Species composition and structure of the woody vegetation of the Middle Casamance region (Senegal)

Forest Ecology and Management 111 (1998) 249±264

Species composition and structure of the woody vegetation of the Middle Casamance region (Senegal) Joris De Wolf 1,* Department of Plant Production, University of Gent, Coupure links 653, 9000 Gent, Belgium Received 14 October 1997; accepted 26 May 1998

Abstract The woody vegetation of the Middle Casamance region in Senegal was studied by means of a ®eld survey consisting of 141 detailed releves of the tree and shrub crown covered by species. A classi®cation was obtained by isolation of groups of releves in the ordination diagrams obtained by detrended correspondence analysis (DCA). Associated releves were visually differentiated and removed from the data set to be used in a subsequent DCA. Four such runs were needed to differentiate 12 vegetation-types. The eight most common ones were described in more detail through (i) species composition based on cover by woody species and diversity indices, and (ii) physiognomy based on ground cover of trees and shrubs, basal area and tree height. Direct ordination (canonical correspondence analysis, CCA) shows the importance of topography and rainfall in the differentiation of these types, but also indicates the existence of a continuum of intermediate forms. Most area is occupied by types that can be designated as woodlands with a well-developed tree stratum (15% to 65% cover) and shrubby undergrowth, resulting in total ground cover by woody plants between 30% and 70%. Only on wetter sites and in some other particular cases, vegetation can be denser and be de®ned as dry forest with canopy cover up to 90%. # 1998 Elsevier Science B.V. Keywords: Correspondence analysis; Savanna; Senegal; Vegetation; Woodland

1. Introduction This vegetation study is a part of larger study aimed at de®ning management models for sustainable fuelwood supply in the region. The management of natural vegetation is often more viable than plantations of fast growing trees in semi-arid Africa (Shepherd, 1992; Ffolliott et al., 1995). The management basically *Corresponding author. Tel.: +254-35-51164; fax: +254-3551592; e-mail: [email protected] 1 Present address: ICRAF, Maseno Agroforestry Research Centre, P.O. Box 25199 Kisumu, Kenya. 0378-1127/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0378-1127(98)00347-8

includes cutting rotations well-spaced in time, diameter and species restrictions, and protection against ®re (Ffolliott et al., 1995). Ideally, sites for fuelwood production should be selected to maximise annual wood increment, but also to reduce environmental hazard by avoiding ecologically valuable vegetation-types (i.e. highly divers or refugia for rare species) or types that are more suitable for other purposes like fruit collection or cattle grazing. Few studies have concentrated on this area's vegetation. References to this area can be found in large scale overviews (Africa or West Africa) like the ones by Monod (1957) or White (1983) or older, more

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descriptive reports (AubreÂville, 1938, 1948; Trochain, 1940). On the country's scale, two remote sensing studies are available. Stancioff et al. (1986) published maps using Landsat images, including a 1:500,000 vegetation maps. Frederiksen and Lawesson (1992) studied the Senegalese vegetation through NOAAAVHRR images. The only detailed study of the Casamance region based on ample ®eld observations is the one of Boudet (1970). It focuses on the grazing possibilities, with an emphasis on the herbal layer. The forest of Bakor has been mapped for reasons of wood exploitation, based on stand height and density but not on species composition (PFRK, 1990). A detailed description of the Kalounayes Forest Reserve situated about 200 km westwards from our study zone, is available (Vanden Berghen, 1984) whereas Adam (1968) presented the vegetation of the Niokolo-Koba National Park, southeastern Senegal, about 200 km to the east of the zone of the present study, but with a similar rainfall regime. The aim of the present study was to elucidate the ¯oristic structure, physiognomic characteristics and diversity of the existing woody vegetation and to evaluate the obtained typology with regard to the two major environmental variables, precipitation and topographic position. 2. The study area The study area (Fig. 1) is situated in the part of the catchment area of the Casamance river situated between 148350 and 158350 east and generally referred to as Middle Casamance. The Casamance river ¯ows in the southern part of Senegal (the Casamance region), in an area that is situated between 30 and 50 m above sea level, lacking any pronounced relief. The plain is intersected by shallow valleys that drain rainwater only after heavy rainstorms. The Casamance river and some of its major tributaries are the only ones to carry water throughout the rainy season (June to October) and even these stop ¯owing in the dry season, leaving not much more than some stagnant water in the deeper parts of the river bed. The soils in the north of the study area are mainly haplic acrisols (sols ferrugineux tropicaux lessiveÂs) that are gradually replaced by haplic ferralsols (sols faiblement ferralitiques) and leptosols in the south. They are

acidic (pH 4±5) and have a clayey±sandy texture. Most valleys consist of eutric regosols and eutric cambisols, whereas in the few larger drainage channels ¯uvisols can be found (Baldensprenger et al., 1968). The climate is Sudano-Guinean, with a very marked transition between the dry and rainy season. Precipitation in the study area steadily increases from northeast (annual average of 800 mm) to southwest (annual average of 1050 mm), but is variable from year to year (600±1400 mm yearÿ1 in Kolda; data for the period 1951±1992). During the dry season (November to May), maximum temperatures normally range between 338C and 428C, while it cools down to between 98C and 228C during the night. Extreme temperatures for the Kolda meteorological station are 78C and 448C. During the rainy season, however, temperatures are less extreme ranging generally between 238C at night and 328C during the day (Boudet, 1970; Ministry of Agriculture, Kolda, unpublished data, 1993). The prevailing vegetation outside the valleys is woodland (sensu White, 1983) consisting of trees that normally do not exceed 20 m height with crowns that do not form a closed canopy. The total cover of perennial vegetation generally varies between 40% and 60%. Bombax costatum, Cordyla pinnata, Combretum glutinosum, Combretum nigricans, Pterocarpus erinaceus and Lannea acida are the dominant species. The herbal layer, dominated by grasses (especially Andropogon, Brachiaria and Digitaria species) attains height of 2.5 m by the end of the rainy season. Large surfaces are occupied by up to 8 m tall bamboo thickets (Oxytenanthera abyssinica). Almost every dry season the vegetation is subjected to ®res, lit by herdsmen to renew the grass layer, by hunters to drive animals out of hiding, or accidentally during clearing of adjacent farm ®elds (Rose Innes, 1971; Kane, 1992). White (1983) classi®ed the vegetation in the north of the zone as Sudanian undifferentiated woodland and the southern part as a mosaic of lowland Guineo±Congolean rain forest and secondary grasslands. In the valleys, if not used for agriculture, riverine forests prevail. These have variable species composition depending on water availability, but are often typi®ed by the presence of Mitragyna inermis, Piliostigma thonningii and a multitude of Guinean species

