Suchergebnisse

The Mampu agroforestry zone on the Batéké plateau in the Democratic Republic of Congo, which has been managed with Acacia auriculiformis shade trees for over twenty years by local communities, supplies subsistence products and fuel wood to Kinshasa. Thanks to international grant funding, this agroforestry system, which integrates traditional slashand- burn cultivation, has been replicated in many places across the RDC, but its performance has never been assessed. The aim of this study was to estimate Acacia auriculiformis production in terms of total biomass and usable biomass for charcoal (stems and branches more than 4 cm in diameter) as part of the agroforestry system. To do so, two local allometric equations for total and usable biomass were adjusted from destructive testing data. Using existing inventory data (n = 112 plots), we identified significant structural heterogeneity throughout the rotation period (8-10 years) but also among plots of the same age. Despite this heterogeneity, which may be accounted for by environmental conditions on site and/or by differences in the handling of plot management techniques, production is comparable to that observed at other sites, averaging 145 tonnes per hectare over 10 years. The Mampu agroforestry system has many advantages, including direct services creating rural employment and combined production of subsistence goods and charcoal, but also indirect services such as avoided deforestation and carbon sequestration. The system's sustainability and dissemination should nevertheless be discussed. ; Peer reviewed

Forest ecosystems play a major role in atmospheric carbon sequestration and emission. Comparable organic carbon stock estimates at temporal and spatial scales for all forest pools are needed for scientific investigations and political purposes. Therefore, we developed a new carbon stock (CS) estimation procedure that combines forest inventory and soil and litter geodatabases at a regional scale (southern Belgium). This procedure can be implemented in other regions and countries on condition that available external carbon soil and litter data can be linked to forest inventory plots. The presented procedure includes a specific CS estimation method for each of the following forest pools and subpools (in brackets): living biomass (aboveground and belowground), deadwood (dead trees and snags, coarse woody debris and stumps), litter, and soil. The total CS of the forest was estimated at 86 Tg (185 Mg ha-1). Soil up to 0.2 m depth, living biomass, litter, and deadwood CSs account, respectively, for 48, 47, 4, and 1 % of the total CS. The analysis of the CS variation within the pools across ecoregions and forest types revealed in particular that: (1) the living biomass CS of broadleaved forests exceeds that of coniferous forests, (2) the soil and litter CSs of coniferous forest exceed those of broadleaved forests, and (3) beech stands come at the top in carbon stocking capacity. Because our estimates differ sometimes significantly from the previous studies, we compared different methods and their impacts on the estimates. We demonstrated that estimates may vary highly, from -16 to ?12 %, depending on the selected methods. Methodological choices are thus essential especially for estimating CO2 fluxes by the stock change approach. The sources of error and the accuracy of the estimates were discussed extensively. ; Peer reviewed

