Topic 15- Forest Succession

Primary Succession

• Starts from bare rock

• Glacier retreat or volcanic event

Secondary Succession

• Starts from disturbance

• Fire, logging, windstorm, flood, etc

Successional Pathways

• Species traits can alter successional trajectories

Ex: Fire tolerance vs competitive ability

Definition of Biological Legacies

• Biologically generated elements of a pre-disturbed ecosystem that persists in some form after a disturbance occurs

Elements of biological legacies

• Living organisms or portions of organisms that survive a disturbance

-Intact plans/animals, rhizomes, and dormant spores/seeds

• Organic Debris

-Standing dead tree (snags), fallen logs (course woody debris), large soil aggregates, dense mats of fungal hyphae

• Biotically derived patterns in soils and understories

• Biological legacies are among the most important ecological outcomes of disturbances

Models of old field succession

Relay floristics

Initial floristic composition

Models of forest stand replacement

• all trees are already present in the stand or colonize soon after the disturbance

• Dominant overstory tree changes as stem number decreases and vertical stratification of the species progresses

• Old growth may be attained in 200-500 years depending on site

1st stage of succession

• Stand initiation (Establishment)

Plants continually colonize for several years

Plants come in as newly disbursed seeds, buried seeds, or resprouts

2nd stage of forest succession

• Canopy Closure

Sharp decrease in light to the forest floor

Stem exclusion (Thinning)

3rd stage of forest succession

• Understory reinitiation (Transition or Mature Forest Stage)

Canopy begins to senesce, insect and pathogen damage and physical disturbances increase

Space created allows some understory trees to enter the canopy (Advanced Regeneration)

4th stage of succession

• Old growth

Second and third generation of understory trees reach the canopy

Greatest amount of turnover

Structural and biological diversity maximized

Gap dynamics

-Most common natural type of forest succession

• Size and location of the gap will have large impacts on succession within the gap

• Distance from other gaps may be important

Small gaps

• May be filled by crown growth of existing canopy trees

• Allow for advancement of overtopped or co-dominant crown class neighboring trees

• Allow for recruitment of new individuals into the open space

Large Gaps

• If there is enough light, mid-tolerant and shade intolerant species can establish and reach the overstory

• Distinct community than in non gap location

Invasive Species

loss of one species due to invasive species, gives rise to another dominant species, changing the trajectory of the succession.

Trajectory on an ecosystem depends on

• ecological strategies of species

• Interactions among species

• Site characteristics

• Disturbance regimes

• Invasion dynamics