States and Transitions in Succession

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

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

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

Cyclic Patterns creating a “Dynamic Equilibrium” 1979 - Pre-burn 1980- 1st 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

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

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. 1989 - Westoby, Walker and Noy-Meir

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

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 <bbestelm@nmsu.edu> 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.

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

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

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.

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