Biodiversity of ligneous species in semi-arid to arid zones of southwestern Niger according to anthropogenic and natural factors

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1 Agriculture, Ecosystems and Environment 105 (2005) Biodiversity of ligneous species in semi-arid to arid zones of southwestern Niger according to anthropogenic and natural factors Larwanou Mahamane a,, Saadou Mahamane b a Département de Gestion des Ressources Naturelles (DGRN), Institut National de Recherches Agronomiques du Niger (INRAN), BP 429 Niamey, Niger b Département de Biologie végétale, Université Abdou Moumouni de Niamey, BP Niamey, Niger Received 13 December 2002; received in revised form 1 March 2004; accepted 5 March 2004 Abstract Floristic diversity, abundance and disappearance of woody species were investigated at four sites of southwestern Niger, two in the semi-arid, two in the arid zone. The woody vegetation was assessed on transects from villages to bush, the measured parameters being number of individuals, height, diameter and number of stems. Questionnaires were used in the field to record information provided by the population for the disappeared species. Sites in the semi-arid zone were species richer than those in the arid zone. In terms of land-use there was a significant difference among and within sites and the number of trees increased with the distance from the village to the bush. A total of 23 species had completely disappeared from the study sites, i.e. five from the semi-arid and 18 from the arid zone. Thirteen different causes of tree species loss were recorded, six of them being of anthropic and seven of natural origin. Four disappeared species were consumed as leaves, buds, flowers and fruits while others were used as firewood and timber. All disappeared species were part of the traditional pharmacopeia. The plantation of useful species is suggested to improve re-introduction Elsevier B.V. All rights reserved. Keywords: Diversity; Disappeared species; Semi-arid zone; Arid zone; Niger 1. Introduction In the Sahelian countries and particularly in Niger, the woody species occupy a significant place in the dynamics of the ecosystems and in the socio-economic life of the human population (Saadou, 1990; Gerard et al., 1997). The current Sahelian landscape is a result of natural and cultural factors, the biological diversity of woody species being linked to physio-geographical and climatic alterations. An es- Corresponding author. Tel.: ; fax.: addresses: larwanou@caramail.com, m.larwanou@coraf.org (L. Mahamane). timated plant species occur in Niger including species of angiosperms (Saadou, 1998) while the flora of the southwestern part of the country is poorly known. The degradation of the terrestrial ecosystems became apparent during the drought years 1973 and 1984 as a loss of woody and herbaceous perennial species, along the valleys and on the plains (Grouzis, 1988). The natural regeneration of the woody species is threatened by the scarcity of rain, their abnormal distribution in time and space, along a south north gradient in particular (Ozer and Erpicum, 1995). Like many Sahelian countries, Niger shows signs of a reduction in the biological diversity of tree /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.agee

2 268 L. Mahamane, S. Mahamane / Agriculture, Ecosystems and Environment 105 (2005) species (Larwanou, 1998). This regression is due to a number of factors including drought (Wijkman and Timberlake, 1990), high demographic pressure and overexploitation of the environment (Grouzis and Albergel, 1986; Mainguet, 1990). A study was undertaken in four villages of southwestern Niger to better understand the diversity, abundance, and distribution of woody species with a view to propose possible solutions. 2. Materials and methods The sites selected in southwestern Niger were under semi-arid (sites I and II) and arid (sites III and IV) climate, between 200 and 300 m a.s.l. Annual average rainfall ranged between 500 and 700 mm in the semi-arid, mm in the arid zone. The soils were in general sandy with various signs of degradation. Two sites were mainly used for agriculture, another two for pastoralism. In the agricultural sites, land preparation for farming played a major role in the degradation of woody species diversity, whereas animal pressure mainly affected woody species in the pastoral sites (Sinclair and Fryxell, 1985; Stiles, 1995; Ganaba and Guinko, 1998). Three types of land-use were identified in the sites, i.e. (1) natural undisturbed formations, (2) fallows and (3) parklands with scattered trees. Investigations were carried out during the rainy season (August), during which all species can be identified. The inventory occurred on transects from the villages to the bush as described by Yamba (1994). The transects were chosen to cover all land-use types and the various physiographical units. On each transect, plots 50 m 50 m were defined, the first 300 m from the village, the following at 500 m distance. All woody species were recorded in each plot. Adult trees and regenerations (plants < 0.5m) were listed separately for each species. The following parameters were measured: (1) height of the largest stem; (2) diameter at 0.2 m for trees < 1.30 m high; (3) diameter at 1.30 m for trees > 1.30 m high; (4) number of stems. The Sympson index (I) was determined at each plot, and extrapolated by site or type of land-use as I = N(N 1) q i=1 n i(n i 1) where N is the total number of individual trees inventoried, q the number of species inventoried, and n i the total number of individual trees for species i. The reciprocal of the index was used to appreciate diversity. Differences were analyzed by ANOVA general linear model (GLM). For disappearing species, data were collected in a survey of the same transects to record information provided by the population. In each village, an assembly was convened around the head of the village and questions were asked using a questionnaire prepared for the circumstance. Old, average-aged and young people were asked to list species disappeared, threatened, the uses of each species, the past situation, the causes of disappearance and possible solutions. The number of people involved varied between villages from 123 to 210. Transects were followed up to the limit of the village to confirm information. The model by Guarino (1997) based on the distribution of a taxon, a score being allotted to each factor was used, a rare species having a score of 10, an abundant one a score of 0. A score of 15 was allotted to species which had disappeared from the area. Data were analyzed using SPSS software. A cross-tabulation was made to determine the percentage of answers given to each parameter, Pearson s coefficient of correlation being used to determine any correlation. 3. Results There was a significant difference (P <0.01) for the average number of species per plot (L.S.D. = 1.24). Sites located in the semi-arid zone were floristically richer (5.36 and 6.20 species) than those of the arid region (3.90 and 4.10 species). Sympson index of diversity for sites I IV was 0.29, 0.19, 0.12 and The number of species varied significantly (P < 0.001) between land-uses (L.S.D. = 0.94). Parklands had more tree species per unit area than the other land-uses. The smallest number of species was recorded on site III. Fallow were a transition between parklands and natural formations with species num-

