Influence of anthropic and eco-hydrological factors on the floristic diversity of the herbaceous vegetation around the temporary ponds in Ferlo, Northern Senegal

Aims Temporary ponds play substantial roles in preserving biodiversity in the Sahel. This study investigated the influence of anthropic and eco-hydrological factors on the plant diversity around two grazed ponds and one ungrazed pond in Ferlo, northern Senegal. Methods Two phytoecological transects were established per pond. On each transect, homogeneous herbaceous vegetation units were identified and characterized (herbaceous cover, exposure, soil texture, soil moisture and pH, average height, and slope). This study evaluated the floristic diversity of the herbaceous vegetation around the temporary ponds in Ferlo (Senegal). Although it focused on three ponds, the phytoecological analysis showed heterogeneity of the floristic structure related to environmental factors. The herbaceous flora recorded in the ungrazed temporary pond was more diverse and dense than in the grazed ponds. These results demonstrated the significant influence of grazing, which favors establishing species with a high regrowth capacity. In contrast, the Wendou Marodi pond, protected by fences for decades, had a more species-rich community and contained more species specialized to the pond‘s habitat conditions. The species composition of the ungrazed Wendou Marodi pond was closer to that of the semi-protected Belel Lougal pond. The vegetation‘s structure and composition are also potentially influenced by abiotic factors such as soil moisture, texture, slope, and sun exposure. This study demonstrates the necessity of maintaining a landscape mosaic comprising ungrazed ponds and grazed ponds reserved exclusively for livestock watering to conserve the floristic diversity. Since there are many temporary ponds in Widou Thiengoly, the only wetlands in the center of Ferlo, it would be essential to implementing such management to preserve the floristic diversity.


INTRODUCTION
In the Sahelian and Sudano-Sahelian zone in Africa, the landscapes depend essentially on geomorphological factors (Penning de Vries and Djitèye 1982). These factors significantly influence the redistribution of water that accumulates around depressions where a great diversity of flora and fauna coexist (White 1970). Temporary ponds are found in the singular and remarkable wetlands (Quezel 1982, Allem et al. 2018, harboring an exceptional species richness and a diversity of habitat and reproduction niches (Sajaloli andDutilleul 2001, Jourdas 2013).
In Senegal, in the northern part commonly known as the Ferlo, the topography, although dominated by flat areas, reveals longitudinal clayey depressions, occasionally limestone where temporary ponds are formed during rainy seasons (Michel 1969). The area is characterized by many small endorheic ponds of great economic, ecological, and social value.
These are particularly important micro-habitats in the management of Ferlo's pastoral systems because of their positive influence on grassland productivity and water availability for domestic needs and livestock watering (Sy 2009). Because of their socio-economic importance for local populations, ponds remain frequented, and heavily exploited microhabitats suffer from anthropic impacts that can prevent their proper functioning.
Within the framework of the fight against climatic change and the conservation of biodiversity, the preservation of ponds is becoming indispensable in the future socioenvironmental resilience of the Ferlo ecosystems. In this context, this study was carried out to understand the influence of anthropic and eco-hydrological factors on the floristic diversity of the herbaceous vegetation around the ponds. Specifically, the aim was to determine the A c c e p t e d M a n u s c r i p t 5 floristic composition of the herbaceous vegetation and to assess the impact of grazing and ecological factors on the diversity of the herbaceous vegetation.

Study site
The study was carried out in Ferlo in northern Senegal in the Widou Tiengoly station (15°58'30''N et 15°17'90''O) from August 2018 to September 2019. The presence of many very diverse temporary ponds, the development of postoral activities, the passage of the Great Green Wall (GGW) route, and the influence of the various agriculture-and environmentrelated development projects installed in the area justified why we chose this zone. The climate of Widou Thiengoly is of arid Sahelian type. It is characterized, according to Niang et al. (2014), by the alternation of a long dry season of nine months (from October to June) and a short rainy season of three months (from July to September). The Widou station has received a yearly average rainfall of 371.67 mm over the last fifty years (Ndiaye et al. 2015) with an average annual temperature of around 27.73°C (Ndiaye et al. 2013).
The study area's topography is flat with tropical and reddish-brown sub-arid soils made of sandy materials with low clay content (Ndiaye et al. 2013). The absence of rugged landscapes limits the development of a hydrographic network with an organized flow. The number of ponds in the Ferlo is estimated to be at least 363, including many small endorheic temporary ponds with a very short lifespan (Lebel andRedelsperger 2007, Sarr 2009 A c c e p t e d M a n u s c r i p t 6 (auct.) (Akpo 1992, Niang 2009). The tree stratum is marked by the strong presence of thorny trees such as Balanites aegyptiaca (L.) Del, Acacia tortilis raddiana (Forst.) and Acacia senegal (L.), while the shrub stratum is mainly composed of Calotropis procera (Ait.) and Boscia senegalensis (Niang 2009, Ndiaye et al. 2015. Although fruit gathering is increasingly practiced thanks to wild fruit trees such as Balanites aegyptiaca (L.) Del, Sclérocarya birrea (A. Rich), and Zizyphus mauritiana Lam, livestock farming remains the dominant socio-economic activity (Niang 2009).

