Pollen-rain–vegetation relationships along a forest–savanna transect in southeastern Cameroon

Abstract

Modern soil and litter samples from southeastern Cameroon, collected along a continuous forest-savanna transect were analysed for pollen content to define modern pollen-vegetation relationships. The pollen results, completed and compared with botanical inventories, leaf area index and basal area measurements performed in the same area, clearly registered the physiognomy, the main floristic composition and floral richness of the two sampled ecosystems. Distortions were observed between sampled vegetations and their pollen rain, related to important differences in pollen production and dispersal of plant species: this is a general feature in many tropical regions. The pollen data in the area studied reflected well the recent transgression of forest versus savanna. This permitted us to define inside the forest ecosystem more successional vegetation communities than the botanical surveys allowed. Keywords calibration;  Cameroon;  modern pollen spectra;  palynology;  vegetation communities In Africa, until recently, the study of modern pollen deposition has been largely used to provide modern references for quantitative reconstructions of biomes (Elenga et al., 2000, Jolly et al., 1998 and Jolly et al., 2000 and climatic parameters (Peyron, 1999 and Peyron et al., 2000) at key periods of the Quaternary (6000 and 18 000 14 C B.P.), or along continuous temporal sequences (Bonnefille et al., 1992, Chalié, 1992 and Vincens et al., 1993. Pollen sampling was generally made under stable and mature formations, without anthropogenic influences, representative of the main vegetation types occurring today in Africa (White, 1983). In some regions, long pollen transects have been established (e.g. Bonnefille and Riollet, 1987, Bonnefille et al., 1993, Lézine and Hoogheimstra, 1990, Vincens et al., 1997 and Ybert, 1975, but they were always discontinuous, i.e. without relations and interferences between sampled vegetation types.This paper presents the most recent palynological research carried out in southeastern Cameroon, in the Kandara area, as part of the ECOFIT program (CNRS-ORSTOM). The work is based on pollen analyses of soil and litter samples collected along a continuous transect representative of the recent dynamics (recent centuries) between forest and savanna. Replacement of savanna by forest has been demonstrated in many regions of Central Atlantic Africa. For example, in Ivory Coast and in Congo, long-term experiments and models have shown that, during recent centuries, forests have been transgressive owing to climatic conditions favourable to their expansion and that savanna vegetation is today mainly maintained by the dual effects of fires and water stress in soils (Menaut et al., 1990, Schwartz et al., 1986 and Schwartz et al., 1996. In Cameroon, more precisely in the Kandara area, the recent researches undertaken by Achoundong et al., 1996, Kamgang Beyala et al. (1996), Kamgang Kabeyene Beyala (1998) and Youta Happi (1998, have also clearly demonstrated that the forest is today spreading over ...

Methodological concerns for analysis of phytolith assemblages: Does count size matter?

“…Emphasis was placed on these studies because they report a large number of assemblages, provide all, or part of, the raw data, and link their phytolith data with estimates of tree cover based on Leaf Area Index (LAI), pollen data. Moderate Resolution Imaging Spectroradiometer satellite data (MODIS), or all three (Barboni et al, 2007;Bremond et al, 2005aBremond et al, , 2005bCoumac et al, 2002;Vincens et al, 2000). Also, the definition of D/P has changed since earlier modem analogue work in Africa (e.g., Alexandre et al, 1997).…”

Modern pollen rain–vegetation relationships along a forest–steppe transect in the Golestan National Park, NE Iran

“…La plupart des études consacrées aux relations végétation actuelle/pluie pollinique ont été menées sur des milieux à végétation homogène et non sur des végétations de transition (Davis, 1983(Davis, /1984. Cependant, la connaissance de la pluie pollinique actuelle dans les milieux d'écotone est importante en paléoécologie car elle peut permettre d'améliorer l'interprétation des diagrammes polliniques provenant de régions qui, au cours de leur histoire, furent dans cette situation écologique intermédiaire (Liu et al, 1999;Vincens et al, 2000). Dans le nord de l'Iran, les écotones sont fréquents dans la zone de transition qui sépare les forêts hyrcaniennes sud-caspiennes des biomes semi-arides adjacents.…”

Central African biomes and forest succession stages derived from modern pollen data and plant functional types

“…They all have been exclusively extracted from surface soil (in savanna) or litter (in forest) samples collected following the Wright method (1967) widely used in African modern pollen studies (eg. Jolly et al, 1996;Lézine and Edorh, 1991;Bonnefille et al, 1993 ;Vincens et al, 1997Vincens et al, , 2000Elenga et al, 2000b). The location of the study modern pollen samples is given in Fig.…”

“…Location of modern pollen samples in Atlantic Central Africa, derived vegetation and reconstructed biomes and succession stages (column A, authors of the data: (a) Vincens, unpublished; (b) Lebamba et al, 2009; (c) Vincens et al, 2000; (d) Jolly et al, 1996; (e) Ngomanda, unpublished; (f) Elenga et al, 2000b) important in the canopy. The Guineo-Congolian wet rain forest represents the climax ecosystem of the central part of the study area.…”