State and Transition Modeling Workshop

Traditional range site descriptions described a single persistent vegetation state referred to as the climax plant community. Range condition was determined by comparing the species composition of present vegetation to the climax vegetation for the site. Ecological site descriptions will utilize the state and transition model to describe the dynamics of rangeland vegetation associated with each site. State and transition model offers a method to organize and communicate complex vegetation management information. Lack of common understanding as to what constitutes a state, transition and threshold has resulted in a tremendous variability in state and transition models that are being developed. A couple of examples of state and transition models that have been developed will be reviewed. One model was developed utilizing a broad approach in defining the different states and the other model was developed with a more detailed approach describing states that were important from a management point of view.

As state and transition models are being developed, need to remember for whom we are developing these models and what information we want the model to provide. We need to agree on common definition for state, transition and threshold. We also need to develop a method to describe those plant communities that are important from a management point of view.

State and transition models provide a repository for all of the information that we know about the ecological dynamics of the site. They are excellent communication tools to discuss the dynamics of the site with a land manager. Once developed, the state and transition model will assist land managers in making timely, well informed management decisions.

Concepts of non-equilibrium ecology have been apparent in the literature for a long period of time. During much of the past century however linear models, focused on systems at or near equilibrium, have driven succession theory. Some of these models have dealt with multiple pathways while others have depicted only one pathway as a linear continuum. The quantitative climax approach developed in the late 1940’s is one such model and has been the workhorse of rangeland ecology and management since its development. This model was developed in the Great Plains and has been applied to rangelands across the U.S. In the Great Plains, mountain grasslands, and various other systems, the quantitative climax model often works very well. However, many instances have been identified where the quantitative climax approach has not adequately described observed vegetation dynamics. As a result, many ecologists have looked to non-equilibrium ecological concepts and developed state and transition models in an effort to more adequately describe vegetation dynamics.

Although the terms state, transition and threshold are commonly used in rangeland ecology, they have been defined by different individuals at various points in time. As a result, a universal set of definitions linking all of the terms together into a framework for state and transition models is lacking in the literature. A review of the literature reveals at least two different applications of the term ‘state’. One is a very broad interpretation where states encompass multiple expressions of plant communities, or seral stages of plant communities, among which reversible relationships are maintained. An alternative interpretation is of much more specific states that approximate seral stages or condition classes. Thresholds have been interpreted at least two ways, depending primarily on how authors have interpreted states. With specific states, thresholds are often placed between some, but not all states. In many of the broad applications of state, a threshold must be crossed in order for a state change to occur.

A variety of state and transition model applications present in the literature were analyzed paying particular attention to the advantages each approach offered to the management of rangeland systems. These advantages were then considered and used to aid in the formulation of draft definitions and a proposed conceptual model linking the concepts of states, transitions and thresholds together. States are defined in terms of abiotic and biotic factors that form the foundation and framework of rangeland systems. Transitions and thresholds are defined relative to states and the proposed conceptual model provides visual representation of how the various components of non-equilibrium ecology may interact. It is recognized and appreciated that the quantitative climax model is appropriate in many systems and therefore, the proposed conceptual model was designed to build upon, rather than to replace it. At the present time, the authors of this effort view it as the next step in the continued development of non-equilibrium ecological models for rangeland applications, and not as a finished product.

Dr. W. C. Krueger and Dr. Tamzen Stringham, Department of Rangeland Resources
Oregon State University, Corvallis, Oregon

Pat Shaver, USDA-Natural Resources Conservation Service
Grazing Lands Technology Institute, Corvallis, Oregon

• A summary of the presentation made at the workshop is below.  Topics included an example of a modified soil; existing soil survey methods for identifying modified soils; new soil classification and database activities that address changing soil properties; and suggestions for including soil information in state and transition models.
• The pertinent question is “what kinds of soil changes cause an ecological site to become a new site?”  Soil properties change both in response to management and natural events or cycles. Those changes that effect the ability of the soil (site) to support the original suite of states (plant communities) should be considered when assigning an ecological site. In the example of the black grama grassland shift to mesquite duneland, the soil modification occurs as both erosion and deposition.  In some landscapes, the soil in the shrub inter-space is eroded and the dune soil is wind deposited soil material.  In other landscapes, the soil in the inter-space has an intact soil profile with an A horizon that extends under the dune soil as a buried soil. In both cases, the soils are no longer the same as the original soil under the black grama grassland and can no longer support the original plant community. Examples of soil changes also occur in plant communities other than black grama-mesquite.
• Current methods of identifying modified soils in soil survey include phases of soil series, unnamed inclusions, new soil series and new soil classification.  The genetic link between the original soil and the modified soil is only maintained through the phase naming method. (eg. Alpha soil and Alpha soil, eroded).
• The International Committee for Anthropogenic Soils has been charged with developing a provisional classification scheme for human modified soils. When proposed, criteria for classifying anthropogenic soils should be evaluated for their impact on rangeland soils and procedures for assigning ecological sites.
• The capacity of soil to function in rangeland systems depends on both 1) inherent (static) soil features and 2) dynamic soil properties that are susceptible to change in response to management. Some “essentially” static properties include texture, mineralogy, horizon sequence, soil depth, slope, and aspect. Examples of dynamic properties include soil moisture and temperature, organic matter, nutrients, topsoil depth, aggregation, mineral crusts, salinity, soil microbes and soil fauna.
• The NRCS Soil Survey Center and Soil Quality Institute are currently doing initial evaluation work for the development of a “Use-dependent Soil Property Database.”  The database will include near-surface dynamic soil properties and their measured or estimated values under various land uses and management systems. The various states in a site could be included.
• Some suggestions for including information within state and transition models to reflect changes in soil properties are:  1) a representative soil series or phase of soil series for each state, 2) information that describes the genetic linkage between the original soil and the modified soil of a new site, and 3) dynamic soil properties that define thresholds or early warning indicators of vegetation changes. The challenges will be to determine the dynamic soil information needed to predict vegetation changes and the functioning level of rangelands, and then to gather the soil property data.  Possible benefits of including dynamic soil information in STM’s include: 1) documentation of soil properties for various states, 2) reference “values” for rangeland inventories and planning, and 3) aides for the prediction of vegetation and soil stability changes.

The Ecological Site Description database is an internet based program for entering, storing, and retrieving ecological site description information. Ecological site descriptions for rangeland and forestland can be entered into the database. Once a site description has been entered it is immediately available for viewing by anyone. Entering or editing data requires a login and password. These will be distributed to appropriate specialists in each state. The remainder of the presentation will review the different sections of the ecological site description, data elements associated with each section, and how the data can be entered into the site. With the increased use of the internet there is a greater demand to have information such as the Ecological Site Description available and easily accessible.