Readings

1. Readings • Chapter 11 textbook • Connell, J. H., and R. O. Slatyer. 1977. Mechanisms of succession in natural communities and their role in community stability and organization. The American Naturalist 111:1119-1144.

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. 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)?