Peet, Robert K. and Christensen, Normal L. 1987. Competition and Tree death.
BioScience 37(8):586-95.
Summary
This paper lays out a general model of forest dynamics for forest regrowth following disturbance based on post disturbance
work in a variety of divergent forest type. This model contains 4 phases: establishment, thinning, transition, and steady-state.
I. Establishment
Establishment occurs immediately following disturbance (e.g. abandonment of an agricultural field, after a wildfire, or
following logging) and sometimes gives rise to a geometric rise in vegetation . This stage progresses with the rapid growth of one or a
few quick growth spp (often pine dom.) which form a closed canopy (in pied 5-15 yrs) The key traits of this phase are:
*Low competition, allowing establishment of numerous seedlings and spp. In the Piedmont this often results in more pine
seedlings than those of any other spp.
*Rapid growth- due to the availability of resources and low competition
II. Thinning
Once the closed canopy forms, competition becomes intense, most overtopped individuals die. Thinning continues until
canopy spp (pines) have reached full size and overtopped indiv. are mostly dead (~60yrs). Key traits of this phase:
*Region near the ground becomes rel. free of veg. due to die back
*The number of trees in the stand decreases and biomass increases.
* Mortality prob. highest for those with a competitive disadvantage. Size, vigor, and prox. to competitors have been shown
to have a high predictive value. At the end of thinning the stand will have a rel. narrow, normal dist of size classes with few small size
class indiv.
* Biomass will peak late in the thinning phase a level out during the final 2 stages.
III. Transition
Begins when the canopy trees begin to die. This creates a canopy gap in which new seedlings become established. The
duration and character of the transition phase is highly dep. On the average lifespan post thinning and the degree of synchrony of
mortality or remaining initial cohort. In the SE, few pine seedlings become dominant in these patches due to their size and hardwoods
begin to increase in dominance. Key traits:
*Seed availability and gap size are key traits that determine which spp successfully exploit the patch.
*Tree death is density independent and a consequence of env. factors (wind, insects, fire, etc.)
IV. Steady state
Dominated by hardwoods (in SE). Indiv of varying age, but often located in rel. even aged patches (coincident w/ prev.
canopy openings). The first 3 phases of the model are repeated in miniature within ea. gap. In the steady state phase, the forest will
have a reverse J distribution of size classes- many small stems and fewer stems in each subsequent size-class.
Applicability
Model applies to essentially all forests where one or only a few trees spp dominate the initial regrowth. The applicability of
the first 2 stages to high diversity tropical forests is not yet clear. Extension of the model to communities of lower stature or woodland
like physiognomy is more problematic.
Patterns of tree mortality
* Trees that become est. during the initial phase have a sigmoidal survivorship curve
* The increase in mortality during thinning is directly related to the initial density of stems.
* Seedling mortality is low during establishment phase, virtually complete during thinning, and intermediate but spatially
highly variable in the steady state. All these are relative against a background of high absolute mortality.
* Yoda et al. (1963) –3/2 law: Mean weight of a plant in a thinning pop is proportional to density to –3/2 power. As the
transition phase approaches, rate of biomass prod. will decrease so the slope will fall back towards –1. This applies well to even-aged
pops. of a single spp. In mixed stands, the over all thinning component seems to range a bit more around the –3/2 value. The
component spp. of these pop. however do not individually follow the –3/2 law.
Spatial Patterns
In steady state forest different size classes have different spatial patterns:
* small trees have a clumped pattern. As trees in crease in size that goes from a clumped to a random to a one that
approaches uniform.
Gleason, H. A. 1926. The Individualistic Concept of the Plant Association. Bulletin of
the Torrey Botanical Club 53:7-26.
Summary
This paper reads more like an essay than a journal article; showing I guess how science and our approach to it has changed.
Consequently, this’ll read more like a paragraph than an outline, but I’ll do my best.
Gleason begins by stating that scientists have made many observations and measurements of the natural word which we can
take to be accurate. From there, he says, it follows that we would depart from these direct observations to more general deductions
about the nature of plant associations and that we would invent terms and methods for indicating the difference between these
associations, and attempt to lump these associations in turn into broader groups. The problem he says is that the areas of “structural
uniformity” which we include in an association are uniform “only [to] a matter of degree” and that different ecologists may divide the
same area up differently depending on individual differences in definition.
From here he raises a variety of points as to why associations are arbitrary or at best problematic:
1. Associations defined by vegetation, while they might be easily defined in the northern latitudes where there are few
dominant spp, are impossible to define in the spp rich tropical forests.
2. Areas with the same association (as defined by vegetation) do not always have the same environment.
3. Areas with the
same environment do not always have the same associations
4. It is not always possible to define the geographical boundaries of an association
5. Some transition zones contain spp not found in either of the abutting associations.
6. Spp. of a plant associated with a particular plant community are frequently fond within many other types of associations.
7. There are variations in the vegetational structure from year to year within every plant association due to rainfall and
other env. conditions.
8. The temporal duration of an association is limited and it is often difficult to identify the temporal boundaries of an
association.
Following all of this and an example of heading down the Mississippi and trying to determine where the
associational boundaries are as you head from spring to gulf, he concludes; “Surely our belief in the integrity of the association and
the sanctity of the association-concept must be severely shaken. Are we not justified in coming to the general conclusion, far removed
from the prevailing opinion, that as association is not an organism, scarcely even a vegetational unit, but merely a coincidence?”
Gleason here turns from “negative evidence” against plant associations to “positive evidence” for the individualistic concept
of the development on plant communities. He does this based on spp. dispersal:
Spp disperse individually, independently from the other spp within their association. The plant spp. in a given area are
therefore the result of the migration of independent spp. and as there is no such thing as a “typical” association. This is illustrated by
the example of dune colonization. The spp. found on each dune are determined by chance alone and their appearance and
disappearance form a dune is due to their individual environmental tolerances.
“In conclusion, it may be said that every species of plant is a law unto itself, the distribution of which in space depends upon
its individual peculiarities of migration and environmental requirements. . . . The behavior of the plant offers itself no reason at all for
the segregation of definite communities. Plant associations, the most conspicuous illustration of the space relation on plants, depend
solely on the coincidence of environmental selection and migration over an area of recognizable extent and usually for a time of
considerable duration. A rigid definition of the scope or extent of the association is impossible, and a logical classification of
association is impossible, and a logical classification of associations into larger groups, or into successional series, has not yet been
achieved.”
Short bio-place in history/context
Gleason published this paper in response to attempts to classify vegetation into discrete, repeating communities. His ideas
were however, largely ignored for a long time in preference for those of Clements (36) who theorized that plant communities were
individual, identifiable units that replaced each other in a regular pattern till a climax community was established. Later studies using
fossilized pollen (REF) and OTHER THINGS showed that plant communities are indeed temporary and that they have not always
existed. Today, the pendulum stands somewhere in between the ideas of Clements and Gleason. Communities are
recognized(generally) to exist, as it their temporally transitory nature, their muddy edges, etc.