Humus profiles under main vegetation types in a rock savanna (Nouragues inselberg, French Guiana)

The aim of our study was to describe succession related changes in humus profiles on moderate slopes of a tropical inselberg (Nouragues, French Guiana). Nine humus profiles were collected in a stratified manner under two main communities on well-drained sites: carpets of Pitcairnia geyskesii (Bromeliaceae) and shrub thickets of Clusia minor(Clusiaceae), the latter including two stages of its dynamic development. The 53 sampled layers were analysed by an optical method, the volume ratio of 109 classes of litter/humus components being quantified by a count point method. Correspondence analysis (CA) revealed marked differences among humus forms. Pitcairnia carpets were characterized by the dominance of cyanobacteria which formed crusts with low faunal activity, except when they were colonized by enchytraeid worms. With advancing succession, we observed that leaf litter did not accumulate but rather was incorporated into organo-mineral excrements of macro-invertebrates under C. minor. The late developmental stage of Clusia thickets, characterised by the establishment of Myrcia saxatilis (Myrtaceae), showed a thick layer of undecayed litter and near absence of organo-mineral aggregates. The humus form varied from mor in cyano-bacterial crusts to tropical moder (with a few mull features) in Clusia thickets, but comparisons among humus profiles revealed more complex successional processes than expected on the basis of the composition of plant and soil animal communities.