LIGNUM - A TREE MODEL-BASED ON SIMPLE STRUCTURAL UNITS

The model LIGNUM treats a tree as a collection of a large number of si mple units which correspond to the organs of the tree. The model descr ibes the three dimensional structure of the tree crown and defines the growth in terms of the metabolism taking place in these units. The ac tivities of physiological processes can be explicitly related to the t ree structures in which they are taking place. The time step is 1 year . The crown of the model tree consists of tree segments, branching poi nts and buds. Each pair of tree segments is separated by a branching p oint. The buds produce new tree segments, branching points and buds. T he tree segments contain wood. bark and foliage. A model tree consisti ng of simple elements translates conveniently to a list structure: the computer program implementing LIGNUM treats the tree as a collection of lists. The annual growth of the tree is driven by available photosy nthetic products after respiration losses are accounted for. The photo synthetic rate of foliage depends on the amount of light. The amount o f photosynthates allocated to the growth of new tree segments is contr olled by the light conditions and the amount of foliage on the mother tree segment. In principle, the biomass relationships of the tree part s follow the pipe model hypothesis. The orientation of new tree segmen ts results from the application of constant branching angles. LIGNUM h as been parametrized for young Scots pine (Pinus sylvestris L.) trees. However, the model is generic; with a change of parameter values and minor modifications it can be applied to other species as well. (C) 19 96 Annals of Botany Company