Journal of Environmental Science and Engineering B 1 (2012) 49-57

Formerly part of Journal of Environmental Science and Engineering, ISSN 1934-8932 D DAVID PUBLISHING

Climate Variability in : Potential Impacts on Vegetation Distribution and Productivity

Ali Mahamane1, 2, Boubé Morou2, Maïnassara Zaman-Allah2, Mahamane Saadou1, 2, Karim Saley2, Yacoubou Bakasso1, Issoufou Sama Wata3, Abdoulaye Amadou Oumani2 and Sandrine Jauffret4 1. Department of Biology, Faculty of Sciences, University A. Moumouni (UAM), 10662, Niger 2. Faculty of Sciences, University of Maradi, Maradi 465, Niger 3. National Center of Ecological and Environmental Surveillance (CNSEE) Ministry of Hydraulics, the Environment and Fight Against the Desertification, Niamey 11870, Niger 4. NARGES (Nature, Resources and Global Environmental Surveillance), Le Coulet Pré d’Engauri 04200 Les Omergues, France

Received: June 30, 2011 / Accepted: August 17, 2011 / Published: January 20, 2012.

Abstract: Since 2003, the sites of the national environmental monitoring system (DNSE) of Niger, set up by the long term ecological monitoring observatories network (ROSELT) with the support of the Sahel and Sahara Observatory (OSS), were used to collect ecological data with harmonized methods for spatio-temporal comparisons purpose. Floristic and phytoecological data were collected using the phytosociological methodology of Braun-Blanquet (1932). Ecosystem vital attributes used included the specific diversity, alpha diversity, equidistribution, biological types and herbaceous phytomass. At the whole system scale, the analysis revealed that the specific diversity, the alpha diversity and the phytomass values were higher in less disturbed biotopes of the north soudanian and south sahelian bioclimates where the rainfall rate is relatively high. Regarding the north sahelian and saharian bioclimates, the topography may play a critical role in the redistribution of this phytodiversity. Besides, the distribution of the biological types showed the prevalence of therophytes (56.8 ± 11%) regardless of the bioclimate and, to a lesser extent, the perennial species (26.5 ± 7.3%), the later group showing higher values for the north soudanian bioclimate.

Key words: Ecological monitoring, ecosystem vital attributes, alpha diversity, environmental gradient, ROSELT-Niger.

1. Introduction degradation especially during the dry seasons [5]. The recent drought events severely disrupted the balance of In tropical regions, plants species play a crucial role ecosystems in this region. In the ecosystems that were in the activities of local communities as well as in soil already fragile due to anthropic disturbance, the fertility restoration [1, 2]. In Niger, since the drought impacts of climatic drought were even more severe [6]. episodes of 1973-74 and 1984-85, the vegetation In the context of climate variability, it is particularly showed important variation due to water deficit [3]. important to improve our understanding on how plant According to Ozer et al. there was a shift southward of species respond to environmental changes. Our isohyets [4] and 2/3 of the country surface was hypothesis was that the vegetation characteristics (vital considered being affected [3]. This area was mainly attributes) would vary along an environmental gradient covered with very sparse steppic vegetation threatened north-south. by various human activities including overgrazing and The aim of this work was to analyze the vegetation rainfed extensive farming that increased soil vital attributes in several bioclimates to better understand the process of desertification and adaptation Corresponding author: Ali Mahamane, associate professor, of plant species to climate variability and relate the main research fields: botany, phytosociology. E-mail: [email protected]. floristic variation with climate and human activities.

