To be able to exactly describe the similarities and differences of vegetation in certain areas, classifying the vegetation only in communities or formations is not enough. Therefore more classes are needed. The classification according the horizontal layers is based on the heights of plants and their relations to each other. Every population in one community has own area of height which extends to horizontal direction.
In comparison with populations the vegetation horizons create a biologically validated comparison of different vegetation groups and their parts. Defining the populations and vegetation horizons creates a division and systematization of plant communities on an ecological basis.
The volume 34 of Acta Forestalia Fennica is a jubileum publication of professor Aimo Kaarlo Cajander.