Plant succession

The Concept Succession is the natural, orderly change

in plant and animal communities that occurs over time. If left undisturbed, an open field may become an "aging" forest in 150-300 years. However soil conditions, climate, permafrost, topography, and natural and unnatural forces may affect the pattern of succession

Weather, fire, flood, insects, and human activity can disrupt the forest, altering the pattern of succession

The Stages

Herb Stage In this stage it contains seed plants whose

stems wither away to the ground each winter.

insects and small rodents feed on the grasses, herbaceous plants, and seeds

As the diversity of plants increases so does the variety of wildlife species.

Shrub Stage usually low woody plants with several

permanent stems instead of a single trunk Larger variety of wild-life attracts predators

Young Forest Stage the quantity and diversity of shrubs and

herbaceous plants decreases With less variety in food available, the

number of wildlife species decreases. birds of prey nest in the trees of a young

forest

Mature Forest Stage hardwood trees begin to die, opening the

canopy for the growth of spruce Shrubs return and make more food and

cover available to songbirds, game birds, and rodents.

Climax Forest Stage dead trees called snags provide homes for

woodpeckers and other cavity-nesting birds. Porcupines, squirrels, spruce grouse, and several songbird species are typical of old stands of spruce trees.

Types of Succession Primary Secondary Allogenic Autogenic Progressive Retrogressive

Primary Establishment of plants on land previously

not cultivated.

Secondary Invasion by plants on land that that was

previously vegetated. Possible causes include natural or human

such as fire logging, cultivation or hurricanes

Allogenic Change in Environmental conditions E.g.. Salt marsh to woodland. The environmental changes the

composition of the plant community

Autogenic Where both plant community and

environment change Caused by the activities of plants over time E.g.. Eruption of Mt. St.Helens

Progressive Where community becomes more complex

with time Contains more species Contains more biomass over time.

Retrogressive The community regresses. Becomes more simplistic Contains fewer species and less biomass Some retrogressive successions are

allogenic in nature E.g. Introduction of grazing animals results

in degradation of farmland.

The Facilitation Model "pioneer species" establish a presence on the site

of a disturbance. They modify a site, for instance, by regenerating

the soil with organic material making the area more attractive for invasion by other species.

Eventually, new species move in, edging out the pioneers.

This process may repeat itself several times, until the ecosystem reaches the climax stage.

The Tolerance Model all species involved in succession are equally

capable of establishing themselves on a recently disturbed site

but those capable of attaining a large population size quickly are likely to become dominant

Unlike the facilitation model, the tolerance model does not depict earlier inhabitants as preparing the site biologically.

this model is more akin to natural selection.

The Inhibition Model all species have equal opportunity to

establish populations after a disturbance some of the early species actually make the

site less suitable for the development of other species

An example of this is when plants secrete toxins in the soil, thus inhibiting the establishment and growth of other species

Climax When a biological community reaches a state of stability and

is in equilibrium with environmental conditions Dominant species in a climax community are those that are

tolerant of the biological stresses that come with competition frequent enough events of disturbance within small sections

of the biological community may prevent climax from even occurring

Once reached change will slow down bringing an end to the stages of succession.

Climax remains a theoretical notion.

What can cause Succession Seismic Events: earthquakes, tidal waves, or

volcanic eruptions hurricanes or tornadoes. the movement of glaciers or even of plates in

Earth's crust wildfires or sudden infestations of insects Humans causes such as plowing up ground,

logging, clearance for construction even by causing explosions on a military reservation or battlefield