50 Climate Variability in Niger: Potential Impacts on Vegetation Distribution and Productivity

2. Material and Methods (6) Azawak, (7) , and (8) . The main characteristics of these observatories are 2.1 Observatories presented in Table 1 and Fig. 2. observatory Niger is an entirely landlocked West African country covered the whole wildlife reserve of Tamou which is covering an area of 1,267,000 square kilometers which adjacent to the W National Park. The vegetation encompass several bioclimatic zones. In 1997, the consisted of a shrub savanna on plateaus and Vitellaria Ministry of environment and desertification control paradoxa parks in farm lands. - (ME/LCD), with the support of the Sahel and Sahara observatory is the area of distribution of tiger bushes in Observatory (OSS), has established the long term Niger. The predominant species on plateaus were ecological monitoring observatories network Combretum micranthum, C. Nigricans and Guiera (ROSELT). In relation to this network, the country was senegalensis while in valleys Acacia albida and divided into eleven (11) observatories based on major Neocarya macrophylla parks were common. agro-ecological systems (Fig. 1). These observatories Regarding the -Gaya observatory, the composed the national environmental monitoring vegetation is characterized by several facies, among system (DNSE). In this study, 8 of the 11 observatories other, shrub savanna on plateaus, Vitellaria paradoxa were considered. They included, from west to east: (1) and Parkia biglobosa parks and the ronier palm trees in Tamou, (2) Torodi-Tondikandia, (3) Falmey Gaya, (4) the Dallol Bosso and Maouri where Borassus

Tahoua_ North Tillabéri, (5) Maradi--Dakoro, aethiopum was the key component. -North

(1) Tamou; (2) Torodi-Tondikandia; (3) Falmey Gaya; (4) Tahoua_ North Tillabéri; (5) Maradi-Mayahi-Dakoro; (6) Azawak; (7) Zinder; (8) Diffa and (9) Aïr Tenere not include in this study. Fig. 1 Geographical location of the observatories network of the long term ecological monitoring system (ROSELT) of Niger.

Climate Variability in Niger: Potential Impacts on Vegetation Distribution and Productivity 51

Table 1 Geographical coordinates and average rainfall in the observatories. Reference station Latitude Longitude Rainfall (mm) Tamou 12.45 N 2.10 E 579.9 Torodi-Tondikandia 13.29 N 2.10 E 518 Falmey-Gaya 11.53 N 3.27 E 722.1 Tahoua-Tillabéri Nord 14.12 N 12.01 E 386 Maradi-Mayahi-Dakoro 13.59 N 7.42 E 336.1 Aza wak 14.54 N 5.15 E 340.8 Zinder 13.48 N 9.00 E 345.6 Diffa 13.25 N 12.47 E 222.7

Fig. 2 Variation of the average rainfall in the observatories of Tamou, Torodi, Gaya, Tillaberi, Mayahi, Taoua, Diffa and Zinder.

Tillabéri observatory is mainly composed of Acacia includes the Damagram Mounio system where the tortilis steppes in the region of Tahoua and a mixed vegetation is largely represented by more or less wooded park in the area of Tillaberi. The observatory is wooded steppe of Acacia tortilis in the North and an distinguished by the importance of land reclamation Acacia albida park in the farm lands of the south. Diffa activities in response to the serious state of degradation observatory is composed of the dune system of the of that area. Chad Lake Basin characterized by the alternation of Maradi-Mayahi-Dakoro observatoiry encompass the relatively fixed dunes and sparse oases. The vegetation Goulbi N'kaba system where Hyphaene thebaica consists of wooded steppe of Acacia tortilis and shrub population is dense in the valleys. The sandy terraces steppe of Leptadenia pyrotechnica. are characterized by a shrubby woody group with 2.2 Method Piliostigma reticulatum, Combretum glutinosum, Guiera senegalensis as predominant species. The Plant diversity is one of the vital attributes of an Acacia tortilis steppes were found in the northern part ecocomplex [7]. To analyze the vital attributes of of the region. vegetation, 8 to 15 permanent stations were described In the Azawak observatory, the vegetation was for each observatory. mainly composed of Acacia tortilis steppes. The area is 2.2.1 Vital Attributes known for the presence of the pond structuring An attribute or indicator is, by definition, a a wetland of international renown. Zinder observatory parameter or a value that provides information about a