Caractériser la dynamique d'une forêt est essentiel pour la gestion forestière. Les houppiers des arbres forment un élément clé de cette dynamique ; mais, en forêt tropicale, les mesurer n'est pas simple. Cette étude teste l'utilisation d'images aériennes à haute résolution pour estimer la croissance diamétrique des arbres, en intégrant des mesures fines des houppiers détectés. Des ortho-images de 10 cm/pixel de résolution ont été obtenues à l'aide d'un drone à aile fixe sur une parcelle de 9 ha, installée dans la forêt de Yoko en République démocratique du Congo. Les inventaires menés sur les arbres de DHP ≥ 10 cm en 2008 et en 2016 ont permis d'avoir accès à différentes caractéristiques dendrométriques individuelles, dont le diamètre des arbres et leur tempérament, et de calculer des accroissements diamétriques. Des modèles linéaires mixtes ont été calibrés pour prédire l'accroissement de 163 arbres identifiés à la fois sur le terrain et sur les ortho-images en utilisant les variables quantifiées uniquement sur le terrain et/ou à partir de variables mesurées sur les ortho-images. Les images aériennes ont permis de détecter 23,4 % des arbres de DHP ≥ 10 cm inventoriés au sol, et représentant 75,1 % de la biomasse aérienne du peuplement. La probabilité de détection des arbres a varié en fonction de leur DHP : de 0,09 pour les arbres de DHP < 30 cm à 0,97 pour les arbres de DHP ≥ 60 cm. Les variables quantifiées par télédétection ajoutées aux variables de terrain ont permis d'améliorer significativement la prédiction de l'accroissement diamétrique. Les meilleurs modèles d'estimation des accroissements diamétriques contiennent notamment un terme caractérisant la dimension du houppier des arbres qui n'a pu être mesuré que par télédétection. Parmi les variables déterminées par télédétection, la superficie convexe du houppier est apparue la plus performante dans les modèles, et s'avère ainsi être la mesure la plus intéressante pour décrire la compétition entre les houppiers. Ces résultats ouvrent des perspectives pour construire de nouveaux outils d'acquisition de données au service de l'aménagement forestier. ; Characterising forest dynamics of a forest is essential to its management. Tree crowns are a key factor in these dynamics, but measuring them in tropical forests is not an easy matter. This study tested the use of highresolution aerial imagery to estimate the tree diameter growth by incorporating detailed measurements of the detected tree crowns. Ortho-images at a resolution of 10 cm/pixel were captured by a fixed-wing drone over a 9 ha plot in the Yoko forest in the Democratic Republic of Congo. Inventories conducted on trees ≥ 10 cm diameter at breast height (DBH) in 2008 and 2016 provided access to a variety of tree dendrometric characteristics, including DBH and species temperament, and allowed the calculation of diameter increments. Mixed linear models were calibrated to predict diameter increment of 163 trees identified both on the ground and on the ortho-images, using variables quantified on the ground only and/or from variables measured from the orthoimages. From the aerial images, we were able to detect 23.4% of the trees with DBH ≥ 10 cm listed in the ground inventories, representing 75.1% of the stand's aerial biomass. The probability of detecting the trees varied with their DBH, from 0.09 for trees with DBH < 30 cm to 0.97 for trees with DBH ≥ 60 cm. Predictions of diametric growth improved significantly when the variables quantified by remote sensing were added to the ground variables. The best models for estimating diameter increment include, in particular, a term characterising the size of tree crowns, which can only be measured by remote sensing. Of the variables determined by remote sensing, convex crown area was the most successfull in the models and therefore appears to be the most accurate variable to describe competition between tree crowns. These results open up possibilities to build new tools of data acquisition to support forest planning. ; Peer reviewed

