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

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 sin ks 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 ar e available to them. The widely accepted, basic mechanism of assimilate mov ement by mass-flow, although conceptually simple, has so far proved too com plex for practical modeling purposes in whole tree systems. Four main model ing approaches can be found in current models: (i) models involving empiric ally 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 m ain sub-classes: "hierarchical" and "proportional" models. These different model classes can be conceptually closer to each other than is readily appa rent. They are presented in relation to their generality and ability to acc ount for complex architectures or responses to environmental changes. The f eedback relationship of allocation to growth is pointed out.