Application of the functional-structural tree model LIGNUM to sugar maple saplings (Acer saccharum Marsh) growing in forest gaps

LIGNUM is a functional-structural model that represents a tree using four modelling units which closely resemble the real structure of trees: tree segments, tree axes, branching points and buds. Metabolic processes are explicitly related to the structural units in which they take place. Here we adapt earlier versions of LIGNUM designed to model growth of conifers for use with broad-leaved trees. Two primary changes are involved. First, the tree segment for broadleaved trees consists of enclosed cylinders of heartwood, sapwood and bark. Leaves consisting of petioles and blades are attached to the segments. Secondly, axillary buds and rules governing their dormancy are included in the model. This modified version of LIGNUM is used to simulate the growth and form of sugar maple saplings in forest gaps. The annual growth of the model tree is driven by net production after respiration losses are taken into account. The production rate of each leaf depends on the amount of photosynthetically active radiation it receives. The radiation regime is tracked explicitly in different parts of the tree crown using a model of mutual shading of the leaves. Forest gaps are represented by changing the radiation intensity in different parts of the model sky. This version of LIGNUM modified for use with broad-leaf, deciduous trees and parameterized for sugar maple, yields good simulations of growth and form in saplings from different forest gap environments. (C) 2001 Annals of Botany Company.