L'utilisation des tarifs de cubage fait partie du quotidien des aménagistes et gestionnaires forestiers. Toutefois, elle se fait généralement au mépris des conditions d'application (zone géographique, gamme de diamètres), ce qui peut engendrer des erreurs dans les estimations de volume. L'objectif de cette étude est de tester la validité des tarifs de cubage actuellement disponibles pour trois essences des forêts du sud-est du Cameroun, dont ceux utilisés par l'administration nationale et, le cas échéant, de proposer des tarifs adaptés pour la zone d'étude. Trois espèces commerciales sont concernées : le sapelli, Entandrophragma cylindricum, le tali, Erythrophleum suaveolens, et l'assaméla, Pericopsis elata. Les données dendrométriques ont été collectées par échantillonnage destructif de 43 arbres régulièrement répartis sur toute la gamme de diamètres. Le volume du fût a été calculé par la méthode des billons successifs et des tarifs de cubage linéaires et non linéaires ont été ajustés aux données par la méthode des moindres carrés pondérés. Les résultats montrent que le meilleur modèle de cubage est non linéaire pour les trois espèces. Les équations utilisées jusqu'à présent par l'administration forestière sous-estiment significativement le volume des arbres et les erreurs d'estimation sont d'autant plus grandes que la taille de l'arbre est importante. Les tarifs de cubage proposés, une fois validés, devraient pouvoir à l'avenir être utilisés pour une meilleure estimation du volume des arbres dans la zone d'étude. Considérant les enjeux internationaux auxquels adhère le Cameroun (FLEGT et REDD+), il est crucial de disposer d'outils performants d'estimation du volume des arbres. Dans ce contexte, il s'avère important que l'administration forestière camerounaise puisse mener à bien un vaste programme de révision des tarifs de cubage. ; Volume tables are routinely used by forest planners and managers, but generally with no regard for the conditions in which they are applied (geographical zone, diameter range), which can cause errors in estimations of timber volumes. The aim of this study is to test the validity of currently available volume tables, including those used by the government, for three commercial tree species in the forests of south-eastern Cameroon, and if required to propose tables suited to the study zone. The three species concerned are sapelli, Entandrophragma cylindricum, tali, Erythrophleum suaveolens, and assamela Pericopsis elata. Dendrometric data were collected by destructive sampling of 43 trees that were evenly distributed across the diameter range. Stemwood volume was calculated through successive butt-log measurements and linear and non-linear volume tables were adjusted by means of the weighted least-squares method. The results show that the best volume tables for all three species are non-linear. The equations used by the forests authority until now significantly under-estimate standing timber volumes and the estimation errors increase with the size of the trees. The proposed volume tables, once validated, should in future produce more accurate estimations of standing timber in the study zone. Given the international challenges involving Cameroon (FLEGT and REDD+), it is essential that it should have effective tools for estimating standing timber volumes. It is therefore important for Cameroon's forests authority to conduct a large-scale programme to revise its volume tables. ; La utilización de tablas de cubicación forma parte de las actividades cotidianas de los responsables de la ordenación y manejo forestal. No obstante, dichas tablas suelen emplearse sin tener en cuenta las condiciones de aplicación (zona geográfica, clases de diámetro), lo que puede provocar errores en las estimaciones volumétricas. El objetivo de este estudio es probar la validez de las tablas de cubicación actualmente disponibles para tres especies de los bosques del sudeste de Camerún, incluyendo las tablas empleadas por la administración nacional, y, llegado el caso, proponer tablas adaptadas al área estudiada. Las tres especies comerciales concernidas son: el sapelli, Entandrophragma cylindricum, el tali, Erythrophleum suaveolens, y la assamela, Pericopsis elata. La recogida de datos dasométricos se efectuó por muestreo destructivo de 43 árboles regularmente distribuidos en las diferentes clases de diámetro. Se calculó el volumen de fuste por el método de trozas sucesivas y las tablas de cubicación lineales y no lineales se ajustaron a los datos mediante el método de mínimos cuadrados ponderados. Los resultados muestran que el mejor modelo de cubicación en las tres especies es el no lineal. Las ecuaciones actualmente empleadas por la administración forestal subestiman significativamente el volumen de los árboles y, cuanto mayor es el tamaño del árbol, tanto más importantes son los errores de estimación. Las tablas de cubicación propuestas, una vez validadas, deberían poder utilizarse para una mejor estimación del volumen de árboles en la zona estudiada. Teniendo en cuenta los envites internacionales que envuelven a Camerún (FLEGT y REDD+), es esencial contar con herramientas eficientes para estimar el volumen de los árboles. En este contexto, es importante que la administración forestal camerunesa pueda llevar a cabo un amplio programa de revisión de las tablas de cubicación. ; Peer reviewed

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. ; Peer reviewed

Reliable tree height-diameter (H-D) allometric equations are a key tool for the estimation of forest productivity and Above Ground Biomass (AGB). Most existing H-D allometric equations developed for the tropical region are based on large-scale multi-species datasets, and their use to derive information on productivity and AGB at the species level is prone to uncertainties. The single-species H-D allometric equations available are mainly focused on monocultures or stands with simple tree species mixtures and did not account for the site effects. Here we measured the height and diameter of 2,288 trees of the emergent tree species Pericopsis elata (Harms) Meeuwen in the Democratic Republic of the Congo (DRC) and in Cameroon. We first examined how accurate multispecies H-D allometric equations are in predicting the total height of P. elata. We then tested whether single-species H-D allometric equations vary between sites. We developed the first H-D allometric equation of P. elata and tested whether and how stand-level and environmental variables induce changes in H-D allometric relationship of P. elata at the regional level. We additionally evaluated whether tree-level variables are important at the local level where climate and stand development stage are expected to be less variable. We found that pantropical, regional and local H-D allometric equations significantly underestimate the total height of P. elata. The local multi-species H-D allometric equation developed for Yangambi showed the highest underestimation in all the studied sites. This result supports the need for an H-D allometric equation specific for P. elata. The species-level H-D allometric equation developed showed significant underestimations for trees from the disturbed and undisturbed forests in DRC, while overestimations were observed for similar sites in Cameroon. Using a mixed-effect H-D allometric equation, we showed that even within a single species, a substantial variation exists between sites. This variation showed to be driven by the differences in the maximum asymptotic height (Hmax) between sites. We found that P. elata trees are taller and attain higher Hmax in DRC than in Cameroon. The basal area showed to be a significant covariate accounting for the site effects at the regional-scale where climate variables showed minor effects. However, at the local-scale, none of climate or stand variables showed to be significant. Local-scale variation showed to be associated with differences in light availability, highlighting the potential of management options that shape the local environment in driving species productivity. ; Peer reviewed

