EVALUATION OF THE RESTORATION PROCESS OF A PLANTATION: CASE OF BENICHAB (MAURITANIA)

Ahmedou Soule 1,2 , AhmedouVadel Salihi 2 , Mohamed Mahmoud Abidine 2 , Mohamed Yahya Lafdal 4 , Pierre Begat 3 and Et Anthony Mills 3 . 1. Ecole Normale Supérieure de Nouakchott/Centre de Recherche pour la Valorisation de la Biodiversité (CRVB). 2. Unite de recherche Biodiversité et Valorisation des Ressources Végétales (BVRV). 3. C4 EcoSolutions Climate Change and Environmental 9B Mohr Road, Tokai, 7945, Cape Town, South Africa. 4. National Cell of Environmental Observation and Drylands (CNOZA) / MEDD. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 16 October 2018 Final Accepted: 18 November 2018 Published: December 2018


Figure 1:-Situation of the study area in mauritania
The Benichab site, subject of the study, has two subsites, A and B. Our study focuses on sub-siteA only. Referring to the only weather station in the Wilaya, located in Akjoujt since 1931, there is no real rainy season in the region (  The climate is dry all year long. Indeed, Benichab is subject to an arid climate of adesert type. Rainfall is rare and irregular (Fig. 3). Rainfall averages rarely exceed 150 mm per year.
The peak of rare rains is observed during the period August-September, or even a prolongation of exceptional rainfall until January, with the influence of the Mediterranean rains. The average annual rainfall is 58 mm at Akjoujt. It is very variable from one year to another.
943 The average monthly relative humidity recorded at the Akjoujt station ranges from around 30% in May to 44% in August. It is on average 35.5%. Benichab knows a thermal regime of continental type marked by high temperatures in summer. The annual average is of the order of 29.1 °C. The daily thermal amplitude is very strong.
The annual evaporation is 4949 mm at Akjoujt. The month with the highest evaporation is May (545 mm), and the month with the lowest evaporation is December (335 mm). Benichab is characterized by the frequency of constantly blowing winds. The prevailing winds are from North to North East.
Rainfall was in deficit in the last two years, when it was respectively 22 mm in 2016 and 35 mm in 2017 (source ONM), which greatly affected the survival, vigor and growth of seedlings.
The site on which our study was conducted is formed by a plantation with several species of 65.02 ha (1042 m x 624 m)  1  . It is located east of the city of Benichab ( fig.4).

Sampling design:-
The plots studied were set up from March 2016 using identical methods and techniques. Thus, all plants were nursed for a few months before being planted in sub-site A (Fig 4). The quality of the plants at the time of transplanting is a major factor in the success and start-up of the plantations. Unfortunately, we do not have very precise data on the quality of the plants, the mode and frequency of their watering, or their age at the time of transplanting. Nevertheless, we assume that these factors were identical. Only two rows of Prosopis juliflora along the fence and serving as windbreaks benefited from a special watering (drip irrigation). As a result, they were not considered in this study.

Data analysis:-
The data collected was processed by the Microsoft EXCEL software to calculate structural parameters (basal area, cover, density, etc.).
The ecological characterization of the plants was carried out by evaluating the population structure of the plantation (in diameter and height), the total density, the basal area, the vigor and the survival rate of the species. The basal area G, expressed in m 2 / ha, is the surface density of the sum of the cross sections of the individuals: G = ∑

45
Where n is the number of individuals of each species in the plot, S is the area of the inventory plot (in m²), and di is the diameter of an individual i (in mm). In addition to these species planted in the site, the natural vegetation consists of a few feet of Maerua crassifolia, Capparis decidua and Acaia tortilis.

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The inventoried woody flora is rich in 13 species divided into 12 genera belonging to 6 families ( Table 1). The Fabaceae occupy the first place with38.5% of the inventoried species. Asclepiadaceae and Capparaceae have 2 species each. The other three families (Combretaceae, Rhamnaceae and Balanitaceae) have only one species each. Acacia tortilis is the most common species; it is present in all inventoried plots. Balanitesa egyptiaca and Prosopis juliflora are found in 17 and 50% of the surveys, respectively.
Acacia tortilis and Prosopis juliflora are the two most used species for reforestation. They alone account for nearly 95% of the reforested area.

Characteristics of the plantations:-
The average values of the different measured dendrometric parameters (height, stem circumference, crown diameter), vigor and survival parameters were used to characterize the plantation and to estimate the ability of the species to adapt to the environment ( Table 2).The following table shows dendrometric characteristics of the main species found at the study site. Plant survival rate:-In arid environments, the key factor in ensuring the establishment of a newly planted plant is proper watering because after planting, the available water in the rhizosphere is quickly depleted.
Newly planted plants require more maintenance than established plants  5  , they must be watered frequently and protected from animals to promote growth during the early stages of development.
In our experimental conditions, the average survival rate for the entire plantation is 54.5%. However, there is a significant difference between species (Fig 5). The highest survival rate was observed in Prosopis juliflora (58.7%), followed by Acacia tortilis (54.5%) and Balanites aegyptiaca (38.5%), respectively. Prosopi sjuliflora and Acacia tortilis do not show significant differences. It is important to note that plant survival is related to their vigor and environmental conditions at the time of and after planting.The slight superiority of Prosopis juliflora can be attributed to the high vigor of the plants at the time of planting. Indeed, the majority of planted Prosopis plants come from Nouakchott. They were more vigorous than those raised in nurseries locally because of the very difficult environmental conditions in Benichab. Italso benefited from maintenance and replenishment because of the availability of the plants.
Moreover, after planting, it has been noticed that the mortality of Ziziphus mauritiana plants is very high. All the plants are dead. This is why these dendrometric characteristics have not been studied. This high mortality confirms that this species, which is not native to the Benichab region, is not adapted to these conditions.

