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Vegetation change and succession on rangelands was historically measured or modeled with the Range Condition and Range Succession Models. These two models are structurally similar but differ in their terminology. Both models viewed plant succession, both before and after disturbance, as a linear process (Figure 2). Following a disturbance, early seral species typically inhabit an ecological site. With time, the plant community progresses through a series of changes toward a potential natural community, or climax community. All plant communities are subject to many stresses that influence the composition and structure of the vegetation. Under the Range Succession and Condition models, little or no stress resulted in vegetation change toward the potential natural community (PNC). Increasing stress above some community specific level would prevent succession toward the PNC and could return the community to some prior composition, lower on the successional ladder.

To describe the stage of succession at any given time, both models compared the current plant composition to the PNC or climax community described. The percent of overlap between the current community and the PNC determined the stage of plant succession and condition of the site. The four categories used were early seral or poor condition; mid-seral or fair condition; late seral or good condition; and the potential natural community or excellent condition. Seral stage descriptors formed the basis of the Range Succession model and condition descriptors the Range Condition model.

Table 1. Vegetation composition by annual biomass production for three Wyoming sagebrush ecological sites (Loamy 8-10 inch) and their range condition and succession scores.

The data in Table 1 show that a potential plant community can have several very different seral stages with the same classification for range condition class or seral stage. Site 1 is a shrub-dominated site with relatively few perennial grasses. Site 2 is predominately a bunchgrass site with few shrubs. Species and particularly life-forms often respond to fire and other disturbances quite differently. The response of both seral stages to the same management action or the same catastrophic disturbance probably will not be the same, despite both having the same classifications for range condition and successional status. One of the serious flaws of the range succession and condition class models is that the same descriptive classification — for example, good/late — can represent two different phases of plant succession and therefore two very different response potentials to management and disturbance. If the descriptive classifier is the only data product retained by the land manager, the manager will not know the response potential of the ecological site following disturbance.

Figures 3a-3c are pictures of the same sagebrush ecological site in three different community phases. Figure 3a is the potential natural community. The community phases in Figures 3b and 3c are both late seral/good condition communities but with very different plant communities. The shrub-dominated site (Figure 3b) will have substantial bare ground after a disturbance and would be very susceptible to invasion by non-native annual grasses or other weeds. The other two sites (Figures 3a and 3c) have sufficient perennial bunchgrasses to remain bunchgrass sites after a disturbance and resist the invasion of annual grasses. The need to better understand the management opportunities and hazards associated with the different community phases for any ecological site’s reference community led to the development of state and transition models.

Written by Brad Schultz, Extension Educator, Winnemucca, Nevada

State and transition models were developed to help land managers make better decisions when managing vegetation for a suite of potential land uses — for example, livestock forage and wildlife habitat — and/or ecosystem services such as erosion control, water infiltration, and wildfire risk reduction. They provide a framework (Figure 8) for describing, understanding, predicting, and controlling ecosystem dynamics; the integrated goal of science and management. State and transition models help understand complex systems, with multiple interactive drivers for changes — competition, precipitation, and response to management actions — that operate at different intensities, frequencies, and durations across different scales of time and space.

Figure 9 is a state and transition model for a Loamy 8-10 inch ecological site from central Nevada. The reference community (PNC) is a Wyoming sagebrush-bunchgrass community. Indian ricegrass (Achnatherum hymenoides) and Thurber’s needlegrass (Achnatherum thurberianum) are the predominant bunchgrasses, representing 30% to 50% of the total annual biomass production. Bottlebrush squirreltail (Elymus elymoides) and Sandberg bluegrass (Poa sandbergii) are minor but constant perennial bunchgrasses in the community. Cheatgrass (Bromus tectorum), an invasive annual grass can establish on sites that are disturbed or mismanaged. Wyoming big sagebrush (Artemisia tridentata wyomingensis, a non-sprouting shrub, is the most common shrub, and at a mid- to late-seral stage has an annual biomass production of 25% to 35% of the total community production.

