Succession secondaire post-culturale en systeme forestier tropical semi-decidu de Cote d'Ivoire: approche phytosociologique integree et systemique
Kassi, Justin; Decocq, Guillaume
Given the current global destruction of tropical rain forests, it is a great challenge for ecologists to understand processes underlying successional pathways and their associated changes in forest structure and biodiversity. In this study, we intensively surveyed forest composition and structure along a chronosequence of old fields, which were abandoned from 5 to 30 years ago, and compared them with the adjacent stands of both undisturbed and disturbed 'primary' forest. All stands (N = 80) were included into an old-growth forest matrix (Sanaimbo forest, Ivory Coast) among 2 soil types (ferralitic and hydromorphic). We recorded vascular plant species of each vegetation layer into randomly disposed nested-plots (1500 m2), following a phytosociological approach and distinguishing two spatio-temporal organization levels: synusia and phytocoenosis. All relevés were submitted to hierarchical clustering and detrended correspondence analysis, as a help into successively grouping relevés into synusiae, and synusiae into phytocoenoses. Finally, a qualitative modelisation of postcultural secondary succession was built up using a space-for-time substitution procedure and a systemic approach.Undershrub species quickly invaded old fields and initiated forest regeneration. Secondary succession accumulated species rapidly and overlaid structural compartments. Clear discontinuities appeared along the chronosequence, that allowed us to distinguish several synusiae and phytocoenoses. Within 30 years following agriculture discontinuation, the structure of the young secondary forest showed a closer resemblance to the primary forest on ferralitic soils than did the floristic composition and richness. We conclude that the regenerative power of semi-deciduous tropical forests is high on ferralitic soils, even if the time taken for secondary forests to revert to 'primary' forests probably exceeds several centuries, and if we cannot fully exclude an alternative successional pathway. Conversely, old-growth forests on hydromorphic soils appeared to be weakly resilient and their destruction might be irreversible. Thus, these species-rich forests should be targeted as a conservation priority.