Vegetation Structure, Diversity and Site Conditions in the South of the Diambour Forest Reserve, Southeastern Senegal (West Africa)

TROPICS Vol. 17 (1) Issued November 30, 2007

Vegetation structure, diversity and site conditions in the south of the Diambour Forest Reserve, southeastern Senegal (West Africa)

Diané ABDOULAYE and Kazue FUJIWARA

Graduate School of Environment and Information Sciences, Laboratory of Vegetation Science Yokohama National University, 79–4 Tokiwadai, Hodogaya-ku, Yokohama-shi, 240–8501, Japan. Tel: +81–45–339–4355, Fax: +81–45–339–4370, E–mail: [email protected]

ABSTRACT The influence of site conditions ecosystems (Heinrich and Moldenhauer, 2002). Non– and human activities on vegetation composition equilibrium views of savannas invoke variable rainfall, was studied in savannas of the Diambour Forest herbivory and fire as determinants of tree demography Reserve of southeastern Senegal. Vegetation interacting in complex ways at different times and places inventories were carried out in areas of (Pellew, 1983; Dublin et al. 1990; Prins and van der homogeneous vegetation in savanna woodland, tree- Jeugd, 1993; Scholes and Archer, 1997; Higgins et al. shrub savanna, grass savanna and gallery forest 2000). Opinions on the forces shaping such communities based on the Braun-Blanquet (1964) methodology. have changed fundamentally over the past two decades. At each site, substrate data (soil types and Competition is now seen as the main process structuring moisture content), micro-topographic features and communities or patterns of species distribution and disturbance factors (cutting, fire and cattle grazing) abundance (Crawley, 1997). Episodic disturbances, such were recorded. The vegetation of the Diambour as drought, floods and fire, occur so frequently that Forest Reserve was classified into six communities, species assemblages never reach competitive equilibrium seven subcommunities and two other subunits. (Hutson, 1979; Pickett and White, 1985). Other ecological The drier savanna areas affected by fire tend to be processes, such as herbivory or predation, may modify or richer in species than the wetter parts restricted to override competitive hierarchies (Louda, 1982, 1989). the riverside and depressions with tree cutting and In the Diambour Forest Reserve (DFR), Senegal, cattle grazing. The communities on drier sites and West Africa, three decades of human occupation, , affected by fire show higher values of Simpson s involving agriculture, cattle grazing and fire management, index of diversity and species richness. The lower have drastically affected vegetation characteristics. Some diversity of wetter sites is caused by tree cutting, researchers have studied the nearby Niokolo–Koba micro-topography and soil with a high content of National Park (NKNP)(Adam, 1966, 1971; Ba et al. 1997; laterite and other clays. Riverside communities Mbow et al. 2003; Nielsen et al. 2003; Schneider and are mostly composed of microphanerophytes, Sambou, 1982; Sonko, 2000; Traoré, 1997); nevertheless, while the communities of drier sites show more little attention has been given to the influence of human diversified life-form spectra. This paper is the first disturbances (tree cutting activities, cattle grazing phytosociological study of the Diambour Forest and fire) on the composition and structure of savanna Reserve (DFR) in Senegal. vegetation. Disturbance factors and environmental conditions are generally important determinants of Key words: cattle, Diambour Forest Reserve, fire, vegetation communities. In 2001, the southern part of grazing, savanna, Senegal, soil, species diversity, the DFR sheltered 28 human settlements and more than species composition. 2000 cattle, with a new village created every year (Diané, 2001). There are few studies on the DFR but none on its current floristic composition that consider the ongoing INTRODUCTION human disturbance. The goal of this study is to clarify the Human activities such as shifting cultivation, extensive current vegetation structure of the DFR and its relation cattle breeding, and burning have long affected African to environmental and disturbance factors. savanna ecosystems. Sediment dating and archeological findings prove early human impact on dry savanna 26 Diané ABDOULAYE and Kazue FUJIWARA

savanna dominated by Combretaceae species; savanna STUDY SITE woodland dominated by Fabaceae and Caesalpiniaceae Diambour Forest Reserve (DFR) is located about 531 species; and riverside gallery forest dominated by km from Dakar in southeastern Senegal (West Africa), Symmeria paniculata, Mitragyna inermis and Borassus , , , between 13˚12 and 13˚41 N latitude and 13˚02 and 13˚ flabellifer. , 31 W longitude (Fig. 1). Its general elevation is about 46 m above sea level. The terrain consists of low, rolling plains and depressions dissected by the hydrographic MATERIAL AND METHODS system of the Niériko River. Soils are mainly ferric and Sixty sample-plots were surveyed in the main vegetation gleyic Luvisols, Lithosols, eutric Regosols and vertic areas: grass savanna, tree-shrub savanna, woodland Cambiosols (Frederiksen and Lawesson, 1992). savanna, and gallery forest. A plot size of 5 x 5 m was The Diambour Forest covers an area of 121,500 ha chosen in the grass savanna (2 relevés), and 50 x 50 m and was classified as a nature reserve in 1968, in order to in the woody savannas (woodland and tree-shrub types) preserve the biodiversity and the gallery forest along the and gallery forest-scrub (58 relevés), depending on Niériko River. Due to the extensive surface of the DFR, areas of homogeneous vegetation. The plot size in grass this study was limited to the southern part of the reserve savannas and woody formations was determined after which presents more diversified vegetation types and the establishment of the species/area curve. Fieldwork more disturbance forms. The climate, representing the was based on the Braun-Blanquet method of vegetation Sudanese savanna belt of sub-Saharan Africa, is primarily survey (1964), as described also by Mueller-Dombois determined by the seasonal movement of the Inter- and Ellenberg (1974) and Fujiwara (1987). In each plot, Tropical Convergence Zone (ITCZ). The climate is thus all species were recorded in all layers in which they characterized by a dry season of 6 to 7 months (November occurred, along with cover and sociability degree, plus to May) and a rainy season of 5 to 6 months (May to environmental features of the site (topography, soil October) (Rigina and Rasmussen, 2003). The annual types and moisture content). Fieldwork was conducted rainfall was 1109 mm in 2004 and occurred mainly within during October-November 2004 (at the end of the only 75 days. Temperature varies significantly between rainy season) and in May 2005 (at the end of the dry day and night, with an annual average of 28˚C. The season). The same plots could not be surveyed in both vegetation of the DFR consists of various savanna types: seasons due to the widespread inﬂuence of ﬁre and other an extensive grass savanna dominated by Poaceae species management activities. Plots were randomly established (Pennisetum pedicellatum, Andropogon pseudapricus, based exclusively on vegetation and site homogeneity. Eragrostis tremula and Popeguinea arrecta); tree-shrub At each relevé site, soil volumetric water content was

