Contribution of new radiocarbon dates to track the impact of past anthropogenic disturbances on current vegetation in Central Africa

Poster (2012, July)

Introduction: Understanding current Central Africa vegetation patterns faces the scarcity of data about their past evolution. However, a growing hypothesis suggests that past human activities could have ... [more ▼]

Introduction: Understanding current Central Africa vegetation patterns faces the scarcity of data about their past evolution. However, a growing hypothesis suggests that past human activities could have had a substantial influence on vegetation (Van Gemerden et al. 2003, Brncic et al. 2009). Indeed, by creating large openings (fig. 1), they might have triggered the expansion of light-demanding species currently suffering from a lack of regeneration. This lack of saplings could have been caused by the change in land use since colonization with the sedentarization of shifting cultivators. Aim of the study: To investigate the potential relationship between past anthropogenic disturbances and present vegetation. Material & methods: Fieldworks combining anthracological and ecological approaches have been undertaken in Northern Congo and South-Eastern Cameroon (fig. 2). We excavated thirty 150-200 cm deep pits under different forest covers and vegetation types to identify evidences of past human presence (i.e potsherds, fig. 3, slags from metallurgy, anthropogenic pieces of charcoals and anthropophilous charred seeds). Charcoals and seeds (oil palm Elaeis guineensis, fig. 4, Canarium schweinfurthii) in combination with artifacts have been dated. Results: A set of 38 new radiocarbon dates ranging from 15,200 cal BP to present time have been obtained. They confirm the existence of important past fire events in a region where natural ones seldom occur. Together with artifacts, our findings support the few already available dates documenting evidences of past human activities in Central African rainforests. On the 36 most recent dates (fig. 5), the majority belongs to the 2,300-1,400 cal BP period (61%). The whole semi-deciduous forest zone is concerned by this period of intense disturbances with a high rate of fragmentation. It follows the last great arid phase ca. 2,500 cal BP and might be linked to the iron workers expansion. Another pool of dates between 650 and 250 cal BP (33%) associated with potsherds might be correlated to a dry phase contemporary to the Little Ice Age in Europe. That last group of dates points out the potential positive impact of anthropogenic disturbances connected to a dry climatic event on light-demanding species populations. Indeed, this is consistent with the fact that most of current light-demanding trees have a higher number of stems around 100 cm dbh. Conclusion: Our multidisciplinary approach allowed new insights into the link between human history and vegetation dynamics in Central Africa. Further investigations should be conducted to go deeper into the understanding of the evolution of Central African rainforests and to improve the management of currently logged light-demanding species resulting from the LIA period. Thirty new dates from sixteen soil profiles are forthcoming. References: Brncic T., Willis M., K. J., Harris D. J., Telfer M. W. & Bailey M. W. 2009. Fire and climate change impacts on lowland forest composition in northern Congo during the last 2580 years from palaeoecological analyses of a seasonally flooded swamp. The Holocene, 19, 79-89. Reimer P. J., Baillie M. G. L., Bard E., Bayliss A., Beck J. W., Blackwell P. G., Bronk Ramsey C., Buck C. E., Burr G. S., Edwards R. L., Friedrich M., Grootes P. M., Guilderson T. P., Hajdas I., Heaton T. J., Hogg A. G., Hughen K. A., Kaiser K. F., Kromer B., McCormac F. G., Manning S. W., Reimer R. W., Richards D. A., Southon J.R., Talamo S., Turney C. S. M., van der Plicht J., & Weyhenmeyer, C. E. 2009. IntCal09 and Marine09 radiocarbon age calibration curves, 0-50,000 years cal BP. Radiocarbon, 51(4), 1111-1150. Van Gemerden B. S., Olff H., Parren M. P. E, Bongers F. 2003. The pristine rain forest? Remnants of historical human impacts on current tree species composition and diversity. Journal of Biogeography, 30, 1381-1390. [less ▲]

Perturbations récentes dans le Bassin du Congo : Contribution de l’anthracologie à une restitution paléoenvironnementale

Poster (2012, May 22)

Predicting alpha diversity of African rain forests: models based on climate and satellite-derived data do not perform better than a purely spatial model

in Journal of Biogeography (2011), 38(6), 1164-1176

Aim Our aim was to evaluate the extent to which we can predict and map tree alpha diversity across broad spatial scales either by using climate and remote sensing data or by exploiting spatial ... [more ▼]

