Evaluation of structure and natural regeneration status of woody plant species in sudanian domain : Case of eastern part of National Park of Sena Oura, Chad

Many scientific studies confirmed that plants have an important ecological role maintaining the productivity of the environment and regulating the global climate. In order to valorize the wild phytogenetic resources for the efficient in situconservation and sustainable use in Sudano-zambezian region, a study was carried out in Sudanian domainprovidingquantitative informations on the community structure and natural regeneration status of woody plant species.The study site is the eastern part of National Park of Sena Oura in Chad. Adults plants (trees and shrubs) were systématically collected in 10 linear transects (20 m x 1000 m). Juvenile plants (saplings and seedlings) weresystematically collected within 40 plots (20 m × 20 m). These plots were randomly stablished in the transects, at a rate of four plots per transect.In total, 84 adults plants species grouped in 58 genera and 29 families and 66 juvenile plants species grouped in 45 genera and 27 families were inventoried. Bell andreverse J-shaped patterns of selected woody species were identified.The stand regeneration status wasgood.The stand regeneration rate were SRR = 52.29% and Hymenocardia acida (SIR = 17.95%), Combretum collinum (SIR = 14.12%), Annona Senegalensis (SIR = 6.67%) and Isoberlinia doka (SIR = 6.22%) had the most important specific index of regeneration. The specific structures showed that the structure of the total stand is the result of the dynamics of all species and their interactions.The global stand regeneration status was good.The obtained results provided quantitative informations on the community structure and natural regeneration status of woody plant species for the efficient conservation and sustainable use.

). Many scientific studies confirmed that tropical ecosystems are the most rich in biodiversity, particulary in plant biodiversity. These plants have an important ecological role maintaining the productivity of the environment and regulating the global climate (Quijas et al. 2012). They provide food for humans and animals, serve as medicine basis and construction materials as well as combustible. Somes wild fruits are edible. They are sold locally or sold after being processed locally (Todou et al. 2017b).They also protect the land from wind and water erosion, stabilizing the water cycle, facilitate the process of evaporation. They serve to absorb carbon dioxide to reduce global warming, give off oxygen, and renew the atmosphere.The main mission of protected areas is to conserve plant populations, specific diversity or genetic diversity. They also act as buffers against anthropogenic or natural uncertainties, including natural disasters and climate change. Despite their richness in biodiversity and their usefulness for humanity, tropical ecosystemsare facing a lot of pressure from human as well as natural phenomenon to ensure sustainable improvement of the resources http://dx.doi.org/10.22161/ijeab/3.5.2 ISSN: 2456ISSN: -1878 www.ijeab.com Page | 1572 (Aubertin and Vivien 1998).Humans are still dependent for a large part of his natural environment. They get all that is necessary for their daily survival. The impacts of anthropogenic activities are negative on biodiversity (Akpagana and Bouchet 1995). On the other hand, in almost all sub-saharan african countries, in situ conservation policies lack rigor in their design and implementation and have failed. For example, protected areas are generally abandoned and are transformed into plantations, feeding areas and zones of anarchic woodcutting (Akpagana and Bouchet 1994). Currently,few studies on plant biodiversity and its natural regeneration are carried out in sudano-zambezian region.It is known that studies on floristic composition, structure and natural regeneration in forests are instrumental in the sustainability of forests since they play a major role in the conservation of plant species, and the management of forest ecosystems as a whole (Ssegawa and Nkuutu 2006). These studies are imperative to identify and develop the potential for innovations derived from plant richness, particularly those of the developing countries and these studies becomes more imperative in the face of the ever increasing threat to the forest ecosystem. They are also help to understand the dynamics of woody vegetation (Adane 2011). In easthern part of NPSO, Todou et al. (2017a) had study floristic composition, diversity and ecological importance of woody plants. The resultsgave a general idea of the diversity of ligneous plants in the Sudanian area considering all its phytogeographical sectors.The present study enables to fill some of the information gaps especially of structure and natural regeneration. The idea is to provide quantitative informations on the forest restoration and specific information for conservation measureswhich may be applicable during forest management in all soudano-zambezian region. The main objective is tocontribute to efficient conservation and sustainable use of the sudano-zambezian wild plant resources.The specific objectives are: (1) to evaluate stand structure and individual structure of most abundant species as well as (2) to evaluate the natural regeneration status.

II. MATERIALS AND METHODS Study area
National Park of Sena Oura is located in the Department of Mayo-Dallah, Mayo-Kebbi West Region. It is located between 8°25'43'' and 9°13' 06'' North latitude and 13°58'47'' and 15°30'09'' East longitude. It is located at an altitude of between 350 and 671 m and it is cross-border with the Bouba-Ndjidda National Park in Cameroon ( fig.  1). This study was carried out in the eastern part of the Park, limited to the western part by the river 'mayo sena oura. The NPSO is located in the Sudanian domain (sudanozambezian region) according to the phytogeographic subdivision (Letouzey 1985). The climate is of the tropical sudano-guinean type with a dry season which extends from October to April and a rainy season from May to September. Annual cumulative rainfall is about 900 to 1200 mm per year. The hydrographic network consists of rivers flowing between July and September (Bemadjim 2014). The vegetation is a wooded savanna identical to that of the National Park of Bouba Ndjidda but with the particularity of sheltering in the zone of confluence of the streams, vegetation of guineo-sudanian type and forests gallery along the rivers.