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Fig. 1. Location of the study site. (modified from Mbow et al. 2003) Vegetation structure, diversity and site conditions in Senegal 27 randomly recorded in 5 points at 20 cm depth using DM each community together with the standard deviation. , 18 device with electronic sensor, and the average value Simpson s diversity index (D) was calculated using the was taken to represent the relevé site. Due to the embank formula: morphology of the Niériko river and the generally flat 2 landscape, we distinguished four main topographical D = 1 / Σ Pi (i = 1, 2, 3, ..., q) units: the riverside (RS) for plots located just next to where q is the total number of species and Pi is the the water; the riverbank (RB) for the upper sites of the relative mean cover value in the i-th species (Magurran, embankment; the depressions (D) correspond to the 1988; Kuroda et al. 2006). The cover-abundance values +, lower terrain within the flat savanna sites (FL). The soil 1, 2, 3, 4, 5 were converted into mean cover values 0.1, 2.5, type within each relevé was visually estimated: sandy (S), 15.0, 37.5, 62.5 and 87.5%, respectively (Mueller-Dombois clay (C), laterite (L), rocky (R), gravel (G), silt (Si) and and Ellenberg, 1974; Kuroda et al. 2006). sandy-loam (SL). Clay soils in the DFR present usually dark or bright color and are sticky when humid while the laterite, also sticky when humid, is exclusively reddish RESULTS with sesquioxides rich profile (Helgren and Butzer, As a result of the phytosociological analysis, considering 1977). Gravel soils are concretionary materials derived also disturbance factors and environmental conditions, from the erosion of the hardpan while rocky sites present the savanna vegetation in the DFR can be classified into very coarse material. Sandy, sandy-loam and silt soils are six communities, seven sub-communities and two other derived from the metasedimentary series of southeast sub-units (Table 1). Senegal (Bassot, 1966) and present fine materials. Sandy soils are widely distributed with an important proportion Symmeria paniculata-Strophanthus sarmentosus of quartz material while sandy-loam soil associate sand, community clay and organic material and are considered as better The Symmeria paniculata-Strophanthus sarmentosus soils. Silt soils, also composed of fine material, are often community occurs along rivers, on laterite or other clay associated with watering sites in the DFR. The DBH soils. The stands are on low ground and are often flooded (diameter at breast height) of trees was measured in each during the rainy season. The community is dominated by layer and the height determined using Haglof Vertex III a dense scrub of Symmeria paniculata, which represents V1–4. Samples of unknown species were taken for later 70% of the total vegetative cover and has a maximum , identification at the Institut Fondamental d Afrique Noire height of 6.2 m. The DBH range is 2.3–9.3 cm. On the (IFAN) of Cheikh Anta Diop University, Dakar. Altitude, ground level are Strophanthus sarmentosus and Cyperus aspect and location were recorded at each plot using a sp., giving a total cover of 10% for the herb layer (H). Global Positioning System (GPS). Laterite and other clay soils have poor water storage and Along with the vegetation survey, visible disturbance infiltration capacity. The bad drainage conditions and tree features were also investigated and recorded: fire (F), cutting affecting this community may explain the poor cutting (Cu), and signs of cattle grazing (Ca), such as species composition, yielding only six differential species: defoliation. Symmeria paniculata, Apocynaceae sp., Strophanthus sarmentosus, Mitragyna inermis, Rytigynia senegalensis and Acacia sieberiana. DATA ANALYSIS The vegetation data were analyzed by tablework Erythrophleum guineense-Oxytenanthera according to Braun-Blanquet methods of vegetation abyssinica community classification and analysis (Braun-Blanquet, 1964), The Erythrophleum guineense-Oxytenanthera abyssinica with successive raw table, frequency table, and partial, community was surveyed on riverbank sites mostly differential and synopsis tables. affected by erosion. These sites also have clay soils , Simpson s index (1949) was used to compare with poor drainage characteristics. The community diversity among communities. Its calculation was presents few differential species, mainly Erythrophleum exclusively based on relevés collected from woody guineense, Oxytenanthera abyssinica, Borassus flabellifer, savannas sites and gallery forest sites (58 relevés) to Dichrostachys cinerea, Saba senegalensis, Urginea avoid the variation of plot size. The diversity index was altissima, Gardenia ternifolia, Cassia tora and Sclerocarya calculated for each relevé and the mean was taken for birrea. The maximum height of the community is 19 m 28 Diané ABDOULAYE and Kazue FUJIWARA

Table 1. Summary table of savanna communities in the DFR.

1. Symmeria paniculata-Strophanthus sarmentosus community 2. Erythrophleum guineense-Oxytenanthera abyssinica community 3. Piliostigma thonningii-Mitragyna inermis community a. Ceiba pentandra subcommunity b. Borassus flabellifer subcommunity c. Grewia flavescens subcommunity 4. Grewia flavescens-Sterculia setigera community a. Lonchocarpus sepium subcommunity b. Annona senegalensis subcommunity 5. Ormocarpum bibracteatum-Prosopis africana community 6. Hibiscus asper-Cassia tora community a. Sclerocarya birrea subcommunity b. Combretum nigricans subcommunity b-1. Hygrophila odora under-unit b-2. Waltheria indica under-unit Communities 3 4 6 1 2 5 Subcommunities a b c a b a b Topography RS RB D D-RS FL FL FL FL FL FL FL FL C SL SL-G G-R SL C Soil L C C G C G Si C R C C S Cu Cu Cu F-Cu F Disturbances Cu Cu F F − F − Ca Ca Ca Ca Ca Total number of releves 3 4＊ 2 2 8 4 7 1 6 9 10＊ 2 2 Number of releves surveyed in rainy season 2 2 0 1 0 0 0 0 5 9 10 2 2 Number of releves surveyed in dry season 1 2 2 1 8 4 7 1 1 0 0 0 0 Mean number of species 6 9 14 21 13 14 21 14 15 14 19 31 42 Differential species of communities and sub-units Family Layers Symmeria paniculata Benth. Polygo. S 2(4) ・ 1(+) ・・・・・・Apocynaceae sp. Apocy. H 2(+) ・・・・・・Strophanthus sarmentosus DC. Apocy. H 1(+) ・・・・・・Erythrophleum guineense G. Don. Ces. T1 ・ 1(3) ・・・・・・I(+) I(+) ・・ Oxytenanthera abyssinica (A.Rich) Munro. Poa. H ・ 2(2) ・・・ 1(1) ・・ I(3) ・ I(+) ・・ Piliostigma thonningii (Schum.) Milne-Redh. Ces. T2 ・・ 2(+-2) 1(+) V(+-3) ・・・ III(+-2) I(+) II(+-2) ・ 1(2) Mitragyna inermis (Willd.) O. Ktze. Rub. T2 2(1) ・ 2(3-4) 2(2) III(2-4) ・・・・・ I(+) ・・ Diospyros mespiliformis Hochst. Ex A.DC. Eben. T1 ・・ 2(+-2) 1(2) II(+-2) ・ I(+) ・・・・・・Grewia lasiodiscus K. Schum. Tilia. T1 ・・ 2(2-3) ・ II(2-3) ・・・・・ I(+) ・・ Ceiba pentandra (L.) Gaertn. Bom. T1 ・・ 1(3) ・・・・・・Cordia rothii Roem. et Sch. Boragi. T2 ・・ 1(2) ・・・・・・Euphorbia heterophylla L. Euph. H ・・ 1(2) ・・・ I(+) ・・・・・・Ficus sp. Mora. T1 ・・ 1(2) ・・・・・・Cola cordilfolia (Cav.) R.Br. Ster. T1 ・・ 1(2) ・・・・・・Borassus flabellifer L. Arec. T1 ・ 1(+) 1(3) 2(+-3) ・・・・・・Rytigynia senegalensis Blume Rub. T2 1(+) ・ 1(1) 1(2) ・・・・・・Grewia flavescens Juss. Tilia. S ・・・・ II(+) 3(+-4) IV(1-3) ・ I(+) II(+-4) I(+-3) 1(+) 1(+) Sterculia setigera Del. Ster. T1 ・・・・ II(+) 2(2-4) IV(2) ・・・・ 1(2) 1(+) Dichrostachys cinerea (L.) Wight & Arn. Mimo. S ・ 1(+) 1(3) ・ III(+-4) 1(4) III(+) ・ III(+-2) I(+) ・・ 1(1) Saba senegalensis (A.DC.) Pichon Apocy. C ・ 1(2) ・・ II(+-2) ・・・ I(2) ・・・・ Parkia biglobosa (Jacq.) Benth. Mimo. T1 ・・・・ II(3) ・・・ I(+) ・・・・ Khaya senegalensis (Desr.) A. Juss. Meli. T1 ・・・・ II(2-3) ・・・・ II(+) ・・・ Lonchocarpus sepium DC. Faba. T2 ・・・・ I(1) 3(+-2) ・・・・・・Combretum velutinum DC. Combr. S ・・・・・ 2(+-1) ・・・・・・Burkea africana Hook. Cesal. T2 ・・・・・ 2(+) ・・・・・・Annona senegalensis Pers. Anno. S ・・・・・・V(+-2) ・・ II(+) II(+-2) 1(+) ・ Lonchocarpus laxiflorus Guill. & Perr. Faba. T2 ・・・・・・V(+-2) ・・ I(+) ・・・ Asparagus Pauli-Gulielmi Solms-Laub. Lilia. H ・・・・・・IV(+-2) ・・・・・・Urginea altissima Bak. Lilia. H ・ 2(+) ・・・・ IV(+-2) ・ II(+) II(+) II(+-2) 1(+) 1(+) Maytenus senegalensis (Lam.) Exell Sima. S ・・・・・ 1(+) IV(+) ・ I(+) ・・・・ Andira inermis (Wright) DC. Faba. T1 ・・・・・・III(+-2) ・・・・・・Gardenia ternifolia Schum. & Thonn. Rub. S ・ 1(+) ・・ I(+) ・ III(+-1) 1(1) ・ I(+) I(+) ・ 1(+) Hymenocardia acida Tul. Euph. S ・・・・・・III(+) ・ I(+) I(+) II(+-2) 1(+) ・ Lannea velutina A. Rich. Anac. T2 ・・・・・ 1(+) III(1-2) ・・ II(+-3) I(+) 1(2) ・ Securidaca longipedunculata Fres. Polyga. S ・・・・・・III(+-1) ・・ II(+) I(+) ・ 1(+) Terminalia avicennioides G & Perr. Combr. T2 ・・・・・・III(1-2) ・・・ I(+) ・・ Syzygium guineense (Willd.) DC. Myrt. S ・・・・・・III(+-1) ・・・ I(+) ・・ Lannea microcarpa Engl. & Kr. Anac. T1 ・・・・・・III(1-2) ・・・・・・Combretum Fragrans F. Hoffm. Combr. S ・・・・・・III(+-2) ・・・・・・Ormocarpum bibracteatum (Hochst.) Bak Faba. T2 ・・・・・・1(3) ・・・・・ Prosopis africana (G. & Perr.) Taub. Mimo. T1 ・・・・・・1(2) ・ Ⅱ(+) Ⅰ(+) 1(2) ・ Hibiscus asper Hook Malv. H ・・・・・・IV(+-3) Ⅲ(+-4) Ⅲ(+-3) 2(2-3) 2(+) Cassia tora L. Ces. H ・ 2(+) ・・・・・・IV(+) Ⅰ(2-3) Ⅰ(+-3) 1(+) 1(+) Andropogon pseudapricus Stapf. Poa. H ・・・・・・Ⅴ(2-5) Ⅲ(+-4) Ⅰ(5) ・・ Sclerocarya birrea (A. Rich.) Hochst. Anac. T2 ・ 1(+) ・・ II(+-1) ・・・ Ⅴ(+-2) ・ Ⅰ(+) ・・ Alysicarpus ovalifolius (S. & Th.) Faba. H ・・・・・・Ⅰ(5) ・・・ 1(+) Acacia polyacantha (Willd.) Mimo. S ・・・・・・Ⅰ(2) ・・・・ Combretum nigricans Lepr. ex Guill & Per Combr. S ・・・・・ 1(+) III(+) 1(2) Ⅰ(+) Ⅳ(+-4) Ⅳ(+-2) 1(1) 1(2) Cochlospermum tinctorium A. Rich. Coch. H ・・・・・・I(2) ・ Ⅳ(+-2) Ⅳ(+-5) 2(2) 1(2) Pennisetum pedicellatum Trin. Poa. H ・・・・・・Ⅱ(+-3) Ⅳ(+-5) Ⅴ(+-4) 2(2) 1(+) Pobeguinea arrecta (Stapf) J.- Fel Poa. H ・・・・・・Ⅰ(+) Ⅳ(+-4) Ⅲ(+-4) 2(3-5) ・ Acacia sieberiana DC. Mimo. T2 1(+) ・・ 1(+) ・・・・・・Ⅲ(+-3) 2(+) 2(+-2) Hygrophila odora (Nees) T. Anders Acan. H ・・・・・・1(+) 1(+) Asparagus africanus Lam. Lilia. H ・・・・・・2(+) 1(+) Eragrostis tremula Hochst. Poa. H ・・・ 1(1) ・・・・・・2(+) 1(+) Indigofera garckeana Vatke Faba. H ・・・・・・2(+) ・ Vigna sp. Faba. H ・・・・・・1(+) 1(+) Vegetation structure, diversity and site conditions in Senegal 29

