Characterization and impact of wood logging on plant formations in Ngaoundéré District , Adamawa Region , Cameroon

This study was conducted to characterize the different plant formations (shrubby, arborescent and woody savanna) and to ascertain the impact of wood logging on the floral diversity in the guinea savanna zone of Ngaoundéré District, Adamawa Region, Cameroon. The “Point-Centered Quarter (PCQ) Method” was used on 120 sites measuring 50 × 50 m. Results showed that according to the types of wood logging in the different plant formations, the species generally had an over-scattered distribution, and only the protected savannas had a gregarious distribution. The increased wood logging affects savannas’ stability and the disappearance of the floral biodiversity which are consequently responsible for the accelerated degradation. This is an alarming situation which enhances the progress of desert and the loss of biodiversity in the guinea savanna of the Adamawa Region, Cameroon. It is advocated that a concerted effort between the government and the local population should be established to protect and save the biodiversity in the guinea savanna of the Adamawa Region, Cameroon.


INTRODUCTION
The biodiversity is often used as a contracted shape of by the local population (Rippstein, 1985;Yonkeu, 1993; the biological diversity (Ndam, 1998;China et al., 2003). Ndjidda, 2001;Tchotsoua, 2006). In the North-Cameroon, It groups together the generic and specific diversity, the it has been asserted that the regression of the forest is populations and ecosystems and bases itself on the due to the combined effects of wood logging, bush fire specific wealth and the relative abundance of the species. and overgrazing (Ntoupka, 1994(Ntoupka, , 1998. In Cameroon, This biodiversity is actually endangered by the wood the impact of wood logging on the distribution of the floral logging phenomenon which took an unequalled scale a diversity is well known in the Southern part (Sonké, 1998; quarter of century ago in the African savannas leading to Guedje, 2002;Zacfack, 2005), but in the guinea savanna the accelerated degradation of the natural resources of Adamawa which is intermediate to the forested south which constitute the productive basic capital. In the and the Sahelian North, there is a paucity of information Adamawa Region of Cameroon, several authors thought on the wood logging activities. Chouaibou (2006) reporthat the causes of the transformation and the degradation ted the distribution of Parkia biglobosa in the district of of the guinea savanna could be due to overgrazing, over-Ngaoundéré which is the capital of the Adamawa Region, population led by human migration from the Far North Cameroon; however, this region is among those that are region of Cameroon, agriculture and exploitation of wood threatened by the anthropological wood logging, bush fire and overgrazing.
Since the signing of the conventions on the presservation of the biological diversity and the use of biological resources in a long-lasting way in 1992 in Rio de Janeiro, Brazil, these conventions are yet to be implemented in our sub-Saharan guinea savannas, where many of the plant species are either cut or harvested by men for several uses. Thus, the present study was carried out to characterize the different plant formations and to estimate the impact of wood logging on the floral diversity in the guinea savanna zone of Ngaoundéré District, Adamawa Region, Cameroon.

Study area
The study was undertaken in ten (10) villages namely: Béka Hooseré, Onaref, Wakwa, Tizon, Beskewal, Ngaohora, Borongo, Dang, Darang and Mban-Mboum all located in the Ngaoundéré district of the Adamawa Region, Cameroon ( Figure 1). These villages are located at about 10 km for the shortest and 60 km for the farthest distance from Ngaoundéré the capital city of Adamawa Region, Cameroon. Ngaoundéré is located at latitude 7° 19' N and longitude 13° 34' E. Its population was estimated at about 230,000 inhabitants in 2001 (Tchotsoua, 2006) with an increase rate of 2.81% per annum. The main ethnic groups are the Fulbés, Mbororos, Gbayas, Mboums, Dourous, Yemyems, Hausas and the Koutinés. The economic activities of the local inhabitants are mainly animal husbandry and land farming. The soil of the area belongs to the geo-morphological domain of the plateau of Adamawa. They are characterized by sedimentary, volcanic, granitic and metamorphic rocks.
The vegetation of the Adamawa corresponds to a typical Sudanoguinea savanna constituted with shrubby, arborescent and woody savannas. These savannas are dominated by Daniellia oliveri and Lophira lanceolata (Letouzey, 1986). The precipitations are maximal in August and practically null from November to February. The hygrometric is maximal in August with a monthly average humidity of 81.38%.

Choice of the different wood logging zones in the guinea savannas of Adamawa Region, Cameroon
To choose the different wood logging, interviews were conducted with group of persons. The prospections with the population in the site were made. The types of wood logging in the savannas depended on the degree of accessibility to the site (absence or proximity to easy access road), the distance to the village (0 to 0.5, 0.5 to 1, 1 to 2, 2 to 4, 4 to 6, > 6 km) and the percentage of the wood cut. At the end of prospection, four types of wood logging were selected: i) Pilot or witness logging (T0): made up with natural formation where the estimated percentage of wood logging is less or equal to 10%. They are generally protected areas by the inhabitants; ii) Weak logging (T1): vegetation where the percentage of wood logging is between 11 and 25%; iii) Average logging (T2): vegetation where the percentage or wood logging is between 26 and 50%; iv) Complete or total logging (T3): vegetation where more than 50% of woods are cuts.

