A matter of warts: a taxonomic treatment for Drypetes verrucosa (Putranjivaceae, Malpighiales) and a new cauliflorous species from Cameroon and Nigeria, D. stevartii

Background and aims – Specimens of a new tree species in the genus Drypetes (Putranjivaceae) distributed in Cameroon and eastern Nigeria, D. stevartii, were associated with D. verrucosa, another tree species endemic to Gabon, due to its warty fruits and to the overall morphological resemblances of both species. Material and methods – The present study is based on the study of 20 gatherings of D. verrucosa and 26 gatherings of D. stevartii. Morphological observations on herbarium specimens belonging to the new species and D. verrucosa were carried out in order to describe them. Key results – This treatment includes the detailed descriptions of these two species, the typification of their names, a comparative table summarizing their main morphological differences, an identification key, photographs of both, as well as information about their distribution, habitat, and phenology. Preliminary IUCN Red List assessments show that both D. verrucosa and D. stevartii are ‘Near Threatened’ species

Closing a gap in tropical forest biomass estimation: taking crown mass variation into account in pantropical allometries

Accurately monitoring tropical forest carbon stocks is a challenge that remains outstanding. Allometric models that consider tree diameter, height and wood density as predictors are currently used in most tropical forest carbon studies. In particular, a pantropical biomass model has been widely used for approximately a decade, and its most recent version will certainly constitute a reference model in the coming years. However, this reference model shows a systematic bias towards the largest trees. Because large trees are key drivers of forest carbon stocks and dynamics, understanding the origin and the consequences of this bias is of utmost concern. In this study, we compiled a unique tree mass data set of 673 trees destructively sampled in five tropical countries (101 trees > 100 cm in diameter) and an original data set of 130 forest plots (1 ha) from central Africa to quantify the prediction error of biomass allometric models at the individual and plot levels when explicitly taking crown mass variations into account or not doing so. We first showed that the proportion of crown to total tree aboveground biomass is highly variable among trees, ranging from 3 to 88 %. This proportion was constant on average for trees  1 Mg) and reduced the range of plot-level error (in %) from [−23; 16] to [0; 10]. The disproportionally higher allocation of large trees to crown mass may thus explain the bias observed recently in the reference pantropical model. This bias leads to far-from-negligible, but often overlooked, systematic errors at the plot level and may be easily corrected by taking a crown mass proxy for the largest trees in a stand into account, thus suggesting that the accuracy of forest carbon estimates can be significantly improved at a minimal cost.This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by Copernicus Publications on behalf of the European Geosciences Union. The published article can be found at: http://www.biogeosciences.net

Consistent patterns of common species across tropical tree communities

Trees structure the Earth’s most biodiverse ecosystem, tropical forests. The vast number of tree species presents a formidable challenge to understanding these forests, including their response to environmental change, as very little is known about most tropical tree species. A focus on the common species may circumvent this challenge. Here we investigate abundance patterns of common tree species using inventory data on 1,003,805 trees with trunk diameters of at least 10 cm across 1,568 locations1,2,3,4,5,6 in closed-canopy, structurally intact old-growth tropical forests in Africa, Amazonia and Southeast Asia. We estimate that 2.2%, 2.2% and 2.3% of species comprise 50% of the tropical trees in these regions, respectively. Extrapolating across all closed-canopy tropical forests, we estimate that just 1,053 species comprise half of Earth’s 800 billion tropical trees with trunk diameters of at least 10 cm. Despite differing biogeographic, climatic and anthropogenic histories7, we find notably consistent patterns of common species and species abundance distributions across the continents. This suggests that fundamental mechanisms of tree community assembly may apply to all tropical forests. Resampling analyses show that the most common species are likely to belong to a manageable list of known species, enabling targeted efforts to understand their ecology. Although they do not detract from the importance of rare species, our results open new opportunities to understand the world’s most diverse forests, including modelling their response to environmental change, by focusing on the common species that constitute the majority of their trees.Publisher PDFPeer reviewe

Co-limitation towards lower latitudes shapes global forest diversity gradients

The latitudinal diversity gradient (LDG) is one of the most recognized global patterns of species richness exhibited across a wide range of taxa. Numerous hypotheses have been proposed in the past two centuries to explain LDG, but rigorous tests of the drivers of LDGs have been limited by a lack of high-quality global species richness data. Here we produce a high-resolution (0.025° × 0.025°) map of local tree species richness using a global forest inventory database with individual tree information and local biophysical characteristics from ~1.3 million sample plots. We then quantify drivers of local tree species richness patterns across latitudes. Generally, annual mean temperature was a dominant predictor of tree species richness, which is most consistent with the metabolic theory of biodiversity (MTB). However, MTB underestimated LDG in the tropics, where high species richness was also moderated by topographic, soil and anthropogenic factors operating at local scales. Given that local landscape variables operate synergistically with bioclimatic factors in shaping the global LDG pattern, we suggest that MTB be extended to account for co-limitation by subordinate drivers

