Éditeur(s) : HAL CCSD Cambridge University Press (CUP)
Résumé : The objective of this study was to analyse the factors explaining spatial variation in soil respiration over topographic transects in a tropical rain forest of French Guiana. The soil of 30 plots along six transects was characterized. The appearance of the 'dry to the touch' character at a depth of less than 1.2 m was used to discriminate soils exhibiting vertical drainage from soils exhibiting superficial lateral drainage and along with colour and texture, to define five classes from well-drained to strongly hydromorphic soils. Spatial variation in soil respiration was closely related to topographic position and soil type. Increasing soil water content and bulk density and decreasing root biomass and soil carbon content explained most of the decrease in soil respiration from the plateaux (vertically drained hypoferralic acrisol) to the bottomlands (haplic gleysol). These results will help to stratify further field experiments and to identify the underlying determinants of spatial variation in soil respiration to develop mechanistic models of soil respiration.
ISSN: 0266-4674

hal-01032046
https://hal.archives-ouvertes.fr/hal-01032046
DOI : 10.1017/S0266467406003415

Éditeur(s) : Pergamon-elsevier Science Ltd
Résumé : Total mercury (HgT) and monomethylmercury (MMHg) were investigated in a tropical head watershed (1 km(2)) of French Guiana. The watershed includes a pristine area on the hill slopes and a former gold mined flat in the bottomland. Concentrations of dissolved and particulate HgT and MMHg were measured in rain, throughfall, soil water and at three points along the stream. Samples were taken in-between and during 14 storm events at the beginning and middle of the 2005 and 2006 rainy seasons. Dissolved and particulate HgT concentrations in the stream slightly increased downstream, while dissolved and particulate MMHg concentrations were low at the pristine sub-watershed outlet (median = 0.006 ng L-1 and 1.84 ng g(-1), respectively) and sharply increased at the gold mined flat outlet (median = 0.056 ng L-1 and 6.80 ng g(-1), respectively). Oxisols, which are dominant in the pristine area act as a sink of HgT and MMHg from rain and throughfall inputs. Hydromorphic soils in the flat are strongly contaminated with Hg (including Hg droplets) and their structure has been disturbed by former gold-mining processes, leading to multiple stagnant water areas where biogeochemical conditions are favorable for methylation. In the former gold mined flat high dissolved MMHg concentrations (up to 0.8 ng L-1) were measured in puddles or suboxic soil pore waters, whereas high dissolved HgT concentrations were found in lower Eh conditions. Iron-reducing bacteria were suggested as the main methylators since highest concentrations for dissolved MMHg were associated with high dissolved ferrous iron concentrations. The connection between saturated areas and stagnant waters with the hydrographic network during rain events leads to the export of dissolved MMHg and HgT in stream waters, especially at the beginning of the rainy season. As both legal and illegal gold-mining continues to expand in French Guiana, an increase in dissolved and particulate MMHg emissions in the hydrographic network is expected. This will enhance MMHg bio-amplification and present a threat to local populations, whose diet relies mainly on fish.
Water Research (0043-1354) (Pergamon-elsevier Science Ltd), 2011-04 , Vol. 45 , N. 8 , P. 2659-2669

Droits : 2011 Elsevier Ltd All rights reserved.
http://archimer.ifremer.fr/doc/00035/14628/12134.pdf
DOI:10.1016/j.watres.2011.02.022
http://archimer.ifremer.fr/doc/00035/14628/

