The role of soil characteristics on forest structure and dynamics in extremely poor tropical soils

The long-term implications of over-exploitation and degradation in the tropics are less well known than those of deforestation. Yet, robust information on the lasting impact of degradation on forest characteristics and tree species diversity as well as their recovery potential are essential. This case study focuses on the coastal rainforest of the Mondah peninsular (Gabon). The forest was frequently exploited for more than 200 years and put under conservation in 1971. Recovery of the forest was assessed by forest inventories performed in 1993 and 2011. Replicate analysis revealed that for trees with dbh&#8805;10cm, the mean tree diameter, height, basal area as well as the above-ground biomass and carbon were significantly higher in 2011. On the stand level, 12 % less stems were counted in 2011 while the stand basal area, above-ground biomass and carbon were 31%, 41% and 40% higher. The thickening and heightening of the forest stand resulted in a shift in the stem diameter and height distribution, respectively: In 2014, less trees were found with a dbh<30cm while more trees were observed in the higher diameter classes up to 60cm dbh. In 1993, the vertical stratification of the forest comprised 66%, 28% and 6 % trees in the understory, medium, and canopy layer, respectively. In 2011, only 55% of the trees stuck in the understory, while 36% and 9% of the trees reach the medium and canopy layer, respectively. In 2011, 868 stems dbh&#8805;10cm were represented by 94 tree species, while in 1993 the ratio was 85 species to 779 trees with dbh&#8805;10cm. Abundance and dominance of the most important tree-species also changed between 1993 and 2011, e.g. the early pioneer tree Musanga cecropioides - most abundant in 1993 - was no longer found in the dense forest, while the long-lived pioneer Aucoumea klaineana became most abundant in 2011. Additional results referring to trees with a dbh< 10cm as well as a more detailed analysis of tree-species diversity will be provided in the session. Literature references were used to benchmark the present state of the forest and confirm the high regeneration potential of degraded coastal forests in Gabon. (Texte intégral)