Le rôle des facteurs environnementaux dans la structure des communautés d'arbres des forêts de plaine d'Afrique centrale

Tree communities vary at different spatial scales and are influenced by environmental factors. To understand the role of environmental factors in tree communities requires a scale-wise analysis. The overall objective of this study was to analyse the influence of environmental drivers (soil and climate) on three aspects of forest community structure (floristic patterns, tree height–diameter relationship and leaf functional traits) at regional (>100 km2) and local (0.5 km2) spatial scales.This study was conducted in Cameroon, Gabon and the Democratic Republic of the Congo, all belonging to the lowland ( one individual ha-1 per site) and by many “scarce species” (≤ one individual ha-1 per site). The gradients in soil composition (soil texture, phosphorus and organic fertility) explained 15.42 % (76/493) mostly scarce tree species while the climatic gradients (rainfall, temperature and precipitation seasonality) explained 26.37 % (130/493) mostly abundant tree species. Tree basal area and the climatic gradient improved the prediction of tree height with only 18.9 % model error as opposed to 20.2 % and 21.7 % model error from two existing pantropical height–diameter models (Chave and Feldpausch, respectively). The new height–diameter models also improved aboveground biomass prediction and revealed that the pantropical models consistently overestimated biomass by 25 and 55 Mg ha-1. At the local scale in Korup, soil fertility was the main gradient and significantly correlated with the leaf functional traits (leaf area, specific leaf area, leaf phosphorus and nitrogen contents). Up to 33 % of these traits were significantly either over-dispersed or under-dispersed (non-random distribution) confirming the influence of abiotic filtering by the soil fertility gradient.In the prospect of possible climate change, there may be a shift in species dominance for tree species in eastern Cameroon forests while the effects of soil on the scarce species pool may limit the risk of extinction. This study thus presents an improved height-diameter model to predict the heights of Central African forest trees by integrating the effects of some easy-to-retrieve ecological predictors. At the local scale, leaf functional traits reveal the effect of soil gradient in the distribution of species and community assembly.Les communautés d'arbres varient à différentes échelles spatiales et sont influencées par des facteurs environnementaux. Pour comprendre le rôle des facteurs environnementaux dans la structuration des communautés d'arbres, une analyse par échelles spatiales est nécessaire. L'objectif global de cette étude est d'analyser l'influence des facteurs environnementaux (sol et climat) sur trois aspects de la structure des communautés d'arbres, à savoir les patrons floristiques, les hauteurs des arbres et les traits fonctionnels foliaires, à échelle spatiale régionale (>100 km2) et locale ( un individu ha-1) et par de nombreuses « espèces rares » (c.-à-d. avec une moyenne par site ≤ 1 individus ha-1). Les gradients de fertilité (phosphore et matière organique) et de texture du sol expliquent la répartition de 15,42 % (76 / 493) des espèces, pour la plupart rares, alors que les gradients climatiques (température et la saisonnalité) expliquent la répartition de 26,37 % (130 / 493) des espèces, majoritairement qualifiées comme abondantes. L‟intégration dans le modèle de la surface terrière des arbres et du gradient climatique améliore la prédiction de la hauteur des arbres avec une erreur associée de 18,9 %, contre 20,2 % et 21,7 % d'erreur associée pour les deux modèles pantropicaux préexistants. Le nouveau modèle hauteur-diamètre proposé améliore également la prédiction de la biomasse aérienne alors que les deux anciens modèles pantropicaux ont tendance à surestimer la biomasse de 25 et 55 Mg ha-1. A l'échelle locale (Korup), la fertilité du sol est le gradient principal qui explique la distribution des espèces ; il est significativement corrélée aux traits fonctionnels foliaires mesurés (surface foliaire, surface foliaire spécifique, teneur en phosphore et en azote des feuilles). Jusqu'à 33 % de la distribution des traits était soit sur-dispersée soit sous-dispersée (une distribution non aléatoire), confirmant l'influence de la filtration abiotique par le gradient de fertilité du sol.Dans la perspective d'un éventuel changement climatique, ces résultats démontrent qu‟à l‟échelle régionale il pourrait y avoir un changement dans la dominance des espèces d‟arbres tandis que l‟influence marquée du sol sur la distribution d'espèces rares pourrait limiter le risque d'extinction de ces dernières. En outre, cette étude présente un modèle de hauteur-diamètre amélioré pour prédire les hauteurs et la biomasse des arbres d'Afrique centrale en intégrant certains prédicteurs écologiques faciles à obtenir. Enfin, il a été montré qu‟à l‟échelle locale l‟étude des traits fonctionnels foliaires peuvent révéler des processus de filtration abiotique par le gradient de fertilité du sol

