Abstract
AimThe accurate mapping of forest carbon stocks is essential for understanding the global carbon cycle, for assessing emissions from deforestation, and for rational land-use planning. Remote sensing (RS) is currently the key tool for this purpose, but RS does not estimate vegetation biomass directly, and thus may miss significant spatial variations in forest structure. We test the stated accuracy of pantropical carbon maps using a large independent field dataset.LocationTropical forests of the Amazon basin. The permanent archive of the field plot data can be accessed at: http://dx.doi.org/10.5521/FORESTPLOTS.NET/2014_1MethodsTwo recent pantropical RS maps of vegetation carbon are compared to a unique ground-plot dataset, involving tree measurements in 413 large inventory plots located in nine countries. The RS maps were compared directly to field plots, and kriging of the field data was used to allow area-based comparisons.ResultsThe two RS carbon maps fail to capture the main gradient in Amazon forest carbon detected using 413 ground plots, from the densely wooded tall forests of the north-east, to the light-wooded, shorter forests of the south-west. The differences between plots and RS maps far exceed the uncertainties given in these studies, with whole regions over- or under-estimated by > 25%, whereas regional uncertainties for the maps were reported to be < 5%.Main conclusionsPantropical biomass maps are widely used by governments and by projects aiming to reduce deforestation using carbon offsets, but may have significant regional biases. Carbon-mapping techniques must be revised to account for the known ecological variation in tree wood density and allometry to create maps suitable for carbon accounting. The use of single relationships between tree canopy height and above-ground biomass inevitably yields large, spatially correlated errors. This presents a significant challenge to both the forest conservation and remote sensing communities, because neither wood density nor species assemblages can be reliably mapped from space.
Highlights
Amazonia contains half of all remaining tropical moist forest (Fritz et al, 2003)
The field plots with our best estimate of aboveground biomass (AGB) (PDHρ) show a robust trend of increasing AGB with increasing latitude, longitude and distance along a SW–NE line (Fig. 2a–c; the parameters of the best-fit lines are given in Table S1; input plot biomass data are available in Lopez-Gonzalez et al, 2014)
The two remote-sensing maps, RS1 (Saatchi et al, 2011) and RS2 (Baccini et al, 2012), show very different spatial patterns of AGB distribution across Amazonia, compared to each other and compared to field plots distributed across the region (Figs 2 & 3)
Summary
The total vegetation carbon storage of Amazon basin tropical forests has been subject to a wide range of estimates (Houghton et al, 2001; Malhi et al, 2006; Saatchi et al, 2007) These have varied from 58 Pg C (Olson et al, 1983) to 134 Pg C (Fearnside, 1997, scaled to whole basin), there is some general consensus in the middle of this range [e.g. 93 ± 23 Pg C (Malhi et al, 2006), 86 ± 17 Pg C (Saatchi et al, 2007) and 89 Pg C (FAO, 2010)]. Though REDD+ is not yet operational, voluntary-sector afforestation/reforestation and REDD+ projects already exist, with REDD+ credit sales equal to $85 million
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