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The estimation of the quality of the plants is difficult to achieve. The condition of the plantations was observed for each plant according to three criteria: high, medium or low vigor. In general, according to the inventory data, nearly three-fifths of the plants have a weak vigor (58.8%), particularly Balanites aegyptiaca, of which 80% of the plants are of low vigor. The other two most used tree species for reforestation, Acacia tortilis and Prosopis juliflora, show moderate to high vigor (Fig.6).

Figure 6:-Vigor of plantations
Basal area:-The basal area of the plantation, that is, the sum of the basal areas of all the plants in it, provides a good evidenceof the density of the plantation. It is evaluated globally at 0.012 m 2 / hawhich is variable according to the species. Table  3 shows the basal areas of the main species. Among these species, Acacia tortilis has the highest basal area (0.01 m² / ha), which represents more than 82.6% of total basal area. But these figures are very low because of the youth of the plantation.

Structure:-Distribution according to the height of the plants:-
The distribution of plant numbers according to height is represented in Figure 7. The average height is 31.82 cm. The plantation distribution is unimodal (mode = 10 -20 cm) and is better adapted to a polynomial curve of degree 6. This curve represented a predominance of individuals whose height is less than or equal to 50 cm (88%). However, individuals with a height greater than or equal to 50cm represent only (12%).

Figure 7:-Repartition of the plantation by height classes
The average height of the A. tortilis population is 28.9 cm. The distribution of height classes is unimodal (mode = 10-20cm) (Fig. 8). It shows that individuals less than 50 cm tall predominate (81.1%). This indicates that most individuals are small. These values are quite comparable to plantation averages. The trend line that better expresses the population distribution of A. tortilis is a polynomial (polygonal) function of degree 6. The average height of the Prosopis juliflora population is 46.43 cm. The distribution of height classes is unimodal (mode = 20-40cm) (Fig 9).
The trend line that better expresses the population distribution of Prosopis juliflora is a polynomial (polygonal) function of degree 5.
949 Figure 9:-Population distribution of Prosopis juliflora by height classes The population structures of P. juliflora and A. tortilis show the distribution of species according to their size in the plantation. They are essential for studying the growth dynamics of these species. In an even-aged plantation, the structure of the population has individuals that are distributed substantially according to Gaussian law, to give the graphical representation of a bell curve, the top of which represents the average tree  7  . This is not the case for population distributions of the two main plantation species with asymmetrical curves. The large disparity in the size of the trees is certainly linked to the high mortality of the plants. Asymmetrical structures reveal a lack of plantation monitoring  7  , these interventions should have contributed to improving tree growth.
In general, over the years, there is a reduction in the number of stems in small diameter classes in favor of higher diameter classes, which is a normal process in forest plantation.

Size distribution of plants:-
The demographic structure of the plantation was determined by histograms of the distribution of individuals according to their distribution by diameter classes. The histogram is established with the abscissa classes diameter and the ordinate of the percentage corresponding to each class. We considered the same amplitude of diameter classes (3mm) for all species although this value does not have the same meaning for all species 8  .
The plantation distribution by diameter class adjusts to a polynomial function of degree 4. The diameter class distribution of the Acacia tortilis population is shown in Figure 11 with a bell-like shape, centered on the circumference class (3-6 mm and 6-9 mm). The distribution of this population is unimodal (3-6 mm mode) with an average circumference of 6.77 mm.

Conclusions:-
The success of plantations depends on many factors, some of which are related to the physiology of plants and others to the specific characteristics of the physical environment (climatic conditions, frequency of watering, etc.). Species with a high survival rate and a low proportion of mortality are those that are better adapted to the planting site. Thus, the species that seem to be best adapted are:Acacia tortilis and Prosopis juliflora showed higher performance than other species tested for both survival and growth (size and diameter) after one year of planting. Among the criteria studied, the principle of survival is chosen as the main criterion of tolerance of species to environmental conditions. With a survival rate of over 50% after one year of planting, the success of Benichab's plantation is assured. As for growth, it is satisfactory despite climatic constraints. These useful results for the study of dynamics show the need to set up a simple monitoring system to assess plantation dynamics. In an arid 951 environment like Benichab where climatic conditions are severe, the choice of species is decisive. Thus, in the current context, we recommend Acacia tortilis as the main species to which we can associate other native species on which the study of the propagation conditions is in progress. These are: Ziziphus lotus, Acacia ehrenbergiana, Hammada scoparia, Maerua crassifolia and Capparisdeciduaswhich would be well adapted to the environmentand can provide a variety of ecosystem services.