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.

Keywords

Africa African Humid Period climate Holocene Nigeria vegetation turnover

Introduction

Africa houses some of the most diverse ecosystems of the world, ranging from wet rainforests to dry deserts, with unique flora and fauna that have fascinated researchers for decades (Huntley, 1988; Klopper et al., 2002; Linder, 2014; Newmark, 2002; Pottinger and Burley, 1992; United Nations Environment Programme, 2002). On a global scale, Africa has also played a major role in the evolution of human and terrestrial biological resources due to its Gondwanan heritage, and a combination of long-term changes in climate and human interactions with the biota have created unique cultural landscapes across the continent (Ekblom et al., 2019; Raven, 1979; Raven and Axelrod, 1974). Unfortunately, the majority of these ecosystems today are facing the threat of disappearance and diversity loss due to a rapidly changing climate and rising human/economic impacts (Adeoye and Ayeni, 2011; Enuoh and Ogogo, 2018; Maranz, 2009; Midgley et al., 2002; Mmom and Arokoyu, 2010; Myers et al., 2000; Rakotomanana et al., 2013; Verschuren et al., 2002).

Understanding long-term ecosystem changes and stability under varying climatic regimes in the past is key to sustainable management and conservation of ecosystem processes under the currently changing climate. Fossil pollen records offer the possibility to investigate centennial–millennial scale changes in vegetation composition and diversity (Marquer et al., 2014; Vegas-Vilarrúbia et al., 2011; Willis et al., 2010; Wingard et al., 2017). Quantitative studies of fossil pollen records have been used to assess past regional- and continental-scale vegetation and diversity changes in Europe, North America and Australia (e.g. Adeleye et al., 2020; Carcaillet et al., 2010; Connor et al., 2019; Giesecke et al., 2019), thanks to the large number of fossil and modern pollen records available in these regions via public repositories. Extensive studies of past vegetation, fire regimes and climate have also been carried out in many African sites (e.g. Adeonipekun et al., 2017; Burrough and Willis, 2015; Dupont and Schefuß, 2018; Lézine et al., 2013; Sowunmi, 2004; Vogelsang et al., 2002; Waller et al., 2007); however, studies targeted at understanding regional and continental scale patterns of vegetation dynamics are still limited on the continent, possibly due to the generally limited number of available/accessible pollen records. This is especially true of Nigeria (western Africa).

Nigeria has one of the highest densities of ecoregions in western Africa and vegetation distribution in the region cuts across various environmental gradients (CILSS, 2016). On a regional scale, vegetation in Nigeria reflects a transition (ecotone) between the tropical rainforests and savanna-desert belts of Africa (CILSS, 2016), making the region sensitive to long-term climatic shifts and therefore suitable for studying the dynamics of major African biomes in response to climate. Although pollen data across Africa is scarce compared to other continents, the collection of sites in Nigeria provides a rare opportunity to compare different site records to investigate spatio-temporal palaeovegetation changes in the region. Existing pollen records in Nigeria mostly span the Holocene and generally suggest a shift from humid forests/woodland (African Humid Period: ~12,000–5000 years ago) to dry open forests/savanna (the last ~5000–4000 years) (e.g. Salzmann and Waller, 1998; Sowunmi, 2004; Waller et al., 2007). In this study, we synthesize a total of 13 existing pollen records in Nigeria and nearby regions, and our aim is to quantitatively assess Holocene regional rates of vegetation change (turnover) under different climatic regimes and identify the timing of significant vegetation shifts for the first time. This will provide insights into continental-scale vegetation and climatic changes in Africa during the Holocene.

