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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.

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.

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; 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.

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.

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.

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.

[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.

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

[1] This paper discusses the quality and the accuracy of the Joint Research Center (JRC) fraction of absorbed photosynthetically active radiation (FAPAR) products generated from an analysis of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data. The FAPAR value acts as an indicator of the presence and state of the vegetation and it can be estimated from remote sensing measurements using a physically based approach. The quality of the SeaWiFS FAPAR products assessed in this paper capitalizes on the availability of a 6-year FAPAR time series over the full globe. This evaluation exercise is performed in two phases involving, first, an analysis of the verisimilitude of the FAPAR products under documented environmental conditions and, second, a direct comparison of the FAPAR values with ground-based estimations where and when the latter are available. This second phase is conducted following a careful analysis of problems arising for performing such a comparison. This results in the grouping of available field information into broad categories representing different radiative transfer regimes. This strategy greatly helps the interpretation of the results since it recognizes the various levels of difficulty and sources of uncertainty associated with the radiative sampling of different types of vegetation canopies.

Light models provide an interesting way to analyse the influence of the forest canopy on understory biological processes but need a detailed description of tree crowns, requiring many field measurements. This study proposes supplying light models with only stem diameter and tree position and reconstructing crowns using diameter-related allometric relations. First, the diameter-related relations for total height, crown base height and mean crown radius were established for each species. Second, two reconstruction methods were compared: a simple isotropic method and a more sophisticated method, the Crown Reconstruction by Overlap Minimisation method. The latter method gave better results than the simpler one, even if some small bias was not completely resolved in the darkest areas. However, using crown centre position instead of stem position resolved this bias., Reconstruction de la forme des houppiers à partir du diamètre des arbres et de leur position pour alimenter un modèle de lumière. I. Algorithmes et comparaison des simulations de l'éclairement. Les modèles de lumière constituent un bon moyen pour analyser l'influence du couvert forestier sur les processus biologiques sous-couvert. Malheureusement ils nécessitent une description détaillée des houppiers des arbres, ce qui implique des mesures de terrain assez lourdes. Cet article propose d'alimenter les modèles de lumière uniquement avec le diamètre et la position des arbres, et de reconstituer les houppiers en utilisant des relations allométriques en fonction du diamètre à 1,30 m. Les relations allométriques sont tout d'abord établies pour la hauteur totale, la hauteur de base de houppier et le rayon moyen du houppier, pour chaque espèce. Ensuite deux méthodes de reconstruction des houppiers sont comparées : une méthode simple dite isotrope et une plus sophistiquée, la méthode de reconstruction des houppiers par minimisation des chevauchements. Cette dernière donne de meilleurs résultats, même si un léger biais subsiste dans les zones les plus sombres. Cependant le fait d'utiliser la position des centres de houppiers au lieu de celle des troncs permet de complètement supprimer ce biais.

International audience; Afin d'évaluer et de suivre les dépérissements du Hêtre, une méthode de notation multicritère est proposée. Elle repose sur l'observation des symptômes rencontrés lors des phases de dépérissement : transparence, mortalité de branches, perte de ramification fine... La notation de 9 critères élémentaires permet de décrire le houppier à un instant donné. L'attribution d'un indice global de dépérissement peut être réalisée à partir d'un modèle combinant les critères élémentaires.

International audience; Les insectes qui vivent aux dépens des hêtres vivants sont au nombre de plusieurs dizaines. Tous les organes ont été colonisés au cours de la lente coévolution entre les insectes et leurs hôtes. Certaines espèces sont spécifiques du Hêtre, d'autres sont plus polyphages. Mais c'est surtout dans les hêtres morts que s'exprime la diversité des insectes. Le cortège des insectes saproxyliques du Hêtre (insectes qui vivent à un stade de leur développement dans un arbre mort ou aux dépens d'autres organismes saproxylophages) comprend plusieurs centaines d'espèces. Au final, le peuplement entomologique d'une hêtraie se compose de plusieurs milliers d'espèces dont moins d'une dizaine sont économiquement dommageables.

