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In French Guiana, the forest sector (including private and public actors) will face strategic choices to respond to the increasing demand in wood supply for the coming decades. Indeed population in French Guiana is currently growing at a rate of around 2.5% per year and should exceed 500 000 inhabitants by 2050 leading to an increased consumption of material, energy and space. Tropical humid forests that cover 90% of the territory will be at the center of production and conservation concerns. Strategic choices, based on strong political decisions, include the volume of wood to be produced locally, the type of production (timber vs fuelwood) and the origin (plantation vs natural forest). We conducted a work to analyze the opportunities of the forestry sector to meet these challenges. All wood supply is currently provided through the management of natural forests. An alternative issue would be to produce wood from forest plantation. Experimental tree plantations have been developed since the 70's in French Guiana. More than 30 local species and 26 exotic species have been tested to evaluate their potential in tree plantation. Up to now no plantation were established at lager scale in French Guiana. We identified various scenarios to reach the wood supply by 2050. The scenarios were elaborated in two phases. First, we interviewed key actors of the forestry sector who gave their view on that perspective. It appeared that tree plantation is considered as a real opportunity for most of the actors. Second, species performance in tree plantation trials was carefully analyzed through mortality rates and tree productivity. Six species (four locals and two exotics) were identified with a high potential for sustainable forest plantation. Sustainability of natural forest management was also assessed based on the results provided by the Paracou experimental plot. Four main scenarios were identified. Wood production is calculated for both managed forests and forest plantation and duration of each production cycle is provided. For each scenario, surface dedicated for natural forest management and tree plantation is estimated. Results are discussed in the light of the sparing and sharing strategies. The four scenarios are providing a framework of discussions that could help the actors to focus on the long term and to imagine the likely impacts of current policy choices on conservation of natural forests.

The seasonal climate drivers of the carbon cycle in tropical forests remain poorly known, although these forests account for more carbon assimilation and storage than any other terrestrial ecosystem. Based on a unique combination of seasonal pan-tropical data sets from 89 experimental sites (68 include aboveground wood productivity measurements and 35 litter productivity measurements), their associate canopy photosynthetic capacity (enhanced vegetation index, EVI) and climate, we ask how carbon assimilation and aboveground allocation are related to climate seasonality in tropical forests and how they interact in the seasonal carbon cycle. We found that canopy photosynthetic capacity seasonality responds positively to precipitation when rainfall is < 2000 mm per year (water-limited forests) and to radiation otherwise (light-limited forests); on the other hand, independent of climate limitations, wood productivity and litterfall are driven by seasonal variation in precipitation and evapotranspiration respectively. Consequently, lightlimited forests present an asynchronism between canopy photosynthetic capacity and wood productivity. Precipitation first-order control indicates an overall decrease in tropical forest productivity in a drier climate. (Texte intégral)

Over the past decade, much attention has been devoted to the development of forest biomass estimation methods at a stand scale, leading to the establishment of allometric models (Chave, et al. 2014). These allometric equations use a unique wood specific gravity value (WSG) per species, but neglect the within tree variations of WSG found by others (Wiemann & Williamson, 1989). The main objectives of this study are to (1) illustrate the diversity of radial and vertical patterns of WSG variation within tree trunks belonging to different ecological groups (from pioneer to sciaphilic species) and to (2) develop an estimation method of trunk biomass taking into account WSG variations. We sampled 33 young trees (10

Background: Tree above ground biomass (AGB) is currently estimated by tree-level allometrical models that take into account, tree volume estimated from proxy variables of tree size (DBH) and species average wood specific gravity (WSG). These methods are common and realistic from a practical point of view. However, they do not take into account deviance from fixed allometrical trajectories and spe-cies or tree level WSG variations. Here, we present Xtrawood software that allows computation of tree AGB according to structure and WSG variations. Method: Xtrawood reconstructs tree structure and integrates WSG variations by merging tree structure and WSG data measured at different position in trees, leading to the computation of global AGB and visualization of WSG variation along tree structure. Tree structure is measured according to stem dimensions (length, diameter) and positions within tree, and encoded in Multiscale Tree Graph format (MTG). WSG data is made of radial WSG profiles (1 measure each 0,5 cm from pith to bark) sampled at different heights within whole tree. Xtrawood output are illustrated using a dataset collected on an Amazonian forest "biomass dominant species", Dicorynia guianensis Amsh., also known to exhibit substantial WSG gradients along both radial and vertical axis. 9 trees ranging from 15 to 60 cm DBH were measured by climbers. Each tree was felled and samples were collected at different positions (3 in trunk, 1 to 5 in crown) to record WSG radial profiles. Results: Xtrawood allows computation of tree volume, but also visualization of WSG variations in tree as well as inference of WSG radial profiles at different heights. Output variables are decomposed according to different tree scale and locations (axis, trunk/crown) and easy to extract. Xtrawood results will be compared to those of standard estimation method and can be used to identify positions in trees where WSG value leads to the better estimate of tree AGB. Conclusion: Xtrawood produces AGB estimate with data from intensive measurements practices. The sampling protocol, used here, remains destructive and time-consuming because Xtrawood is not directly dedicated to forest managers, but to help calibration of realistic sampling strategies. Moreover, Xtrawood offers a way to understand relationships between tree development, WSG variations within tree structure and biomass accumulation in the context of natural forests or plantations. A software demo is available at coffe break. (Texte intégral)