A checklist of vascular plants of the W National Park in Burkina Faso, including the adjacent hunting zones of Tapoa-Djerma and Kondio

Abstract Background The W National Park and its two hunting zones represent a unique ecosystem in Burkina Faso for biodiversity conservation. This study aims at providing a detailed view of the current state of the floristic diversity as a baseline for future projects aiming at protecting and managing its resources. We combined intensive inventories and distribution records from vegetation plots, photo records and herbarium collections. New information This is the first comprehensive checklist of vascular plants of the Burkina Faso part of the transborder W National Park. With 721 documented species including 19 species new to Burkina Faso, the Burkina Faso part of the W National Park is, so far, the nature reserve with most plant species in Burkina Faso. To a large extent, this may be assigned to its high habitat diversity and moderate degree of disturbance, but also to a relatively large area within an even larger complex of neighbouring protected areas, as well as comprehensive spatial, ecological and seasonal sampling efforts. However, as a World Heritage Site and regarding the current general context of insecurity, the W National Park and the entire WAP Complex require more international attention in order to ensure its conservation.


Introduction
In times of growing land use pressure in West Africa, protected areas play an increasingly important role in the conservation of wildlife as well as plant species (Clerici et al. 2007, Zoungrana et al. 2018. The W-Arly-Pendjari Complex (WAP Complex) is the largest transborder complex of West African nature reserves shared amongst the three countries Benin, Burkina Faso and Niger and a World Heritage Site of the UNESCO (UNESCO World Heritage Committee 2017). The three partner countries seek to further optimise and formalise joint management systems for the regional park, which are already partially in place (Miller et al. 2015). Remote sensing studies (Schulte to Bühne et al. 2017) showed vegetation cover within the protected areas to remain relatively stable, while decreasing in the surroundings between 2000 and 2013. Due to agricultural expansion and intensification, especially in the context of cotton farming, the WAP Complex is becoming increasingly isolated (Clerici et al. 2007, Konrad 2015, leaving little space for near-natural habitats in the surrounding communal area, being composed of croplands, fallows of different ages and only few savannas in sites unsuitable for agriculture. The complex is well-known for its role in the conservation of large mammals and typical Sudanian savanna ecosystems (e.g. , see next paragraph for more details). It is a continuous area of nine protected areas which include the trinational complex of W Regional Park (named after the course of the river Niger forming a "W" in this area), Arly National Park in Burkina Faso and Pendjari National Park in Benin connected and enlarged by protected areas of other categories and hunting reserves (Konrad 2015). The present checklist has been produced to fill this gap by providing a comprehensive and detailed view of the floristic diversity of the W National Park including the surrounding hunting reserves in Burkina Faso as a baseline for future projects aiming at protecting and managing the resources of the WAP Complex. We assume that the core area has a high plant diversity due to the geographical and climatic setting, as well as the diversity of its habitats.

Study area
The W National Park (235,000 ha) and its two hunting reserves, Kondio (also called Kourtiagou 51,000 ha) and Tapoa Djerma (30,000 ha) are located in eastern Burkina Faso, between 12.4° and 11.4° N and 1.8° and 2.4° E (Fig. 1). The W National Park in Burkina Faso, together with its equivalents in Benin and Niger, constitute a transfrontier park that is designated "wetland of international importance" under the Ramsar Convention in 2007. Altogether, they are part of the larger W-Arly-Pendjari transfrontier complex shared amongst the three countries Benin, Burkina Faso and Niger and one of the most important West African transboundary Biosphere Reserves of UNESCO-MAB. The national park belongs to IUCN Category II, whereas the hunting reserves belong to Category IV. Both categories act to protect large terrestrial and aquatic ecosystems, as well as threatened and rare species (both fauna and flora), serving as refuges and enhancing resilience. The National Park and the hunting zones are managed by prescribed fires ignited in October or November every year to mitigate the effect of accidental late fire and also to stimulate an off-season re-growth of perennial herbs for wildlife and to open the vegetation and increase the visibility of animals for tourists.
The W National Park and the two adjacent hunting zones of Burkina Faso are located in the Sahelo-Sudanian climate zone with an average rainfall of 750 to 950 mm, annual mean temperatures between 26 and 29°C and a dry season of 6-7 months. It belongs to the Pendjari-Mekrou biogeographical sector, shared between the Volta basin in its southern part and the Niger basin in the north. In the central western part of W National Park, eastern outliers of the Gobnangou range reach into the park and eastern outliers of the Atakora mountains are located on the southern edge of Kondio reserve and W National Park. The largest rivers in the reserves are the Mekrou on the eastern border and the Tapoa in the north, both tributaries of the Niger. In addition to the artificial water points, there are also numerous natural pools, with permanent and/or temporary water regimes (PNUD 2004). The vegetation is composed of a mosaic of various types of savannas i.e. woodland, grass, shrub and tree savanna, dry and gallery forests (Fontès and Guinko 1995), mostly distributed along elevation and soil gradients (Nacoulma et al. 2011) .

