State and Transition Modeling: an Ecological Process Approach

J. Range Manage. 56: 106 -113 March 2003 State and transition modeling: An ecological process approach

TAMZEN K. STRINGHAM, WILLIAM C. KRUEGER, AND PATRICK L. SHAVER

Authors are assistant professor and professor, Department of Rangeland Resources, Oregon State University, Corvallis, Ore. 97331; and rangeland manage - ment specialist, USDA, Natural Resources Conservation Service, Grazing Land Technology Institute, Corvallis, Ore. 97331.

Abstract Resumen

State-and-transition models hold great potential to aid in Los modelos de estados-y- transición presentan un gran understanding rangeland ecosystems’ response to natural and/or potencial para ayudar a entender la respuesta de los ecosis- management-induced disturbances by providing a framework temas de pastizal a los disturbios naturales y/o inducidos por el for organizing current understanding of potential ecosystem manejo al proveer una estructura para organizar el dynamics. Many conceptual state-and-transition models have conocimiento presente de las dinámicas del potencial del ecosis- been developed, however, the ecological interpretation of the tema. Muchos modelos conceptuales de estados-y-transición model’s primary components, states, transitions, and thresholds, han sido desarrollados, sin embargo, la interpretación ecológi- has varied due to a lack of universally accepted definitions. The ca de los componentes principales del modelo: estados, transi- lack of consistency in definitions has led to confusion and criti- ciones y umbrales han variado debido a la carencia de defini- cism indicating the need for further development and refinement ciones universalmente aceptadas. La falta de consistencia en las of the theory and associated models. We present an extensive definiciones ha conducido a confusión y critica indicando la review of current literature and conceptual models and point out necesidad de un mayor desarrollo y refinamiento de la teoría y the inconsistencies in the application of nonequilibrium ecology los modelos asociados. Nosotros presentamos una revisión concepts. The importance of ecosystem stability as defined by the extensiva de la literatura actual y modelos conceptuales y pun- resistance and resilience of plant communities to disturbance is tualizamos las inconsistencias en la aplicación de los conceptos discussed as an important concept relative to state-and-transition de la ecología de no equilibrio. La importancia de la estabilidad modeling. Finally, we propose a set of concise definitions for del ecosistema, definida como la resistencia y resilencia de las state-and-transition model components and we present a concep- comunidades vegetales a los disturbios, se discute como un con- tual model of state/transition/threshold relationships that are cepto importante relativo al modelaje de estados-y- transición. determined by the resilience and resistance of the ecosystems’ Finalmente, proponemos un grupo de definiciones concisas primary ecological processes. This model provides a framework para los componentes del modelo de estados-y-transición y pre- for development of process-based state-and-transition models for sentamos un modelo conceptual de las relaciones de management and research. estados/transiciones/umbrales que están determinadas por la resilensia y resistencia de los principales procesos ecológicos del ecosistema. Este modelo provee un marco para el desarrollo de Key Words: state, transition, threshold, modeling, ecological, modelos de estados-y-transición basados en procesos para process manejo e investigación.

Applied ecology disciplines, such as range management, are necessarily organized around a response model based on theoretical supposition. Thus, the litmus test for an ecological or mecha- 1986, Tausch et al. 1993). After 50 years of applying the quanti- nistic model is its ability to predict the consequences of natural tative climax model of Dyksterhuis (1949) to rangeland manage- disturbances and/or management activities with acceptable preci- ment its predictive capabilities have come under scrutiny. The sion over timescales relevant to management. Traditional theories inability of the model to incorporate multiple pathways of change of plant succession leading to a single climax community have has led some ecologists to abandon the model completely been found to be inadequate for understanding the complex suc- (Wilson 1984, Smith 1988). The recognition of this inadequacy cessional pathways of semi-arid and arid rangeland ecosystems has generated a search for an alternative theory that more correct- considering timescales important for making management adjust- ly reflects the observed dynamics of rangeland ecosystems. As ments (West 1979, Westoby 1980, Anderson 1986, Foran et al. many scientists were questioning the validity of the climax model, Westoby et al. (1989) developed a foundational discus- Authors wish to thank L.E. Eddleman, N.E. West, M. Westoby, W.A. Laycock, sion and conceptual model based on non-equilibrium ecology. and G.L. Peacock for insightful review. Numerous scientists have utilized these concepts as a basis for This article is submitted as Technical Paper No. 11874 Oregon Agricultural Experiment Station, Corvallis, Ore. the development of conceptual models of vegetation dynamics Manuscript accepted 8 Oct. 2002. which incorporate multiple successional pathways, multiple