Éditeur(s) : HAL CCSD Public Library of Science
Résumé : We examined tree-soil habitat associations in lowland forest communities at Paracou, French Guiana. We analyzed a large dataset assembling six permanent plots totaling 37.5 ha, in which extensive LIDAR-derived topographical data and soil chemical and physical data have been integrated with precise botanical determinations. Map of relative elevation from the nearest stream summarized both soil fertility and hydromorphic characteristics, with seasonally inundated bottomlands having higher soil phosphate content and base saturation, and plateaus having higher soil carbon, nitrogen and aluminum contents. We employed a statistical test of correlations between tree species density and environmental maps, by generating Monte Carlo simulations of random raster images that preserve autocorrelation of the original maps. Nearly three fourths of the 94 taxa with at least one stem per ha showed a significant correlation between tree density and relative elevation, revealing contrasted species-habitat associations in term of abundance, with seasonally inundated bottomlands (24.5% of species) and well-drained plateaus (48.9% of species). We also observed species preferences for environments with or without steep slopes (13.8% and 10.6%, respectively). We observed that closely-related species were frequently associated with different soil habitats in this region (70% of the 14 genera with congeneric species that have a significant association test) suggesting species-habitat associations have arisen multiple times in this tree community. We also tested if species with similar habitat preferences shared functional strategies. We found that seasonally inundated forest specialists tended to have smaller stature (maximum diameter) than species found on plateaus. Our results underline the importance of tree-soil habitat associations in structuring diverse communities at fine spatial scales and suggest that additional studies are needed to disentangle community assembly mechanisms related to dispersal limitation, biotic interactions and environmental filtering from species-habitat associations. Moreover, they provide a framework to generalize across tropical forest sites.
ISSN: 1932-6203

hal-01260289
https://hal.archives-ouvertes.fr/hal-01260289
PRODINRA : 342930
DOI : 10.1371/journal.pone.0141488

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : In habitat mosaics, plant populations face environmental heterogeneity over short geographical distances. Such steep environmental gradients can induce ecological divergence. Lowland rainforests of the Guiana Shield are characterized by sharp, short-distance environmental variations related to topography and soil characteristics (from waterlogged bottomlands on hydromorphic soils to well-drained terra firme on ferralitic soils). Continuous plant populations distributed along such gradients are an interesting system to study intrapopulation divergence at highly local scales. This study tested (1) whether conspecific populations growing in different habitats diverge at functional traits, and (2) whether they diverge in the same way as congeneric species having different habitat preferences. Phenotypic differentiation was studied within continuous populations occupying different habitats for two congeneric, sympatric, and ecologically divergent tree species (Eperua falcata and E. grandiflora, Fabaceae). Over 3000 seeds collected from three habitats were germinated and grown in a common garden experiment, and 23 morphological, biomass, resource allocation and physiological traits were measured. In both species, seedling populations native of different habitats displayed phenotypic divergence for several traits (including seedling growth, biomass allocation, leaf chemistry, photosynthesis and carbon isotope composition). This may occur through heritable genetic variation or other maternally inherited effects. For a sub-set of traits, the intraspecific divergence associated with environmental variation coincided with interspecific divergence. The results indicate that mother trees from different habitats transmit divergent trait values to their progeny, and suggest that local environmental variation selects for different trait optima even at a very local spatial scale. Traits for which differentiation within species follows the same pattern as differentiation between species indicate that the same ecological processes underlie intra- and interspecific variation.
ISSN: 0305-7364

hal-01032430
https://hal.archives-ouvertes.fr/hal-01032430
DOI : 10.1093/aob/mct176

Éditeur(s) : HAL CCSD Ecole nationale du génie rural
Résumé :  
A l'échelle de l'arbre, la fertilité chimique des sols semble expliquer la localisation majoritairement superficielle du système racinaire, alors que le type de drainage explique la forte variabilité de sa répartition verticale. Les racines profondes, malgré leur faible proportion, jouent un rôle important pour extraire l'eau en saison sèche. A l'échelle du peuplement, les variations latérales du sol influent sur des processus tels que la compétition inter-individuelle et la sylvigénèse. Pour les sols de terre ferme, plus les conditions édaphiques sont contraignantes, plus les gros arbres sont rares dans le peuplement forestier. Celui-ci est dense lorsque le sol est très aminci, moins dense en situation hydromorphe. Le sol influence nettement la composition floristique des peuplements. Mais d'autres facteurs tels la dynamique sylvigénétique ou la dynamique propre à chaque population ont un poids très important. Des résultats divergents conduisent à formuler de nouvelles hypothèses, parmi celles-ci l'influence majeure de l'épaisseur du profil d'altération.
ISSN: 0035-2829

hal-01031882
https://hal.archives-ouvertes.fr/hal-01031882