Carbon allocation among tree organs: A review of basic processes and representation in functional-structural tree models

Abstract

-Carbon assimilates flow from "source" areas such as leaves to "sink" areas where they are taken up and used. The assimilate fluxes from sources to sinks are mainly dependent on the source-sink distances and on the respective abilities of the different sinks to take up and use the assimilates that are available to them. The widely accepted, basic mechanism of assimilate movement by mass-flow, although conceptually simple, has so far proved too complex for practical modeling purposes in whole tree systems. Four main modeling approaches can be found in current models: (i) models involving empirically determined allocation coefficients; (ii) models based on growth rules, including functional balance or "goal-seeking" principles; (iii) transport-resistance models; (iv) models based on relative sink strength, with two main sub-classes: "hierarchical" and "proportional" models. These different model classes can be conceptually closer to each other than is readily apparent. They are presented in relation to their generality and ability to account for complex architectures or responses to environmental changes. The feedback relationship of allocation to growth is pointed out.assimilate / partitioning / source / sink / model Résumé -La répartition du carbone entre organes chez les arbres : processus de base et représentation dans les modèles « structure-fonction ». Les assimilats carbonés circulent de zones « sources » telles les feuilles, vers des « puits » où ils sont préle-vés et utilisés. Ces flux d'assimilats dépendent principalement des distances entre sources et puits ainsi que des capacités respectives des différents puits à prélever et utiliser les assimilats disponibles. Il est largement admis aujourd'hui que le mécanisme de base de la translocation est un flux de masse. Mais malgré sa simplicité conceptuelle, la simulation de ce processus implique des calculs trop complexes pour une modélisation pratique. On trouve actuellement quatre approches modélisatrices principales : (i) l'utilisation de coefficients d'allocation empiriques ; (ii) la mise en oeuvre de règles de croissance, notamment des équilibres fonctionnels et autres principes téléonomiques ; (iii) l'analogie électrique avec des résistances ; (iv) des règles de répartition basées sur les « forces » relatives des différents puits (modèles hiérarchiques et modèles proportionnels). Ces diverses classes de modèles sont parfois conceptuellement moins éloignées les unes des autres qu'il n'y paraît. Elles sont présentées en relation avec leur généralité et leur capacité à rendre compte d'une architecture complexe ou des influences de l'environnement. On souligne l'importance des rétroactions entre allocation et croissance. assimilats / allocation / source / puits / modèleAnn. For. Sci. 57 (2000) [521][522][523][524][525][526][527][528][529][530][531][532][533] 521

Concepts of Modelling Carbon Allocation Among Plant Organs

“…2 is used to distribute dry matter among a group of organs, and only if supply exceeds demand of these organs (e.g., fruits, leaves and stems) assimilates are distributed to other organs (e.g., roots) (Wermelinger et al 1991;Grossman and DeJong 1994). These models, which are often used for trees, are also called hierarchical models (Lacointe 2000). Sometimes organ relative growth rate instead of absolute growth rate is used as a measure for sink strength.…”

“…The simulation of carbon allocation is one of the weakest features of current crop growth models. The different approaches of modelling carbon allocation have been reviewed by several authors, e.g., Marcelis (1993b), Cannell and Dewar (1994), Marcelis et al (1998) and Lacointe (2000). In this paper, which is partly based upon an earlier review (Marcelis et al 1998), we will briefly discuss five concepts of modelling carbon allocation among plant organs.…”

Mechanistic Modelling of Carbon Partitioning

“…Assuming a similar TR description for nitrogen flow between root and shoot, with bi-substrate (C; N) growth kinetics, the well-known shoot:root balance follows Lacointe (Lacointe 2000 and references therein). The commonly observed distance effects, such as a source supplying the nearest available sink, are qualitatively explained by the resistance term.…”

“…Respiratory needs have usually been handled independently from all other Cdemands, by first removing this from the pool of available C and then distributing the remaining C between the sinks according to the rules of the model (Lacointe 2000). For example, Grossman and DeJong's PEACH model (1994) calculates the total C acquired during a time step, then removes the plant's entire maintenance respiration need from this pool.…”

“…While qualitative descriptions of partitioning are available, there is still no accepted mechanistic description (Wardlaw 1990;Marcelis 1993;Lacointe 2000;Le Roux et al 2001). Plant growth models, therefore, use algorithms derived from observational data.…”