52 Climate Variability in Niger: Potential Impacts on Vegetation Distribution and Productivity phenomenon or its state [8]. These attributes also help greater is the number of species contributing to the land to quantify the extent of impact of various biotic and cover. It is expressed as: abiotic stresses. In the case of vegetation, they can be H ' E  divided into two groups depending on whether they log 2 S describe the structure of an ecosystem or its where H’ is the Shannon index and S is the specific functioning. In this work, the vital attributes used to richness. describe the characteristics of the different ecosystems The assessment of spatial changes in plant diversity were: specific richness, alpha diversity, plant biomass, was done using these diversity indices. biological and phytogeographic types. The point 2.2.1.4 Biological Spectra quadrats as defined by Daget and Poissonnet [9, 10] was Biological spectra were described for each used to characterize the state of the vegetation cover. observatory. These were Mesophanerophyte (MSPH), 2.2.1.1 Specific Richness Microphanerophyte (MCPHS), Nanophanerophyte Specific richness represents the number of different (Nnph), climbing Phanerophytes (Phgr), species in a given area [11]. The data regarding the list Chamaephytes (Ch), Geophytes (G), of plant species in each observatory were collected Hemicryptophytes (Hey), Hydrophytes (Hy) and using floristic relevés. To assess the changes that Therophytes (Th). occurred in the floristic richness at the scale of the 2.2.1.5 Phytogeographical Spectra observatory and analyze its evolution, species The chorology was characterized from the following composition was analyzed using quantitative elements: GC-SZ-Sah.S: species with Guineo parameters. Congolese-and Sudano Zambezian Saharo Sindian 2.2.1.2 Alpha Diversity distribution, i: introduced species, GC-SZ i: species Alpha diversity (α-diversity) is the biodiversity with Guineo Congolese-Zambezian Sudano within a particular area, community or ecosystem, and distribution, Sah. S: species with Saharo Sindian is usually expressed as the species richness of the area. distribution, SZ: species with Sudano Zambezian It was derived from the frequency of species found distribution, SZ-Sah.S: species with Sudano along the lines of quadrat points [12]. Alpha diversity Zambezian Saharo Sindian distribution, SZ-Sah.S-Med: was computed using the indices of biological diversity species with Sudano Zambezian Saharo Sindian and (H’) and equitability according to the Shannon-Weaver mediterranean distribution. method [13]. The diversity index H' is defined by the 2.2.1.6 Phytomass formula: Aerial phytomass represents the weight of living or s non-living aboveground plant material, per unit area H    p i log 2 p i i  1 and at a given time. It is expressed in kg DM/ha. The With S = total number of species; pi = (ni/N), measurement of plant biomass is essential to assess the relative frequency of species (Σpi = 1); ni = relative amount of resources available within the observatory, frequency of species i in the sampling unit; N = sum of while regular monitoring of plant biomass would relative specific frequencies. enable assessment of primary production based on 2.2.1.3 Equitability Index different production parameters (time, rainfall) and Equitability index [12] was used to assess the productivity, provided that the required protocols were weight of each species in the land cover. This used. parameter allows to describe the distribution of species 2.2.2 Analysis of Climatic Parameters in a given community. The greater this index, the To determine the bioclimates in the different

Climate Variability in Niger: Potential Impacts on Vegetation Distribution and Productivity 53 observatories, ombrothermic diagram was built using agro-pastoralism. In fact, overgrazing and intensive rainfall and average temperatures for each observatory farming decreases the flora diversity. According to [14]. This diagram allowed to identify the wet and dry Legendre P. & Legendre L. [12], the Shannon index months and thus to explain partly the differences values are high for species with comparable recovery between observatories. In general, a given month is ability while being low when only some of the species considered wet when its rainfall is more than twice its have high recovery ability. Regarding the equitability average temperature. index, it tends to 0 when a species has very strong recovery ability and tends to 1 when all species are 3. Results and Discussion equally important. 3.1 Diversity Indices 3.2 Biological Types When the species composition was considered alone, the observatories of the northern Sudan (Tamou, In the Tamou observatory, the chamaephytes were Falmey-Gaya, Torodi-Tondikandia, relatively frequent (Table 3). This reflects not only the Maradi-Mayahi-Dakoro) had significantly more taxa status of wildlife reserve which reduces the impact of than those in the Sahelian bioclimatic zone (Tahoua, human activities compared to other observatories, but Tillaberi-North, Azawak, Zinder and Diffa). In also the higher amount of rainfall in that area (579.9 addition Tamou and Maradi-Mayahi-Dakoro mm). This allowed the development of observatories presented the highest number of taxa microphanerophytes which composed the tiger and because of the presence of forest reserves. Indeed, the spotted bushes. The nanophanerophytes fall in all the Tamou observatory is a wildlife reserve adjacent to the observatories while the geophytes were the least W National Park of Niger protected from human represented. Overall, the biological spectrum was activities. In purely Sahelian zone, the number of taxa dominated by the therophytes. This therophytisation is in the Azawak is dependent on the presence of the a characteristic of the arid zones [15]. This Tabalak pond (Table 2). In addition, α-diversity was phenomenon, which reflects a strategy to cope with higher in the Sudanese observatories compared to the adverse conditions, has been observed in many arid northern observatories. The Diffa observatory which zones in northern Sahara [8, 15]. corresponds to a dune system showed a lower 3.3 Phytogeographical Types α-diversity. By contrast, in the Tahoua-North Tillabéri observatory the α-diversity was relatively high, partly The species with Sudano-Zambezian (SZ) and because of land reclamation activities that were based Guineo-Congolese and Sudanese-Zambezian (GC-SZ) on soil and water conservation techniques. Indeed, distribution were predominant for the observatories these restoration activities increased the number of located in Western part of the country. However for the ecotones with a subsequent increase in the number of observatories in eastern and northen part of the country, microclimates, thereby improving the floristic richness. there was a high frequency for species with SZ-and The Torodi-Tondikandia observatory corresponding to SZ-Sah.S Sah.S-Med distribution. The introduced the tiger bush system, as well as the Tahoua-North species (i) were more common in observatories where Tillabéri observatory, presented relatively higher planting woody species took place to limit the diversity than those of observatories where farming environmental degradation following the droughts of activities were common. This indicated that the 1973-74 and 1984-85. These sites were characterized rehabilitated stations had a relatively greater diversity by intense human activities (planting for reforestation, than the observatories characterized by an intensive market gardening etc..) (Table 4).