Aim Our aim was to evaluate the extent to which we can predict and map tree alpha diversity across broad spatial scales either by using climate and remote sensing data or by exploiting spatial autocorrelation patterns. Location Tropical rain forest, West Africa and Atlantic Central Africa. Methods Alpha diversity estimates were compiled for trees with diameter at breast height ‡ 10 cm in 573 inventory plots. Linear regression (ordinary least squares, OLS) and random forest (RF) statistical techniques were used to project alpha diversity estimates at unsampled locations using climate data and remote sensing data [Moderate Resolution Imaging Spectroradiometer (MODIS), normalized difference vegetation index (NDVI), Quick Scatterometer (QSCAT), tree cover, elevation]. The prediction reliabilities of OLS and RF models were evaluated using a novel approach and compared to that of a kriging model based on geographic location alone. Results The predictive power of the kriging model was comparable to that of OLS and RF models based on climatic and remote sensing data. The three models provided congruent predictions of alpha diversity in well-sampled areas but not in poorly inventoried locations. The reliability of the predictions of all three models declined markedly with distance from points with inventory data, becoming very low at distances > 50 km. According to inventory data, Atlantic Central African forests display a higher mean alpha diversity than do West African forests. Main conclusions The lower tree alpha diversity in West Africa than in Atlantic Central Africa may reflect a richer regional species pool in the latter. Our results emphasize and illustrate the need to test model predictions in a spatially explicit manner. Good OLS or RF model predictions from inventory data at short distance largely result from the strong spatial autocorrelation displayed by both the alpha diversity and the predictive variables rather than necessarily from causal relationships. Our results suggest that alpha diversity is driven by history rather than by the contemporary environment. Given the low predictive power of models, we call for a major effort to broaden the geographical extent and intensity of forest assessments to expand our knowledge of African rain forest diversity. [less ▲]

The ayous Triplochiton scleroxylon K. Schum., large commercial and anthropogenic tree species of African moist semi-deciduous dense forest, on natural decline

Poster (2009)

The origin of the forests in the North of the Congo Republic: an anthraco-pedological contribution.

in Damblon, F.; Court-Picon, M. (Eds.) Geological survey of Belgium professional papers N. 303-2008/1 (2008)

The vegetation of the Sangha region in the North of the Congo Republic is composed of a mosaic of forest formations including semi-deciduous humid dense forests characterized by Ulmaceae and Sterculiaceae ... [more ▼]

The vegetation of the Sangha region in the North of the Congo Republic is composed of a mosaic of forest formations including semi-deciduous humid dense forests characterized by Ulmaceae and Sterculiaceae, evergreen forests with Gilbertiodendron dewevrei and woodland with Marantaceae. Palynological analyses proved that this region witnessed an arid phase 2500 years ago. This was associated with an important reduction of the forest cover and an equal extension of savannahs that could have facilitated the migrations of the Bantou people and the propagation of the shifting cultivation agriculture in the whole region which is, still at the moment, sparsely populated. In order to verify climatic and anthropological influences on the present composition of the forest vegetation, a pedological investigation has been set up in the region of Pokola (1.5° N; 16.5 °E) comprising 43 drillings and 12 pits along a gradient covering the different forest types. Charcoal has been found in 84% of the cores in all the forest formations especially between 30 cm and 60 cm depth. Charred oil palm seeds (Elaeis guineensis), often a culture accompanying species, have been found in 60% of the pits. C14 analysis revealed dates between 600 and 2200 years BP. Pedological analyses showed the association between semi-deciduous humid dense forest and red ferralsols, between Marantaceae woodlands and ochre ferralsols (poorer than the former) and between Gilbertiodendron dewevrei stands and pseudogleys and gleysols. The identification of the botanical taxon of charcoal fragments from tropical regions deals with important limitations due to scarce reference collections that are far from being complete. Therefore it is crucial to combine a thorough knowledge of the different vegetation types and their character species with a wood anatomical study. Given the hypothesised strong influence of man induced perturbations and past climatic changes, these results emphasize the importance of edaphic factors in differentiating the forest stands in the North of the Congo Republic. They also permit a better understanding of a potential evolution of the forests in this region driven by the actual climatic changes. [less ▲]