1. Readings Chapter 11 textbook
Connell, J. H., and R. O. Slatyer Mechanisms of succession in natural communities and their role in community stability and organization. The American Naturalist 111:

2. Outline Intermediate disturbance hypothesis
Dynamic equilibrium hypothesis
Basic characteristics of succession
Mechanisms of succession
Disturbance and succession in range and forest ecosystems: scale
Large scale disturbance (fire, logging, landslides etc)
Gap dynamics and replacement models
Herbivory and holistic grazing management

3. Intermediate disturbance hypothesis
Recall that:
Disturbance is creation of space
Plant species classified as r- or K selected
Intermediate disturbance hypothesis: at very high and very low rates of disturbance, the pool of species adapted to conditions is small. At intermediate levels, frequencies, or intensities, both r- and K species could survive, hence diversity highest.

4. Intermediate disturbance hypothesis
Species diversity
Low disturbance High disturbance

5. Dynamic equilibrium hypothesis
An extention of intermediate disturbance –Huston 1979, Am. Nat 113:81-101
Species coexistence (therefore diversity) determined by RATE at which competitive equilibrium reached.
Therefore, disturbance increases richness in high productivity environments and reduces it in low productivity environments.

6. Succession
Directional plant change after disturbance (2o) or on new substrate (1o)
May be AUTOGENIC (driven by plants) or ALLOGENIC (driven by environment)

7. Characteristics of successional communities
Some common characteristics of communities as succession proceeds (table 11.3 text)
Often increase in diversity and biomass (though only to a point – recall I.D.H.)
Increase in structural complexity
Increase in resistance, decrease in resilience
Switch from r to K species
Decrease in productivity
Decrease in light available
More mesic environment (less stress)
Nutrients tied up in biomass; tight, slow cycling

8. Mechanisms of succession
Relay Floristics (Clements): succession requires arrival at and site preparation by previous species. 6 steps: nudation, migration, ecesis, competition, reaction, and stabilization.
Initial Floristic Composition (Egler): plants involved in succession are already at site; differences in rate of establishment result in ‘seres’.

9. Other explanations Connell and Slatyer 1977:
Facilitation (RF; site preparation)
Tolerance (IFC; plants don’t interact much)
Inhibition (succession slowed by current vegetation)
Grime 1977: C-S-R life history
Shift in composition from R to C to S through time
Higher productivity, larger change in composition
Tilman 1985: Resource ratio hypothesis
Shift in biomass and resource availability allows change in competitive interactions throughout succession (different species “win” based on ability to deplete resources)

10. Forest Succession Three mechanisms:
Catastrophic disturbance (fire, landslide, clearcut, insect outbreak)
Gap dynamics (small-scale succession in treefall gaps)
Continuous recruitment without disturbance
What sorts of species or species interactions might drive each of these mechanisms?

11. Several ways of looking at secondary succession in forest stand
Successional pathways may differ based on landscape properties, environment, etc (see handout)
Several ways of looking at secondary succession in forest stand
Stand development (initiation, stem exclusion, understory reinitiation, old growth)
Population model (establishment, thinning, transition, steady-state)

12. Catastrophic disturbance
“Traditional” secondary succession
Previously discussed mechanisms of succession apply, including facilitation
Secondary succession models of stands based on this disturbance mechanism
Understory intolerant species dominate early
Transition to more tolerant species
Handout – fire in PIPO forest in Montana.

13. Gap dynamics
“Regeneration process whereby small-scale, localized disturbance gaps form in the canopy, as one or a few trees die, and release resources of light, nutrients, water, etc. for colonization of the gap by seedlings or saplings of the same or replacement species”.
Smaller scale, less change in environmental conditions

14. Gap dynamics Can affect structure and composition of forest
Important in mesic forests with low rates of disturbance and long canopy turnover times (e.g. Appalachian mts – 0.9% disturbance and 125-year canopy replacement time)
Tend to find understory tolerant species
Seedling banks may come into play
Similar to tolerance or IFC models of succession
See handouts

15. Gap dynamics
Note: can model canopy composition/structure dynamics using transition matrices.
Different forest species have different probabilities of colonizing treefall gaps of different sizes.
Given a starting composition, rate of gap formation for each species, and probability of replacement for each species, can model canopy composition.

16. Disturbance in rangeland
Rangeland disturbances?
Role of disturbance?
Plant responses to disturbance?

17. Disturbance in rangeland
Rangeland disturbances:
Fire
Soil disturbance (e.g. prairie dogs, hoofprints)
herbivory
Role of disturbance:
Reduce competition
Create regeneration microsites (Grubb 1979)
Release resources
Plant responses to disturbance:
Resprouting, seedling germination in microsites, increase or decrease.

18. Plant functional groups
Functional group: usually defined based on
Role of species in ecosystem (eg N-fixer)
Physiognomy or growth form (e.g. bunchgrass, rhizomatous grass)
Response to disturbance (e.g. increaser, decreaser)
Characteristics of increasers and decreasers?

19. Disturbance in rangeland
“Chronic disturbance” in grazed rangeland: dynamic equilibrium or non-equilibrium model.
Intermediate disturbance hypothesis: intermediate grazing pressure – increased productivity and diversity
Dynamic equilibrium hypothesis: more productive sites can handle higher grazing pressure

20. Parallels with forest succession?
Gap dynamics at small scale? (e.g. level of a bite or hoofprint)
“catastrophic” disturbance – heavy grazing, fire, flood.
Differences between responses and successional mechanisms under chronic grazing and intense rotational grazing (“holistic management” style)?