Botanical specimens have been collected from the PNPB area since 2001 by a variety of collectors, and they are most notably deposited in LBV, MO, and WAG (herbarium acronyms follow Thiers continuously updated). Although the area was traversed by several Europeans during the colonial era, who provided vegetation descriptions and descriptions of fire use in the 1800s (de Brazza 1887; Guiral 1889; de Chavannes 1935), botanical collections from these trips appear to be more localised around cities outside the study area (e.g. Baudon 1929). Botanical collections in Africa were first made from the coasts and rivers starting in the 1780s; the nearby Franceville area was collected by Le Testu late in his career (Raynal 1968). However, some interior areas, including the Plateaux Batéké, have only recently been studied (Sosef et al. 2017), notably by the authors of this paper. Other parts of the Republic of Congo were collected in the 1950–1970s, especially for studying savanna ecology, but these works often did not list their specimens (e.g. Koechlin 1961) and many of their specimens are missing from databases.

The majority of plant collections were identified at BR, LBV, MO, and the National Herbarium of the Netherlands (NHN), and registered in the databases at LBV, MO, and WAG. Database records were obtained in 2016 from the MO and WAG databases and again from MO in 2021 for the area comprising PNPB and including a 25 km limit of the Park. There were no specimens from the Republic of Congo in the circumscribed area. This represented 1,921 collections that have been collected in all seasons, from 2001 to 2019, in part due to residential collection (by the first author) from 2006 to 2008.

Endemic species were determined by their classification in the Checklist of Gabon (Sosef et al. 2006), and research in the botanical literature to understand their distributions, notably in recent treatments of the Flore du Gabon and recent species descriptions.

Collection density was analysed by importing all collection coordinates, where reported, into QGIS v.3.16.4 (QGIS Development Team 2021). A 2.0 km² grid was overlaid on the study area, a size that provides a minimum amount of collections per area to be informative; collections densities were given corresponding colours for the following number of specimens per grid: 1–9; 10–26; 27–55; 56–107; 108–238; and 239–379.

A subset of the savanna flora from the checklist was studied to understand the impact of fire on forb diversity and geoxylic suffrutices, based on the 183 savanna species from a previous study derived from the study area data set (Walters et al. 2012). Our study focused on annually, dry-season burned areas outside the PNPB and comparing it to an area that had not been burned for three years inside the PNPB. Savannas are generally annually burned by local people (Walters 2015); to find an unburned control area, only a single area in the PNPB was available. Three years was the maximum possible time that could be identified for areas that had not been recently burned, in consultation with local people, local resident conservation staff, and Firemapper (University of Maryland 2006), which indicated that the area had last burned in 2004 (three years prior to the field study). In the burned plots, outside of the park, the study areas were burned by local hunters every dry season (June–September). Fire typically stimulates flowering within two years (Lamont and Downes 2011), making this comparison feasible of annually burned versus an area not burned in three years. However, a longer period of no burning would have been preferable.

In each area, five hilltop sites at approximately 600 m in elevation were selected and revisited each season for one year. There is variation in sand type by stratigraphy (Koechlin 1957; Schwartz 1990) and rainfall between hilltops and valleys (Mpounza and Samba-Kimbata 1990; Walters 2010). Hilltop sites were selected to control for these differences. To understand the differences in forb diversity and geoxylic suffrutices in burned and unburned areas, a method sensitive to changes in herbaceous communities was used (Barnett and Stohlgren 2003; see Supplementary file 3). In each of the sites, in each season, plots were selected using a random walk method. Sites were not permanent and each season were established in the same hilltop areas. A random azimuth was ascertained and then followed a random number of steps. In each site, three circular plots of a radius of 7.32 m were established (measurements converted from feet to meters). Each plot consisted of three sub-plots measuring 1 m² each and placed at 4.57 m from the centre point at 30°, 150°, and 270°. In each subplot, the percentage of cover per species of forb and geoxylic suffrutex was recorded, and percentage cover by grasses and sedges (all species mixed for these two categories). Finally, all forbs and woody species unrecorded in the three subplots but found in the circular plot were listed. In this study, only the presence of forbs and geoxylic suffrutices was used from the data set. This was done because the focus of this part of the study was to understand how fire influences forb diversity specifically, since forbs, in savanna studies are understudied, and often lumped together, making their contribution to biodiversity or link to management difficult to ascertain (Siebert and Dreber 2019).