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Fig. 1. Situation of the study area.

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like Erythrophleum suaveolens, Dialium guineense, Sarcocephalus latifolius or Anthostema senegalensis. The study zone is mainly populated by sedentary Fulani people, who have their villages and farmlands in the valley and use the woodlands on the plateau as grazing land and source of fuelwood and other forest products (e.g. fruits or ®bres). Population pressure is much higher in the south than in the north, because of the higher rainfall and the presence of fertile valleys in the south. Throughout the study zone the settlements are concentrated along the seasonal rivers. In the southern part, only the foreÃt classe de Bakor is spared of high population densities, and therefore, less disturbed, basically because of its protected status, but also because it is a trypanosomiasis-prone area (PeÂllisier, 1966). According to the land-use system in this zone (Leemans, 1993) major interference with vegetation can be expected from man-lit ®res, timber and ®re wood collection, and from cattle grazing. Whereas wood collection has obviously negative effects, grazing may have a positive effect on woody biomass production through shrub encroachment (Boutrais, 1980; Breman and Kessler, 1995), but probably modi®es vegetation physiognomy and species diversity. Human disturbance of this vegetation is the main subject of De Wolf (1998). 3. Methods 3.1. Floristics Plant identi®cation was carried out using Flore du SeÂneÂgal (Berhaut, 1967); Flore illustreÂe du SeÂneÂgal vol. 1±6 (Berhaut, 1971±1976) and Guide de terrain de ligneux saheÂliens et soudano-guineÂens (Geerling, 1982) and checked in the IFAN Herbarium in Dakar. An identi®cation key was developed and used in this study (De Wolf and Van Damme, 1994). Nomenclature follows that of Geerling (1982). The classi®cation of the families is based on Mabberley (1990), who follows generally the Cronquist's system (Cronquist, 1981). Voucher specimens are deposited at the Herbarium of University of Gent, Department of Botany . 3.2. Data collection Data were collected during the rainy seasons of 1992 and 1993. A total of 141 sites was selected for

sampling following a strati®ed random sampling method whereby strata were based on a visual delimitation of homogeneous zones on a colour composition SPOT image of November 1986. For practical reasons, the actual selection took the site accessibility into account, although a minimum distance (at least 100 m) from major roads was respected to reduce human in¯uence. Selected sites were located with GPS with a resolution of 50 m. The woody vegetation was assessed using the line intercept method (Mueller-Dombois and Ellenberger, 1974). This method was chosen after ®eld comparison with the point centred quarter method and the Bitterlich method (Mueller-Dombois and Ellenberger, 1974; Bonham, 1989). It gives a reliable estimate of the percentage crown cover by assessing the proportion of a sample line that is covered by a canopy. Compared on 1 ha plot, it proved to be the quickest and most suitable survey method for the vegetation under study which comprises a lot of multistemmed shrubs and has a low canopy. With a sample line of 300 m (three parallel lines of 100 m each), the crown cover of 17 out of 19 species was estimated within 5% limits of the values obtained by an extensive measurement of all crown diameters in two perpendicular directions on 1 ha plot (De Wolf and Van Damme, 1994). The method also yielded accurate cover estimates in a tropical dry forest of Madagascar (Andrianarivo, 1993). The three parallel sample lines were 30 m apart, which was suf®cient to avoid large trees being counted twice on two adjacent lines. The combination of three lines, 30 m apart and 100 m long, results in an approximate sampling area of 1 ha per site. All woody vegetation higher than 1 m was recorded. For each sampled canopy, species and growth form (shrub, sapling, tree) were recorded. `Shrub' was de®ned as an erect woody plant taller than 1 m, branched low above the ground level, without one of the branches being clearly dominant. In this way, we obtained data on cover by species, total upperstorey cover (sum of all tree covers irrespective species), total understorey cover (sum of all shrub and sapling covers irrespective species) and total ground cover (accumulated cover of shrubs, saplings and trees without double counting overlapping canopies). Besides vegetation data, environmental data were recorded. A complete overview of all measured

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factors and their relation with the vegetation pattern is given by De Wolf (1998). Position on the toposequence and geographical position are the factors with by far the largest in¯uence on the vegetation and will be the only ones discussed in this paper. Position on the toposequence of the site was estimated on a scale from 1 (xeric) to 5 (mesic). The geographical position, recorded by GPS, was used to calculate expected yearly rainfall by interpolation between data from two known meteorological stations, as rainfall in the study zone shows a regular and linear increase from northeast to southwest in the study zone, not in¯uenced by differences in altitude (Boudet, 1970). For 18 sites, diameters at breast height (dbh 1.3 m) of all trees and shrubs higher than 2 m were measured. The sample area was variable according to the girths of the trees. All stems with dbh >31.8 cm were measured in 7200 m2 (30024 m) strips, stems between 6.4 and 31.8 cm dbh on 3600 m2 (30012 m) and all stems <6.4 cm dbh on 1800 m2 (3006 m). Total basal area was calculated from these data. Tree heights were measured with a Suunto clinometer.

