Abstract
Abstract. Spatially-explicit maps of aboveground biomass are essential for calculating the losses and gains in forest carbon at a regional to national level. The production of such maps across wide areas will become increasingly necessary as 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 more generally. However, mapping biomass over high-biomass tropical forest is challenging as (1) direct regressions with optical and radar data saturate, (2) much of the tropics is persistently cloud-covered, reducing the availability of optical data, (3) many regions include steep topography, making the use of radar data complex, (5) while LiDAR data does not suffer from saturation, expensive aircraft-derived data are necessary for complete coverage. We present a solution to the problems, using a combination of terrain-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 savanna to closed-canopy tropical forest, is topographically complex, has no recent contiguous cloud-free high-resolution optical data, and the dense forest is above the saturation point for radar. Our 100 m resolution biomass map is derived from fusing spaceborne LiDAR (7142 ICESat GLAS footprints), 96 ground-based plots (average size 0.8 ha) and an unsupervised classification of terrain-corrected ALOS PALSAR radar data, from which we derive the aboveground 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 which are independent as they were not used for the GLAS-biomass estimation. We estimate an uncertainty of ±25% on our carbon stock value for the park. This error term includes uncertainties resulting from the use of a generic tropical allometric equation, the use of GLAS data to estimate Lorey's height, and the necessity of separating the landscape into distinct classes. As there is currently no spaceborne LiDAR satellite in operation (GLAS data is available for 2003–2009 only), this methodology is not suitable for change-detection. This research underlines the need for new satellite LiDAR data to provide the potential for biomass-change estimates, although this need will not be met before 2015.
Highlights
Tropical forest ecosystems have a variety of values, monetary and otherwise, that vary markedly with the scale considered
We wished to develop a site-specific relationship between diameter at breast height (DBH) and stem height, based on 3673 stem DBH and heights measured over four a) PALSAR HH
This relationship was applied to the stems measured in 2009, allowing aboveground biomass (AGB) to be calculated from the Chave et al (2005) equation involving DBH, wood density and height
Summary
Tropical forest ecosystems have a variety of values, monetary and otherwise, that vary markedly with the scale considered. Many of these, especially the more general benefits at a larger scale, are often not included in decisions relating to whether a forest area remains forest or is converted to another land-use (Stern, 2008; Engel et al, 2008). The fate of an area of forest has tended to be controlled by the opportunity to liquidate its considerable timber value (Geist and Lambin, 2002) by destructively harvesting its trees, or clearing the land to convert it to a more productive land-use, e.g. agriculture (though in many cases local people derive no benefit from conversion (Rodrigues et al, 2009)). The 2010 Cancun Agreement, agreed at the UNFCCC COP-16 Conference, has created an international framework for valuing forest carbon within the Reducing Emissions from Deforestation and forest Degradation (REDD+) scheme. REDD+ transfers through government-government agreements and through the voluntary sector have already started, and are accelerating (Clements et al, 2010)
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