Mapping tropical forest biomass with radar andspaceborne LiDAR in Lopé National Park, Gabon: overcoming problems of high biomass and persistent cloud

[摘要] Spatially-explicit maps of aboveground biomass are essential for calculatingthe losses and gains in forest carbon at a regional to national level. Theproduction of such maps across wide areas will become increasingly necessaryas international efforts to protect primary forests, such as the REDD+(Reducing Emissions from Deforestation and forest Degradation) mechanism,come into effect, alongside their use for management and research moregenerally. However, mapping biomass over high-biomass tropical forest ischallenging as (1) direct regressions with optical and radar data saturate,(2) much of the tropics is persistently cloud-covered, reducing theavailability of optical data, (3) many regions include steep topography,making the use of radar data complex, (5) while LiDAR data does not sufferfrom saturation, expensive aircraft-derived data are necessary for completecoverage.

We present a solution to the problems, using a combination ofterrain-corrected L-band radar data (ALOS PALSAR), spaceborne LiDAR data(ICESat GLAS) and ground-based data. We map Gabon's Lopé National Park(5000 km²) because it includes a range of vegetation types from savannato closed-canopy tropical forest, is topographically complex, has no recentcontiguous cloud-free high-resolution optical data, and the dense forest isabove the saturation point for radar. Our 100 m resolution biomass map isderived from fusing spaceborne LiDAR (7142 ICESat GLAS footprints), 96ground-based plots (average size 0.8 ha) and an unsupervised classificationof terrain-corrected ALOS PALSAR radar data, from which we derive theaboveground biomass stocks of the park to be 78 Tg C (173 Mg C ha⁻¹).This value is consistent with our field data average of 181 Mg C ha⁻¹,from the field plots measured in 2009 covering a total of 78 ha, and whichare independent as they were not used for the GLAS-biomass estimation. Weestimate an uncertainty of ±25% on our carbon stock value for thepark. This error term includes uncertainties resulting from the use of ageneric tropical allometric equation, the use of GLAS data to estimateLorey's height, and the necessity of separating the landscape into distinctclasses.

As there is currently no spaceborne LiDAR satellite in operation (GLAS datais available for 2003–2009 only), this methodology is not suitable forchange-detection. This research underlines the need for new satellite LiDARdata to provide the potential for biomass-change estimates, although thisneed will not be met before 2015.

[学科分类] 地球化学与岩石