Building on what has been learned, the green box in Figure 9 is a resilient sagebrush-bunchgrass state, with seven community phases within the resilient state. Depending on the specific management goals for the site there could be more or fewer phases. The arrows connecting the green and yellow boxes are pathways between community vegetative phases within the resilient state of the Wyoming Big Sagebrush Ecological Site. The green boxes represent very resilient vegetative phases that present little or no ecological risk to the integrity of the state. That is, any community phase they change toward will not result in transition to a different vegetative state. If a disturbance or management action removes the desired sagebrush, the perennial bunchgrasses are dense enough to reoccupy the site and exclude the annual invasive grasses, the sprouting shrubs, or few juniper trees.

The framework needed to develop a state and transition model for vegetation management and change.

Figure 8. The framework needed to develop a state and transition model for vegetation management and change. Courtesy of Brad Shultz

Management actions and/or normal successional pathways across decades could result in phase changes from communities described in the green boxes to communities in the yellow boxes. These may or may not be desired community changes, depending on management goals. Regardless, the changes represent increased risk to the integrity or resilience of the ecological site. These added biological risks are from increased sprouting shrubs, sagebrush cover above 15% to 20%, associated declines in the density of bunchgrasses, and the establishment of juniper trees. An increase in the woody species above some site-specific level is an ecological risk because large woody plants out-compete the smaller bunchgrasses for soil moisture, and the bunchgrasses eventually decline. At some point in the successional process, there will be insufficient bunchgrasses to completely occupy the site immediately after a catastrophic disturbance, and the site will likely transition to a cheatgrass community.

The yellow boxes represent community phases that may meet management goals but are communities with an increased risk of transition across a threshold to an undesired alternative state. The yellow lines that approach the threshold represent the reversible portion of the transition process. A change in management at this stage of plant succession may return the community to a phase with more resilience and less ecological risk. Once the transition crosses the threshold, none of the alternative states can move back into a desired phase of the ecological site without extensive and expensive inputs.

The black arrows between the community phases are pathways between each phase. The specific management actions and/or ecological processes that interact to change the vegetation from one phase to another can be documented and used to direct management toward the desired community phase.

Figure 9. State and transition model for a Loamy 8-10 ecological site in Central Nevada. Courtesy of Brad Shultz

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.

Plant communities on rangelands typically are composed of a mixture of grasses, forbs, and shrubs. Some rangelands, such as many ponderosa pine forests or pinyon-juniper woodlands, have an overstory of trees and an understory of grasses, forbs, and/or shrubs. All plant communities, regardless of their location, change across time — a process called plant succession. The changes may be in species composition, life-forms (grasses, forbs, shrubs, trees) and life cycles (annual, biennial, perennial); the abundance of each species; the relative proportion of the life-forms present; the size, density, shape, or arrangement of plants (structure); or combinations of these features.

Vegetation change involves both the direction and the rate of change. A plant community can move toward more desired or productive communities or toward less desired communities. The former can be considered as management opportunities and the latter as hazards that should be avoided. Understanding plant succession is important because the composition of plants within plant communities has three important influences:

1. How landscapes function — for example, water cycle, nutrient cycle, soil formation,
2. The type and amount of products or resources and services society can develop and produce, and
3. Identification of management opportunities and hazards.

Figure 1 (below) shows some of the ecological changes that result from plant succession across several to many decades on a sagebrush-bunchgrass rangeland.

Periodic fire is the typical disturbance on shrub-grass rangelands that maintains the balance between the perennial grasses and forbs (perennial herbaceous) and the woody shrubs and trees. This is particularly true for shrubs that do not sprout after a disturbance. Fire removes the non-sprouting shrubs and permits an abundance of other, more fire-adapted species. The lack of fire eventually results in shrub-dominated communities and a corresponding decline of the other species. The shift from a predominately perennial herbaceous life-form to woody species changes both structural and functional components of the ecosystem. The key management question is: Can the change to shrub dominance be reversed, completely or partially, to meet management goals and objectives for desired resources?