tions and calculations of the physiognomy and diversity parameters. The vegetation structure is described using total ground cover, upperstorey cover and understorey cover and the ratio between the two latter parameters. These values were pooled for each vegetation-type by giving the extreme values for the type, as well as by the median value within the type. Shannon and Simpson indices of diversity were computed following the formulas given by Legendre and Legendre (1984). Cover data per species expressed as a percentage of the total cover, were used as an input. These indices, as well as the number of species in the sample, were presented in the same way as the cover data. Canonical correspondence analysis (CCA) was used to link the vegetation-types with topographical position and estimated rainfall. This analysis was carried out by means of CANOCO 3.12, calculating the position of the species as weighted mean of the plots.

3.3. Data analysis

4.1. Floristics

The 141 sites were grouped through a divisive approach using an iterative process involving four detrended correspondence analyses (DCA), using the data on cover per species. At each step, a scatter diagram based on the ®rst two axes of the ordination was drawn. Associated releves deviating from the bulk of the releves were visually differentiated and removed, while the rest was submitted to a subsequent DCA. The DCAs were carried out by a default run of CANOCO 3.12 (ter Braak, 1988a, 1990), by using all species (57) present in at least ®ve releves. The detailed process and the consecutive DCAs are described by De Wolf and Van Damme (1994). A default run of the classi®cation programme TWINSPAN (Hill, 1979) almost fully corroborated the classi®cation obtained by the DCA runs. TWINSPAN put only some releves into an another or in an intermediate category, but yielded the same general classi®cation. The releves that were found to be intermediate between two groups or the ones for which a disagreement existed between the two classi®cation methods were left out of association descrip-

A total of 146 indigenous or naturalised woody species were identi®ed during this study, belonging to 109 genera and 40 families (Table 1). The best represented family was Leguminosae with 40 species of which 17 belong to the Caesalpinioideae subfamily. The Rubiaceae had 12 species. This might be a considerable underestimation of the total species within this family for the region, because this family is know to be better represented in the gallery forests (Geerling, 1982; Berhaut, 1971±1976), which were not studied in detail. Only 11 Combretaceae species were found but, coverwise it was the most important family. Another important family was Anacardiaceae, whereas Moraceae, Euphorbiaceae and Apocynaceae contributed little to total cover, although represented by several species.

4. Results

4.2. Classification The division based on the consecutive DCAs is given in Fig. 2. The ®rst DCA separated two groups from the bulk of the releves, while a second DCA with

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Table 1 Overview of the families encountered in the study and their importance in regard of numbers of species and abundance Family subfamily

Number of species

Number of genera

Percentage of total cover in releves

Leguminosae Caesalpinioideae Mimosoideae Papilionideae Rubiaceae Combretaceae Moraceae Anacardiaceae Euphorbiaceae Apocynaceae Bombacaceae Tiliaceae Sapindaceae Capparaceae Annonaceae Sterculiaceae Loganiaceae Meliaceae Verbenaceae Bignoniaceae Chrysobalanaceae Simaroubaceae Other

40 17 11 12 12 11 9 8 8 6 3 3 3 3 2 2 2 2 2 2 2 1 25

29 11 9 9 10 4 1 5 7 6 3 1 3 3 2 2 2 2 1 2 2 1 23

34.3 14.8 6.5 13.0 2.1 34.1 1.9 5.8 0.8 0.7 10.1 0.9 0.1 ± 3.5 2.8 1.0 0.7 0.5 0.1 0.1 0.3 0.2

the remaining releves distinguished another ®ve groups, leaving 70 releves. A good indication of a clear discontinuity along an axis of a DCA ordination is the asymmetrical distribution of the releves, namely some samples are at an extreme end of that axis, while the rest clusters around the origin (Noy-Meir and Whittaker, 1977; Kenkel and OrloÂci, 1986). This was the case in the ®rst two ordination scattergrams. The remaining 70 releves were submitted to third DCA, which resulted in another four groups, although discontinuities were less clear. The biggest remaining group was split into two by fourth DCA, which brings the total of vegetation-types thus de®ned to 12 ®nal groups. Not all of these 12 groups will be discussed in detail. The ®rst group of seven releves which was detached from the rest by the ®rst DCA, consisted of very mesic sites on the banks of the bigger semipermanent rivers. The species composition and structure within this group was very heterogeneous and the

releves contained several Guinean species (e.g. D. guineense, E. suaveolens, A. senegalensis, Cola cordifolia) absent in the rest of the survey. This clearly indicates its unique status, which can be explained by its speci®c soil moisture conditions. A second vegetation-type which is not discussed in detail are the bamboo thickets. Bamboo (O. abyssinica) invades all other vegetation-types and could be found throughout the study zone and along the whole topographic sequence. This extremely broad niche was also described by Adam (1968). Because O. abyssinica ¯owers gregariously and dies off afterwards, the groups characterised by the dominance of this species were not considered as a true vegetation-type. Within the bamboo thickets, we distinguished two subgroups, mainly based on the degree of dominance of the bamboo. In the ®rst, 35±60% of the woody cover consisted of bamboo, while in the second this percentage ranged between 15% and 25% (see Fig. 2). A third group that is not covered in detail consisted of thickets of Acacia macrostachya. This group of ®ve releves was also very heterogeneous in terms of accompanying species and site characteristics, but can often be found on seasonally water-logged shallow soils over ironstone. The kind of vegetation is often been referred to as `bowal' (Adam, 1968; White, 1983). In most cases the extent of the thickets was small and often <1 ha (i.e. the size of the sample area). The remaining eight types were put into three groups, (1) dry forests and woodlands of mesic sites, (2) woodlands of the xeric plain with low rainfall and (3) dry forests and woodlands of xeric plains with high rainfall. The species composition of each type is summarised in Table 2, while physiognomic and diversity parameters are presented in Tables 3 and 4. 4.2.1. Dry forests and woodlands of mesic sites 4.2.1.1. The Pterocarpus erinaceus±Securinega virosa-type (A1). This vegetation-type is found on wetter sites, often on deep soils on the gentle slopes of broad valleys. Under undisturbed conditions, it is a dry forest with up to 91% ground cover, but this can be reduced to 47% in case of disturbance. Both upperstorey and shrub layers are well-developed. The upperstorey is dominated by P. erinaceus,

