Abstract :
[en] Logging in Central Africa plays a key economic role by generating employment and income for States and local populations. When carried out in compliance with legal standards, its impact on forest cover remains limited, with only one to two trees harvested per hectare every 25 to 30 years.
However, this model relies on a restricted number of species (around thirty), some of which exhibit low regeneration capacity in natural forest. The most affected are light-demanding species. This results in the progressive depletion of their populations and a decline in profitability over the relatively short term. Therefore, developing pragmatic silvicultural approaches based on assisted or artificial regeneration is necessary to ensure the long-term sustainability of timber harvesting. They can also prevent the conversion of forest land to other more lucrative activities.
The general objective of this thesis is to identify the most effective silvicultural practices for ensuring the regeneration of logged species within forest concessions in Central Africa. More specifically, it seeks to determine the factors influencing the survival and growth of high-value local timber species planted in openings created by logging operations, such as logging gaps, former log yards, and degraded areas along roads. The research was conducted in two forest concessions in south-eastern Cameroon that initiated enrichment activities 19 years ago.
Following the general introduction (Chapter 1), which presents the context of the thesis, Chapter 2 provides a meta-analysis of the performance of 89 local species from the Guineo-Congolian region. The findings show that tree survival depends on plantation age, with mortality being highest during the first seven years. Growth, on the other hand, depends on the planting method and the species’ guild. It is greatest in plantations established in degraded areas and clear-cut for pioneer and non-pioneer light-demanding species.
Chapter 3 describes the study area, including both the environmental setting and the logging practices applied.
Chapter 4 analyses the survival and growth of 15 planted tree species over a period of 14 to 19 years in logging gaps in south-eastern Cameroon. Five species – Detarium macrocarpum Harms, Erythrophleum suaveolens (Guill. & Perr.) Brenan, Terminalia superba Engl. & Diels, Baillonella toxisperma Pierre and Prioria oxyphylla (Harms) Breteler – had survival rates above 80 %. In contrast, all seedlings of Entandrophragma cylindricum (Sprague) Sprague planted have died after 15 years. Terminalia superba exhibited the highest diameter (16 mm/year) and height (103 cm/year) growth. Lovoa trichilioides Harms and B. toxisperma had diameter increments higher than 2 mm/year. This study recommends enriching logging gaps with T. superba and, if regular maintenance is ensured, also planting B. toxisperma, E. suaveolens, and D. macrocarpum.
Chapter 5 assesses the success of enrichment planting established on former log yards. It examines the performance of 11 planted tree species over a period of one to six years, in relation to log yard size and the impact of soil ripping with bulldozer claws down to 40 cm. Annual seedling survival ranged from 59 % to 98 % depending on the species. The highest diameter growth was recorded in Terminalia superba (21 mm/year) and Pterocarpus soyauxii Taub. (12 mm/year). Soil ripping had a positive effect on T. superba, while log yard size had no effect on species performance. The study recommends enriching former log yards with T. superba and P. soyauxii. Since soil ripping has limited effect on most species, investment should instead focus on the removal of competing vegetation.
Chapter 6 examines the performance of 30 planted tree species monitored over a period of up to 15 years under different silvicultural practices in degraded forest areas. Annual survival rates ranged from 67 % to 99 %, depending on species, saplings age, and clearing methods of competing vegetation. Piptadeniastrum africanum (Hook.f.) Brenan had the highest survival rate (99 %) after 14 years. Diameter increment depends on species, proximity to open areas (e.g., roads), tree age, plot preparation, and clearing methods of competing vegetation. The highest mean diameter increments (10 – 13 mm/year) were recorded for Bobgunnia fistuloides (Harms) JHKirkbr. & Wiersema, Triplochiton scleroxylon K.Schum., Distemonanthus benthamianus Baill. and Pseudospondias microcarpa (A.Rich.) Angl.
Finally, Chapter 7 discusses the overall findings, provides recommendations to optimize the performance of planted trees, and highlights perspectives for the sustainable management of forest concessions. It is recommended to enrich timber-production forests in Central Africa with local species. Fully open areas, including deforested sites outside forest concessions, could be restored with species that are difficult to establish in degraded forest areas. The influence of fauna on seedling mortality, especially for E. cylindricum, deserves further investigation to develop strategies tailored to herbivory-sensitive species. For species with irregular fruiting or whose seeds are heavily predated, the study of conservation methods is valuable. Experiments combining genotypic, phenotypic, and environmental factors would help to better identify the determinants of performance in vulnerable species. Finally, analyses of carbon and economic balances, complemented by long-term projections, would allow for a more precise assessment of the sustainability and profitability of enrichment planting.
