Large-scale, man-created or natural disturbances play a major role in determining forest structure and species composition in many areas of North America and probably other temperate and tropical forests. Studies suggest a single group of species is not predestined to inhabit an area; forest physiognomic appearances assumed to imply all-aged succession often occur in single-age class stands; and recruitment of new stems into a forest often follows a disturbance rather than being a constant occurrence. After distrubances, forests develop through general physiognomic stages: 'stand initiation', 'stem exclusion', 'understory reinitiation', and 'old growth'. Disturbance severity and frequency determine which species will dominate the forest.