Table 1. (continued). Waltheria indica L. Ster. H ・・・・・・Ⅰ(+) ・・ 2(+) Feretia apodanthera Del. Rub. H ・・・・・・2(+) Ocimum basilicum L. Lami. H ・・・ 1(2) ・・・・ Ⅰ(3) Ⅰ(4) Ⅰ(+) ・ 2(+) Pandiaka heudelotii (Moq.) Hook Amar. H ・・・・・・2(+) Urena lobata L. Malv. H ・・・ 1(+) ・・・・ Ⅰ(3) Ⅰ(2) Ⅰ(+) ・ 2(+) Common savanna species Combretum glutinosum Perr. Combr. S ・ 3(+-3) 2(+-2) 2(+) IV(+-4) 4(3-4) V(2-3) 1(3) Ⅴ(+-2) Ⅴ(+-3) Ⅴ(+-3) 2(2-3) 2(2-3) Pterocarpus erinaceus Poir. Faba. T1 ・ 3(+) 1(3) 2(2) V(+-2) 3(+-2) IV(2-3) 1(+) IV(+-4) Ⅲ(+-3) Ⅳ(+-3) 2(+-4) 1(+) Lannea acida A. Rich. Anac. T1 ・ 2(+) ・ 1(1) IV(+-3) 4(+-2) IV(+-2) 1(1) Ⅲ(+-3) Ⅲ(+-3) Ⅲ(+-2) 1(+) 1(+) Bombax costatum Pell. & Vuill. Bom. T1 ・ 1(2) ・ 1(+) III(+-2) 4(+-5) V(+-3) ・ IV(+-3) Ⅴ(+-3) Ⅴ(+-4) 2(2-3) 2(2-3) Hexalobus monopetalus (A. Rich.) E. & D Anno. S ・・・ 1(2) III(+-2) 1(2) V(+-3) ・ Ⅲ(+-3) Ⅱ(+) Ⅳ(+-3) 1(+) 1(+) Strychnos spinosa Lam. Loga. S ・ 1(4) ・ 1(+) I(+) 3(+-4) V(2-3) 1(3) Ⅱ(+-3) Ⅲ(+-3) Ⅳ(+-3) 1(1) 1(+) Cordyla pinnata (Lepr.) Milne-Redh. Ces. T1 ・ 2(2) ・・ II(+) 4(+-2) V(+-2) 1(+) Ⅱ(+-2) Ⅲ(+-2) Ⅴ(+-3) 2(+-3) 2(+-2) Terminalia macroptera Guill. & Perott. Combr. T1 ・・・ 1(+) IV(1-3) 1(+) IV(+-2) ・ Ⅰ(2) Ⅰ(+) Ⅳ(+-2) 1(2) 2(+-3) Cassia sieberiana DC. Mimo. T2 ・ 1(3) 1(+) 2(+) IV(+-2) ・・ 1(+) Ⅲ(+-2) Ⅰ(+) Ⅰ(+) ・ 1(+) Vitex madiensis Oliver Verbe. S ・ 1(2) ・ II(+-1) 3(+-2) V(+-3) 1(3) Ⅰ(+) Ⅰ(+) Ⅳ(+-2) 2(+-2) 1(2) Sansevieria senegambica Bak. Agav. H ・ 2(+-2) ・ 1(3) I(2) ・ I(2) ・ Ⅲ(+-2) Ⅰ(4) Ⅰ(+) 1(+) 1(3) Combretum micranthum G. Don. Combr. S ・・・ 2(+-2) IV(+-4) 4(+-3) ・・ Ⅰ(+) Ⅰ(3) Ⅰ(+-3) ・ 1(2) Detarium microcarpum G. & Perr. Ces. T1 ・・・・ I(+) 1(+) III(+-2) 1(3) Ⅰ(3) Ⅰ(+) Ⅰ(+) 1(2) ・ Acacia ataxacantha DC. Mimo. S ・ 2(+) 1(2) 1(+) II(+) 2(2-3) I(1) ・ Ⅰ(+) ・ Ⅰ(3) ・・ Other savanna species Combretum sp. Combr. S 1(+) 1(+) ・・ I(+) 1(2) II(+) 1(+) ・ Ⅰ(+) ・・・ Indigofera sp. Faba. H ・・・・・・Ⅰ(+-5) 1(+) 1(+) Pennisetum violaceum (Lam.) L. Rich. Poa. H ・・・ 1(2) ・・・・・・Ⅰ(+) 1(+) 2(+) Piliostigma reticulatum (DC.) Hochst. Ces. T2 1(+) ・ 1(+) ・ I(2) ・ II(+) ・・ Ⅰ(+) ・ 1(+) 2(+) Ziziphus mucronata Willd. Rhamn. S ・ 1(+) 1(2) 1(+) I(+) ・・・ Ⅰ(+) Ⅰ(+) ・・ 1(+) Grewia bicolor Juss. Tilia. T2 ・・・ 1(+) I(+) ・・・ Ⅲ(+-3) Ⅰ(+) ・ 1(+) ・ Guiera senegalensis J.F Gmel. Combr. S ・・ 1(3) 2(+) II(+-3) ・・・・・・1(+) ・ Mitracarpus scaber Zucc. Rub. H ・・・・・・I(+) ・ Ⅰ(3) Ⅰ(2) Ⅰ(+-2) ・ 1(+) Tamarindus indica L. Ces. T1 ・・・・ I(+) ・・・ Ⅰ(+) Ⅰ(3) ・・ 1(+) Baissea multiflora A.DC. Apocy. C ・・・・・・II(+) ・・ Ⅰ(+) ・ 2(+) 1(+) Anogeissus leiocarpus (DC.) G. & Perr. Combre T1 ・ 1(3) ・ 1(2) I(2) ・・・・・・1(3) Ficus dicranostyla Mildbr. Mora. T1 ・・ 1(+) ・・・ I(2) ・・ Ⅰ(+) ・・・ Combretum tomentosum G. Don Combr. S ・・・・ Ⅰ(1) 1(2) III(+-2) ・・・ Ⅰ(+) ・・ Spondias mombin L. Anac. T1 ・・ 1(2) 2(2) II(+-2) ・・・・・・Afrormosia laxiflora (Benth.) Harms. Faba. T2 ・・・・ I(+) ・ I(+) ・・・・ 1(+) ・ Afzelia africana Sm. ex Pers. Ces. T1 ・・・・・・I(2) ・ Ⅰ(2) ・・・ 1(+) Tephrosia deflexa Bak. Faba. H ・・・・・・Ⅰ(+) ・ Ⅰ(+) ・ 1(+) Rhynchosia minima (L.) DC. Faba. H ・・・・・・Ⅰ(+) Ⅰ(+) ・ 1(+) Sesbania pachycarpa DC. Faba. H ・・・ 1(+) ・・・・・ Ⅰ(+) ・ 2(+-3) ・ Ximenia americana L. Olaca. S ・・・・・・II(+) ・・ Ⅰ(+) Ⅰ(+) ・・ Rubiaceae sp. Rub. H ・・・・・・Ⅰ(+) ・ 1(+) Securinega virosa (Roxb.) Baill. Euph. S ・・・・・ 1(+) ・・ Ⅰ(3) ・・・・ Neocarya macrophylla (Sabine) Prance Myrt. T2 ・・・・ Ⅰ(3) 1(+) ・・・・・・Boscia angustifolia A. Rich. Cappa. S ・・・・・・Ⅰ(+) ・・ 1(+) Brachiaria distichophylla Stapf. Poa. H ・・・ 1(2) ・・・・・・Ⅰ(+) ・・ Cassia nigricans Vahl. Ces. H ・・・・・・Ⅰ(+) ・ Ⅰ(+) ・・ Drypetes floribunda (Mull. Arg.) Hutch. Euph. H ・・・・・・Ⅰ(+) Ⅰ(+) ・・ Dioscorea preussii Pax. Diosc. H ・・・ 1(1) ・・・・・・2(+) Crescentia cujete L. Bigno. H ・・・・・・Ⅰ(+) Ⅰ(+) ・・ Cyperus sp. Cyp. H 1(+) ・・ 1(+) ・・・・・・Grewia villosa Willd. Tilia. S ・・・・・・Ⅰ(+) ・ 1(+) Ipomoea coscinosperma Hochst. Convu. H ・・・・・・Ⅰ(+-3) ・ 1(+) Stereospermum kunthianum Cham. Bigno. S ・・・・・・1(+) 1(+) Triumfetta pentandra A. Rich. Tilia. H ・・・・・・Ⅰ(+) Ⅰ(+) ・・ Vigna racemosa (G. Don) H. & D. Faba. H ・・・・・・Ⅰ(+) ・ 1(+) Acanthospermum hispidum DC. Aster. H ・・・・・・1(+) Albizia ferruginea (G. & Perr.) Benth. Mimo. T1 ・・・・・・I(2) ・・・・・・Amarantaceae sp. Amar. H ・・・ 1(+) ・・・・・・Blainvillea gayana Cass. Aster. H ・・・・・・1(+) ・ Boerhavia diffusa L. Nyct. H ・・・・・・Ⅰ(+-3) ・・ Borreria verticillata (L.) G. F. Mey. Rub. H ・・・・・・Ⅰ(+) ・・ Combretum lecardii Engl. & Diels Combr. S ・・・・・・Ⅰ(+) ・・ Commelina sp. Com. H ・・・ 1(+) ・・・・・・Compositeae sp. Comp. H ・・・ 1(+) ・・・・・・Corchorus trilocularis L. Tilia. H ・・・・・・1(+) ・ Crotalaria goreensis G. & Perr. Faba. H ・・・・・・Ⅰ(+) ・・ Crossopterix febrifuga (Afz.) Benth. Rub. S ・・・・・・Ⅰ(+) ・・・ Cucumis melo var. agrestis Naud. Cucur. H ・・・・・・Ⅰ(+) ・・ Cyperus compressus L. Cyp. H ・・・・・・Ⅰ(+) ・・ Desmodium hirtum G. & Perr. Faba. H ・・・・・・1(+) Desmodium sp. Faba. H ・・・・・・1(+) ・ Eriosema afzelii Bak. Faba. H ・・・・・・1(+) Erythrina senegalensis DC. Faba. T2 ・・・・・・Ⅰ(+) ・・・ Gardenia triacantha DC. Rub. S ・・・・・・1(+) Hermannia tigrensis Hochst. Ster. H ・・・・・・1(+) Icacina senegalensis A. Juss. Verbe. H ・・・・・・1(+) Indigofera secundiflora Poir. Faba. H ・・・・・・1(+) Ipomoea eriocarpa R. Br. Convu. H ・・・・・・1(+) 1(+) Ipomoea ochracea (Lindl.) G. Don. Convu. H ・・・・・・1(+) ・ 30 Diané ABDOULAYE and Kazue FUJIWARA

