References of "Fayolle, Adeline"
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See detailHow tree architecture varies across coexisting tropical tree species and relates to functional traits?
Loubota Panzou, Grâce Jopaul ULg; Ligot, Gauthier ULg; Loumeto, Jean-Joël et al

Conference (2017, February 06)

Architecture refers to the overall shape of a tree and the spatial position of its components. Tree height determines the position in the forest canopy and access to light, while the amount and spatial ... [more ▼]

Architecture refers to the overall shape of a tree and the spatial position of its components. Tree height determines the position in the forest canopy and access to light, while the amount and spatial distribution of the foliage depend on the depth and the width of the crown. The aim of this study is to understand how tree architecture varies across coexisting tropical tree species and relates to functional traits. Forty five coexisting tree species were sampled in the semi-deciduous forests of Northern Congo. Species were classified according to ecological strategies, specifically regeneration guilds: shade bearers (27 species), non-pioneers light demanding (14 species) and pioneers (4 species). For each species, 14–72 trees (968 trees in total) were measured over a large range of diameter (10–162 cm). At the tree level, we measured the diameter (D in cm), height (H in m), crown radius (Cr in m) and crown depth (Cd in m) and crown exposure index (CEI) was visually estimated. At species level, architectural traits (Dmax, Hmax, Crmax and Cdmax), life history traits (dispersal mode, phenology and guild) and functional traits (wood density and light requirement) were obtained. We investigates the H-D, Cr-D and Cd-D allometric relationships at the tree level using linear mixed models on log-transformed data with species as a random effect on both slope and intercept. We used the multivariate analysis to quantify the relationship between architectural, functional traits and life history traits. Based on AIC, we found that the best linear mixed model was the one with two species random parameters (intercept and slope) for H-D and Cr-D allometries, while the best model for Cd-D allometry was the one with only a random intercept. Thus, our results showed a significant variation in tree allometry between coexisting species. The interspecific variation in H-D allometry was related to light requirement while Cr-D and Cd-D allometries were more related to dispersal mode and wood density, respectively. The confirmed the existence of three ecological strategies (shade bearers, non-pioneers light demanding and pioneers) in tropical forests, specifically in Central Africa. Architectural traits were the main traits that differentiate between ecological strategies. Architectural traits are therefore strong predictors of ecological strategies of coexisting tropical tree species. [less ▲]

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See detailPresent-day central African forest is a legacy of the 19th century human history
Morin, Julie ULg; Fayolle, Adeline ULg; Favier, Charly et al

in eLife (2017)

The populations of light-demanding trees that dominate the canopy of central African forests are now aging. Here, we show that the lack of regeneration of these populations began ca. 165 ya (around 1850 ... [more ▼]

The populations of light-demanding trees that dominate the canopy of central African forests are now aging. Here, we show that the lack of regeneration of these populations began ca. 165 ya (around 1850) after major anthropogenic disturbances ceased. Since 1885, less itinerancy and disturbance in the forest has occurred because the colonial administrations concentrated people and villages along the primary communication axes. Local populations formerly gardened the forest by creating scattered openings, which were sufficiently large for the establishment of light-demanding trees. Currently, common logging operations do not create suitable openings for the regeneration of these species, whereas deforestation degrades landscapes. Using an interdisciplinary approach, which included paleoecological, archaeological, historical, and dendrological data, we highlight the long-term history of human activities across central African forests and assess the contribution of these activities to present-day forest structure and composition. The conclusions of this sobering analysis present challenges to current silvicultural practices and to those of the future. [less ▲]

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See detailThe influence of spatially structured soil properties on tree community assemblages at a landscape scale in the tropical forests of southern Cameroon
Vleminckx, Jason; Doucet, Jean-Louis ULg; Morin, Julie ULg et al

in Journal of Ecology (2016)

Species distribution within plant communities results from both the influence of deterministic processes, related to environmental conditions, and neutral processes related to dispersal limitation and ... [more ▼]