Species numbers from the three subplots were grouped together into a single cumulative plot; this became the unit for comparison. Each treatment (burned, unburned) was visited in the long-dry (July 2007), mid-rainy (December 2007), and late-rainy (April 2008) seasons producing two sets of data (burned and unburned sites) across three seasons. Thus, for each of the three seasons, 30 plots could be compared; for each burn treatment, 45 plots could be compared. A total of 90 plots were available for comparison (Table 1).

Normality of the distribution of species per treatment was tested by calculating the mean and standard deviation per treatment and then determining if 70% of the values fell within this range (Fowler et al. 1998). Variances were homogenous according to both Levene’s and Bartlett’s tests. A two-factor ANOVA was conducted using Systat v.12.0 (Systat Software Inc. 2007), with two burn treatments (annual and no-burn) and three season treatments (dry, mid-rainy, and late-rainy seasons). Each treatment contained five hilltop blocks, each containing 15 plots for a total of 45 plots (with 44 degrees of freedom per treatment-season). Post-hoc Tukey’s tests were conducted amongst the seasons. Species diversity based on individual species was compared using the Jaccard Similarity Index on species lists by season and by burn treatment. The Jaccard Similarity Index is computed as follows:

Where c is the number of species in common, a is the number of species only occurring in treatment i and b is the number of species only occurring in treatment j (Gotelli and Ellison 2004).

The checklist is the first published checklist of the PNPB and comprises 612 species (615 taxa), representing 105 families (Supplementary file 1). Fabaceae, Rubiaceae, and Poaceae are the top three families, with the top ten families representing 55% of all species (Table 2). The top nine genera comprise Cyperus, Psychotria, Campylospermum, Vigna, Landolphia, Dalbergia, Vernonia, Utricularia, and Millettia (Table 3). The most notable finds are the endemic species and geoxylic suffrutices. Seven species are endemic or near endemic to the Plateaux Batéké forest-savanna mosaic: Asclepias occidentalis, Eriosema batekense, Memecylon batekeanum, Memecylon sitanum, Millettia viridiflora, Sorindeia batekeensis, and Psychotria callensii. Seventeen geoxylic suffrutices include Anisophyllea quangensis, Chamaecrista mimosoides, Cryptolepis oblongifolia, Dolichos subcapitatus, Eriosema glomeratum, Eriosema laurentii, Eriosema pellegrinii, Eriosema shirense, Glossostelma lisianthoides, Ipomoea linosepala, Kalaharia schaijesii, Landolphia owariensis, Macrotyloma biflorum, Parinari capensis, Vernonia daphnifolia, Vernonia guineensis, and Vernonia potamophila.

Four species of the checklist have mostly Congolian distributions but reach the edge of their western-most distribution in the gallery forests of the Plateaux Batéké, as confirmed by recent collections: Dracaena waltersiae (Damen et al. 2018), Dewevrella cochliostema (Leeuwenberg 1985), Monodora laurentii (Couvreur 2008), and Combretum pellegrinianum (Jongkind 2021).

Of the 1,914 collections from the data set, 1,897 were able to be mapped, with coordinates. Of these, 70% have been collected within the PNPB, and largely in the northern half, with significant collections around the following localities of the park: the “Débarcadère” in the north, Lac Loulou to the east, the Project Protection des Gorilles main camp and Mbié Cirque in the centre, and Bai Djobo in the western forest. The focus on the northern part of the PNPB is due to the difficulty in access: the park is largely bounded by rivers with no bridges, lacks roads, and must be crossed by foot, quad, or boat. The remaining 30% of the collections were made in the proposed area for PNPB to be extended, including around Ekouyi-Mboma village, due to the resident collection by the first author in that area. Collections also include the Kele la Kalami rock outcrop, Lewou Cirque, and the forb-fire study area (see Fig. 1) and additional collections in the Kessala forest area to the west and Boumango area by other collectors (Fig. 3).

