Successional changes in plant communities were recognized and described well before the 20th century. Frederick E. Clements' observations were developed into theory while he created the original vocabulary (see below) and published the first scientific explanation for the process of succession in his book, Plant Succession: An Analysis of the Development of Vegetation. It is very interesting to note that sixty years earlier, Henry David Thoreau described forest succession for the first time in his book, The Succession of Forest Trees
Trees play a major role in creating terrestrial plant cover when conditions develop to the point where some bare-ground and soil is present. Trees grow alongside grasses, herbs, ferns and shrubs and compete with these species for future plant community replacement and their own survival as a species. The process of that race toward a stable, mature, "climax" plant community is called succession which follows a successional pathway and each major step reached along the way is called a new seral stage.
Primary succession typically occurs very slowly when site conditions are unfriendly to most plants but where a few unique plant species can catch hold and thrive. Trees are not often present under these initial harsh conditions. Plants and animals resilient enough to first colonize such sites are the "base" community that kick starts the complex development of soil and refines the local climate. Site examples of this would be rocks and cliffs, dunes, glacial till and volcanic ash.
Both primary and secondary sites in initial succession are characterized by full exposure to the sun, violent fluctuations in temperatures, and rapid changes in moisture conditions. Only the hardiest of organisms can adapt at first.
Secondary succession tends to happen most often on abandoned fields, dirt and gravel fills, roadside cuts and after poor logging practices where disturbance has occurred. It can also start very rapidly where the existing community is completely destroyed by fire, flood, wind or destructive pests.
Clements' defines the succession mechanism as a process involving several phases when on completion is called a "sere". These phases are: 1.) development of a bare site called Nudism; 2.) introduction of living regenerative plant material called Migration; 3.) establishment of vegetative growth called Ecesis; 4.) plant competition for space, light and nutrients called Competition; 5.) plant community changes that effect the habitat called Reaction; 6.) final development of a climax community called Stabilization.
Forest Succession in More Detail
Forest succession is considered a secondary succession in most field biology and forest ecology texts but also has its own particular vocabulary. The forest process follows a time-line of tree species replacement and in this order: from pioneer seedlings and saplings to transition forest to young growth forest to mature forest to old growth forest.
Foresters generally manage stands of trees that are developing as part of a secondary succession. The most important tree species in terms of economic value are a part of one of several seral stages below the climax. It is therefore important that a forester manage his forest by controlling the tendency of that community to move toward a climax species forest. As presented in the forestry text, Principles of Silviculture, Second Edition, "foresters use silvicultural practices to maintain the stands in the seral stage that meets society's objectives most closely."