54 Climate Variability in Niger: Potential Impacts on Vegetation Distribution and Productivity

Table 2 Diversity indices in the observatories. Observatories Diversity indices Tamou Torodi Gaya Tillaberi Mayahi Tahoua Zinder Diffa Number of species (S) 168 145 154 163 64 132 120 63 Shannon Index (H’) 3.7 3 3.6 2.8 3.8 2.9 2.7 1.9 Equitability Index (E) 0.6 0.7 0.8 0.6 0.8 0.6 0.7 0.5

Table 3 Biological types (%) in the observatories. Observatories Biological types Tamou Torodi Gaya Tillabéri Mayahi Tahoua Zinder Diffa Mesophanerophyte (Msph) 2.4 0.6 5.6 3.1 1.8 2.3 4.2 3.2 Microphanerophyte (Mcph) 13.7 11 11.1 32.8 13.5 13.6 8.3 9.5 Nanophanerophyte (Nnph) 2.4 3.2 2.8 3.1 4.9 6.8 5.8 6.3 Phanerophyte grimpant (Phgr) 0.6 1.9 0 1.6 3.1 2.3 1.7 4.8 Chamephyte (Ch) 31 28.6 4.2 1.6 6.7 8.3 4.2 6.3 Geophyte (G) 4.2 1.3 1.4 3.1 4.3 2.3 3.3 0 Hemicryptophytes (He) 1.2 1.9 5.6 1.6 3.7 4.5 2.5 3.2 Hydrophyte (Hy) 0.6 3.2 0 0 0 16.7 0 1.6 Therophyte (Th) 43.9 48.3 69.3 53.1 62 43.2 70 65.1 Total (%) 100 100 100 100 100 100 100 100

Table 4 Phytogegraphical types (%) in the observatories. Observatories Phytogeographical types Tamou Torodi Gaya Tillabéri Mayahi Tahoua Zinder Diffa GC-SZ 36.4 35 33.3 26.2 33 27.4 31.9 25.8 GC-SZ-Sah.S 12.4 13.7 8.3 21.3 10.8 19.5 12.6 19.4 i 0.8 0.9 0 3.3 4 13.3 1.7 3.2 Sah.S-Med 0 0 0 0 1.7 0 1.7 4.8 SZ 38 31.6 40.3 16.4 28.4 15.9 32.8 16.1 SZ-Sah.S 12.4 17.9 18.1 32.8 21.5 22.1 18.5 29 SZ-Sah.S-Med 0 0.9 0 0 0.6 1.8 0.8 1.7 Total 100 100 100 100 100 100 100 100 GC-SZ: Guinéo-Congolaise et Soudano-Zambézienne; GC-SZ-Sah.S: Guinéo-Conglaise et Soudano-Zambézienne et Saharo-Sindienne; i: introduites; Sah.S-Med: Saharo-Sindienne et Méditerranéenne; SZ: Soudano-Zambézienne; SZ-Sah.S: Soudano-Zambézienne et Saharo-Sindienne; SZ-Sah.S-Med: Soudano-Zambézienne, Saharo-Sindienne et Méditerranéenne.

