Department of Applied Biological Sciences, Ghent University, 9000 Gent, BELGIUM.
The tropical forest of Central Africa is the second largest and probably the best preserved stretch of rain forest on Earth, yet the least known. Increasing demographic growth and economic interests are major threats to this ecosystem. Accurate knowledge on community dynamics and on the ecology of tree species growing in this forest is thus urgently needed to underpin conservation and management practices.
The Reserve of Luki at the extreme West of the Democratic Republic of Congo was a privileged site to study this ecosystem, moreover concealing spectacular biological collections and datasets. From the community level to the minute anatomy of wood this dissertation gives an overview of the ecology of trees of the Central African rain forest from 1948 until today. First, the history of community dynamics in a 200 ha forest plot was studied to highlight tendencies in the variations of species diversity and biomass content. Then the biological rhythms of five selected tree species and functional groups of species were examined to get a better understanding of natural ecosystem processes in relation with climate variations.
Long-term inventory data revealed 58 years of forest dynamics after an initial transformation thinning treatment and continuous forest use until today. Perturbations maintained at a moderate level by the protected status of the Reserve seem to have favoured species diversity and biomass sequestration in this forest. Besides, most tree species were found to have annual rhythms of leaf and reproductive phenology but in a wide array of patterns, from synchronous annual peaks to continuity. Direct and indirect associations with intra-annual and supra-annual climate variations suggest that changes in environmental conditions will affect the phenological rhythms of tropical trees. Dendrochronological analyses proved annual ring formation for the five study tree species and positive correlation between growth and rainfall. For the three understory species radial growth was found to associate with precipitation during the rainy season but in a different month for each species. For canopy species strong heterogeneity of growth patterns was found, between species and between individuals of the same species. A more detailed study of radial wood growth by use of cambial marking experiments showed that individual sensibilities to the type of substrate and fine plasticity of cambial activity in response to environmental changes are possible causes for this growth variability. Cambial dormancy in tropical trees may not be strict like in trees of temperate regions, but highly plastic and triggered by endogenous factors as well as climate variations.
This heterogeneity of responses to environmental changes between species and between individuals of the same species growing in the same site supports the idea that plurality is a key concept in species-rich rain forests. As a consequence, studying the diverse components of this heterogeneous mix remains extremely challenging and requires repeated efforts on the long run. Protecting this natural resource that is so far from being understood is therefore of utmost importance.