Dynamique des populations d'azobé, Lophira alata Banks ex C. F. Gaertn., et implications pour sa gestion durable au Cameroun

L'impact de l'exploitation forestière sur le renouvellement du stock de bois d'oeuvre reste largement méconnu en Afrique centrale du fait du manque de données sur la dynamique des populations d'arbres exploités. C'est en particulier le cas pour l'azobé, Lophira alata, un bois d'oeuvre de grande valeur commerciale. L'objectif de cette étude est double : quantifier l'effet du type forestier et de l'exploitation sur la croissance de l'azobé et simuler l'évolution du stock exploitable et du taux de reconstitution après exploitation. Nous avons effectué pendant trois ans un suivi annuel de la croissance et de la mortalité dans trois types forestiers au Cameroun : en forêt sempervirente, en forêt mixte et en forêt semi-caducifoliée. Le recrute-ment a été étudié uniquement en forêt mixte, sous deux conditions : en milieu exploité et non exploité. Nous avons calibré, avec ces données, un modèle matriciel de Usher. Douze scénarios d'exploitation ont été simulés en faisant varier le diamètre minimum d'exploitation de 60, 70 et 80 cm, et l'intensité de prélèvement des arbres de 100 à 40 %. La croissance de l'azobé est influencée à la fois par le type forestier et l'exploitation. Les arbres de forêt sempervirente ont une croissance plus faible jusqu'à 50 cm de dia-mètre, alors que la croissance maximale prédite pour ce type forestier est la plus élevée. L'exploitation a par ailleurs stimulé la croissance. Enfin, l'exploitation de l'azobé ne respecte pas le principe du rendement soutenu : ses taux de croissance à long terme varient entre 0,54 et 0,83 %. Pour garantir la durabilité de son exploitation au Cameroun, une sylviculture dynamisant la croissance des futurs arbres exploitables, ainsi que leur régé-nération, s'avère indispensable

Evaluation de la diversité génétique et de la régénération à faible échelle au sein des forêts exploitées du bassin du Congo

Background In the Congo Basin most of the light-demanding timber tree species display a deficit of natural regeneration which is a major handicap for sustainable production and certification. Whilst the majority of scientists investigate abiotic and biotic factors explaining that pattern, we hypothesize that tree population density or individual spatial isolation may also affect the tree fitness through inbreeding. In this study, we integrate ecological and genetic approaches to characterize the regeneration potential of a set of priority timber species by (i) estimating pollen dispersal distances at various tree population densities, and (ii) evaluating the impact of increasing spatial isolation on mating characteristics and tree fitness. The ultimate goal is the proposal of minimum population density that prevents inbreeding consequences. Method This ongoing study focuses on 10 timber species (Pericopsis elata, Milicia excelsa, Baillonella toxisperma, Entandrophragma cylindricum, E. utile, E. angolense, E. candollei, Afzelia bipindensis, Erythrophleum suaveloens, Terminalia superba). The data collection was carried out in the logging concession granted to Pallisco in Cameroon. We established two 400-ha plots, where all individuals (DBH > 10 cm) of the target species were inventoried and mapped. A sample of leave or cambium was collected for each of these individuals, as well as for seedlings to characterize patterns of gene flow using genetic tools (nuclear microsatellites). Dispersal agents were identified by direct observations and camera traps. Germination success was characterized in nursery for seeds collected on trees under an increasing isolation gradient. Results Main dispersal agents (wind, bat, rodent) and predators (rodent) were identified for all the species. The gene flow and germination data is still being analyzed and the main results will be presented in the poster. Conclusion Our data will allow characterizing the reproductive biology of a set of important timber species from the Congo basin. These information will strengthen sustainable forest management and the application of certification by adjusting harvesting norms through the use of scientifically-relevant data. In particular, we will tentatively define a maximum distance to be maintained between two adults to allow a qualitative reproduction.Aide à l'application des normes FSC sur la régénération et la diversité génétique des essences du bassin du Cong

Enrichment of Logging Gaps with a High Conservation Value Species (Pericopsis elata) in a Central African Moist Forest

In central Africa, most of the timber species require high light at the seedling stage for survival and growth. Forest managers face a regeneration shortage of these light-demanding timber species. To achieve long-term sustainability, there is a need for enrichment methods combining low cost and high species performance. The aim of this study was to assess the performance of Pericopsis elata seedlings in enriched logging gaps in Cameroon. Over five years; the survival and size of each seedling was monitored in 27 logging gaps that were either left without maintenance or cleared. Gaps were relatively small with an average total area of 155 m2. We found that planted seedlings of P. elata performed well in logging gaps. Even without any maintenance 61% of the planted seedlings survived after five years with an average annual diameter increment of 0.28 cm. P. elata appeared to be a good candidate species for enrichment in logging gaps. We demonstrated that the seedlings of P. elata tolerated a wide range of soil conditions but that their performance was strongly influenced by light availability (gap clearance), suggesting potentially improved performance of P. elata in high light environments such as in plantation or larger gaps

Wood description and timber use investigation of Pachyelasma tessmannii (Harms) Harms

peer reviewedIn Central Africa more than 75% of the total log production is focussed on only four timber species, whose populations are progressively being depleted. Reducing the impact on those flagship species by diversifying the exploitation could lead to better forest management in the long term. Pachyelasma tessmannii is a species whose trees are among the largest in the forests of Central Africa. Yet the properties of its wood are poorly documented. The aim of this study is to evaluate the possibility of using this species with a view to diversify forest production in Central Africa. Its physical and mechanical properties, its natural durability, and quantified radial variation were investigated. By using Hierarchical Clustering on Principal Component (HCPC), heartwood was classified among 98 other Central African timber species. P. tessmannii has a wavy grain and a coarse texture with a wide well-discernible sapwood. The wood is “heavy” with disadvantageous dimensional stability parameters. All mechanical properties are “medium”, except for “low” impact bending strength. Heartwood is very durable against white and brown rot. All properties were significantly influenced by radial variation, except for axial compression strength. According to the HCPC, P. tessmannii should be suitable for stairs (inside/outside), flooring, decking, veneer (back and face of plywood), sliced veneer, furniture (inside/outside), exterior panelling, cabinetry, and joinery (inside/outside). The results obtained concerning the radial variation of basic density could suggest that the species is light-demanding. Considering that the sustainable exploitation of light-demanding species is often confronted with the problem of their lack of regeneration in closed-canopy rainforests, further studies are needed before promoting this species on international markets.Essence à Haut Potentiel de valorisation (EHPval

Enrichment of Central African logged forests with high-value tree species: testing a new approach to regenerating degraded forests

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