Succession is a continuous change in the species composition, structure, and function of a forest through time following disturbances. A. Primary succession occurs at sites where soil must be developed and thus takes a far longer to complete than secondary succession, which occurs where relatively undisturbed soil already exists. B. The second stage of forest development is much shorter in boreal forests than it is in tropical forests. C. Old growth forest differs from earlier succession stages in that most soil nutrients come from leaves and fine root debris rather than from dead trees and other coarse woody debris. D. Early in secondary succession resources are relatively abundant and vegetation increases until canopy closure, after which competition for resources brings about increased plant mortality. E. With a rising tree mortality, openings in the canopy develop, leading to layered plant growth beneath the overstory canopy, and eventually a climax stage is reached. F. The effective suppression of grassland wild fires has provided opportunities for the ecotone to proceed into a forest.