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Caractériser la dynamique d’une forêt est essentiel pour la gestion forestière. Les houppiers des arbres forment un élément clé de cette dynamique ; mais, en forêt tropicale, les mesurer n’est pas simple. Cette étude teste l’utilisation d’images aériennes à haute résolution pour estimer la croissance diamétrique des arbres, en intégrant des mesures fines des houppiers détectés. Des ortho-images de 10 cm/pixel de résolution ont été obtenues à l’aide d’un drone à aile fixe sur une parcelle de 9 ha, installée dans la forêt de Yoko en République démocratique du Congo. Les inventaires menés sur les arbres de DHP ≥ 10 cm en 2008 et en 2016 ont permis d’avoir accès à différentes caractéristiques dendrométriques individuelles, dont le diamètre des arbres et leur tempérament, et de calculer des accroissements diamétriques. Des modèles linéaires mixtes ont été calibrés pour prédire l’accroissement de 163 arbres identifiés à la fois sur le terrain et sur les ortho-images en utilisant les variables quantifiées uniquement sur le terrain et/ou à partir de variables mesurées sur les ortho-images. Les images aériennes ont permis de détecter 23,4 % des arbres de DHP ≥ 10 cm inventoriés au sol, et représentant 75,1 % de la biomasse aérienne du peuplement. La probabilité de détection des arbres a varié en fonction de leur DHP : de 0,09 pour les arbres de DHP < 30 cm à 0,97 pour les arbres de DHP ≥ 60 cm. Les variables quantifiées par télédétection ajoutées aux variables de terrain ont permis d’améliorer significativement la prédiction de l’accroissement diamétrique. Les meilleurs modèles d’estimation des accroissements diamétriques contiennent notamment un terme caractérisant la dimension du houppier des arbres qui n’a pu être mesuré que par télédétection. Parmi les variables déterminées par télédétection, la superficie convexe du houppier est apparue la plus performante dans les modèles, et s’avère ainsi être la mesure la plus intéressante pour décrire la compétition entre les houppiers. Ces résultats ouvrent des perspectives pour construire de nouveaux outils d’acquisition de données au service de l’aménagement forestier.

Common allometric patterns have been reported across the tropics and good performance on independent data was retrieved for the most recent pantropical model predicting tree aboveground biomass (AGB) from stem diameter, wood density and total height. General models are undoubtedly useful for the estimation and monitoring of biomass and carbon stocks in tropical forests, however specific allometry, allocation, and traits, are at the core of many models of vegetation dynamics, and there is lack of such information for some regions and species. In this study, we specifically evaluated how size-dependent changes in above-ground biomass and biomass allocation to crown relate to other allometric and life-history traits for tropical tree species. We gathered destructive data available in eight terra firme forest sites across central Africa and the combined dataset consisted of 1,023 trees belonging to 54 tropical tree species phylogenetically dispersed, with only two congeneric species. A huge body of field and laboratory measurements was used for computing AGB and crown mass ratio (CMR) at the tree level, and to derive key allometric traits at the species level. For the latter, species-specific relationships between tree diameter and total height, crown exposure to light, wood density, and bark thickness were fitted for 50 species. Our results show interspecific variation in the relationships relating tree diameter to both AGB and CMR, and including species traits in a multi-specific AGB model confirmed that interspecific variation in biomass allometry is primarily determined by species wood density. We also showed that the allocation of biomass to crown increases linearly with tree diameter for most species, and that interspecific variation in the CMR model is associated with the species dispersal mode and maximum height. Trait covariations among our set of tropical tree species widespread and/or locally abundant in central Africa, revealed a continuum between large-statured species, which tended to be light-demanding, deciduous and wind-dispersed, and species with opposite attributes. Information on allometry, allocation, and traits provided here could further be used in comparative ecology and for parameterizing dynamic and succession models. Also importantly, the species-specific AGB models fitted for major tree species, including most timber species of central Africa, will help improve biomass estimates.

Tropical forests play a key role in regulating the terrestrial carbon cycle and climate change by storing a large amount of carbon. Yet, there is considerable uncertainty about the amount and spatial variation of aboveground biomass (AGB), especially in the relatively less studied African tropical forests. In this study, we explore the local-scale variation and determinants of plot-level AGB, between and within two types of forests, the Celtis and Manilkara forests, growing under the same climate but on different geological substrates in the northern Republic of Congo. In each forest site, all trees ≥10 cm diameter were censured in 36 × 1-ha plots and we measured tree height and crown size using a subsample of 18 × 1-ha of these plots. We developed height-diameter and crown-diameter allometric relationships and tested whether they differed between the two sites. For each 1-ha plot, we further estimated the AGB and calculated structural attributes (stem density and basal area), composition attributes (wood density) and architectural attributes (tree height and crown size), the latter being derived from site-specific allometric relationships. We found strong between-site differences in height-diameter and crown-diameter allometries. For a given diameter, trees were taller in the Celtis forest while they had larger crown in the Manilkara forest. Similar trends were found for the sixteen species present in both forest sites, suggesting an environmental control of tree allometry. Although there were some between-site differences in forest structure, composition and architecture, we did not detect any significant difference in mean AGB between the Celtis and the Manilkara forests. The AGB variation was related to the heterogeneous distribution of large trees, and influenced by basal area, height and crown dimensions, and to a lesser extent wood density. These forest attributes have strong practical implications on emerging remote-sensing technologies for carbon monitoring in tropical forests.

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SYNERGIE [Axe_IRSTEA]TETIS-ATTOS; International audience; Finding suitable models of canopy reflectance in forward simulation mode is a prerequisite for their use in inverse mode to characterize canopy variables of interest, such as leaf area index (LAI) or chlorophyll content. In this study, the accuracy of the three-dimensional reflectance model DART (Discrete Anisotropic Radiative Transfer) was assessed for canopies of different genotypes of Eucalyptus, having distinct biophysical and biochemical characteristics, to improve the knowledge on how these characteristics are influencing the reflectance signal as measured by passive orbital sensors. The first step was to test the model suitability to simulate reflectance images in the visible and near infrared. We parameterized DART model using extensive measurements from Eucalyptus plantations including 16 contrasted genotypes. Forest inventories were conducted and leaf, bark, and forest floor optical properties were measured. Simulation accuracy was evaluated by comparing the mean top of canopy (TOC) bidirectional reflectance of DART with TOC reflectance extracted from a Pleiades very high resolution satellite image. Results showed a good performance of DART with mean reflectance absolute error lower than 2%. Intergenotype reflectance variability was correctly simulated, but the model did not succeed at catching the slight spatial variation for a given genotype, excepted when large gaps appeared due to tree mortality. The second step consisted of sensitivity analysis to explore which biochemical or biophysical characteristics influenced more the canopy reflectance between genotypes. Perspectives for using DART model in inversion mode in these ecosystems were discussed. [ABSTRACT FROM PUBLISHER]

