-We analyze functional-structural tree models (FMSs) that are an outgrowth of developments in process-based models (PBMs) on the one hand, and morphological models on the other. Existing morphological and functional-structural models are briefly reviewed. We introduce the idealized elementary unit (IEU) that can be used as the basic component of a FSM, and pinpoint the processes that have to be accounted for. The distribution of metabolites and growth is identified as one of the main focal points to be investigated in conjunction with FSMs, and the different approaches that can be applied in constructing the model component for this process are presented. Finally, we analyze the computational requirements of FSMs, discuss the challenges they pose, and assess their applicability in a number of different tasks. functional-structural model / process-based model / morphological model / tree structure / L-system Résumé -Les composants des modèles fonctionnels et structuraux des arbres. Cet article a pour objet de faire une synthèse des approches réalisées dans le cadre de la modélisation fonctionnelle et structurale des arbres (FSM). Ces modèles résultent du couplage entre la modélisation du fonctionnement écophysiologique d'arbres, d'une part, et la modélisation des processus morphologiques, d'autre part. Après une brève présentation des approches existantes, nous introduisons la notion « d'unité élémentaire idéale » (IEU) qui peut être considérée comme la composante fondamentale des FSM au regard de la souplesse qu'elle confère dans l'articulation des processus. La distribution des métabolites et la croissance sont ensuite abordées comme étant les processus à résoudre de façon prioritaire dans le développement des FSM, et les différentes approches pouvant être mises à contribution dans la construction de ces modèles sont discutées. Enfin nous analysons les besoins en programmation des FSM, discutons des avancées nécessaires et évaluons leur adéquation à la résolution d'objectifs divers. modèle fonctionnel-structural / modèle de fonctionnement / modèle morphologique / structure des arbres / L-system

ABSTRACT

Root system architecture determines many of the vital functions of a tree, e.g. stability of anchorage and resource uptake. The shoot:root ratio is determined through the allocation of resources. Studies on below-ground architectural elements in boreal mixed forests are relatively scarce despite the fact that knowledge on below-ground interactions and allocation changes in relation to stand developmental stage and soil fertility is needed both in ecological and silvicultural research. In this study, sixty tree root systems of three different tree species, Betula pendula, Picea abies and Pinus sylvestris, were excavated in five mixed forest stands in order to quantify differences between the species and sites in terms of rooting behaviour. Root architecture differed greatly between the species, implying different solutions for the functions of root systems. Half of the P. sylvestris had developed a taproot as a response to anchorage needs, while P. abies correspondingly had pronounced secondary growth of proximal roots. Betula pendula had the most extensive root system, illustrating the greater demand of deciduous trees for water. Betula pendula was also the most sensitive to soil fertility: it favoured exploration on the poorest site, as illustrated by the high total root length, whereas on the most fertile site its strategy was to efficiently exploit soil resources through increased branching intensity. The results obtained in this study provide basic knowledge on the architectural characteristics of boreal tree root systems for use by forestry professionals and modellers.

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