Ecological Succession and Community Dynamics - Ecological Systems - page 40

eventually leads to a White Spruce ( Picea glauca ) forest that is quite different in its
composition and character from the Black Spruce ( Picea marina ) forest seen on
north-facing slopes. To some degree, Clements and his colleagues used terms such
as disclimax, etc. (see above) to handle these sorts of cases in their monoclimax
theory of succession.
A.S. Watt
One of Tansley's outstanding students was Alexander Stuart Watt. He developed
his doctorate in 1924 on the dynamics of Beech ( Fagus sp.) forests [ 38 ]. He noted
the structure of the beech forest was at the scale of the canopy controlled by
a dominant large tree best characterized as a cyclical process of the regeneration
in the patch of well-lighted area that followed the death of a canopy tree, the growth
of these recruits and their competition with one, and the development of a dominate
canopy tree in that location. The death of this large tree would then reinitiate the
cycle. By 1947, Watt had become the president of the British Ecological Society
and delivered one of the most important papers in ecology as his presidential
address. This presidential address and the subsequent paper represented
a synthesis of what nowadays might be called the “big picture” science view of
his doctoral advisor, A.G. Tansley, with Watt's own insights based of his field
experience and broad ecological interests. In his address, Watt represented the plant
community as a “working mechanism” of interacting plant processes such as
regeneration, growth, and death on individual plants producing the broader-scale
pattern of plant communities. He stated, “The ultimate parts of the community are
the individual plants, but a description of it in terms of the characters of these units
and their spatial relations to each other is impractical at the individual level. It is,
however feasible in terms of the aggregates of individuals and species which
form different kinds of patches: these patches form a mosaic and together constitute
the community. Recognition of the patch is fundamental to an understanding of the
structure as analyzed here” [ 39 ]. To illustrate this communality in the underlying
working dynamics of vegetation, he compared a diverse array of plant communities
- heathlands, grasslands, bogs, montane vegetation and forests.
As was the case with Gleason, Watt noted that one must account for the results
of individual plants interacting with one another, but he also felt such an account-
ing was “impractical at the individual level.” Twenty years later, the explosive
expansion of computational power that still continues to date has made such
computations more and more feasible. With the development of increasingly
powerful digital computers, starting in the 1960s and continuing to the present,
several different scientific disciplines (physics, astronomy, ecology) independently
began to apply computers to the tasks of “book-keeping” the changes and
interactions of individual entities. Forest “gap” models (which will be discussed
below) were one of this class of individual-based models or IBM's that brought
Watt's insights on vegetation dynamics into a form that could be projected using
computer simulation techniques.