Vincent Droissart

Seeing Central African forests through their largest trees

Large tropical trees and a few dominant species were recently identified as the main structuring elements of tropical forests. However, such result did not translate yet into quantitative approaches which are essential to understand, predict and monitor forest functions and composition over large, often poorly accessible territories. Here we show that the above-ground biomass (AGB) of the whole forest can be predicted from a few large trees and that the relationship is proved strikingly stable in 175 1-ha plots investigated across 8 sites spanning Central Africa. We designed a generic model predicting AGB with an error of 14% when based on only 5% of the stems, which points to universality in forest structural properties. For the first time in Africa, we identified some dominant species that disproportionally contribute to forest AGB with 1.5% of recorded species accounting for over 50% of the stock of AGB. Consequently, focusing on large trees and dominant species provides precise information on the whole forest stand. This offers new perspectives for understanding the functioning of tropical forests and opens new doors for the development of innovative monitoring strategies.

Exploring the floristic diversity of tropical Africa

BackgroundUnderstanding the patterns of biodiversity distribution and what influences them is a fundamental pre-requisite for effective conservation and sustainable utilisation of biodiversity. Such knowledge is increasingly urgent as biodiversity responds to the ongoing effects of global climate change. Nowhere is this more acute than in species-rich tropical Africa, where so little is known about plant diversity and its distribution. In this paper, we use RAINBIO – one of the largest mega-databases of tropical African vascular plant species distributions ever compiled – to address questions about plant and growth form diversity across tropical Africa.ResultsThe filtered RAINBIO dataset contains 609,776 georeferenced records representing 22,577 species. Growth form data are recorded for 97% of all species. Records are well distributed, but heterogeneous across the continent. Overall, tropical Africa remains poorly sampled. When using sampling units (SU) of 0.5°, just 21 reach appropriate collection density and sampling completeness, and the average number of records per species per SU is only 1.84. Species richness (observed and estimated) and endemism figures per country are provided. Benin, Cameroon, Gabon, Ivory Coast and Liberia appear as the botanically best-explored countries, but none are optimally explored. Forests in the region contain 15,387 vascular plant species, of which 3013 are trees, representing 5–7% of the estimated world’s tropical tree flora. The central African forests have the highest endemism rate across Africa, with approximately 30% of species being endemic.ConclusionsThe botanical exploration of tropical Africa is far from complete, underlining the need for intensified inventories and digitization. We propose priority target areas for future sampling efforts, mainly focused on Tanzania, Atlantic Central Africa and West Africa. The observed number of tree species for African forests is smaller than those estimated from global tree data, suggesting that a significant number of species are yet to be discovered. Our data provide a solid basis for a more sustainable management and improved conservation of tropical Africa’s unique flora, and is important for achieving Objective 1 of the Global Strategy for Plant Conservation 2011–2020.

RAINBIO: a mega-database of tropical African vascular plants distributions

Abstract The tropical vegetation of Africa is characterized by high levels of species diversity but is undergoing important shifts in response to ongoing climate change and increasing anthropogenic pressures. Although our knowledge of plant species distribution patterns in the African tropics has been improving over the years, it remains limited. Here we present RAINBIO, a unique comprehensive mega-database of georeferenced records for vascular plants in continental tropical Africa. The geographic focus of the database is the region south of the Sahel and north of Southern Africa, and the majority of data originate from tropical forest regions. RAINBIO is a compilation of 13 datasets either publicly available or personal ones. Numerous in depth data quality checks, automatic and manual via several African flora experts, were undertaken for georeferencing, standardization of taxonomic names and identification and merging of duplicated records. The resulting RAINBIO data allows exploration and extraction of distribution data for 25,356 native tropical African vascular plant species, which represents ca. 89% of all known plant species in the area of interest. Habit information is also provided for 91% of these species.