Among the important concepts in plant succession on rangelands are the ecological site, community resilience, and community resistance. An ecological site is a unique, identifiable, and repeatable patch of vegetation on a landscape. It has specific (homogeneous) biological, physical, and chemical characteristics and the potential to produce a distinctive kind and amount of vegetation. An ecological site is a product of all of the environmental factors that influence the development of soils and vegetation, including disturbance regimes. Vegetation on an ecological site is not static and can be one of several seral communities, with the same species present but in different amounts. If the biological, physical, or chemical conditions that create an ecological site change, the site can degrade to a different bio-physical state. Resilience refers to the ability of a plant community to return to prior composition and structure after a disturbance. Resistant communities, or one of their seral stages, occupy the site after it has been disturbed. Resistance refers to a plant community’s ability to avoid being changed from its current state.

Written by Brad Schultz, Extension Educator, Winnemucca, Nevada

There are no clear boundaries to definitively classify rangelands. However, grouping rangeland plant species into vegetation types provides a framework for managers to assess the ecological status and trend of plant communities. Vegetation can be classified on a hierarchical scale, the broadest of which is based on climatic, physiographic, and edaphic factors across large geographical regions such as grasslands, deserts, and shrublands. Vegetation types can further be divided into groups based on major plant species. Examples include the sagebrush steppe, salt-desert shrub, juniper woodland, intermountain bunchgrass, shortgrass prairie, and tallgrass prairie plant associations. Some types of land classification systems commonly used by managers to guide research and policies and as a tool for communication are: Ecological Sites; Rangeland Vegetation Types; Rangeland Habitat Types; Major Land Resource Areas. 

Ecological Sites

An ecological site is a distinct kind of rangeland that has a certain potential to produce a distinct plant community. It is used to describe units of land that require a unique management strategy based on kind of soils, climate and topography found on the site. Currently, the most widely accepted classification system for rangelands is the Ecological Site.

What is an ecological site? An ecological site is a unique, identifiable, and repeatable patch of vegetation and soil on a landscape. Each ecological site is the product of the environmental factors that influence the development of the soil and vegetation, including disturbance regimes. An ecological site has specific homogeneous biological, physical, and chemical characteristics and has the potential to produce a distinct mix of plant species with similar amounts of annual biomass of vegetation. The basic premise is that a specific soil type based on depth, texture, horizonation, water-holding capacity, pH, salinity, and other factors is inhabited by specific plant species. The soil, therefore, is the foundation of an ecological site. If the foundation undergoes a dramatic change, the structure above will have a corresponding change.

On rangelands, ecological sites form the basic classification unit for categorizing different plant communities and their associated soils. A key concept for an ecological site is the potential natural community (PNC). The PNC is a complex concept that includes the typical disturbance regime that affects the ecological site. Many rangeland plant communities endure a periodic stand-replacing disturbance — for example, fire — that does not dramatically alter the soil but decreases or removes some plant species and lets others immediately increase. The first plant community that establishes after a stand-replacing disturbance usually undergoes vegetation change over time. If the evolved disturbance regime has an average of 50 years between events the community that typically is present after 40 to 50 years of succession is considered the potential natural community. Vegetation on an ecological site, therefore, is not static, and several community phases of the PNC may develop and eventually succeed one another. Rangeland scientists have developed a conceptual approach called state and transition models to help describe changes in community composition, vegetation structure, and ecological function with regard to management actions and environmental condition.

The biological, physical, or chemical conditions that create an ecological site can also change. When their change is sufficient to add, remove, or change the intensity, frequency, or duration of the ecological processes — for example, plant competition, hydrology, disturbance regime — that maintain an ecological site, the site can degrade to a different ecological site.

What is an ecological site description? Each ecological site has defined components that are described by range management specialists and soil scientists. The complete assemblage of these components into one document forms an ecological site description (ESD). Ecological site descriptions are intended to be clear descriptions of the features that characterize the site, making it unique and different from other ecological sites.

