Phytogeography or plant geography is the study of plant dispersal patterns, which one then tries to explain on historical and geological grounds as a result of the formation of continents, climate changes (e.g. ice ages), and the formation of mountain ranges and river basins. Such gradual changes eventually isolate plant species, which then evolve independent of their relatives, but dependent on local conditions. Phytosociology, or vegetation science, focuses on natural plant communities, usually on a much smaller scale and more focused on the present. Here characteristic species and differentiating species and their abundance (the numbers in which they occur) play an important role in distinguishing the various vegetation associations.
As far as forests are concerned, many typologies have been drawn up in both scientific disciplines, which are therefore always to some extent related to the places where tree species occur naturally. In vegetation science, natural forest types are often distinguished on the basis of differences in the herbaceous layer, since the tree layer in contemporary forests is more often than not decisively altered by human intervention.
The geographical arboretum has been organized on the basis of insights from the above-mentioned disciplines, although Professor Bommer, the founder, and his successor U. Liénard did not always apply the same logic. They made different choices depending on their current research interests and the availability of plant material. Some sections refer to very large and botanically varied areas of provenance, and can therefore hardly be representative, e.g. China. To the much smaller country of Japan, on the other hand, three sections have been devoted. Some sections are composed with a specific forest area in mind, for example: The Djebel Babor range in Algeria. Other sections of the arboretum are built around one particular tree species, e.g. the Araucarias from the Andes range in Chile.