Are modern pollen data representative of west African vegetation?☆
Introduction
Surface pollen data have been widely used to test interpretations of fossil pollen records as well as reconstructions of plant distributional history and related climate. The assumption of this approach is that surface pollen records accurately reflect local plant distribution and land surface conditions making it possible to perform continental scale synthesis of plant distribution (e.g., Huntley and Birks, 1983 in Europe; Webb and Yeracaris, 1978 in northern America) or global biome distributions (Prentice et al., 1996) from pollen data to evaluate climate model simulations (e.g., Braconnot et al., 2000). In sub-Saharan Africa, a considerable effort has been made over the past few decades to increase our understanding of the difference in the representation of pollen from a species to another characterized by specific pollen production and dispersal mechanisms from a species to another (e.g., Watrin et al., 2007). Since the pioneer work of Hedberg (1954) from the mountains of east Africa, numerous studies of modern pollen deposition have been conducted regionally (e.g., Bonnefille, 1972, Hamilton, 1972, Maley, 1981, Laseski, 1983, Vincens, 1984, Ritchie, 1986, Lézine and Hooghiemstra, 1990, Scott and Cooremans, 1992, Marchant and Taylor, 2000, Vincens et al., 2006, Lebamba et al., 2009) paving the way for the reconstruction of main African biomes (Jolly et al., 1998, Prentice et al., 1992, Elenga et al., 2000a, Elenga et al., 2000b). Following Gajewski et al. (2002), it is now clear that, despite of the different sampling methods used in these regional studies (soil, moss polters, river and lake sediments), the modern pollen data set of the African Pollen Database (APD) can be considered as a valid tool for environmental characterization on a continental scale.
This article focuses on west Africa, a sector which has undergone drastic environmental changes in the recent past as illustrated by the switch from “Green Sahara” to the present-day desert 4500 years ago (Kröpelin et al., 2008). In order to better constrain vegetation reconstructions and climate models in this sector, we analyze surface pollen data recovered along a north–south transect from the equatorial forest domain to the Saharan desert, with a special focus on transition zones between tropical forests, savannahs and xeric regions, which are currently poorly simulated by all Dynamic Global Vegetation Models (DGVM). Our data set includes several new samples to improve precise description and interpretation of the west African vegetation (White, 1983). We use recent works by Gajewski et al. (2002), Vincens et al., 2006, Vincens et al., 2007, Hély et al. (2006) and Watrin et al. (2007) for quality control on taxonomy and pollen–plant–climate relation to discuss pollen-vegetation relation as well as to improve plant functional types (PFTs) and biome definitions in west Africa.
Section snippets
Pollen data
The set of modern pollen data from west Africa used in this work includes 452 samples from 13 vegetation types defined by White (1983) and located between 5°S and 26°N (Fig. 1; Table S1). The vegetation types considered in this paper are:
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Guineo–Congolian (wet) rain forest (1a)
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Guineo–Congolian (dry) rain forest (2)
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Swamp forest (8)
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Guineo–Congolian mosaic of rain forest and secondary grassland (11a)
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Sudanian Isoberlinia woodland (27)
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Sudanian undifferentiated woodland (29a)
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Sudanian undifferentiated
Methods
Reconstruction of west African vegetation from pollen data has been performed using two independent methods: statistical analysis and biomisation. After homogenization of the nomenclature (Vincens et al., 2007), a total of 1075 pollen taxa was obtained. Our study does not include aquatic (including Cyperaceae) and mangrove pollen types (Rhizophora, Conocarpus erectus, Avicennia and Acrostychum aureum) or exotic pollen types (from planted or introduced plants). The remaining 618 tree, shrub and
The Guineo–Congolian region
Samples from the Guineo–Congolian lowlands mainly come from a mosaic of rain forest and secondary grassland (type 11a) in Senegal (Lézine and Hooghiemstra, 1990), Togo (Edorh, 1986), Benin (Tossou, 2002), Nigeria (Agwu, 1986), Cameroon (Bengo, 1992), and Congo (Elenga, 1992). Highly variable percentages of Poaceae and ferns reflect the mosaic like character of most of the Guineo–Congolian vegetation at the northern edge of the forest and/or in secondary grassland (Fig. 2, Fig. 3). Combretacae,
Concluding remarks
This study is part of an extensive effort to make pollen data from tropical Africa available for quantitative reconstructions of climate parameters, vegetation types and biomes. Difficulties linked to samples collected from different depositional environments and sampling methods (Gajewski et al., 2002), nomenclature homogenisation (Vincens et al., 2007), plant diversity and pollen-plant-climate relation (Watrin et al., 2007) have already been investigated in detail and regional studies (
Acknowledgements
This is a contribution of the African Pollen Database (APD) project and the Laboratoire des Sciences du Climat et de l'Environnement (publication no. 3142). Our research is financially supported by the Centre National de la Recherche Scientifique (CNRS) and the Agence Nationale de la Recherche (ANR) in the frame of the “Sahelp” Project. We would like to thank D. Jolly, R. Magioncalda, D. Lewden and V. Lézine for data entry, H. David for drawing, J.-Y. Peterschmitt, F. Aptel, G. Buchet and J.-P.