Table 1. (continued). Ipomoea involucrata P. Beauv. Convu. H ・・・・・・1(+) Ipomoea nil (L.) Roth. Convu. H ・・・・・・1(+) Lablab niger Medik. Faba. H ・・・・・ 1(+) ・・・・・・Leptadenia sp. Ascle. H ・・・・・・1(+) Nauclea latifolia Sm. Rub. H ・・・・・・Ⅰ(+) ・・ Pavetta crassipes K. Schum. Rub. H ・・・・・ 1(+) ・・・・・・Setaria pallidifusca Stapf. & Hubb. Poa. H ・・・・・・1(+) Setaria sp. Poa. H ・・・・・・1(+) Sida alba L. Malv. H ・・・ 1(+) ・・・・・・Sida rhombifolia L. Malv. H ・・・・・・1(+) Striga senegalensis Benth. Scro. H ・・・・・・Ⅰ(+) ・・・ Tephrosia lathyroides G. & Perr. Faba. H ・・・・・・1(+) ・ Tephrosia pedicellata Bak. Faba. H ・・・・・・1(+) Terminalia albida Sc. Elliot Combr. S ・・・・・・1(+) ・・・・・ Vitex doniana Sw. Verbe. S ・・・・・・1(+) Wahlenbergia campanuloides (Del.) Vatk Campa. H ・・・・・・1(+) NOTE: RS: riverside; RB: riverbank; D: depression; FL: flat; Cu: cutting; Ca: cattle; F: fire; C: clay; L: laterite; SL: sandy-loam; S: sandy; G: gravel; R: rocky; Si: silt.

Numbers before parentheses refer to frequency class which is summarized depending on the number of relevés within the community. For less than 5 relevés, the frequency class cannot be determined. For this case the actual number of relevés is used : 4, 3, 2, 1. From 5 relevés and above, the frequency class which depends on the number of relevés of the community is organized as the following : 100−81% : V ; 80−61% : IV ; 60−41% : III ; 40−21% : II ; 20−11% : I ; 10−6% : + ; 5% : r Numbers between parentheses refer to the cover value and/or range. +: less than 5% and few individuals ; 1: less than 5% but some individuals ; 2: 5−25% ; 3: 25−50% ; 4: 50−75% ; 5: 75−100%.

Asterisk (＊) for number of relevés means that a plot of 5×5 m is included in that community. These plots (of 5×5 m) were surveyed during the rainy season.

