References of "Réjou-Méchain, Maxime"
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See detailGeological Substrates Shape Tree Species and Trait Distributions in African Moist Forests
Fayolle, Adeline ULg; Engelbrecht, Bettina; Freycon, Vincent et al

in PLoS ONE (2012), 7(8), 1-10

Background: Understanding the factors that shape the distribution of tropical tree species at large scales is a central issue in ecology, conservation and forest management. The aims of this study were to ... [more ▼]

Background: Understanding the factors that shape the distribution of tropical tree species at large scales is a central issue in ecology, conservation and forest management. The aims of this study were to (i) assess the importance of environmental factors relative to historical factors for tree species distributions in the semi-evergreen forests of the northern Congo basin; and to (ii) identify potential mechanisms explaining distribution patterns through a trait-based approach. Methodology/Principal Findings: We analyzed the distribution patterns of 31 common tree species in an area of more than 700,000 km² spanning the borders of Cameroon, the Central African Republic, and the Republic of Congo using forest inventory data from 56,445 0.5-ha plots. Spatial variation of environmental (climate, topography and geology) and historical factors (human disturbance) were quantified from maps and satellite records. Four key functional traits (leaf phenology, shade tolerance, wood density, and maximum growth rate) were extracted from the literature. The geological substrate was of major importance for the distribution of the focal species, while climate and past human disturbances had a significant but lesser impact. Species distribution patterns were significantly related to functional traits. Species associated with sandy soils typical of sandstone and alluvium were characterized by slow growth rates, shade tolerance, evergreen leaves, and high wood density, traits allowing persistence on resource-poor soils. In contrast, fast-growing pioneer species rarely occurred on sandy soils, except for Lophira alata. Conclusions/Significance: The results indicate strong environmental filtering due to differential soil resource availability across geological substrates. Additionally, long-term human disturbances in resource-rich areas may have accentuated the observed patterns of species and trait distributions. Trait differences across geological substrates imply pronounced differences in population and ecosystem processes, and call for different conservation and management strategies. [less ▲]

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See detailEnvironmental filtering of dense-wooded species controls above-ground biomass stored in African moist forests
Gourlet-Fleury, Sylvie; Rossi, Vivien; Réjou-Méchain, Maxime et al

in Journal of Ecology (2011), 99(4), 981-990

1. Regional above-ground biomass estimates for tropical moist forests remain highly inaccurate mostly because they are based on extrapolations from a few plots scattered across a limited range of soils ... [more ▼]

1. Regional above-ground biomass estimates for tropical moist forests remain highly inaccurate mostly because they are based on extrapolations from a few plots scattered across a limited range of soils and other environmental conditions. When such conditions impact biomass, the estimation is biased. The effect of soil types on biomass has especially yielded controversial results. 2. We investigated the relationship between above-ground biomass and soil type in undisturbed moist forests in the Central African Republic. We tested the effects of soil texture, as a surrogate for soil resources availability and physical constraints (soil depth and hydromorphy) on biomass. Forest inventory data were collected for trees ≥20 cm stem diameter in 2754 0.5 ha plots scattered over 4888 km². The plots contained 224 taxons, of which 209 were identified to species. Soil types were characterized from a 1:1 000 000 scale soil map. Species-specific values for wood density were extracted from the CIRAD’s data base of wood technological properties. 3. We found that basal area and biomass differ in their responses to soil type, ranging from 17.8 m² ha-1 (217.5 t ha-1) to 22.3 m² ha-1 (273.3 t ha-1). While shallow and hydromorphic soils support forests with both low stem basal area and low biomass, forests on deep resource-poor soils are typically low in basal area but as high in biomass as forests on deep resource-rich soils. We demonstrated that the environmental filtering of slow growing dense-wooded species on resource-poor soils compensates for the low basal area, and we discuss whether this filtering effect is due to low fertility or to low water reserve. 4. Synthesis. We showed that soil physical conditions constrained the amount of biomass stored in tropical moist forests. Contrary to previous reports, our results suggest that biomass is similar on resource-poor and resource-rich soils. This finding highlights both the importance of taking into account soil characteristics and species wood density when trying to predict regional patterns of biomass. Our findings have implications for the evaluation of biomass stocks in tropical forests, in the context of the international negotiations on climate change. [less ▲]

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See detailDetecting large-scale diversity patterns in tropical trees: can we trust commercial forest inventories?
Réjou-Méchain, Maxime; Fayolle, Adeline ULg; Nasi, Robert et al

in Forest Ecology & Management (2011), 261(2), 187-194

In this paper we seek to identify the floristic determination biases contained in large-scale commercial inventories conducted by logging companies and to determine whether this impacts on the observed ... [more ▼]

