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

Abstract

International audienceThe 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

Successional patterns on tropical inselbergs: A case study on the Nouragues inselberg (French Guiana)

“…Parmentier (2003), studying three inselbergs in Central Africa, did not describe any woody vegetation except summit and fringe forest areas. Jeník and Hall (1976) and Oumorou and Lejoly (2003) described woody plant communities on slopes of inselbergs in West Africa, but they attributed the existence of woody areas to thickness of the soil which, based on our own observations, should not be considered as a cause but rather as a consequence of shrub development (Kounda-Kiki et al, 2006;Vaçulik et al, 2004). Hambler (1964) and later Isichei and Longe (1984) described the seral and seasonal development of Afrotrilepis mats on Nigerian inselbergs, but without any appearance of woody vegetation except in particular areas such as ravines and fringes of the summit forest, probably because of recurrent fires during the dry season.…”

“…The common development of soil and vegetation has been studied by Sarthou and Grimaldi (1992), Vaçulik et al (2004) and Kounda-Kiki et al (2006, 2008. We will focus our story on the vascular vegetation of 'rock savanna', omitting fringe and summit forested areas.…”

“…2A). Organic soils (1-20 cm depth), acid (pH 4.0-5.5), with a high faunal and microbial activity, are formed under patches of woody vegetation where humidified organic matter is prevented from flushing by a dense root network (Kounda-Kiki et al, 2006;Sarthou and Grimaldi, 1992). The summit of the inselberg, and the ecotone zone surrounding it, are covered with a low forest rich in epiphytes and understory plants (Larpin, 2001).…”

Stability of plant communities along a tropical inselberg ecotone in French Guiana (South America)

“…This xerophilous vegetation is composed of numerous small scattered patches of epilithic wind-and bird-disseminated herb species and shrubs. Two widespread plant communities give a typical aspect to the landscape (Sarthou, 2001): a herbaceous community dominated by the bromeliad Pitcairnia geyskesii established directly on granite and a shrub community dominated by the Clusiaceae Clusia minor forming thickets 2-8 m tall, with an organic soil (Kounda-Kiki et al, 2006). Thickets in the contact zone with the low forest are likely denser and more diversified (Sarthou, , 2001.…”

“…Shallow soils and steep slopes cause a rapid run-off of water. Organic matter and roots accumulate locally under shrub thickets (Kounda-Kiki et al, 2006, 2008. In the low forest, the temperature is more buffered and soils are deeper and mostly made of mineral matter overlaid by a thin litter layer.…”