Experimental technique
The point-centered quarter (PCQ) technique described by Farid et al. (2006), Kevin (2007) and Tchobsala (2010Tchobsala ( , 2011 was used in this study. This technique consisted in choosing a direction at random in the savannas under study (Figure 2).

Experimental design
The study was a split-plot design with 3 factors (shrubby savanna, arborescent savanna and woody savannas) ( Table 1). The pieces were numbered from 1 to 12, delimitated by numbered cement terminals or wood stakes. One hundred and twenty sites (3 types of savannas × 4 types of cuts × 10 villages) were selected with 30 sites for each treatment.

Calculation of the relative frequency (Fr):
Fr (%) = , with A = number of the statements containing species; B = total number of the statements.

Calculation of the relative density (Dr):
Dr (%) = , with C = number of individuals of a species; D = total number of the individuals encountered on a considered surface.

Calculation of the important value of curtis (IV):
IV (%) = Fr + Dr + Dre, with Fr = relative frequency, Dr = relative density and Dre = relative dominance. The (IV) varies between 0 and 300%.

Analysis of species distribution, species abundance and species dispersal
To analyze the distribution of species abundance, the models of Pichod-Viale, 1993), while the model of Pareto (Frontier and Pichod-Vilae, 1993) used to analyze the structure of wet tropical dense forest was adopted to analyze the structure of the guinea savanna observed in the study which is a zone of transition between the wet tropical forest and the sahelian savanna. The species distribution is the distribution of the number of trees species by class diameter. To study the horizontal organization of the plant communities, we used the dispersal parameter of all the individuals of the community. The closest neighbors method described by Clark and Evan (1954) was used to analyze the dispersal of all the population. This method allows specifying the way and the degree of remoteness of the random distribution of individuals of a given population.
If R = 1, the distribution is random, If R < 1, the distribution is grouped, If R > 1, the distribution is over-scattered.
The statistical difference between rat and rob can be appreciated by using the formula C = (rob-rat) /∂r, t = 0.26136Nd 1/2 , with ∂r as the standard error on the average distance observed in the case of a The analysis of the distribution of the individuals of the same species within the inventories was made by the "run test" method (Siegel, 1956). We considered the value (1) for the pre-sence and the value (0) for the absence. "Run test" examines the distribution of the species along the transect, allows to determine if we observe more or less sequences "run" than in the case of a random distribution. Significance levels were considered at P≤0.05, P≤ 0.01 and P≤0.001.

Characterisitics of the different plant formations in Ngaoundéré and environs
In whole, three types of plant formations were observed in the area: the shrubby, arborescent and woody savannas. The number of trees alive decreased from 241,860 trees/ha/year in the shrubby to 133,980 trees/ha/year in the woody savanna; however, the number of dead trees also decreased from 141,210 trees/ha/year in the shrubby savanna to 131,242 trees/ha/year in the arborescent savanna, but with a drastic decrease in the woody savanna. The mortality rate of these individuals is more important in shrubby and arborescent savannas. It is observed that in the three plant formations, the number of trees alive is almost twice the number of dead trees (Figure 3). This rate decreases in woody savannas. Indeed, shrubby and arborescent savannas undergo a strong pressure of wood cuttings and pastures.

Relative frequency, relative dominance, relative density and relative important values of the tree species in the different plant formations
Of all the 4,320 points of reading, Hymenocardia acida (58.01%) recorded the most important relative value. This species was the most important tree species in the savanna with a relative frequency of 18.54%, relative dominance of 8.56% and relative density of 30.96% (Table  3). Lannea acida (0.06%), Carissa edulis (0.05%) and Mytragina inermis (0.3%) had the least important values.

Tchobsala and Mbolo 105
Relative frequency, relative dominance, relative density and relative important value of trees' genera in the different plant formations Sixty two (62) genera were inventoried in the plant formations. The genera Hymenocardia (58.06%) had the most important relative value of all the genera encountered in the plant formations, while the genera Antidesma (0.07%) and Mytragina (0.03%) having the least values (Table 4).

Relative frequency, relative dominance, relative density and relative important value of the trees' families in the plant formations
Of the 34 families listed, Hymenocardiaceae (58.06%), Cesalpiniaceae (54.74%) and Annonaceae (28.38%) had the most relative important value, but Myrsinaceae (0.53%), Olacaceae (0.49%) and Tiliaceae (0.23%) families were sparsely found ( Table 5).  Distribution, density and recovery of the species according to the types of savannas and wood logging Table 6 represents the results of distribution, density and recovery of the species in relation to the plant formations and wood logging. The distance between the individuals

Distribution, recovery and density of the different types of plant formations in relation to distances from the villages surveyed
On the sites, the species were randomly distributed within 0.5 to 1 km (R = 0.99) ( Table 7). They are grouped for the distances > 6 km (R = 0.91); 4 to 6 km (R = 0.93) and of 2 to 4 km (R = 0.71) and over-scattered for the distances of 0 to 0.5 km (R = 1.34) and of 1 to 2 km (R = 2.39) with regards to the villages.