State of the art on tropical tree allometry

In the recent decades, a renewed interest has been observed for the estimation and monitoring of forest biomass and carbon worldwide, and specifically in the tropics with the ongoing negotiations under the UNFCCC for the implementation of the REDD+ mechanism. Because all methods to estimate biomass and carbon stocks contained in tropical forests rely on an allometric equation to convert inventory data into biomass estimates, tropical tree allometry recently received great attention from scientists and research sponsors. Pantropical allometric models early developed in the 1980s, and 2000s, were recently revised and a global consensus is emerging toward a universal approach to estimate biomass and carbon stocks in tropical forest using generic allometric models. Many local allometric models were also recently established, specifically in places previously under-sampled, such as the forests of the Congo basin. We believe that the newly collected data that have and will have strong practical implications for the estimation of forest biomass and carbon stocks are also extremely important to explore the between-site and between-species variations in tropical tree allometry. Despite major advances in our understanding of tropical tree allometry, an integrative view on tropical tree allometry, its variation and ecological (and evolutionary) significance, is yet to arise. As an introduction to the session, we will present the interest of analyzing between-site and between-species variations in tropical tree allometry, integrating methods and spatial scales, and bridging disciplines and approaches to reach a unifying view

Multiple Stable Dominance States in the Congo Basin Forests

International audienceUnderstanding the dynamics of dominant tree species in tropical forests is important both for biodiversity and carbon-related issues. We focus on the Congo Basin (East of Kisangani) to investigate the respective roles of topographic/soil gradients and endogenous dynamics in shaping local variations in dominance. We used a dataset of 30 1-ha plots, in which all trees above 10 cm diameter at breast height (DBH) were censused. Soil samples were analyzed for standard pedologic variables and a digital elevation model permitted to infer topography and hydromorphy. We found that this forest is characterized by variations in the abundance of three dominant species: Petersianthus macrocarpus (P.Beauv.) Liben (PM), Gilbertiodendron dewevrei (De Wild.) J.Leonard (GD) and Julbernardia seretii (De Wild.) Troupin (JS). These variations occur independently of substratum or topography variations. At plot level, the local relative abundance never reached 50%, the threshold for monodominance proposed in the literature, but rather progressively increased to reach higher values for canopy trees (>60 cm DBH), where this threshold could be exceeded. We found no sign of shifting compositional dynamics, whereby the dominant species would switch between the canopy and the undergrowth. Our results, therefore, support the possibility of the existence of stable dominance states, induced by endogenous processes, such as biological positive feedbacks fostering monodominance. We also document a strong relation between monodominance level and alpha diversity, when giving more weight to abundant species which is expected (R 2 = 0.79) but also when giving more weight to rare species (R 2 = 0.37), showing that monodominance influences tree species richness patterns. Structural differences existed between groups, with the PM group having more (and on average smaller) stems and lighter wood on average, but paradoxically also higher biomass and basal area

A matter of warts: a taxonomic treatment for Drypetes verrucosa (Putranjivaceae, Malpighiales) and a new cauliflorous species from Cameroon and Nigeria, D. stevartii

Background and aims – Specimens of a new tree species in the genus Drypetes (Putranjivaceae) distributed in Cameroon and eastern Nigeria, D. stevartii, were associated with D. verrucosa, another tree species endemic to Gabon, due to its warty fruits and to the overall morphological resemblances of both species. Material and methods – The present study is based on the study of 20 gatherings of D. verrucosa and 26 gatherings of D. stevartii. Morphological observations on herbarium specimens belonging to the new species and D. verrucosa were carried out in order to describe them. Key results – This treatment includes the detailed descriptions of these two species, the typification of their names, a comparative table summarizing their main morphological differences, an identification key, photographs of both, as well as information about their distribution, habitat, and phenology. Preliminary IUCN Red List assessments show that both D. verrucosa and D. stevartii are ‘Near Threatened’ species

Land use has little influence on the soil seed bank in a central African moist forest

International audienceRegeneration of tropical forest occurs naturally and mainly through early secondary succession by soil seed banks. Here, we analyzed seed bank variation in density and diversity along a degradation gradient. We also explored the relationship (similarity in composition) between the seed and seedling banks and the overstory vegetation. The investigated forests were located at Dja Fauna Reserve and its surroundings, south-eastern Cameroon. We sampled 33 one-ha forest plots scattered in a protected area, in a logging concession and in community forests. Within each plot, we inventoried the seed bank in the litter and in four successive soil layers of 5 cm depth interval, and the standing vegetation. Seed density significantly varied with sampling depth but not between land use types. A total of 181 morphospecies were identified. Trees species accounted for 46% of seeds with pioneer species dispersed by animals dominating the seed bank. Species diversity and composition strongly varied with the sampling depth; the litter seed bank was more similar in composition to the soil seed bank than to the seedling bank, sapling, and forest stands. We highlighted the presence of a dense and diverse seed bank in each land use type. We also showed the differentiation of each sampling depth in terms of species composition, thus revealing their complementarity in the dynamics of natural regeneration. The dominance of pioneers in the seed bank and weak relationship with the overstory are universal for tropical forests and logically follow from forest succession after disturbance. Abstract in French is available with online material