106 JOURNAL OF RANGE MANAGEMENT 56(2) March 2003 steady states, thresholds of change, and discontinuous and irreversible transitions (Archer 1989, Friedel 1991, Laycock 1991, Fuhlendorf et al. 1996, Stringham 1996, Rietkerk and Van de Koppel 1997, Davenport et al. 1998, Oliva et al. 1998, Petraitis and Latham 1999, Plant et al. 1999, West 1999, West and Young 2000, Stringham et al. 2001). However, the ecological interpretation of Westoby’s model has varied due to a lack of universally accepted definitions of the key concepts. The lack of consistency in definitions has led to confusion and criticism indicating the need for further development and refinement of the theory and associated models (Iglesias and Kothmann 1997). The USDA Natural Resources Conser- vation Service (NRCS) adopted the use of state-and-transition vegetation dynamics in describing rangeland ecological sites. The attempt to use this concept illustrated the inconsistency in the definitions and con- Fig. 1. Broad applications of the state-and transition concepts. Derived from the Society for cepts. The NRCS recognizes the need for Range Management, Task Group on Unity in Concepts and Terminology (1995). The consistency in the application of the con- plane labeled SCT (site conservation threshold) represents a change from 1 ecological site cepts (USDA 1997). For management to to another and may also be considered a threshold between 2 states. The individual boxes utilize the non-equilibrium ecological or ovals represent plant communities or seral stages that exist within 1 site. model the definitions of model objects must be succinctly stated and validated. change the system does not stabilize until As defined, Friedel’s thresholds mirror the transition is complete. Westoby et al.’s (1989) definition of per- Quantitative approaches to ecological sistent or irreversible transitions. However, Background thresholds have been presented by May the use of thresholds in current state-and- (1977), Wissel (1984) and Rietkerk and transition models has not been consistent van de Koppel (1997). Archer (1989) nor clear on whether thresholds exist Westoby et al. (1989) was the first to introduced the qualitative concept of a between all states or only a subset of states. apply the use of state-and-transition termi- transitional threshold. He modeled the Conceptual models, based on these nology to non-equilibrium theory for the expansion of a woodland community into ideas, have incorporated states and transi- purpose of producing a management a grassland domain using a transitional tions but not always thresholds. As a focused model that describes vegetation threshold as the boundary between the result, there have been both a broad inter- dynamics in a non-linear framework as an respective grassland and shrub domains. pretation of states, more or less separated alternative to the linear continuum process Whisenant (1999) proposed a model of by thresholds, and a narrow interpretation incorporated in the quantitative climax degradation based on the stepwise degra- of states that approximate seral stages or model. The authors defined a “state” as an dation concept of Milton et al. (1994). phases of vegetation development. alternative, persistent vegetation commu- Whisenants model is similar to Archer’s, Broadly applied, states are climate/soil/- nity that is not simply reversible in the lin- which incorporates 2 transition thresholds, vegetation domains that encompass a large ear successional framework. We interpret the first being controlled by biotic interac- amount of variation in species composi- Westoby’s transitions as trajectories tions and the second by abiotic limitations. tion. Specifically a grassland state would between states with the characteristic of The concept of a transitional threshold as include many seral stages of the overall the transition being either transient or per- used by both Archer and Whisenant is sim- grassland community. These seral stages sisting. Transitions between states are ilar to the persistent transition as the suc- are within the amplitude of natural vari- often triggered by multiple disturbances cessional processes shift from grass con- ability characteristic of the state and repre- including natural events (e.g., climatic trolled to shrub controlled, however, in sent responses to disturbances that do not events or fire) and/or management actions Whisenant’s (1999) model the focus is on force a threshold breach. Westoby et al. (grazing, farming, burning, etc.). ecological processes not vegetative groups. (1989), Archer (1989), and Archer and Transitions may occur quickly, as in the Friedel (1991) focused on the concept of Smeins (1991) provided examples of this case of catastrophic events like fire or thresholds of environmental change broad definition of state where domination flood or slowly over an extended period of between domains of relative stability. She of successional processes determine the time as in the case of a gradual shift in defined a threshold as a boundary in space boundary of the state (e.g. grass controlled weather patterns or repeated stresses like and time between 2 domains or states, succession versus shrub controlled succes- frequent fire. Regardless of the rate of which is not reversible on a practical time sion). The Society for Range Manage- scale without substantial inputs of energy. ment, Task Group on Unity in Concepts

