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 ¹⁴C 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.