LIGNUM: a model combining the structure and the functioning of trees

The model LIGNUM treats a tree as a collection of a large number of simple units that correspond to the organs of a tree. The model describes the three-dimensional structure of the tree crown and derives growth in terms of the metabolism taking place in these units. The time step is one year. The structural units are: tree segments, branching points and buds. Tree segments are separated by branching points. 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 trees as a collection of lists. The annual growth of the tree is driven by the available photosynthetic products after accounting for respiration losses. The photosynthetic rate of foliage depends on the amount of intercepted light. The amount of photosynthates allocated to the growth of new tree segments is controlled by the light conditions and the amount of foliage of the mother tree segment. The biomass relationships of the tree parts follow, e.g. from a pipe model hypothesis. The orientation of new tree segments results from application of simple branching rules. LIGNUM has been parametrized for young Scots pines (Pinus sylvestris L.).