States and Transitions in Succession

Plant Community Succession (Initial Ideas – F.E. Clements)

Natu

ral Po

ten

tial Large Variation

Between Years

Climax or PotentialNatural Community

Pioneer/Early LateSeral Stage

Plant Community Succession(Initial Ideas -- Clements)

Natu

ral Po

ten

tial

Pioneer/Early LateSeral Stage

Above average rainfall

Successional

tendency

Succession

Drought

Grazing Pressure

Retrogression

Climax or PotentialNatural Community

Example -Succession in the western juniper woodland

Grassland after fire Stand initiation juniper

Open young juniper Young multistory juniper

Old multistory juniper

Mountain bigsagebrush steppe

Cyclic Patterns creating a “Dynamic Equilibrium”1979 - Pre-burn

1989

1983

1980- 1st year after fire

1994

Problems with Succession Model

Wyoming big sagebrush/bluebunch wheatgrass steppe near Mtn. Home, ID

Wyoming big sagebrush/bluebunch wheatgrass steppe, central Idaho

• multiple pathways of succession• multiple stable vegetation types• no single and certain end-point

Two examples of stable states for same ecological site

Problems with Succession Model

• multiple pathways of succession• multiple stable vegetation types• no single and certain end-point

State and Transition Models

1989 - Westoby, Walker and Noy-Meir

State A

State D

State C

State B

Focus on States of communities and Transitions between states

Creosotebush community -this ecological site may have always been creosotebush dominated, or degraded long ago.

States and Transitions depend on biotic & abiotic conditions

States are relatively stable but some states are more stable than others

States and Transitions depend on biotic & abiotic conditions

Higher Elevation

Central Idaho

Lower Elevation Central Idaho

Higher Elevation Southwestern Idaho

Juniper Present

Cheatgrass Risk

Low Cheatgrass Risk

States and stability depends on factors such as elevation & climate

Example - Sagebrush Steppe States

Fire transition

Succession transitionImproper grazing transition

Transitions= compositional change resulting in the change in plant community (state)

Grassland Open Sagebrush

Closed Sagebrush

DepletedSagebrush

Thresholds = transitions that are nearly irreversible & sometimes unpredictable.

Sagebrush Steppe State

Annual Grass Dominated State

Threshold

Grassland Open Sagebrush

Closed Sagebrush

DepletedSagebrush

Grazing reduces likelihood of fire

The information required to develop these models: • potential alternative vegetation states on a site• potential transitions between states • opportunities to achieve favorable transitions between

vegetation states and hazards to avoid unfavorable transitions

Rangelands are Dynamic!

Rangeland Ecology and Management

•But, dynamic patterns can be describe in state and transition models.

•We manage transitions