The format for ecological site descriptions changes with time because research and management regularly create new knowledge about the environmental variables and ecological processes that affect the formation of distinct soils and their associated PNC. Important ecological and environmental components of updated ESDs are: 1) the major land resource area (MLRA) in which the ecological site occurs, 2) physiographic features, 3) climatic features, 4) influencing water features, 5) representative soil features, and 6) plant communities. Except for the MLRA, the ESD will provide substantial detail for each parameter. The intent of an ESD is to provide landowners and managers with the environmental and ecological information they need to develop management plans, management goals, and management actions for the rangelands they manage. The content of an ESD provides the end users much of the information they need to understand the potential productive capability of their rangeland, the constraints that limit management options, and potential hazards that may occur.

ESDs are developed and housed by the USDA Natural Resources Conservation Service. Recently updated versions are often available on the Internet. Older versions, especially those still in previous formats, usually are not available electronically but can be obtained from state and field level offices of the Natural Resources Conservation Service. More detail about ESDs can be found in Chapter 3 of the NRCS National Range and Pasture Handbook.

How are ecological site descriptions developed? Ecological site descriptions are used to make decisions about how to manage rangelands. Data and information about ecological sites are combined with other resource information to improve management decisions. One of the first steps is to identify and map the soils and ecological sites on a heterogeneous landscape. Once the soils and ecological sites are mapped, managers can assess the vegetative composition and annual production of the seral communities relative to the potential plant communities that could occupy the site.

How are ecological site descriptions used? Ecological sites are distinct units of the soil-vegetation complex, but many management questions and issues occur across landscapes occupied by several ecological sites. Interrelationships among ecological sites can be analyzed at different spatial scales and the results applied at the scale appropriate for the management questions and issues being addressed. Boltz and Peacock (2002) describe six uses of ecological site descriptions: 1) describing the interactions among soils, vegetation, and land management; 2) a foundation to assess the condition of current resources and monitor changes; 3) a framework to assess management opportunities and predict the outcome of management decisions; 4) a framework for identification of knowledge gaps in vegetation dynamics; 5) a common framework for communication of resource information among disciplines, agencies, and organizations; and 6) a framework for transferring experience and knowledge.

Written by Brad Schultz, Extension Educator, Winnemucca, Nevada

The vegetation on rangeland is always changing toward one of several or more plant communities. Understanding why vegetation changes and how to manipulate that change is critical for rangeland managers to ensure that rangelands continue to provide the goods and services needed by society.

Early research about changes in plant communities across time, or plant succession, on rangelands resulted in the belief that vegetation change was linear. That is, community A changed into community B, which ultimately changed into community C, and so forth. After many decades, rangeland scientists and managers realized that vegetation change is not always linear. Changes in management did not always result in the return to the previous, more desired plant community, and normal disturbances often resulted in new plant communities that were stable and responded very little to management actions. In essence, the response of the plant communities to management and disturbance was non-linear.

Land managers need to understand which plant communities can ultimately occupy a site, given the current plant composition, the inherent potential of the soil on the site to produce specific plant communities, the probable climatic patterns and environmental conditions or constraints that will occur, and the suite of management actions available within the aforementioned constraints. To describe the non-linear vegetation changes that occur on most rangelands, rangeland scientists developed a conceptual approach called state and transition models. State and transition models use box-and-arrow diagrams to describe and understand non-linear vegetation change, or plant succession. That is, changes in community composition, vegetation structure, and ecological function are illustrated that do not follow the one-dimensional pathway, forward or backward, described in the previous paragraph.

The specific plant communities that ultimately develops depends upon which management actions are applied; the intensity, frequency, and duration of those actions; and how those management actions interact with environmental conditions and their variation across time. To help you understand the concepts behind and advantages of state and transitions models we will briefly discuss plant succession on rangelands, why the previous models used to describe plant succession on rangelands do not work and can result in poor management decisions, the components of a state and transition model, and the structure of a complete state and transition model. Data and information from a Wyoming sagebrush ecological site will be used to illustrate the problems with previous plant succession models and the components of state and transition models.

