-The amount of transmitted light in the understories of forest stands affects many variables such as biomass and diversity of the vegetation, tree regeneration and plant morphogenesis. Therefore, its prediction according to main tree or stand characteristics, without the need for difficult and costly light measurements, would be most useful for many different users and scientists. Transmitted global solar radiation was measured using tube solarimeters in the understories of 204 plots of even-aged coniferous stands of four species (Pseudotsuga menziesii, Picea abies, Larix sp. and Pinus sylvestris) in a wide range of ecological and management conditions in the temperate climate zone. From these data, a range of simple models based on the Beer-Lambert law was built and fitted to predict mean stand radiation transmittance from basic stand traits and management features: stand basal area, stand age, time since last thinning, and last thinning intensity. Forest managers can use it to predict understory light availability and adapt their silviculture to various objectives.coniferous forest / solar radiation / model / basal area / stand management Résumé -Simulation de l'éclairement relatif dans le sous-bois de peuplements réguliers de conifères en forêts tempérées. La quantité de lumière disponible dans le sous-bois des forêts affecte de nombreux processus tels que la production de biomasse et la diversité de la végétation, la régénération des arbres et la morphogénèse des plantes. Prédire cette quantité sans avoir à effectuer de mesures de lumière délicates et coûteuses serait donc d'un grand intérêt pour différents utilisateurs et chercheurs. Le rayonnement solaire global transmis a été mesuré avec des solarimètres dans le sous-bois de 204 parcelles de peuplements réguliers de quatre espèces de conifère (Pseudotsuga menziesii, Picea abies, Larix sp. et Pinus sylvestris) dans diverses conditions écologiques et de gestion en climat tempéré. A partir de ces données et en utilisant le formalisme de la loi de Beer-Lambert, plusieurs modèles ont été bâtis et ajustés simulant la transmission de l'éclairement sous couvert en fonction des caractéristiques dendrométriques simples des peuplements étudiées et de leur gestion : surface terrière et âge du peuplement, durée depuis la dernière éclaircie et intensité de celle-ci. Ces outils pourraient être facilement utilisés par les gestionnaires forestiers pour estimer le niveau d'éclairement sous couvert et ainsi adapter leur sylviculture à divers objectifs.forêt de conifères / éclairement / modèle / surface terrière / gestion des peuplements

Light models for vegetation canopies based on the turbid medium analogy are usually limited by the basic assumption of random foliage dispersion in the canopy space. The objective of this paper was to assess the effect of three possible sources of non-randomness in tree canopies on light interception properties. For this purpose, four threedimensional (3-D) digitized trees and four theoretical canopies -one random and three built from fractal rules -were used to compute canopy structure parameters and light interception, namely the sky-vault averaged STAR (Silhouette to Total Area Ratio). STAR values were computed from (1) images of the 3-D plants, and (2) from a 3-D turbid medium model using space discretization at different scales. For all trees, departure from randomness was mainly due to the spatial variations in leaf area density within the canopy volume. Indeed STAR estimations, based on turbid medium assumption, using the finest space discretization were very close to STAR values computed from the plant images. At this finest scale, foliage dispersion was slightly clumped, except one theoretical fractal canopy, which showed a marked regular dispersion. Taking into account a non-infinitely small leaf size, whose effect is theoretically to shorten self-shading, had a minor effect on STAR computations. STAR values computed from the 3-D turbid medium were very sensitive to plant lacunarity, a parameter introduced in the context of fractal studies to characterize the distribution of gaps in porous media at different scales. This study shows that 3-D turbid medium models based on space discretization are able to give correct estimation of light interception by 3-D isolated trees, provided that the 3-D grid is properly defined, that is, discretization maximizes plant lacunarity.

A simplified method for building three-dimensional (3D) mock-ups of peach trees is presented. The method combines partial digitizing of tree structure with reconstruction rules for non-digitized organs. Reconstruction was applied at two scales: leaves on current-year shoots (CYS) and shoots on 1-year-old shoots (OYOS). Reconstruction rules make use of allometric relationships, random sampling of shoot attribute distribution and additional hypotheses (e.g., constant internode length). The method was quantitatively assessed for two training systems (tight goblet and wide-double-Y), at a range of spatial scales. For this purpose, light interception properties of reference and reconstructed mock-ups were compared. Mock-up quality depended on scale. Foliage reconstruction on CYS was unsuitable for generating a given CYS. Similarly, CYS reconstruction on OYOS was unsuitable for generating a given OYOS. This is because generic rules derived at the population scale do not consider specific foliage or shoot attributes of a given CYS or OYOS. In contrast, foliage reconstruction on CYS was able to generate OYOS mock-ups having light properties similar to the reference mock-ups. The same held for CYS reconstruction on OYOS for light capture properties at the tree scale. The CYS reconstruction on OYOS was also suitable for deriving OYOS distribution as a function of light interception ability. Reconstruction rules were successfully used to build the vegetation neighborhood of a reference shoot. The proposed method could therefore be used to make 3D tree mock-ups usable for a range of some, but not all, light computations. Because the simplified method allows large time savings, it could be used in virtual experiments requiring large numbers of replicates, such as comparative studies of tree genotypes or training systems.

The CERES‐Maize model is the most widely used maize (Zea mays L.) model and is a recognized reference for comparing new developments in maize growth, development, and yield simulation. The objective of this study was to present and evaluate CSM‐IXIM, a new maize simulation model for DSSAT version 4.5. Code from CSM‐CERES‐Maize, the modular version of the model, was modified to include a number of model improvements. Model enhancements included the simulation of leaf area, C assimilation and partitioning, ear growth, kernel number, grain yield, and plant N acquisition and distribution. The addition of two genetic coefficients to simulate per‐leaf foliar surface produced 32% smaller root mean square error (RMSE) values estimating leaf area index than did CSM‐CERES. Grain yield and total shoot biomass were correctly simulated by both models. Carbon partitioning, however, showed differences. The CSM‐IXIM model simulated leaf mass more accurately, reducing the CSM‐CERES error by 44%, but overestimated stem mass, especially after stress, resulting in similar average RMSE values as CSM‐CERES. Excessive N uptake after fertilization events as simulated by CSM‐CERES was also corrected, reducing the error by 16%. The accuracy of N distribution to stems was improved by 68%. These improvements in CSM‐IXIM provided a stable basis for more precise simulation of maize canopy growth and yield and a framework for continuing future model developments.

A three-dimensional digitizing method was used to assess the canopy structure of six Festuca arundinacea (FA)-Trifolium repens (TR) mixtures during the installation stage. Virtual canopy images were synthesized and used to derive light interception and partitioning between species. Computations from images were compared with a simple light model based on Beer's law, in order to analyse within-and between-species foliage dispersion. The total leaf area index of the mixtures ranged from 0·6 to 4·5. The fraction of FA foliage overtopping TR was 9-30%. The mean inclination of FA and TR was 66 and 57 °°°° , respectively. Within-species dispersion parameters of FA and TR were about 0·8 and 1·0, namely clumped and random foliage dispersion, respectively. Although FA was sown in rows, between-species dispersion was random. Lower leaf inclination and lesser clumping in TR compensated foliage overtopping by FA, so that light partitioning between FA and TR (about 80 and 20%, respectively) was similar to the species contribution to total canopy foliage. Since between-species dispersion was random, a simple two-layer light model based on Beer's law provided correct estimations of light partitioning (RMSE = 0·05), although light interception by FA was slightly overestimated because of its clumped dispersion.