Written by Brad Schultz, Extension Educator, Winnemucca, Nevada
State and transition models have four primary components: states, transitions, thresholds, and triggers. The reference vegetative state refers to a recognizable and repeatable plant community (ecological site) that typically occupies a specific type of soil. The abundance of the specific plant species largely depends upon the topography, the specific characteristics of the soil, climatic conditions and its variability, the composition of the previous plant community when it was last disturbed, the type and intensity of the last disturbance, time elapsed since the last disturbance, historic and current land uses, and herbivory (wild and domestic). Management that removes important species or life-forms from a vegetative state and/or introduces non-native plants that alter the state's ecological processes and functions typically results in a different vegetative state. This is illustrated in Figures 4a-4c, with photos from several Wyoming sagebrush (Artemisia tridentata wyomingensis) communities in Nevada.
Figure 4a shows a Wyoming sagebrush site with an intact understory of perennial bunchgrasses and forbs. Removal of the shrubs from a fire or other catastrophic disturbance would result in a community of mostly bunchgrasses: the earliest phase of secondary succession on most sagebrush-grass ecological sites. The bunchgrasses provide sagebrush-bunchgrass states with good ecological resilience to disturbance. Figure 4b is a Wyoming sagebrush community without the perennial bunchgrasses in the understory. Following removal of the shrubs, bunchgrasses cannot reoccupy this site without significant external inputs (e.g., seeding, weed control). This community is unlikely to return to the previous sagebrush-bunchgrass state because cheatgrass (Bromus tectorum), an introduced annual grass, may rapidly occupy the site (Figure 4c). The sagebrush community in Figure 4b is resistant to change back to the sagebrush-bunchgrass state, regardless of the management actions applied. Furthermore, it lacks ecological resilience (i.e., change back to a sagebrush-bunchgrass state) if disturbed. The sudden loss of the sagebrush component in this community would leave the ground barren and prone to invasion by undesired weeds which results in the common transition to a cheatgrass community (Figure 4c). No simple change in management can reestablish the perennial bunchgrasses missing from either community (Figures 4b and 4c); therefore, they have become alternative vegetative states
Loamy WY sagebrush
Figures 4a-4c. Three vegetative states in a loamy 8-10 inch (precipitation zone) Wyoming sagebrush ecological site in Nevada. The community in Figure 4a has an extensive understory of perennial bunchgrasses that maintain site resilience. Figure 4b shows a community without the bunchgrasses or other perennial herbaceous species. Figure 4c shows a sagebrush-bunchgrass community that lost the bunchgrasses, burned, and transitioned to a cheatgrass community. Courtesy of Brad Shultz.
Many vegetative states are composed of two or more vegetative phases (community phase) of the potential natural community (PNC). This is particularly true if both herbaceous and woody life-forms inhabit the PNC. A community phase is one of several plant communities that may occur within a state. Two phases of the same community (ecological site) typically reflect different proportions of the species that occur in the PNC. Whether the specific plant community on a site is a phase of the reference community of a vegetative state or a different vegetative state depends upon how the community responds to potential management actions. Community phases that typically respond to common management actions (grazing management, brush control, etc.) by changing to a different desired phase in an acceptable time frame (from a management perspective) are part of the suite of community phases within one vegetative state. When a typical management action cannot change a plant community to a more desired community, and the desired vegetation change requires intensive and expensive external inputs, that community has transitioned to an alternative vegetative state. Figure 5 shows two phases of a black sagebrush-perennial bunchgrass state in west-central Utah. The phase to the right of the fence has been ungrazed for over 60 years and is mostly black sagebrush with widely scattered native bunchgrasses. The vegetative phase on the left is predominately bunchgrasses with widely scattered shrubs. The bunchgrass phase has received dormant-season grazing for 60 years. A simple change in the season and intensity of grazing could shift the shrub-grass phase to more grasses or the grass-shrub phase to more shrubs. The shrub-grass phase to the right of the fence (Figure 5) is slowly losing its bunchgrass component due to competition from the shrubs. Without a change in management to reduce competition from the shrubs, the shrub-grass community phase will become a shrub state with limited management options.
Figure 5. A black sagebrush–bunchgrass vegetation state with two vegetative phases in west central Utah. Courtesy of Brad Shultz.
A vegetative transition is a directional change in a plant community from one state to an alternative state. Most transitions are toward undesirable or less desirable states. Transitions to more desired states are possible but typically require extensive external inputs to the community. Transitions do not reflect annual differences in a community or the expected seral changes within a state (i.e., the community phase changes previously discussed). Many ecological processes, management actions, and disturbances may occur individually, collectively, or sequentially. The cumulative effect of these processes and events may cause the vegetation to change from one state to another or to recycle among the different phases of a state. The ecological processes that interact to cause transitions typically occur across long periods. For example, the change associated with the loss of bunchgrasses on a sagebrush-bunchgrass site often takes decades (Figures 4a and 4b), but the ecological effect often becomes clear-cut only after a sudden intense disturbance, such as fire (Figure 4c).
A threshold is the conceptual boundary between two or more vegetative states (Figure 6). Since vegetation cannot rapidly move from a less to more desired state without substantial external inputs to the system, thresholds once crossed are irreversible boundaries. Once a threshold is crossed, the ecological constraints created by interactions among many environmental variables interact to prevent the vegetation from returning to the previous desired state. Prior to crossing a threshold, desired vegetation change usually can occur with a change in management or the application of a single treatment to the vegetation, provided the action mimics a missing or altered natural process. These actions deflect the transition (i.e., its reversibility) away from the threshold and toward a more desired phase of the desired state. After a threshold is crossed, reestablishment of the previous state is very costly and requires the use of several and/or complex, risky, difficult, and expensive management actions.
Figure 6. The top diagram is a conceptual model of a threshold between two states. The bottom diagram shows the concept for a sagebrush-bunchgrass site in Nevada. Courtesy of Brad Shultz.
Thresholds are crossed because the ecological processes that interact to maintain the plant-soil matrix in a desired vegetation state are degraded, removed, or altered enough to change the function of the site. Changes in vegetation composition and structure interact to affect vegetation and ecological function (e.g., biomass production, water-holding capacity, erosion potential, susceptibility to invasive species, etc.). Examples of different functional thresholds being approached and crossed are shown in Figure 7.
Triggers are events that cause transitions to occur. These events may be caused by natural or human activities and are extreme or unusual events that may occur suddenly or over long periods. Sudden events may be fire or a flood. Examples of long-term events include poor grazing management that removes desired plants, the exclusion of fire from a fire-dependent ecosystem, or the increase of woody vegetation to the point it eliminates desired grasses and forbs. Triggers are often sudden, culminating events (e.g., fire) that interact with either the removal of or a change in the intensity, frequency, duration, or scope of long-term ecological processes that form transitions and thresholds. The collective interaction of the change in ecological processes and the triggering event result in the non-reversible transition to a less desired state.
The top diagram is a conceptual model of a threshold between two states. The bottom diagram shows the concept for a sagebrush-bunchgrass site in Nevada.
Figures 7a-7c. Three examples of different thresholds being approached and crossed. Figure 7a represents a functional threshold (incision and erosion) that if crossed results in complete loss of the meadow riparian area. Figure 7b represents a biological threshold being approached and eventually crossed, due to declining bunchgrass composition. The competition process has been altered; thus, a biological threshold has been crossed. Figure 7c represents crossing both a biological (species and life-form composition) and physical (soil loss from erosion). Courtesy of Brad Shultz.