The forest comprises smaller subsystems and so on (Oldeman, 1990). The forest is layered from an overall mosaic consisting of eco-units (i.e., the unit which started its development at the same moment and on the same surface) and their subsystems, which contain coexisting animals, fungi, plants, and abiotic factors.
There is the formula ’System = Collection of subsystems + Cohesion’.
Oldeman, R.A.A. (1990). Forests: Elements of Silvology. Berlin Heidelberg: Springer-Verlag.