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.
The forests comprise large areas of relatively even-aged stands, often composed of 1 or 2 dominant species occurring in a mosaic of age classes determined by previous disturbance. The actual species are determined by regional climate, local site conditions and timing and nature of the disturbance. Sites can be highly sensitive to disturbance. Even-aged stands of jack pine often grow on shallow soils that can be damaged during harvesting. Harvesting black spruce on deep organic soils can leave deep ruts that are not conducive to regenerating black spruce.