Characterization and Risk Assessment of the Collapse of the Woody Stand of Ecosystems of the Fathala Forest (Saloum Delta Biosphere Reserve-Senegal)

The objective of this study is to characterize and assess the risk of collapse of woody plant formations in the Fathala forest. In recent years, this forest has suffered a sharp reduction in its plant cover to the point of compromising the survival of populations of certain animal species such as the Red colobuses. The methods used are respectively constituted by the transect method, the dendrometric statements method and that of establishing the red list of ecosystems of the IUCN. The specific richness comprises 56 species divided into 47 genera and 22 families. The density is higher in the unfenced area (369 ind/ha in clear forest and 53 ind/ha in gallery forest) compared to the fenced area (160 ind/ha in clear forest versus 48 ind/ha in gallery forest). A study of the plant formations shows a strong degradation of the ecosystems passing from a clear and dry Sudanese forest to a wooded savannah. The cover rate in the fenced area is 20% in gallery forests and 25% in clear forests; in the unfenced area, it is 19% and 23% in gallery forests and clear forests, respectively. The application of the IUCN criteria shows an annual rate of collapse is −6 ind/ha in gallery forests and −4 ind/ha in clear forests. The annual rate of cover collapse would be −3.75% and −2.9% for gallery forests and clear forests, respectively. These results classify the forest in the Critically Endangered category. The main factors responsible for this degradation are, among others, anthropogenic actions and climatic pejoration. These results could constitute a basic tool for undertaking an improvement in the management of this forest, which is a living environment for an animal species. of certain species with drier affinities is a good indicator of changes in the state of woody vegetation.


Introduction
Senegal's forest formations, like those of many Sahelian countries, are prone to relatively significant degradation, marked by a decrease in wooded areas, a reduction in the density of species and a change in their structure and function [1].
The alteration of ecosystems is more marked in the Sudano-Sahelian zone due to the combined action of climatic pejoration and increasingly significant anthropogenic pressures on plant and soil resources [2]. This state of degradation has not spared protected areas as the Fathala Forest [3]. These important decreases in biodiversity observed in the Fathala Forest are linked to the drought, which has become permanent and generalized in all West African countries for more than two decades [4]. According to [4], the degradation of the Fathala Forest is very rapid and the forest galleries are very threatened. The savannah becomes more and more open with a decrease in the number of species and rarefaction of forest species. Anthropogenic pressure, such as abusive cutting of the trees by local people, as well as the selective exploitation of wood by the sawmill (established in the Fathala Forest before its integration into the Park in 1986), are the main causes of the decrease in tree density, especially of Guinean essences. Initial analyses by [5] blamed the impact of late fires and overgrazing for the degradation of the Fathala Forest. This practice causes strong changes in the physiognomy and composition of the vegetation and prevents the regeneration of forest essences [5]. These changes also affect protected areas such as the Fathala Forest, which is home to a northern population of Colobus badius temminckii from Africa [3]. The climatic causes of this degradation, including drought, are difficult to control. However, better management and control of anthropogenic actions can mitigate threats to these ecosystems [2]. In order to better plan actions to slow the rate of decline, secure the future functions of these ecosystems for human use, and promote investment in ecosystem management [6], it would be necessary to understand the risks that threaten this biodiversity. For this purpose, the International Union for the Conservation of Nature (IUCN) has set up criteria for evaluating ecosystems to develop a better plan of action to slow the rate of decline, to guarantee the future functions of ecosystems for human use and to promote investment in their management [6]. Through the identification of species threatened with extinction, the IUCN Red List criteria offer information to governments and society on the current status of biodiversity [7] and trends in extinction risk [8]. These criteria also provide data to formulate conservation priorities and management strategies [9]. Knowledge of the characteristics of this plant population makes it possible to understand these ecosystems with worrying degradations in order to propose urgent rehabilitation strate-