Species distribution within plant communities results from both the influence of deterministic processes, related to environmental conditions, and neutral processes related to dispersal limitation and stochastic events, the relative importance of each factor depending on the observation scale. Assessing the relative contribution of environment necessitates controlling for spatial dependences among data points. Recent methods, combining multiple regression and Moran's eigenvectors maps (MEM), have been proved successful in disentangling the influence of pure spatial processes related to dispersal limitation, pure environmental variables (not spatially structured) and spatially structured environmental properties. However, the latter influence is usually not testable when using advanced spatial models like MEM. To overcome this issue, we propose an original approach, based on torus-translations and Moran spectral randomizations, to test the fraction of species abundance variation that is jointly explained by space and seven soil variables, using three environmental and tree species abundance data sets (consisting of 120, 52 and 34 plots of 0·2 ha each, located along 101-, 66- and 35-km-long transect-like inventories, respectively) collected in tropical moist forests in southern Cameroon. The overall abundance of species represented by ≥30 individuals, and 27% of these species taken individually, were significantly explained by fine-scale (<5 km) and/or broad-scale (5–100 km) spatially structured variations in soil nutrient concentrations (essentially the concentration of available Mn, Mg and Ca) along the 120-plots area. The number of significant tests considerably decreased when investigating the two smaller data sets, which mostly resulted from low statistical power rather than weaker floristic and/or edaphic variation captured among plots. Synthesis. Our results provide evidence that tree species turnovers are partly controlled by spatially structured concentrations in soil nutrients at scales ranging from few hundreds of metres to c. 100 km, a poorly documented subject in Central African forests. We also highlight the usefulness of our testing procedure to correctly interpret the space-soil fraction of variation partitioning analyses (which always accounted here for the most important part of the soil contribution), as this fraction was sometimes relatively high (R2 values up to c. 0·3) but nearly or not significant. [less ▲]

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See detailHow tree architecture varies across coexisting tropical tree species and relate to ecological strategies?
Loubota Panzou, Grâce Jopaul ULg; Ligot, Gauthier ULg; Loumeto, Jean-Joël et al

Conference (2016, December 14)

Architecture refers to the overall shape of a tree and the spatial position of its components. Tree height determines the position in the forest canopy and access to light, while the amount and spatial ... [more ▼]

Architecture refers to the overall shape of a tree and the spatial position of its components. Tree height determines the position in the forest canopy and access to light, while the amount and spatial distribution of the foliage depend on the depth and the width of the crown. The aim of this study is to understand how tree architecture varies across coexisting tropical tree species and relates to ecological strategies. Forty five coexisting tree species were sampled in the semi-deciduous forests of Northern Congo. Species were classified according to ecological strategies, specifically regeneration guilds: shade bearers (27 species), non-pioneers light demanding (14 species) and pioneers (4 species). For each species, 14–72 trees (968 trees in total) were measured over a large range of diameter (10–162 cm). At the tree level, we measured the diameter (D in cm), height (H in m), crown radius (Cr in m) and crown depth (Cd in m) and crown exposure index (CEI) was visually estimated. At species level, architectural traits were estimated at juvenile tree with diameter of 10 cm (H10, Cr10 and Cd10) and at adult stature with maximum diameter (Hmax, Crmax and Cdmax), life history traits (dispersal mode, phenology and regeneration guild) and functional traits (wood density and light requirement) were obtained. Our results showed a significant variation in tree allometry between coexisting species. The interspecific variation was related to light requirement (H-D allometry), dispersal mode (Cr-D allometry) and liana infestation (Cd-D allometry). Large-statured tree species were light demanding, deciduous and wind dispersed, while small-statured tree species were evergreen, dense wooded, and animal dispersed. Architectural traits strongly differed between regeneration guilds. [less ▲]

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See detailBiomasse et stocks de carbone des forêts tropicales africaines (synthèse bibliographique)
Loubota Panzou, Grâce Jopaul ULg; Doucet, Jean-Louis ULg; Loumeto, Jean-Joël et al

in Biotechnologie, Agronomie, Société et Environnement = Biotechnology, Agronomy, Society and Environment (2016), 20(4), 508-522

Introduction. Quantifying the biomass and carbon stocks contained in tropical forests has become an international priority for the implementation of the REDD+ mechanism. Forest biomass is estimated at ... [more ▼]