En Afrique, la phénologie reproductive des arbres tropicaux, majoritairement annuelle, semble présenter une certaine régularité. Cette étude documente les variations intra- et interannuelles de la phénologie reproductive des arbres de la forêt du Mayombe à partir des données historiques de la Réserve de Luki (République démocratique du Congo). Le diamètre de reproduction des espèces exploitées pour le bois, encore largement méconnu, a été également examiné pour les espèces suffisamment représentées dans les données. Le suivi phénologique de 3 642 arbres appartenant à 158 espèces et 39 familles a été réalisé tous les 10 jours de 1948 à 1957. Les statistiques circulaires ont été utilisées pour tester le synchronisme de la phénologie entre arbres, à l'échelle de la communauté, pour la forêt dans son ensemble, et individuellement pour 87 espèces, dont 35 espèces bien représentées (n ≥ 20 arbres), 16 espèces commerciales et 36 autres espèces. Des régressions logistiques ont permis de déterminer le diamètre de fructification (minimum et régulier) de ces espèces. Pour la majorité des espèces, la floraison était régulière, annuelle et largement saisonnière (81,6 %, 71 espèces). Les pics étaient plus marqués pour la floraison que pour la fructification, plus étalée dans le temps, bien que significativement agrégée temporellement. La majorité des arbres et des espèces fleurissaient entre décembre et février, pendant la petite saison sèche, bien que des fleurs et des fruits étaient observables toute l'année à l'échelle de la communauté. Seules 13 espèces ont montré une relation significative entre le diamètre et la reproduction, parmi lesquelles sept espèces de canopée, cinq de sous-bois et une héliophile. Pour ces espèces, la moyenne du diamètre minimum de reproduction était de 17,3 cm. ; Peer reviewed

A diversity of phenological strategies has been reported for tropical tree species. Defoliation and seasonal dormancy of cambial activity inform us on how trees cope with water stress during the dry season, or maximize the use of resources during the rainy season. Here, we study the matching between leaf phenology (unfolding and shedding) and cambial activity for Prioria balsamifera, a key timber species in the Democratic Republic of Congo. In particular, we (i) evaluated the seasonality of cambial activity and synchrony of phenology among trees in response to climate and (ii) identified the seasonality of leaf phenology and its relation with cambial phenology. The study was conducted in the Luki Man and Biosphere Reserve, located in the Mayombe forest at the southern margin of the Congo Basin. Historic defoliation data were collected every ten days using weekly crown observations whereas recent observations involved timelapse cameras. Cambial pinning was performed on ten trees during 20 months and radius dendrometers were installed on three trees during 13 months. Tree rings were measured on cores from 13 trees and growth synchrony was evaluated. We found that P. balsamifera defoliates annually with a peak observed at the end of the dry season and the beginning of the rainy season. The new leaves unfolded shortly after shedding of the old leaves. The peak defoliation dates varied across years from September 12 to November 14 and the fraction of number of trees that defoliated at a given time was found to be negatively correlated with annual rainfall and temperature; during the dry season, when precipitation and temperatures are the lowest. Wood formation (radial growth), was found to be highly seasonal, with cambial dormancy occurring during the dry season and growth starting at the beginning of the rainy season. Individual ringwidth series did not cross date well. The within species variability of leaf phenology and cambial rhythms provides indication about resistance of the population against climatic changes. ; Peer reviewed