International audience; The transparency of trees is the most important indicator for a forest health assessment. This paper presents an efficient method for calculating the crown transparency coefficient from tree binary images. This coefficient is based on the automated quantification of the deep indentation, macro-hole and micro-hole densities. Circular structuring elements are introduced, among other things, to automatically find the significant biological size. The symmetric tree convex hull and the tree smoothed contour are defined to delineate the reference areas necessary to evaluate the above-mentioned densities. Statistical thresholds are proposed to eliminate human operator subjectivity, especially in the automated identification of anatomical elements such as soft and deep crown-indentations or macro and micro crown-holes. A point-wise transparency map is produced to better appreciate the origin of the visible skylight areas in the crown. The crown micro-hole density is calculated from the 0.1-to-0.5 transparency points, the crown macro-hole density from the 0.5-to-1 transparency points. We finally opt for weighting of the above three densities with regard to the importance of the symptoms they describe for a more relevant crown transparency coefficient. A comparative study on several trees from full-size and half-size binary images showed that our method is similar overall to the DSO and less sensitive to scale reduction.

Since the early 1990s, ash dieback due to the invasive ascomycete Hymenoscyphus fraxineus is threatening Fraxinus excelsior in most of its natural range. Previous studies reported significant levels of genetic variability in susceptibility in F. excelsior either in field or inoculation experiments. The present study was based on a field experiment planted in 1995, 15 years before onset of the disease. Crown and collar status were monitored on 777 trees from 23 open-pollinated progenies originating from three French provenances. Health status was modeled using a Bayesian approach where spatiotemporal effects were explicitly taken into account. Moderate narrow-sense heritability was found for crown dieback (h2 = 0.42). This study is first to show that resistance at the collar level is also heritable (h2 = 0.49 for collar lesions prevalence and h2 = 0.42 for their severity) and that there is significant genetic correlation (r = 0.40) between the severities of crown and collar symptoms. There was no evidence for differences between provenances. Family effects were detected, but computing individual breeding values showed that most of the genetic variation lies within families. In agreement with previous reports, early flushing correlates with healthier crown. Implications of these results in disease management and breeding are discussed.

UMR AGAP - équipe AFEF - Architecture et fonctionnement des espèces fruitières; Numerous agronomical applications of remote sensing have been proposed in recent years, including water stress assessment at field by thermal imagery. The miniaturization of thermal cameras allows carrying them onboard the unmanned aerial vehicles (UAVs), but these systems have no temperature control and, consequently, drifts during data acquisition have to be carefully corrected. This manuscript presents a comprehensive methodology for radiometric correction of UAV remotely-sensed thermal images to obtain (combined with visible and near-infrared data) multispectral ortho-mosaics, as a previous step for further image-based assessment of tree response to water stress. On summer 2013, UAV flights were performed over an apple tree orchard located in Southern France, and 4 dates and 5 h of the day were tested. The 6400 m(2) field plot comprised 520 apple trees, half well-irrigated and half submitted to progressive summer water stress. Temperatures of four different on-ground stable reference targets were continuously measured by thermo-radiometers for radiometric calibration purposes. By using self-developed software, frames were automatically extracted from the thermal video files, and then radiometrically calibrated using the thermal targets data. Once ortho-mosaics were obtained, root mean squared error (RMSE) was calculated. The accuracy obtained allowed multi-temporal mosaic comparison. Results showed a good relationship between calibrated images and on-ground data. Significantly higher canopy temperatures were found in water-stressed trees compared to well-irrigated ones. As high resolution field ortho-mosaics were obtained, comparison between trees opens the possibility of using multispectral data as phenotypic variables for the characterization of individual plant response to drought.

In order to quantify the effects of forests to oil palm conversion occurring in the tropics on land–atmosphere carbon, water and energy fluxes, we develop a new perennial crop sub-model CLM-Palm for simulating a palm plant functional type (PFT) within the framework of the Community Land Model (CLM4.5). CLM-Palm is tested here on oil palm only but is meant of generic interest for other palm crops (e.g., coconut). The oil palm has monopodial morphology and sequential phenology of around 40 stacked phytomers, each carrying a large leaf and a fruit bunch, forming a multilayer canopy. A sub-canopy phenological and physiological parameterization is thus introduced so that each phytomer has its own prognostic leaf growth and fruit yield capacity but with shared stem and root components. Phenology and carbon and nitrogen allocation operate on the different phytomers in parallel but at unsynchronized steps, separated by a thermal period. An important phenological phase is identified for the oil palm – the storage growth period of bud and "spear" leaves which are photosynthetically inactive before expansion. Agricultural practices such as transplanting, fertilization and leaf pruning are represented. Parameters introduced for the oil palm were calibrated and validated with field measurements of leaf area index (LAI), yield and net primary production (NPP) from Sumatra, Indonesia. In calibration with a mature oil palm plantation, the cumulative yields from 2005 to 2014 matched notably well between simulation and observation (mean percentage error = 3 %). Simulated inter-annual dynamics of PFT-level and phytomer-level LAI were both within the range of field measurements. Validation from eight independent oil palm sites shows the ability of the model to adequately predict the average leaf growth and fruit yield across sites and sufficiently represent the significant nitrogen- and age-related site-to-site variability in NPP and yield. Results also indicate that seasonal dynamics of yield and remaining small-scale site-to-site variability of NPP are driven by processes not yet implemented in the model or reflected in the input data. The new sub-canopy structure and phenology and allocation functions in CLM-Palm allow exploring the effects of tropical land-use change, from natural ecosystems to oil palm plantations, on carbon, water and energy cycles and regional climate. Open-Access Publikationsfonds 2015 peerReviewed

1.Architectural traits that determine the light captured in a given environment are an important aspect of the life‐history strategies of tropical tree species. In this study, we examined how interspecific variation in architectural traits is related to the functional traits of 45 coexisting tree species in central Africa. 2.At the tree level, we measured tree diameter, total height and crown dimensions for an average of 30 trees per species (range 14–72, total 968 trees) distributed over a large range of diameters (up to 162 cm). Using log‐log models, we fitted species‐specific allometric relationships between tree diameter, height and crown dimensions. At the species level, we derived architectural traits (height and crown dimensions) at 15 cm and maximum diameters from species‐specific allometries. The architectural traits were then related to functional traits, including light requirements, wood density, leaf habit, and dispersal mode. 3.Among the 45 coexisting tree species, we identified strong variations in height and crown allometries, along with architectural traits derived from these species‐specific allometries. There was a positive correlation among architectural traits, suggesting that large‐statured canopy species were taller and had larger and deeper crowns than small‐statured understory species at all ontogenic stages. The relationships between architectural and functional traits highlighted a continuum of species between the large‐statured canopy species and the small‐statured understory species. In this moist and seasonal forest, large‐statured canopy species tended to be light‐demanding, wind‐dispersed, deciduous and large contributors to forest biomass (high basal area), while small‐statured understory species tended to be shade‐tolerant, animal‐dispersed, evergreen and most abundant in terms of stem density. 4.Our results highlighted strong architectural differences among coexisting tropical tree species in central Africa. The relationships between architectural and functional traits provided insights into the life‐history strategy of tropical tree species.