Distribution, density and recovery of the species in the sites with easy access (T 3 )
The recovery of trees is maximal in arborescent savanna with T 3 (1154.21 m 2 ha -1 year -1 ) from 2 to 4 km of the villages. In the arborescent savanna, the farthest distance from the villages was characterized by encountering the big trees such as D. oliveri and Cesalpinia sp (25 to 60 cm of diameter) that are not cut by lumberjacks. These species have a very important recovery in order of 40 to 60 m 2 by individual. In the whole of the sites and according to the distances with regard to the villages, trees have average distances varying between 1.55 and 6.29 m, with R oscillating between 0.31 and 2.92 (Table 8).

Specific dispersal of the species in the various plant formations in Ngaoundéré and environs
The distribution of the species was observed inside every type of savanna with treatments. The species which were present at least 10 times in 360 points of reading were held for the analysis of the specific dispersal. Among these species, only 15 on 102 species were inventoried by the method of distance between the species and were held for the analysis of the gregariousness. A total of 82 species represented scattered or over-scattered distributions. The species which have a significantly grouped distribution (p < 0.05) were Annona senegalensis in T 1 , T 2 and T 3 ; H. acida in T 2 and T 3 and Psorospermum febrifigum in T 1 in shrubby savanna. In the arborescent savannas, only H. acida represented a grouped distribution (T 1 and T 2 ) ( Table 9).

Scientific names Shrubby savanna Arborescent savanna
highly significance. These species are over-scat-ever, our results are very different from those tered in the outer-urban zone of Ngaoundéré reported by Sonké (1998) who found 90 gregabecause of the fragility of the ecosystems. How-rious species in the reserve of the biosphere of Dja. The strong pasture are at the gregarious specie In the peri-urban zone of Ngaoundéré, the women are the first to be responsible for wood logging activities which they use for energy and other culinary task. The organization of rural markets of the firewood constitutes a major threat to plant species in all the different plant formations in Ngaoundéré. In the peri-urban zone of Ngaoundéré, women are more involved in wood logging activities more than the loggers, local farmers and therapeutists. Mapongmetsem and Akagou (1997) showed that the situation of the firewood is already alarming in Adamawa and even worsened these recent years with the multiform economic crisis which Cameroon is passing through. This crisis involved people to have an increased quest for charcoal and firewood, thus increasing the rate of taking away significant quantities of wood from the natural formations. In addition to the firewood, peasants cut wood like non-woody forest products selectively. The wild fruit-lofts like Vitellaria paradoxa, Tamarindus indica, Syzygium guineense, Ximenia americana, Vitex doniana, S. guineense and Parkia biglobosa are generally cut for human consumption. Gudjé (2002) reported that, the taking away of trees as not ligneous family products contributes to the destruction of vegetation cover. For the traditional pharmacopeia, Piliostigma thonningii and Securidaca longepedunculata are requested.
The local population appreciates S. longepedunculata for the treatment of rheumatisms; likewise pastoralists in the dry tropical zones use it to increase the availability of fodder at the end of the dry season and the rainy season. These pastoralists have a practice of cutting the highest branches, to lay them down on the ground and to place them at the disposal of their cattle and the smaller livestock (Ntoupka, 1999). The disappearance of the plant species mainly is due to the wood logging for heating and charcoal, the intensification of agriculture, the traditional pharmacopeia, the construction of the houses, the beekeeping through wood used in the hives. Species like H. acida, S. guineense spp, D. oliveri, Terminalia spp, Strychnos spinosa and P. biglobosa are over-exploited. The distribution of the species according to plant formations and the distances with regards to villages showed that the difficulty of access was one of the reasons. It can be noticed that the species are over-scattered in shrubby savannas (0 to 0.5 km), arborescent (> 6 km) and afforested (1 to 2 km).
With regards to recovery of the plant species, T 3 was observed having the highest activities of wood loggings which are very important; this consequently influenced the rate of regeneration which was also high. The sum of the recovery of the big trees and the regeneration of trees after logging is at the origin of the biggest recovery of trees in the outer-urban savanna of Ngaoundéré. However, the recovery of the species by hectare is more important in woody savanna.

CONCLUSION AND RECOMMENDATIONS
Wood logging activities represents a direct effect on the state of the individuals and consequently imposes an over-scattered distribution of the species in the different types of plant formations according to treatments. The number of the wood logging plant individuals was very important in shrubby savannas and afforested with the treatment T 3 . H. acida has an important value in the Ngaoundéré savannas. It is the best regenerating plant in the vegetation and resists bushfire and wood logging in the guinea savanna of Adamawa. To manage our savannas, it is important to limit the wood logging around villages because of the over-dispersion and disappearrance of the endemic multi-purpose species in the zone. If the population does not become aware of wood logging activities and managing their forest heritage, we shall arrive at a total eradication of the gregarious endemic species of the region of Adamawa. It would be desirable that in the Sudano-Sahelian zone, victim of wood's logging begins to practise annual and seasonal wood logging rotation according to the types of plant formations. High practice of the wood logging should be sanctioned by the population itself and by the government. Other studies on wood logging activities should be carried out in the northern zone of Cameroon.