In this paper we seek to identify the floristic determination biases contained in large-scale commercial inventories conducted by logging companies and to determine whether this impacts on the observed patterns of alpha and beta diversity. The study focused on floristic data recently collected by industrial timber companies in the tropical forests of the Central African Republic (28,229 0.5-ha plots spread over 14,000km2). A subset of these plots (n = 1107) was later re-sampled for controlling purposes by experienced botanists. The proportion of agreement between the two samplings was assessed for each species and independently for small and large trees, and at genus and family resolutions. Unsurprisingly, large trees and common species were more accurately identified than small trees and rare species. We found that the quality of the floristic determination increased slightly from species to families. We also detected a significant variation between concessions in the quality of the floristic determination that was more dependent on working conditions during forest inventories than on field workers. Contrary to a widespread belief, we did not find a strong bias toward commercial species, showing that commercial inventory data could also be valid for non-commercial species in ecological studies. Finally, we found that both alpha and beta diversity patterns in commercial inventories were highly consistent with those of the re-sampled inventory. This latter result shows that commercial inventories are well suited to detect large-scale patterns of floristic variation. Large-scale commercial inventories could thus play an important role in the identification of large-scale patterns in tropical tree diversity. This could enhance our ability to manage tropical forests by designing representative reserve networks and developing management plans that integrate diversity patterns at the landscape scale. [less ▲]

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See detailSpatial aggregation of tropical trees at multiple spatial scales
Réjou-Méchain, Maxime; Flores, Olivier; Bourland, Nils ULg et al

in Journal of Ecology (2011), 99

1. In tropical forests, species distribution patterns may be strongly context-dependent owing to local stochasticity of recruitment and⁄ or to the specific history and environment of each site. Recent ... [more ▼]

1. In tropical forests, species distribution patterns may be strongly context-dependent owing to local stochasticity of recruitment and⁄ or to the specific history and environment of each site. Recent studies have reported, however, that the degree of spatial aggregation of tropical tree species is partly determined by some species traits irrespectively of site conditions, at least at a very local scale (<200 m). 2. Here, we used standardized large-scale forest inventories of five Central African tropical forests (9670 0.5-ha plots spread over 5550 km2) to quantify the spatial aggregation of 106 tropical tree species at larger spatial scales. For this purpose, we developed a new statistic to quantify the respective contributions of different spatial scales to the aggregation patterns, and we tested whether patterns were consistent across sites. We finally asked whether species characteristics related to dispersal ability, to response to disturbances and to biogeographical range could significantly explain aggregation patterns. 3. Although aggregation patterns varied substantially among sites within each species, they displayed inter-site consistencies (21–24%of the total variance explained by species identity) at the local scale (0.2–1 km) and at the mesoscale (1–10 km) but not at the landscape scale (>10 km). At the two former scales, upper taxonomical levels (family and⁄or order) significantly explained variation in the degree of species aggregation, while at the landscape scale, aggregation was entirely contingent on the site considered. Few species characteristics, except dispersal syndromes and wood density, were able to significantly explain aggregation patterns. 4. Synthesis. One of our most striking results is the high context dependence of species aggregation patterns, whatever the spatial scale considered. However, we showed that species distribution patterns can be predicted, to an extent, at spatial scales much larger than previously investigated in this context. Such patterns may be explained by traits displaying phylogenetic conservatism [less ▲]

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See detailPeut-on expliquer les patrons d’agrégation spécifiques d’arbres tropicaux à partir de leurs traits ? Une analyse de l’échelle locale à l’échelle régionale.
Rejou-Mechain, Maxime; Bourland, Nils ULg; Flores, O. et al

in Laboratoire d'Ecologie de la FUSAGx (Ed.) ECOVEG 5 - Cinquième colloque d'écologie des communautés végétales (2009, April 08)

Les processus qui génèrent la répartition spatiale des arbres tropicaux ont fait l’objet de nombreuses études. Trois processus semblent déterminants pour la répartition des végétaux : la dispersion ... [more ▼]

Les processus qui génèrent la répartition spatiale des arbres tropicaux ont fait l’objet de nombreuses études. Trois processus semblent déterminants pour la répartition des végétaux : la dispersion limitée, l’histoire de la zone (perturbations anthropiques et histoire climatique) et la différenciation de niche. Leur importance relative est aujourd’hui largement débattue et dépend certainement de l’échelle spatiale considérée. Dans cette étude notre objectif général est de comprendre quels sont les principaux mécanismes qui génèrent les patrons spatiaux des espèces et à quelles échelles ils interviennent. Les données traitées proviennent d’inventaires botaniques réalisés à grande échelle (460000 ha) par des sociétés forestières, en Afrique Centrale (Cameroun et République Centrafricaine). Des mesures d’agrégation spatiale sont réalisées sur 121 espèces d’arbre (individus > à 30 cm de diamètre), dans 4 sites différents et à 3 échelles spatiales : l’échelle locale (0-1 km), l’échelle intermédiaire (1-10 km) et l’échelle du paysage (> à 10 km). Nous testons la relation entre l’agrégation spatiale spécifique et une série de caractéristiques propres aux espèces : le type de diaspore, la tolérance à l’ombre, le type biologique, le système de reproduction et le type phytogéographique. Nos résultats montrent que les patrons d’agrégation spécifiques sont cohérents à travers les sites, jusqu’à l’échelle intermédiaire, mais que les caractéristiques propres aux espèces permettent peu de les prédire. La variation de l’agrégation entre sites pour une même espèce est toutefois très importante et suggère que les patrons d’agrégation observés sont en grande partie dépendants des sites et de leur histoire. Nos résultats suggèrent que les patrons spatiaux spécifiques sont liés à une dispersion limitée à une échelle très locale alors que les patterns observés à des échelles supérieures sont principalement dus à l’histoire des sites et dans certains cas à une forte hétérogénéité environnementale. [less ▲]

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