State and Transition Model (STM)
Ecological dynamics describe the changes to vegetation and soils, and the causes of those changes that can occur on an ecological site. Details on the alternative states, ranges of variability within states, and the processes that cause plant community shifts within states as well as transitions among states are described in the text and diagram of a State and Transition model (STM).
STMs synthesize literature and the knowledge tied to particular ecological sites to distinguish changes in vegetation and soils that are easily reversible versus changes that are subject to thresholds beyond which reversal is costly or impossible. They are a means of communicating about plant succession, ecological thresholds, non-equilibrium dynamics, and functional and structural change in response to disturbances and management actions. STMs describe the following:
All possible states, community phases (i.e., easily-reversible variants of states), and transitions between communities and states.
Patterns, causes, and indicators of transitions between communities within an ecological site.
Relationships between vegetation, soil, animals, hydrology, disturbance (e.g., fire, lack of fire, grazing and browsing, drought, unusually wet periods, insects and disease), and management actions.
Existing soil-vegetation relationships.
STMS also document historical vegetation and dynamics as well as restoration outcomes, and measurements of ecosystem properties and processes occurring within states (e.g., cover, soil aggregate stability, erosion rates, net primary production). They also provide a conceptual understanding of:
Ecological dynamics that can occur on an Ecological Site;
Drivers and disturbance mechanisms of ecosystem change, such as impacts of animals, wildland fire or lack of fire, recreation activities, etc.; and
Management actions that can influence change.
State and Transition Model (STM) Purpose
A state and transition model (STM) is a diagram that depicts our current understanding of the dynamics of an ecological site. They identify the different plant associations or “states” that may exist on a given ecological site and how other site characteristics, such as hydrology and soil stability, might change with them. STMs describe the environmental conditions, disturbances and management actions that cause vegetation to change from one group of plant species to a different set of species, and the management actions needed to restore plant communities to a desired composition. They help the Bureau of Land Management (BLM) identify where the land is currently (its present state) and what potential alternative states it could inhabit, and provide ideas about how to move to a more desirable state and avoid unwanted transitions.
State and Transition Model (STM) Development
STMs are developed using four kinds of information:
Inventory data of soil properties and vegetation;
Historical reconstructions using long-term monitoring data, historical records, or photography;
Recent monitoring data, including responses to climate variability and management interventions; and
Process-based research and studies that test for the mechanisms causing or constraining ecosystem responses.
Ecological Site Descriptions (ESDs) and their associated STMs are developed using expert knowledge, available inventory and monitoring data, previous range site descriptions, soil survey information and research to help confirm model assumptions for STMs. STMs look like and include the following:
Stable groupings of plant species or “states” (boxes) and the pathways of change (transition) between states (arrows).
The arrows between the large boxes, or states, signify that a threshold has been crossed. This means that new ecological processes characterize the site, and it will take active management to shift back to the previous state.
Vegetation states (larger boxes) may contain several communities which are called plant community phases (or plant associations).
Plant community phases (smaller boxes) can easily transition from one to another in short time frames.
The narrative section of the STM model gives a written description of each of these states that explains what plants we would find in each state, how useful each state is for achieving specific management objectives, and other site characteristics.
The description of each transition helps provide an understanding of what the Bureau of Land Management (BLM) might have to do to move from one state (box) to a more desirable state.
Click here to see an example of an STM.
State and Transition Model (STM) Content
The state is a suite of community phases that interact with the environment to produce a characteristic composition of plant species, functional and structural groups, soil functions, and a characteristic range of variability. Alternative states differ in the operation of one or more primary ecological processes including the water cycle, nutrient cycle, the process of energy capture and transformation (energy flow).
A reference state is recognized in each STM describing ecological potential and natural or historical range of variability of the ecological site. The reference state includes information on dynamic soil properties and native plant and animal populations. Due to natural disturbance and climatic processes, reference conditions can be represented by more than one community phase depending on the time period in which an ecological site is observed. In some ecological sites (for example some grass-dominated sites) only one community phase may be recognized in the reference state. Ecological sites featuring natural fire regimes will most likely have more than one community phase in the reference state. In such cases, a cessation of natural disturbance may lead to transitions.
Transitions describe the biotic or abiotic variables or events that contribute directly to loss of state resilience and result in shifts between states. A transition can be triggered by natural events (e.g., climatic events or fire), management actions (e.g., grazing, burning, fire suppression, recreational use) or both.
Whereas transitions describe the drivers and mechanisms of shifts between states, the term “threshold” indicates a set of conditions separating two states where conditions sufficient to modify ecosystem structure and function beyond the limits of ecological resistance or resilience results in the formation of alternative states. Changes that warrant the use of intensive management practices and restoration technologies to return to the previous state are considered to be transitions.
Changes in dynamic soil properties can parallel changes in community phases or transitions between states. These coupled changes may be due to feedbacks between plant cover and composition and dynamic soil properties. Thus, dynamic soil properties can be used to help understand the risk of transition, especially when it is not easily understood by observing vegetation.
Transitions may be reversible, but may become irreversible and may go from a “disturbance” to an alternative state” for example by overgrazing, lengthening of fire cycles (e.g., from 50-60 to 70-90 years), and prolonged drought. Alternative states with significant disturbances such as shortening fire cycles to 15-20 year frequencies and the introduction of invasive annual grass species may result in irreversible transitions.
Restoration pathways describe the environmental conditions and practices that are required to recover a state that has undergone a transition. Ecosystem properties that promote restoration might be seed sources, species composition, nutrient distribution, and hydrologic function which influence the probability of success of prescriptions restoration. Environmental conditions may include relatively high rainfall years. Management practices include chemical/mechanical treatments, planting, prescribed fire and grazing management.
Community phases are unique assemblages of plants and associated dynamic soil property levels that can occur over time within a state. In states that attain equilibrium, community phases are equivalent to seral or successional stages that may undergo orderly changes in response to natural disturbance, management, and succession. In states that do not attain equilibrium, community phases may shift from one to the other in complex ways depending on the interactions of climate, natural disturbance, and management. Community phases included in a single state may have similar floristic or functional groups, but may differ in dominant or subordinate species. However, the community phases may be quite different in floristic or functional groups when disturbance drivers, for example fire, cause changes in plant composition and structure. Collectively, the community phases represent the range of variation within a state, including conditions that place the state at risk of a transition.
Community pathways between community phases within a state describe the causes of shifts between community phases. Community pathways can include the concepts of episodic plant community changes as well as succession and serial stages. In contrast to transitions between alternative states, shifts in community phases are easily reversed due to succession, natural disturbances, short-term climatic variation, and facilitating practices such as grazing management.