Introduction. Quantifying the biomass and carbon stocks contained in tropical forests has become an international priority for the implementation of the REDD+ mechanism. Forest biomass is estimated at three successive levels: the tree, the stand and the region level. This paper reviews the state of the art regarding the estimation of biomass and carbon stocks in tropical African forests. Literature. This review highlights the fact that very few allometric equations, equations used for estimating the biomass of the tree using non-destructive measurements (diameter, height), have been established for tropical African forests. At the stand level, the review highlights the spatial and temporal variations in biomass between forest types in Central and Eastern Africa. While biomass recovery after a disturbance (logging, for instance) is rather quick, a great deal of uncertainty still remains regarding the spatial variation in biomass, and there is no consensus on a regional biomass map. The quality of biomass mapping in tropical Africa strongly depends on the type of remotely-sensed data being used (optical, RADAR or LIDAR), and the allometric equation used to convert forest inventory data into biomass. Conclusions. Based on the lack of precision of the available allometric equations and forest inventory data and the large spatial scale involved, many uncertainties persist in relation to the estimation of the biomass and carbon stocks contained in African tropical forests. [less ▲]

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See detailTerrestrial photogrammetry: a non-destructive method for modelling irregularly shaped tropical tree trunks
Bauwens, Sébastien ULg; Lejeune, Philippe ULg; Fayolle, Adeline ULg

in Methods in Ecology and Evolution (2016)

1. Irregularly shaped trees including trees with buttresses, flutes or stilt roots are frequent in tropical forests. The lack of an international standard tomeasure the diameter of such trees leads to ... [more ▼]

1. Irregularly shaped trees including trees with buttresses, flutes or stilt roots are frequent in tropical forests. The lack of an international standard tomeasure the diameter of such trees leads to high uncertainties in biomass estimation, tree growth and carbon budget monitoring. 2. In this study, we developed a new method based on terrestrial close-range photogrammetry for measuring andmodelling irregular stems. This approach is cheap and easy to implement in the field as it only requires a camera and a graduated rod. We validated the approach with destructive cross-sectionmeasurements along the stem of three buttressed trees. To demonstrate the broader utility of this method, we extended the validated approach to 43 additional trees belonging to two species: Celtis mildbraedii (Ulmaceae) and Entandophragma cylindricum (Meliaceae). Based on the three dimensional models, we computed shape indices for each tree, and we analysed the stem morphology of the two species. Finally, we analysed some standardized predictors for the estimation of above-ground biomass. 3. We found a high concordance between diameters derived from the photogrammetric process and destructive diameter measurements along the stem for the three calibration trees. We found that C. mildbraedii develop much stronger irregularities than E. cylindricum.We also identified a large intraspecific variation in trunk morphology for E. cylindricum. The basal area at 1 3 mheight (Darea130) seems to be amore robust predictor for biomass estimates (lowest Akaike information criterion and relative squared error) than diameter measured above buttresses (DAB) or diameter at breast height estimated from available taper model. Finally, Darea130 might be estimated with a good precision [root mean square error (RMSE) < 5%] with linear model based on the field measurements DABand the perimeter of the convex hull of the buttresses at 1 3 mheight (Dconvhull130). 4. In this study, we showed the high potential of the photogrammetry for measuring and modelling irregular stems. Photogrammetry could then be used as a non-destructivemeasurement tool to produce correction factors for standardizing the diameter of irregular stems at a reference height which is a key issue in tree growthmonitoring and biomass change estimation. [less ▲]

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See detailHautes Valeurs de Conservation (HVC) dans les Unités Forestières d'Aménagement du Cameroun : concepts, choix et pratiques
Daïnou, Kasso ULg; Bracke, Charles; Vermeulen, Cédric ULg et al

Book published by Presses Agronomiques de Gembloux (2016)

Le système de certification FSC (Forest Stewardship Council) vise à promouvoir la gestion durable des forêts. Il repose sur un ensemble de normes dont une est particulièrement complexe à mettre en oeuvre ... [more ▼]