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Fig. 2. Divisive classification of all 141 sites into 12 vegetation-types obtained through iterative use of 4 runs of DCA. Numbers represent the total number of samples entered for the respective DCA run (indicated in the tree), the number of samples that could not be classified by that run (indicated at the bottom) and the number of sites finally grouped in the respective vegetation-types. The vegetation-types in bold were retained for further description.

accompanied by Prosopis africana and/or Terminalia macroptera. Individuals of Parkia biglobosa, Terminalia laxiflora and Khaya senegalensis are also common. These tree species grow here to heights of 18 m and more (up to 31 m for K. senegalensis). Also Lannea acida and L. velutina reach their biggest size in this setting. The presence of big trees results in high total basal areas (15± 17 m2 haÿ1). The shrubby undergrowth is typified by Securinega virosa and M. inermis, normally mixed with Combretum collinum, C. glutinosum and P. thonningii.

This type can be divided into two subtypes. One is characterised by the association Prosopis africana± Lannea velutina, the other by Terminalia macroptera± Lannea acida. 4.2.1.2. The Daniellia oliveri±Parkia biglobosa-type (A2). This type is also found near valleys, but on steeper slopes with shallow soils or rocky outcrops. Ground cover varies between 63% and 76%. Trees are usually less high than in the previous type and the shrub stratum is less developed. Total basal area ranges between 12 and 15 m2 haÿ1. The tree

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Table 2 Abundance (expressed as % canopy cover) by species in the eight common vegetation-types distinguished in the study area, with indication of the usual habit of the species Key vegetation-types

Abundance

A1: Pterocarpus erinaceus±Securinega virosa A2: Daniellia oliveri±Parkia biglobosa A3: Combretum glutinosum±Pterocarpus erinaceus B1: Bombax costatum±Strychnos spinosa B2: Sterculia setigera±Hexalobus monopetalus C1: Afzelia africana±Detarium microcarpum C2: Cordyla pinnata±Combretum nigricans C3: Combretum nigricans±Combretum glutinosum

0 1 2 3 4 5 6 7

Mitragyna inermis Kuntze Piliostigma reticulatum Hochst Khaya senegalensis A. Juss Ziziphus mauritiana Lam. Sarcocephalus latifolius Bruce Terminalia laxiflora Engl. Prosopis africana Taub. Parkia biglobosa Benth. Securinega virosa Baill. Piliostigma thonningii Milne-Redhead Cassia sieberiana DC. Sclerocarya birrea Hochst Annona senegalensis Pers. Pericopsis laxiflora Harms Trichilia emetica Vahl. Daniellia oliveri Hutch. et Dalz. Allophylus cobbe Rausch Swartsia madagascariensis Desv. Bridelia micrantha Baill. Maytenus senegalensis Exell. Dombeya quinquiseta Exell Hymenocardia acida Tul. Lonchocarpus laxiflora Guill. et Perr. Securidaca longipedunculata Fresen Lannea velutina A. Rich Quassia undulata D. Dietz Pterocarpus erinaceus Poir. Terminalia macroptera Guill. et Perr. Lannea microcarpa Engl. et K. Krause Erythrina senegalensis DC. Combretum lecardii Engl. et Diels Guiera senegalensis J.F. Gmelin Dichrostachys cinerea Wight et Arn Detarium microcarpum Guill. et Perr. Holarrhena floribunda Dur. et Schinz. Combretum collinum Fresen Gardenia erubescens Stapf. et Hutch. Ozoroa insignis Del. Entada africana Guill. et Perr. Acacia macrostachya Reichenb. ex Benth. Bombax costatum Pellegr. et Vuillet

A1 1 1 1 1 1 2 5 2 3 3 2 2 1 0 1 0 0 0 0 0 1 0 2 1 3 1 6 4 0 0 1 0 1 1 1 3 1 0 0 3 3

absent in all plots <0.5% of total cover in 3/4 of the plots variable but <0.5% of total cover in 1/2 of the plots variable but >0.5% of total cover in 1/2 of the plots >0.5% of total cover in 3/4 of the plots >2% of total cover in 3/4 of the plots >5% of total cover in 3/4 of the plots >10% of total cover in 3/4 of the plots A2 1 1 1 1 1 1 3 3 1 3 1 2 1 3 3 6 2 1 1 1 0 3 0 0 4 2 6 2 0 0 0 0 1 5 2 2 1 0 0 2 1

A3 1 0 1 1 1 1 2 2 1 2 2 1 1 1 1 2 1 1 1 1 1 1 0 1 3 1 4 4 1 1 1 1 1 2 1 2 1 0 1 2 3

B1 0 0 0 0 0 0 1 1 1 1 1 0 1 1 1 0 0 0 0 1 1 1 1 0 3 1 4 2 1 1 1 1 1 2 1 3 1 1 1 2 5

B2 0 0 0 0 0 0 1 1 1 1 1 0 1 1 1 0 0 0 0 0 0 1 1 1 2 1 4 3 1 0 0 0 1 2 1 3 1 1 1 2 3

C1 0 0 0 0 0 0 1 2 0 1 0 0 1 3 1 3 0 1 0 0 0 2 0 1 1 1 4 2 0 0 0 0 0 5 1 3 0 0 0 1 3

C2 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 2 2 0 0 0 0 1 2 2 2 1 0 1 2 5

