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

Abstract

International audienceWe questioned whether and how plant communities vary in space and time along an inselberg-rainforest ecotone in relation to present-day warming and whether biotic and non-biotic factors could explain the observed patterns. The study took place on a granitic inselberg in the French Guianan (South America) rainforest (Nouragues Natural Reserve: 4°5'N, 52°41'W). In a diachronic study (1995-2005) embracing a severe El-Niño event in 1997, we analysed vegetation structure and composition along three transects subsuming whole environmental and topographical variations in the transition zone from shrub vegetation at the fringe of open-rock vegetation to tall-tree rainforest. Data were analysed by PCA. Major variations in species and trait distribution were described in the low forest, with two floristic types evidenced by first PCA component and verified by cluster analysis: one with floristic composition reminiscent of open-rock vegetation but with higher and continuous canopy, the other typical of the low forest. There is no clear-cut boundary between typical open-rock and low forest vegetation. Variation in species composition of typical low forest was evidenced by second PCA component, which displayed differences according to slope and altitude. Small (~1.5 m), although significant, shifts in the spatial distribution of plant species pointed to possible slow encroachment of typical low forest vegetation in the absence of disturbance. However, the stability of species and trait distribution was remarkable within the 10-yr interval considered, despite an otherwise recorded decrease in species richness and recruitment. The boundary between typical low forest and open-rock-like vegetation coincided with the spatial limit of the mineral soil above granite. Despite demographic accidents due to severe El Niño events, plant communities at the fringe of a tropical inselberg are stable at short-time both in composition and spatial distribution. In the absence of strong disturbances such as wildfire and further erosion, soil availability for roots could be interpreted as an environmental constraint to the successional development of forest vegetation. Soil development might thus act as an ecological barrier to forest encroachment, which could only be alleviated by erosion recovery, as otherwise demonstrated

Plant diversity on granite/gneiss rock outcrop at Pedra do Pato, Serra do Brigadeiro State Park, Brazil

“…With greater soil depth, species adapted to higher moisture availability become dominant, with exclusion of dry-tolerant species. A great number of species (150 species) was also recorded on granite inselberg in the Nouragues Natural Reserve in French Guiana on a similar gradient (Sarthou et al 2010) and in ironstone (127 species) in the Serra dos Carajás, Amazonia-Brazil (Nunes et al in press). The diversity of habitats represents a variation in the resource availability and it correlates with a variation not only in the floristic composition, but also in the diversity of functional traits as a response to the different populations present on rock outcrop (de Paula et al 2015).…”

The role of outcrops in the diversity of Patagonian vegetation: Relicts of glacial palaeofloras?

“…Many of these studies were conducted mainly in Africa and North America (e.g., in Africa: Burke, 2003a;Changwe and Balkwill, 2003;Porembski et al, 1996;in North America: Burbank and Platt, 1964;Wyatt and Fowler, 1977). Other studies were carried out within tropical South America in Brazil, Venezuela and French Guiana (e.g., Gröger and Barthlott, 1996;Porembski et al, 1998;Meirelles et al, 1999;Jacobi et al, 2007;Sarthou et al, 2010). Several of the studies focus on the application of island theory (MacArthur and Wilson, 1967;MacArthur, 1972) to outcrop biodiversity (Wyatt and Fowler, 1977), deal with succession and speciation processes (Burbank and Platt, 1964;Houle and Phillips, 1989;Williamson et al, 1997;Sarthou et al, 2009Sarthou et al, , 2010 or have shown very specific adaptations to these extreme environments (e.g.…”

IAPT chromosome data 28

“…Methods for chromosome analysis are according to Guerra & Souza (2002). Inselbergs, also known as rock outcrops, are granitic or gneiss geological formations from pre-Cambrian that stand out in a flat landscape (Sarthou & al., 2010). These formations are characterized by the presence of severe edaphoclimatic conditions in relation to the surrounding environment, which are important selective pressures for plant evolution and speciation.…”