(IF 3.95; Q1); International audience; The rapid expansion of perennial crops is a major threat to biodiversity in Southeast Asia. The biodiversity losses related to the conversion of forest lands to oil palm or rubber plantations (RP) are well documented by recent studies. However, the impact of the conversion from intensively managed annual crops to perennial crops on soil biodiversity has not yet been addressed. This study aims at assessing the impact on soil biodiversity of a) the short-term effect of land use conversion from cassava crop to RP, and b) the long-term effect of RP ageing. Soil biodiversity (bacterial, fungal and macrofaunal), microbial activities and pedoclimatic characteristics were measured over a chronosequence of 1–25 years old of RP compared to cassava fields, the former crop, in Thailand. The conversion from cassava to young RP (1–3 yr) had a significant effect on microbial biomass and activities and fungal composition, but did not impact the bacterial and macrofaunal diversity. This effect of land use conversion could be explained by the change in land management due to the cultivation of pineapple in the inter-row of the young RP. Canopy closure appeared to be the main driver of soil biota shifts, as most of the biotic parameters, composition, abundance and activities were significantly modified after 7 years of RP. The changes of composition in older rubber plantations originated from the dominance of Trichoderma (fungi), Firmicutes (bacteria), and earthworms. Old rubber plantations (23–25 yr) harboured the highest microbial and macrofaunal biomass; however, they were also characterised by a significant decrease in bacterial richness. The change in pedoclimatic conditions across the rubber chronosequence, i.e. increase in soil moisture, litter and organic carbon content, was a stronger driver of soil biota evolution than land use conversion. The macrofaunal composition was more resistant to land use conversion than the bacterial composition, whereas the microbial biomass was sensitive to land use conversion, but showed resilience after 20 years. However, bacterial, fungal and macrofaunal diversity, macrofaunal and microbial biomass and microbial activities were all sensitive to RP ageing.

The paradigm of functional-structural models (FSPM) assumes that studying the detailed organisation of plant structure allows a better understanding of functional processes; in particular the way plants capture light for performing photosynthesis. However, much attention must be paid toward the consistency between virtual plants and plants in the field in terms of size and geometry to accurately evaluate light interception. This paper thus aimed at i) assessing the capacity of a 3D architectural model based on oil palms (Elaeis guineensis) to accurately represent plants structural characteristics at both the scale of the individual plant and the cultivated plot and ii) employing the validated 3D mock-ups to investigate how light interception efficiency varies among progenies that exhibit different architectures. Innovative indicators related to plant geometry and topology were derived from terrestrial LiDAR scanners (TLS) and hemispherical photographs (HP) in order to assess a 3D plant model. Indicators such as plant height, width and volume, gap fractions and solid angle projections were established from field measurements and were compared to equivalent indicators that had been extracted from virtual TLS (VTLS) and virtual HP (VHP) simulated on 3D mock-ups. Indicators were then evaluated for their significance in terms of light interception. Progeny effect on light interception efficiency was finally evaluated for five progenies. The structural indicators estimated from VTLS and VHP were significantly correlated with equivalent indicators estimated from TLS and HP, respectively, and with simulated outputs related to light interception. Light interception efficiencies estimated from validated 3D mock-ups differed significantly among the five progenies under study, most notably along plant development. Our results highlight the relevance of combining TLS- and HP-derived indicators to evaluate the reliability of virtual 3D reconstruction of plants in relation to light capture, at both the plant and plot scales. The study paves the way for further investigations aiming at unravelling the relationships between oil palm architecture and the physiological processes driving its production.(Resume d'auteur)

International audience; Heterotrophic soil respiration (SRH) alone can contribute up to 50% of total ecosystem respiration in tropical forests. Whereas the abiotic controls of SRH have been extensively studied, the influence of plant traits is less well characterised. We used field experiments and a modelling approach to test the relative influence of plant traits on SRH in lowland tropical forest in French Guiana. We measured leaf- and root litter traits for five common tree species and conducted a root decomposition experiment to evaluate the influence of root chemistry on decay rates. We measured SRH in trenched plots and used our field measurements to parameterize and test the Century model of soil C dynamics. Overall, the Century model performed well in simulating SRH, and species-specific root decomposition in Century corresponded well to decomposition rates measured in situ. Root litter characterized by low lignin-to-nitrogen ratios decomposed more rapidly than low-quality root litter during the first 6 months. Model runs over different time scales revealed that litter quality substantially influenced SRH on an annual time-scale by determining the rates of root- and leaf litter decomposition. However, litter mass had an overriding influence on SRH over the longer term in 20-year model runs. Synthesis Using simple plant trait data to parameterise the Century model, we were able to accurately simulate changes in SRH in a lowland tropical forest. Our results suggest that this approach could be used to predict changes in tropical soil C dynamics under global change scenarios by including data on changes in plant productivity and C inputs to the soil (for example litterfall and root turnover).

International audience; Despite advances in Earth observation and modeling, estimating tropical biomass remains a challenge. Recent work suggests that integrating satellite measurements of canopy height within ecosystem models is a promising approach to infer biomass. We tested the feasibility of this approach to retrieve aboveground biomass (AGB) at three tropical forest sites by assimilating remotely sensed canopy height derived from a texture analysis algorithm applied to the high-resolution Pleiades imager in the Organizing Carbon and Hydrology in Dynamic Ecosystems Canopy (ORCHIDEE-CAN) ecosystem model. While mean AGB could be estimated within 10% of AGB derived from census data in average across sites, canopy height derived from Pleiades product was spatially too smooth, thus unable to accurately resolve large height (and biomass) variations within the site considered. The error budget was evaluated in details, and systematic errors related to the ORCHIDEE-CAN structure contribute as a secondary source of error and could be overcome by using improved allometric equations.

