The conditions across larger ecosystem units often override those of smaller units and smaller units emerge in the context of larger systems. Non-adjacent units may become networked if they share common biota. At coarse scales, the important factors are largely abiotic, while at finer scales both biotic and abiotic factors are important. The structure and function of ecosystems change spatially and temporally. Some abiotic properties such as soil fertility and climate are relatively stable over time and encourage certain assemblages of trees and other biota.
Biotic conditions can change with time.
Ecosystem units may support trees that are young, mature or old and they may support communities that are early, late, or mid successional. Narrowly defined units can be used to assess forest growth, succession and health in relation to various physical conditions such as rainfall, temperature and soil fertility but excessive detail must not be allowed to confuse major issues. The Boreal Forest Region in the area north of the Great Lakes includes a vast area of Canadian Shield that is characterized by deep to shallow tills and glacial lake deposits. The whole area is underlain largely by granite rock that influences the topography and soils. Soils are mainly podzols except in large clay belt areas in the northeast.