C3 0 0 0 0 0 0 1 0 0 1 0 0 1 0 0 2 0 0 0 0 0 1 0 0 1 2 3 1 0 0 0 0 0 2 1 2 0 0 0 1 2

habit sh sh ht sh sh ht ht ht sh lt sh ht hc/sh lt ht ht sh lt lt/sh sh lt sh/lt ht sh/hc lt ht ht ht lt lt sh sh/hc sh ht sh/hc sh sh hc lt sh ht

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Table 2 (Continued ) Terminalia avicennioides Guill. et Perr. Crossopterix febrifuga Benth. Sterculia setigera Del. Ximenia americana L. Hexalobus monopetalus Engl. et Diels Grewia venusta Fresen Afzelia africana Smith ex Pers. Combretum molle R. Br. ex G. Don Pavetta sppa Lannea acida A. Rich Vitex madiensis Oliv. Combretum glutinosum Perr. ex DC. Stereospermum kunthianum Cham. Burkea africana Hook. f. Erythrophleum africana Harms Ficus glumosa Del. Strychnos spinosa Lam. Cordyla pinnata Milne-Redhead Oxytenanthera abyssinica Munro Xeroderris stuhlmannii Mendonc,a et E.P. Sousa Grewia lasiodiscus K. Schum. Combretum nigricans Lepr. ex Guill. et Perr.

1 1 1 1 1 1 0 1 0 2 1 3 0 0 1 1 1 2 0 1 1 2

2 1 1 0 4 0 2 0 0 4 2 5 0 1 2 1 2 3 1 3 4 4

2 2 2 1 2 0 1 0 0 4 1 5 1 1 2 2 2 3 2 1 1 3

2 1 2 1 2 1 0 0 0 4 2 6 1 1 1 1 4 4 2 2 1 2

3 2 4 1 6 1 1 1 1 3 1 6 1 0 2 3 2 4 1 2 1 4

2 1 3 0 4 0 7 1 0 3 2 5 1 2 2 1 3 3 1 3 3 5

1 1 2 1 3 0 2 1 1 3 1 4 1 1 2 2 2 5 1 2 1 7

1 1 2 1 2 0 0 0 0 4 1 5 0 0 2 1 2 4 1 3 3 7

sh/lt lt/sh ht sh sh/lt sh/hc ht lt sh/hc lt sh/hc lt/sh lt ht ht ht sh/lt ht bmb ht sh/lt sh/hc

a The species P. crassipes K. Schum. and P. oblongifolia Bremek could not be distinguished when vegetative and were therefore treated as one taxon. hc: woody hemicryptophyte (all above ground parts die-off during dry season); sh: shrub (woody, low branching plant without clear dominant main stem; lt: low tree (tree generally <8 m tall); ht: high tree (tree generally >8 m tall); bmb: bamboo.

stratum is typically composed of D. oliveri, P. erinaceus, P. biglobosa, Detarium microcarpum and Xeroderris stuhlmannii. Individuals of L. acida, Quassia undulata and Pericopsis laxiflora are common. Less typical for this vegetation, but often present, are T. macroptera, P. africana, Afzelia africana, Sclerocarya birrea, Erythrophleum africanum and Trichilia emetica. The shrub layer is composed of P. thonningii, L. velutina, Grewia lasiodiscus, Hexalobus monopetalus, Strychnos spinosa, Acacia macrostachya, Annona senegalensis, Allophylus cobbe and the ubiquitous C. glutinosum and C. nigricans. This type is normally species-rich and has on average the highest diversity indices (Table 4). 4.2.1.3. The Combretum glutinosum±Pterocarpus erinaceus-type (A3). These woodlands take an intermediate position between the two previous types of wetter sites and the types of the drier plains (see next). They form a transition both in structure and in species composition. Ground cover

varies between 40% and 80%, whereas total basal area drops to 9±12 m2 haÿ1. The species listed in the previous types are still present, but are now dominated by C. glutinosum and B. costatum. Only P. erinaceus and T. macroptera can maintain the high abundance figures of previous types. Very frequent are C. pinnata, L. acida and E. africanum. The understorey is often dominated by C. nigricans or sometimes by A. macrostachya. Within this type some releves are more closely related to the Daniellia oliveri±Parkia biglobosa-type, whereas others refer to the Pterocarpus erinaceus±Securinega virosa-type. 4.2.2. Woodlands of the xeric plains with low rainfall 4.2.2.1. The Bombax costatum±Strychnos spinosatype (B1). The woodlands of this type are situated on the dry plains in the north of the study zone and are by far the most extended type north of 13th parallel. Ground cover varies between 30% and 70% and basal areas between 8 and 11 m2 haÿ1. The upperstorey is often not higher than 12 m with a

9 5 20

15 15

6 19 7

Xeric sites north Bombax±Strychnos Sterculia-Hexalobus

Xeric sites south Afzelia±Detarium Cordyla±Combretum Combretum

Number of plots

Mesic sites Pterocarpus±Securinega Daniellia±Parkia Combretum±Pterocarpus

Vegetation-type

54±82 34±71 26±58

30±70 30±73

47±91 63±76 40±80

70 53 52

50 56

74 72 53

15±51 13±45 16±71

2±39 10±33

11±91 34±47 12±92

23 26 36

22 26

50 44 27

Med

Range

Range

Med

Understorey (a) (%)

Crown cover (%)