Forces that Shape Rangelands. Rangelands are a dynamic landscape, composed of many resources, that produce many products. The rangeland landscape and its resources are constantly being modified by a suite of non-human forces, including:

• Grazing

• Fire

• Climate or Weather

Humans also modify rangelands directly through development (e.g., energy, mining, and transporation and communications infrastructure) and recreation. People also affect the other forces of change by introducing invasive species, controlling or igniting fires, managing grazing and potentially impacting the climate and weather patterns through human caused changes in atmospheric chemistry .

Managers need a way to predict how management practices or natural disturbance will impact the vegetation on rangelands, so they developed State and Transition Models. State and transition models are box-and-arrow diagrams used to describe vegetation change, or plant succession, from a specific disturbance based on the current vegetation community, the soils and climate of a site.

Rangelands are dynamic ecosystems that  produce a wide variety of goods and services desired by society, including livestock forage, wildlife habitat, water, mineral resources, wood products, wildland recreation, open space and natural beauty. In order to continue to provide those goods and services, rangelands must function ecologically, in other words, they must be able to capture water and nutrients and convert them into plants

Written by Rachel Frost, Montana State University

Herbicides that interfere with and/or disrupt the biochemical or physiological processes unique to plants. Herbicides typically decreases the growth, competitiveness, and/or seed production of unwanted plant species while providing opportunities for desired species to increase in number, size and productivity. Herbicides, therefore, are valuable tools for controlling unwanted plants on rangelands.

When properly applied, herbicides can provide a window of opportunity for desired plants by removing or suppressing populations of weeds. However, using herbicides alone to control weeds seldom results in successful long-term control. Once weed species are removed from a site, the area must be revegetated with desired species that can competitively exclude the potential weed species. Herbicides are most valuable when they are one component of an integrated weed management plan that focuses on replacing of weeds with desired species, proper grazing management, and management actions that reduce the risk of new infestations.

• How Herbicides Work
• Successful Weed Management with Herbicides
• Herbicide Mode of Action
• Herbicide Fate in the Environment

How Herbicides Work

Written by Rachel Frost, Montana State University

Application. Most herbicides are applied either pre-emergence, before the weeds emerge from the soil and begin to grow, or post-emergence, when weeds are already growing and easily located. Both pre- and post-emergent herbicides are used on rangelands. Pre-emergent herbicides have a high degree of soil activity and are readily absorbed by the roots of both seedlings and mature plants. The target plants are killed shortly after they germinate and/or shoots emerge from the soil. Pre-emergence herbicides are effective in controlling annual weeds, such as cheatgrass, and may be applied following disturbances like wildfire to prevent weed establishment from seeds.

Formulation. A herbicide formulation is how a particular herbicide is packaged for distribution. The formulation includes both active ingredients — the chemical that harms or kills the plant — and inert ingredients such as solvents that enable them to penetrate leaf tissue. The most common formulation of rangeland herbicides is a concentrated liquid that can be diluted with water and sprayed on the target species. Water is the most commonly used carrier of herbicides for rangeland application. It is cheap, universally available, and works with a wide variety of herbicides. However, hard water — water with high levels of calcium and magnesium salts — can decrease the activity of certain herbicides with an ionic charge. A few rangeland herbicides are formulated as dry granular material or pellets that are applied directly to the soil surface without dilution in water. Because it is not diluted prior to application, the active ingredient in dry formulations is much less concentrated. These herbicides require precipitation to move the active ingredient into the soil to the root system, where it is absorbed and translocated to the plant's growing points.