International audience; Heartwood/sapwood profile and the tradeoff between trunk and crown increment in a natural forest: the case study of a tropical tree (Dicorynia guianensis Amsh., Fabaceae) Key message Sapwood area and the radial growth rate of the trunk follow the same pattern at breast height, with an initial increase and subsequent constant value, resulting from the increasing growth allocation toward the crown rather than tree decline. Heartwood area and heartwood volume in the trunk increase more rapidly after this shift occurs. Abstract Sapwood (SW) and heartwood (HW) are two functionally distinct classifications of wood in perennial stems for which quantities can vary greatly in tropical trees. Numerous positive correlations have been found between the radial growth rate (RGR) and SW quantity; however, variations in the SW/HW quantities have not been studied in light of the ontogenetic variation of RGR. Wood core sampling, intensive measurements of tree structure (number of branches, stem volumes), and radial growth monitoring were performed on an abundant and highly exploited tree species in French Guiana (Dicorynia guianensis) to investigate the relationship between RGR, SW/HW quantity, tree structure, and their variations on the course of a tree's ontogeny. SW area and RGR followed the same pattern of variation throughout tree development, both increasing first and reaching a steady state after 50 cm DBH (diameter at breast height). After this value, we observed a strong increase in both the HW area and HW volume increment, concomitant with a more rapid increase in crown volume. The stabilization of RGR for trees with DBH >50 cm was related not to a tree's decline but rather to an increasing wood allocation to the crown, confirming that RGR at breast height is a poor indicator of whole-tree growth for bigger individuals. We also confirmed that HW formation is an ontogenetic process managing SW quantity that is continuously and increasingly produced within the crown as the tree grows. This study highlights the effect of growth-mediated ontogenetic changes on the localization of water and carbohydrate storage within a tree, resulting from SW and HW dynamics throughout tree ontogeny.

International audience; There are strong uncertainties regarding LAI dynamics in forest ecosystems in response to climate change. While empirical growth & yield models (G&YMs) provide good estimations of tree growth at the stand level on a yearly to decennial scale, process-based models (PBMs) use LAI dynamics as a key variable for enabling the accurate prediction of tree growth over short time scales. Bridging the gap between PBMs and G&YMs could improve the prediction of forest growth and, therefore, carbon, water and nutrient fluxes by combining modeling approaches at the stand level. Our study aimed to estimate monthly changes of leaf area in response to climate variations from sparse measurements of foliage area and biomass. A leaf population probabilistic model (SLCD) was designed to simulate foliage renewal. The leaf population was distributed in monthly cohorts, and the total population size was limited depending on forest age and productivity. Foliage dynamics were driven by a foliation function and the probabilities ruling leaf aging or fall. Their formulation depends on the forest environment. The model was applied to three tree species growing under contrasting climates and soil types. In tropical Brazilian evergreen broadleaf eucalypt plantations, the phenology was described using 8 parameters. A multi-objective evolutionary algorithm method (MOEA) was used to fit the model parameters on litterfall and LAI data over an entire stand rotation. Field measurements from a second eucalypt stand were used to validate the model. Seasonal LAI changes were accurately rendered for both sites (R-2 = 0.898 adjustment, R-2 = 0.698 validation). Litterfall production was correctly simulated (R-2 = 0.562, R-2 = 0.4018 validation) and may be improved by using additional validation data in future work. In two French temperate deciduous forests (beech and oak), we adapted phenological sub-modules of the CASTANEA model to simulate canopy dynamics, and SLCD was validated using LAI measurements. The phenological patterns were simulated with good accuracy in the two cases studied. However, IA/max was not accurately simulated in the beech forest, and further improvement is required. Our probabilistic approach is expected to contribute to improving predictions of LAI dynamics. The model formalism is general and suitable to broadleaf forests for a large range of ecological conditions. (C) 2014 Elsevier B.V. All rights reserved.

Competition with neighboring trees of different species can affect crown size and shape. However, whether intra-specific differences in crown characteristics in mixed stands compared to pure stands are dependent on site conditions remains poorly understood. We used terrestrial laser scanning (TLS) to examine the differences in Fagus sylvatica crown characteristics at four sites, each of which contained pure stands of F. sylvatica and their mixture with Pinus sylvestris. These sites covered the area where the mixture occurs in Europe from south to north, representing a gradient of F. sylvatica productivity, defined as the mean increment of annual volume growth in pure F. sylvatica stands. Despite the large range in productivity, F. sylvatica trees in mixtures had larger crowns regardless of site conditions, with a higher proportion of their crown volume in the lower canopy compared to trees in pure stands. Larger crown volumes were related to higher live crown ratios and greater crown expansion, depending on the site. The magnitude of the mixing effect was variable among the crown characteristics evaluated, but overall our findings provide evidence that for a given species combination and density, the effect of mixture increased in the two most productive sites. TLS-derived novel crown metrics revealed that the mixing effect was affected by productivity, which was not captured by traditionally measured crown variables. © 2017 Elsevier B.V.

The development of new breeding strategies to find more sustainable and productive systems is a major challenge to cope with ceaseless increasing demands for vegetable oils, notably palm oil. Optimizing plant architecture to increase radiation interception efficiency could be an option for enhancing potential oil palm production. Indeed, studies in cereals showed great improvement of yields by altering plant architecture, in combination with agronomic practices. By analogy, we proposed to investigate the influence of oil palm architecture on the capacity of the plant to intercept light, by using 3D reconstructions and model-assisted evaluation of radiation-use efficiency. The first objective of this study was to analyse and model oil palm architecture and light interception taking into account genetic variability. A second objective was to explore the potential improvement in light capture and carbon assimilation by manipulating oil palm leaf traits and propose architectural ideotypes. Data were collected in Sumatra, Indonesia, on five progenies (total of 60 palms), in order to describe the aerial architecture from leaflet to crown scales. Allometric relationships were applied to model these traits according to ontogenetic gradients and leaf position within the crown. The methodology allowed reconstructing virtual oil palms at different stages over plant development. Additionally, the allometric-based approach was coupled to mixed-effect models in order to integrate inter and intra progeny variability through progeny-specific parameters. The model thus allowed simulating the specificity of plant architecture for a given progeny while including observed inter-individual variability. The architectural model, once parameterized for the different progenies, was then implemented in AMAPstudio to generate 3D mock-ups and estimate light interception efficiency, from individual to stand scales. Model validations were performed at different scales. Firstly at organ scale, the geometry of the stem, the leaves and the leaflets were compared between virtual mock-ups and actual plants measured in the field. Secondly, at plant scale with indicators derived from terrestrial laser scanning (TLS) to assess crown dimensions and porosity. These indicators integrated topological and geometrical information related to the amount of light intercepted by an individual. Finally, validations were performed at plot scale using hemispherical photographs (HP) to assess the variability of canopy openness for the five studied progenies. Significant differences in leaf geometry (petiole length, density of leaflets and rachis curvature) and leaflets morphology (gradients of leaflets length and width) were detected between and within progenies, and were accurately simulated by the modelling approach. The comparison of plant area obtained from TLS and virtual TLS highlighted the capacity of the model to generate realistic 3D mock-ups. The architectural variabilities observed at plot scale between and within progenies were also satisfactory simulated. Finally, light interception estimated from the validated 3D mock-ups showed significant variations among the five progenies. Sensitivity analyses (Morris method and metamodelling approach) were then performed on a subset of architectural parameters in order to identify the architectural traits impacting light interception efficiency and potential carbon assimilation over plant development. Daily carbon assimilation was estimated with a photosynthesis model coupled to the radiative balance model, which enabled to integrate the temporal and spatial variations of photosynthetic organ irradiances. The most sensitive parameters over plant development were those related to leaf area (rachis length, number of leaflets, leaflets morphology), although fine attribute related to leaf geometry showed increasing influence when canopy got closed. In adult stand, optimized carbon assimilation was estimated on plants presenting a leaf area index (LAI) between 3.2 and 5.5 m2.m−2, with erect leaves, short rachis and petiole and high number of leaflet on rachis. Four ideotypes were identified with respect to carbon assimilation, exhibiting specific geometrical features that optimize light distribution within plant crown and reduce mutual shading among plants. In conclusion, this study highlighted how a functional-structural plant model (FSPM) can be used to virtually explore plant biology. In our case, the 3D model of oil palm, in its conception and its application, enabled the detection of the architectural traits genetically determined and influencing light interception. The limited number of traits revealed in the sensitivity analysis and the combination of traits proposed through ideotypes could guide future breeding programs. Forthcoming work will be dedicated to integrate in the modeling approach other physiological processes such as stomatal conductance and carbon partitioning. The improved FSPM could then be used to test different scenarios, for instance in climate change context with low radiations or frequent drought events. Similarly, the model could be used to investigate different planting patterns and intercropping systems and propose new multi-criteria ideotypes of oil palm.