40±94 16±56 8±39

18±67 16±55

20±78 50±62 12±75

Range

58 38 22

39 43

52 57 41

Med

Upperstorey (b) (%)

0.20±1.06 0.31±2.26 0.73±4.65

0.05±1.62 0.23±1.82

0.19±3.19 0.61±0.88 0.24±3.23

Range

Ratio a:b

0.40 0.67 1.42

0.66 0.64

1.17 0.84 0.84

Med

14.7 (1) 8.3±11.6 (3) 7.3 (1)

8.2-10.5 (4) 12.6 (1)

15.1±17.2 (3) 12.4±15.4 (2) 8.6±12.2 (3)

Basal area (m2 haÿ1) Range (n)

Table 3 Physiognomy of the eight most common vegetation-types in the Middle Casamance: total crown cover, cumulative cover per stratum and ratio between the strata (range and median value) and total basal area (range or single measured value; n: number of measurements for basal area, because measured on a subsample only)

258 J. De Wolf / Forest Ecology and Management 111 (1998) 249±264

J. De Wolf / Forest Ecology and Management 111 (1998) 249±264

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Table 4 Diversity parameters (number of species, Shannon index and Simpson index) for the eight most common vegetation-types identified in the Middle Casamance (range and median value) Vegetation-type

Numbers of plots

Number of species

Shannon index

Range

Med

Range

Med

Range

Med

Mesic sites Pterocarpus±Securinega Daniellia±Parkia Combretum±Pterocarpus

9 5 20

13±24 24±35 13±29

16 26 20

0.61±1.02 1.14±1.28 0.79±1.16

0.87 1.20 1.01

0.61±0.90 0.90±0.94 0.78±0.92

0.82 0.92 0.87

Xeric sites north Bombax±Strychnos Sterculia±Hexalobus

15 15

12±25 13±24

21 19

0.72±1.15 0.81±1.12

0.89 0.98

0.74±0.91 0.79±0.91

0.84 0.85

Xeric sites south Afzelia±Detarium Cordyla±Combretum Combretum

6 19 7

11±28 12±22 5±21

22 15 15

0.75±1.11 0.58±1.04 0.38±1.02

0.94 0.87 0.74

0.75±0.89 0.60±0.89 0.51±0.83

0.78 0.82 0.68

maximum of 18 m (B. costatum). Most common tree species are C. glutinosum, B. costatum, C. pinnata, P. erinaceus and L. acida. The understorey is characterised by S. spinosa, Terminalia avicennioides and Vitex madiensis. It differs from the more southern types by the lower cover percentages of C. nigricans and from the more mesic types by the lower cover percentages of T. macroptera and P. africana. 4.2.2.2. The Sterculia setigera±Hexalobus monopetalus-type (B2). This type is structurally similar to the previous type. The CCA associates it with slightly more xeric conditions than the previous type, probably because of more sandy soils. Species composition is also very similar, but is characterised by the strong association between a tree species, Sterculia setigera, and an understorey species, H. monopetalus, and a reduced abundance of B. costatum and S. spinosa as compared to the previous type. Crown cover ranges between 30% and 73%. The basal area was assessed in only one site belonging to this type and was 12.6 m2 haÿ1. 4.2.3. Dry forest and woodlands of xeric plains with high rainfall 4.2.3.1. The Afzelia africana±Detarium microcarpum-type (C1). This type is the only one on

Simpson index

xeric sites which deserves to be designated as dry forest. It has an upperstorey (54±82% cover) dominated by compact and wide crowns of A. africana. Characteristic is the absence of tall grasses, so common for the other types. Some less common species like D. microcarpum, D. oliveri or P. laxiflora can often be found in this type, together with more widely distributed species like C. nigricans, C. glutinosum, B. costatum, P. erinaceus and H. monopetalus. The area occupied by this type is very restricted; it could only be found in small patches on shallow soils. The big girths of the A. africana trees result in high basal areas (14.7 m2 haÿ1). 4.2.3.2. The Cordyla pinnata±Combretum nigricanstype (C2). This type is the most common vegetationtype south of the 13th parallel. It is a woodland vegetation with a ground cover between 34% and 71% an upperstorey which normally does not reach higher than 15 m and a relatively dense understorey. Measured basal areas range between 8 and 12 m2 haÿ1. Common tree species are C. pinnata, B. costatum, C. glutinosum, L. acida and P. erinaceus. The understorey is dominated by C. nigricans, with C. collinum, C. glutinosum (as shrub), A. macrostachya and H. monopetalus as secondary species. Two subtypes can be distinguished: one characterised by C. pinnata as dominant tree and H. mono-

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petalus as important shrub, and one by B. costatum as dominant tree and A. macrostachya and Cassia sieberiana as shrubs. 4.2.3.3. The Combretum glutinosum±C. nigricanstype (C3). This type is characterised by a shrub layer completely dominated by C. nigricans and C. glutinosum, the latter seldomly developing into a tree in this type. The understorey often has a much greater ground cover than the tree layer, which is usually only composed of a few C. pinnata or L. acida trees. It has the lowest diversity indices of all types discussed here. Crown cover varies between 26% and 58%. For only one site, the basal area was assessed, resulting in 7.3 m2 haÿ1, the lowest of all measured sites. On that site only 11 trees with dbh >21 cm were found per hectare, while in the previous type normally >50 trees haÿ1 of that size can be found.