Contrasted allometries between stem diameter, crown area, and tree height in five tropical biogeographic areas

Key messageAcross five biogeographic areas, DBH-CA allometry was characterized by inter-site homogeneity and intra-site heterogeneity, whereas the reverse was observed for DBH-H allometry.AbstractTree crowns play a central role in stand dynamics. Remotely sensed canopy images have been shown to allow inferring stand structure and biomass which suggests that allometric scaling between stems and crowns may be tight, although insufficiently investigated to date. Here, we report the first broad-scale assessment of stem vs. crown scaling exponents using measurements of bole diameter (DBH), total height (H), and crown area (CA) made on 4148 trees belonging to 538 species in five biogeographic areas across the wet tropics. Allometries were fitted with power functions using ordinary least-squares regressions on log-transformed data. The inter-site variability and intra-site (sub-canopy vs. canopy trees) variability of the allometries were evaluated by comparing the scaling exponents. Our results indicated that, in contrast to both DBH-H and H-CA allometries, DBH-CA allometry shows no significant inter-site variation. This fairly invariant scaling calls for increased effort in documenting crown sizes as part of tree morphology. Stability in DBH-CA allometry, indeed, suggests that some universal constraints are sufficiently pervasive to restrict the exponent variation to a narrow range. In addition, our results point to inverse changes in the scaling exponent of the DBH-CA vs. DBH-H allometries when shifting from sub-canopy to canopy trees, suggesting a change in carbon allocation when a tree reaches direct light. These results pave the way for further advances in our understanding of niche partitioning in tree species, tropical forest dynamics, and to estimate AGB in tropical forests from remotely sensed images.

Taxonomy of Atlantic Central African Orchids 1. A New Species of Angraecum sect. Pectinaria (Orchidaceae) from Gabon and Equatorial Guinea

Abstract During a recent survey of Atlantic central African orchids, we collected four orchid specimens in Rio Muni (Equatorial Guinea) that share the general morphology at Angraecum gabonense, the most frequent member of Angraecum section Pectinaria in Central Africa, but differ in leaf shape and flower size. Further inspection of specimens deposited at the Wageningen herbarium and cultivated in their greenhouse led to the discovery of additional specimens from the Monts Doudou area in Gabon. Comparison with other Angraecum specimens indicate that these collections represent a new species, which we describe here as Angraecum atlanticum, the fifth species of Angraecum section Pectinaria recorded in central Africa. The new species is restricted to submontane forests covering the mountain chain situated along the coasts of Gabon and Equatorial Guinea. The distinguishing features of the species include its leaves, which are more widely spaced than A. gabonese, and are 2.2 mm wide; its petals and sepals, which are slightly longer than the lip; its spur, which is somewhat inflated in the middle; and its larger ovary. Information on the ecology, phenology and distribution of Angraecum atlanticum is presented, along with a preliminary conservation assessment using the IUCN Red List Categories and Criteria.

Synopsis of the Genus Chamaeangis (Orchidaceae), Including Two New Taxa

Abstract A synopsis of Chamaeangis (Orchidaceae) with a taxonomic key to the genus is provided. Twelve taxa are recognized. Detailed examination of the specimens revealed two novelties: Chamaeangis spiralis from Nigeria and Cameroon, and Chamaeangis lecomtei var. tenuicalcar, a new variety from Ivory Coast, Ghana, and Gabon. Chamaeangis auciflora is shown to be synonymous with Chamaeangis letouzeyi, which is here redescribed in detail. Chamaeangis thomensis is neotypified, and Chamaeangis lecomtei var. lecomtei is lectotypified. Chamaeangis gracilis is treated as an excluded species. Several new country records of Chamaeangis are provided.

New orchid records from atlantic central Africa

Several papers dealing with Orchidaceae from Atlantic Central Africa have recently been published. However, orchid distributions in this area remain poorly known. Over the last decade, we have conducted an intensive fieldwork programme in Cameroon and Equatorial Guinea. This has enabled us to collect 22 taxa that are new national records. Ten taxa are new records for Cameroon, eight for Equatorial Guinea and four for both countries. One of these taxa is also a new record from Nigeria. Fifteen taxa are endemic to the Lower Guinea area and six were previously known only from the type locality.