Choice of ponds
Most of the temporary ponds in the Ferlo are filled by runoff from slopes. They often can benefit from management to ensure a sufficient filling for use in various activities. Thus, based on the type of activities developed and the management type, three ponds were chosen as shown in Fig. 1: Wendou Marodi or the Lion's pond, Belel Sow, and Belel Lougal or the hole pond (artisanal surface wells). The Wendou Marodi pond has a shallow anthropic pressure. It has been fenced for decades to host the Ferlo Pastoral Self-Promotion Project PAPF) funded by the German government from 1975 to 2008 (Ka 2016). In 2008, the fence was reinforced by the Great Geen Wall project. The Great Green Wall appears to be a pan-African-driven reforestation project from Senegal in the West of Africa to Djibouti in the East of African to combat climate change and provide socio-economic means to habitants of the areas it crosses (Ka 2016). Belel Sow is a private pond owned by a family head from the village Belel Sow. It is very protected, but the human-made action is very regular. The local population dug Belel Lougal to exploit the water of a small superficial alluvial aquifer. It is undoubtedly a grazed pond, but its accessibility is limited by many holes representing a threat to animals.
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Survey method of herbaceous plants
The herbaceous vegetation was studied during the rainy season when most herbaceous plants are recognizable (August-September-November), following a phytoecological approach (Diallo et al. 2009, Mangara et al. 2010, Mboup 2014, Faye et al. 2020a). The sampling was carried out along a flooding or drought gradient at each pond. One transect was perpendicular to the axis of runoff flow in the pond, and the other was arranged longitudinally from the upstream end of the pond to its center (Fig. 2a). In total, six transects (two per pond) of 40 m to 82 m long, depending on the size of the pond (Fig. 1), were established. After placing the transects, we first visually identify the different homogenous vegetation groupings (Fig. 2b) and the soil texture's uniformity. We then ensure that each grouping has a homogenous and repeatable floristic composition with no discontinuity.
Following that, the list of herbaceous plants, their average height, the exposure to the sun, the cover of the herbaceous stratum (established according to the Braun-Blanquet's scale 1932) were determined in each delimited grouping. A critical step in the survey consisted of describing certain abiotic factors such as the slope, the soil moisture content, and soil texture that could influence the structure and the distribution of herbaceous plants around ponds. Soil samples were collected from the different units to measure the soil pH at a 2:5 soil-todistilled water slurry using a pH meter.

Data analysis
All data collected from the three sampling periods (August, September, November) were entered and processed using the Excel spreadsheet and the free software R studio (Xie et al. 2018) for Discriminating Factorial Analysis (DFA), Principal Component Analysis (PCA), and floristic composition. The PCA was carried out on the eight measured parameters (ground cover, average height of species, number of species, slope, texture, humidity, A c c e p t e d M a n u s c r i p t 8 exposure to the sunlight, pH) on transects to identify the ecological factors that best characterize each pond. The species names were updated, referring to several online sources such as the African Plants Database, West African Plants, The Plant List, African Plants, and the Burkina Faso vascular plants catalog (Thiombiano et al. 2012). Several parameters such as species richness, specific contribution, the diversity index (Shannon and Weaver 1963), and the regularity index (Pielou 1966) were evaluated and defined. The specific contribution of species i (C Si ) is calculated as (Barmo et al., 2020) : ∑ where Fsi specific frequency the «i» species. 55 represents the total number of species The diversity index used to assess the heterogeneity and diversity of a biotope is expressed in bits and generally varies from 0 to 5. It was calculated following the equation: ∑ .
Finally, the regularity or equitability index (varying between 0 and 1), which expresses the distribution of species in a unit, was calculated using the formula: with representing the maximum Shannon specific diversity, and S was the total number of species. The phytoecological study was carried out using data collected in September since, during this period, all biotic and abiotic conditions were met.
The soil was humid because the ponds were filled with water and the plant species were welldifferentiated.
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Composition and floristic diversity of herbaceaous plants in ponds
The three ponds contained 55 herbaceous plant species belonging to 41 genera and 22 families. Species were unevenly distributed in the ponds (Table 1)