(tree layer 1). The average number of species is 9. Crown Tree cutting is the only disturbance on such sites. coverage by trees is 20–40%. The DBH range is 91.2– The Grewia flavescens subcommunity is localized 126.6 cm for canopy tree (T1) layer, 11.0–33.0 cm for on flatter sites under cutting and cattle grazing. This understory tree (T2) layer and 5.3–6.9 cm for the shrub subcommunity is exclusively composed of woody species (S) layer. Disturbances affecting this community are and is differentiated by Grewia flavescens, Dichrostachys cattle grazing and tree cutting. cinerea, Sterculia setigera, Saba senegalensis, Parkia biglobosa and Khaya senegalensis. Piliostigma thonningii-Mitragyna inermis community Grewia flavescens-Sterculia setigera community This community is found both on riversides and in Located on flat savanna areas, the Grewia flavescens- depressions within the drier savanna areas and occurs Sterculia setigera community occurs on various soil types mainly on clay soils, secondarily on sandy-loam and silt (gravel, rocky, sandy-loam, clay) under the influence soils. The maximum height of the T1 layer is 21.5 m, of fire. This community is differentiated by Grewia with a total cover of 30%. The community is differentiated flavescens, Sterculia setigera and Dichrostachys cinerea. by Piliostigma thonningii, Mitragyna inermis, Diospyros Understory tree (T2) and shrub (S) layers dominate in mespiliformis and Grewia lasiodiscus. This community was this community, with respectively 25% and 40% cover with classified into three subcommunities: Ceiba pentandra heights of 5.3–14.2 m. The Grewia flavescens-Sterculia subcommunity, Borassus flabellifer subcommunity, and setigera community is composed of two subcommunities Grewia flavescens subcommunity. with different micro-site conditions. The Ceiba pentandra subcommunity is differentiated The Lonchocarpus sepium subcommunity occurs by Ceiba pentandra, Cordia rothii, Euphorbia heterophylla, on sandy-loam, gravel and rocky sites. Despite the Ficus sp. and Cola cordifolia. It occurs on a depression occurrence of all disturbances factors (grazing, cutting with clay soil. Tree cutting and cattle grazing are the and fire), this subcommunity presents a total species main disturbances at this site. The Ceiba pentandra number of 14, which is higher than those of riverside and subcommunity is characteristic of clay soils in depressions riverbank communities. Total cover in this subcommunity within savanna ecosystems that have tall forest trees and varies from 50% for tree and shrub layers to 5% for the an understorey composed of Dichrostachys cinerea and herb layer. Acacia ataxacantha. The second subcommunity of Annona senegalensis is The Borasssus flabellifer subcommunity is much richer (average number of species of 21) and occurs differentiated by two species: Borassus flabellifer and exclusively on fire-affected sites. This subcommunity Rytigynia senegalensis. This subcommunity is found both suggests the existence of fire-stimulated or fire-resistant along rivers and in depressions, on silt and clay soils. species in southeastern Senegalese savannas. It is mainly Vegetation structure, diversity and site conditions in Senegal 31 composed of thorn-shrub species generally under 6 m in ±SD) with a mean height of 1.9±0.9 m (Fig. 2). At the height and with an average cover of 25%. The drainage end of the dry season, marked by six months of climatic condition-generally determined by soil types-is favorable, stress, the herbaceous cover is considerably reduced with gravel, rocky and clay soils. and the savanna vegetation is dominated by the woody layers. The shrub layer becomes dominant (31.0±12.7%), Ormocarpum bibracteatum-Prosopis africana followed by the T1 layer (27.0±9.5%) and T2 layer (18.0 community ±6.1%). The herb layer is still present (8.0±3.4%) despite This is a part of drier, flat savanna communities occurring the stress, but is restricted to riversides and depression on gravel soils under the influence of fire. The total areas where water is available throughout the year. number of species is 14 and consists of woody species Species richness shows the same seasonal variation as for (tree layer 2 and shrub species essentially) reaching a the cover and height of the layers. Herbaceous species total cover of 30%, with a maximum height of 10.9 m. are dominant at the end of the wet season with mean number of species of 11.0±5.7 followed by shrub, T2 and Hibiscus asper-Cassia tora community T1 species (8.0±3.1, 6.0±2.2 and 5.0±1.4, respectively). This community is dominated by grass and other herb At the end of the dry season, woody species tend to be species, including Hibiscus asper, Cassia tora, and the more common, especially in the shrub layer, with a mean tall-savanna herb species Pennisetum pedicellatum, number of species of 7.0 ± 1.8 while the herbaceous Andropogon pseudapricus, Popeguinea arrecta and species become almost non existent (1.7±0.8). Ocimum basilicum, which may reach 3 m in height at Seasonal analysis of the savannas of the DFR shows the end of the rainy season. The herb cover within this two main variations: 1) a large dominance by grass and community varies from 25 to 75%, with an average height other herb species, both in cover and total number of of 2 m. This community includes few woody species and species, at the end of the growing season (rainy season); occurs extensively on sandy, sandy-loam, clay and gravel and 2) a somewhat equivalent dominance by the shrub soils with fire as the exclusive disturbance form. The layer, both in total cover and species richness, at the end occurrence of fire in this community is favored by the of the dry season. continuous herbaceous carpet, which provides fuel and fosters the fast spread of savanna fires. In the absence of Community diversity and life forms , fire, the Hibiscus asper-Cassia tora community tends to be As for species richness, Simpson s index of diversity richer on gravel sites, with the highest mean number of tends generally to increase from the riverside species recorded (42 species). communities toward the drier-site communities that are The Sclerocarya birrea subtype of this community affected by fire but have better drainage conditions (Table occurs on sandy-loam and clay soils with a mean number 2). The Symmeria paniculata-Strophanthus sarmentosus species of 15, a height range of 4.2–14.2 m and no community has the lowest diversity index (1.3 ± 0.5) disturbance factors. The second subtype, of Combretum and species richness with only 9 species recorded. The nigricans is dominated by herb species (mean of 26 Riverbank community of Erythrophleum guineense- species), and occurs on flat sites with clay, sand and Oxytenanthera abyssinica also presents lower diversity gravel soils. In this subcommunity the herb cover is 25 value compared to others communities from flat and dry –75%, and the tree layer reaches 18 m, with an average savanna areas. High diversity-index values were found on cover of 30%. drier savanna areas, in the overwhelmingly herbaceous The sensitivity of these two subcommunities to communities of Hibiscus asper and Cassia tora (5.5 ± fire may be a factor of their structural composition, 2.0), within Piliostigma thonningii-Mitragyna inermis dominated by grass and other herb layers. At the end of community (5.7 ± 2.0) and Grewia flavescens-Sterculia the rainy season, and with the lower moisture content of setigera community (7.0 ± 1.7). The typical community the vegetation, the tall savanna herbs represent fuel for of Ormocarpum bibracteatum-Prosopis africana of dry fast–spreading savanna fires. sites shows an index of 6.6 with a total of 14 species. The high diversity of drier savanna sites, as compared Savanna vegetation structure and species richness to wetter areas, may be explained by the better drainage The vegetation structure is dominated by grass and other conditions, with diverse soil types and fewer disturbance herb species at the end of the wet season. The herb layer factors. The common disturbance in the dry savanna sites reaches an average cover value of 35.0 ± 19.3% (mean is fire. 32 Diané ABDOULAYE and Kazue FUJIWARA

�� 30

15 �� 10

�� 30

18 16 14 12 10 8

�� 6 4 2 0 �1 �2 � �

Fig. 2. Mean height, cover and number of species of vegetation layers at the end of rainy and dry seasons. Vertical lines represent SD. T1: tree layer 1 (tree canopy layer); T2: tree layer 2 (understorey tree layer); S: shrub layer and H: herb layer. Vegetation structure, diversity and site conditions in Senegal 33

As for species richness and structural composition, abundant within drier savanna communities. the vegetation communities of the DFR show a gradient of generally increasing diversity from the wetter sites to microtopography and soil moisture the drier ones. Soil moisture content was successively analyzed for Microphanerophytes are the most widespread life rainy season and dry season (mean ± SD) within each form in the Diambour Forest Reserve and dominated community. This analysis shows a lower seasonal within all communities, followed by mesophanerophytes variability of soil moisture in the riverside community of (Fig. 3). Tall, large megaphanerophyte trees are found Symmeria paniculata-Strophanthus sarmentosus with 40.0 on riverbanks and in depressions while annual plants are ± 0.1% in the rainy season and 34.0% in the dry season.