Study region

Nigeria lies in the tropics in the southern part of western Africa, occupying a total area of about 923,768 km²between the latitudes 4°N and 14°N and longitudes 2°E and 15°E (Figure 1). The climate is characterized by wet and dry seasons, which are mainly modulated by the positions of the Intertropical Convergence Zone (ITCZ) and accompanied by tropical continental and maritime airmasses (Metz, 1991; Salau, 2017). The dry season occurs between November and February in the south and between October and April in the north, while the rest of the year in the respective regions are wet seasons. The dry season, especially in the north, is also accompanied by the tropical continental airmass (Harmattan) from the Sahara Desert, while the wet season is accompanied by the tropical maritime airmasses originating over the southern Atlantic Ocean. Annual temperatures are generally higher in the northern part than in the wet, humid southern part of Nigeria, where temperature variability is damped by high rainfall (Salau, 2017; Figure 1). Annual temperature required for plant growth is highest in the southern part and decreases towards the north (Eludoyin and Adelekan, 2012). Southern Nigerian vegetation is mainly characterized by rainforests, freshwater swamps and mangrove forests, the central part is characterized by Guinean savanna and the northern part is characterized by Sudanian and Sahelian savanna vegetation (Figure 2).

Figure 1.

(a) Map of Africa showing the location centred on Nigeria, western Africa and other surrounding regions mentioned later in the discussion, which include Chad (CH), Mali (MA), Burkina Faso (BF), Niger (NG), Dahomey Gap (DG), Cameroon (CA) and Congo (CO). (b) The climate map of Nigeria showing 10-year mean annual temperature and 10-year mean daily precipitation. Climate map was generated on the NOAA Physical Science Laboratory online platform using the NCEP/NCAR and GPCP data sets, respectively (https://psl.noaa.gov/cgi-bin/data/composites/printpage.pl).

Figure 2.

Major vegetation zones of Nigeria (adapted from Adeonipekun et al., 2017) and locations (black hollow circles) of pollen records used in this study, including sites of pollen records in nearby regions (solid red stars). Bal Lake (BL), Kajemarum Oasis (Kaj.O), Jikariya Lake (JL), Kaigama Oasis (Kai.O), Kuluwu Oasis (Kul.O), Tilla Lake (TL), Ohe Pond (OP), Ofuabo Creek (OC) and Badagry Creek (BC). Red hollow circles represent locations of other late-Holocene records in Southern Nigeria later mentioned in the discussion. Sahel, Sudan and Guinea savannas are transitional ecoregions between the Sahara Desert and northern boundary of the equatorial rainforests. Woody growth increases southward in the ecoregions, with grass dominating the Sahel, a mixture of grass, shrubs and trees (scattered) in the Sudan, and forest-grassland-shrubland mosaics in the Guinea savannas. Guinea savannas were derived as a result of human land use.

Methods

Pollen record selection

Pollen records spanning at least the last ~6000 years, with a maximum sampling resolution of 500 years were selected across Nigeria. This criterion yielded six records for northern Nigeria and three records for southern Nigeria (Table 1). The majority of the records have at least one radiocarbon date per 2000–3000 years, while two sites (Badagry and Ohe) from southern Nigeria have one radiocarbon date per 4000–6000 years in some parts of the records. Two pollen records (Bal and Tilla) were drawn from the Neotoma Database, while pollen diagrams (taxa with least 5% abundance) of other unavailable sites were digitized using the Quintessa Graph Grabber 2.0.2 (Javed et al., 2020). Due to the limited number of records in Nigeria, sites from nearby regions (Benin, Cameroon and Niger) were also included in our selection. Cameroon and Niger pollen records were obtained from the Neotoma Database (Williams et al., 2018), while Dahomey pollen data was digitized.

Table 1.

Nigerian pollen records used in this study (1–9), including location, elevation, catchment type, estimated basal and top ages, sample resolution and data source. Also shown are additional records selected from nearby regions of Benin, Cameroon and Niger (10–13), as well as other pollen records in Nigeria that did not meet the criteria for this study (14–22). Dates were recalibrated using IntCal20 dataset (Reimer et al., 2020). Northern Nigerian sites include 1–6 and southern sites include 7–9.

