Many successional trajectories follow a basic four-stage development pattern. The first of these stages, "stand initiation", occurs after a major disturbance and involves many species arriving in the area of abundant light and nutrients. The second stage, "stem exclusion", describes the growth and competition of these species as resources become less available; likely one or a few species outcompetes and becomes stand-dominating. The third stage, "understory reinitiation", involves further disturbance and the creation of gaps; at this point stratification develops, with layers of canopy, midstory, and understory appearing. The final stage, known as "old-growth", is the extension and completion of the understory reinititation; a complex multi-aged and multi-layered forest has developed.