The pines cannot persist because their seedlings can't grow in the shade created by their parents and other trees. The oak and hickory seedlings can grow in the shade of their parents and so persist. One of the methods ecologists use to determine whether a community is at climax is to see if the young plants are the same species as the older plants. In a seral community, the old plants will typically be different from the young plants. During primary succession, soil develops.
Many species attempt to colonize newly available space but only certain species are able to do so.
These pioneer species modify the environment and make it suitable for the species in the next seral stage, which further modify the environment to make it suitable for the next stage, and so on. Each seral stage facilitates the next seral stage until the climax community which no longer modifies the environment. Tolerance model All species that can tolerate conditions on the site are present from the start. Succession is largely the process of tolerant species finding the site, but early arrivers modify the site so that some early arrivers can no longer tolerate the conditions. Succession proceeds until only species that can tolerate the conditions are present.