In the Azawak observatory, in addition to the Echinochloa stagnina prairie in the Falmey-Gaya introduction of exotic species such as Prosopis observatory. In the northern most area, juliflora of Australian origin, there was an important productivity was lower due to low rainfall. For irrigated farming system on the Tabalak pond slopes example, in the Acacia tortilis steppes located in the and in the valley. area of , the yearly rainfall was often less than 250 mm. 3.4 Primary Productivity In addition, for the same bioclimatic conditions, Productivity was higher for stations located in the productivity was higher in the protected area. This is a northern Sudanian zone (Tamou, typical example of the Torodi-Tondikandia Torodi-Tondikandiaand, Falmey-Gaya, Table 5). The observatory that included protected and non protected highest values were recorded for wetlands such as the fallows since 1994 [16].

Climate Variability in Niger: Potential Impacts on Vegetation Distribution and Productivity 55

Table 5 Shrub phytomass in the studied observatories (with n as number of samples). Observatories Land cover units Phytomass (g·m-²) Tamou Combretum shrub-savana 284.646 ± 92 (n = 4) Guiera senegalensis in protected fallow 287.55 ± 93.5 (n = 4) Tondikandia Guiera senegalensis in non protected fallow 191.7 ± 32 (n = 11) Gaya Guiera senegalensis in fallows 264.36 ± 116.6 (n = 5) Mayahi Fallows 160 ± 75.8 (n = 10) Acacia tortilis steppes 70.5 ± 16.14 (n = 4) Zinder Fallows 113.3 ± 79 (n = 3) Diffa Acacia tortilis and Leptadenia pyrotechnica shrub-steppes 214.3 ± 131.4 (n = 4)

3.5 Influence of Some Climatic Parameters on the compartments of the southernmost, corresponding to Characteristics of Vegetation the northern Sudanese bioclimate, were having the highest specific richness [3]. The number of wet months varied from south to In this zone, the length of the rainy season and the north. Analysis of the ombrothermic diagrams for each amount of rainfall resulted in a higher systematic observatory allowed grouping the observatories in 3 diversity and primary productivity. Similarly, the categories: (1) The North-Sudanian bioclimate for perennial species have the highest densities in the which 4 to 5 months are wet. It includes the wettest bioclimate. observatories of Tamou, Falmey-Gaya and The north sahelian bioclimate is populated by Torodi-Tondikandia (Fig. 3); (2) The Sahelian therophytes and microphanerophytes, which reflected a bioclimate characterized by 3 wet months and covering landscape of shrub-steppe vegetation. These two types the observatories of Tillabéri, Maradi, Zinder and Dakoro-Mayahi; (3) The North Sahel bioclimate with are the most adapted biological types to an area where only 2 wet months and comprises the observatories of nine months per year were dry. Azawak and Diffa. In these observatories which are For all observatories, the rainy season begins in distributed to the edge limit of the desert, the short April with highly variable amplitude. The therophytes duration of the rainy season is associated with erratic have a dominant share in the observatories of the eastern rainfall. Overall, the specific richness showed a part of the country. The short rainy season favored the decreasing gradient from south to north, i.e. from development of therophytes [8]. The phanerophytes wetter to dryer areas [17]. The phytogeographical showed an increasingnorth-to-south gradient.