Selectivity. The selectivity of an herbicide is determined by the plant's ability to metabolize the active ingredient and render it harmless. The different metabolic processes in plants are capable of inactivating certain herbicides and rendering them harmless. For example, grass species are capable of metabolizing phenoxy herbicides, while broadleaf plants are not. Phenoxy herbicides, therefore, kill broadleaf plants but have no effect on most mature grass plants. Seedlings are the growth stage most susceptible to chemical control. An herbicide that does not harm mature plants may kill most of the seedlings of the same species. It is important that anyone applying an herbicide read the herbicide's product label before they apply the chemical. The product label will inform the applicator of the herbicide's level of selectivity and how that may change with plant maturity or the dosage applied. Understanding an herbicide's selectivity is important because land managers often want to remove one or more species without adversely affecting the non-target species.

Translocation. Once an herbicide has been absorbed by a plant, the movement of the active ingredient throughout the plant is referred to as translocation. Systemic herbicides are absorbed by the plant's roots and/or foliage and are translocated to distant parts of the plant, including root buds, growing points (meristematic tissue), and other reproductive structures. The active ingredient typically accumulates in these critical growth areas and interrupts the important metabolic processes that keep the plant alive. Systemic herbicides are very effective at controlling perennial weeds that regrow from buds on the roots or root crowns. To kill perennial plants that resprout from belowground buds, all of the buds have to be killed. Systemic herbicides that are translocated through the phloem are most commonly used on rangeland. Contact herbicides have very limited movement within the plant and kill only the tissue that comes in direct contact with the herbicide. Although contact herbicides can effectively top-kill a perennial plant, the herbicide's inability to be translocated to the root system allows the plant to regrow the following year. Contact herbicides are most effective against annual plants.

Successful Weed Management with Herbicides

Written by Rachel Frost, Montana State University

Proper attention to the following three basic principles will improve the effectiveness of herbicides and decrease potential negative impacts to non-target species, the environment, and the applicator:

• Choose the right herbicide for the job - Herbicides vary in the way that they affect plants and in the type of plants that they affect. Non-selective herbicides kill or suppress all vegetation, while selective herbicides kill some plants but not others. For example, a broadleaf spectrum herbicide like 2,4-D kills only broadleaf forbs and shrubs and does not harm grasses and sedges.
• Apply the herbicide at the right time - Herbicides need to be applied at the correct stage of plant growth to maximize effectiveness. For example, annual weeds should be treated before flowering to prevent seed set. Movement of the herbicide through the plant's system to the roots is essential for perennial weed control, so herbicides are most effective when applied to perennial plants when they are moving carbohydrates to the roots.
• Use the proper application technique - Herbicides can be mixed in a variety of formulations, including liquids, powders, or pellets. The label is a legal document that provides information on the proper use and application of herbicides. Labels provide detailed information on the correct formulation, rate of application, recommended carriers or additives, and safety precautions for that specific herbicide.
  • Additional site characteristics such as soil type, slope, and the existing vegetation — both target and non-target plants — should also be considered when selecting the herbicide and planning the application process.
  • Always read and follow the herbicide label directions.
  • ​Check with your local weed professional or Cooperative Extension agent for help in selecting the proper herbicide and application procedure for your target species. Remember to calibrate your sprayer to ensure accuracy in application rates and to save money.

Herbicide Mode of Action

Written by Rachel Frost, Montana State University

The mechanism by which herbicides actually kill plants is known as the herbicides’ mode of action. Each mode of action focuses on a specific site of action, which is usually a single enzyme or enzyme pathway that is essential for plant growth. Herbicides kill plants by inhibiting or affecting these essential enzymes or pathways.

Growth-regulating herbicides. The most widely used herbicides in rangelands are growth-regulating herbicides. Growth-regulating herbicides upset the normal hormonal balance that regulates processes such as cell division, cell enlargement, protein synthesis, and respiration. Although usually applied to the foliage, this group of versatile herbicides is also effective in the soil. This increases the effectiveness of the herbicide as any of the chemical that does not land on the foliage can be percolated into the soil with rain and taken up by the weed roots. Growth-regulating herbicides can be classified as phenoxy herbicides, benzoic acids, and carboxylic acids.

Photosynthesis inhibitors. Photosynthesis inhibitors do just what their name implies: They inhibit photosynthesis, preventing plants from converting light energy from the sun into sugars used for food. Photosynthesis herbicides are used primarily to control broadleaf plants but can be marginally effective against annual grasses.

