Forest Gap models share the following principles (Bugmann, 2001):

(i) Forests are represented as a collection of small patches. The forest successional stage and age vary across patches.
(ii) 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.
(iii) All patches are homogeneous in size and resource level (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). Intra- or interspecific interactions are simulated for all trees in a patch rather than tree-by-tree, as tree positions are not included.
(iv) Leaves are modeled as thin disks on top of each tree. Trees standing within one patch compete for light due to asymmetric shading effects of larger tree canopies on smaller ones.

Following the gap model approach, FORMIX was applied to tropical forests in South-East Asia. FORMIX accounts for biomass and tree numbers in five distinct canopy layers (each layer has some representative trees similar to size class models).

FORMIND is the process- and individual-based successor of the FORMIX model, in which the concept of distinct layers was discarded. FORMIND was developed in the late 1990’s to simulate tropical forest dynamics more realistically than before.

The individual-based approach of forest gap models offer a high degree of flexibility to be adapted to different environments (e.g., temperate forests) or even other ecosystem types (e.g., temperate grassland – GRASSMIND)