Succession

I.  Concepts
	The biotic response to a disturbance through time.  Defined as a
directional change in communities following a disturbance or during
colonization of newly formed or exposed habitat, that leads to a
relatively stable climax community.
	Distinct communities that form during succession are called seres
or seral stages.

	2 general categories of succession:
	1) primary succession- occurs on newly formed or newly exposed
substrate, with no organic matter present and no portion of any previous
existing communities- e.g. sand dunes, lava flows, land exposed following
retreat of glaciers

	2) secondary succession- following a disturbance where vegetative
influence of previous community is not removed e.g. some plants may
survive above and below ground, seeds remain alive in soil;  e.g.
post-fire, forest clearing

	Key difference between the two is presence of vegetative
components for succession, and reservoir of organic matter (resource-
nutrient storage).  Resouce availability is greater during secondary
succession.  Primary succession is starting from scratch.

II.  Facilitation, inhibition, and chance- Processes controlling
succession
	Throughout the early 20th century there were numerous descriptive
studies of succession in different communities.  Following the 1920's
attempts have been made to generalize about what happens during
succession, in terms of the environment and biotic interactions.
	F. Clements was one of the first to formally present a model of
succession; in it he proposed succession was an orderly process, with each
sere facilitating the establishement of the next sere .  Facilitation was
proposed as the process driving the majority of succession.  Facilitation
was generally considered to be accomplished by amelioration of a more
harsh climate following disturbance - e.g. post fire environment- hot
soils, high PET
	Others, notably H. Gleason, suggested that chance events, such as
the presence or absence of certain plant propagules, nature of the
disturbance, played a very important role, and that inhibition was a more
important process. e.g. allelopathy may be important in old field
succession in OK

	More extensive research since the mid-1900's has indicated the
following are important in determining succession:

	1) facilitation- including both ameliration of climate, and
increasing soil resource supply
	2) inhibition- competitive interactions, herbivory/predation, and
allelopathy
	3) tolerance:
		a) ability to persist through a disturbance- like pines
and fires
		b) plant adaptations to the environment- harsh post
disturbance environment and changing environment during succession-  e.g.
relation of seed size to tolerance of low light following germination

	4) life-history characteristics and population processes - similar
to r- and K- selection; how much reproduction, how fast reproductive
maturity reached

III.  Climax Community

	Often an arbitrarily determined condition of a seemingly
self-replacing community.
	Existence of climax community relates back to succession- given
that several seres are possible at a given stage, due to chance events and
the nature of the disturbance, would there always be the same end point?
	Proponent of the single climax theory was Clements, who proposed
that there were only 14 climax communities in the continental US.  For
example, in SW Wisconsin Maple-Basswood forests were the only recongized
climax; but at the U of Wisconsin, the Curtis lab found that the final
climax community was dependent on soil characteristics; continuum concept,
where the climax varies according to subtle site characteristics.

	Now widely recognized that a mosaic of communities, some at climax
stage, others not, exist across landscapes, as a result of:
	1) small-scale disturbances
	2) different successional paths, although in general the climax
varies less than successional seres do for any given location,
	3) site variation in microclimate and soils

	Some climax communities appear to be stable, self-replacing, but
in fact are dependent on disturbance for their maintenance- e.g. Boreal
Forest, African Savannas, chaparral, tall grass prairie.

	Others may self-replace for what seems like a long time, but in
fact lead to a different community through time e.g. in Pacific NW, 700-
1000 year old Douglas Fir forests are eventually replaced by hemlock
forests.