Amino acid synthesis inhibitors. There are two main types of amino acid synthesis inhibitor herbicides: selective and non-selective. Selective amino acid synthesis inhibitors control both broadleaf plants and grasses, have both soil and foliar activity, and are virtually non-toxic to mammals and most non-vegetative life forms. Amino acid synthesis inhibitors kill the plant by binding to a specific enzyme and preventing the plant from synthesizing essential amino acids. Non-selective amino acid inhibitors, such as Roundup, control a broad range of plants, including grasses, sedges, and forbs. They bind tightly to soil clay and organic matter and do not move through the soil profile.

Herbicide Fate in the Environment

Written by Rachel Frost, Montana State University

What happens to herbicides after they have been applied to rangeland? Do they remain in the soil, eventually accumulating to hazardous levels? Or do they move into the environment, causing off-site problems? In reality, most herbicides do not survive in the soil for very long, nor do they move very far through the soil from one area to another. The environmental persistence of a herbicide depends on the rate of application, the method of application, soil type, weather, and characteristics of the chemical. Still, herbicides are subject to the same biological processes of decomposition as any other compound.

Written by Rachel Frost, Montana State University

Mechanical control on rangelands is defined as "the use of a tool to remove or destroy above and/or below ground plant material." There are numerous different ways to treat rangeland mechanically. The form of mechanical control that is best suited to a particular situation depends on:

1. characteristics of the plants
2. the size of the infestation (this is directly related to cost)
3. availability of the equipment
4. soil characteristics
5. topography
6. current and intended land use

Mechanical tools commonly employed to manage rangeland vegetation:

Chains. The use of chains to alter rangeland vegetation is called "chaining". Chaining involves pulling a large chain, most often a section of marine anchor chain, between 2 tractors. Vegetation is either pulled out of the ground by the roots or broken off at ground level. Chaining is most effective on trees as herbaceous material is generally not seriously damaged. Chaining provides relatively quick results and allows large areas to be treated at a relatively low cost, however, it causes substantial soil disturbance that can lead to compaction or erosion. Trees or shrubs that resprout may require additional treatment with herbicides or fire to actually kill the plants.

Root Plows. A root plow is a heavy-duty, V-shaped blade that is pulled behind a tractor to sever the roots of trees below the bud zone. Root plowing can control up to 90% of target plants when properly implemented; i.e. the blade is at the correct depth and it is done at the proper time of year. Unfortunately, root plowing also kills most herbaceous plants on site, especially when operating in areas of high tree density. Selective plowing can be used to sculpt the vegetation, improve wildlife habitat and enhance multiple use values on rangeland with fertile soils.

Mowers. Mowing is generally applied to herbaceous vegetation to immediately remove biomass from an area. It can be used to prevent seed production in patches of invasive weeds or to increase visibility and ease of travel such as along highways or in recreation areas. Mowing causes minimal soil disturbance, but does not necessarily kill the target plant and may cause harm to desirable vegetation as well. Repeated mowing may reduce resprouting trees and brush, however, it is most often combined with other treatments to achieve maximum control. Mowing is not suitable for rocky ground or areas with dense stands of large diameter trees.

Rakes. Rakes are often used after chaining or root plowing to smooth and prep the site for revegetation. Specially designed rakes allow trees and brush to be stacked with minimal soil in the pile.

Written by Rachel Frost, Montana State University

Cultural management tools involve changing the timing, intensity or duration of a land use or management action to achieve a desired vegetation composition or structure. Targeted grazing, fire and reseeding or planting desirable vegetation are all forms of cultural control. These management practices can be implemented at different seasons, intensities or durations to accomplish clearly defined vegetation management goals. Cultural control is most often employed to manage invasive herbaceous weeds. Planting desirable, competitive plants that can capture soil and moisture resources, preventing weeds from obtaining these resources, and limiting their abundance is an example of cultural control on rangelands.