Le système de certification FSC (Forest Stewardship Council) vise à promouvoir la gestion durable des forêts. Il repose sur un ensemble de normes dont une est particulièrement complexe à mettre en oeuvre en Afrique Centrale, le principe 9 traitant des Hautes Valeurs de Conservation (HVC). Ce principe devrait être interprété aux échelons nationaux afin de prendre en compte les spécificités de chaque pays. Bien que des ouvrages aient déjà été élaborés par diverses organisations, aucun ne cible particulièrement les grandes concessions forestières. Au Cameroun, ces concessions ou Unités Forestières d’Aménagement (UFA), représentent pourtant 40 % du domaine forestier national. Le présent guide ambitionne de fournir aux acteurs de la gestion forestière au Cameroun les connaissances les plus pertinentes afin de leur permettre d’identifier, de gérer et de suivre les Hautes Valeurs de Conservation dans les UFA. Il se démarque des précédents guides par plusieurs points : (i) une revue bibliographique détaillée est fournie sur le sujet épineux de l’identification de chaque HVC, et l’opinion des auteurs y est mise en exergue; (ii) la démarche d’identification est appuyée par les références les plus pertinentes, évitant au gestionnaire de se disperser dans sa quête de documentation; (iii) sur la base de leur expérience, les auteurs proposent une série de menaces pouvant affecter les HVC, de mesures de gestion et d’indicateurs de suivi. L’approche développée se base sur des méthodes empiriques et pragmatiques d’une part et, d’autre part, sur des études scientifiques. Cet ouvrage devrait constituer une base intéressante pour une interprétation solide des HVC au Cameroun. De plus, bien que ciblant les UFA camerounaises, il pourrait inspirer d’autres acteurs forestiers œuvrant dans le Bassin du Congo. [less ▲]

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See detailUsing herbarium records to explore the ecological differentiation between closely‐related tree species in tropical Africa
Gorel, Anaïs ULg; Duminil; Doucet, Jean-Louis ULg et al

Poster (2016, June 20)

Background: Tree hypothesis are invoked to explain species distribution and evolutionary history of tree clades in tropical Africa: 1) The forest refuge hypothesis postulates that contractions of lowland ... [more ▼]

Background: Tree hypothesis are invoked to explain species distribution and evolutionary history of tree clades in tropical Africa: 1) The forest refuge hypothesis postulates that contractions of lowland forests during the climatic oscillations of the Pleistocene could have driven allopatric speciation between fragmented populations; 2) The ecological gradient hypothesis states that environmental gradients promote parapatric speciation; 3) The vanishing refuge hypothesis reconciles the two previous hypotheses and postulated a diversification process through climate-driven habitat fragmentation and exposure to new environments. Disentangling the respective influence of environmental and historical factors requires information on phylogeny, as well as information on geography and the environmental space used by species. In this study, we aimed to determine the environmental factors constraining the distribution of African tree species in order to explore ecological divergence and speciation processes. Method: We focused on three African Erythrophleum species (Fabaceae, Caesalpinioideae) that are economically and socially important, providing timber and non-timber resources. Erythrophleum ivorense, Erythrophleum suaveolens and Erythrophleum africanum also show contrasted distributions in Africa. To determine species climatic niche, we used a combination of species presence data gathered from 606 herbarium records and environmental factors (19 BIOCLIM variables). We used Species Distribution Models (SDM, MaxEnt algorithm) in combination with similarity metrics to quantify the degree of niche divergence between species. Results: We showed that the distribution of Erythrophleum species are substantially determined by climate (especially annual rainfall and temperature range) and support the ecological gradient hypothesis. Moreover, the main traits (e.g. wood density and leaf area) and growth rates previously reported among Erythrophleum species confirmed a differential adaptation to drought. Conclusion: Herbarium data provide valuable information on the distribution of species over the whole range. In tropical regions where extensive inventories data are extremely rare, herbarium records in combination with presence-only SDM offer opportunities to explore speciation processes. [less ▲]

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See detailThe determinants of tropical forest deciduousness: disentangling the effects of rainfall and geology in central Africa
Ouedraogo, Dakis-Yaoba ULg; Fayolle, Adeline ULg; Gourlet-Fleury, Sylvie et al

in Journal of Ecology (2016)

1. Understanding the environmental determinants of forests deciduousness i.e. proportion of deciduous trees in a forest stand, is of great importance when predicting the impact of ongoing global climate ... [more ▼]

