Lowland rainforest response to hydrological changes during the last 1500 years in Gabon, Western Equatorial Africa

Abstract

International audiencePollen and δ13CTOM data obtained from two contrasting lake sequences (Lakes Kamalété and Nguène), located 200 km apart in the lowland rainforest of Gabon, provide complementary local and regional 1500-yr records of high resolution (15–30 yr) vegetation change. A combination of aquatic, semi-aquatic and terrestrial pollen showed in both records that the tropical rainforest increased during periods of high rainfall and decreased during drought intervals. The strong fluctuations of water balance at decadal scale during the “Medieval Warm Period” (not, vert, similar 1100–800 cal yr BP) coincided with a noticeable increase in shade-intolerant taxa, indicating recurring rainforest canopy disturbance. The δ13CTOM signal showed high-amplitude variations in both records, which positively correlates with the rainforest dynamics and local vegetation changes. The similar trends in both the pollen and the δ13CTOM signals between these sites demonstrate the regional broadly synchronous timing of shifting hydrological conditions. The largely positive co-variation between strong fluctuations of hydrological conditions and changes in rainforest structure and composition indicate that regional climatic change is probably the driving force for major rainforest dynamics in Gabon. Any significant anthropogenic impact on vegetation has not been clearly identified, and this issue still needs to be resolved independently by obtaining detailed archeological records across the interval 1400–800 BP, which currently seem to be extremely rare or not easily available

Elemental weathering fluxes and saprolite production rate in a Central African lateritic terrain (Nsimi, South Cameroon)

“…However, the South Cameroon Plateau was not always covered with rainforest and had heavy rainfall. Paleohydrological and paleoclimatological investigations covering the last 10,000 yr interglacial period (Holocene) have pointed out that the region has undergone alternating wet and drought periods leading, for the rainforest, to either its expansion during wet intervals or its contraction during drought periods with set up of savanna (Ngomanda et al, 2007(Ngomanda et al, , 2009. The long term precipitation changes in monsoon regime during the past 20,000 yrs in Central Africa are mostly related to changes in ocean circulation and sea surface temperature patterns, which are important to modulate atmospheric moisture transport (Schefusz et al, 2005;Weldeab et al, 2005).…”

Fire management in a changing landscape: a case study from Lopé National Park, Gabon

“…Although anthropogenic savannah fires are commonplace, Lopé National Park (a UNESCO World Heritage site), is one of only two protected areas in Gabon to use a prescriptive fire programme to manage its savannahs, which it has done since 1993. Fire has been used by humans in Gabon for thousands of years, and Lopé's savannahs are thought to be relicts of a dynamic vegetation history linked to historic human migration events and past climatic conditions (Maley, 2001;Oslisly, 2001;White, 2001;Ngomanda et al, 2007). Human fire activity combined with a dry climate is thought to have maintained large areas of savannah between 2000-3000 years ago (Oslisly & Peyrot, 1992;Peyrot et al, 2003;White, 1995).…”

Past environmental and climatic changes during the last 7200 cal yr BP in Adamawa plateau (Northern-Cameroun) based on fossil diatoms and sedimentary carbon isotopic records from Lake Mbalang

“…In central African subequatorial regions, proxy data inferred important disturbances in the periphery of the equatorial rain forest belt with possible appearance of included savannas (Ngomanda et al, 2009a, b) and complete dryness of lakes as Lake Sinnda in south Congo by 4400 yr BP (Vincens et al, 1994;Bertaux et al, 2000). In inner forest block, lakes were less affected by this climatic change (Vincens et al, 1999;Ngomanda et al, 2007;Kossoni et al, 2009). This period is characterised in Lake Mbalang by the maintenance of indicators of stable water table in agreement with the palynological and δ 13 C ORG.…”

“…Concordant data from low and high altitudes in western Cameroon (Maley and Brenac, 1998;Reynaud-Farrera et al, 1996;Nguetsop et al, 1998;Stager and Anfang-Sutter, 1999;Vincens et al, 1999;Ngomanda et al, 2007Ngomanda et al, , 2009b; Kossoni and Giresse, 2009) suggest that climatic changes were also controlled by modifications in the vertical structure of the atmosphere (Nguetsop et al, 2004). The present stable air layer situated at the base of the monsoon flux in JulyAugust could have extended on the western Cameroon lowlands and mid altitude areas during the greatest part of the year, entailing the almost suppression of convective rains before 3000 cal yr BP.…”

“…Available paleoclimatic records of the last 3000 yr in the tropical zones of Africa, close to the Atlantic coast of Gabon, West-Cameroon and South-Congo (Ngomanda et al, 2009a;Nguetsop et al, 2004;Vincens et al, 1999), suggest significant modifications in abundance and/or seasonal distribution of rainfall in response to north south shift of the Intertropical Convergence zone (ITCZ). Thus, climatic changes affected in the past water resources that impacted on human population and vegetation landscape of central and north tropical Africa.…”