Across five biogeographic areas, DBH-CA allometry was characterized by inter-site homogeneity and intra-site heterogeneity, whereas the reverse was observed for DBH-H allometry. Tree crowns play a central role in stand dynamics. Remotely sensed canopy images have been shown to allow inferring stand structure and biomass which suggests that allometric scaling between stems and crowns may be tight, although insufficiently investigated to date. Here, we report the first broad-scale assessment of stem vs. crown scaling exponents using measurements of bole diameter (DBH), total height (H), and crown area (CA) made on 4148 trees belonging to 538 species in five biogeographic areas across the wet tropics. Allometries were fitted with power functions using ordinary least-squares regressions on log-transformed data. The inter-site variability and intra-site (sub-canopy vs. canopy trees) variability of the allometries were evaluated by comparing the scaling exponents. Our results indicated that, in contrast to both DBH-H and H-CA allometries, DBH-CA allometry shows no significant inter-site variation. This fairly invariant scaling calls for increased effort in documenting crown sizes as part of tree morphology. Stability in DBH-CA allometry, indeed, suggests that some universal constraints are sufficiently pervasive to restrict the exponent variation to a narrow range. In addition, our results point to inverse changes in the scaling exponent of the DBH-CA vs. DBH-H allometries when shifting from sub-canopy to canopy trees, suggesting a change in carbon allocation when a tree reaches direct light. These results pave the way for further advances in our understanding of niche partitioning in tree species, tropical forest dynamics, and to estimate AGB in tropical forests from remotely sensed images.

Context In tropical areas, studies based on the retrospective analysis of tree development have focused principally on growth ring research. The interpretation of primary growth markers is overlooked although it opens perspectives to provide long time-series on tree-crown development. Aims This study focused on Parkia velutina, an emergent tree of neotropical rain forests. Our objectives were (1) to characterize the phenological cycle of this species, and (2) to identify temporally interpretable morphological and anatomical markers. Methods We collected dominant branches in 14 adult trees and identified growth markers that limit longitudinal and radial increments. We coupled this approach with a 2-year phenological survey of 20 trees. Results Leaf shedding, growth unit elongation and growth ring formation define the phenological cycle. At tree scale, this cycle is synchronous and affects all axes. At population scale, trees can be desynchronized. This cycle is annual despite some slight variability. Successive growth units and growth rings are easily identifiable. Conclusion Dating a branch by counting the number of growth units or growth rings is possible in many years with a reasonable error. Nevertheless, estimating their precise month of formation in order to study climatic influences remains difficult.

International audience; By harvesting scattered large trees, selective logging increases light availability and thereby stimulates growth and crown expansion at early-life stage among remnant trees. We assessed the effects of logging on total and merchantable bole (i.e., lowest branch at crown base) heights on 952 tropical canopy trees in French Guiana. We observed reductions in both total (mean, -2.3 m) and bole (mean, -2.0 m) heights more than a decade after selective logging. Depending on local logging intensity, height reductions resulted in 2-13 percent decreases in aboveground tree biomass and 3-17 percent decreases in bole volume. These results highlight the adverse effects of logging at both tree and stand levels. This decrease in height is a further threat to future provision of key environmental services, such as timber production and carbon sequestration.

Canopy height is a key variable in tropical forest functioning and for regional carbon inventories. We investigate the spatial structure of the canopy height of a tropical forest, its relationship with environmental physical covariates, and the implication for tropical forest height variation mapping. Making use of high-resolution maps of LiDAR-derived Digital Canopy Model (DCM) and environmental covariates from a Digital Elevation Model (DEM) acquired over 30,000 ha of tropical forest in French Guiana, we first show that forest canopy height is spatially correlated up to 2500 m. Forest canopy height is significantly associated with environmental variables, but the degree of correlation varies strongly with pixel resolution. On the whole, bottomland forests generally have lower canopy heights than hillslope or hilltop forests. However, this global picture is very noisy at local scale likely because of the endogenous gap-phase forest dynamic processes. Forest canopy height has been predictively mapped across a pixel resolution going from 6 m to 384 m mimicking a low resolution case of 3 points˙km2. Results of canopy height mapping indicated that the error for spatial model with environment effects decrease from 8.7 m to 0.91 m, depending of the pixel resolution. Results suggest that, outside the calibration plots, the contribution of environment in shaping the global canopy height distribution is quite limited. This prevents accurate canopy height mapping based only on environmental information, and suggests that precise canopy height maps, for local management purposes, can only be obtained with direct LiDAR monitoring., SCOPUS: ar.j, info:eu-repo/semantics/published