4.3. Ordination The CCA was carried out on the 111 sites of the eight types discussed before, taking into account the 55 species which occurred at least ®ve times in these 111 sites. The data matrix contained 1973 non-zero values (34%). The ®rst ordination axis had a constrained eigenvalue of 0.217 and was positively correlated with the topographic position (rˆ0.750), while the second axis with a constrained eigenvalue of 0.110, was negatively correlated with precipitation (rˆÿ0.722). The types are indicated by the various symbols used in Fig. 3. The ®rst axis (topography) separates the two most mesic types from the xeric sites, while the Combretum glutinosum±Pterocarpus erinaceus-type takes an intermediate position. The Afzelia africana±Detarium microcarpum-type contains some rather mesic sites as well. Among the

Fig. 3. Ordination diagram of the first two axes of CCA of the 111 releves of the eight treated vegetation-types, with precipitation and topographic position as environmental variables; eigenvalues of the axes: 0.217 and 0.110 (*: Pterocarpus±Securinega; *: Daniellia±Parkia; }: Combretum±Pterocarpus; &: Bombax±Strychnos; &: Sterculia±Hexalobus; ~: Afzelia±Detarium; !: Cordyla±Combretum; r: Combretum; ‡: non-classified)

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261

Fig. 4. CCA species ordination corresponding to the releve ordination diagram of Fig. 3. (acma: Acacia macrostachya; afaf: Afzelia africana; anse: Annona senegalensis; boco; Bombax costatum; brmi: Bridelia micrantha; buaf: Burkea africana; casi: Cassia sieberiana; coco: Combretum collinum; coni: C. nigricans; copi: Cordyla pinnata; crfe: Crossopterix febrifuga; daol: Daniellia oliveri; demi: Detarium microcarpum; dici: Dichrostachys cinerea; enaf: Entada africana; eraf: Erythrophleum africanum; figl: Ficus glumosa; gaer: Gardenia erubescens; grla: Grewia lasiodiscus; hemo: Hexalobus monopetalus; hofl: Holarrhena floribunda; hyac: Hymenocardia acida; khse: Khaya senegalensis; laac: Lannea acida; lami: L. microcarpum; lave: L. velutinum; lola: Lonchocarpus laxiflora; miin: Mitragyna inermis; oxab: Oxytenanthera abyssinica; pabi: Parkia biglobosa; pasp: Pavetta spp; pela: Pericopsis laxiflora; pith: Piliostigma thonningii; praf: Prosopis africana; pter: Pterocarpus erinaceus; quun: Quassia undulata; scbi: Scleocarya birrea; selo: Securidaca longipedunculata; sevi: Securinega virosa; stse: Sterculia setigera; stku: Stereospermum kunthianum; stsp: Strychnos spinosa; swma: Swartzia madagascariensis; teav: Terminalia avicennioides; tela: T. laxiflora; tema; T. macroptera; trem: Trichilia emetica; vima: Vitex madiensis; xest: Xeroderris stuhlmannii).

mesic sites, the second axis (precipitation) differentiates the Pterocarpus erinaceus±Securinega virosatype from the Daniellia oliveri±Parkia biglobosatype, whereas it divides the xeric sites into low rainfall and high rainfall-types. The Combretum glutinosum± Pterocarpus erinaceus-type can be found over the whole rainfall range comprised in the survey. The abundance of species in the vegetation-types and their strength as an indicator of the environment, can be deducted from the species ordination (Fig. 4). M. inermis, S. virosa, P. africana and P. thonningii are important indicators for the wetter sites, while

P. erinaceus and T. macroptera take a less extreme position on the ®rst axis because they can also be found on drier sites, albeit less abundantly. In the left upper corner of the scatterplot, P. biglobosa and D. oliveri characterise the type named after these species. D. microcarpum is abundant in both the latter type and in the Afzelia africana±Detarium microcarpum-type, hence its intermediate position. The species below the zero position of the ®rst axis, typify the dryer sites. Central on the second axis is the ubiquitous C. glutinosum. On the positive side of this axis, species characteristic for the northern types can be found:

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S. setigera, B. costatum, T. avicennioides and H. monopetalus. On the negative side, C. nigricans and A. africana are noteworthy. S. spinosa, takes a slightly negative position on this axis, although it is characteristic for one of the drier types. This position can be explained by its high abundance in the Afzelia africana±Detarium microcarpum-type (see Table 2), which pulls the species score to the negative end. 5. Discussion The vegetation of the Middle Casamance region (with the exception of the riparian forests and the bamboo and A. macrostachya formations which have not been considered here) can be differentiated into eight main types. These types are different in both species composition and structure. The various vegetation-types can be separated along topographic and rainfall gradients by the use of CCA, suggesting the importance of both environmental variables. The absence of clear gaps between sites in the scatter diagram (Fig. 3), however, indicates a gradual change of species composition. The distinction among types depends, in a lot of cases, on differences in abundance of species and not on presence±absence criteria. Most species have rather wide niches. The validity of the classi®cation as it is given here, might therefore, be questionable or regarded as arti®cial. However, the variation between types in the ®eld is clear, both in species composition and structure. The dif®culty lies in where to mark the boundaries for purpose of classi®cation between types that turn from one into another via a diffuse transition. Wherever that boundary is put, it will always be arbitrary and arti®cially sharp. The physiognomic parameters do not provide a clear distinction either, since the ranges of the various vegetation-types overlap (Table 3). Nevertheless the more mesic-types (A1±A3) and the Afzelia africana± Detarium microcarpum-type (C1) have generally higher crown cover and basal area than the other types, justifying their designation as `forest.' The other types are similar to each other for most parameters. Only the Combretum glutinosum±C. nigricans-type has a low tree cover, low basal area and high shrub cover despite the relatively high rainfall in the zone where this type is found. Moreover, this type

