A quantitative synthesis of Holocene vegetation change in Nigeria (Western Africa)

Abstract

Understanding long-term (centennial–millennial scale) ecosystem stability and dynamics are key to sustainable management and conservation of ecosystem processes under the currently changing climate. Fossil pollen records offer the possibility to investigate long-term changes in vegetation composition and diversity on regional and continental scales. Such studies have been conducted in temperate systems, but are underrepresented in the tropics, especially in Africa. This study attempts to synthesize pollen records from Nigeria (tropical western Africa) and nearby regions to quantitatively assess Holocene regional vegetation changes (turnover) and stability under different climatic regimes for the first time. We use the squared chord distance metric (SCD) to assess centennial-scale vegetation turnover in pollen records. Results suggest vegetation in most parts of Nigeria experienced low turnover under a wetter climatic regime (African Humid Period), especially between ~8000 and 5000 cal year BP. In contrast, vegetation turnover increased significantly under the drier climatic regime of the late-Holocene (between ~5000 cal year BP and present), reflecting the imp role of moisture changes in tropical west African vegetation dynamics during the Holocene. Our results are consistent with records of vegetation and climatic changes in other parts of Africa, suggesting the Holocene pattern of vegetation change in Nigeria is a reflection of continental-scale climatic changes.

Macrotermes termite mounds influence the spatial pattern of tree species in two African rainforest sites, in northern Congo. But were they really forests in the past?

“…Our results contribute to the assumption that the origin of the LHRC is probably a combination of climate change and human impacts (Bayon et al 2019;Adeleye et al 2021). Both these conditions were met in Loundoungou where savannah termite mounds can still be observed but not in Mokabi where forest termite mounds are present.…”

“…However, recently some authors have shown flexibility in their statements and have put forward less clear-cut hypotheses. Adeleye et al (2021) and Bayon et al (2019) claim that paleoclimatic change was the main factor of the LHRC, but that anthropogenic impact cannot be excluded from the scenario since it is very difficult to quantify. In our study, evidence of past human occupation, such as shards of pottery, scorias, and charcoal, was found in Loundoungou (Freycon et al 2018), consistent with the presence of old savannah termite mounds.…”

Impact of farming on African landscapes

“…Orbital parameters alone insufficiently explain such anomalies in the long sedimentary records. The spatial discontinuity of the termination event as captured by models (Hopcroft and Valdes, 2021) and the proxy records (Adeleye et al, 2021) support the notion that local disturbance ecologies, potentially associated with human technologies, could have accelerated the tempo and magnitude of some threshold crossing events during the terminal AHP in northern Africa.…”