International audience; Background and Aims: Morphological variation in light-foraging strategies potentially plays important roles in efficient light utilization and carbon assimilation in spatially and temporally heterogeneous environments such as tropical moist forest understorey. By considering a suite of morphological traits at various hierarchical scales, we examined the functional significance of crown shape diversity and plasticity in response to canopy openness.Methods: We conducted a field comparative study in French Guiana among tree saplings of 14 co-occurring species differing in light-niche optimum and breadth. Each leaf, axis or crown functional trait was characterized by a median value and a degree of plasticity expressed under contrasting light regimes.Key Results: We found divergent patterns between shade-tolerant and heliophilic species on the one hand and between shade and sun plants on the other. Across species, multiple regression analysis showed that relative crown depth was positively correlated with leaf lifespan and not correlated with crown vertical growth rate. Within species displaying a reduction in crown depth in the shade, we observed that crown depth was limited by reduced crown vertical growth rate and not by accelerated leaf or branch shedding. In addition, the study provides contrasting examples of morphological multilevel plastic responses, which allow the maintenance of efficient foliage and enable effective whole-plant light capture in shaded conditions under a moderate vertical light gradient.Conclusions: This result suggests that plastic adjustment of relative crown depth does not reflect a strategy maximizing light capture efficiency. Integrating and scaling-up leaf-level dynamics to shoot- and crown-level helps to interpret in functional and adaptive terms inter- and intraspecific patterns of crown traits and to better understand the mechanism of shade tolerance.

International audience

The importance of measuring forest biophysical properties for ecosystem health monitoring and forest management encourages researchers to find precise, yet low-cost methods especially in mountainous and large areas. In the present study, geoscience laser altimeter system (GLAS) on board Ice, Cloud, and land Elevation Satellite (ICESat) was used to estimate three biophysical characteristics of forests located in the north of Iran: 1) maximum canopy height ( ${\text {H}_{{\max}}}$ ); 2) Lorey’s height ( ${{\text H}_{\text{Lorey}}}$ ); and 3) forest volume (V). A large number of multiple linear regressions (MLR) and also random forest (RF) regressions were developed using different sets of variables including waveform metrics, principal components (PCs) produced from principal component analysis (PCA) and wavelet coefficients (WCs) generated from wavelet transformation (WT). To validate and compare models, statistical criteria were calculated based on a fivefold cross validation. Best model concerning the maximum height was an MLR ( ${\text {RMSE}} = {5.0}\;{\text m}$ ) which combined two metrics extracted from waveforms (waveform extent “ ${{\text W}_{\text{ext}}}$ ” and height at 50% of waveform energy “ ${{\text H}_{{50}}}$ ”), and one from digital elevation model (terrain index, TI). The mean absolute percentage error (MAPE) of maximum height estimates was 16.4%. For Lorey’s height, a simple MLR (including ${{\text W}_{\text{ext}}}$ and TI) represented the highest performance ( ${\text {RMSE}} = {5.1}\;{\text m}$ , ${\text {MAPE}} = {24.0}\% $ ). Generally, MLR models had a better performance when compared to the RF models. In addition, the accuracy of height estimations using waveform metrics was greater than those based on PCs or WCs. Concerning forest volume, regression models estimating volume directly from GLAS data led to a better result ( ${\text {RMSE}} = {128.8}\;{{\text m}^{3}}/\text {ha}$ ) rather than volume– ${{\text H}_{\text{Lorey}}}$ relationship ( ${\text {RMSE}} = {167.8}\;{{\text m}^{3}}/\text {ha}$ ).

Carbon allocation to the cambium along the stem is represented by a reaction-diffusion model along a continuous sink. Verti- cal variations of stem area increment along the stem are then theoretically connected to partitioning coefficients between tree compart- ments, at different spatial scales. This model is very sensitive to environmental growth conditions, and demonstrates the importance of topology and geometry in models of secondary tree growth. An analytical resolution is proposed to describe the vertical profile of stem area increment between crown basis and soil level. An empirical parametric equation is derived from this theoretical model. The 3 para- meters of this equation are related to the internal and environmental conditions of the tree. These parameters can be used as indicators in order to study the variability of stem taper. This equation is separately fitted on data from two experiments, with different silviculture and site quality, for Picea abies of different ages. Variation in the parameters is discussed according to growth conditions. This equation is further integrated in order to predict stem volume increment. Finally, some simple characteristic heights are derived from this function as indicators of functional crown basis. These heights are systematically calculated to predict crown recession. They are finally compared to heights measured during field work.

Afin de pallier le manque de simulateurs adaptés à la sylviculture locale et intense du hêtre (détourage d'arbres d'avenir), nous avons construit un modèle d'arbre à partir d'hypothèses écophysiologiques. Ce modèle linéaire relie la croissance de la tige à la taille du houppier. Le rapport de proportionnalité est nommé rendement du houppier. Ce paramètre macroscopique peut être obtenu comme combinaison de quelques paramètres écophysiologiques élémentaires (rendement photosynthétique, coefficient d'allocation à la tige, coefficients de respiration etc.). La simplicité du modèle permet d'analyser les variations du rendement du houppier dans un essai sylvicole. On montre à cette occasion que : - l'éclaircie entraîne un effet direct d'augmentation du rendement du houppier ; - la refermeture du couvert et les fluctuations climatiques entraînent d'importantes différences de rendement entre périodes successives ; - pour montrer un effet de l'âge, la structure mathématique du modèle doit être améliorée, notamment par une description plus fidèle de la respiration d'entretien et de la géométrie des cernes annuels, In order to design a simulator of local, intensive silviculture to the benefit of future crop trees in Common Beech, an individual-tree growth model was built using ecophysiological assumptions. This linear model links stem growth and crown size. The proportionality factor is named crown efficiency. This macroscopic parameter may be obtained as a combination of several elementary ecophysiological parameters (photosynthetic efficiency, allocation coefficient to stem, respiration coefficients etc.). The simple structure of the model allows the analysis of crown efficiency variations in a thinning trial. We show that: - thinning directly increase crown efficiency; - canopy closure and climatic fluctuations lead to large differences of crown efficiency between successive periods; - in order to show an effect of age, the mathematical structure of the model should be improved, e.g. by a better account of maintenance respiration and geometry of the annual ring.