3 L. Mahamane, S. Mahamane / Agriculture, Ecosystems and Environment 105 (2005) Table 1 Average number of tree species and of individual trees per land-use and study site Sites Land-use Proportion of land-use (%) Species number ± S.D. Inverse Sympson index Tree number ± S.D. I Natural formations ± ± 70.2 Parkland ± ± 46.2 Fallow ± ± 91.8 II Natural formations ± ± 29.2 Parkland ± ± 24.8 Fallow ± ± 38 III Natural formations ± ± Parkland ± 25 IV Natural formations ± ± 21.5 Parkland ± ± 15.3 Fallow ± 12.6 bers varying according to sites particularly at site IV. Sympson index confirmed this finding. In all sites, parklands had high, natural formations intermediate and fallows a low tree diversity (Table 1). There was a significant difference (P <0.001) according to the distance to the village (L.S.D. = 2.06). Apart from site I, there was a significant difference (P <0.0001) between distances to the village in terms of average number of individuals which decreased due to human activities near the village (Fig. 1). At sites I and II under agricultural land-use the number of woody species varied according to the distance and diversity was higher at m than at any other distance. From m on, the number of species was more constant. At sites III and IV, species numbers were more or less uniform, except at m distance on site IV. The abundance of woody species differed significantly (P <0.0001) between sites and was significantly higher in sites I and III compared to sites II and IV (L.S.D. = 26.44). Guiera senegalensis was the main tree species of the four sites followed in order of decreasing importance by Combretum micranthum and Combretum glutinosum. Species like Acacia raddiana, Acacia laeta, Acacia ataxacantha, Faidherbia albida, Calotropis procera, Maerua crassifolia and Sclerocarya birrea only occurred in the two arid sites (Table 2). The average number of individuals per plot varied significantly between types of land-use and sites. In Fig. 1. Number of individuals of woody species from village to the bush.

4 270 L. Mahamane, S. Mahamane / Agriculture, Ecosystems and Environment 105 (2005) Table 2 Average proportion of individual woody species in the sites (percentage of total number of individuals) Species Proportion (%) Site I Site II Site III Site IV G. senegalensis C. micranthum C. glutinosum Balanites aegyptiaca 3 3 Hyphaene thebaïca 2 Piliostigma reticulatum Combretum nigricans 1 5 Boscia senegalensis Acacia nilotica Gardenia sokotensis 0 Combretum aculeatum 2 Ziziphus mauritiana Mitragyna inermis 1 A. raddiana 2 M. crassifolia 1 3 A. laeta 1 A. ataxacantha 1 F. albida 2 C. procera 1 S. birrea 1 the semi-arid zone, natural formations had on average more individuals than the other types of land-use followed by parklands (Table 1). There was a significant positive correlation between the average species and adult individual numbers (P <0.001) even though the coefficient was low (0.25). In the semi-arid zone, some 18% of the disappeared species were used for human consumption aginst 82.35% in the arid zone essentially as leaves but also as flowers, and fruits (fresh and dry). All disappeared species were consumed by animals mainly as leaves but also as buds, flowers, fruits and pods. All disappeared species were also used for various services, to 62.5% as firewood, and 37.5% for timber and other services. All species were also part of the traditional pharmacopeia in the bioclimatic zone, as leaves (35.6%), bark (40.33%) and roots (24.07%). A total of 23 different woody species had disappeared in the four villages, an information confirmed by the population and field situation (Table 3). More species (80%) had disappeared in the arid zone, 13 from site III and 8 from site IV. The causes most frequently mentioned by the population to explain disappearance were low rainfall (31.3%), drought (20.9%), Table 3 Species disappeared from the study sites in southwestern Niger Species Site I Site II Site III Site IV Acacia macrostachya Adansonia digitata Albizia chevalieri Annona senegalensis Anogeissus leiocarpus Cadaba farinosa Cassia sieberiana Cissampelos mucronata Combretum nigricans Dyospiros mespilliformis Ficus ingens Grewia bicolor Grewia flavescens Grewia tenax Khaya senegalensis Lannea acida Prosopis africana Pterocarpus erinaceus Sclerocaya birrea Sterculia setigera Stereospermum kunthianum imenia americana Total exploitation for pharmacopeia (13.4%), bush fire (10.4%), winds (7.4%), lack of natural regeneration, ageing of trees and drying of water ponds. The most significant causes were considered to be natural. The only anthropic causes were exploitation for pharmacopeia, bush fire and the lack of natural regeneration. Among the possible solutions proposed by locals, plantation of trees in the villages and direct sowing in the arid zone were mainly mentioned (63.9%), followed by sensitizing of population (9.8%), availability of seed (6.6%), and fencing (4.9%). 4. Discussion and conclusions Loss of diversity was more of a concern in the semi-arid than in the arid zone. The results showed that the diversity was higher in the parklands than in the other types of land-use. In addition to bioclimatic considerations (Grouzis, 1988), human activities and grazing pressure had a negative effect on plant diversity, especially near the villages.

