Abstract
While considerable research has focused on using either L-band or P-band SAR (Synthetic Aperture Radar) on their own for forest biomass retrieval, the use of the two bands simultaneously to improve forest biomass retrieval remains less explored. In this paper, we make use of L- and P-band airborne SAR and in situ data measured in the field together with laser scanning data acquired over one hemi-boreal (Remningstorp) and one boreal (Krycklan) forest study area in Sweden. We fit statistical models to different combinations of topographic-corrected SAR backscatter and forest heights estimated from PolInSAR for the biomass estimation, and evaluate retrieval performance in terms of R2 and using 10-fold cross-validation. The study shows that specific combinations of radar observables from L- and P-band lead to biomass predictions that are more accurate in comparison with single-band retrievals. The correlations and accuracies between the combinations of SAR features and aboveground biomass are consistent across the two study areas, whereas the retrieval performance varied for individual bands. P-band-based retrievals were more accurate than L-band for the hemi-boreal Remningstorp site and less accurate than L-band for the boreal Krycklan site. The aboveground biomass levels as well as the ground topography differ between the two sites. The results suggest that P-band is more sensitive to higher biomass and L-band to lower biomass forests. The forest height from PolInSAR improved the results at L-band in the higher biomass substantially, whereas no improvement was observed at P-band in both study areas. These results are relevant in the context of combining information over boreal forests from future low-frequency SAR missions such as the European Space Agency (ESA) BIOMASS mission, which will operate at P-band, and future L-band missions planned by several space agencies.
Highlights
Forests are relevant as natural carbon storage in the global carbon cycle and play an important role for climate change mitigation [1,2,3]
The forest height from PolInSAR improved the results at L-band in the higher biomass substantially, whereas no improvement was observed at P-band in both study areas
This was investigated in a boreal and a hemi-boreal forest in Sweden with a stand-level aboveground biomass ranging from 28 t ha−1 to 183 t ha−1 and 11 t ha−1 to 287 t ha−1
Summary
Forests are relevant as natural carbon storage in the global carbon cycle and play an important role for climate change mitigation [1,2,3]. After launch, expected around 2021, the BIOMASS mission will use for the first time a space borne P-band synthetic aperture radar (SAR) to retrieve forest height and biomass estimates with global coverage of tropical and partial coverage of boreal forests. This band was selected because of the good sensitivity of P-band backscatter to forest biomass in different biomes and high temporal coherence which supports complementary forest height retrieval using PolInSAR techniques [6,7,8,9,10,11,12,13,14]
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