Achille Biwole

Soil Charcoal to Assess the Impacts of Past Human Disturbances on Tropical Forests

The canopy of many central African forests is dominated by light-demanding tree species that do not regenerate well under themselves. The prevalence of these species might result from ancient slash-and-burn agricultural activities that created large openings, while a decline of these activities since the colonial period could explain their deficit of regeneration. To verify this hypothesis, we compared soil charcoal abundance, used as a proxy for past slash-and-burn agriculture, and tree species composition assessed on 208 rainforest 0.2 ha plots located in three areas from Southern Cameroon. Species were classified in regeneration guilds (pioneer, non-pioneer light-demanding, shade-bearer) and characterized by their wood-specific gravity, assumed to reflect light requirement. We tested the correlation between soil charcoal abundance and: (i) the relative abundance of each guild, (ii) each species and family abundance and (iii) mean wood-specific gravity. Charcoal was found in 83% of the plots, indicating frequent past forest fires. Radiocarbon dating revealed two periods of fires: “recent” charcoal were on average 300 years old (up to 860 BP, n = 16) and occurred in the uppermost 20 cm soil layer, while “ancient” charcoal were on average 1900 years old (range: 1500 to 2800 BP, n = 43, excluding one sample dated 9400 BP), and found in all soil layers. While we expected a positive correlation between the relative abundance of light-demanding species and charcoal abundance in the upper soil layer, overall there was no evidence that the current heterogeneity in tree species composition can be explained by charcoal abundance in any soil layer. The absence of signal supporting our hypothesis might result from (i) a relatively uniform impact of past slash-and-burn activities, (ii) pedoturbation processes bringing ancient charcoal to the upper soil layer, blurring the signal of centuries-old Human disturbances, or (iii) the prevalence of other environmental factors on species composition.

High spatial resolution of late-Holocene human activities in the moist forests of Central Africa using soil charcoal and charred botanical remains

Palaeoecological and archaeological studies have demonstrated that human populations have long inhabited the moist forests of central Africa. However, spatial and temporal patterns of human activities have hardly been investigated with satisfactory accuracy. In this study, we propose to characterize past human activities at local scale by using a systematic quantitative and qualitative methodology based on soil charcoal and charred botanical remains. A total of 88 equidistant test-pits were excavated along six transects in two contrasting forest types in southern Cameroon. Charred botanical remains were collected by water-sieving and sorted by type (wood charcoals, oil palm endocarps and unidentified seeds). A total of 50 Accelerator Mass Spectrometry 14C dates were also obtained. Results showed that charred macroremains were found at multiple places in the forest, suggesting scattered human activities, which were distributed into two main periods (Phase A: 2300–1300 BP; Phase B: 580 BP to the present). Charred botanical remains indicated two types of land-use: (1) domestic, with oil palm endocarps most often associated with potsherds (villages) and (2) agricultural, with charcoal as probable remnant of slash-and-burn cultivation (fields). Oil palm endocarp abundance decreased with distance from the identified human settlements. Our methodology allowed documenting, at high resolution, the spatial and temporal patterns of human activities in central African moist forests and could be applied to other tropical contexts.