ARTICLE Received 31 Jan 2013 | Accepted 9 Jul 2013 | Published 5 Aug 2013 DOI: 10.1038/ncomms3269 Conventional tree height–diameter relationships significantly overestimate aboveground carbon stocks in the Central Congo Basin Elizabeth Kearsley1,2,3, Thales de Haulleville3,4, Koen Hufkens2, Alide´ Kidimbu5,6, Benjamin Toirambe3, Geert Baert7, Dries Huygens2,8,9, Yodit Kebede10,11, Pierre Defourny10, Jan Bogaert4, Hans Beeckman3, Kathy Steppe1, Pascal Boeckx2 & Hans Verbeeck1 Policies to reduce emissions from deforestation and forest degradation largely depend on accurate estimates of tropical forest carbon stocks. Here we present the first field-based carbon stock data for the Central Congo Basin in Yangambi, Democratic Republic of Congo. We find an average aboveground carbon stock of 162±20 Mg C ha À 1 for intact old-growth forest, which is significantly lower than stocks recorded in the outer regions of the Congo Basin. The best available tree height–diameter relationships derived for Central Africa do not render accurate canopy height estimates for our study area. Aboveground carbon stocks would be overestimated by 24% if these inaccurate relationships were used. The studied forests have a lower stature compared with forests in the outer regions of the basin, which confirms remotely sensed patterns. Additionally, we find an average soil carbon stock of 111±24 Mg C ha À 1, slightly influenced by the current land-use change. 1 Laboratory of Plant Ecology, Department of Applied Ecology and Environmental Biology, Faculty of Bioscience Engineering, Ghent University, Ghent 9000, Belgium. 2 Isotope Bioscience Laboratory—ISOFYS, Department of Applied Analytical and Physical Chemistry, Faculty of Bioscience Engineering, Ghent University, Ghent 9000, Belgium. 3 Laboratory for Wood Biology and Xylarium, Royal Museum for Central Africa, Tervuren 3080, Belgium. 4 Biodiversity and Landscape Unit, Gembloux Agro-Bio Tech, University of Lie`ge, Gembloux 5030, Belgium. 5 De´partement d’Ecologie et Gestion des Ressources Ve´ge´tales, Faculte´ des Sciences, Universite´ de Kisangani, BP 2012 Kisangani, Democratic Republic of Congo. 6 Herbarium National, INERA, BP 28 Yangambi, Democratic Republic of Congo. 7 Department of Plant Production, University College Ghent, Ghent 9000, Belgium. 8 Institute of Agricultural Engineering and Soil Science, Faculty of Agricultural Sciences, Universidad Austral de Chile, Valdivia 5090000, Chile. 9 Instituto Multidisciplinario de Biologı´a Vegetal - IMBIV, Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas de Argentina, Facultad de Ciencias Exactas, Fı´sicas y Naturales, Universidad Nacional de Co´rdoba, Co´rdoba 5000, Argentina. 10 Earth and Life Institute, Universite´ catholique de Louvain, Louvain-la-Neuve 1348, Belgium. 11 Wageningen University and Research Centre, Farming Systems Ecology Group, 6708 PG Wageningen, The Netherlands. Correspondence and requests for materials should be addressed to E.K. (email: [email protected]). NATURE COMMUNICATIONS | 4:2269 | DOI: 10.1038/ncomms3269 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3269 ropical forests represent a crucial carbon pool as they, regions15,16, with its strength lying in the large sample size of reportedly, store 40–50% of all terrestrial carbon1,2 and tropical trees compared with the other equations. Nevertheless, Tinfluence the global carbon balance when changes are the use of pan-tropical equations12,13 in unstudied areas needs to induced due to climate change3 and anthropogenic disturbances. be done with care as it could produce systematic errors in carbon In order to understand and quantify tropical forest carbon uptake stock estimates, specifically if not all variables, namely diameter, and losses, accurate carbon stock reference data are needed4, wood density and tree height, are accounted for. as well as estimates of carbon fluxes resulting from both Here we report the first inventory-based carbon data obtained natural processes and land-use changes1,5. The accuracy of from a central location in the Congo Basin, situated in the these reference data is essential for the implementation of UNESCO Man and Biosphere (MAB) reserve at Yangambi (YGB), climate mitigation policies to reduce emissions resulting from Democratic Republic of Congo (DRC) (Fig. 1), with estimates for deforestation and forest degradation (REDD, UN Framework both aboveground live tree and soil carbon stocks. Thirteen Convention on Climate Change, www.un-redd.org)6,7. Currently, 1-ha plots are established in intact old-growth forest, including the uncertainty associated with these data is high and there are two forest types namely, mixed semideciduous (n ¼ 8) and