1. Understanding the environmental determinants of forests deciduousness i.e. proportion of deciduous trees in a forest stand, is of great importance when predicting the impact of ongoing global climate change on forests. In this study, we examine (i) how forest deciduousness varies in relation to rainfall and geology, and (ii) whether the influence of geology on deciduousness could be related to differences in soil fertility and water content between geological substrates. 2. The study was conducted in mixed moist semi-deciduous forests in the northern part of the Congo basin. We modelled the response of forest deciduousness to the severity of the dry season across four contrasting geological substrates (sandstone, alluvium, metamorphic and basic rocks). For this, we combined information on forest composition at genus level based on commercial forest inventories (62 624 0.5 ha plots scattered over 6 million of ha), leaf habit, and rainfall and geological maps. We further examined whether substrates differ in soil fertility and water-holding capacity using soil data from 37 pits in an area that was, at the time, relatively unexplored. 3. Forest deciduousness increased with the severity of the dry season, and this increase strongly varied with the geological substrate. Geology was found to be three times more important than the rainfall regime in explaining the total variation in deciduousness. The four substrates differed in soil properties, with higher fertility and water-holding capacity on metamorphic and basic rocks than on sandstone and alluvium. The increase in forest deciduousness was stronger on the substrates that formed resource-rich clay soils (metamorphic and basic rocks) than on substrates that formed resource-poor sandy soils (sandstone and alluvium). 4. Synthesis. We found evidence that tropical forest deciduousness is the result of both the competitive advantage of deciduous species in climates with high rainfall seasonality, and the persistence of evergreen species on resource-poor soils. Our findings offer a clear illustration of wellknown theoretical leaf carbon economy models, explaining the patterns in the dominance of evergreen versus deciduous species. And, this large-scale assessment of the interaction between climate and geology in determining forest deciduousness may help to improve future predictions of vegetation distribution under climate change scenarios. In central Africa, forest is likely to respond differently to variation in rainfall and/or evapotranspiration depending on the geological substrate. [less ▲]

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See detailEnrichment of Central African logged forests with high-value tree species: testing a new approach to regenerating degraded forests
Doucet, Jean-Louis ULg; Daïnou, Kasso ULg; Ligot, Gauthier ULg et al

in International Journal of Biodiversity Science, Ecosystem Services and Management (2016)

In natural forests of Central Africa, several studies indicate a dramatic decrease in commercial trees, including species of concern for conservation. Enrichment planting with these species will favor ... [more ▼]

In natural forests of Central Africa, several studies indicate a dramatic decrease in commercial trees, including species of concern for conservation. Enrichment planting with these species will favor both the long-term recovery of their populations and biodiversity conservation in logged forests. In this study, we analyzed the survival and growth of 23 species in plantations. Fourteen 0.2–1.1 ha mixed species plantations consisting of single-species 15 × 15 m blocks were studied for 5 years in a logging concession of southeastern Cameroon. The plantation design considered both species light requirements and sensitivity to damage by pests. To identify the best species for enrichment planting, we assessed both species performance and plantation costs. We also tested for relationships between species traits and species performance. Mean annual diameter growth increments ranged from 1.67 to 42.9 mm. No significant relationship was found between growth and survival. Herbivory by wild Bovidae was the main cause of mortality and should be carefully considered in rehabilitation efforts. We found a significant negative relationship between wood density and maximum growth rate. The other traits tested were not good predictors of species performance in plantations. The two best-performing species, Triplochiton scleroxylon and Terminalia superba, could reach the minimum cutting diameter during a 30-year cutting cycle. Costs were high and mechanized site preparation is suggested to reduce them. Widespread adoption of such plantations will only occur if financial incentives or national regulations for assuring regeneration are implemented. [less ▲]

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See detailTaller trees, denser stands and greater biomass in semi-deciduous than in evergreen lowland central African forests
Fayolle, Adeline ULg; Loubota Panzou, Grâce Jopaul ULg; Drouet, Thomas et al

in Forest Ecology & Management (2016), 374

Accurate height-diameter allometry is crucial for the estimation of forest biomass and carbon stocks. Tree height measurements over a large range of diameters and species are urgently needed in the ... [more ▼]

