The functional structural plant models (FSPMs) can be defined as models that combine descriptions of metabolic (physiological) processes with a presentation of the 3D structure of a plant. They contain usually the following components 1) Presentation of the plant structure in terms of basic units, 2) Rules of morphological development and 3) Models of metabolic processes that drive the plant growth. The main emphasis in these applications has been individual plants.

The model LIGNUM has been developed as a research tool that addresses tree (plant) growth by realizing the three components and that can be applied to several tree species. In the following we shortly describe how the three FSPM model components have been realized in the LIGNUM model. We are using the STL template library of C++ to define a blueprint of a tree that can be instantiated by actual representations of the species-specific components. We are using four generic algorithms for traversing the data structure of the tree and to make calculations. L-systems are used for specifying the morphological development of the trees.

LIGNUM has been applied to a number of tree species. We describe the use of it in three different applications. First, we used LIGNUM to simulate individuals of different leaf area to find distributions of leaf angle and leaf photosynthetic capacity that maximized net carbon gain at the whole-plant level in sugar maple. Second, the growth simulation of short-rotation eastern cottonwood required the incorporation of a biochemically-derived photosynthesis submodel, nested time steps for simulating physiological processes, structural development, and annual biomass production. Third, simulations of pine forest stands consisting of individual trees show how different alternatives of shoot properties and production of new buds affect stand dynamics.