Presentation of the Study Area
The study was carried out in the Fathala Forest located between 13˚41'N latitude and 16˚30'W longitude ( Figure 1). It is located in the Sudanese climatic zone.
This Sahelo-Sudanese type climate is characterized by two seasons during the year: a rainy season which extends from late June to mid-October, and a dry season of 7 to 9 months from October to May.  The geographic coordinates of each floristic survey were determined using GPS in order to materialize the different sites ( Figure 1). This working method is comparable to that used by [3] and [13]. The inventory of all plant species in all the plots was carried out. The botanical samples were identified in the field using the "Flore du Sénégal" [14] and the book: "Arbres, arbustes et lianes d'Afrique de l'Ouest" [15]. American Journal of Plant Sciences

Data Treatment
The data obtained were processed using EXCEL, XLStat and Minitab.16 software which were used to classify the numerical data and to construct the graphs.
These software were used to calculate a certain number of ecological parameters, the specific richness, the real density, the relative frequency, the specific diversity of the woody stand of the Fathala Forest, to characterize the woody vegetation and to appreciate its degree of homogeneity.
The specific richness was evaluated from the total richness and the average specific richness. The total specific richness is the total number of species in the stand considered in a given ecosystem [16]. The average specific richness corresponds to the average number of species per survey for a given sample. Assessment of diversity: the diversity index (H') from Shannon Weaver (1949) and the regularity from Pielou (E) were used. The Shannon index is the most commonly used, it expresses the relative importance of species in a given environment. Its value gives an estimate of the uncertainty with which we can correctly predict the species to which the next individual collected belongs. The index is minimum when all individuals belong to the same species. It is maximum when each individual represents a distinct species [18]. It is given by the following relation: According to [19], the Pielou regularity index appears to be a more rigorous comparison term. It is between 0 and 1 and tends towards 0 when all the individuals belong to a single species. It tends towards 1 when each species is represented by the same number of individuals. Its value is obtained using the following formula: , S being the total specific richness.
Recovery: it is given by the ratio as a percentage of the sum of the areas of the crown of all the trees projected vertically to the ground on the sampled area.
The regeneration rate of the stand is given by the percentage ratio between the total number of young plants (circumference < 10 cm) and the total number of the stand [20]: Total number of young plants RRS 100 Total number of the stand = × The total population of the stand comprising both young plants and adult plants.
The specific importance of regeneration is obtained from the percentage ratio between the number of young plants of a species and the number of young plants counted [21].

Number of young plants of a species SIR
100 Total number of young plants counted = ×

Assessment of the State of Ecosystems and Application of Criteria Defined by IUCN
The IUCN developed ecosystem collapse risk assessment method can be applied to any consistent classification of ecosystems. A generic risk assessment protocol developed by IUCN requires clearly well-defined assessment units. It requires the flexibility to assess risks across contrasting ecosystems. The biological and environmental characteristics, as well as the organization scales are very varied, and the levels of knowledge available are different.

Specific Richness
The diversity of a plant stand is often measured by the specific richness. In our study we encountered 56 species divided into 47 genera belonging to 22 botanical families (Table 1). Also good in unfenced and fenced areas, the specific richness is similar between gallery forests and clear forests. The average specific richness is 4.31 species per survey in the Fathala Forest. It varies depending on the area and the plant formation; this being higher in clear forests than in gallery forests (Table 1).

Frequential Analysis
The diversity of the woody flora was appreciated by using the distribution of species in the different taxonomic groups (genera, families). The centesimal frequencies are variable in the different zones ( Table 2). In addition, in the Fathala forest no species are present in all the surveys that is to say at a frequency of 100% (Table 2).

Density
The observed density (real density) of the different areas is consigned in Table 3.
In the unfenced area, the observed density is 53 feet/hectare at the level of forest galleries and 369 feet/hectare at the level of clear forests. At the level of the fenced area, it is 48 feet/hectare at the level of galleries forest and 160 feet/hectare at the level of clear forests (Table 3).
• in gallery forests and clear forests, respectively (Table 3).

