Performances des essences indigènes dans les plantations : une synthèse pour la région Guinéo-Congolaise

editorial reviewedIn the rainforests of the Guineo-Congolian region, several native tree species have been tested in plantations with different silvicultural methods and objectives. The results of these experiments remained scattered, hampering our ability to identify the key drivers of variability in the survival and growth of planted tree species. Here, we synthesize the literature by conducting a systematic review. From 45 selected studies, we compiled a database on 89 native tree species planted in different forest types (evergreen, semideciduous and transition). The data included plantation age, survival, height and diameter growth. For each planted species, we gathered information on the planting method (understorey, line planting, gap, degraded area, regrowth and clear-cut), and species functional traits (species guild, dispersal mode, wood density and leaf phenology). We modelled tree survival, height and diameter growth using linear mixed-effect models. We found that tree survival depended mainly on plantation age, and that the mortality rate was the highest during the seven first years. In contrast, tree survival did not significantly depend on the planting method or species traits. In the studied plantations, height and diameter growth significantly depended on the planting method and species guild. Diameter growth was negatively correlated with wood density. Pioneer, non-pioneer light-demanding and shade bearer species grew faster in diameter when planted in degraded areas and clear-cuts. Pioneer species grew the fastest in gaps, where they also grew faster than the other species. Although we did not find an effect of forest type on tree survival and growth, the variability between sites was substantial. This study provided empirical evidence that planting methods need to be adapted to the species guild.UFA-Reforest (CRIS: 2020/419-588)13. Climate actio

Origin and populations dynamics of the tree species of Central African rainforests, the case of Lophira alata Banks ex Gaertn CF (Ochnaceae)

