FORMIND (forest model individual-based) is an individual based forest gap model that is able to simulate the growth of species-rich forests. It was developed in the late 1990s to simulate forest dynamics of tropical forests.
Forest dynamics and classical forest gap models
Gaps in a forest arise from disturbances, often resulting from large dying trees, ranging from a large branch breaking off and dropping from a tree, to a tree falling over, to landslides bringing down large groups of trees. Once a gap has formed, light-demanding tree species can establish and a succession of different tree species occurs over time.
Gap models have been used to study forest succession and to investigate the mechanisms underlying the long-term dynamics of forest ecosystems (stand level modelling).
Forest Gap models share the following principles: Forests are represented as a collection of small patches. The forest successional stage and age vary across patches. Patches are independent of their neighborhoods and do not interact with other patches. Thus, dynamic processes such as tree recruitment, growth and mortality are calculated separately for each patch. All patches are assumed to be homogeneous regarding the resource conditions (i.e., light reaching the upper canopy). The size of one patch is usually chosen according to the extent of the largest possible tree crown (e.g., 20 m x 20 m). All trees in a patch are interacting whereby tree positions are not relevant. Trees standing within one patch compete for light due to asymmetric shading effects of larger tree canopies on smaller ones. Crown and stem shapes are modeled by different geometries (e.g. cylinder form).
Initially, forest gap models were applied to temperate forests. For more complex forests as found in the tropics, gap models have to account for higher species-richness. Additionally, natural forests normally consist of patches in different successional stages. Their dynamics are mainly governed by falling trees creating gaps. Moreover, human-induced disturbances (e.g., logging or fragmentation) affect the overall dynamics of forests worldwide. To include these processes, forest models needed to combine complexity and structural realism with efficient computing.
FORMIND model development
FORMIND was developed in the late 1990s to simulate tropical forest dynamics more realistically than before. In FORMIND, physiological processes such as photosynthesis and respiration are simulated at the tree level (process-based model). Dying large trees can fall down and create gaps in the forest. Forest stands of several hundred hectares can be simulated over a time period of a few centuries on an annual time step. The simulation area is a composite of 20-m × 20-m patches typical for forest gap models. In FORMIND, these patches interact via seed dispersal and the falling of large trees. The basic model includes four main process groups: tree growth, tree mortality, recruitment of tree seedlings, and competition between trees.
Beside this, four important aspects are incorporated into FORMIND which are intended to be applicable to tropical forest sites. This distinguishes FORMIND from classical forest gap models: The high tree species-richness in tropical forests is covered by the concept of plant functional types (PFTs), i.e., grouping of tree species. Disturbances, both on small and large scale, caused by either natural reasons (e.g. tree falling, edge effects) or anthropogenic activities (e.g. tree logging) can be investigated in their effects on forest structure and dynamics. The calculated carbon balance of individual trees is based on a detailed process-based approach that enables upscaling to estimate carbon fluxes for small or large forest areas. The current version of FORMIND uses dynamic global vegetation model approaches to simulate the carbon cycle of forests on a continental scale (e.g. for the whole Amazon,). The model can be merged with remote sensing data (e.g. forest height from Lidar or Radar). The model can be also applied at temperate and boreal sites.
Modeled processes
Tree growth: Tree biomass growth is determined by a physiology-based tree carbon balance that includes leaf photosynthesis, maintenance and growth respiratory costs. An increase in tree biomass results in increments in height, stem diameter, stem volume and leaf area using allometric relationships.