Forest progression modes in littoral Congo, Central Atlantic Africa

Aim To understand the persistence of a forest-savanna mosaic in places where rainfall data suggest that forest take-over should take place. To study the various modes of forest encroachment, and the role of human activities to hamper it. Location Data were collected on several forest-savanna ecotones in the coastal region of the Republic of Congo. The sites were chosen to illustrate the differing principal modes of forest expansion, corresponding to different levels of anthropic pressure. Methods The study sites were situated on five transects perpendicular to the ecotone (total sampled area: 1.7 ha) and 10 forest clumps in savanna (with diameters from 3 to 20 m). Along the transects botanical identification, diameter measurement and cartography were performed, while leaf area index was measured at a high resolution (every metre) along two of them. Collected data were analysed using a continuous quantification approach, which is much more useful than classical quadrat analysis. Time calibration of progression rates was performed using a simple model of the growth of the characteristic pioneer species, Aucoumea klaineana. Results The two main different modes are reflected in different successional patterns. The edge diffusion is slow (its rate is evaluated to c. 1 m year(-1)) and is characterized by a progressive increase in large-diameter tree density and shade-tolerant tree density away from the ecotone. Conversely, savanna to forest phase transition by coalescence of clumps exhibits high tree density remnants distributed in established forest. The composition of these remnants is compatible with that of the forest clumps in savannas. Main conclusions Three functional groups of pioneer trees are distinguished: some occupy the edge (edge pioneer), others establish clumps of forest in savanna (clump pioneers) and the longer-living A. klaineana ensures the transition to 'mature' forest. The two different observed patterns (linear edge progression and clump coalescence) can be understood with the use of a model of forest-savanna dynamics, 'FORSAT'. The two control parameters are the annual rainfall and the frequency of man-made fires in each savanna. In particular, an increase in the fire frequency can lead to a shift from the coalescence regime to the edge progression one.