International audience ; Africa is forecasted to experience large and rapid climate change1 and population growth2 during the twenty-first century, which threatens the world's second largest rainforest. Protecting and sustainably managing these African forests requires an increased understanding of their compositional heterogeneity, the environmental drivers of forest composition and their vulnerability to ongoing changes. Here, using a very large dataset of 6 million trees in more than 180,000 field plots, we jointly model the distribution in abundance of the most dominant tree taxa in central Africa, and produce continuous maps of the floristic and functional composition of central African forests. Our results show that the uncertainty in taxon-specific distributions averages out at the community level, and reveal highly deterministic assemblages. We uncover contrasting floristic and functional compositions across climates, soil types and anthropogenic gradients, with functional convergence among types of forest that are floristically dissimilar. Combining these spatial predictions with scenarios of climatic and anthropogenic global change suggests a high vulnerability of the northern and southern forest margins, the Atlantic forests and most forests in the Democratic Republic of the Congo, where both climate and anthropogenic threats are expected to increase sharply by 2085. These results constitute key quantitative benchmarks for scientists and policymakers to shape transnational conservation and management strategies that aim to provide a sustainable future for central African forests.

International audience ; Africa is forecasted to experience large and rapid climate change1 and population growth2 during the twenty-first century, which threatens the world's second largest rainforest. Protecting and sustainably managing these African forests requires an increased understanding of their compositional heterogeneity, the environmental drivers of forest composition and their vulnerability to ongoing changes. Here, using a very large dataset of 6 million trees in more than 180,000 field plots, we jointly model the distribution in abundance of the most dominant tree taxa in central Africa, and produce continuous maps of the floristic and functional composition of central African forests. Our results show that the uncertainty in taxon-specific distributions averages out at the community level, and reveal highly deterministic assemblages. We uncover contrasting floristic and functional compositions across climates, soil types and anthropogenic gradients, with functional convergence among types of forest that are floristically dissimilar. Combining these spatial predictions with scenarios of climatic and anthropogenic global change suggests a high vulnerability of the northern and southern forest margins, the Atlantic forests and most forests in the Democratic Republic of the Congo, where both climate and anthropogenic threats are expected to increase sharply by 2085. These results constitute key quantitative benchmarks for scientists and policymakers to shape transnational conservation and management strategies that aim to provide a sustainable future for central African forests.

International audience ; Africa is forecasted to experience large and rapid climate change1 and population growth2 during the twenty-first century, which threatens the world's second largest rainforest. Protecting and sustainably managing these African forests requires an increased understanding of their compositional heterogeneity, the environmental drivers of forest composition and their vulnerability to ongoing changes. Here, using a very large dataset of 6 million trees in more than 180,000 field plots, we jointly model the distribution in abundance of the most dominant tree taxa in central Africa, and produce continuous maps of the floristic and functional composition of central African forests. Our results show that the uncertainty in taxon-specific distributions averages out at the community level, and reveal highly deterministic assemblages. We uncover contrasting floristic and functional compositions across climates, soil types and anthropogenic gradients, with functional convergence among types of forest that are floristically dissimilar. Combining these spatial predictions with scenarios of climatic and anthropogenic global change suggests a high vulnerability of the northern and southern forest margins, the Atlantic forests and most forests in the Democratic Republic of the Congo, where both climate and anthropogenic threats are expected to increase sharply by 2085. These results constitute key quantitative benchmarks for scientists and policymakers to shape transnational conservation and management strategies that aim to provide a sustainable future for central African forests.

International audience ; Africa is forecasted to experience large and rapid climate change1 and population growth2 during the twenty-first century, which threatens the world's second largest rainforest. Protecting and sustainably managing these African forests requires an increased understanding of their compositional heterogeneity, the environmental drivers of forest composition and their vulnerability to ongoing changes. Here, using a very large dataset of 6 million trees in more than 180,000 field plots, we jointly model the distribution in abundance of the most dominant tree taxa in central Africa, and produce continuous maps of the floristic and functional composition of central African forests. Our results show that the uncertainty in taxon-specific distributions averages out at the community level, and reveal highly deterministic assemblages. We uncover contrasting floristic and functional compositions across climates, soil types and anthropogenic gradients, with functional convergence among types of forest that are floristically dissimilar. Combining these spatial predictions with scenarios of climatic and anthropogenic global change suggests a high vulnerability of the northern and southern forest margins, the Atlantic forests and most forests in the Democratic Republic of the Congo, where both climate and anthropogenic threats are expected to increase sharply by 2085. These results constitute key quantitative benchmarks for scientists and policymakers to shape transnational conservation and management strategies that aim to provide a sustainable future for central African forests.