JOURNAL OF RANGE MANAGEMENT56(2) March 2003 107 and Terminology (1995) developed a graphical depiction of the broad applica- Late Seral tion of states with multiple vegetative Sagebrush Steppe stages diagrammed within one state (Fig. 1). Milton et al. (1994) and Whisenant (1999) de-emphasized the species component of the ecosystem within their models, Depauperate Late focusing instead on the functional integrity Native Perennial Seral Sagebrush and self-repair thresholds of the site for Grasses dominate determining state boundaries. In the broad Steppe definition of state the natural variability Threshold characteristic of plant communities within a site is the result of, and contributes to, Introduced the current functional integrity of the site’s Brush and grass pastures primary ecological processes (hydrology, introduced annuals nutrient cycling, and energy capture). The narrower interpretation of state allows for far less variation in plant community composition. States are typically Introduced grass depicted as seral stages or phases of vege- Cheatgrass and/or pastures with tation development. In the narrow applica- medusahead shrub reinvasion tion of the model a state change does not necessarily represent a movement across a threshold as envisioned by Friedel (1991). Figure 2 represents the narrow interpretation of states as adapted from West (1999). Boxes represent states and arrows indicate ? the transitions between states. Note that many of the transitions are reversible, however, the threshold indicates a persis- Fig. 2. Specific, or narrow, application of states with each state (box) representing 1 phase or seral stage of vegetation development. Transitions between states are indicated by tent transition. Other examples of specific arrows and the dashed line represents a threshold. The dashed transitional line signifies or narrow applications of states are pre- the requirement of substantial energy input to move the state back across the threshold. sented by Weixelman et al. (1997), Oliva Modified from West (1999) and West and Young (2000). et al. (1998), Allen-Diaz and Bartolome (1998), West (1999), and West and Young recover after it has been disturbed. Thus, dynamics. A state change, on the other (2000). The specific approach to state-and- fully functioning ecosystems are both hand, requires a shift across a boundary or transition modeling may be the reason for resistant to change and resilient or able to threshold, defined by a change in the statements that such models are structural- recover without external energy inputs integrity of the site’s primary ecological ly similar to traditional linear climax-seral thereby maintaining stability while allow- processes, resulting in a different potential stage models. The significant difference ing for fluctuating combinations of plant set of plant communities. being the description of communities as species over time. States, by definition are discrete entities as opposed to the continu- relatively stable (Westoby et al. 1989), um concept of the quantitative climax therefore it follows that a state change is Rangeland Ecological Processes model (Iglesias and Kothmann 1997). only possible when a threshold is crossed. Accepting this concept points out the con- Ecological processes functioning within fusion that is apparent in the current a normal range of variation will support a Ecological Resistance and attempts to produce state-and-transition suite of specific plant communities. The Resilience models. The specific or narrow approach important primary processes are (1) has produced models, which depict state hydrology (the capture, storage, and redis- The concept of stability as defined by changes occurring without having crossed tribution of precipitation); (2) energy cap- the resistance and resilience of plant com- a threshold. Often such changes are dia- ture (conversion of sunlight to plant and munities have been discussed in the litera- grammed as reversible and perhaps occur animal matter); and (3) nutrient cycling ture for sometime and offer important without the input of management (the cycle of nutrients through the physical insights for state-and-transition models resources (Fig. 2). Rather than consider and biotic components of the environment (Margalef 1969, Verhoff and Smith 1971, these vegetation dynamics as state changes (Pellant et al. 2000, Whisenant 1999). Holling 1973, May 1977, Noy-Meir and it is more appropriate to consider them as Pellant et al. (2000) defines the function- Walker 1986). Resistance is defined as the phase shifts or plant community dynamics ing of an ecosystem by “the degree to ability of the system to remain the same within a state. Therefore, within a state which the integrity of the soil, vegetation, while external conditions change whereas there exists the potential for a large varia- water, and air, as well as the ecological resilience is the ability of the system to tion in species composition, which is processes of the rangeland ecosystem, are merely a reflection of plant community

