Drawing ecological insights from a management-oriented forest inventory in French Guiana

Reliable ecological information at the landscape scale is generally lacking for tropical rain forests, although extensive areas have been sampled by forest inventory to estimate timber resources. We used the data provided by a 12,240 ha management-oriented forest inventory in the lowland rain forest of French Guiana to document species/environment relationships and to characterise the spatial variation of the floristic composition. The forest inventory encompassed 22,023 trees larger than 7.5 cm diameter measured in 4110.3 ha sampling plots spread over a systematic grid with 500 m x 400 m spacing between plot centres. In each plot, all the trees above 37.5 cm diameter have been recorded, while the trees between 7.5 and 37.5 cm diameter have been recorded in smaller sub-plots. Each sampling plot was characterised using semi-quantitative ecological descriptors relating to topography, remnants of lateritic cuirasses, presence of hydromorphic soils, etc. Preliminary analyses revealed that most of these variables could be accounted for by topographical categories subdivided in relation to presence/absence of hydromorphic soils. However, stand structure, expressed by the distribution of trees in diameter classes, proved fairly independent on such categories. Floristic information was based on a refined vernacular nomenclature (291 taxa) with collection of herbarium specimens that allowed us to equate 59 taxa with known botanical species. We produced a reduced floristic table expressing the distribution of the 59 botanical species within the 411 plots, and a complete floristic table featuring the distribution of all the vernacular taxa. The main floristic gradients were extracted from these tables via correspondence analysis (CA) and non-symmetric correspondence analysis (NSCA), a complementary approach that emphasises frequent species. Analogous constrained ordinations (CAIV, i.e. canonical correspondence analysis, and NSCAIV), based on the approximation of the floristic tables by ecological variables (i.e. topography and stand structure) were also used. All analyses yielded consistent results pointing towards a major floristic gradient closely linked to topography, and to secondary gradients related to stand structure. The environmental variables had significant and non-redundant explanatory powers for floristic composition. Two main geographical partitions of the forest were revealed, one reasonably accounted for by environmental variables, the other remaining insufficiently explained. In tropical rain forests, inventories could be a valuable source of ecological information, at the price of reasonable effort oriented towards enhanced vernacular nomenclatures (collection of herbarium specimens, training of tree-spotters); in situ recording of simple environmental variables (e.g. slope, topography), and data analyses based on complementary ordination methods. (C) 2002 Elsevier Science B.V. All rights reserved.