Forests, being an ecological system, are subject to the species succession process. There are "opportunistic" or "pioneer" species that produce great quantities of seed that are disseminated by the wind, and therefore can colonize big empty extensions. They are capable of germinating and growing in direct sunlight. Once they have produced a "closed canopy", the lack of direct sun radiation at the soil makes it difficult for their own seedlings to develop. It is then the opportunity for shade-tolerant species to become established under the protection of the pioneers. When the pioneers die, the shade-tolerant species replace them. These species are capable of growing beneath the canopy, and therefore, in the absence of disturbances, will stay. For this reason it is then said the stand has reached its climax. When a disturbance occurs, the opportunity for the pioneers opens up again, provided they are present or within a reasonable range.