The forbs and geoxylic suffrutices in this part of the study represent a subset of savanna species in the checklist: 49 hilltop species (see Supplementary file 2), comprising 28% of the overall savanna flora, based on the 183 savanna species from a previous study derived from the study area data set (Walters et al. 2012). When comparing species numbers alone, by season and burn treatment, tested in a two-way ANOVA, only season was significantly different. Neither burning alone nor the interaction between burning and season was significant (Table 4).

When Tukey’s HSD was used to determine which seasons were most significant, the dry season differed significantly from both rainy season in terms of species numbers, while the rainy seasons were not significantly different from one another (Table 5). Species unique to the dry season were five, mid-rainy season has one unique species, and the late-rainy season had three.

To understand if burning affects species diversity within the burn treatments, species lists from the 90 plots (see Supplementary file 2) of burn treatments were compared using the Jaccard Similarity Index. The dry season is the most divergent, having only 32% species in common with the other seasons (but 45% for mid-rainy season and 50% for the late-rainy season). However, when species lists were pooled by season alone, the dry and late-rainy season were the most dissimilar, with 36% in common (for 44% for dry-mid rainy; 50% for mid- and late-rainy). Overall, whether considering species or season, the species overlap was always 50% or less.

However, it is insufficient to consider only species numbers and seasons. To understand if some species were only found in the burned or unburned plots, across the 90 plots, species were compared by burn treatment and season: 17 species are found only in burned areas, representing 35% of the species in the fire study, while three occur only in unburned areas. The dry season has the highest number of unique species, while the late- and the mid-rainy seasons comprise fewer. In order to understand how fire affects these 17 species, the botanical and ecological literature for each species was consulted to understand if the species exhibits Fire Stimulated Flowering (FSF) (Table 6). Several species were present in both burned and unburned plots, in nearly all seasons, including geoxylic suffrutices (e.g. Anisophyllea quangensis, Cryptolepis oblongifolia, Chamaecrista mimosoides, Kalaharia schaijesii, Parinari capensis) as well as Dissotis brazzae and the endemic Eriosema batekense.

The results from this part of the study show that while season heavily influences savanna forb diversity, fire is also important in the life cycle of many forbs and geoxylic suffrutices, and that savanna burning promotes forb species.

This study shows that the floristic diversity, across 615 species, comprises seven endemic species and 17 geoxylic suffrutices, as well as four Congolian species which reach their limits in the study area. Furthermore, the area, though represented by 1,914 collections, shows a collection bias for the northern part of the PNPB.

Previous analysis of the area’s flora showed both a strong element of Lower Guinea species (Walters et al. 2006) and a strong affinity with the Congolian flora (Wieringa and Sosef 2011). Although floristic studies of other national parks in Gabon have been conducted, the savanna component is often understudied, with some notable exceptions such as the case of the Parc National de Loango, where a distinct coastal savanna flora was described (Harris et al. 2012). However, when the species lists from Loango, Pongara (Dauby et al. 2008), and Lopé National Parks were combined, distinct savanna floras emerged, showing the coastal sites to be separate from Lopé and the PNPB; of the 183 known species in Plateaux Batéké savannas, the authors noted that 26% were nationally rare according to the Star Rating (Walters et al. 2012).

Eriosema batekense and Kalaharia schaijesii have been described in the past decade or so (van der Maesen and Walters 2011; Bamps 2013), and show that within their limited distribution they are very abundant across vast savanna areas (sensu Rabinowitz et al. 1986), demonstrating a previous lack of attention to herbaceous species and the area in general. Asclepias occidentalis, a Plateaux Batéké endemic not in the fire plots, and rarely collected, flowers only after burning (Goyder 2009). Other species where fire stimulates flowering could be additionally found in future collections.

