Summary1. Predicting stand structure parameters for tropical forests from remotely sensed data has numerous important applications, such as estimating above-ground biomass and carbon stocks and providing spatial information for forest mapping and management planning, as well as detecting potential ecological determinants of plant species distributions. As an alternative to direct measurement of physical attributes of the vegetation and individual tree crown delineation, we present a powerful holistic approach using an index of canopy texture that can be extracted from either digitized air photographs or satellite images by means of two-dimensional spectral analysis by Fourier transform. 2. We defined an index of canopy texture from the ordination of the Fourier spectra computed for 3545 1-ha square images of an undisturbed tropical rain forest in French Guiana. This index expressed a gradient of coarseness vs. fineness resulting from the relative importance of small, medium and large spatial frequencies in the Fourier spectra. 3. Based on 12 1-ha control plots, the canopy texture index showed highly significant correlations with tree density ( R 2 = 0·80), diameter of the tree of mean basal area ( R 2 = 0·71), distribution of trees into d.b.h. classes ( R 2 = 0·64) and mean canopy height ( R 2 = 0·57), which allowed us to produce reasonable predictive maps of stand structure parameters from digital aerial photographs. 4. Synthesis and applications . Two-dimensional Fourier analysis is a powerful method for obtaining quantitative characterization of canopy texture, with good predictive ability on stand structure parameters. Forest departments should use routine forest inventory operations to set up and feed regional databases, featuring both tree diameter figures and digital canopy images, with the ultimate aims of calibrating robust regression relationships and deriving predictive maps of stand structure parameters over large areas of tropical forests. Such maps would be particularly useful for forest classification and to guide field assessment of tropical forest resources and biodiversity.

Summary1. Considerable debate surrounds the extent to which tropical forests can be managed for resource extraction while conserving biodiversity and ecosystem properties, which depend on functional composition. Here we evaluate the compatibility of these aims by examining the effects of logging on taxonomic and functional diversity and composition in a tropical forest. 2. Twenty years after selective logging, we inventoried 4140 stems regenerating in logging gaps and adjacent undisturbed areas, and we integrated a database of 13 functional traits describing leaf and wood economics of tropical trees. 3. We found no differences in taxonomic and functional richness among habitats, but logging gaps had significantly higher taxonomic and functional evenness. 4. Logging also effected striking, long-term changes in both species and functional composition. In particular, the xylem density of recruits in logging gaps was 6% less than in unlogged forests, leaves were 11% less tough and had 6-13% greater mineral nutrient concentrations. 5. Synthesis and applications. Our results suggest that managers of tropical forests should limit overall surface area converted to logging gaps by creating fewer, larger gaps during selective logging, to reduce impacts on the taxonomic and functional composition of the regenerating stand.

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Question: What are the relative roles of tree growth, mortality and recruitment in variations of aboveground biomass in tropical forests? Location: Paracou, French Guiana. Methods: We quantified the contribution of growth, recruitment and mortality to total biomass of stands (trees DBH _ 10 cm) in six 6.25-ha permanent plots over 16 yr. Live biomass stocks and fluxes were computed for four separate size classes. Results: All plots showed increasing biomass stocks over the study period, with an average value of 10.9Mgha_1 yr_1. Plots aggrading biomass were characterized by either minor biomass losses due to mortality or substantial increases in the biomass of large trees (DBH _ 60 cm). Conclusions: Within the study period, the rarity of mortality events could not counter-balance the slow permanent increase in biomass, resulting in an apparent increase in biomass. Accounting for such rare events results in no net change in biomass balance. (Résumé d'auteur

In beech (Fagus sylvatica L.), the number of leaf primordia preformed in the buds determines the length and the type (long versus short) of annual growth units, and thus, branch growth and architecture. We analyzed the correlation between the number of leaf primordia and the hydraulic conductance of the vascular system connected to the buds. Terminal buds of short growth units and axillary buds of long growth units on lower branches of mature trees were examined. Buds with less than four and more than five leaf primordia formed short and long growth units, respectively. Irrespective of the type of growth unit the bud was formed on, the occurrence of a large number of leaf primordia was associated with high xylem hydraulic conductance. Xylem conductance was correlated to the area of the outermost annual ring. These results suggest that organogenesis and primary growth in buds correlates with secondary growth of the growth units and thus with their hydraulic architecture. Possible causal relationships between the variables are discussed.

International audienceIn coral islands, groundwater is a crucial freshwater resource for terrestrial life, including human water supply. Response of the freshwater lens to expected climate changes and subsequent vegetation alterations is quantified for Grande Glorieuse, a low-lying coral island in the Western Indian Ocean. Distributed models of recharge, evapotranspiration and saltwater phytotoxicity are integrated into a variable-density groundwater model to simulate the evolution of groundwater salinity. Model results are assessed against field observations including groundwater and geophys-ical measurements. Simulations show the major control currently exerted by the vegetation with regards to the lens morphology and the high sensitivity of the lens to climate alterations, impacting both quantity and salin-ity. Long-term changes in mean sea level and climatic conditions (rainfall and evapotranspiration) are predicted to be responsible for an average increase in salinity approaching 140 % (+8 kg m −3) when combined. In low-lying areas with high vegetation density, these changes top +300 % (+10 kg m −3). However, due to salinity increase and its phytotoxicity, it is shown that a corollary drop in vegetation activity can buffer the alteration of fresh groundwater. This illustrates the importance of accounting for vegetation dynamics to study groundwater in coral islands

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A qualitative architectural analysis conducted on young beech (Fagus sylvatica L.), benefiting from the protection of a leaf canopy or exposed to full sunlight, revealed architectural characteristics susceptible of influencing the arrangement of vertical lateral axes entering in direct competition with the trunk and contributing to the formation of forked axes. The distinct structures of the forked trees are dependent on the absence or the presence of a superior forest canopy. Forks are formed when the following events occur in nature: monocyclic growth (formation of a single growth unit per year) and polycyclic growth (formation of numerous growth units per year) of the terminal meristem originating from the main axis, destruction of this terminal meristem, and large increases or decreases in the annual growth rate. None of these characteristics alone will determine the arrangement of a forked branch. Nonetheless, the annual fluctuations in growth rate seem to have the strongest influence. A quantitative analysis conducted on two 18-year-old groups exposed to either full sunlight (no canopy) or shade (under dense high forest) revealed that the frequency in vertical lateral axes is greater in full sunlight (0.18 – 0.45) than in shade (0.03 – 0.1). Similarly, the percentage of polycyclic shoots is greater in full sunlight (51%) than in shade (14%). However, this polycyclic growth does not seem to have a direct influence on the formation of vertical lateral axes. On the other hand, there may be an intermediary influence on apical mortality since in full sunlight the percentage of seasonally traumatized stems is greater (31%) than in shade (21%). The apical mortalities seem to stimulate the organization of the vertical branches. However, this formation of vertical lateral axes seems mainly controlled by the fluctuations in annual growth rates and more specifically by the rate of growth reached by the plant. In full sunlight, the annual stems have an average size (40 – 75 cm) notably superior to 50 cm. It is beyond this threshold of development that the observed trees produced greater annual frequencies in the appearance of vertical lateral axes. In the shade, the average size of annual stems ranged from 20 to 55 cm, but seldomly surpassed 50 cm. The annual frequency in the appearance of vertical lateral axes is negligeable. Key words: beech, Fagus sylvatica, architecture, forked branch, growth, forest canopy. [Journal translation]