1. States and Transitions in Succession

2. Plant Community Succession (Initial Ideas – F.E. Clements)
Climax or Potential Natural Community
Large Variation Between Years
Natural Potential
When a perturbation (disturbance or stress as we will refer to it) causes a major ecosystem function to deviate from its normal operating range, the degree of deviation is a measure of relative resistance stability, whereas the time required for recovery is a measure of relative resilience stability.
Notes from Steve Bunting – University of Idaho
Pioneer/Early
Late
Seral Stage

3. Plant Community Succession (Initial Ideas -- Clements)
Above average rainfall
Successional tendency
Progressive Succession
Climax or Potential Natural Community
Drought
Grazing Pressure
Retrogression
Natural Potential
Successional communities usually increase in diversity and biomass over time, referred to as progressive succession.
In contrast, retrogressive succession, or retrogression, results in decreased biodiversity and biomass.
Notes from Steve Bunting – University of Idaho
Pioneer/Early
Late
Seral Stage

4. Example -Succession in the western juniper woodland
Mountain big
sagebrush steppe
Grassland after fire
Stand initiation juniper
This sequence of photographs summarizes a secondary successional chronosequence leading to a mature juniper woodland in southwestern Idaho. The process illustrated probably takes more than 600 years.
We cannot actually observe a 600-year successional sere. Chronosquences are often employed where space (studying many sites), using sites of different ages, is substituted for successional time on a single site.
Notes from Steve Bunting – University of Idaho
Open young juniper
Young multistory juniper
Old multistory juniper

5. Cyclic Patterns creating a “Dynamic Equilibrium”
Pre-burn
st year after fire
1994
The sequence of photos illustrates a 15-year period of vegetation succession following a prescribed fire in mountain big sagebrush/Idaho fescue steppe. The density and coverage of sagebrush has nearly developed back to the pre-burn level during this time.
Notes from Steve Bunting – University of Idaho
1983
1989

6. Problems with Succession Model
multiple pathways of succession
multiple stable vegetation types
no single and certain end-point
Two examples of stable states for same ecological site
Here are two examples of different community types occupying the same habitat type (Wyoming big sagebrush/bluebunch wheatgrass). The photo on the left is dominated by cheatgrass and bottlebrush squirreltail, and the photo on the right is dominated by Wyoming big sagebrush and bluebunch wheatgrass. Both communities are relatively stable.
Notes from Steve Bunting – University of Idaho
Wyoming big sagebrush/bluebunch wheatgrass steppe near Mtn. Home, ID
Wyoming big sagebrush/bluebunch wheatgrass steppe, central Idaho

7. Problems with Succession Model
multiple pathways of succession
multiple stable vegetation types
no single and certain end-point
State and Transition Models
Here are two examples of different community types occupying the same habitat type (Wyoming big sagebrush/bluebunch wheatgrass). The photo on the left is dominated by cheatgrass and bottlebrush squirreltail, and the photo on the right is dominated by Wyoming big sagebrush and bluebunch wheatgrass. Both communities are relatively stable.
Westoby, Walker and Noy-Meir

8. Focus on States of communities and Transitions between states
State A
State D
State B
State C

9. States and Transitions depend on biotic & abiotic conditions
Patterns in Chihuahuan Desert state-and-transition models MLRA 42 Ecological Site Description Workshop (Alpine TX) by Brandon Bestelmeyer, USDA-ARS Jornada Experimental Range
States are relatively stable but some states are more stable than others
Creosotebush community -this ecological site may have always been creosotebush dominated, or degraded long ago.

10. States and Transitions depend on biotic & abiotic conditions
Juniper Present
Low Cheatgrass Risk
Higher Elevation Southwestern Idaho
Higher Elevation Central Idaho
Cheatgrass Risk
Lower Elevation Central Idaho
States and stability depends on factors such as
elevation & climate

11. Example - Sagebrush Steppe States
Grassland
Open
Sagebrush
Closed
Depleted
In some areas of the Great Basin we can observe a third state- that dominated by annual grasses such as cheatgrass or ripgut brome. The movement back into either the sagebrush state or the woodland state is difficult in these environments.
Transitions= compositional change resulting in the change in plant community (state)
Fire transition
Succession transition
Improper grazing transition

12. Sagebrush Steppe State
Annual Grass
Dominated State
Threshold
Grassland
Open
Sagebrush
Closed
Depleted
In some areas of the Great Basin we can observe a third state- that dominated by annual grasses such as cheatgrass or ripgut brome. The movement back into either the sagebrush state or the woodland state is difficult in these environments.
Grazing reduces likelihood of fire
Thresholds = transitions that are nearly irreversible & sometimes unpredictable.

13. 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

14. Rangelands are Dynamic!
Rangeland Ecology and Management
Rangelands are Dynamic!
But, dynamic patterns can be describe in state and transition models.
We manage transitions 14