, Table 2. Number of species, Simpson s index of diversity and soil moisture content (mean values ± standard deviation) in floristic communities: Symmeria paniculata –Strophanthus sarmentosus comm.; Erythrophleum guineense –Oxytenanthera abyssinica comm.; Piliostigma thonningii –Mitragyna inermis comm.; Grewia flavescens–Sterculia setigera comm.; Ormocarpum bibracteatum–Propsopis africana comm.; Hibiscus asper–Cassia tora comm. A total of 58 relevés of 50×50 m were used. Floristic communities S.P–S.S E.G–O.A P.T–M.I G.F–S.S O.B–P.A H.A–C.T Number of relevés 3 3 12 11 1 28 Number of species 6.0±0.0 9.3±1.1 14.7±3.8 19.0±4.6 14.0 18.1±9.1 , Simpson s index of diversity 1.3±0.5 2.5±0.9 5.7±2.0 7.0±1.7 6.6 5.5±2.0 Soil moisture in rainy season (%) 39.9±0.0 38.7±1.3 41.8 – – 32.2±5.9 Soil moisture in dry season (%) 34.0 33.3±0.4 2.4±6.2 0.0±0.0 0.0 0.0

100�

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60� ��

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0� 1 2 3 4 5 6 ��

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Fig. 3. Life-forms spectra of savanna communities in the DFR. 34 Diané ABDOULAYE and Kazue FUJIWARA

The riverbank community of Erythrophleum guineense- and depressions). Generally, on sites where water is Oxytenanthera abyssinica also shows a lower sensitivity available, the number of species tends to increase as to the seasonal variation of soil moisture with 38.8 ± compared to drier sites. The opposite situation has been 1.4% in the rainy season and 33.3 ± 0.4% at the end of found in the DFR, where communities on wetter areas the dry season. The lower variability of soil moisture with laterite and clay soils tend to be poorer in species content between seasons in the riverside and riverbank than dry sites with soils of sandy, sandy-loam, gravel and communities contrasts with an important seasonal rocky structures. variability within the flat and drier savanna communities The lower species occurrence in depressions and of Piliostigma thonningii-Mitragyna inermis, Grewia riverside sites seems due to soil type and, in particular, to flavescens-Sterculia setigera, Ormocarpum bibracteatum- the poor drainage characteristics of laterite and other clay Prosopis africana and Hibiscus asper-Cassia tora with soils. moisture content as high as 41.8% in the rainy season and The lower species numbers within depression and as low as 0.0% in the dry season (Table 2). riverside areas may also be a consequence of selective The analysis of soil moisture content in these savanna grazing by cattle. During the dry season, with the communities with different microtopography shows a decrease of water resources, cattle herds are maintained strong variability between the rainy and dry seasons; and near water sources. Some cattle owners prefer a daily between riverside and riverbank communities and drier divagation of the herd between pastures and watering and flat savanna sites. sites. The daily movements or the presence of herds may damage and harden the upper surface of the soil, limiting species occurrence near water sources. DISCUSSION AND CONCLUSION Drier and flat savanna sites with more diverse soil In terms of composition and characteristics, the DFR types (gravel, sandy, rocky, sandy-loam) and fire as a showed six main vegetation units along a gradient from common disturbance form, present various plant species riversides to drier savannas areas. These communities and life-form spectra. These findings show, as have many are influenced by the seasonal variation of rainfall and soil other studies, that fire effect is not always negative in the moisture content. Riverside and riverbank communities savanna. Our study shows a clear relationship between which were surveyed in both seasons appear to be less the floristic diversity of savanna vegetation, soil type, sensitive to soil moisture content and to seasonal variation. disturbance factors and micro-topography. Generally, But the drier savanna communities of Piliostigma and for a long time, the composition and structure of thonningii-Mitragyna inermis, Grewia flavescens-Sterculia savanna vegetation have been explained as a result of the setigera and Ormocarpum bibracteatum-Prosopis africana degradation of former dry forest and rainforest through which were surveyed mainly in dry season appear to be human disturbance. This study suggests that, alongside the seasonal variation of the herb and grass dominated disturbance factors, various environmental conditions, community of Hibiscus asper-Cassia tora mostly surveyed particularly soil texture and drainage characteristics, are during the rainy season. Moreover, the six communities also important in determining the floristic composition of classified in this study could be summarized into three savannas, at least in southeastern Senegal. The influence main communities: the riverside community of Symmeria of micro-site conditions has been given little theoretical paniculata-Strophanthus sarmentosus; the riverbank consideration in the savanna literature (Vetaas, 1992) community of Erythrophleum guineense-Oxytenanthera but appears to be important for the floristic composition abyssinica and the drier and flat savanna community of savannas. A broad-scale continental study of African represented by Piliostigma thonningii-Mitragyna inermis, savannas investigated the relative importance of resource Grewia flavescens-Sterculia setigera and Ormocarpum availability (water, nutrients) and disturbance regimes bibracteatum-Prosopis africana communities in the (fire, herbivory) in regulating woody cover (Sankaran et dry season and replaced by Hibiscus asper-Cassia tora al., 2005). In this recent study at continental scale, the community in the rainy season. woody cover of savannas tended to increase with lower The common disturbance form on the riverside, clay content in the soil. The riverside and depression riverbank and depression sites is tree cutting. The poor communities have abnormally fewer species, as compared species richness observed in riversides and depressions to dry savanna sites with better soil conditions. Also in communities corresponds generally to the location of Bolivian savannas, a soil texture-moisture gradient was laterite and clay-dominated sites (riversides, riverbanks identified as the prime factor determining variation in the Vegetation structure, diversity and site conditions in Senegal 35 floristic composition of vegetation communities (Haase, savanna species to annual fires, more research seems 1990). important on the physical and biological characteristics Fire constitutes a serious issue in protected areas of the common shrub species. Moreover, study of the fire in Senegal, where it is considered as a management tool return interval, through experimental and“ controlled” (Mbow et al. 2004). Ecological studies within the NKNP fire, can be of particular importance in investigating the showed that fire-in the absence of other human activities positive and negative influence of fire in the savannas of (i.e. agriculture, breeding, natural resources extraction, southeast Senegal. etc.) – is the main cause of vegetation degradation (Sonko, 2000). In our study, fire affects the dry savanna ACKNOWLEDGEMENTS This paper arose from communities more than riverside and depression a deep concern about savannas of southeastern Senegal. communities, which is consistent with former studies This investigation has been made possible through the in the same geographical region. Mbow et al. (2003), support of many people, both in Senegal and Japan. assessing fire risk in NKNP, found that it was high in We would like to express our sincere thanks to Agnès tree-shrub, woodland and shrub savanna because of the Thiaw, Sadia Badji, Bakary Fofana, Mamadou Badji, and fuel continuity and low in flooded areas along rivers, Alamouta Sadiakhou for field assistance and Mr. Koma on plateau outcrops, and in wet areas corresponding to and Mr. Diop of IFAN for plant identification. We are gallery forest. especially grateful to Dr. Paul Ndiaye of Cheikh Anta The Grewia flavescens-Sterculia setigera community Diop University and to Professor Elgene Box of Georgia occurs almost exclusively with dense cover on fire- University for reviewing this paper. We finally would like affected sites and shows the highest diversity index to thank the authorities of the National Park of Niokolo- compared to other communities (7.0±1.7). The Annona Koba for facilitating our research. senegalensis subcommunity is restricted to drier savanna areas affected by fire. The dense cover and high sociability levels within this subcommunity suggest an REFERENCES adaptation of these species to annual savanna fires. These Adam, J.G. 1966. Composition floristique des principaux species show more vitality and higher cover values on types de végétation du Sénégal. Journal of West fire-affected sites than within undisturbed areas. The African Science Association, 11: 87–97. frequency of fire in southeast Senegal – a fire-prone Adam, J.G. 1971. Le milieu biologique, flore et végétation. region (Nielsen et al., 2003) – may have led to a strategic In Le Niokolo Koba, premier grand Parc National adaptation of the species occurring in the Annona de la République du Sénégal. Dakar, Editions GIA senegalensis subcommunity. Among the most distinct (Groupe Inter Africain). Pp. 43–62. fire adaptations are the suites of characters associated Ba, A.T., Sambou, B., Ervik, F., Goudiaby, A., Camara, with fire life-history: the scheduling of reproduction and C. & Diallo, D. 1997. Végétation et flore. Parc mortality, the development of a thicker bark, multiple Transfrontalier du Niokolo Badiar. Union reproductive episodes, better seed production, and Européenne-Niokolo Badiar, ISE. 157 pp. adaptive seed protection of plants in fire-prone areas Bassot. J.P. 1966. Etude géologique du Sénégal Oriental (Bond and Van Wilgen, 1996). et de ses confins Guinéo-Maliens. Mémoires BRGM. In the Brazilian cerrado, Hoffmann and Solbrig Paris. 322 pp. (2002), investigating the differential response of specific Bond W.J. & van Wilgen B.W. 1996. Fire and Plants. sub-shrub, shrub and tree species, found that topkill and Chapman & Hall 263 pp. mortality of plants tended to be significantly greater in Braun-Blanquet, J.1964. Pflanzensoziologie. Grundzuege high-intensity burns than in low-intensity burns. They der Vegetationskunde. 2nd edition. Springer-Verlag, concluded that the positive or negative influence of fire Vienna. 865 pp. on savanna communities depends largely on the intensity Crawley, M.J. 1997. The structure of plant communities. and frequency of fires. Plant Ecology (ed.M.J. Crawley), 2nd edn. Blackwell, Fire is becoming a major factor influencing the Oxford. Pp. 475–531. structure and species richness of savanna communities. Diané A. 2001. Anthropisation et transformations du Nevertheless, its effect may be very negative through milieu dans le sud-ouest de la Forêt classée du , high frequency and intensity. Diambour : Etude de l occupation humaine et de For better understanding of the adaptation of son impact sur le cadre naturel. Université Gaston 36 Diané ABDOULAYE and Kazue FUJIWARA