The canopy of Central African rainforests is characterized by the abundance of light-demanding trees belonging to several species, suggesting that favorable conditions for their regeneration once prevailed. Today this isn’t the case in light of the demographic structure of their populations, which shows a lack of regeneration. The appearance of such species in rainforests is the result of significant and sustainable openings in the forest canopy. Such events may have occurred with climatic changes and / or large-scale anthropogenic disturbances. The risk of their populations decline is increased by the fact that most of them are timber species. This is typically the case of Lophira alata Banks ex Gaertn C.F., which is one of the most important Central African trees harvested, and is considered vulnerable by the IUCN. In the current context where the preservation of the biodiversity and the integrity of the forest cover is a major concern, the present PhD aims to understand the history and ecological requirements of L. alata in order to propose the sustainable management strategies. The history of L. alata has been studied using an approach integrating both anthracology and archeology. We found a large body of evidences (pottery fragments, palm nuts and charcoal) in the forests soil of southern Cameroon indicating human occupation and widespread fires over two periods : 2200 - 1500 years BP, and around 300 years BP. The age of the most recent events coincides with the probable age of the tallest specimens of L. alata. To determine the light requirements of the species and to better define its regeneration strategy, a quantitative study on the guild of juveniles was conducted for two years. The results show that L. alata has both the characteristics of a pioneer species and those of a non-pioneer species. Indeed, at low irradiances (1% of full sunlight), the plants mortality rate was very low and their biomass showed no signs of decline, a behavior observed in all non-pioneer species. By contrast, the response of leaf traits, patterns of biomass allocation, and the maximum growth of plants in the intermediate (23-43% of the relative irradiance) and high (100%) irradiance levels, according to the population origins, are typical of all pioneer species. These results emphasize the importance of the population origins when studying the response to light and the need to define a more encompassing classification system. In terms of the dynamics of mature populations, environmental conditions affect the diameter growth of trees, with better growth being realized in the evergreen forest. Nevertheless, even under these favorable conditions, simulations of the exploitable stocks and their recovery rates indicated a significant decline during the first three cutting cycles, which could lead to a decline in economic activity associated with this species. Our results allowed us to suggest a set of recommendations for the sustainable management of L. alata.La canopée des forêts denses humides d’Afrique centrale se caractérise par l’abondance d’arbres héliophiles appartenant à plusieurs espèces, ce qui indiquerait que des conditions favorables pour leur régénération ont jadis prévalu. Aujourd’hui, ce ne serait plus le cas au regard de la structure démographique de leurs populations, laquelle témoigne généralement de déficits de régénération. L’installation de telles espèces en forêt dense humide suggère des ouvertures importantes du couvert forestier. De tels évènements auraient pu survenir lors de changements climatiques et/ou de perturbations anthropiques d’ampleur. Le risque de diminution de leurs populations est accru par le fait que de nombreuses espèces sont exploitées pour le bois d’oeuvre. C’est typiquement le cas de Lophira alata Banks ex Gaertn C.F., un des plus importants bois d’oeuvre du Cameroun, jugé vulnérable par l’IUCN. Dans le contexte actuel où la préservation de la biodiversité et de l’intégrité du couvert forestier fait l’objet de toutes les attentions, la présente thèse a pour objectif de comprendre l’histoire et l’écologie de L. alata afin de proposer des stratégies de gestion durable. L’histoire des populations de L. alata a été appréhendée par une approche intégrant l’anthracologie et l’archéologie. Elle a mis en exergue un faisceau d’indices (fragments de poterie, de noix de palme et de charbons de bois) dans le sol des forêts du Sud du Cameroun témoignant d’une occupation humaine et des feux généralisés à deux périodes : 2 200 - 1 500 ans BP, et plus récemment, il y a environ trois siècles. L’âge des évènements les plus récents semble coïncider avec l’âge probable des plus grands spécimens de L. alata. Afin de déterminer les besoins en lumière de l’espèce et de mieux définir sa stratégie de régénération, une étude quantitative du tempérament des juvéniles a été réalisée pendant deux ans. Les résultats révèlent que L. alata présente à la fois les caractéristiques d’une espèce pionnière et celles d’une espèce non pionnière. En effet, à de faibles éclairements (1% d’éclairement relatif), la mortalité des plants était très faible et la biomasse n’affichait aucun signe de diminution, un comportement observé chez les espèces non pionnières pouvant se maintenir dans le sous-bois. Par contre, la réponse des traits foliaires et des patrons d’allocation de la biomasse, ainsi que la croissance maximale des plants dans les éclairements intermédiaires (23-43% d’éclairement relatif) et élevés (100%), en fonction des provenances, étaient caractéristiques des espèces pionnières. Il en résulte un vaste champ d’investigation quant à la classification du tempérament des espèces tropicales. En terme de dynamique des populations adultes, les conditions environnementales des sites influencent la croissance en diamètre des arbres, avec un meilleur développement en forêt sempervirente. Néanmoins, même dans ce milieu favorable, les simulations du taux de reconstitution et du stock exploitable montrent une importante baisse au cours des trois premières rotations, avec pour conséquence une probable diminution à terme de l’activité économique autour de cette espèce.Nos résultats permettent de formuler un ensemble de recommandations relatives à la gestion durable des populations de L. alata. Elles sont principalement liées à la nécessité de considérer des données spécifiques et locales lors de l’aménagement forestier et de développer des programmes de sylviculture adaptés

Caractérisation écologique de Lophira alata (azobé), une essence commerciale jugée vulnérable, en vue de développer des stratégies sylvicoles efficaces

Ekki, Lophira alata Banks ex C.F.Gaertn., is an African timber species occurring in tropical rainforests. Of important commercial value, this species is logged for its excellent timber properties and has been classified by the IUCN as «vulnerable" species. As for many other tropical tree species, the commercial exploitation of Ekki is confronted to an insufficient knowledge about the species’ ecological profile. With a distribution range limited to the Guinea-Congolian region, Ekki often displays insufficient natural regeneration, but the biotic and abiotic factors explaining this deficiency have been hardly characterized. This lack of knowledge makes it difficult to model the population dynamics on the long term. The reproductive biology of Ekki and the genetic variability of its population remains poorly studied too. To address this situation, a fundamental and applied research is being undertaken in the forest concessions managed by Wijma Cameroun SA., covering over 250,000 hectares of rainforest in Southern Cameroon. This study aims to answer four essential questions: (1) what is the phylogenetic relationship between L. alata and L. lanceolata, (2) how paleoclimate and human disturbances impact its distribution and abundance, (3) how to these populations respond to logging over several rotation periods, (4) does logging promote or inhibit the natural regeneration of the species. Several study devices have thus been installed to collect the data needed to improve the characterization of Ekki’s ecology and to conceive silvicultural strategies

