting it to large footprints separated by hundreds of meters. Although there are no current ongoing missions, the future ICESat-2, GEDIand MOLIproducts will be used for vegetation mapping [23].There has also been growing interest in the use of microwave synthetic aperture radar (SAR) to estimate AGB. SAR offers some advantages over LiDAR, allowing continuous coverage over vast areas and consistent and frequent acquisitions achievable from spaceborne platforms [15]. Microwave pulses are transmitted to the Earth's surface at shallow incidence angles (compared to airborne LiDAR scanning), and the backscattered signal received in return can be used to interpret information of land surface structure [24]. Backscatter is characterized by polarized modes of the transmitted and received signals (e.g., horizontal send ...

Understanding ‘saturation’ of radar signals over forests

“…4); an observation seen in the theoretical modelling of scattering regime transitions as larger stems and branches transition from Rayleigh to Mie/Optical scattering 26, 27 . Similarly, at high AGV ranges, an increase in forest height resulted in a negative AGV-backscatter trend; a similar negative trend is observed in numerous empirical studies 29, 30, 40–43 . The finding is also supported by the explanation that scattering is dominated by dense upper layers of canopy rather than deeper layers, hence decreasing trunk-ground double-bounce scattering 28 , particularly , and increasing signal extinction 25, 44 .…”

Canopy Height and Above-Ground Biomass Retrieval in Tropical Forests Using Multi-Pass X- and C-Band Pol-InSAR Data

“…Especially in tropical regions, cloud coverage is a reoccurring issue that aggravates monitoring based on multispectral satellite data. The extrapolating of accurate forest inventories or regional LiDAR-derived biomass estimations with large-scale satellite imagery represents an appropriate compromise [12][13][14][15]. Solberg et al [16] and Englhart et al [17] investigated the suitability of airborne laser scanning (ALS) for extrapolating biomass reference data from field plots.…”

Influence of Slope Aspect on above Ground Biomass Estimation using ALOS-2 Data

“…[6]- [8] studies indicated that radar backscatter saturates when the level of biomass increases to a certain point. The saturation levels depend on the wavelengths (such as C, L, Pbands), polarization, incidence angle and the characteristics of vegetation stand structure and ground conditions.…”