Ecological characteristics of ponds
The principal component axis referred to as dimensions, varied from 53.6% (Dim1) to 16.7% (Dim2). The first two dimensions that explain 70.3% of the information were retained for the analysis and the interpretations of results (Fig. 3)  Belel Sow ponds, respectively. Soils in these groupings are sandy, with a pH close to neutrality, and very exposed to the sun. The ground cover was weak, and the herbaceous layer was shallow, but the herbaceous biodiversity was quite important. The second class consisted of five and two groupings from the grazed Belel Lougal and Belel Sow ponds, respectively. Like the first class, the soils were also sandy, and the ground cover and the herbaceous stratum were low. However, the pH was acidic, and the exposure to the sun was weak. The third class was composed of three groupings of the grazed Belel Lougal pond and six groupings of the ungrazed Wendou Marodi pond. In this class, the soils had a silty-clay texture, with a medium exposure to the sun and a medium ground cover. The height of the herbaceous layer was high, and the biodiversity was important. Three groupings from the grazed Belel Lougal pond and nine groupings from the Wendou Marodi pond formed the fourth class. The soils had a silty-clay texture in this class, with medium exposure to the sun and a medium ground cover. The ground cover was more a less important, and the herbaceous layer reasonably high, with a shallow diversity. These homogenous vegetation groupings had the particularity to evolve on a very humid ground, with a very weak slope.
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Influence of ecological factors on the distribution of the herbaceous flora of ponds
The Discriminating Factorial Analysis (DFA) highlighted the existing links between the herbaceous flora of ponds and the ecological factors (texture, pH, slope, exposure, herbaceous cover, humidity, and average height). The DFA varied from 76.2% (Dim1) to 15.5% (Dim2), indicating that these two first dimensions contained most of the information (91.7%) of the DFA (Fig. 5). There were four groups of species with discriminated ecological preferences. The first group located on the positive side of axis  The better soil fertility around the ungrazed Wendou Marodi pond (Faye et al. 2020b) and the more prolonged water stagnation period may justify its species singularity compared to the two other ponds. Moreover, it is known that ponds are original ecosystems whose functioning is tightly linked to the surrounding environment (Jourdas 2013). The ungrazed Wendou Floristic diversity indices are objective criteria for assessing the diversity of a plant community (Ramade 1994, Yoka et al. 2013. The Shannon's diversity index (H') revealed a slightly higher species diversity in the ungrazed Wendou Marodi pond (4.9 bits) than in the grazed ponds of Belel Louga (4.8 bits) and Belel Sow (4.5 bits). The three ponds showed a reasonably high regularity or equitability index of 0.85, 0.83, and 0.78, respectively. Ecohydrological factors such as soil pH, soil texture, soil humidity, and exposure to the sunlight are also putative determinants. The ground cover, average species height, soil humidity, and soil texture positively correlate together. These factors are negatively correlated with the slope, sun exposure, soil pH, and the number of species.
In this study, the principal component analysis grouped the eco-hydrological parameters (pH, soil texture, soil humidity, exposure to the sunlight) into four classes that showed differences in their floristic composition. The first class had dry and sandy soil, steep slopes, sun exposure, and grazed. This biotope was more a less clearly discriminated against the fourth class, which evolved on a clayey soil texture, covered, humid ground with low slopes, and ungrazed. However, the second and third classes harbored common species.
These results matched that of Akpo et al. (1999), Akpo andGrouzis (1996, 2009) The statistical analysis of the phytoecological surveys revealed that the four described classes were correlated with four plant groupings. The Tribulus terrestris grouping comprised species growing in constraining areas with a sandy soil texture, a complete exposition to the sun, and a meager soil moisture. These species are short and less abundant (Thiombiano et al. 2012). The Digitaria horizontalis grouping also had short species with a low ground cover.
They colonize degraded and sandy lands (Agbodan et al. 2019) around villages and camps.
The Chloris barbata grouping contained large and very diverse species found in very humid areas, with often a clay-silty soil texture. The species in these last two groupings can colonize humid or dry areas and covered or uncovered zones. They can be sciaphilic and hygrophilous under trees, xerophilic outside shade, which justify their high diversity (Akpo and Grouzis 2009) in the Sahelian ecosystems. The species forming the Achyranthes aspera grouping were found in low or flat areas with a shallow exposure to the sunlight and a soil pH of around 6. They were very abundant and large but not very species-rich. It is worth noting that the presence of trees is beneficial for the development of other species as demonstrated by Ovalle and Avendano (1987) with the Acacia caven (Mol.) au Chili species association, Weltzin et Coughenour (1990) in the East African savannahs, Akpo and Grouzis (2009)

ACKNOWLEGMENTS
We thank the project -International Human-Environment Observatory (OHMi)‖ of Tessekere for funding this research and providing the logistic in the field.