Although individual trees in real forests should not have necessarily identical shapes, the FORMIND model represents each tree by a cylindrical stem and a cylindrical crown (Fig. 3). The geometry of an individual can be described completely by the following size characteristics: stem diameter (D), height (H), crown diameter (CD), crown length (CL) and crown projection area (CA).
All geometric characteristics can be calculated as a function of the tree’s stem diameter through allometric equations. The tree’s geometry is specific for each plant funtional type, and the competition for light determines how much from the total incoming solar radiation is available for other individuals by shading.
In the FORMIND model, gross photosynthetic production (GPP) is determined for each individual using the solar radiation available to the top of each tree and its total Leaf Area Index (LAI) which is also a function of the tree’s diameter. Total LAI is distributed by several layers, each receiving different light intensities.
To calculate growth we must also calculate the maintenance respiration of individual trees. This is done by modelling it in function of GPP and the individual’s biomass at a certain timestep.
By the means of Net Primary Production (GPP minus maintenance respiration, a function of the total individual’s biomass), the simulated trees produce organic dry matter (biomass) and grow in stem diameter. Growth rate is dependent on the diameter-growth curve, since growth speed is a function of the tree trunk’s diameter, and is also used to calibrate the parameters of the maintenance respiration.
The calculated photosynthesic production of the leaves can be limited by water or temperature, with the activation of the climate module.