Succession

Equilibrium theory (Clements 1916):

disturbance,

migration,

establishment,

competition,

reaction (change in the environment),

and stabilization (domination by long-lived species)

Relay floristics:

One group of plant species establishes and is then replaced by another group until a stable state is achieved.

Initial floristics (Egler 1954):

Species that are initially present have a good chance of staying there.

Drury and Nisbet (1973):

Availability of resources changes over time, species sort themselves out over a resource gradient, and so species composition changes as resources (like light) change.

Connell and Slatyer (1977):

Three models: Facilitation, Tolerance and Inhibition

Pickett et al (1987), and others:

Individual species models (site availability, species availability, species performance)

Methods of Managing Succession

I. Plant and plant part removal

A. Mowing, clipping and cutting

1. Grasslands and meadows

2. Heathlands

3. Semi-desert scrublands

4. Forests

B. Herbicide (Oregon State University herbicide link) (Go to Table of Contents)

1. Timing

2. Effectiveness

3. Rights-of-way

4. Rangelands

5. Heathlands

C. Fire

1. Controlling woody spp. in grasslands

2. Changing dominance of non-woody species in grasslands

3. Southern pine forests

4. Prescribed burning

fuel

weather

firelines

burning techniques

D. Cabling, root plowing

II. Changing Resource Availability

A. Soil Nutrients

Some plants, including most weeds, have very high rates of nutrient uptake and growth response to fertilization.

On the other end of the spectrum are slow-growing plants adapted to infertile environments.

So, soil nutrients determine plant community composition at any given time, and are an important determinant of species turnover during succession.

Examples:

Heathlands: Plants adapted to low nutrient environment. Fertilization results in a decrease of shrubby heathers, increase of grasses.

Disturbed soils: Grasses dominate fertilized plots. Legumes decrease.

Grasslands: Fertilization favors cool season grasses and weeds.

B. Nutrient Exhaustion

Soil nutrient accumulation during succession drives species turnover.

Nutrient inputs come from rainfall, weathering, manure, fertilization, nitrogen fixation, dryfall.

Nutrient outputs are leaching, surface runoff, volatilization, removal of soil or plant biomass.

Most common ways of removing nutrients from established system are grazing, burning and mowing.

Other ways are soil removal, addition of carbon sources for microbial uptake.

C. Water

Water is the most limiting resource in most terrestrial communities.

For management purposes, the most common ways that water is controlled are through irrigation and drawdown/flooding.

Irrigation is a powerful, but expensive method for controlling species performance.

Examples:

Irrigation of grasslands: Warm season grasses favored more than cool season grasses.

Irrigation of disturbed soils: Speeds succession, favors grasses, has short-lived effect.

Irrigation of Swedish pine-heath: Interacts with fertilizer. No effect alone. Limits slow-growing plants, lichen, mosses.

Water-level manipulation of wetlands, ponds, lakes and reservoirs is a powerful method of controlling colonization and species performance.

Drawdown will kill aquatics by desiccation, heat or freezing.

Also, individual species respond to specific water level changes.

Examples:

Wetlands: Annual plants need unflooded mudflats to germinate. Longer and longer reflooding will eliminate annuals and favor perennials.

Swamp forests: Water depth > 1.5 m eliminates trees. At lesser depths there is an increasing number of surviving species. Dry periods favor more mesic tree species, increase diversity.

III. Changing Propagule Availability

Revegetation is managed primary succession that relies strongly on augmenting the propagule supply.

Successional pathways after revegetation are influenced by:

physical and chemical conditions of the substrate,

methods and timing of revegetation,

types of plant species planted or seeded,

cultural practices maintained during community development,

and ingress of volunteer species.

Restoration manages early stages of succession; later stages should also be managed.

A. Seeding and Mulching

Seeding method can influence establishment, even though similar seed mixes are used.

Forbs like drill-seeding, esp. large seeded ones. Grasses establish well regardless of method.

Mulches retard erosion, improve germination, conserve soil moisture, ameliorate soil moisture.

Example: Snowberry in mulched plots had 50:1 better survival than unmulched sites on UBNA.

Native seed mixes:

Keep fertilization rates low.

Screen aggressive alien spp. from seed mixes.

Do not sow aggressive non-native cover crop.

Tree seeds require:

control of seed-eating animals,

preparation of mineral seed bed,

sparse cover crop or no cover crop.

Tree seedlings will reach canopy closure faster than trees from seeds.

Clearing an area around seedlings improves their chances.

B. Topsoiling

May be appropriate following severe soil disturbance.

Stockpile and redistribute, or borrow from adjacent communities.

Source of propagules, if not stockpiled for too long a time.

Inoculation (small amounts) from adjacent communities is effective.

Seeds that germinate from borrowed topsoil may not accurately reflect the composition of the vegetation where you borrow it.

C. Neighborhood

The amount of effort put into increasing propagule availability will depend upon the vegetation condition of the neighborhood.

Surroundings that are like the target community are good; they will seed into your plot.

Uphill, upstream, upwind are good places for them.

Leaving seed trees is a good strategy.