is marked by a low species richness and low diversity indices (Table 4). These characteristics are an indication of vegetation degradation through human impact. On the contrary, high diversity is a characteristic of the Daniellia oliveri±Parkia biglobosa-type and a considerable number of species were only recorded in the sites belonging to this type, indicating that these forest patches have escaped severe human disturbance. Other authors studying the same area, have de®ned similar vegetation-types. The distinction between the denser vegetation of the wetter sites and the woodlands found on the upland, was also made by Boudet (1970) and Stancioff et al. (1986). The latter study points out T. macroptera, P. erinaceus and M. inermis as typifying species for one all-comprising valley vegetation-type, which is probably too general according to our ®ndings. Boudet (1970) distinguishes several valley vegetation-types, although they are always characterised by the presence of P. erinaceus. If the same locations as in this study are considered, two main types exist. The ®rst is typi®ed by the association D. oliveri±Hymenocardia acida±X. stuhlmannii and occurs on the stony fringes of the valleys, the second is marked by the association T. macroptera±P. laxi¯ora and can be found in deep well-drained soils of the valley. In the Kalounayes forest, two subtypes, one dominated by D. oliveri and another dominated by P. erinaceus, has been proposed by Vanden Berghen (1984). However, in that area these dry forests-types are not con®ned to valleys, which might be due to the higher rainfall occurring in that zone (between 1175 and 1765 mm yearÿ1 during the 1955±1965 period; Vanden Berghen, 1984). The north±south change with climate is also mentioned in the other studies. Stancioff et al. (1986) distinguish between two main subdivisions. In the north, they ®nd woodland typi®ed by C. glutinosum in the understorey, and in the south dry forest with less C. glutinosum and more Holarrhena ¯oribunda. The transition is situated around 128520 N, which corresponds to the isohyete of 1000 mm yearÿ1. Boudet (1970) does not give such a clear boundary but he indicates that in general the vegetation south of the Casamance river is dominated by C. nigricans (cover between 5% and 50%) and marked by an important cover of A. africana, H. ¯oribunda and P. laxi¯ora. Breman and Kessler (1995) make a distinction between the northern (600±900 mm yearÿ1) and the

J. De Wolf / Forest Ecology and Management 111 (1998) 249±264

southern Sudan zone (900±1200 mm yearÿ1) based on a personal observations and a wide review of literature on semi-arid vegetation in west Africa. The transition between these two zones is among other marked by an increase in overall cover, by a decreasing abundance of B. costatum and C. glutinosum, an increasing abundance of C. nigricans and the appearance of A. africana, D. oliveri, D. microcarpum and P. biglobosa when moving from north to south. The vegetation map of White (1983), considered as the most authoritative for African vegetation, represents the north of our study zone as `Sudanian undifferentiated woodland' and the south as `mosaic of lowland rain forests and secondary grassland.' While we can agree with the Sudanian woodland in the north, the characterisation of the southern part of the study zone as mosaic of lowland rain forest is arguable. Based on our data and on the literature sources referred to, the borderline between these two map units should be moved southwards, beyond the limits of our study zone. Or, if necessary, a transition zone between woodland and rain forest should be created between the isohyetes of 1000 and 1200 mm yearÿ1 for this part of Africa. The Combretum glutinosum±C. nigricans-type is most probably a degraded form of the Cordyla pinnata±Combretum nigricans-type. In Fig. 2, it can be seen that four sites were intermediate between these two types, indicating the gradual change. It is generally found in the vicinity of high human population concentrations around the town of Kolda (De Wolf, 1998) and shows similarities with fallow vegetation as described by Leemans (1993) and Breman and Kessler (1995) or regrowth vegetation 5 years after clearfelling (Renes, 1991). Stancioff et al. (1986) identi®ed a vegetation-type at the west side of Kolda, characterised by C. nigricans in the understorey, although did not made a link with human pressure. Vanden Berghen (1984) found a similar vegetation-type with C. nigricans as dominant and typifying species as a last stage of the degradation process of the Kalounayes forest. The vegetation classi®cation presented here can thus be linked to earlier descriptions found in the literature. It corroborates the general structure of the classi®cation of Stancioff et al. (1986), although the present data do justify a further division. The same structure can also be found with Boudet (1970), but the lowest level of his classi®cation cannot be sustained

263

on the grounds of our data. This was also the conclusion of Salle (1984) who reviewed the original data of Boudet. No agreement could be found, however, with the classi®cation of Frederiksen and Lawesson (1992), which is far to general and uses rather uncommon species as indicators. Besides providing insight in the ecology and vegetation dynamics, vegetation classi®cations are also useful in assessing potentials for a given area. As mentioned in the introduction, the present study was executed as part of a project for sustainable fuelwood production. When it comes to point out a vegetationtype for fuelwood production, the Combretum glutinosum±C. nigricans-type seems to be by far the most suitable. Its environmental value is low and the Combretum species easily regenerate after being cut back. Their wood forms a highly appreciated fuelwood readily used by the local people. Protection from ®re and a better control of upperstorey timber cutting will probably reverse degradation process and change the vegetation into the Cordyla pinnata±Combretum nigricans-type. The Daniellia oliveri±Parkia biglobosa-type on the other hand should be avoided when fuelwood production is planned, because of its high environmental value. As was pointed out, this type acts as a refuge for numerous species that have almost disappeared from the other types. The xeric-types of the north, with their relatively low shrub cover, are probably more suitable as rangeland and should therefore be maintained as such. Acknowledgements This study was made possible through the sponsorship of the European Union (DG VIII) through the grants B7-5040/91/041 and B7-5040/93/04. Many thanks to Patrick Van Damme, Pol Van Mele and an anonymous reviewer for useful comments on earlier versions of the manuscripts. Thanks also to G. Verstraeten for the SPOT image and to Jan Van Winghem, Diego Van Den Meersschaut, Abdul Balde and Mamadou Balde for help in data collection. References Adam, J.G., 1968. La flore et la veÂgeÂtation du Parc National Niokolo-Koba (SeÂneÂgal). Adansonia, seÂrie 2, 8, 429±459.

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