Rainfall (mm) Rainfall (mm) Rainfall (mm) Rainfall (mm)

56 Climate Variability in Niger: Potential Impacts on Vegetation Distribution and Productivity Rainfall (mm) Rainfall (mm) Rainfall (mm )

Rainfall (mm) Rainfall (mm) Rainfall (mm) Rainfall (mm)

Rainfall (mm) Rainfall (mm) Rainfall (mm) Rainfall (mm)

Fig. 3 Ombrothermic diagrams of the stations used in this study.

the south-to-north rainfall gradient. The diversity 4. Conclusion indices and productivity were significantly higher in This study reported a spatial variability of vital the observatories of the north-sudanese bioclimate attributes of the vegetation in relation to the rainfall in where 4-5 months are wet. But locally, wetlands like the studied observatories. The differences in floristic the surroundings of Tabalak pond in the Azawak diversity and site productivity were closely related to observatory, created favorable conditions that resulted

Climate Variability in Niger: Potential Impacts on Vegetation Distribution and Productivity 57 in relatively high plant diversity and productivity. [7] H. Le Floc, J. Aronson, Ecologie de la restauration. Définition de quelques concepts de base, Natures Sciences On the whole system, the distribution of biological Sociétés volume 2 Hors série (1995) 29‐34. (in French) types showed a predominance of the therophytes in the [8] S. Jauffret, Validation et comparaison de divers north Sahelian bioclimate, a sign of therophytisation indicateurs des changements à long terme dans les explained by a relatively severe environmental écosystèmes méditerranéens arides: Application au suivi de la désertification dans le Sud tunisien, Ph.D. Thesis, degradation mainly due to the impact climatic University of Aix-Marseille III, France, 2001. (in French) conditions. Compared to the agricultural production [9] P. Daget, J. Poissonnet, Une méthode d’analyse systems and areas where exotic species were phytosociologique des prairies, Critères d’application, introduced, the protected areas were very diverse and Annales Agronomiques 22 (1971) 5-41. (in French) [10] C. Floret, Méthodes de mesure de la végétation pastorale: productive due to minor impact of human activities. Pastoralisme et développement, Montpellier (France), References CIHEAM, 1988. (in French) [11] G. Long, Diagnostic phytoécologique et aménagement du [1] J.M. Harmand, Rôle des espèces ligneuses à croissance territoire: I Principes généraux et méthodes, Masson, Paris, rapide dans le fonctionnement biogéochimique de la France, 1974. (in French) jachère: Effets sur la restauration de la fertilité des sols [12] P. Legendre, L. Legendre, Numerical ecology, ferrugineux tropicaux, Bois et Forêts des Tropiques 256 Developments in Environmental Modelling 20 (1998) (1998) 75-79. (in French) 235-245. [2] J.D. Meunier, Le rôle des plantes dans le transfert du [13] S. Frontier, D. Pichod-Viale, Ecosystèmes: Structure, silicium à la surface des continents, C. R. Geoscience 335 fonctionnement, évolution, Collection d’Ecologie, (2003) 1199-1206. (in French) Masson 21, Paris, France, 1993. (in French) [3] M. Saadou, La végétation des milieux drainés nigériens à [14] H. Gaussen, F. Bagnouls, Saison sèche et indice l'est du fleuve Niger, Ph.D. Thesis, Université de Niamey, xérothermique, Université de Toulouse, Faculté des Niger, 1990. (in French) Sciences, 1953. (in French) [4] P. Ozer, M. Erpicum, Méthodologie pour une meilleure [15] Ph. Daget, Sur les types biologiques en tant que stratégie représentation spatio-temporelle des fluctuations adaptative, (Cas des thérophytes), in: Recherches pluviométriques observées au Niger depuis 1905, d’écologie théorique, les stratégies adaptatives, Paris, Sécheresse 6 (1995) 103-108. (in French) 1980, pp. 89-114. (in French) [5] A. Mahamane, A. Ichaou, K.J.M. Ambouta, M. Saadou, B. [16] E. Delabre, Caractérisation et évolution d’écosystèmes Morou, I. Amani, et al., Indicateurs écologiques de la anthropisés sahéliens: les milieux post-culturaux du période optimale de remise en culture de jachères au Niger, sud-ouest nigérien, PhD Thesis, Université P. & M. Curie, Sécheresse 18 (2007) 289-95. (in French) Paris VI, Paris, 1998. (in French) [6] W.R. Teague, S.L. Dowhower, J.A. Waggone, Drought [17] A. Mahamane, M. Saadou, M.B. Dan Jimo, K. Saley, B. and grazing patch dynamics under different grazing Yacoubou, A. Diouf, et al., Plant diversity in Niger: State management, Journal of Arid Environment 58 (2004) of the present studies, Annales Université de Lomé (Togo), 97-117. série Sciences, Tome XVIII, 2009, pp. 81-93. (in French)