Clements (1904, 1917) developed a scheme of processes that drive succession: 1)
 initial causes that produce new or denuded soil on which invasion is possible, 2) ecesic
 processes resulting in migration (arrival at the site) include ecesis (germination, growth,
 and reproduction), aggregation (grouping of offspring about the parent plant),
 competition (interaction of organisms at the site), invasion (movement of plant from one
 area to another) and reaction (the modification of the site by the organisms); and 3)
 stabilization which results in the development of a stable climax (Weaver and Clements
 1929). Clements emphasized the importance in succession of autogenic processes,
 processes controlled from within a system. He viewed reaction, creating environmental
 conditions less favorable to early colonists and more favorable to late seral or climax
 species, as the main driving force of succession.

 Gleason (1927) stressed the unique individualistic behavior of plant species and
 the role of stochastic events in the successional process. He believed successional stages
 resulted from the response of individual species with different environmental tolerances
 to a constantly changing suite of environmental forces. Tansley (1935) criticized
 Clements' assumption that all vegetation change in a particular region would converge
 toward the same type of climax driven primarily by the regional climate. Tansley argued
 that rock type and topographic position may result in a climax that differs from that
 associated with a regional climate. Tansley's terminology (1935) included the term
 autogenicc" succession, referring to controlling processes originating from within the
 biological components of the system, and "allogenic" succession referring to controlling
 influences from the physicochemical components of the system.

 Egler (1954) concluded that the initial floristic composition of a site may largely
 determine the subsequent vegetation. The "initial floristics" model of succession
 suggests that the initial composition of species migrating to and establishing at the site
 determined the direction of later species replacements.

 Margalef (1958, 1963) suggested that the linkages among trophic levels and
populations represented information and that succession represents a natural trend toward
the accumulation of greater information in an ecosystem. Odum (1969) believed that
successional trends and ecosystem properties result from the tendency of ecosystems to
develop toward greater homeostasis. Within limits set by the physical environment,
succession proceeds toward an ecosystem of maximum biomass and diversity.

 Walker and Chapin (1987) suggest that the major factors influencing importance
of successional processes are (1) stage of succession; (2) type of succession (primary,
secondary, or regeneration after disturbance); (3) availability of environmental resources
(particularly water and nutrients;) and (4) type, frequency and intensity of disturbance.

 Since the mid-1970's two major conceptual trends have appeared (1) a shift away
from holistic explanations of successional phenomena postulated by Clements, Odum and
Margalef, toward mechanistic approaches emphasizing proximate causes of vegetative
change; and (2) a shift away from equilibrium paradigms. However, in the shifting-
mosaic steady-state model of succession (Borman and Likens 1979) the forest as a whole