A sharp floristic discontinuity revealed by the biogeographic regionalization of African savannas

Abstract

Aim:In tropical Africa, savannas cover huge areas, have high plant species richness and are considered as a major natural resource for most countries. There is, however, little information available on their floristics and biogeography at the continental scale, despite the importance of such information for our understanding of the drivers of species diversity at various scales and for effective conservation and management. Here, we collated and analysed floristic data from across the continent in order to propose a biogeographical regionalization for African savannas.Location: We collated floristic information (specifically woody species lists) for 298 samples of savanna vegetation across Africa, extending from 18°N to 33°S and from 17°W to 48°E. Taxa:We focused on native woody species. Methods:We used ordination and clustering to identify the floristic discontinuities and gradual transitions across African savannas. Floristic relationships, specificity and turnover, within and between floristic clusters, were analysed using a (dis-)similarity-based approach. Results:We identified eight floristic clusters across African savannas which in turn were grouped into two larger macro-units. Ordinations at species and genus levels showed a clear differentiation in woody species composition between the North/ West macro-unit and the South/East macro-unit. This floristic discontinuity matches to the High (i.e. N&W) and Low (S&E) division of Africa previously proposed by White (1983) and which tracks climatic and topographical variation. In the N&W savannas, the floristic gradient determined by rainfall was partitioned into the Sudanian (drier) and Guinean (wetter) clusters. Within the highly heterogeneous S&E savannas and woodlands, six clusters were identified: Ugandan, Ethiopian, Mozambican, Zambezian, Namibian and South African. Main conclusions:The proposed pan-African classification of savannas and woodlands might assist the development of coordinated management and conservation policies. K E Y W O R D Sbiogeographical regions, clustering, correspondence analysis, distance decay in similarity, floristic clusters, indicator species, rainfall and altitude/temperature gradients, savannas

An interpolated biogeographical framework for tropical Africa using plant species distributions and the physical environment

“…As far as we are aware, the use of a decision tree (CART) method to interpolate a biotically derived biogeographical framework over a large spatial extent is novel, and the approach could be pursued for incompletely sampled taxa and areas outside of tropical Africa. The success of the CART approach is perhaps unsurprising, given the predictive relationship shown between climate and individual plant species distributions in tropical Africa (Blach‐Overgaard, Balslev, Dransfield, Normand, & Svenning, 2015; Deblauwe et al., 2016; Maharjan et al., 2011; McClean et al., 2005), between climate and vegetation physiognomy (Arino et al., 2012), between climate and local‐scale species assemblages across west African forest (Bongers et al., 2004), and between climate and local‐scale species assemblages across savanna and forest biomes of tropical Africa (Fayolle et al., 2014, 2019). Our model is empirical (Guisan & Zimmermann, 2000), employed for the pragmatic purpose of rendering a biogeographical map spatially complete in a way that is plausible and useful.…”

“…We excluded from analysis vague names, hybrids, cultivars and taxa we knew to be introduced or cultivated. Analyses are conducted at the lowest named taxonomic rank, the most commonly used rank used in phytogeographical studies in tropical Africa (Denys, 1980; Fayolle et al., 2014, 2019; Linder et al., 2005, 2012). Only 4.5% of informative records are resolved to infraspecific rank.…”

The mesic savannas of the Bateke Plateau: carbon stocks and floristic composition

“…Additionally, Fayolle et al . () concluded, using the data presented here, that Lefini and Lesio Louna have floristic similarities with Northern and Western African savannas and woodlands. The mixed floristic composition of the Bateke is likely due to its historical spatial geography.…”

“…The similarities with the northern savannas could be explained by the fragmentation of the Congo Basin forest during the Last Glacial Maximum (18000 years ago) (Maley , Fayolle et al . ). Furthermore, the Sangha River Interval provided a large savanna corridor connecting the Sudanian savannas in the north to the Bateke savannas (Maley , Maley & Willis , Bostoen et al .…”

“…The floristic affinities with the south and east could be explained by a connection via a savanna corridor with the Angolan highlands (Fayolle et al . ) and by the Kalahari sands sheets, which could have provided a connection with southern species, although this hypothesis remains uncertain (Walters et al . ).…”