#	Site	Latitude (°N)	Longitude (°E)	Elevation (m asl)	Catchment type	# of ¹⁴C dates	Basal age (cal yr BP)	Top age (cal yr BP)	Samp. res.	Reference
1	Bal	13.3068	10.9493	300–350	Lake	11	~11,000 ± 509	~Present	–	Salzmann and Waller (1998)
2	Kajemarum	13.305	11.0288	300–350	Oasis	4	~11,000 ± 443	~2000	–	Salzmann and Waller (1998)
3	Kaigama	13.2517	11.568	300–350	Oasis	4	~11,000 ± 437	~4000	–	Salzmann and Waller (1998)
4	Kuluwu	13.2173	11.5508	300–350	Oasis	2	~12,000 ± 19	~4000	–	Salzmann and Waller (1998)
5	Jikariya	13.3137	11.077	~345^a	Lake	7	~12,000 ± 27	~Present	~150	Waller et al. (2007)
6	Tilla	10.5444	12.1313	690	Crater lake	18	~12,500 ± 120	~Present	~80	Salzmann (2000)
7	Ohe	6.83593	7.36395	1502	Pond	3	~8000 ± 2	~Present	~200	Njokuocha and Akaegbobi (2014)
8	Ofuabo	4.55	6.43333	~4^a	Mangrove swamp	5	>40,000	~Present	~300	Sowunmi (1981)
9	Badagry	6.43278	2.77472	0.2	Freshwater swamp	6	~10,000 ± 2	~3000	~600	Sowunmi (2004)
10	Dahomey	7.15000	2.43333	9	Lake	12	~9000 ± 20	~Present	~360	Salzmann and Hoelzmann (2005)
11	Bambili	5.925715	10.24339	2273	Crater lake	17	~20,000	~Present	~120	Lézine et al. (2013)
12	Mbalang	7.316667	13.733333	1110	Lake	4	~7300	~Present	~150	Vincens et al. (2010)
13	Mare d’Oursi	14.65787	0.47614	~251^a	Temporary Lake	3	~4500	~Present	~76	Ballouche and Neumann (1995)
14	Unilag	6.516861	3.399378	~3^a	Swamp	0	>8000^b	–	>500	Adekanmbi and Alebiosu (2017)
15	Ologun	6.750083	4.6	~120^a	Coastal wetland	3	~700	~Present	~40	Adekanmbi et al. (2020)
16	Unilag	6.401250	3.517594	~3^a	Coastal swamp	0	18 cm	–	–	Adeleye et al. (2018)
17	Unilag	6.516667	3.398333	~3^a	Freshwater swamp	2	~19,000	<6000	~1200	Adeonipekun et al. (2017)
18	Itowolo	6.613333	3.441944	~3^a	–	2	>250	~Present	~15	Ajikah et al. (2019)
19	Ajido	7.133333	3.400000	~33^a	Coastal wetland	0	51 cm	–	–	Adekanmbi et al. (2017)
20	Vampire	4–6	5–7	~Sea level	Coastal creek	0	400,000 cm	–	–	Ige (2009)
21	Obayi	Close to Ohe		550	Crater Lake	1	>1900	–	>300	Njokuocha (2012)
22	Tse Dura	6.935556	9.263611	~139^a	Rock shelter	3	~800	~400	~36	Orijemie (2018)

Elevation estimated from topographic-map (https://en-gb.topographic-map.com/maps/zrs/Nigeria/) due to unavailability in the original publication. ^bExtrapolated age by investigators.

Estimating past vegetation turnover

Fossil pollen data have been widely used in estimating changes in terrestrial vegetation through time (turnover), which is useful in assessing long-term stability and dynamics in ecosystems (e.g. Adeleye et al., 2020; Carcaillet et al., 2010; Connor et al., 2019; Seddon et al., 2015). In order to quantify Holocene vegetation change in Nigeria, we applied this approach to the 13 selected pollen records from Nigeria and nearby regions. In our turnover analysis for each site, aquatic pollen such as Typha and Nymphaeaceae were excluded, as they reflect localized (catchment-scale) vegetation and our study is focused on regional terrestrial vegetation.

