State & Transition Models
WHAT ARE THEY?
State and Transition (S/T) models are a way of documenting our understanding of how processes create patterns we observe in an ecosystem. They define the different discrete types of vegetation – called “states” – that occur as a result of an area’s characteristics like climate, soils, history, or disturbances. States vary in their ability to resist change and one state can “transition” to another as a result of natural processes - like succession or wildfire - or management practices – like grazing or juniper removal.
HOW DO YOU USE THEM?
By themselves, S/T models are useful because they quantify what is known about the vegetation states of a rangeland ecosystem and what events may lead to changes in the states and how likely those events are to occur. This kind of information, gleaned from the model documentation, can be useful when considering management regimes to maintain or improve rangeland condition or developing restoration activities.
S/T models and maps of current rangeland conditions can be used with simulation modeling tools (Path Landscape and VDDT) to quantify and assess the potential effects of management activities, treatment options, and natural disturbances for managing landscapes toward a desired future condition. The results from scenario modeling inform planning efforts on the landscape-scale impacts of specific, site-scale management opportunities. This information can feed directly into planning documents or inform other procesesses like cost-benefit analyses.
Because scenario models are quantitative they require describing all of the possible transition pathways and states in the S/T models and assigning a likelihood to each transition. For example, S/T pathway diagrams must be created with succession, management, and disturbance pathways as well as disturbance probabilities, all of which should be based on empirical data. It is rarely the case, though that data will be available to quantify many S/T model parameters, and expert opinion must be used to supply missing values. In such cases, sensitivity analysis can be used to determine the influence expert-opinion values may have on the scenario outcomes.
For a good introduction to S/T models, including basic terms and concepts, please read: Bestelmeyer, B.T., K. Moseley, P.L. Shaver, H. Sanchez, D.D. Briske, and M.E. Fernandez-Gimenez. 2010. Practical Guidance for Developing State-and-Transition Models. Rangelands 32:23-30.A state & transition model for the Deep Sand Savannah ecological site.