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Agreement between floristic and soil organic carbon isotope (13C/12C, 14C) indicators of forest invasion of savannas during the last century in Cameroon

Published online by Cambridge University Press:  27 November 2001

BERNARD GUILLET
Affiliation:
Université d'Orléans et CNRS, ISTO-Géochimie Organique, BP 6759, 45067 Orléans 02, France
GASTON ACHOUNDONG
Affiliation:
Herbier National du Cameroun, BP 1601, Yaoundé, Cameroun
JOSEPH YOUTA HAPPI
Affiliation:
Université de Yaoundé, Département de Géographie, BP 1857, Yaoundé, Cameroun
VÉRONIQUE KAMGANG KABEYENE BEYALA
Affiliation:
IRGM, BP 4110, Yaoundé, Cameroun
JACQUES BONVALLOT
Affiliation:
IRD, Centre de Recherche de l'Ile de France, 32 avenue Henri Varagnat, 93143 Bondy cedex, France
BERNARD RIERA
Affiliation:
URA 1183 du CNRS, MNHN, Laboratoire d'Ecologie Générale, 4, avenue du Petit Château, 91800 Brunoy, France
ANDRÉ MARIOTTI
Affiliation:
Université Pierre et Marie Curie, Laboratoire de Biogéochimie des Isotopes, Case 120, 4 place Jussieu, 75252 Paris cedex 05, France
DOMINIQUE SCHWARTZ
Affiliation:
Université Louis Pasteur, CEREG, 3 rue de l'Argonne, 67083 Strasbourg cedex, France

Abstract

In an area of savanna-forest (S-F) mosaic of Cameroon, at Kandara near Bertoua, an enclosed savanna bordered by young semi-deciduous forests was selected for detailed studies of vegetation and soil carbon isotope compositions with a view to estimating the rate of forest advance into savannas. Forest floristic composition and structure were analysed in small plots along two S-F transects and within two large stands. Tree species counts and basal area (BA) measurements gave convergent results defining (1) an edge forest with low BA values that forms an irregular strip parallel to the S-F border, (2) a large colonization zone zone composed of pioneer species (Albizia species, with individuals of very large diameter) and (3) a mature forest composed of abundant Rinorea individuals and large individuals of Triplochiton scleroxylon and Piptadeniastrum africanum. Carbon stable isotopes were deter mined from organic matter of soil profiles sampled at various depths in savanna, colonization-zone and mature forest. In the deep soil horizons (40-50 cm) of the colonization-zone profiles, δ13C values similar to those of the present savanna reveal the past existence of a large-tree savanna. In subsurface horizons (15-20 cm), δ13C values intermediate between those of savanna and mature forest prove the encroachment of the forest ecosystem on savanna. Using 14C measurements, the mean residence time (MRT) of soil organic matter of these last horizons was determined with precision owing to the atmospheric 14C pulse from nuclear bomb tests prior to 1964. By assuming an exponential age distribution of organic compounds and by taking account of MRT and remaining carbon from the savanna, the coloniza tion zone was found to be 60-80 y old. The age of the colonization zone being the same near the present savanna and near the mature forest, it seems that the forest advance was probably not a linear process but would result from the coalescence of Albizia thickets born in savanna.

Type
Research Article
Copyright
2001 Cambridge University Press

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Agreement between floristic and soil organic carbon isotope (13C/12C, 14C) indicators of forest invasion of savannas during the last century in Cameroon
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