Pollen records were divided into 1000-year time bins before conducting a turnover analysis. This was done to reduce the risk artefact introduction in turnover results (Adeleye et al., 2020; Connor et al., 2019; Seddon et al., 2015). Turnover analysis was conducted using the Squared Chord Distance (SCD) dissimilarity metric, with pairs of pollen samples randomly selected (50 replications) from each bin to calculate the average SCD between samples (Adeleye et al., 2020; Connor et al., 2019; Overpeck et al., 1985; Seddon et al., 2015). A Bayesian Change Point analysis was performed on SCD results to identify significant changes in turnover trends. All analyses were done in ‘R’ (R Core Team, 2019) using the ‘analogue’ and ‘bcp’ packages (Simpson, 2007; Wang et al., 2018). Published ¹⁴C dates were recalibrated using IntCal20 dataset (Reimer et al., 2020) and the recalibrated dates were used to guide reported chronologies.

Results

In northern Nigeria, palaeovegetation turnover was low at most sites from ~10,000 to ~5000 cal year BP, with increases after ~5000 cal year BP, including in nearby region (Niger – Mare d’Oursi). However, turnover was also high at Tilla and Jikariya around 10,000 cal year BP. In southern Nigeria, high vegetation turnover was generally observed between ~4000 cal year BP and present. This is also the case in nearby regions, especially Dahomey and Bambili. High turnover prior to ~4000 cal year BP was only observed at Ofuabo.

Bayesian Change Point analysis shows significant shift in overall vegetation turnover (average SCD) in northern Nigeria (and nearby region combined) occurred between ~5000 and 2000 cal yr BP, while significant shift in turnover in southern Nigeria (and nearby regions combined) occurred between ~4000 and 2000 cal year BP (Figure 3).

Figure 3.

Vegetation turnover – Squared Chord Distance (SCD) results for each site in northern and southern Nigeria, western Africa (black), as well as sites from nearby regions (blue), including Niger (Mare d’Oursi), Benin (Dahomey) and Cameroon (Mbalang, Bambili). Also presented is the average SCDs (green) and posterior probability of change points – CP (green) for norther and southern Nigeria (combined with nearby regions). Orange-shaded zones indicate periods of significant shifts in regional vegetation turnover, as identified by change point analysis. Each SCD point represents dissimilarity between vegetation in adjacent time bins, for instance, the average SCD point at 2000 cal yr BP in southern Nigeria represents the dissimilarity between vegetation at 3000 and vegetation at 1000 cal year BP. See Figure 1 for site locations.

Discussion

Our analysis broadly shows some regional consistency in rates of palaeovegetation change across Nigeria, especially during the late-Holocene, indicating that the vegetation has often responded to large-scale drivers such as Holocene climatic change. Deviations from these regional trends at individual sites are likely to be linked to local factors, such as land-use change. In the following discussion, we examine the limitations of the data and discuss possible drivers of long-term vegetation change for the Northern and Southern regions of Nigeria.

The distribution of pollen records in Nigeria is generally sparse and the majority of pollen records has one or more of the following limiting factors: (1) poor chronological control, (2) low sampling resolution and (3) limited temporal span (late-Holocene). Nonetheless, the selected pollen records in this study allowed the reconstruction of past regional vegetation turnover in Nigeria, and potential sources of error are acknowledged, which may include the method used in acquiring pollen data sets as well as the number and distribution of sites considered.

The quality of pollen data and their chronologies, especially the digitized pollen data, may have potentially influenced the timing of vegetation turnover recorded. Given the chronological uncertainty, reported dates in this study should be taken as approximations and not absolute dates. The spatial distribution of pollen data may have also biased our results, with an underrepresentation of central and southern vegetation zones. Pollen records from north-eastern Nigeria combined with the record from Niger were taken to represent western African dry tropics (WADT) vegetation history and turnover. These pollen records generally reflect the vegetation history of the Sudan and Sahel savanna zone, considering the relative homogeneity of vegetation across latitudes (Figure 2). This is also the case for southern Nigeria combined with Benin and Cameroon, with pollen records from the regions taken to reflect western African wet tropics (WAWT) vegetation history and turnover. Although there are various types of wet forests in southern Nigeria, the limited number of suitable pollen records from the region limits regional reconstructions for each of the forest zones. Hence the term ‘rainforest sensu lato’ is used later in the discussion of vegetation changes in the region. Rainforest sensu lato includes lowland, freshwater, riverine and semi-deciduous forests (Nigerian Federal Department of Forestry, 2019). Despite these unavoidable limitations, the pollen-based reconstructed regional vegetation turnover in this study provides new insights into the Holocene vegetation history of Nigeria, facilitating comparisons with surrounding regions.

