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

SummaryThe common development of vegetation and soil is a central issue in plant succession. We hypothesized that areas of woody vegetation decay and accumulation on the ground (zones of destruction or ‘micro-chablis’) played a role in the successional development of vegetation patches on tropical inselbergs and that disturbance events 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 (∼1 acre 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. Using a dissecting microscope, a count point optical method for small soil volumes was employed to measure the volume ratio of each kind of humus component (107 categories) in the 62 layers sampled. Micro-morphological data were analysed by correspondence analysis (CA). Humus profiles varied with canopy tree type and revealed traits of past as well as trends for future plant succession. The lack of OL and OF horizons, and the presence of charred material differentiated the zones of destruction from other humus profiles and confirms the impact of spatially limited fires or lightning strikes in the cyclic development of vegetation patches.