LIGNUM: A tree model based on simple structural units

The model LIGNUM treats a tree as a collection of a large number of simple units which correspond to the organs of the tree. The model describes the three dimensional structure of the tree crown and defines the growth in terms of the metabolism taking place in these units. The activities of physiological processes can be explicitly related to the tree structures in which they are taking place. The time step is 1 year. The crown of the model tree consists of tree segments, branching points and buds. Each pair of tree segments is separated by a branching point. The buds produce new tree segments, branching points and buds. The tree segments contain wood. bark and foliage. A model tree consisting 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 photosynthetic products after respiration losses are accounted for. The photosynthetic rate of foliage depends on the amount of light. The amount of photosynthates allocated to the growth of new tree segments is controlled by the light conditions and the amount of foliage on the mother tree segment. In principle, the biomass relationships of the tree parts follow the pipe model hypothesis. The orientation of new tree segments results from the application of constant branching angles. LIGNUM has 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) 1996 Annals of Botany Company