Holocene vegetation turnover in western African dry tropics (WADT)

Climate become warmer and wetter in Africa following the last glacial period (Adkins et al., 2006; Armitage et al., 2015; Cockerton et al., 2014) and the lateglacial–early Holocene climate transition may be reflected by high vegetation turnover in Tilla and Jikariya around 10,000 years ago. Though turnover at this time may have been less significant compared to turnover during the later part of the Holocene. Humid and wetter conditions prevailed from the early to mid- Holocene (African Humid Period) and these conditions supported humid Guinean woodland/forest in northern Nigeria, which today is typical of a dry Sahel and Sudan savanna vegetation (Armitage et al., 2015; Salzmann, 2000; Salzmann and Waller, 1998; Wang et al., 2008; Figure 4). Low vegetation turnover at most sites after ~8000 cal year BP until ~5000–3000 cal year BP suggests the humid Guinean vegetation (forest-grassland mosaic/wooded-grassland) at this time was mostly stable. Palaeoecological records from surrounding Sahel (e.g. Chad, Burkina Faso, Mali) and the Saharan region north of Nigeria reflect a similar expansion of humid vegetation in the early–mid Holocene (Amaral et al., 2013; Ballouche and Neumann, 1995; deMenocal and Tierney, 2012; Eichhorn and Neumann, 2014). Increased insolation during this period is thought to have driven higher monsoonal rainfall than today in northern Africa, which in turn promoted vegetation cover and higher lake levels across the region, including in Nigeria (Armitage et al., 2015; deMenocal and Tierney, 2012; Salzmann and Waller, 1998). The early Holocene low turnover in WADT was likely a reflection of stable humid vegetation in many parts of northern Africa during the African Humid Period.

Figure 4.

Regional vegetation turnover change points for northern and southern Nigeria in relation to climatic changes inferred from dust influx record (Cockerton et al., 2014), Chad Lake level record (Armitage et al., 2015), July insolation for 15°N (Berger and Loutre, 1991), Bosumtwi Lake level, Ghana (Shanahan et al., 2006), Gulf of Guinea sea surface temperature – SST (Shanahan et al., 2006) and salinity – SSS (Weldeab et al., 2007), and Mauritanian coast sea-level oscillation (Einsele et al., 1974). Dust influx record is the average influx of Jikariya and Kajemarum Oases, northern Nigeria. Periods of significant changes are shaded in light grey.

Guinean vegetation reached its maximum extent in northern Nigeria in the mid- Holocene, which overlaps with the maximum northward expansion of monsoon rainfall before its decline after 6000 cal year BP (Armitage et al., 2015; Patricola and Cook, 2007; Salzmann, 2000; Salzmann and Waller, 1998; Wang et al., 2008; Figure 4). Guinean vegetation decreased steadily with rainfall decline from mid- Holocene, with the expansion of dry Sahelian and grassland vegetation (Armitage et al., 2015; Cockerton et al., 2014; Salzmann, 2000; Salzmann and Waller, 1998; Street-Perrott et al., 2000; Wang et al., 2008; Figure 4). This vegetation shift is reflected in the significant change in turnover (increase) between ~5000 and 2000 cal year BP, which was likely caused by the deteriorating climate (aridification) during this period (Hély et al., 2014; Figure 4). Abrupt regional aridification, along with decreasing summer insolation, is thought to have driven low rainfall and lake levels across the region at this time (Armitage et al., 2015). Significant aridity likely halted the northward expansion of humid Guinean vegetation, pushing its limit towards the southern sector of northern Nigeria (Figure 2), with a significant vegetation reassembly and turnover (Cockerton et al., 2014; Hély et al., 2014; Street-Perrott et al., 2000). A major increase in fire and human agricultural activities was also recorded in the Sahel and Sudan from about 3000 cal year BP and would have contributed to high vegetation turnover at several sites, as Guinean forests were progressively altered to promote a more open savanna and grassland vegetation (Ballouche and Neumann, 1995; Marlon et al., 2013; Salzmann and Waller, 1998).