Berger. 117 pp. Saint-Louis (Unpublished) Magurran A.E. 1988. Ecological diversity and its Dublin, H.T., Sinclair, A.R.E & McGlade, J. 1990. measurement. Princeton University Press.125 pp. Elephants and fire as causes of multiple stable states Mbow, C., Goita, K., Benie, G.B. 2004. Spectral indices in the Serengeti-Mara woodlands. Journal of Animal and fire behavior simulation for fire risk Assessment Ecology, 59: 147–1164. in savanna ecosystems. Remote Sensing of Frederiksen P. & Lawesson J.E. 1992.Vegetation types Environment, 91: 1–13. and patterns in Senegal based on multivariate Mbow, C., Sambou, B., Ba, A.T., Goudiaby, A. 2003. analysis of field and NOAA-AVHRR satellite data. Vegetation and fire readiness in main morphological Journal of Vegetation Science, 3: 535–544. units of Niokolo Koba National Park (Southeast Fujiwara K. 1987. Aims and methods of phytosociology or Senegal). Danish Journal of Geography, 103: 55–62. “Vegetation Science”. Plant Ecology and Taxonomy Mueller-Dombois, D.R., Ellenberg, H. 1974. Aims and to the memory of Dr. Satoshi Nakanishi. Kobe method of vegetation ecology Wiley, New York. 547 Geobotanical Society, Kobe. Pp. 607–628. pp. Haase, R. 1990. Community composition and soil Nielsen, T.T., Rasmussen, K., Mbow, C., & Toure, properties in northern Bolivian savanna vegetation. A. 2003. The fire regime of Senegal and its Journal of Vegetation Science, 1: 345–352. determinants Danish Journal of Geography, 103: 43 Heinrich J. & Moldenhauer K.M. 2002. Climatic and –53. anthropogenic induced landscape degradations of Pellew, R.A.P. 1983. The impacts of elephants, giraffe West African dry Savanna environments during the and fire upon the Acacia tortilis woodlands of the Later Holocene. Quaternary International, 93–94: Serengeti. African Journal of Ecology, 21: 41–74. 127–137. Pickett, S.T.A. & White, P.S. 1985. The ecology of natural Helgren, D.M. & Butzer K. 1977. Paleosols of the disturbance. Academic Press, New York. 472 pp. southern Cape coast, South Africa: implications for Prins, H.H.T. & van der Jeugd, H.P. 1993. Herbivore laterite definition, genesis and age. Geographical population crashes and woodland structure in East Review, 67: 430–445. Africa. Journal of Ecology, 81: 305–314. Higgins, S.I., Bond, W.J. & Trollope, W.S.W. 2000. Fire, Reed M.S. & Clokie, M.R.J. 2000. Effect of grazing and resprouting and variability: a recipe for grass-tree cultivation on forest plant communities in Mount coexistence in savanna. Journal of Ecology, 88: 213– Elgon National Park, Uganda. African Journal of 229. Ecology, 38: 154–162. Hoffman W.A., Solbrig, O.T. 2003. The role of topkill in Rigina, O. & Rasmussen, M.S. 2003. Using trend line and the differential response of savanna woody species principal component analysis to study vegetation to fire. Forest Ecology and Management, 180: 273– changes in Senegal 1986–1999 from AVHRR-NDVI 8 286. km data. Danish Journal of Geography, 103: 31–42. Hutson, M. 1979. A general hypothesis of species Sankaran, M., Hanan, N.P., Scoles, R.J., Ratnam, J., diversity. American Naturalist, 113: 81–101. Augustine, D.J., Cade, B.S., Gignoux, J., Higgins, Kuroda, A., Mukai, S., & Toyohara, G. 2006. Floristic S.J., Le Roux, X., Ludwig, F., Ardo, J., Banyikwa, F., composition and community structure of dense Bronn, A., Bucini, G., Caylor, K.K., Coughenour, undergrowth vegetation formed by evergreen M.B., Diouf, A., Ekaya, W., Feral, C.J. & Februa. perennial ferns, Dicranopteris linearis and 2005. Determinants of woody cover in African Gleichenia japonica (Gleicheniaceae). Vegetation savannas. Nature, 438: 846–849. Science, 23: 25–36. Schneider, A. & Sambou, K. 1982. Prospection botanique Louda, S.M. 1982. Distributional ecology: variation in dans les Parcs Nationaux du Niokolo-Koba et de plant recruitment over a gradient in relation to Basse Casamance. Recherches Scientifiques dans insect seed predation. Ecological Monographs, 52: les Parcs Nationaux du Senegal. Mémoires de , 25–41. l IFAN, 92: 101–122. Louda, S.M. 1989. Differential predation pressure: Scholes, R.J. & Archer, S.R. 1997. Tree-grass interactions a general mechanism for structuring plant in savannas. Annual Review of Ecological System, communities along complex environmental 28: 517–544. gradients. Trends in Ecology and Evolution, 4: 158– Serneels, S. & Lambin, E.F. 2001. Impact of land- 159. use changes on the wildebeest migration in the Vegetation structure, diversity and site conditions in Senegal 37

northern part of the Serengeti-Mara ecosystem. Vetaas, O.R. 1992. Micro-site effects of trees and shrubs Journal of Biogeography, 28: 391–407. in dry savannas. Journal of Vegetation Science, 3: 337 Simpson, E.H. 1949. Measurement of diversity. Nature, –344. 688: 163. Sonko, I. 2000. Etude des régimes de feux dits précoces Received 6th May 2006 et de feux tardifs sur la flore et la végétation Accepted 18th June 2007 ligneuses des plateaux du Parc National du Niokolo Koba (Sud-Est Sénégal). Thèse de Doctorat de troisième cycle. Institut des Sciences de , l Environnement, FST-UCAD-Dakar. Traoré, S.A. 1997. Analyse de la flore et de la végétation de la zone de Simenti (Parc National de Niokolo- Koba), Senegal Oriental. Thèse de troisième cycle. FST-UCAD-Dakar.

Photo 1. Symmeria paniculata site close to Photo 2. Riverside communities along the Niériko River. Niériko River.

Photo 3. Land preparation under shifting Photo 4. Tree-shrub savanna after burning at cultivation. the beginning of the rainy season. 38 Diané ABDOULAYE and Kazue FUJIWARA

APPENDIX 1: List of species. No. Botanical names Family Vegetation layer 1 Acacia ataxacantha DC. Mimosaceae S 2 Acacia polyacantha (Willd.) Mimosaceae S 3 Acacia sieberiana Mimosaceae T2 4 Acanthospermum hispidum DC. Asteraceae H 5 Afrormosia laxiflora (Benth.) Harms. Fabaceae T2 6 Afzelia africana Sm. ex Pers. Caesalpiniaceae T1 7 Albizia ferruginea (G. & Perr.) Benth. Mimosaceae T1 8 Alysicarpus ovalifolius (S. & Th.) Fabaceae H 9 Amaranthaceae sp. Amaranthaceae H 10 Andira inermis (Wright) DC. Fabaceae T1 11 Andropogon pseudapricus Stapf. Poaceae H 12 Annona senegalensis Pers. Annonaceae S 13 Anogeissus leiocarpus (DC.) G. & Perr. Combretaceae T1 14 Apocynaceae sp. Apocynaceae H 15 Asparagus africanus Lam. Liliaceae H 16 Asparagus Pauli-Gulielmi Solms-Laub. Liliaceae H 17 Baissea multiflora A.DC. Apocynaceae C 18 Blainvillea gayana Cass. Asteraceae H 19 Boerhavia diffusa L. Nyctaginaceae H 20 Bombax costatum Pell. & Vuill. Bombacaceae T1 21 Borassus flabellifer L. Arecaceae T1 22 Borreria verticillata (L.) G. F. Mey. Rubiaceae H 23 Boscia angustifolia A. Rich. Capparidaceae S 24 Brachiaria distichophylla Stapf. Poaceae H 25 Burkea africana Hook. F. Caesalpiniaceae T2 26 Cassia nigricans Vahl. Caesalpiniaceae H 27 Cassia sieberiana DC. Mimosaceae T2 28 Cassia tora L. Caesalpiniaceae H 29 Ceiba pentandra (L.) Gaertn. Bombacaceae T1 30 Cochlospermum tinctorium A. Rich. Cochlospermaceae H 31 Cola cordilfolia (Cav.) R.Br. Sterculiaceae T1 32 Combretum Fragrans F. Hoffm. Combretaceae S 33 Combretum glutinosum Perr. Combretaceae S 34 Combretum lecardii Engl. & Diels Combretaceae S 35 Combretum micranthum G. Don. Combretaceae S 36 Combretum nigricans Lepr. ex Guill & Perr. Combretaceae S 37 Combretum tomentosum G. Don Combretaceae S 38 Combretum velutinum DC. Combretaceae S 39 Combretum sp. Combretaceae S 40 Commelina sp. Commelinaceae H 41 Compositae sp. Compositae H 42 Corchorus trilocularis L. Tiliaceae H 43 Cordia rothii Roem. et Sch. Boraginaceae T2 44 Cordyla pinnata (Lepr.) Milne-Redh. Caesalpiniaceae T1 Vegetation structure, diversity and site conditions in Senegal 39

