**1. Introduction**

Land degradation has become a global problem affecting at least a quarter of all terrestrial biomes and agro-ecologies, and occurring in many low-income as well as industrialized countries [1]. Understanding and assessing the underlying processes of land degradation is important to develop suitable land management measures and policies. Land degradation involves many interrelated processes such as soil erosion, depletion of soil nutrients, loss of biodiversity, deforestation, loss of ecosystem services etc. [2].

Many studies examined the impacts of land degradation on vegetation. In general, the methodologies used consisted in statistically testing differences in certain measures of vegetation structure, biodiversity and/or ecosystem services collected over different states or intensities of degradation of a given environmental component. Some authors examined the diversity and changing composition of plant communities of different land use and land cover types under different grazing pressure intensities [3–5]. Others have addressed the difference in species diversity between forest successional stages [6, 7] or between concretion soil, sand-clay soil and *Bowal*, (considered as the final of land degradation) [8]. *Bowa*l (plural *bowé*) comes from the *fulfulde* language spoken in Guinea and refers to degraded lands found on hardened ferruginuous soils also known as ferricretes [8]. However, we are not aware of any studies that have attempted to assess the impacts on vegetation (structure and diversity) of soil degradation defined as physical soil degradation classes.

Soil is a key resource that manages the cycle of water, cycle of carbon, plant growth and distribution, fauna and geochemicals [9–11]. Soils play an important role in mountainous areas often characterized by steep slopes and shallow soils. In Benin, the mountainous Atacora region is confronted with different soil degradation processes. Increased human activities (unsustainable agriculture, livestock grazing, fuelwood and tree cutting), combined with steep slopes, shallow soils and heavy rainfall had led to soil degradation [12–16].

Into the mountainous Atacora region, previous study in Ref. [17] had examined various indicators of land degradation and found that soils could be classified into 4 soil degradation categories i.e. light, moderate, high, and extreme degradation. However, nothing is known about the impacts of soil degradation classes on vegetation. Up to now investigations about phytodiversity into the mountainous region have mainly focused on characterization of plant communities and assessment of species diversity through phytosociological surveys [18–20]. There is need to fill a gap in scientific researches and to contribute to sustainable land management in the study area by enhancing the knowledge of land degradation processes.

For the assessment of plant diversity, different methods and indices are available, including vegetation structure, floristic composition and specific richness, chorological types, life forms and dispersal types of diapores which are good indicators of the state of vegetation health [7, 21–23]. The aim of the present study was to explore the impacts of soil degradation classes on vegetation namely vegetation structure, floristic composition, species richness, chorological types, life forms and dispersal types of diapores.
