Ancient origin of a rainforest in Gabon as revealed by carbon isotope data of vegetation and soil

Abstract

Holocene climate change caused disturbances of the Central African rainforest belt, with decrease in forest area during drier conditions followed by expansion during wetter periods. Today, human impact is a driver for forest degradation in the Congo Basin. For assessing the quality and quantity of forest degradation, information on biodiversity and carbon stocks from reference ecosystems is needed which (1) were not affected by Holocene climate change and (2) lack direct human impacts like logging or shifting cultivation. The Birougou Mountains in Gabon, which were devoid of direct human impact over the last centuries, may resemble such a reference ecosystem. This paper tests the hypothesis that the Birougou Mountains served as refugium for rainforest vegetation during periods of Holocene climate change. For 34 forest plots within the Birougou Mountains and 27 savannah plots within an adjacent forest savannah mosaic, we present stable carbon isotope data from current vegetation cover, organic inputs to soil organic carbon (SOC), bulk SOC and the humic acid fraction of SOC. Additionally, we estimated the mean age of the SOC humic acid fraction by 14 C dating. The data indicated a continuous forest cover in the Birougou Mountains since -at least -the Holocene climate optimum, but a change from forest to savannah vegetation in the forest savannah mosaic. Results confirm the forest of the Birougou Mountains as a refugium of rainforest vegetation and suitable reference for the natural potential regarding biodiversity and ecosystem carbon stocks.

What determines plant species diversity in Central Africa?

“…The centres of species richness and of weighted endemism presented in this thesis coincide well with previously hypothesized LGM forest refugia in the Crystal Mountains, western parts of the Chaillu Massif and Doudou Mountains (Maley, 1996, Pietsch & Gautam, 2013, Sosef, 1996. In order to further improve the understanding of Quaternary rainforest dynamics in Africa, studies on the location of hypothesized LGM forest refugia are recommended, preferably by inferring the LGM species richness pattern using SDMs.…”

“…In the past 2.5 million years, 21 cycles of global glaciations took place: 9 major and 12 minor. From fossil pollen records and carbon isotope data of vegetation and soils it is concluded that, in response to these climate changes, rainforests contracted into forest refugia during drier and cooler glacials and expanded again during wetter and warmer interglacials (Maley, 1996, Pietsch & Gautam, 2013, Plana, 2004. Where contraction of the rainforests resulted in a reduction of surface by 54% for Amazonian rainforests, African rainforests were reduced by as much as 84% (Anhuf et al, 2006).…”

“…Where contraction of the rainforests resulted in a reduction of surface by 54% for Amazonian rainforests, African rainforests were reduced by as much as 84% (Anhuf et al, 2006). The location of the forest refugia has been identified based on the composition of pollen in lake sediments (Maley, 1996), carbon isotope data (Pietsch & Gautam, 2013), and the presence of endemic species (White, 1979), or species that are known to be bad dispersers, such as e.g. Begonia species (Sosef, 1994), Caesalpinioid legume trees with ballistic seeds (Leal, 2004, Wieringa, 1999 and some Rubiaceae taxa (Robbrecht, 1996).…”

Morphometric analysis within populations of Hylomyscus walterverheyeni (Rodentia: Muridae) confirms complex relationships between small rodent populations of West Central African forests

“…Several species present in Birougou Mountains are important internationally for conservation. Carbon isotope analysis of soil organic carbon indicates that Birougou Mountains have been continuously covered with rainforest vegetation since the Holocene climate optimum, serving as a refuge for Congo Basin flora and fauna [13]. It is subject to an equatorial transition climate, with a long dry season from July to September.…”