Data collection
The inventory was done during four months (April-May-June-July 2016).Data collection for adult plants (adult trees and shrubs) was done in 10 linear transects (20 m x 1000 m).These transects were established about more than 500 m one away from each other in order to cover the eastern part of the Park and to represent the maximum of species. In total, 20 ha were surveyed. Juvenile plants (s aplings and seedlings) collection was done within 40 plots (20 m × 20 m)randomly stablished in the transects,at a rate of four plots per transect. Within each plot, all juvenile plants were systematically counted. Scientific identification of the most common species was done directly in the field whenever possible. Some specimens were collected in order to authentify scientific names in laboratory of Agriculture and Development Research Institute (IRAD) in Maroua and by botanists of University of Maroua (Cameroon).

Data analysis
The stand structure and specific structure of the eight most abundant adult plant species were assessed through histograms constructed by using the density of individuals (Y-axis) categorized into height and diameters classes (Peters 1996). All recorded data of plots were pooled and the total number of species were tallied. Using the pooled data, number of species, of genera and of families were calculated in order to evaluate their richness.

III. RESULTS AND DISCUSSION Stand structure of adult plants based on height class
The height class frequency distribution of the stand exhibited a tendency towards a reverse J-shaped distribution ( fig. 2).The adjustment of the distribution of all adult plants to the mathematical model gave an exponential function (y = 4350.6e -0.74x ), highly significant (p <0.001, R² = 0.94), with y = numberof species and x = the center of diameter class.About 44.02% (1670 individuals)were the highest between 6 and 11 m height class. However,23.29%(800 individuals) were inferior to6 m height. This class was represented by shrubs and also juvenile trees. It can therefore be deduced that up to 76.71% of plants were trees more than 11 m height.In this case where all plants species were grouped, reverse Jshaped distribution indicate that there were less and less species possessing greater individuals than 11 m height.This structure which decreased exponentially from small height classes to great height classes testifies to the stability of the total stand, characterized by regular natural regeneration.  In bell-shaped pattern the distribution of individuals of a species in the middle diameter classes is high and low in lower and higher diameter classes. According to Feyera et al. (2007), bell-shaped pattern indicates a poor reproduction and recruitment of species which may be associated with intense competition from the surrounding trees. This similar situation was observed by Kuma and Shibru (2015) in Oda Forest of Humbo Carbon Project (Ethiopia). Inverted J-shaped pattern has shown high distribution of individuals of a species in the lower diameter classes and a gradual decrease towards the higher classes. It has shownalso good reproduction and recruitment potential of the species.Analysis structures for adult plant species could provide more realistic and specific information for conservation measures.    (Table 1).Thisstand diversity was moderated with fairly good equitablility according Yédomonhan (2009) analysis. The Shannon index was usually found to fall between 1.5 and 3.5 and is rarely above 5.0 (Magurran 2004 (Kuma and Shibru 2015). Combretaceae (13 species), Caesalpinaceae (9 species) and Mimosaceae (8 species) specifically dominated in the studied area (Table 2). It was same to adult plants that Combretaceae (16 species), Caesalpinaceae (10 species) and Mimosaceae (10 species) were specifically dominated in the same site (Todou et al. 2017a).The so called new species could be present in the forest, more or less far from the study site, but dispersed by frugivorous animals. This could justify the important ecological role of animals in a forest ecosystem. Natural Regeneration is the establishment of trees from seeds that fall and germinate in situ (Harmer 2001). It is the basis for understanding the dynamics of woody vegetation. It involves recruitment, juvenile mortality and different stages of development, and survival (Traoré 1997). It can be vegetative or by natural seedling, but in easthern part of NPSO, natural regeneration was assessed by the importance of sapling. The general regeneration important, stand regeneration rate were SRR = 52.29%.This value is not negligible because it is higher than average. It indicates a stability of the total stand assumed in the structural study. Stand regeneration rate of easthern part of NPSO was slightly lower than that of Ngom et al. (2013) in Ferlo Biosphere Reserve (northern Senegal). That was 72%.The importance of regeneration according different species was represented by Specific Index of Regeneration (SIR). Species with the greatest potential for regeneration wereHymenocardia acida (SIR = 17.95%) Combretum collinum (SIR = 14.12%), Annona senegalensis (SIR = 6.67%) and Isoberlinia doka (SIR = 6.22%). These species alone accounted for up to 45% of regeneration. Species with low regeneration potential   (Table 2).According Ngom et al. (2013), species with the highest specific index of regeneration are species which regenerate readily by stump in the absence of bushfires. This is the case of Hymenocardiaceae and Combretaceae.

IV.
CONCLUSION At the end of this study, it was demonstrated the structure of all plants species grouped together was reverse Jshaped distribution characterized by regular natural regeneration. Structure of eight selected species demonstrated two types of patterns of population structure: bell-shaped pattern for two s pecies and reverse J-shaped pattern for six species. These specific structures showed that the structure based on the height of the total stand is the result of the dynamics of all species and their interactions.The diversity of juvenile plants stand was moderated with fairly good equitablilitysimilar to adult plants diversity. The global stand regeneration was good that species with the greatest potential for regeneration wereHymenocardia acida, Combretum collinum, Annona senegalensis and Isoberlinia doka. But some species with no regeneraion were recorded too and 11 species were represented by only juvenile plantsproving the ecological role of frugivorous animals.These results of structure and natural regeneration status provided quantitative informations on the community structure and natural regeneration status of woody plant species for the efficient conservation and sustainable use.They may be applicable during forest management.