Ce travail de thèse explore la signification écologique des variations inter et intraspécifiques de la croissance et des traits fonctionnels des espèces d’arbres de forêt tropicale guyanaise en relation avec les conditions d’éclairement. Nous démontrons que le syndrome de traits associé à la niche de régénération (durée de vie des feuilles plus longue et surface massique plus grande pour les espèces les plus tolérantes à l’ombre) est valide pour 14 espèces non pionnières et non strictement sciaphiles. La plasticité de la surface massique exprimée au stade juvénile chez les espèces les moins spécialisées pourrait refléter une adaptation à l’hétérogénéité de l’éclairement rencontrée au stade jeune et au cours des stades ontogéniques successifs. Les analyses multivariées mettent en évidence un lien étroit entre la durée de vie des feuilles, la profondeur relative du houppier et la niche de régénération. La plasticité de la profondeur du houppier observée chez les espèces les plus héliophiles ne peut pas être interprétée comme une réponse d’évitement de l’auto-ombrage. Nous montrons qu’elle correspond plutôt à un changement des taux de croissance et de mortalité des feuilles et des branches imposé par les conditions d’éclairement et se produisant à l’échelle de la plante entière. En ce qui concerne la variabilité intra-spécifique de la croissance des arbres adultes, nous avons observé une faible contribution de la compétition pour la lumière et un effet important de l’espèce. Nos résultats indiquent que ce patron de réponse résulte de la partition des espèces le long du gradient lumineux vertical et de la forte abondance d’espèces de petite taille peu sensibles à la compétition du fait de leur adaptation aux conditions lumineuses du sous-bois. La différentiation de niche de régénération et la stratification verticale des arbres adultes étayent l’hypothèse d’une partition de niche vis à vis de la ressource lumineuse dans les trois dimensions de la forêt tropicale humide., This study explores the ecological significance of inter- and intraspecific variations of growth and functional traits found in tropical tree species of French Guiana in relation to light regime. We demonstrate that the syndrome of leaf traits associated with light -niche (longer life span, higher leaf mass per area of more shade tolerant species) holds among a set of 14 species comprising no pioneer nor any shade specialist species. Our results further suggest that plasticity of leaf mass per area expressed at the sapling stage may reflect adaptation to the spatial heterogeneity of light conditions encountered both at the sapling stage and across ontogenetic stages. Multivariate analysis provides evidence of a close linkage between leaf lifespan, relative crown depth and light-niche optimum. The plasticity in relative crown depth observed in shade-intolerant species can not be interpreted in terms of self-shading avoidance. Rather, crown depth adjustment was found to be a consequence of changes in growth rates and mortality rates of leaves and branches imposed by current light conditions and occurring at the whole-plant level. At the adult stage, our results point to competition for light making an unexpectedly low contribution to individual tree growth variations, contrasting with a major effect of species identity. We argue that this pattern of variation is likely to result from the vertical niche partitioning of species and the high abundance of small-statured species that are moderately responsive to light competition (consistent with their adaptation to understorey conditions). Light-niche differentiation of juvenile trees and vertical stratification of adult trees provide strong evidence for light niche partitioning in the three-dimensional space of tropical forests.

In tropical rainforests, the sustainability of selective logging is closely linked to the extent of collateral stand damage. The capacity to measure the extent of such damage is essential for calculating carbon emissions due to forest degradation under the Reducing Emissions from Deforestation and Forest Degradation (REDD+) process. The use of remote sensing to detect canopy gaps in tropical rainforests is an attractive alternative to ground surveys, which are laborious and imprecise. In French Guiana, the detection of logging-related gaps using very high spatial resolution optical satellite images produced by the Système Pour l'Observation de la Terre (SPOT) 5 sensor is carried out by Office National des Forêts (ONF) (French National Forestry Agency). Gaps are detected using a segmentation method based on computer-assisted photointerpretation. Detection has been automated to improve and accelerate the process. We developed an automatic method, which involves estimating segmentation thresholds using a statistical approach. The principle of the method presented in this article is to model the forest's spectral signature by using a Gaussian distribution and calculate a divergence between that theoretical signature and the image histogram in order to detect gaps that constitute a reduction of forest cover. The segmentation threshold between gap and forest is thus no longer defined in the original radiometric area but as a discrepancy between theoretical distribution and histogram. Computing the divergence to define the threshold made it possible to efficiently automate the detection of all gaps and skid trails with a surface area greater than 100 m2. The proportion of misclassified points measured during field surveys is 12%, which is a high level of precision. The proportion of misclassified points obtained is 12%. This tool could be used to assess the quality of logging operations or biomass loss in other areas where the forest is undergoing deterioration while still remaining predominant in the landscape.

Seasonal variations in environmental conditions influence the functioning of the whole ecosystem of tropical rain forests, but as yet little is known about how such variations directly influence the leaf gas exchange and transpiration of individual canopy tree species. We examined the influence of seasonal variations in relative extractable water in the upper soil layers on predawn leaf water potential, saturated net photosynthesis, leaf dark respiration, stomatal conductance, and tree transpiration of 13 tropical rain forest canopy trees (eight species) over 2 yr in French Guiana. The canopies were accessed by climbing ropes attached to the trees and to a tower. Our results indicate that a small proportion of the studied trees were unaffected by soil water depletion during seasonal dry periods, probably thanks to efficient deep root systems. The trees showing decreased tree water status (i.e., predawn leaf water potential) displayed a wide range of leaf gas exchange responses. Some trees strongly regulated photosynthesis and transpiration when relative extractable water decreased drastically. In contrast, other trees showed little variation, thus indicating good adaptation to soil drought conditions. These results have important applications to modeling approaches: indeed, precise evaluation and grouping of these response patterns are required before any tree-based functional models can efficiently describe the response of tropical rain forest ecosystems to future changes in environmental conditions. (Resume d'auteur)

National audience

L'objectif de cette thèse est l'étude rétrospective du développement de la couronne de plusieurs grands arbres de la forêt tropicale humide guyanaise. Nous avons retenu des espèces possédant des tempéraments écologiques, des modèles architecturaux et des cycles phénologiques variés, mais présentant toutes au moins un marqueur morphologique ou anatomique susceptible d'être utilisé dans le cadre d'une analyse rétrospective du développement des axes qui composent la couronne. Nous avons décrit des branches sommitales représentatives de la croissance en hauteur et de la reproduction des individus. Parallèlement, nous avons mis en place un suivi phénologique mensuel afin de pouvoir confronter les résultats issus des descriptions rétrospectives aux résultats du suivi phénologique. L'analyse rétrospective permet de reconstituer l'histoire des branches sommitales sur 10 à 15 cycles de développement en moyenne, mais il est possible de remonter jusqu'à 40 cycles chez certaines espèces. Les informations recueillies nous renseignent sur l'organisation intrinsèque du cycle phénologique de l'organisme ainsi que sur les modalités de construction de la couronne et de reproduction de l'espèce. Le fait de pouvoir reconstituer l'histoire des différents méristèmes individuels permet dans certains cas de faire des hypothèses sur leur fonctionnement dans l'espace et dans le temps au sein des structures ramifiées. Finalement, la méthode autorise la comparaison d'espèces dont les cycles phénologiques ont des dimensions temporelles absolues différentes dans un cadre temporel relatif identique dont " l'unité ", la " pousse ", correspond à l'expression morphologique du cycle phénologique dans son ensemble. L'analyse rétrospective de l'appareil caulinaire des arbres tropicaux apparait donc comme une méthode à la fois souple et robuste, complémentaire des suivis de croissance et des suivis phénologiques plus classiques. Les résultats obtenus sont originaux et nouveaux dans le domaine de l'écologie tropicale et ils ouvrent des perspectives intéressantes.