Holocene vegetation turnover in western African wet tropics (WAWT)

Sea level was high on the African coast (Nouakchottian marine transgression) between ~7000 and 5000 years ago (Einsele et al., 1974) and may have contributed to the observed vegetation turnover in Ofuabo in WAWT during this period. Though low turnover in other coastal sites (Badagry and Dahomey) suggests vegetation change in response to sea-level rise during this interval was minimal or spatially variable in WAWT coasts during the mid- Holocene (Figure 4).

Lake level and sea surface salinity records suggest a drier climate prevailed in WAWT from ~4000 cal year BP (Shanahan et al., 2006), which may have driven a significant shift in vegetation turnover at most sites between ~5000 and 2000 cal year BP, with increased turnover from this period. Pollen records show a gradual decline in forests during this period. Arid conditions may have translated into a loss of suitable habitat for wet forest expansion, causing a major shift from close to open forests, with increased understory communities. Marked changes in sea level around this time (Jaritz et al., 1977; Sowunmi, 2004; Weldeab et al., 2007) may have also contributed to high vegetation turnover in coastal areas (Badagry and Dahomey; Figure 4). Mangrove forest reduced significantly in Badagry and Dahomey around this time due to changes in salinity and soil hydrology related to lower sea levels (Salzmann and Hoelzmann, 2005; Sowunmi, 2004). This collapse in mangrove forest has also been recorded in other wet tropical African sites during the late-Holocene (Caratini and Giresse, 1979; Tossou, 2002).

High, but less significant vegetation turnover in most WAWT sites after ~2000 cal year BP may represent further decline in rainforest, probably related to the intensification of arid conditions (Figure 4). Human impact may have also contributed to the high turnover via land clearance and agricultural activities, as inferred from archaeological records (Alabi, 1999; Orijemie, 2018; Sowunmi, 1981). Southern Nigerian pollen records (Njokuocha and Akaegbobi, 2014; Sowunmi, 2004), including other low-temporal resolution late-Holocene records that are not included in this study (Adekanmbi et al., 2020; Adeleye et al., 2018; Ajikah et al., 2019; Njokuocha, 2012; Orijemie, 2014, 2018), show the replacement of rainforest by secondary regrowth in the late-Holocene, indicative of anthropogenic disturbance. The collapse in rainforest is also evident in pollen records from central Africa (Congo) and other sites in Cameroon, which show the replacement of closed forests by open forests, secondary forests and savannas in the last ~3000–1000 years in response to increasingly dry conditions and anthropogenic impacts (Elenga et al., 1994; Lebamba et al., 2016; Reynaud-Farrera, 1997). This pattern of regional vegetation change suggests turnover in WAWT during the late-Holocene was mainly driven by aridification (and human land use), a pattern observed throughout the African wet tropics. In fact, this late-Holocene rainforest decline has been described as a period of rainforest crises in Africa (Giresse et al., 2020).

Conclusion

Our vegetation turnover estimates for Nigeria suggest major vegetation shift in the region during the Holocene, occurred between ~5000 and 2000 years ago, and was mainly driven by moisture changes. Low vegetation turnover prevailed during the early–mid Holocene African Humid Period, while significantly high turnover prevailed after the termination of the humid period. Our result is not only consistent with records of vegetation and climatic change in the surrounding regions of northern, western and central Africa, but also reflect the important role of continental-scale changes in moisture regimes in the dynamics of African tropical vegetation during the Holocene.

Footnotes

Acknowledgements

Three pollen data were obtained from the Neotoma Paleoecology Database (). The work of data contributors, data stewards and the Neotoma community is gratefully acknowledged.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Matthew Adesanya Adeleye

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