Accurate height-diameter allometry is crucial for the estimation of forest biomass and carbon stocks. Tree height measurements over a large range of diameters and species are urgently needed in the tropics, specifically in central Africa, for the development of locally derived height-diameter allometric equations and the conversion of forest inventory data into biomass estimates, and for the validation of remotely sensed canopy height that mostly rely on a few specific field sites. In this study, we aimed to identify the variation in height-diameter allometry of tropical trees between forest types and among species in central Africa, and we examined the consequences for biomass estimation. Height and diameter were measured for a total of 521 trees over a large range of diameters in two forest types in southern Cameroon, 10–240 cm in the evergreen forest and 11–182 cm in the semi-deciduous forest. A total of ten allometric models including asymptotic and non-asymptotic models were fitted to the heightdiameter data. Measured tree diameters, grouped into 10 cm wide diameter classes up to 150, from commercial forest inventory data (0.5 ha plots, n = 2101 and n = 5152, respectively in the evergreen and in the semi-deciduous forests) were converted into biomass estimates using general allometric models with and without including our site-specific height-diameter allometry. Though debated in the literature, our results supported a saturation of tree height with tree diameter both at site and species level, with asymptotic models better depicting the height-diameter allometry. Height-diameter allometry significantly differed between forest types and these local height-diameter equations also differed from published equations. For a given diameter, trees tended to be taller in the semi-deciduous forest than in the evergreen forest, as already reported between moist and wet forests in pantropical studies. Similar trends were reported within species for the three species shared by both forest types, suggesting an environmental control of tree allometry. Because of the low performance of the bioclimatic stress variable to predict tree height and of the slight soil differences between the two forest types, the environmental determinants of height-diameter allometry remain to be explored. In addition to tree allometry variation, structural differences (basal area and density) were also identified between the two forest types using commercial forest inventory data at genus level, and both allometry and forest structure (taller trees and denser stands) contributed to the greater biomass per hectare of the semi-deciduous forest. [less ▲]

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See detailL’aménagement forestier au Congo engendre-t-il plus de déforestation ?
Karsenty, Alain; Cerutti, P.; Doucet, Jean-Louis ULg et al

E-print/Working paper (2016)

Un article publié dans Land Use Policy début 2016 arrive à la conclusion a priori étonnante que la déforestation serait, au Congo, plus élevée dans les concessions forestières avec des plans d’aménagement ... [more ▼]

Un article publié dans Land Use Policy début 2016 arrive à la conclusion a priori étonnante que la déforestation serait, au Congo, plus élevée dans les concessions forestières avec des plans d’aménagement que dans celles qui n’en ont pas. L’analyse d’évaluation d’impact qui a conduit ces chercheurs à un tel résultat se base sur un appariement de parcelles sélectionnées aléatoirement dans des concessions avec et sans plans d’aménagement. Ces chercheurs indiquent que le réseau de routes forestières plus développé dans les concessions aménagées serait un des facteurs explicatifs. L’autre facteur serait le développement local lié aux cahiers des charges des plans d’aménagement, lequel conduirait à une augmentation de la population dans ces concessions et à une déforestation accrue. Notre groupe d’une vingtaine de chercheurs connaissant bien la problématique de l’aménagement forestier en Afrique centrale s’est penché à son tour sur cette question et a analysé la déforestation au niveau des concessions sur le même intervalle de temps. Nos résultats montrent, cette fois, que la déforestation est moins importante dans les concessions avec un plan d’aménagement que dans les autres. Et si l’on compare à production égale la déforestation dans des concessions avec et sans plan d’aménagement, il apparaît que les UFA aménagées sont environ deux fois plus « efficaces », c’est-à-dire qu’on observe deux fois moins de perte de couvert forestier par mètre-cube produit. Nous en concluons qu’il est nécessaire d’analyser précisément la dynamique des différents facteurs de déforestation, et éviter d’imputer mécaniquement à l’aménagement forestier un rôle excessif dans l’évolution dans un sens ou dans l’autre du taux de déboisement. Enfin, toute évaluation doit rappeler que les effets de l’aménagement forestier doivent être mesurés sur le long terme : l’objectif de l’aménagement est de permettre une mise en valeur forestière durable, en conservant l’essentiel du capital productif pour éviter, autant que possible, la conversion à d’autres usages après les cycles de coupe initiaux. [less ▲]

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See detailError in the estimation of emission factors for forest degradation in central Africa
Picard, N.; Henry, M.; Fonton, N.H. et al

in Journal of Forest Research (2016)

The implementation of forest-based projects to mitigate greenhouse gas emissions requires the estimation of emission factors (here the difference in biomass stocks between two forest types). The ... [more ▼]

