Carbon stocks and tree allometries in the savannahs of the Plateau Batéké, central Africa

Tree allometric equations that are used to predict the biomass of a tree from its dendrometrical characteristics are key elements for the estimation of forest carbon stocks. Few equations compared to the diversity of forest ecosystems are currently available for the tropical forests of the Congo Basin. Pantropical equations are available but a pending question is whether they remain valid for all types of forest ecosystems in central Africa. The Plateau Batéké that covers a large area across Congo, Gabon and the Democratic Republic of Congo is dominated by savannahs that contrast with the dense rain forest often found under this tropical wet climate. Four 1-ha plots were inventoried in a savannah of the Plateau Batéké in Congo. Ninety trees from the five most abundant species were destructively sampled for aboveground biomass, while belowground biomass was measured for a sub-sample of 36 of these trees. The relationship between height (H in m) and diameter at breast height (dbh, D in cm) was modelled by a Mitscherlich model: H = 6.755-5.570 exp(−0.0946 D). The relationship between biomass (B in kg) and D, H and wood density (ρ in g cm−3) was modelled as: B = 0.6229 × (ρD2H)0.7031. This locally fitted allometric model had a bias of −2%, while a commonly used pantropical equation had a bias of −38%. Nevertheless, the bias of the pantropical equation at the Batéké site was of the same order as the bias of this pantropical equation at other sites used to establish it. The shoot-root ratio was 0.272. The savannah of the Plateau Batéké had a basal area of 6.32 m2 ha−1 and an aboveground woody biomass of 11.7 Mg ha−1 while belowground woody biomass accounted for an additional 3.4 Mg ha−1. The estimate of the aboveground woody biomass dropped by 50% when using the pantropical allometric equation. Insofar as site differences in biomass allometry are negligible in the pantropical equation, so is the difference between the pantropical equation and the locally fitted biomass equation. Nevertheless, the difference between the estimate of aboveground biomass using the locally fitted equation and the pantropical equation is large enough to call for a further examination of site differences in pantropical approaches.