The ecologist community of the 20th century was divided over the nature of plant succession on both an ecological and a geological timescale. During the last glacial advance of the Pleistocene Epoch ice sheets covered nearly all of Canada and the northern third (more or less) of the United States. Beginning approximately 18,000 years ago, these continental glaciers began to melt back, uncovering a landscape that had been entirely denuded of vegetation (not to mention soil). The pattern of re-vegetation of these post-glacial landscapes was the focus of the debate.
Frederic Clements, a botanist with the Carnegie Institution, developed a theory of plant succession in early 20th century that introduced the idea of a “climax community” to the professional lexicon. Clements described a series of intermediate communities (seres) that occupied a given location between the reintroduction of vegetation (pioneer species) and the climax (final) stage. In addition to series beginning with the absence of vegetation, the progression of communities could also start after recovering from a disturbance. The most well-known example of this is “old field succession,” which describes the recovery of a natural landscape on abandoned agricultural land.
Clements descriptive work was underlain with the belief that the species within communities were inter-adapted, and that all members of a particular seral stage would be reintroduced to a given locality at essentially the same time and come to dominate the place in a unison manner. Clements thought of plant communities as analogous to organisms. In an organism all the organs work together to make the body function; in a plant community all the species work together (associations) to make energy flow the community properly.
Henry Gleason, a younger contemporary of Clements, initially endorsed and employed Clementsian concepts in his early research in the Midwest, but after moving to the New York Botanical Garden, Gleason began to develop a new theory of succession. Gleason first questioned the analogy between an organism and a plant association. His own fieldwork suggested that plants were not found together in a statistically significant manner. Rather, a given plant species’ own ecology determined its distribution, regardless of what other species were present.
This “individualist” model of plant distribution did not gain much ground in the ecology community for several decades. In the 1950s further work by Robert Whittaker began the overturning of the Clementsian model in the professional community, but the Clementsian terminology was used in ecology classes right through the rest of the 20th century. While the plant ecology community works on a timescale of hundreds of years at most, the paleoecology community, employing the sedimentary record, works on a timescale of thousands (to millions) of years.
In addition to the actual organic remains of plant parts (leaves, bark, flowers etc.), plant communities of the past can be reconstructed from fossil pollen. Bogs are a primary source of fossil pollen because they are isolated from other water bodies, and the pollen that collects in them represents only the immediate vicinity and is therefore more likely to represent a single plant community rather an amalgamation of several. However, pollen recovered from small ponds and larger lakes is also used in the absence of bogs and fens.
Thompson Webb of Brown University led a large-scale effort to use pollen records to document the recovery of North American plant communities in the wake of the retreat of Pleistocene ice sheets. Beginning in the 1970s and culminating the 1980s with the COHMAP (Cooperative Holocene Mapping Project), Thompson’s group collected pollen records (dated with 14C with an accelerator mass spectrometer) and mapped the northward migration of plant species over several thousand years.
The primary conclusion germane to this discussion is that Gleason’s ideas were borne out by the pollen data. In the aftermath of continental glaciation, Clements climax communities did not march northward together as an association. Instead between the windswept barrenness of the glacial front and the “climax forest community” of the present day, all kinds of non-analogous plant communities occupied the landscape at any given location.