The implementation of forest-based projects to mitigate greenhouse gas emissions requires the estimation of emission factors (here the difference in biomass stocks between two forest types). The estimation of these quan- tities using forest inventory data and allometric models implies different sources of errors that need to be prioritized to improve the precision of estimation. Using data from permanent sample plots in a tropical moist forest in central Africa and considering four allometric models with equal likelihood, the largest source of error in the estimate of the difference of biomass between intact and logged-over forest was that due to the model choice (40 % of the sum of squares). The error due to the model choice did not cancel out in the difference due to an interaction between the model’s prediction and the diameter structure of the forest. The variability in biomass between plots was the second largest source of error, but was underestimated because of post-stratification. The error due to the model choice could be reduced by weighting the models’predictions. [less ▲]

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See detailTerrestrial photogrammetry: A non-destructive method for modeling irregularly shaped tropical tree trunks
Bauwens, Sébastien ULg; Fayolle, Adeline ULg; Gourlet-Fleury, Sylvie et al

Computer development (2016)

1. Irregularly shaped trees including trees with buttresses, flutes or stilt roots are frequent in tropical forests. The lack of an international standard to measure the diameter of such trees leads to ... [more ▼]

1. Irregularly shaped trees including trees with buttresses, flutes or stilt roots are frequent in tropical forests. The lack of an international standard to measure the diameter of such trees leads to high uncertainties in biomass estimation, tree growth and carbon budget monitoring. 2. In this study, we developed a new method based on terrestrial close range photogrammetry for measuring and modeling irregular stems. This approach is cheap and easy to implement in the field as it only requires a camera and a graduated rod. We validated the approach with destructive cross section measurements along the stem of three buttressed trees. To demonstrate the broader utility of this method, we extended the validated approach to 43 additional trees belonging to two species: Celtis mildbraedii (Ulmaceae) and Entandophragma cylindricum (Meliaceae). Based on the 3D models, we computed shape indices for each tree, and we analyzed the stem morphology of the two species. Finally, we analyzed some standardized predictors for the estimation of above-ground biomass. 3. We found a high concordance between diameters derived from the photogrammetric process and destructive diameter measurements along the stem for the three calibration trees. We found that C. mildbraedii develop much stronger irregularities than E. cylindricum. We also identified a large intra-specific variation in trunk morphology for E. cylindricum. The basal area at 1.3m height (Darea130), seems to be a more robust predictor for biomass estimates (lowest AIC and RSE) than diameter measured above buttresses (DAB) or DBH estimated from available taper model. Finally, Darea130 might be estimated with a good precision (RMSE < 5 %) with linear model based on the field measurements DAB and the perimeter of the convex hull of the buttresses at 1.3 m height (Dconvhull130). 4. In this study, we showed the high potential of the photogrammetry for measuring and modeling irregular stems. Photogrammetry could then be used as a non destructive measurement tool to produce correction factors for standardizing the diameter of irregular stems at a reference height which is a key issue in tree growth monitoring and biomass change estimation. [less ▲]

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See detailClosing a gap in tropical forest biomass estimation : taking crown mass variation into account in pantropical allometries
Ploton, Pierre; Barbier, Nicolas; Takoudjou Momo, Stéphane et al

in Biogeosciences (2016), 13

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 ... [more ▼]

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 < 10Mg (mean of 34 %) but, above this threshold, increased sharply with tree mass and exceeded 50% on average for trees _45 Mg. This increase coincided with a progressive deviation between the pantropical biomass model estimations and actual tree mass. Taking a crown mass proxy into account in a newly developed model consistently removed the bias observed for large 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. [less ▲]

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See detailBiomass and carbon stocks of tropical African forests: synthesis and perspectives
Loubota Panzou, Grâce Jopaul ULg; Doucet, Jean-Louis ULg; Loumeto, Jean-Joël et al

Poster (2015, November 26)

Quantifying the biomass and carbon stocks contained in tropical forests has become an international priority for the implementation of the REDD+ mechanism. Forest biomass is estimated through three ... [more ▼]

