Soil charcoal to assess the impacts of past human disturbances on tropical forests

PLoS One. 2014 Nov 12;9(11):e108121. doi: 10.1371/journal.pone.0108121. eCollection 2014.

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Agriculture*
  • Cameroon
  • Charcoal / analysis*
  • Fires
  • Forests*
  • Geography
  • Humans
  • Radiometric Dating*
  • Soil / chemistry*
  • Trees
  • Tropical Climate

Substances

  • Soil
  • Charcoal

Grant support

This study is part of the FRFC project (« Fonds pour la Recherche Fondamentale et Collective», convention n ° 2.4577.10) entitled “Dynamics of tree and herbaceous communities of the tropical forests of Central Africa, in relation with past anthropogenic and climatic disturbance”. J. Vleminckx and J. Morin-Rivat are funded by the “Fonds pour la formation à la Recherche dans l'Industrie et dans l'Agriculture (FRIA)”. The authors gratefully acknowledge the CoForChange project funded through the ERA-net BiodivERsA (http://www.coforchange.eu/fr/). The authors declare that the funders only provided financial support for the field inventories and had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.