Le genre Stolzia (Orchidaceae) en Afrique centrale avec deux nouveaux taxons

Droissart V., Simo M., Sonké B., Cawoy V. & Stévart T. 2009. — Le genre Stolzia (Orchidaceae) en Afrique centrale avec deux nouveaux taxons. Adansonia, sér. 3, 31 (1): 25-40. ABSTRACT The genus Stolzia (Orchidaceae) in central Africa with two new taxa. A taxonomic revision of genus Stolzia (Orchidaceae) occurring in central Africa is given. Nine taxa are recognized. Two new taxa from Cameroon and Equatorial Guinea (Rio Muni) are described and illustrated. Stolzia repens (Rolfe) Summerh. var. cleistogama Stévart, Droissart & Simo differs from the two other varieties of S. repens by its dorsal sepal, which is jointed to lateral sepal on one third of his total length, and by its smaller and non-reflected sepals and petals. Stolzia grandiflora P.J.Cribb subsp. lejolyana Stévart, Droissart & Simo is morphologically close to the type subspecies, endemic to Ethiopia, but differs from it by a longer inflorescence with two flowers that are smaller. The morphological affinity of S. grandiflora subsp. lejolyana appears to lie with the varieties of S. repens (Rolfe) Summerh. However, the new species differs by having inflorescences with two flowers and a longer and thinner peduncle. Stolzia cupuligera (Kraenzl.) Summerh. and S. peperomioides (Kraenzl.) Summerh. are neotypified. The position of S. thomensis Stévart & P.J.Cribb is reconsidered and proposed as S. peperomioides (Kraenzl.) Summerh. subsp. thomensis (Stévart & P.J.Cribb) Stévart, Droissart & Simo. Many new records of Stolzia are given. Key to identification of species and synopsis table of all taxa occurring in central Africa are given.

Morphometrics and molecular phylogenetics of Angraecum section Dolabrifolia (Orchidaceae, Angraecinae)

Species delimitation within Angraecum section Dolabrifolia is problematic due to morphological variability coupled with overlap in many of the characters traditionally used to distinguish species. Recent molecular phylogenetic studies of the genus included three of the five currently described species of the Dolabrifolia group, placing them as sister to continental African species of Angraecum sect. Pectinaria. In preparation for a taxonomic revision of section Dolabrifolia, we analyzed morphological and molecular data to re-assess the circumscription of each of the five currently described species, examined the relationships among members of the section, and assessed their position within the genus. We used 172 alcohol-preserved specimens to perform multivariate analyses on 15 morphological characters. We also collected molecular sequence data from 16 taxa including all members of the section using six DNA regions, and analyzed these data with parsimony and Bayesian methods. The morphometric study revealed five distinct groups, of which four correspond to currently recognized species, while the fifth represents a taxonomic novelty. Angraecum podochiloides is the most distinctive morphologically, recognizable by its narrow leaves bearing white-yellowish flowers. The often confused species Angraecum distichum and Angraecum bancoense are clearly distinguishable by flower size. Molecular phylogenetic analyses indicated that section Dolabrifolia forms a well-supported clade related to the continental African members of section Pectinaria. Four species are well delimited, while the accessions of Angraecum aporoides formed two well-supported clades corresponding to two subclusters revealed by the morphometric study. The recently published Angraecum poppendickianum is shown to be a synonym of A. distichum.

Taxonomic revision of the continental African species of Angraecum section Pectinaria (Orchidaceae)

Abstract Angraecum, the largest genus of the angraecoid orchids, has long been regarded as a natural group possessing a common overall floral morphology, although recent molecular studies have called into question its monophyly. No recent taxonomic revision is available for the genus or its sections, and previous studies (whether based on morphology or DNA sequences) have suffered from a paucity of material from continental Africa. An earlier study suggested that the section Pectinaria was polyphyletic, with one clade, containing the type species, centered in Madagascar and the other in continental Africa. Our recent morphometric and molecular study confirmed the polyphyly of this section, clarified the circumscription of its continental African species, and assessed their monophyly. Here we present a comprehensive taxonomic revision of the five continental African members of Angraecum section Pectinaria, including an identification key, distribution maps, and an assessment of their conservation status (two are threatened, one could become threatened in the near future, and two do not meet the criteria for threatened status). Species richness is highest in Atlantic Central Africa, and A. gabonense is reported for the first time from Equatorial Guinea. Lectotypifications are made for A. pungens, A. subulatum and its synonym, A. canaliculatum.