108 JOURNAL OF RANGE MANAGEMENT 56(2) March 2003 balanced and sustained”. Integrity is Temporal Scale • Vegetation Structure: a component defined as the “maintenance of the func- The definition of threshold as presented resulting from above ground communi- tional attributes characteristic of a locale, by Friedel (1991) indicates that once a ties of living organisms, whose vital including normal variability” (Pellant et threshold has been breached return to the attributes (Noble and Slatyer 1980) al. 2000). Degradation of an ecosystem previous state is precluded within a time competitively capture and utilize the occurs when the integrity of the system is frame relevant to management, without system’s available energy, water, nutri- damaged or lost. Maintenance of a func- substantial inputs of energy. Ecological ents, and space. tional site or repair of a damaged site management models should focus on the The interaction between the structural requires management focused on soil sta- time required to repair damaged ecological attributes of soil and the vegetative commu- bility, nutrient cycling, and the capture, processes not on a time scale predicated nities, through the processes of energy cap- storage and safe release of precipitation. by management. Careful consideration of ture, hydrology and nutrient cycling defines Vegetation goals should be based on the the threshold concept negates the need for the resilience and resistance of the state. concept of vegetation as a tool for main- including management timescales in the taining or repairing damaged ecological definition of ecological thresholds as these Resilience and Resistance processes rather than predefined species thresholds represent a permanent change The stability of a state is defined above groups. Monitoring of species groups may in the function of the state. Thus, restating in terms of resilience and resistance. be a mechanism for evaluating or detecting the threshold definition, independent of Resilience and resistance are inherent change in the site’s ecological processes. management timescales, results in the con- p r operties of an ecosystem that are deter- clusion that once a threshold has been vio- mined by the physical components of the lated return to the prior state is precluded system and the functional capacity of the Clarification of the Concepts without substantial inputs of energy. and Definitions associated ecological processes. Resilience Therefore, under the current climatic con- focuses on how far a system can be dis- ditions and without substantial inputs of placed from equilibrium before return to Spatial Scale energy, state changes are permanent. The equilibrium is precluded. The emphasis is Ecosystems are difficult to define or temporal scale is defined by the perma- placed on the persistence of relationships delimit in space and time. Hierarchy theo- nence of the current climate regime. as they affect the systems ability to adapt ry, as applied to ecological systems, sug- to change (Walker et al. 1981), therefore, gests several levels of organization exist, State resilience relates to the functioning of the i.e., organisms, populations, communities, A state is a recognizable, resistant and system’s ecological processes. Resistance ecosystems, landscapes (Archer and resilient complex of 2 components, the indicates the ability of a system to remain Smeins 1991). Each level of organization soil base and the vegetation structure. The at or near its equilibrium condition by encompasses one or more of the primary vegetation and soil components are neces- maintaining control of its ecological ecological processes that are operating at sarily connected through integrated eco- processes. Thus, the strength of this con- specific spatial and temporal scales. logical processes that interact to produce a trol determines a system’s inherent resis- Although landscape scale management sustained equilibrium that is expressed by tance to change. Consequently, under an may be the goal, our current understanding a specific suite of vegetative communities. existing climate, stability of a state is a of organization function declines with function of the combination of its inherent increasing spatial and temporal scale. resilience and resistance. The ecological site concept has long Soil Base and Vegetation Structure been utilized as an organization level that The base of any rangeland ecosystem is Thresholds and Transitions provides an appropriate spatial scale for the soil resource that has developed Thresholds are points in space and time inventory, evaluation, and management of through time from a specific parent materi- at which one or more of the primary eco- rangelands (USDA 1997). Organisms, al, climate, landscape position, and interac- logical processes responsible for maintain- populations, and communities exist within tion with soil and terrestrial biota. These ing the sustained equilibrium of the state this spatial scale and interact with one factors are the primary determinants of the degrades beyond the point of self-repair. another through the flow of water and ecological site’s capability. The integrity These processes must be actively restored energy, and the cycling of nutrients. An of the soil resource, as reflected by site before the return to the previous state is ecological site has evolved a kind of char- hydrology and nutrient cycling, is directly possible. In the absence of active restora- acteristic plant community such as cool connected to the composition and energy tion a new state, which supports a differ- season shrub-grass or warm season grass- capture process of the above-ground vege- ent suite of plant communities and a new land. Within an ecological site numerous tative component. The interaction between threshold, is formed expressions of the various developmental the soil resource and the associated vegeta- • Thresholds: boundary in space and time stages of the characteristic plant communi- tive community determines the functional between any and all states, or along irre- ty can occur. The concept and definition status of the state’s ecological processes. versible transitions, such that one or of an ecological site fits the large-scale • Soil Base: a component that results from more of the primary ecological process- interpretation of the state-and-transition the interaction of climate, abiotic soil es has been irreversibly changed and model. We define the ecological site as the characteristics, soil biota and topogra- must be actively restored before return minimum scale for definition of a state. phy that determines the hydrologic characteristic and biotic potential of the sys- to a previous state is possible. tem. Transitions are trajectories of change that are precipitated by natural events