45 Crescentia cujete L. Bignoniaceae H 46 Crotalaria goreensis G. & Perr. Fabaceae H 47 Crossopterix febrifuga (Afz.) Benth. Rubiaceae H 48 Cucumis melo var. agrestis Naud. Cucurbitaceae H 49 Cyperus compressus L. Cyperaceae H 50 Cyperus sp. Cyperaceae H 51 Desmodium hirtum G. & Perr. Fabaceae H 52 Desmodium sp. Fabaceae H 53 Detarium microcarpum G. & Perr. Caesalpiniaceae T1 54 Dichrostachys cinerea (L.) Wight & Arn. Mimosaceae S 55 Dioscorea preussii Pax. Dioscoreaceae H 56 Diospyros mespiliformis Hochst. Ex A.DC. Ebenaceae T1 57 Drypetes floribunda (Mull. Arg.) Hutch. Euphorbiaceae H 58 Euphorbia heterophylla L. Euphorbiaceae H 59 Eragrostis tremula Hochst. Poaceae H 60 Eriosema afzelii Bak. Fabaceae H 61 Erythrina senegalensis DC. Fabaceae T2 62 Erythrophleum guineense G. Don. Caesalpiniaceae T1 63 Feretia apodanthera Del. Rubiaceae H 64 Ficus dicranostyla Mildb. Moraceae T1 65 Ficus sp. Moraceae T1 66 Gardenia ternifolia Schum. & Thonn. Rubiaceae S 67 Gardenia triacantha DC. Rubiaceae S 68 Grewia bicolor Juss. Tiliaceae T2 69 Grewia flavescens Juss. Tiliaceae S 70 Grewia lasiodiscus K. Schum. Tiliaceae T1 71 Grewia villosa Willd. Tiliaceae S 72 Guiera senegalensis J.F Gmel. Tiliaceae S 73 Hermannia tigrensis Hochst. Sterculiaceae H 74 Hexalobus monopetalus (A. Rich.) E. & D. Annonaceae S 75 Hibiscus asper Hook Malvaceae H 76 Hygrophila odora (Nees) T. Anders Acanthaceae H 77 Hymenocardia acida Tul. Euphorbiaceae S 78 Icacina senegalensis A. Juss. Verbenaceae H 79 Indigofera garckeana Vatke Fabaceae H 80 Indigofera secundiflora Poir. Fabaceae H 81 Indigofera sp. Fabaceae H 82 Ipomoea coscinosperma Hochst. Convulvulaceae H 83 Ipomoea eriocarpa R. Br. Convulvulaceae H 84 Ipomoea involucrata P. Beauv. Convulvulaceae H 85 Ipomoea nil (L.) Roth. Convulvulaceae H 86 Ipomoea ochracea (Lindl.) G. Don. Convulvulaceae H 87 Khaya senegalensis (Desr.) A. Juss. Meliaceae T1 88 Lablab niger Medic. Fabaceae H 89 Lannea acida A. Rich. Anacardiaceae T1 90 Lannea microcarpa Engl. & Kr. Anacardiaceae T1 40 Diané ABDOULAYE and Kazue FUJIWARA

91 Lannea velutina A. Rich. Anacardiaceae T2 92 Leptadenia sp. Asclepiadaceae H 93 Lonchocarpus laxiﬂorus Guill. & Perr. Fabaceae T2 94 Lonchocarpus sepium DC. Fabaceae T2 95 Maytenus senegalensis (Lam.) Exell Simaroubaceae S 96 Mitracarpus scaber Zucc. Rubiaceae H 97 Mitragyna inermis (Willd.) O. Ktze. Rubiaceae T2 98 Nauclea latifolia Sm. Rubiaceae H 99 Neocarya macrophylla (Sabine) Prance Myrtaceae T2 100 Ocimum basilicum L. Lamiaceae H 101 Ormocarpum bibracteatum (Hochst.) Bak. Fabaceae T2 102 Oxytenenthera abyssinica (A.Rich) Munro. Poaceae S 103 Pandiaka heudelotii (Moq.) Hook Amaranthaceae H 104 Parkia biglobosa (Jacq.) Benth. Mimosaceae T1 105 Pennisetum pedicellatum Trin. Poaceae H 106 Pennisetum violaceum (Lam.) L. Rich. Poaceae H 107 Piliostigma reticulatum (DC.) Hochst. Caesalpiniaceae T2 108 Piliostigma thonningii (Schum.) Milne-Redh. Caesalpiniaceae T2 109 Pobeguinea arrecta (Stapf) J.- Fel Poaceae H 110 Prosopis africana (G. & Perr.) Taub. Mimosaceae T1 111 Pterocarpus erinaceus Poir. Fabaceae T1 112 Rhynchosia minima (L.) DC. Fabaceae H 113 Rubiaceae sp. Rubiaceae H 114 Rytigynia senegalensis Blume Rubiaceae T2 115 Saba senegalensis (A.DC.) Pichon Apocynaceae C 116 Sansevieria senegambica Bak. Agavaceae H 117 Securidaca longipedunculata Fresen. Polygalaceae S 118 Securinega virosa (Roxb.) Baill. Euphorbiaceae S 119 Sesbania pachycarpa DC. Fabaceae H 120 Setaria pallidifusca Stapf. & Hubb. Poaceae H 121 Setaria sp. Poaceae H 122 Sida alba L. Malvaceae H 123 Sida rhombifolia L. Malvaceae H 124 Spondias mombin L. Anacardiaceae T1 125 Sterculia setigera Del. Sterculiaceae T1 126 Stereospermum kunthianum Cham. Bignoniaceae S 127 Striga senegalensis Benth. Scrophulariaceae H 128 Strophanthus sarmentosus DC. Apocynaceae H 129 Strychnos spinosa Lam. Loganiaceae S 130 Symmeria paniculata Benth. Polygonaceae S 131 Syzygium guineense (Willd.) DC. Myrtaceae S 132 Tamarindus indica L. Caesalpiniaceae T1 133 Tephrosia deﬂexa Bak. Fabaceae H 134 Tephrosia pedicellata Bak. Fabaceae H 135 Terminalia albida Sc. Elliot Combretaceae T2 136 Terminalia avicennioides G & Perr. Combretaceae T1 Vegetation structure, diversity and site conditions in Senegal 41

137 Terminalia macroptera Guill. & Perott. Combretaceae H 138 Triumfetta pentandra A. Rich. Tiliaceae H 139 Urena lobata L. Malvaceae H 140 Urginea altissima Bak. Liliaceae H 141 Vigna racemosa (G. Don) H. & D. Fabaceae H 142 Vigna sp. Fabaceae H 143 Vitex doniana Sw. Verbenaceae S 144 Vitex madiensis Oliver Verbenaceae S 145 Waltheria indica L. Sterculiaceae H 146 Wahlenbergia campanuloides (Del.) Vatk. Campanulaceae H 147 Ximenia americana L. Olacaceae S 148 Ziziphus mucronata Willd. Rhamnaceae S T1: tree layer 1 (canopy layer) ; T2: tree layer 2 (understorey tree layer) ; S: shrub layer ; H: herb layer.