Diversity and Dynamics of Plant Communities in Niger River Valley (W Regional Park)

The “W” Regional Park is located in the West African north-sudanian zone (Fig. 1; White, 1983) and covers an area of 1.024.280 ha. The average annual rainfall is 704.7 ± 180 mm with an average temperature of 37 ° C (Fig. 2). This park includes several Precambrian geological structures. In the river valley, the soil is of clayey gley/pseudo-gley type. The banks of this river host a “special vegetation” called "bourgou" by the local population in reference to “bourgoutiere” commonly used in the literature for this type of vegetation (Dulieu, 1989). The phytosociological investigation was conducted in several sites located in this area from 2002 to 2003 during the flooding period and the dry season. Each releve included the complete list of species with their abundance-dominance coefficients (Braun Blanquet, 1932).


Introduction
The "W" Regional Park covers an area adjacent to the border of Benin, Burkina Faso and Niger (Fig. 1). The park hosts diverse flora and fauna. The large part of plant communities of this area remain poorly understood and only few reports on this subject are available (Garba, 1985;Boudouresque, 1995 andCouteron et al. 1992).
The objective of this work was to characterize plant communities along the Niger River bank during the flooding period and the dry season. These two periods play important roles in the ecosystem dynamics.

Materials and methods
The "W" Regional Park is located in the West African north-sudanian zone ( Fig. 1; White, 1983) and covers an area of 1.024.280 ha. The average annual rainfall is 704.7 ± 180 mm with an average temperature of 37 ° C (Fig. 2). This park includes several Precambrian geological structures. In the river valley, the soil is of clayey gley/pseudo-gley type. The banks of this river host a "special vegetation" called "bourgou" by the local population in reference to "bourgoutiere" commonly used in the literature for this type of vegetation (Dulieu, 1989). The phytosociological investigation was conducted in several sites located in this area from 2002 to 2003 during the flooding period and the dry season. Each releve included the complete list of species with their abundance-dominance coefficients (Braun Blanquet, 1932).
Data analysis was performed with Canoco software (ter Braak and Smilauer. 1998). For the different plant communities that were studied, the specific diversity indices of Shannon and Weaver (1949 in Legendre and Legendre, 1998) and Pielou equitability index were calculated.
www.intechopen.com Rainfall (mm) Boundaries Regional sahelian endemism center Regional Soudanian Endemism Center Rainfall (mm) W Regional Park www.intechopen.com The nomenclature after identification of species is referred to in Lebrun andStorck (1991-1997). The reported species were represented by herbarium specimens available in the herbaria of the UAM and ULB (BRLU).

Results
Detrended Correspondance Analysis was performed on a matrix of 42 relevés and 116 species. The first Axis reflected a gradient of water depth (Fig. 3). Near the origin of this axis were located the groups of plants adapted to deep water conditions occurring during flooding period while the dry season plant groups were positioned on its positive side.

Polygono senegalensis Echinochloetum stagninae ass. nova
Polygono Echinochloetum stagninae was defined by 13 releves and 22 species of which four were specific to this association: Echinochloa stagnina, Polygonum senegalensis, Lemnapaucicostata and Azolla pinnata (Table 1). Water depth may exceed 2 m. The pH was neutral and close to 7. The distribution of the biological types showed the predominance of therophytes (33.3%) followed by hydrophytes (28.6%) and phanerophytes (28.6%). Regarding the phytogeographical units, results showed the dominance of species with paleotropical distribution (33.33%) followed by pantropical species (28.57% and Sudan-Zambezian species (23.81%). The number of species per releve varied from 2 to 17 with an average of 4.19 ± 1.86. The Shannon diversity index was 2.69 and the maximum diversity index up to 4.64 while the Pielou equitability was 0.57.

Eichhornietum crassipedis Vanderlyst 1931
Eichhornietum crassipedis Vanderlyst 1931 was distributed along the linear fringe of the Niger River bank. Water depth was quite similar as for Polygono senegalensis Echinochloetum stagninae, since they colonized the same stations.
The distribution of biological types revealed the predominance of hydrophytes (40%), followed by therophytes (20%) and microphanerophytes (20%). The helophytes represented only 10%. Regarding the phytogeographical distribution results showed the dominance of cosmopolitan species. The Eichhornietum crassipedis association consisted of 10 releves and 17 species. The average number of species per releve was 4. The Shannon diversity index was 2.34 and Pielou equitability 0.57. These values indicated a very small number of dominant species within the plant community.