JOURNAL OF RANGE MANAGEMENT56(2) March 2003 109 and/or management actions which degrade the narrow application of the non-equilib- Milton et al. 1994, and Whisenant 1999). the integrity of 1 or more of the states pri- rium approach to states and transitions Plant community phase changes within mary ecological processes. Transitions are (Fig. 5). States are diagrammed as the states, in addition to transitions of change, often composed of 2 separate properties large boxes and are bordered by thresh- thresholds and multiple stable states are that are defined by the state threshold. The olds. Thresholds are the boundaries of any illustrated in Figure 5. The management first property is reversibility and it occurs and all states, but may also occur during and natural mechanisms responsible for within the state. The second property is the transition between states. For a state community phase shifts and transition ini- irreversibility and it occurs once a thresh- change to occur a threshold must be tiation must be defined in terms of ecolog- old has been breached. Transitions are breached. The small boxes within the state ical processes and included in the model vectors of system change that will lead to are referred to as plant community phases description. For example, prolonged a new state without removal of the stres- or seral stages and are joined by communi- drought or overgrazing leads to a reduc- sor(s). The primary difference between the ty pathways that flow in both directions. tion in the perennial herbaceous understo- reversible and irreversible property of a Transitions are reserved for a trajectory of ry. The decrease in perennial understory transition is defined by the systems’ abili- change with the dashed line inside the leads to a decrease in total energy capture ty or inability to repair itself. state indicating the portion of the transi- and nutrient cycling. In addition, the plant • Transition: a trajectory of system change tion that is reversible with minimal input community’s ability to protect the soil away from the current stable state that is from management. Figure 4 illustrates the from raindrop impact and potential soil triggered by natural events, management process of a state change. Once the thresh- erosion declines. The mechanism (or actions, or both. old is crossed the state has lost control of mechanisms) of disturbance have led to a – Reversible Property of the Transition: its primary ecological processes, is no change in the 3 primary ecological trajectory of change that occurs within a longer able to self-repair and will transi- processes and a phase shift as diagrammed state and indicates the system is moving tion to a new equilibrium with a different by community phase pathway P1 (Fig. 5). toward a threshold. Reversal requires ecological capability. The entire trajectory In the case of prolonged drought return to elimination of the stress or stresses from a vegetation phase in State 1, across the late seral sagebrush steppe phase responsible for triggering the transition. the threshold to the formation of State 2 is would gradually occur with a return to a – Irreversible Property of the Transition: considered a transition and represents a normal or above normal precipitation peri- trajectory of change that occurs after a degradation of ecological capability. The od (P2). Increased available moisture threshold has been breached. The sys- portion of the transition contained within leads to an increase in biomass of the tem can no longer self-repair even with the boundary of State 1 is reversible with herbaceous understory that translates into removal of the stressor(s). The system removal of the stressor(s), however, once an increase in energy capture, nutrient will not come to rest until a new equilib- the trajectory crosses the threshold it is not cycling and an improvement in soil pro- rium (i.e., new state) is established that reversible without active restoration tection and site hydrology. The degrada- supports a different suite of plant com- including substantial energy input. tion mechanism of overgrazing would munities. Additional thresholds may occur while the need to be addressed through grazing system is in transition, changing the direc- management with the goal of improving tion of the trajectory away from State 2 the function level of the primary ecologi- Model Structure towards State 3 (Fig. 4). State-and-transi- cal processes. Continued overgrazing tion modeling efforts indicate the first would further decrease the vigor of the The conceptual model, illustrating the threshold is forced by a change in the biot- native herbaceous understory and further above definitions, is represented in ic component of the system whereas addi- impact the community’s ability to main- Figures 3 and 4. The model accommodates tional thresholds would involve changes in tain control of the primary ecological both the quantitative climax approach and the soil resource (Westoby et al. 1989, processes. As the vigor of the native