Definition of ecological profile of Lophira alata (Ekki), a major important African timber species: litterature review and perspectives for future studies

Over two decades, labors made in order to promote sustainable management of African tropical forests are tackled to the lack of knowledge about this complex ecosystem. Ecological parameters of timber species are no more studied, this complicates long-term sustainable forest management. This literature review related to the ecology and silviculture of ekki, Lophira alata Banks ex C.F.Gaertn. (Ochnaceae), a main African timber species recorded as “vulnerable” in the IUCN Red List, will serve as a study’s case. The planned literature review reveals the doubt about its taxonomy, as well as the lack of understanding concerning its reproductive biology, growth conditions, population dynamics parameters, and the spatial distribution of its genetic diversity. The deficiency of knowledge about its ecological needs and the factors which have historically influenced its population dynamics explain why most silvicultural trails provide hazardous and uncertain results. Remedying these gaps in order to improve its ecological characterization and innovative silvicultural trails, would be a significant contribution to the sustainable management of its populations

Le paysage comme porte d'entrée à l'aménagement des terroirs villageois dégradés en péripherie de Kinshasa, capitale de la République Démocratique du Congo

Projet Makala (financement UE

Defining the ecological profile of Ekki (Lophira alata Banks ex Gaertn F., Ochnaceae) in evergreen forests of Cameroon

Ekki or L. alata is a commercial timber species in Guinea-Congolian region considered vulnerable (IUCN, 2011). Indeed, its population structure shows a severe lack of regeneration in some rainforests (Palla et al., 2002). Moreover, in logging areas, insufficient knowledge about its ecological profile limits any reliable modeling of population dynamics on a long term. Consequently, original research, to improve its ecological characterization to ensure its sustainable management is conducted in moist evergreen forest in southern Cameroon. First results on population dynamics of this species show, in plots study of recruitment, a population without major deficit regeneration. On its silvicultural ability, although that estimated on plantations on parks under 2 years old, the trends of growth (ca 30 and 0.5 cm / year respectively in height and diameter) and mortality (3, 3% / year) suggest a positive perspective. Current work in relation to other topics of research should provide further insights into its ecological profile in the evergreen forests of Cameroon

Can the competition dynamics of non-native invaders be reconstructed to reveal historical impact? The case of Cecropia peltata and Musanga cecropioides (Urticaceae) in Cameroon

Measuring impacts of non-native species is challenging, particularly for long-established invasions in poorly characterised native ecosystems. Recreating historical population dynamics using dendrochronological methods is a popular approach for addressing conservation and forest management questions. However, these tools are rarely applied for addressing questions of invasion. Classical dendrochronology is also not well suited to many tropical tree species as they do not produce growth rings. Here we test the validity of an alternative approach, based on the analysis of multiple morphological markers, that could allow for both the recreation of historical invasion dynamics as well as quantify the impacts of invasion on the recipient native ecosystem. We validate this method on the Cecropia peltata invasion in Cameroon and its potential impacts on a native tree, Musanga cecropioides. We were able to compare architectural development over time in a mixed secondary forest where the two species co-occur and revealed differences in performance (growth, branching and flower production) between the two tree species. Our results revealed more sustained growth during early establishment and an early sexual maturity for the non-native C. peltata, compared to its native counterpart. A lower density of M. cecropioides was observed in the region of co-occurrence, supporting a hypothesis of competitive exclusion of the native species at the juvenile stage. Our work provides a novel way to understand the impacts of plant invasions via retrospective analysis that could well be applied to other invasions globally, opening up more opportunities to prioritise management on an impact-focused basis.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Les forêts vierges du Bassin du Congo - mythe ou réalité ?

Les forêts du bassin du Congo ont longtemps été considérées comme des massifs historiquement épargnés par l'action de l'homme. Des résultats plus ou moins récents de recherches scientifiques remettent en question cette vision des forêt denses humides africaines