Data collection and management
The checklist is a result of extensive field surveys by the first author during the years 2007-2010, (622 records from a species list based on 369 vegetation plots; Nacoulma 2012), as well as photo records (545 records; Dressler et al. 2014 and herbarium collections from the W National Park, including the herbaria of OUA, FR (733 and 583 records via their respective collection databases) and WAG (five records via GBIF). The vegetation plots followed standard sizes of 10 m x 10 m for the herb layer and 10 m x 50 m or 30 m x 30 m for the woody layer shown to be suitable for the characterisation of Sudanian savanna vegetation by Hahn-Hadjali (1998) andOuédraogo (2006). They were taken mostly during the end of the rainy season (September-October) in order to maximise the number of flowering individuals for identification. The smaller plots for herbs were randomly located inside the corresponding plots for woody plants. The locations were chosen by a stratified random design using satellite images and soil maps to best represent the geomorphological units (     The 721 species found in the study area belong to 385 genera and 102 plant families, the families with most species being Fabaceae (126 spp.), Poaceae (102 spp.) and Cyperaceae (44 spp.) (Fig. 3). The genera with most species in the study area are Indigofera (20 spp.), Crotalaria (17 spp.) and Acacia (13 spp.), all belonging to Fabaceae.
Therophytes and phanerophytes are the most prominent life forms, followed by hemicryptophytes, geophytes and chamaephytes (Fig. 4). Only very few hydrophytes and epiphytes are present.
The best represented distribution types in the flora are specific to the vegetation zone, either Sudanian or Sudano-Zambesian (Fig. 5). Species with wider distributions in the tropics (pantropical, paleotropical) or within continental Africa (afrotropical, pluriregional African) follow. The remaining distribution types have smaller shares and comprise either distributions extending to other continents or Madagascar or belong to the neighbouring zones under higher rainfall regimes, typically extending into the Sudan along gallery forests of the larger watercourses.

Discussion
With 721  Several factors might explain this: to a large extent, the high plant diversity can be assigned to habitat diversity and a moderate degree of disturbance, but also to a relatively large area within an even larger complex of neighbouring protected areas. In Burkina Faso, the study sites are known for the diversity of their natural habitats, i.e. the Gobnangou and Atakora mountains, wetlands and gallery forests, termite mounds as well as anthropogenic relics of the past. Anthropogenic relics are still visible with metallurgical sites and baobab vegetation types, being closely linked to human settlements as shown by Duvall (2007).
Thorough spatial, ecological and seasonal sampling have certainly contributed to a good documentation of the area, building on experiences from similar studies in protected areas and focussed collection activities for the documentation of the flora of Burkina Faso (van der Burgt et al. 2010).
Finally, based on different scenarios, recent investigations (Schmidt et al. 2017) identified the southeast of Burkina Faso, around the W National Park, as one of the two hotspots of plant diversity of the country. Our present results confirm and strengthen these scenarios.
There are no species endemic to W National Park and Isoetes jaegeri, the only plant still recognised as endemic to Burkina Faso does not occur here, being known exclusively from the sandstone massif in the southwest of the country. Including the 19 newly-documented species, the W National Park harbours more than one third of the plant diversity known for Burkina Faso, making it an important area for plant conservation in a landscape setting where biodiversity outside of protected areas is increasingly threatened by intensified agriculture (Zoungrana et al. 2018). At the country level,  evaluated the percentage of modelled plant species distributions covered by protected areas and came up with a figure of 77% for Burkina Faso. Our results confirm the importance of these reserves for plant conservation with primary biodiversity data.
The composition of the flora by families, life forms and distribution types follows the same patterns as in other protected areas of the WAP complex (Assédé et al. 2012, Mbayngone et al. 2008, Ouédraogo et al. 2011. The dominance of Fabaceae and Poaceae may be explained by their respective innovations in N-fixation and C -photosynthesis, making them well-adapted to savanna environments. Therophytes and phanerophytes are the dominant life forms here and in the wider area of the West African savanna belt, with therophytes being more important in the drier north and phanerophytes in the wetter south . The large proportion of widespread species in West African savanna areas like the W National Park may be explained by the dominance of the savanna vegetation in a vast continuous band throughout West Africa in combination with the lack of effective barriers, such as higher mountains which separate savanna areas in other parts of Africa (White 1983).
Only few species from Burkina Faso have been Red-listed by IUCN, the three species in the W National Park all being sought-after timber species. The IUCN Red List is a good instrument to target species for conservation measures.However, only few species of the West African Flora have been evaluated, for Burkina Faso less than 20% (400 out of 2099 spp., IUCN 2020). Continued efforts will be necessary to get a better idea on threats and necessary conservation measures and a prerequisite are scientifically reliable assessments, accessible to the community and representing important parts of the region.
The number of 17 introduced plant species (2.3%) is rather low when compared to the 116 introduced species (5.5%) on the country scale (Zizka et al. 2015). It shows the success of these neophytes in protected areas under limited anthropogenic influence and may stimulate research on introduction routes and more in-depth studies on neobiota in Burkina Faso in general.
invitation and Dr. Stefan Dressler of Senckenberg Natural History Museum Frankfurt for identification of herbarium samples.