still large gaps in our knowledge with respect to the actual riparian mono-dominant evergreen Gilbertiodendron dewevrei amount of carbon stored in tropical forests and the stability of (n ¼ 5) forests. In the mixed forests, the species Scorodophloeus this pool4. zenkeri, Panda oleosa, Anonidium mannii, Petersianthus macro- In African tropical forests, in particular, there are major carpus, Staudtia kamerunensis and Erythrophleum suaveolens are challenges with respect to estimating carbon stocks and fluxes. abundant. In addition, to assess the effect of deforestation and Saatchi et al.8 developed a benchmark map of carbon stocks regrowth at local scales, seven plots of 1 ha are established in across the tropics, predicting a gradient in Central Africa with previously deforested areas of mixed semideciduous forests, with highest stocks in western and eastern parts of the Congo Basin. varying regrowth ages after slash-and-burn agriculture, namely This gradient is not always predicted9, and more data is needed fallow (n ¼ 3) next to young (n ¼ 3) and old (n ¼ 1) regrowth for effective mapping, which is dependent on spatially extensive forest. Soil carbon stocks to a depth of 100 cm are quantified for ground-based measurements to reduce regional uncertainties. To all forest types except for the old regrowth forest. A comparison date, African tropical forests have been underrepresented in of aboveground carbon stocks in old-growth forests is made with carbon research mainly due to a lack of resources for scientific AfriTRON sites11 in western and eastern Congo Basin, in order to research, limited logistical support, poor infrastructure and assess whether our first ground-based observations for this periodic political instability9,10. Only a limited number of important region confirm the spatial trends observed by remote inventoried and permanent monitoring plots have been sensing8,9 and to quantify possible intrinsic differences in carbon established given the considerable extent of the African tropical stocks. forest11. The few available inventoried plots are unevenly distributed, with a high concentration in the outer regions of the Congo Basin, leaving the centre of the basin unexplored10,11. Results Biomass estimates for these plots have been extrapolated to the YGB aboveground carbon stocks are lower. With an above- entire African tropical forest based on pan-tropical allometric ground carbon stock in live free-standing woody stems (mini- equations12,13. These equations are generally parameterized mum diameter 10 cm) in old-growth forest at YGB of without data from the African continent due to the lack 162±20 Mg C ha À 1, our estimates are 19% lower and sig- thereof13. Only the study by Feldpausch et al.14 has recently nificantly different (Po0.001, Df ¼ 2) from estimates at the expanded the frequently used pan-tropical Chave et al.13 equation AfriTRON sites11 (Fig. 2) in the Congo Basin. Lewis et al.11 with African data from the western Congo Basin. The robustness report an average of 202 Mg C ha À 1 for African tropical and accuracy of this equation has been noted for some African forest, and an even higher average in the Congo Basin, with 5 10152025303540 10 10 Cameroon [EJA] Cameroon [DJA−M] Cameroon [DJK] Cameroon [DJA] Cameroon [DJA−A] Central African Repuplic [BKK−LLL] 5 5 Cameroon [DJA−D] DR Congo [EDO−ITU] ) ° Cameroon [CAM] DR Congo [LEN] DR Congo [YGB] Latitude ( 0 0 Gabon [OVG] Gabon [LOP−LWW] Gabon [EKO] Gabon [MAK] −5 −5 510152025303540 Longitude (°) Figure 1 | Site locations. MODIS composite (NASA, Blue Marble) with the YGB study site and AfriTRON sites superimposed11. The sites are separated into different regions: squares indicate western Congo Basin sites, circles indicate eastern Congo Basin sites, and the cross indicates the Central Congo Basin site in YGB. In the scale bar, one block equals 250 km. 2 NATURE COMMUNICATIONS | 4:2269 | DOI: 10.1038/ncomms3269 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3269 ARTICLE Cameroon−CAM observed lower aboveground carbon stocks, we compared our Cameroon−DJA data in more detail with datasets from sites in the Dja Faunal Cameroon−DJA−M Reserve in Cameroon in the west of the Congo Basin and in the Ituri Dynamics Plots in the east, with a distinction of mixed and Cameroon−DJA−A mono-dominant G. dewevrei forest types (Table 2). For these Cameroon−DJA−D areas, carbon stocks, basal area, stem density and wood density Cameroon−DJK were available from literature, but explicit height data are lacking. Cameroon−EJA In addition, data on soil and climate are available. By comparing C.Af.Rep.−BKK−LLL mixed and mono-dominant plots separately, we were able to get a Gabon−EKO