Domain
Broadest entity or level in classification, equivalent to the five thermic Köppen - Trewartha climatic groups and including the tropical, subtropical, temperate, boreal and polar domain.
Ecological Zone
Defined as a zone or area with broad yet relatively homogeneous natural vegetation formations, similar (not necessarily identical) in physiognomy. Boundaries of the Ecological Zones approximately coincide with Köppen- Trewartha climatic types, which are based on temperature and rainfall. An exception to this definition are "mountain systems", classified as one separate Ecological Zone in each domain and characterized by a high variation in both vegetation formations and climatic conditions.
Mountain systems
Mountain systems are defined as zones/areas that have a distinctly different vegetation (and climate) than the surrounding lowlands at a given latitude. Mountain vegetation is usually lower and the floristic composition is different (with generally fewer species). Additional components to define mountain systems are altitude and steepness of slopes. It is difficult to select specific altitudinal thresholds for defining mountain systems also as the changes in vegetation are often gradual, however they are usually at around 1000 - 1200 meter in the tropics and decrease with higher latitudes.
Physiognomy
Covers all aspects of the structure of vegetation, such as height, (tree) density, thorniness, deciduousness, life forms, etc.
