Humus profiles and successional development in a rock savanna (Nouragues inselberg, French Guiana): A micro-morphological approach infers fire as a disturbance event

Abstract

The common development of vegetation and soil is a central question of plant succession. We asked whether places where aerial parts of woody vegetation die and accumulate on the ground (zones of destruction or 'micro-chablis') played a role in the successional development of vegetation patches on tropical inselbergs and whether causes could be inferred from the analysis of the organic matter accumulated along a successional gradient. The study was conducted in French Guiana (South America). Nine humus profiles (each comprised of a varying number of layers) were selected in shrub thickets (~1a each) representative of three vegetation types of the rock savanna: canopies of pure Clusia minor (Clusiaceae), C. minor in mixture with Myrcia saxatilis (Myrtaceae) and zones of destruction. A count point optical method for small soil volumes was used to measure under a dissecting microscope the volume ratio of each kind of humus component (107 categories) in the 62 layers thus sampled. Micromorphological data were analysed by correspondence analysis (CA). Humus profiles varied according to canopy trees and revealed traits of the past and trends for the future of the plant succession. Zones of destruction differed from other humus profiles by lack of OL and OF horizons and by the presence of charred material, which establishes the role of spatially limited fires or lightning impacts in the cyclic development of vegetation patches.

Humusica 1, article 5: Terrestrial humus systems and forms — Keys of classification of humus systems and forms

“…Nota Bene: Even if a very low soil pH is observed (≤ 4.5) in the equatorial zone, temperature and moisture compensate for unfavourable soil conditions (Sanchez et al, 2003) and a very active Mull humus system occurs in all this area (Lavelle et al, 1993), except in white sand or inselberg sites (with very low base content), where Mor and Moder dominate, respectively (Hartmann, 1970;Klinka et al, 1981;Coomes and Grubb, 1996;Kounda-Kiki et al, 2008). The equatorial Mull shows a large number of roots at its surface (it is often a Rhizo Mull), which can absorb the nutrients thanks to mycorrhizal symbiotic partners (Nasto et al, 2014).…”

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

“…A closer examination of advancing zones of destruction reveals intense woodrotting and xylophagous termite activity (Fig. 2D) and charcoal has been found in all samples taken in the topsoil of destruction zones (Kounda-Kiki et al, 2008). Whether wood-destroying fungi and xylophagous termites occur concurrently after fire (due to lightning strikes) or, more probably, according to a substrate succession starting with the death of woody branches and stems and subsequent fungal development, is still a matter of conjecture, but we always found rotten wood associated with termite consumption.…”

“…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.…”

Humusica 2, article 18: Techno humus systems and global change – Greenhouse effect, soil and agriculture

“…This map shows also that Mull systems tends to be slightly acidic (Andreetta et al, 2016) even under temperate conditions and that, on the other side, Moder systems tend to be less acidic in dry biomes. Even if a very low soil pH is observed (≤ 4.5) in all the equatorial zones, temperature and moisture compensate the acidity (Sanchez et al, 2003) and a very active Mull humus system occurs in all this area (Lavelle et al, 1993), except in white sand or inselberg sites (with very low base and N contents), where Mor and Moder respectively dominate (Coomes and Grubb, 1996;Kounda-Kiki et al, 2008).…”