New floristic inventories of the proposed park extension (see next section) have recently begun and it is expected that new species and records will be discovered, especially if new areas are visited. This study presents a baseline from which to work. Future botanical work should focus on the forest, especially its riverine forests, but also the under-collected areas of the southern part of the PNPB. Our checklist shows few forest flora endemic species, however, Memecylon batekeanum (Stone et al. 2006) forms part of a monophyletic group with M. amshoffiae and M. diluviorum, a clade from riverine forests in Cameroon, Gabon, and Angola (Cabinda) (Stone 2014), potentially indicating a unique diversity for these habitats. An additional rare species to consider is Syngonananthus poggeanus Ruhland, known from the Haut Ogooué, along the Republic of Congo border, from sandy wetlands, and although it is not currently known from the study area, it could be collected there in the future (Phillips 2016: 18).

The interior savannas of Gabon, such as those of PNPB, continue to be explored, with other floristic studies underway around Franceville and Moanda. Those areas are expected to be floristically distinct from the PNBP. For example, from a Haut Ogooué checklist of ferns, largely collected around Franceville, few of the 84 species reported were found in the PNPB (Mundy 2000). Elsewhere in the Kalahari sands savannas, such as in Angola, recent botanical work notes that the flora of the Kalahari sands is particularly underexplored in the Moxico Province (Goyder and Gonçalves 2019). Future botanical work is also planned for similar areas in the Republic of Congo. Further floristic work in these areas should focus on the endemic herbaceous species, geoxylic suffrutices, and the gallery forests.

Given the age of first savanna formation in the Plateaux Batéké of 30,000–70,000 years ago, and a persistence of savanna in many places in the Plateaux area over time, it is not surprising that they contain endemic species and geoxylic suffrutices. However, beyond climatic origins of the savanna habitat and associated flora, anthropogenic factors also contribute to the floristic diversity: people have created village forests or copses, and initiate fire regimes. The Batéké-Alima have long created copses in open savanna (Aubréville 1949: 318). They were created and abandoned over time (Soret 1973; Guillot 1980); however, this stopped with the implementation of the Regroupement policy (or resettlement), which occurred forced villages to “regroup” in larger settlements near roads. The policy was repeatedly implemented from 1910 to the 1960s (Aubame 1947; Sautter 1966; Coquery-Vidrovitch 1972) and was implemented in the study area in the 1950s–1960s (Walters 2010). These copses remain important components of the PNPB landscape, likely maintaining a specific type of diversity, as found in the Parc National de la Lopé, Gabon, where they harbour 16% of the species (Ukizintambara et al. 2007).

In this study, beyond maintaining an open, savanna environment and related diversity, fire appears to be important for floristic diversity in two ways: fire fosters the emergence of some species post-fire and it favours some life forms such as geoxylic suffrutices.

In terms of protection, the area under consideration for the extension would extend the PNPB by adding 344,460 acres north of the park. This proposed area contains some areas considered to be sacred including Mopia Bai and Kele la Kalami (Mont Kalami), a rock outcrop. The rocky outcrops of the area occur at higher elevations in the savanna, typically at 600–700 m. Their vegetation is distinct, along with unique bird species (e.g. the African River Martin, Pseudochelidon eurystomina) and plant species populations including Polystachya dendrobiiflora and Bidens oligoflora. The area is under customary management with restricted access and constitutes a type of protection similar to a Territory of Life (ICCA Consortium 2021), that could be recognised by the government of Gabon as a category V or VI protected area (Dudley 2008), a formalised hunting territory (Cornelis et al. 2017) or as an “Other effective area-based conservation measure” (Dudley et al. 2018; Gurney et al. 2021). The customary hunting and gathering territories in the proposed extension could also be recognised under these measures, thus permitting the PNPB to expand while recognising the role of the Batéké people in creating this cultural landscape and their continued governance of some of its resources.