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2023, South African Journal of BotanySahel environmental variability during the last millennium: Insight from a pollen, charcoal and algae record from the Niayes area, Senegal
2019, Review of Palaeobotany and PalynologyReconstructing past biomes states using machine learning and modern pollen assemblages: A case study from Southern Africa
2019, Quaternary Science ReviewsIdentifying the ‘savanna’ signature in lacustrine sediments in northern Australia
2019, Quaternary Science ReviewsCitation Excerpt :Much larger studies on gradients with more significant contrast have found that rainfall is a major determinant of pollen assemblage. For example, Lu et al. (2011) (southwest China/Tibet; 1202 samples, 15–1840 mm rainfall) and Lézine et al. (2009) (northern Africa; 452 sample, ∼0–2500 mm), found that rainfall amount was the major factor driving changes in the pollen spectra, in each case moderated by a number of other climate and geographic variables. In this study and some of those cited above, a larger sample set may provide more power from which to tease out significant relationships to climate drivers, but the likelihood is that other variables will confound a simple interpretation.
Altitudinal distribution of pollen, plants and biomes in the Cameroon highlands
2018, Review of Palaeobotany and PalynologyCitation Excerpt :The genus Artemisia is present in the upper vegetation belts of the central Saharan massifs (e.g. the Tibesti Mountains) and at the Saharan-Mediterranean boundary (Ozenda, 2004) in northern West Africa. Since Artemisia pollen had never been found in modern samples south of 12.63°N in West Africa (Lézine et al., 2009), it is most unlikely that the pollen grains found in our samples from the Afroalpine grassland come from these remote origins. However, Artemisia is common in the Afroalpine grasslands in East and South Africa (Livingstone, 1967) and it has been defined as a typical fire-follower taxon in dwarf scrub and herbaceous plant communities (Lange et al., 1997).
Composition and diversity of vegetation and pollen spectra along gradients of grazing intensity and precipitation in southern Africa
2018, Review of Palaeobotany and PalynologyCitation Excerpt :Furthermore, studies that assess the indicator value of modern pollen spectra for different types of land-use in African savannas, such as cropland, livestock farming, and settlements are lacking, even though the potential of palynology to track long-term land-use change has long been recognised (Ekblom and Gillson, 2010; Mighall et al., 2012; Msaky et al., 2005). Several studies performed in mesic and humid environments show that modern pollen–vegetation relationships in fragmented landscapes are particularly complex (Bunting et al., 2016; Groenman-van Waateringe, 1993; Lézine et al., 2009). The underrepresentation of key entomophilous taxa in disturbed tropical forests (Lézine et al., 2009; Vincens et al., 2006), the background pollen load in cleared temperate forests (Bunting et al., 2016), and contrasting pollen productivities in temperate grasslands (Groenman-van Waateringe, 1993) contribute strongly to the observed biases.
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Contributors for this paper are: C.O.C. Agwu, M.D. Bengo, T. Edorh, G. El-Ghazali, H. Elenga, A. Frédoux, D. Jolly, J. Maley, J.C. Ritchie, U. Salzmann, E. Schulz, M.G. Tossou, J.-P. Ybert.