Stand Structure
The structure of the woody stand was established by the class distribution of heights and circumference in the different ecosystems of the Fathala Forest.
Distribution according to height: the Weibull distribution with three parameters (position parameter, scale or size parameter, shape parameter) of the woody stand according to the height classes shows: • a straight asymmetric distribution (shape parameter between 1 and 3.6) characteristic of monospecific stands at the level of the clear forest of the unfenced aera and at the level of the gallery forest of the fenced aera ( Figure 2).
This distribution shows a predominance of young or low-height individuals between 1 and 5 m (Figure 2). • an "inverted J" distribution (shape parameter less than 1) at the level of the gallery forest of the unfenced aera and at the level of the open forest of the fenced aera, characteristic of multispecies woody stands with the predominance of individuals whose height is between 2 and 5 m (Figure 2). Distribution according to circumference: the Weibull distribution with three parameters (position parameter, scale or size parameter, shape parameter) of the woody stand according to the circumference classes shows an "inverted J" distribution (lower shape parameter 1) at the level of all of the two areas (unfenced and fenced areas) with a predominance of young individuals (Figure 3).

Regeneration
In order to understand the basis of the dynamics of woody vegetation we evaluated the natural regeneration rate of the stand. It is 97% at the level gallery forests and 67% in clear forests of the unfenced area. In the fenced area, it is 94% for gallery forests against 90% for clear forests ( Table 3). The importance of regeneration according to the different species was determined by calculation the specific index of regeneration (SIR) in the two areas (Table 4). In the unfenced area, the best regeneration was observed in species of the families of Caesalpini-  (Table 4).

Ecosystem Assessment According to the Criteria of the Red List of the International Union for Conservation of Nature (IUCN)
To assess the state of ecosystems and the risk of the collapse at our study site, we synthesized different evidence, causes, mechanisms and pathways of decline into an assessment framework generic risk. To do this, we estimated the level of threat to the ecosystems. The UINC red list is based on five evaluation criteria (A, B, C, D and E), two of which are based on spatial distribution, two others concern the functioning and a last rarely evaluated criterion which takes into account global digital modelling of the risk of collapse. To estimate this "risk", the coverage rate of these different ecosystems was evaluated on the one hand over a specified time space of twenty years ranging from 1969 to 1989, then from 1989 to 2019 and on the other hand the population density was used over a forty-year period from 1972 to 2012. Finally, a projection of these two parameters, the coverage rate and the density, was carried out respectively in 2019 and 2022 (Table 5 & Table 6). Our risk assessment model groups symptoms of collapse into three broad categories. Two of the three mechanisms produce distribution symptoms: • Criterion A: Continuous drop in distribution, which reduces the carrying capacity for dependent biota. This reduction is measured on the basis of one or more of the sub-criteria from A1 to A4. For our study, only the sub-criteria A3 and A4 were applied. Sub-criterion A3 corresponds to the reduction in the population predicted, deduced or assumed in the future. Sub-criterion A4 indicates the reduction in population noticed, estimated, deduced, predicted or assumed, over a period of time that should include both the past and the future, when the causes of the reduction may not have ceased or may not have been understood or are maybe not reversible. When the observation is direct, it is the sub-criterion A4a that is applied [22]. • Criterion B: Restricted expanse predisposes the system to the spatially explicit threats. This criterion is subdivided into two sub-criteria b1 and b2. For our model, the sub-criterion b2 was applied. Sub-criterion b2, area of occupancy (AOO) i.e., the territory occupied by a taxon within the extent of occurrence, excluding wandering individuals [22]. • Criterion C: Degradation of the abiotic environment, reducing the quality of habitat or the diversity of abiotic niches for biota. This criterion is subdivided into three sub-criteria (C1, C2, and C3) [22]. The sub-criteria C1 and C2 werer applied in this study. When the modification of abiotic parameters is observed during the last 50 years, sub-criterion C1 is applied. C2 is the modification of abiotic parameters calculated or estimated over the 50 years or period of 50 years including the present and the future ( Table 5 & Table 6). Besides the recovery rate, the density observed in the two ecosystems of 1972 and 2012 was used to assess the degree of collapse.

