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

Abstract

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.

How Tightly Linked Are Pericopsis elata (Fabaceae) Patches to Anthropogenic Disturbances in Southeastern Cameroon?

“…On the other hand, this innovative approach allows reliable investigations, which appear not to be relevant at different perception levels. Having worked in the same study area, but at larger scales, contrary to our findings (see Figure 3), some authors could not detect correlations between the local distribution of light-demanding species and soil charcoal abundance [53].…”

“…Additional samples can be taken following contour lines as an option for narrowing down the investigation. If charcoal is widespread in forests around the globe [53][54][55], the one located in the superficial soil layers does not always reflect recent human disturbance. The upper soil layer (0-25 cm) contains a mixture of both recent and ancient charcoal [53].…”

“…If charcoal is widespread in forests around the globe [53][54][55], the one located in the superficial soil layers does not always reflect recent human disturbance. The upper soil layer (0-25 cm) contains a mixture of both recent and ancient charcoal [53]. We thus recommend as many 14 C datings as funding permits of charcoal collected within the upper soil layer.…”

“…Working in a forest concession is another innovative aspect of the methodology (but, see [53,66]). Large-scale forest management inventories have already been carried out by logging companies in Central African forests, providing a unique opportunity to identify in advance the best spots to investigate (e.g., high density of light-demanding tall trees).…”

Biochar: Pyrogenic Carbon for Agricultural Use - A Critical Review

“…The same is true for pyrogenic C in other soils where the intentionality of soil improvement is not confirmed, e.g. : Chernozems (Schmidt et al, 1999;Ponomarenko and Anderson, 2001), Mollisols (Glaser and Amelung, 2003) soil charcoal derived from wildfire or anthropic fires (Knicker, 2011;Vleminckx et al, 2014).…”