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The persistence of carbon in the African forest understory

Abstract

Quantifying carbon dynamics in forests is critical for understanding their role in long-term climate regulation1,2,3,4. Yet little is known about tree longevity in tropical forests3,5,6,7,8, a factor that is vital for estimating carbon persistence3,4. Here we calculate mean carbon age (the period that carbon is fixed in trees7) in different strata of African tropical forests using (1) growth-ring records with a unique timestamp accurately demarcating 66 years of growth in one site and (2) measurements of diameter increments from the African Tropical Rainforest Observation Network (23 sites). We find that in spite of their much smaller size, in understory trees mean carbon age (74 years) is greater than in sub-canopy (54 years) and canopy (57 years) trees and similar to carbon age in emergent trees (66 years). The remarkable carbon longevity in the understory results from slow and aperiodic growth as an adaptation to limited resource availability9,10,11. Our analysis also reveals that while the understory represents a small share (11%) of the carbon stock12,13, it contributes disproportionally to the forest carbon sink (20%). We conclude that accounting for the diversity of carbon age and carbon sequestration among different forest strata is critical for effective conservation management14,15,16 and for accurate modelling of carbon cycling4.

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Data availability

The input data and R-scripts to generate the figures and tables are available for download using the following private link: https://figshare.com/s/06c793575d3b52ef5574. Images of wood cores are available using the following link: https://figshare.com/s/e6101fe7d330f8ea140a. This folder also contains all annotation documents needed to visualize growth ring boundaries on the wood samples (please consult the README document for guidelines). Wood samples used to conduct this analysis are stored in the Tervuren xylarium (http://www.africamuseum.be/collections/browsecollections/naturalsciences/earth/xylarium). These samples may be studied, within the Tervuren xylarium, on request addressed to the curator H.B. (hans.beeckman@africamuseum.be) or the corresponding author W.H. (whubau@gmail.com).

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Acknowledgements

Nkulapark: W.H. and T.D.M. were both funded by the Brain programme of the Belgian Federal Government (BR/132/A1/AFRIFORD and BR/143/A3/HERBAXYLAREDD). The PhD project of T.D.M and the tenure track of J.V.d.B. were supported by Ghent University Special Research Fund (BOF). Fieldwork was sponsored by the King Leopold III fund for nature exploration and conservation. B.A.I. is supported by the Institut National pour l’Étude et la Recherche Agronomiques en R.D.Congo (INERA- RDC- Luki) and the École Régionale Postuniversitaire d’Aménagement et de Gestion intégrés des Forêts et Territoires tropicaux (ERAIFT Kinshasa). We thank WWF-RDC (G. Lejeune), INERA and ERAIFT for facilitating fieldwork in the Luki Reserve. We thank the INERA employees (J.-B. Ndunga, J.-M., F. Mbungu Phaka, L. Ngoma, P. Noble), the WWF ecoguards and the students of the Universities of Kinshasa (UNIKIN) and Boma for assistance in the field. For assistance with datasets we thank M. De Groot, K. Lievens, P. Dekeyser, S. Willen and J. Kempenaers. The 23 permanent inventory plots: This paper is also a product of the AfriTRON network, for which we are indebted to hundreds of institutions, field assistants and local communities for establishing and maintaining the plots. This network has been supported by the European Research Council (291585, ‘T-FORCES’ – Tropical Forests in the Changing Earth System, Advanced Grant to O.L.P. and S.L.L.), the Gordon and Betty Moore Foundation, the David and Lucile Packard Foundation, the European Union’s Seventh Framework Programme (no. 283080, ‘GEOCARBON’) and Natural Environment Research Council (NERC) Consortium Grant ‘TROBIT’ (no. NE/D005590/1), ‘BIO-RED’ (no. NE/N012542/1) and a NERC New Investigators Grant, the Royal Society, the Centre for International Forestry (CIFOR) and Gabon’s National Parks Agency (ANPN). We are indebted to the University of Yaounde I, the National Herbarium of Yaounde, Rougier-Gabon, the Marien Ngouabi University of Brazzaville, WCS-Congo, Salonga National Park, WCS-D.R.Congo and the University of Kisangani for logistical support in Africa.

Author information

W.H., T.D.M., J.V.d.B., J.V.A. and H.B. conceived and designed the Nkulapark study and S.L.L. conceived the AfriTRON plot network. T.D.M. and B.A.I. coordinated collection of Nkulapark data and wood cores. T.D.M. and J.V.d.B. measured growth ring series. W.H. carried out the data analysis and wrote the paper. S.L.L., O.L.P., T.R.B. and Y.M. conceived the ForestPlots.net database, and most co-authors helped collecting AfriTRON forest census data. S.L.L., B.S., S.K.B., A.C.S., W.H., T.S., T.R.F., T.S., C.E.N.E. and L.W.W. coordinated forest plots data collection. M.J.P.S., G.L.G., S.L.L., O.L.P., T.R.B. and G.P. contributed tools to analyse and curate data. All co-authors commented on or approved the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to Wannes Hubau.

Supplementary information

  1. Supplementary Information

    Supplementary Figures 1–4

  2. Reporting Summary

  3. Supplementary Table 1

    Number of growth rings, growth-ring formation rate, tree age and mean carbon age of all 55 Nkulapark trees with 1,948 nail traces used for this analysis

  4. Supplementary Table 2

    List of all 23 plots included in this analysis, with geographic coordinates, plot size, dates of first and last census, and the main researchers for each plot

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Fig. 1: Example of a wood core (Greenwayodendron suaveolens, TreeID 765) showing the 1948 nail trace.
Fig. 2: Variation in tree age inferred from growth-ring patterns in 55 trees where nail traces accurately mark the year 1948.
Fig. 3: Estimation of mean carbon age and contribution to above-ground carbon (AGC) sink per crown illumination category for 23 permanent inventory plots in Central Africa.