Éditeur(s) : HAL CCSD European Geosciences Union
Résumé : International audience
Measurements of branch xylem density, D_x, were made for 1466 trees representing 503 species, sampled from 80 sites across the Amazon basin. Measured values ranged from 240 kg m^?3 for a Brosimum parinarioides from Tapajos in West Pará, Brazil to 1130 kg m^?3 for an Aiouea sp. from Caxiuana, Central Pará, Brazil. Analysis of variance showed significant differences in average D_x across the sample plots as well as significant differences between families, genera and species. A partitioning of the total variance in the dataset showed that geographic location and plot accounted for 33% of the variation with species identity accounting for an additional 27%; the remaining "residual" 40% of the variance accounted for by tree to tree (within species) variation. Variations in plot means, were, however, hardly accountable at all by differences in species composition. Rather, it would seem that variations of xylem density at plot level must be explained by the effects of soils and/or climate. This conclusion is supported by the observation that the xylem density of the more widely distributed species varied systematically from plot to plot. Thus, as well as having a genetic component branch xylem density is a plastic trait that, for any given species, varies according to where the tree is growing and in a predictable manner. Exceptions to this general rule may be some pioneers belonging to Pourouma and Miconia and some species within the genera Brosimum, Rinorea and Trichillia which seem to be more constrained in terms of this plasticity than most species sampled as part of this study.
ISSN: 1810-6277

hal-00298004
https://hal.archives-ouvertes.fr/hal-00298004
https://hal.archives-ouvertes.fr/hal-00298004/document
https://hal.archives-ouvertes.fr/hal-00298004/file/bgd-5-2003-2008.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : URL de l'article : http://www.sciencedirect.com/science/article/B6WPN-4XY3KDK-2/2/fd8508af63d82e29b4f9829c65194e26
The impact of climatic changes that occurred during the last glacial maximum and the Holocene on vegetation changes in the Amazon Basin and the Guiana Shield are still widely debated. The aim of our study was to investigate whether major changes in vegetation (i.e. transitions between rainforests and C4 savannas) occurred in northern French Guiana during the Holocene. We measured variations in the δ13C of soil organic matter at eight sites now occupied by forest or savannah. The forest sites were selected to cover two regions (forest refugia and peneplains) which are thought to have experienced different intensities of disturbance during the latest Pleistocene and the Holocene. We found that none of the forest sites underwent major disturbances during the Holocene, i.e. they were not replaced by C4 savannahs or C4 forest savannahs for long periods. Our results thus suggest that tropical rainforests in northern French Guiana were resilient to drier climatic conditions during the Holocene. Nevertheless, geographical and vertical variations in the 13C of SOM were compatible with minor changes in vegetation, variations in soil processes or in soil physical properties.
ISSN: 0033-5894

halshs-00551758
https://halshs.archives-ouvertes.fr/halshs-00551758
DOI : 10.1016/j.yqres.2009.11.007

Éditeur(s) : HAL CCSD
Résumé : The genus Cecropia includes pioneer trees that colonize canopy gaps and cleared areas. It is widely distributed from Mexico to Northern Argentina. Cecropia trees have architecture, which corresponds to the Rauh’s architectural model. In recent studies we have found that C. obtusa/ and C. sciadophylla, in French Guiana, have a strong annual periodicity for growth, branching and flowering processes, with annual emission of 35 nodes for C. obtusa and 25 for C. sciadophylla. Those species show different life histories since C. sciadophylla has longer life expectancy. In addition, C. sciadophylla is found more frequently in forest gaps, although in French Guiana mixed populations are found. The aim of this work was to compare developmental strategies of both species in three different field stations in French Guiana and to determine how these two species coexist during its first years of life. We studied a 20-year old population of /C. sciadophylla/ at Saint Elie’s Road, a 10-year old population of /C. obtusa/ at Paracou’s field station, and a 7-year old mixed population at Counami’s Road. /C. sciadophylla /emits 25 nodes per year in both studied populations, while/ C. obtusa /emits 35 nodes in the monospecific population and 30 nodes in the mixed population. In the monocaule stage, the lower number of nodes emitted by /C. sciadophylla/ compared to /C. obtusa /is compensated with a higher internodal length, which allows finding individuals with similar ages and lengths, but different number of nodes. The branching and flowering processes start on average three years before in /C. obtusa/, however for 12-year old individuals we observed a dominancy of C. sciadophylla over C. obtusa. In the mixed population it is likely that the different life histories would decrease competition effects allowing coexistence of both species during the first stages of life
Botany 2008
Vancouver, Canada

hal-01190209
https://hal.archives-ouvertes.fr/hal-01190209
PRODINRA : 182910