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.