[Notes_IRSTEA]ONLINE FIRST 6 OCTOBRE 2011 [Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVINHandling Editor: Erwin Dreyer; International audience; Context: Light availability in forest understory is essential for many processes; it is, therefore, a valuable information regarding forest management. However, its estimation is often difficult and direct measurements are tedious. Models can be used to compute understory light, but they often require a lot of field data to accurately predict light distribution, particularly in the case of heterogeneous canopies. Aims : The influence of the precision level of crown description was studied with a model (MμSLIM) that can be used with both detailed and coarse parameterization with the aim of reducing field data requirements to a minimum. Methods :We analyzed the deterioration of the prediction quality of light distribution to the reduction of inputs by comparing simulations to transmitted light measurements in forests of increasing complexity in three different locations. Results : With a full set of parameters to describe the tree crown (i.e., crown extension in at least eight directions, crown height and length), the model accurately simulated the light distribution. Simplifying crown description by a geometric shape with a mean radius of crown extension led to deteriorated but acceptable light distributions. Allometric relationships used to calculate crown extension from trunk diameter at breast height seriously reduced light distribution accuracy.

National audience; Dans le cas d’études menées sur la biodiversité dans les peuplements forestiers, nous nous sommes interrogés sur la possibilité d’estimer rapidement et depuis le sol, le volume de bois mort des houppiers de chênes adultes avec une précision acceptable. Pour y répondre nous avons testé depuis le sol, trois méthodes d’estimation rapide du volume et du stade de décomposition du bois mort dans les houppiers sur un échantillon de 12 chênes de réserve. La première méthode «globale» consiste à évaluer visuellement dans son ensemble le pourcentage du volume de branches mortes. La deuxième méthode «branche par branche à l’oeil» estime à l’oeil le diamètre et la longueur de chaque branche morte. La troisième méthode «branche par branche appareillé» mesure le diamètre de chaque branche morte au pentaprisme de Weelher en estimant sa longueur à l'oeil. Les deux dernières méthodes dites «branche par branche» ont été comparées entre elles et à des données de référence obtenues par mesure directe par un grimpeur. Les résultats montrent que la méthode «branche par branche à l’oeil» est la plus adaptée à nos besoins, car elle est rapide et l’incertitude est de l’ordre de 20 % sur les volumes de bois morts et très faible sur les stades de décomposition.

International audience; Decreasing growth rate (Feeley et al., 2007) and large die-back due to drought (Phillips et al., 2009) suggest that tropical forests are suffering recent climate changes. Forest vulnerability to external factors (e.g. air pollution, acid rain) is widely studied in northern countries, while only a few attempts have investigated crown integrity in the Tropics. The method needs to be generic enough to account for the large number of species and crown shapes encountered in tropical forests. In the present study, we developed and tested a novel field method that estimates crown fragmentation (main branch mortality (MB) and secondary branch mortality (SB)), liana infestation (LI) and crown position (CP) in the canopy. The relationship between crown fragmentation and annual growth rate (agr) was investigated through multiple regression. Six out of eight canopy tree species showed significant growth decline with increasing crown fragmentation. Higher probability of death was also found in trees with severe crown fragmentation. The capacity of such crown assessment to depict tree vitality in a forest stand is discussed along with potential applications in both forest science and management

International audience; This paper presents a new framework for modeling light interception by isolated trees which makes it possible to analyze the influence of structural tree organization on light capture. The framework is based on a multiscale representation of the plant organization. Tree architecture is decomposed into a collection of components representing clusters of leaves at different scales in the tree crown. The components are represented by porous envelopes automatically generated as convex hulls containing components at a finer scale. The component opacity is defined as the interception probability of a light beam going through its envelope. The role of tree organization on light capture was assessed by running different scenarii where the components at any scale were either randomly distributed or localized to their actual 3D position. The modeling framework was used with three-dimensional digitized fruit trees, namely peach and mango trees. A sensitivity analysis was carried out to assess the effect of the spatial organization in each scale on light interception. This modeling framework makes it possible to identify a level of tree description that achieves a good compromise between the amount of measurement required to describe the tree architecture and the quality of the resulting light interception model.

La savanisation croissante et la disparition des galeries forestières, au Cameroun, oblige les cacaoculteurs à installer leurs systèmes agroforestiers (SAF) sur savane. L'Action Thématique Programmée CARESYS (CIRAD) vise la "CARactérisation et l'Evaluation des performances agro-écologiques de SYStèmes de culture plurispécifiques de la Zone Tropicale Humide". Une première étude a montré une dynamique de restauration de la fertilité de cacaoyères en savane. La présente étude s'intéresse au bilan radiatif à l'intérieur des strates végétales composant les SAF: canopée d'ombrage, cacaoyers sur des sous-parcelles homogènes pédologiquement. Après un relevé des essences suivi d'une cartographie de l'ambiance radiative, un classement des cacaoyers en fonction de l'ombrage subi fut établi: (i) absence d'ombrage supérieur, (ii) ombrage supérieur faible et (iii) ombrage supérieur fort. Il apparait que la répartition et la dominance des arbres, en fonction de leur force d'ombrage évolue à mesure que la canopée devient mature. Sur les cartographies radiatives, le pourcentage d'ouverture de la canopée est de plus en plus faible à mesure que la parcelle vieillit. Plus l'ombrage domine, plus cette canopée limite la photosynthèse des cacaoyers. Le L.A.I. mesuré est plus développé sous des conditions d'ombrage Faible que Fort pour des cacaoyères en production. La méthode de caractérisation lumineuse des SAF multistrates est opérationnelle et reflète la réalité des conditions radiatives et d'ombrage observées. L'expérimentation pourrait être poursuivie en parallèle de mesures agronomiques (développement et production) et/ou de dégâts (ravageurs, microorganismes).

La génération de paysage naturel virtuel réaliste a beaucoup d'applications, aussi bien en infographie qu'en écologie par exemple. Ce problème est généralement traité en deux étapes. Une distribution des plantes sur le terrain est d'abord calculée en utilisant des modèles simplifiés de plantes. Puis ces modèles sont utilisés pour générer des représentations géométriques détaillées des plantes. En suivant cette méthodologie, nous présentons dans ce travail une réalisation d'un tel processus sur une forêt de hêtres. Pour cela, nos proposons d'utiliser une méthode de distribution stochastique d'arbres basée sur les processus de Gibbs. Une contribution importante de ce travail est la prise en compte la plasticité des houppiers lors de la distribution en considérant notamment des formes asymétriques de houppiers.