States and Transitions in Succession

Plant Community Succession (Initial Ideas – F.E. Clements) Natural Potential Large Variation Between Years Climax or Potential Natural Community Pioneer/EarlyLate Seral Stage

Plant Community Succession (Initial Ideas -- Clements) Natural Potential Pioneer/EarlyLate Seral Stage Above average rainfall Successional tendency Succession Drought Grazing Pressure Retrogression Climax or Potential Natural Community

Example -Succession in the western juniper woodland Grassland after fireStand initiation juniper Open young juniper Young multistory juniper Old multistory juniper Mountain big sagebrush steppe

Cyclic Patterns creating a “Dynamic Equilibrium” Pre-burn st 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 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 transition Improper grazing transition Transitions= compositional change resulting in the change in plant community (state) Grassland Open Sagebrush Closed Sagebrush Depleted Sagebrush

Thresholds = transitions that are nearly irreversible & sometimes unpredictable. Sagebrush Steppe State Annual Grass Dominated State Threshold Grassland Open Sagebrush Closed Sagebrush Depleted Sagebrush 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