Objects within a State

Reversible transition

Threshold of the state

Plant community phases or seral stages within a state

Community pathways

Fig. 3. Conceptual model depicting the objects of 1 state. Note the linear response, retrogression-succession model may be modeled within the state (i.e., a to b to c and vice-versa).

110 JOURNAL OF RANGE MANAGEMENT 56(2) March 2003 Threshold

State 1

State 2 Threshold

Reversible transition

Community Pathway

Irreversible transition

Community Phases or State 3 seral stages within a state

Fig. 4. Conceptual state-transition model incorporating the concepts of community pathways between plant community phases within states, reversible transitions, multiple thresholds, irreversible transitions, multiple pathways of change, and multiple steady states. herbaceous community declines, the site is reduction in the amount of organic materi- Conclusions opened up for invasion by annual species. al being added to the soil and an increase The transition from State 1 towards State 2 in the potential for damage to soil surface Definitions and model concepts as dis- has begun and will continue without the structure from raindrop impact. Return to cussed in this paper are being adopted by removal of the stress from improper graz- State 1 would be impossible without the the USDA Natural Resources Conserva- ing (T1a). At the point in time where use of intensive management inputs. The tion Service as the standard for describing annuals dominate the herbaceous under- practicality of this level of management vegetation dynamics in rangeland ecologi- story and fire frequency intensifies, the would preclude its use. State 3 may be the cal site descriptions. State-and-transition state has crossed a threshold and is transi- practical state of choice. models hold great potential to aid in tioning to a new state (T1b). During this Although many scientists have recog- understanding rangeland ecosystems’ transition phase the plant community may nized the short-comings of the quantitative response to natural and/or management- still retain a minor component of sage- climax model developed by Dyksterhuis induced disturbances by providing a brush; however, this is not representative (1949) there are ecosystems, generally of framework for organizing understanding of a stable state and with increased fire more mesic climates, where the linear of potential ecosystem dynamics. Many frequency the brush will be eliminated and model is appropriate. It is important to real- state-and-transition model applications are the new equilibrium state formed. The ize that any modeling approach is a best-fit available in the literature, although the new state is defined as a Bromus tectorum solution, not a perfect-fit solution. scale of interpretation of the concepts has (cheatgrass) and/or Taeniatherum asperum Therefore, the retrogression-succession varied. We have attempted to review and (medusahead) dominated community with continuum can be modeled within the states clarify a large amount of information into a fire frequency interval of 2 to 3 years. to depict the situation where plant commu- a proposed conceptual model of state/tran- Energy capture has declined and the time nity phases do respond linearly. However, sition/threshold relationships that are period for energy capture has been it is also possible for linear response mech- determined by the resilience and resistance reduced. Nutrient cycling in both the verti- anisms to be pushed past an ecological of the systems’ primary ecological cal and horizontal plane has decreased threshold, resulting in a state change. processes. Most of the components pre- with the shift to a shallow rooted, primari- sented are not new; however, the proposed ly monoculture community. The hydrolo- model attempts to clarify the definitions gy of the site will be impacted through a

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