5 L. Mahamane, S. Mahamane / Agriculture, Ecosystems and Environment 105 (2005) More woody species had disappeared from the arid than from the semi-arid zone. The number of the disappeared species varied according to sites, mainly as a consequence of human activities. Human consumption has hampered diversity of tree species whereas the use of trees as firewood or for the construction of houses are difficult to answer. Most species disappeared from parklands which is a characteristic of the Sahelian system (Ounteini, 1993). The solutions recommended by the populations were realistic and could contribute to reversing the tendency, plantation and direct sowing of species in particular. The disappearance of woody species is a sign of alarm in the arid zone, that should be taken very seriously. References Ganaba, S, Guinko, S., Structure et dynamique du peuplement ligneux de la région de la mare d Oursi (Burkina Faso). In: Bâ, A.T., Madsen, J.E., Sambou, B. (Eds.), Atelier sur la flore, la végétation et la biodiversité au Sahel. Aarhus University Press, Aarhus, AAU, Reports 3, pp Gerard, F., Willen, F., Gerard, M., Laouali, E.M., Pierre, M., Kiri, T., The Niger Household Energy Project: Promoting Rural Fuelwood Markets and Village Management of Natural Woodlands. World Bank Technical Paper No. 362, Washington, DC, 103 pp. Grouzis, M., Structure, productivité et dynamique des systèmes écologiques sahéliens (Mare d Oursi, Burkina Faso). ORSTOM, Paris, 139 pp. Grouzis, M., Albergel, J., Du risque climatique à la contrainte écologique: incidence de la sécheresse sur les productions végétales et le milieu au Burkina Faso. In: Elden, M., Milleville, P. (Eds.), Le risque en Agriculture. Coll. A Travers Champs, Orstom, Paris, pp Guarino, L., Assessing the threat of genetic erosion. In: Maxted, N., Ford-Lloyd, B.V., Hawkes, J.G. (Eds.), Plant Genetic Conservation: The In Situ Approach. Chapman & Hall, London, pp Larwanou, M., Rapport technique d activités. Institut National de la Recherche Agronomique du Niger, 58 pp. Mainguet, M., La désertification: une crise autant socio-économique que climatique. Sécheresse 3 (1), Ounteini, I.A., Les parcs agroforestiers au Niger. Etat des connaissances et perspectives de recherches. Consultation dans le cadre du Réseau ICRAF/SALWA, 76 pp. Ozer, P., Erpicum, M., Méthodologie pour une meilleure représentation spatio-temporelle des fluctuations pluviométriques observées au Niger depuis Sécheresse 1 (6), Saadou, M., La végétation des milieux drainés nigériens à l Est du fleuve Niger. Thèse Doct. Sci. Nat. Univ. de Niamey, Niger, 395 pp. Saadou, M., Evaluation de la diversité biologique au Niger: Eléments constitutifs de la biodiversité végétale. Conseil National de l Environnement pour un développement durable SE/CNEDD. Projet NER/97/G31/A/1G/99 Stratégie Nationale de plan d action Diversité biologique, Niamey, Niger, 138 pp. Sinclair, A.R., Fryxell, J.M., The Sahel of Africa: ecology of disaster. Can. J. Zool. 63, Stiles, D., An overview of desertification as dryland degradation. In: Stiles, D. (Ed.), Social Aspects of Sustainable Dryland Management. Wiley, Chichester, pp Wijkman, J.V., Timberlake, L.V., Natural Disasters Acts of God or Acts of Man? Earthscan, London, 154 pp. Yamba, B., Les ressources ligneuses et problèmes d aménagement forestier dans la zone agricole du Niger. Thèse Doct. Univ. Michel de Montaigne. Bordeaux III, Institut d Aménagement et Ressources Naturelles, 392 pp.