Do plant families with contrasting functional traits show similar patterns of endemism? A case study with Central African Orchidaceae and Rubiaceae

Abstract

International audienceThe influence of functional traits on floristic patterns remains poorly understood in tropical rain forests. This contribution explores whether patterns of endemism of plant species are influenced by their life form and mode of dispersal. We used a comprehensive dataset of 3650 georeferenced plant specimens collected in Cameroon belonging to 115 taxa of Orchidaceae and 207 Rubiaceae endemic to Atlantic Central Africa. Species diversity of each family was compared using raw species richness (SR) and an index of species diversity (S k ) using subsampling procedure to correct for sampling bias. Measures were compared at three scales (square grids of one half-degree and one-degree per side and ecoregions) and according to elevation and continentality gradients. Species similarity between grid cells was measured using the sample-size corrected NNESS index. For both families, SR and S k decreased along the continentality gradient. In forest habitats below 1500 m altitude, both Orchidaceae and Rubiaceae show similar endemism patterns, but they differ in intensity. At higher altitudes, S k is higher for orchids due to the presence of endemic terrestrial taxa in grasslands, where the endemic Rubiaceae flora is rather poor. Substantial endemism observed at the ecoregion level and turnover analysis supported the role of the Sanaga River as a phytogeographical boundary. Similar endemism patterns were observed in lowland forests for Orchidaceae and Rubiaceae, even though Orchidaceae are assumed to have better long distance dispersal capabilities. The dispersal ability of Orchidaceae could be limited by the need of specific mycorhizal fungi for seed germination or host specificity for epiphytic orchid

Congruent phylogeographical patterns of eight tree species in Atlantic Central Africa provide insights into the past dynamics of forest cover

“…The genetic discontinuity between the regions north and south of the Sanaga River described in the present study (Figs 2 and 3) supports the scenario of past fragmentation into at least two main refugia located approximately north and south of the Sanaga River. Interestingly, while a discontinuity has been described both for plant diversity patterns (Droissart et al 2011 for flora belonging to Rubiaceae and Orchidaceae) and for the pDNA diversity pattern (this study), the synthesis of Hardy et al (2013), mostly based on microsatellites, did not reveal any genetic split in this specific area. In fact, the impact of a relatively ancient historical event may still be noticeable with plastid sequences but not with nuclear microsatellites where long-distance pollen dispersal and/or homoplasious mutations may have erased the signature of past population fragmentation.…”

“…Interestingly, while a discontinuity has been described both for plant diversity patterns (Droissart et al . for flora belonging to Rubiaceae and Orchidaceae) and for the pDNA diversity pattern (this study), the synthesis of Hardy et al . (), mostly based on microsatellites, did not reveal any genetic split in this specific area.…”

“…; Droissart et al . ). Among the eight studied species, three were poorly sampled or not sampled at all north of the Sanaga River, marking the limit of their northern range.…”

“…3, respectively, the Cameroonian volcanic line and the Ngovayang massif, see Fig. 1), which have been defined by several authors as two distinct areas of plant species endemism (Cheek et al 2001;Droissart et al 2011). Among the eight studied species, three were poorly sampled or not sampled at all north of the Sanaga River, marking the limit of their northern range.…”

“…It is unlikely that the Sanaga River is a physical barrier in itself for plant dispersal. Droissart et al (2011) instead proposed that the forest along the Sanaga River was strongly fragmented during the drier periods of the Quaternary while forest refugia would have occurred in adjacent mountainous areas (the Cameroonian volcanic line in the north and the Ngovayang massif in the south, see Fig. 1).…”