Articles | Volume 13, issue 5
https://doi.org/10.5194/bg-13-1571-2016
https://doi.org/10.5194/bg-13-1571-2016
Research article
 | 
14 Mar 2016
Research article |  | 14 Mar 2016

Closing a gap in tropical forest biomass estimation: taking crown mass variation into account in pantropical allometries

Pierre Ploton, Nicolas Barbier, Stéphane Takoudjou Momo, Maxime Réjou-Méchain, Faustin Boyemba Bosela, Georges Chuyong, Gilles Dauby, Vincent Droissart, Adeline Fayolle, Rosa Calisto Goodman, Matieu Henry, Narcisse Guy Kamdem, John Katembo Mukirania, David Kenfack, Moses Libalah, Alfred Ngomanda, Vivien Rossi, Bonaventure Sonké, Nicolas Texier, Duncan Thomas, Donatien Zebaze, Pierre Couteron, Uta Berger, and Raphaël Pélissier

Abstract. Accurately monitoring tropical forest carbon stocks is a challenge that remains outstanding. Allometric models that consider tree diameter, height and wood density as predictors are currently used in most tropical forest carbon studies. In particular, a pantropical biomass model has been widely used for approximately a decade, and its most recent version will certainly constitute a reference model in the coming years. However, this reference model shows a systematic bias towards the largest trees. Because large trees are key drivers of forest carbon stocks and dynamics, understanding the origin and the consequences of this bias is of utmost concern. In this study, we compiled a unique tree mass data set of 673 trees destructively sampled in five tropical countries (101 trees > 100 cm in diameter) and an original data set of 130 forest plots (1 ha) from central Africa to quantify the prediction error of biomass allometric models at the individual and plot levels when explicitly taking crown mass variations into account or not doing so. We first showed that the proportion of crown to total tree aboveground biomass is highly variable among trees, ranging from 3 to 88 %. This proportion was constant on average for trees < 10 Mg (mean of 34 %) but, above this threshold, increased sharply with tree mass and exceeded 50 % on average for trees  ≥  45 Mg. This increase coincided with a progressive deviation between the pantropical biomass model estimations and actual tree mass. Taking a crown mass proxy into account in a newly developed model consistently removed the bias observed for large trees (> 1 Mg) and reduced the range of plot-level error (in %) from [−23; 16] to [0; 10]. The disproportionally higher allocation of large trees to crown mass may thus explain the bias observed recently in the reference pantropical model. This bias leads to far-from-negligible, but often overlooked, systematic errors at the plot level and may be easily corrected by taking a crown mass proxy for the largest trees in a stand into account, thus suggesting that the accuracy of forest carbon estimates can be significantly improved at a minimal cost.

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Monitoring forest carbon stocks requires understanding how resources allocation within trees varies across tree size, species and environmental conditions. Using data on tree dimensions and mass, we show that the average tree shape varies along ontogeny, with large canopy trees having a greater proportion of carbon in their crowns (up to 50 %). This variation pattern generates important bias in carbon predictions at both tree and stand levels, which can be corrected using simple crown metrics.
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