International audience; Les forêts hétérogènes guyanaises sont des milieux complexes dont la gestion passe par la compréhension de leur organisation (structure spatiale, horizontale et verticale et diversité spécifique) et de leur dynamique. Dans ce contexte, l'étude des stratégies de développement et d'occupation de l'espace des arbres permet de mieux comprendre comment les différentes espèces exploitent et structurent l'hétérogénéité du milieu. Depuis une trentaine d'années, les travaux menés en architecture végétale ont permis d'établir un contexte méthodologique et conceptuel pour étudier la structure globale de l'arbre et pour comprendre les mécanismes morphologiques qui lui ont donné naissance. Après une présentation de concepts majeurs d'architecture végétale, cet article expose les méthodes employées pour quantifier la structure des arbres et les approches permettant d'intégrer les critères architecturaux dans un diagnostic à l'échelle du peuplement.

• This work aims at developing new tools for rapid assessment of forest health indicators in poplar plantations. • Crown transparency and discoloration were visually evaluated in all trees of four 15 m-radius sub-plots in 32 poplar clonal plantations, which were chosen according to a factorial scheme with three factors: tree age, site quality and understorey vegetation management. A subset of trees was assessed using digital photos processed with a semi-automatic image analysis system (the CROCO software) in order to compare visual and digital crown transparency estimates. • Poplar crown conditions were better in young stands and rich sites. Harrowing understorey vegetation improved tree health in poor sites. Samples of 20 trees per stand provided the same information about crown transparency and discoloration as 60 trees. Calibration curves of digital crown transparency estimates were successfully fitted against visual crown transparency estimates. The same effects of stand age and site quality could be detected with digital crown transparency as response variable. • The use of digital photos processed with CROCO in ca. twenty trees per stand is therefore recommended to accurately and objectively monitor crown condition in clonal poplar plantations., • Une étude a été réalisée pour développer des méthodes rapides et standardisées de suivi de l'état sanitaire des houppiers en plantation de peuplier. • La transparence et la décoloration des houppiers ont été estimées visuellement sur tous les arbres de quatre placettes d'un rayon fixe de 15 m, dans 32 plantations monoclonales de peuplier choisies selon un plan factoriel complet croisant trois facteurs : l'âge des plantations, la fertilité du site et la gestion du sous-bois. La moyenne des notes de transparence et de décoloration des houppiers a été calculée sur un nombre décroissant de placettes par plantation afin de tester l'influence de la taille de l'échantillon sur la qualité de l'estimation. Des photographies numériques du houppier d'un sous-échantillon de peupliers ont été également prises puis traitées à l'aide du logiciel CROCO afin de comparer les estimations visuelles et numériques de la transparence du houppier. • L'état sanitaire des houppiers était meilleur dans les jeunes plantations de peupliers et les sites les plus fertiles. Le hersage de la végétation du sous-bois a permis d'améliorer l'état sanitaire des peupliers dans les sites les plus pauvres. Les valeurs moyenne de transparence et de décoloration du houppier par plantation, calculées à partir des données d'une seule placette de 15 m de rayon – soit une vingtaine d'arbres – ne différaient pas significativement de celles calculées à partir des données de trois placettes et permettaient de détecter les mêmes effets des caractéristiques du peuplement et du site. Des courbes de calibration de la transparence du houppier estimée à partir de photos numériques et du logiciel CROCO ont pu être ajustées aux observations de transparence. Les effets de l'âge des plantations, de la fertilité du site et de la gestion du sous-bois ont pu également être détectés avec ces valeurs numériques calculées. • Il peut donc être recommandé d'utiliser des photos numériques traitées à l'aide du logiciel CROCO pour estimer la transparence du houppier d'une vingtaine d'arbres afin d'évaluer rapidement, précisément et objectivement l'état sanitaire des plantations monoclonales de peuplier.