Discussion
The results of this study provide first information on the state of the woody vegetation in the forest of Fathala. The characterization of the woody stand revealed the presence of 56 species divided into 47 genera belonging to 20 families.
The method of comparison of means [23] was applied to the specific richness by area. On average, we have identified more species in the unfenced area ( The proliferation of these species reduces biodiversity at the forest level. We found a recovery rate of 22% for the whole forest. However, this vegetation cover varies from one area to another. It is important at the level of clear forests compared to gallery forests. This reduction in plant cover observed at level gallery forests could be linked to climatic pejoration and especially to anthropogenic action [2]. These factors reduce the number of Guinean species (trees with large crowns) characteristic of gallery forests to the detriment of species in dry areas. Indeed, trees with large crowns contribute more to the covering up to a certain degree of covering [26]. This vegetation cover is function on the distribution of the woody stand. However, several types of distribution can be used (normal distribution, log-normal distribution, exponential distribution, Weibull distribution) to estimate the parameters from the observed data [28].
The Weibull distribution is more adequate because it is characterized by a great flexibility of use and presents a great variability of shape following to the values taken.
The distribution according to the size (circumference) of all the forests of the two areas shows that the population has a reversed "J" shaped structure trans- Weibull distribution clearly show that this is a habitat where the predominance of a species or a restricted group of species is noted. This characteristic is in adequacy with the ecological characteristics of a strongly disturbed area [29].
The regeneration rate of the plant stand is 87% in the whole forest. It is higher in gallery forests (97% for the unfenced area and 94% the fenced area) than in clear forests (67% for the unfenced area and 90% for the fenced area). This difference could be explained by a good seed germination capacity observed in the forest galleries on account of water potential. The presence of water points in the forest galleries, constitute drinkers for wild animals which play an important role in the dissemination of seeds. Similar results were obtained in the area on natural regeneration of a species (Cordyla pinnata) [1].
Following the characterization of the woody stand, an assessment of the state of the ecosystems was carried out according to criteria for establishing the Red American Journal of Plant Sciences List of ecosystems of the UINC between 1969 and 2019 for the recovery rate and between 1972 and 2022 for density. Ideally, the criteria for establishing the Red List should identify ecosystems whose diversity, ecological functions and/or ecosystem services are risk of disappearing, since these three elements are closely linked and are important for conservation [30]. The Fathala forest located in a Sahelo-Sudanese climate, formerly wooded, has suffered the impact of episodes of repetitive droughts and abusive anthropogenic activity. A decrease in the recovery rate is observed, going from 98% in 1969 to 23% in 1989 at the level of forest galleries and 88% in 1969 to 30% in 1989 according to [27] [31]. From these results, compared to ours, an exhaustive ecosystem assessment study was carried out. In 30 years, the reduction in plant cover was 75% in the gallery forests and 58% in clear forests. The rate of regression would be 3.75%/year for gallery forests and 2.9%/year for clear forests. By referring to this rate of regression, our work on forest assessment forecasts over a period of twenty years ranging from 1989 to 2019, that the regression would be 187.5% in 2019 for gallery forests and 145% for clear forests, if no management plan is undertaken.
The application of Red List assessment criteria of ecosystems translates a level of collapse CR (critically endangered; according to the application of sub-criteria A3 and A4a) and VU (vulnerable; according to the application of the sub-criterion A4C2) [32] for gallery forests and clear forests. In short, depending on the collapse category for the recovery rate parameter, the Fathala forest would be in critical danger. The state of degradation of this ecosystem has already been underlined by [11] [27] and [31].
This impact did not spare the density of the woody stand. An evaluation of the density per hectare showed that this increased from 293 ind/ha in 1972 to 53 ind/ha in 2012 at the level of forest galleries and 517 ind/ha in 1972 to 355 ind/ha in 2012 at the level of clear forests, it went from 517 ind/ha in 1972 to 355 ind/ha in 2012. In forty years, the reduction in density is 240 ind/ha, i.e., a rate of 6 ind/ha/year observed at the level of forest galleries and 148 ind/ha, i.e., a rate of 3.7 ind/ha/year for clear forests. A decrease in the density of more stable natural plant formations is observed in favour of those more adapted ones. This is translated in the level of CR collapse observed as well in gallery forests and in clear forests. Starting from old data (1972) to recent data on the densities of plant formations, it may be predicted that the degradation of ecosystems is very advanced. These changes in vegetation cover and structure, accompanied by the development of species with drier affinities, constitute threats for certain animal species, especially the arboreal species Colobus badius temminckii from Africa who is arboreal.

Conclusions
The wooded surface of the forest has undergone profound modifications, with a decrease in the area both in the clear forest and in the gallery forests. However, it is at the level of gallery forests that the degradation is more accentuated. Today,