| Abstract: |
While tropical forests store large amounts of carbon, their contribution to the Earth’s carbon balance –and consequently their role on climate change mitigation- is still uncertain [1]. Improving the estimation of biomass in tropical forests is urgently needed to bridge this gap. Such advances may come from satellite passive microwave sensors, from which aboveground carbon biomass from woody and leaf components of the canopy can be derived using Vegetation Optical Depth (VOD), a measures of the attenuation of vegetation canopy layers on microwave soil emissions. While previous studies have demonstrated the feasibility of VOD from C-band (4 to 8 GHz) and X-band (8 to 12 GHz), among others, to study forests biomass and carbon balance [2, 3], L-band (1 to 2 GHz) is expected to provide more accurate information because the sensitivity of VOD to vegetation biomass increases with decreasing frequencies [4]. Nevertheless, the improved capacity of L-band to derive biomass estimations has not yet been demonstrated over large spatial extents. The objective of this study is to evaluate the potential of biomass retrieval from VOD at L, C, and X-bands over tropical forests of Peru, southern Columbia, and Panama using VOD datasets from the Advanced Microwave Scanning Radiometer 2 (AMSR2; C and X bands [5]), and from the Soil Moisture Active Passive satellite (SMAP; L-band [6]). They are compared to Aboveground Carbon Density (ACD) benchmark measurements from airborne Lidar provided in [7, 8, 9]. Results show that VOD at L-band has a stronger statistical relationship with ACD than VOD at C and X-bands. The analysis further evaluates to what degree this pattern is consistent for different forest densities and biomass classes.
References:
[1] Baccini, A., Walker, W., Carvalho, L., Farina, M., Sulla-Menashe, D., Houghton, R.A. 2017. “Tropical forests are a net carbon source based on abovegroung measurements of gain and loss.” Science, 358(6360):230-234.
[2] Liu, Y.Y., van Dijk, A.I.J.M., McCabe, M.F., Evans, J.P., de Jeu, R.A.M. 2013. “Global vegetation biomass change (1988–2008) and attribution to environmental and human drivers.” Global Ecology and Biogeography, 22(6):692-705.
[3] Liu, Y.Y., van Dijk, A.I.J.M., de Jeu, R.A.M., Canadell, J.G., McCabe, M.F., Evans, J.P., Wang, G. 2015. “Recent reversal in loss of global terrestrial biomass.” Nature Climate Change, 5:470-474.
[4] Ulaby, F. T., Moore, R. K., & Fung, A. K. (1982). Microwave Remote Sensing Active and Passive-Volume II: Radar Remote Sensing and Surface Scattering and Enission Theory.
[5] Vrije Universiteit Amsterdam (Richard de Jeu) and NASA GSFC (Manfred Owe)(2014), AMSR2/GCOM-W1 surface soil moisture (LPRM) L3 1 day 25 km x 25 km ascending V001, Greenbelt, MD, USA, Goddard Earth Sciences Data and Information Services Center (GES DISC).
[6] Konings, A. G., Piles, M., Das, N., Entekhabi, D. 2017. “L-band vegetation optical depth and effective scattering albedo estimation from SMAP.” Remote Sensing of Environment, 198, 460-470.
[7] Asner, G.P. et al. 2012. “High-resolution mapping of forest carbon stocks in the Colombian Amazon.” Biogeosciences 9:2683-2696.
[8] Asner G.P. et al. 2013. “High-fidelity national carbon mapping for resource management and REDD+.” Carbon Balance and Management 8:7.
[9] Asner, G.P. et al. 2014. “Targeted carbon conservation at national scales with high-resolution monitoring.” Proceedings of the National Acedemy of Siences. Available at: www.pnas.org/cgi/doi/10.1073/pnas.1419550111.
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