Quantifying the biomass and carbon stocks contained in tropical forests has become an international priority for the implementation of the REDD+ mechanism. Forest biomass is estimated through three successive levels: the tree, the stand and the region level. This study reviews the state of the art on the estimation of biomass and carbon stocks contained in tropical African forests. This review highlights that only few allometric equations, equations used for estimating biomass of the tree from non-destructive measurements (diameter, height), have been established for tropical African forests. At the stand level, this synthesis highlights the spatial and temporal variations in biomass between forests types in Central and Eastern Africa. While biomass recovery after disturbance (logging for instance) is rather quick, there is still a lot of uncertainity on the spatial variation in biomass, and there is no consensus on a regional biomass map. The quality of biomass mapping in tropical Africa stronly depends on the various sensors used (optical, RADAR or LIDAR), the allometric equation used to convert forest inventory data and sampling design. Based on the lack of precision of available allometric equations and forest inventory data a large spatial scale, many uncertainties persist on estimating the biomass and carbon stocks contained in the African tropical forests. It is important to develop reference sites (both allometry and forest inventory) to provide accurate biomass estimates for an effective implementation of the REDD+. [less ▲]

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See detailWood Specific Gravity Variations and Biomass of Central African Tree Species: The Simple Choice of the Outer Wood
Bastin, Jean-François ULg; Fayolle, Adeline ULg; Tarelkin, Yegor et al

in PLoS ONE (2015)

Context: Wood specific gravity is a key element in tropical forest ecology. It integrates many aspects of tree mechanical properties and functioning and is an important predictor of tree biomass. Wood ... [more ▼]

Context: Wood specific gravity is a key element in tropical forest ecology. It integrates many aspects of tree mechanical properties and functioning and is an important predictor of tree biomass. Wood specific gravity varies widely among and within species and also within individual trees. Notably, contrasted patterns of radial variation of wood specific gravity have been demonstrated and related to regeneration guilds (light demanding vs. shade-bearing). However, although being repeatedly invoked as a potential source of error when estimating the biomass of trees, both intraspecific and radial variations remain little studied. In this study we characterized detailed pith-to-bark wood specific gravity profiles among contrasted species prominently contributing to the biomass of the forest, i.e., the dominant species, and we quantified the consequences of such variations on the biomass. Methods: Radial profiles of wood density at 8% moisture content were compiled for 14 dominant species in the Democratic Republic of Congo, adapting a unique 3D X-ray scanning technique at very high spatial resolution on core samples. Mean wood density estimates were validated by water displacement measurements. Wood density profiles were converted to wood specific gravity and linear mixed models were used to decompose the radial variance. Potential errors in biomass estimation were assessed by comparing the biomass estimated from the wood specific gravity measured from pith-to-bark profiles, from global repositories, and from partial information (outer wood or inner wood). Results: Wood specific gravity profiles from pith-to-bark presented positive, neutral and negative trends. Positive trends mainly characterized light-demanding species, increasing up to 1.8 g.cm-3 per meter for Piptadeniastrum africanum, and negative trends characterized shade-bearing species, decreasing up to 1 g.cm-3 per meter for Strombosia pustulata. The linear mixed model showed the greater part of wood specific gravity variance was explained by species only (45%) followed by a redundant part between species and regeneration guilds (36%). Despite substantial variation in wood specific gravity profiles among species and regeneration guilds, we found that values from the outer wood were strongly correlated to values from the whole profile, without any significant bias. In addition, we found that wood specific gravity from the DRYAD global repository may strongly differ depending on the species (up to 40% for Dialium pachyphyllum). Main conclusion: Therefore, when estimating forest biomass in specific sites, we recommend the systematic collection of outer wood samples on dominant species. This should prevent the main errors in biomass estimations resulting from wood specific gravity and allow for the collection of new information to explore the intraspecific variation of mechanical properties of trees. [less ▲]

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See detailThe last 1,000 years in the Northern Congo Basin
Morin, Julie ULg; Fayolle, Adeline ULg; Favier, Charly et al

Conference (2015, November)

Review of the events that happened in the northern Congo basin during the last 1,000 yr. Positive impact of human disturbances on the regeneration of light-demanding trees. Negative impact of the European ... [more ▼]

Review of the events that happened in the northern Congo basin during the last 1,000 yr. Positive impact of human disturbances on the regeneration of light-demanding trees. Negative impact of the European colonization and following events on human populations and tree regeneration. [less ▲]

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See detailSeeing Central African forests through their largest trees
Bastin, Jean-François ULg; Barbier, Nicolas; Réjou-Méchain, Maxime et al

in Scientific Reports (2015)

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 ... [more ▼]

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. [less ▲]

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