Leptochloo Stachytarphetetum angustifoliae ass. nova association
Leptochloo coerulescentis Stachytarphetetum angustifoliae developed in the late dry season and beginning of the rainy season on the banks of the river that were sufficiently dewatered to allow the development of an herbaceous layer. It corresponded to a more or less continuous linear strip along the banks that were battered by the waves.
This syntaxon was defined by 10 releves and 34 species of those three were specific to the association: Stachytarpheta angustifolia, Leptochloa coerulescens, Cardiospermum halicacabum ( Table 2). The raw distribution of biological types showed that helophytes were dominant (58% -73%), followed by therophytes (25%) and hydrophytes (22%) in the pondered distribution. Species of Sudanian distribution represented only 8% of the spectrum. As for the weighted spectrum, it was largely dominated by the species of Sudanese-Zambezian distribution (59.4%), species of Sudanian (9%) and Afrotropical species distribution (8.79%). The number of species per survey varied from 3 to 13 with an average of 7.6 ± 3.2. The Shannon diversity index was 3.6 and the maximum diversity index 4.9. The equitability index of Pielou was 0.74. These results suggest an equal distribution and overlap between species.

Cyperetum maculati Mandango 1982 association
Cyperetum maculati grows in the dry season on the sandbanks of the river's main channel.
Cyperus maculatus form clumps of variable size. It is a rhizomatous species which is completely submerged during flooding perio d s . D u r i n g t h i s p e riod of prolonged immersion, the plants were represented by perennial rhizomes. The floristic association has many annuals germinating on wet sand ( Area (m²) 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 Land - Benth.

Discussion
The initial phase of the bourgoutiere included Leptochloa coerulescens, Echinochloa stagnina, Echinochloa pyramidalis, Cyperus cylindrostachyus, Saciolepis africana and Stachytarpheta angustifolia. These plant species were progressively established in the late dry season and early rainy season. Leptochloa coerulescens bear fruits during this period. This phase corresponded to the ecological amplitude of Leptochloo-Stachytarphetetum angustifoliae. With the increase in water level, Polygonum senegalense and Echinochloa stagnina actively developed by vegetative propagation.
The optimal phase or aquatic prairie of bourgoutiere corresponded to Polygono Echinochloetum stagninae. The group covered a broad variable on the shores of the river. During the dry season, Echinochloa stagnina as Polygonum senegalense fall on the dewatered river banks. At that time, both species have spread their seeds. In the riverbed, Cypretum maculata Mandango 1982 was subject to strong variations in relation to alternating periods of flood and dry period. During the dry period characterized with a low water flow in the river, the species happened to complete its cycle. The start of this cycle, as and when the water recedes, is characterized by buds on the stolons. The flooding period was characterized by a progressive invasion by water hyacinth and resulted in the formation of Eichhornietum crassipedis. This determined syntaxon pollution of aquatic stressed environments (Brendonck et al. (2003)). These stations were characterized by variability of plant communities determined by the river water regime (Duvigneaud, 1946). Indeed, this group was not identified by Atta and DanJimo (2003) in the same river valley during the dry season. Gradually, as the water level dropped, hydrophytes population declined and progressively replaced by therophytes. This resulted in a decrease of the number of species.

Conclusion
Bougoutières vegetation plays an important role in the ecosystem of the W regional park. Apart from Eichhornietum crassipedis, the different syntaxons provide forage and habitat for wildlife. These syntaxons were characterized by low diversity indices in relation to dominance effects between species and disturbance (fire bank, harvested biomass) related to human activities.

Acknowledgments
The authors thank the Department of Wildlife Fisheries and Fish the Parc du W du Niger. They also thank for facilitating access to the Free University of Brussels, the University of Niamey, the Project ECOPAS and the African Academy for Science for their permanent support during the different phases of this work. They also thank Mr L. Pauwels the Botanical Garden of Meise, Prof. B. Sinsin Abomey Calavi University of Benin and Prof.. van der Maesen of the National Herbarium of Wageningen who have made valuable contributions in the identification of species. The ecosystems present a great diversity worldwide and use various functionalities according to ecologic regions. In this new context of variability and climatic changes, these ecosystems undergo notable modifications amplified by domestic uses of which it was subjected to. Indeed the ecosystems render diverse services to humanity from their composition and structure but the tolerable levels are unknown. The preservation of these ecosystemic services needs a clear understanding of their complexity. The role of research is not only to characterise the ecosystems but also to clearly define the tolerable usage levels. Their characterisation proves to be important not only for the local populations that use it but also for the conservation of biodiversity. Hence, the measurement, management and protection of ecosystems need innovative and diverse methods. For all these reasons, the aim of this book is to bring out a general view on the function of ecosystems, modelling, sampling strategies, invading species, the response of organisms to modifications, the carbon dynamics, the mathematical models and theories that can be applied in diverse conditions.