The choice of methods and criteria to assess rangelands depends on the questions the assessments are intended to answer. There are many different questions that assessments could be and have been designed to answer including: What is the quality and quantity of the livestock forage produced? Is habitat for wildlife improving or degrading? and What quantity and quality of water can rangeland watersheds be expected to provide? These are all important questions and, for the most part, answers to each of them require different information about rangelands.
Although essential, defining the purpose of national assessments of rangelands is not simply a scientific problem because the motivations for national assessments emanate from the reasons society values rangeland ecosystems. Institutionalizing goals for rangeland assessments, then, unavoidably entails a judgment about what information about rangelands is most important to provide national policymakers, ranchers, environmentalists, and the general public. The capacity of rangelands to satisfy the values of and produce commodities for these diverse groups depends on the integrity of the soil and ecological processes of rangelands. National assessments should provide accurate and accessible information about the status of rangeland ecosystems to all individuals and groups who have an interest in the values and commodities that rangelands provide. Providing this information should be the first step toward formulating decisions about the management and use of federal and nonfederal rangelands.
GOALS FOR NATIONAL ASSESSMENTS
Chapter 1 described the diverse values and commodities that rangelands provide. The capacity of rangelands to sustainably produce com-
modities and satisfy values depends on the interaction of climate, plants, and animals in a particular geological and topographic setting over time. These interactions result in soil development and the production of particular kinds and amounts of vegetation and enable rangelands to adjust to changes in their environment or management. These interactions also give rangelands the ability to resist the destructive effects of such extreme events as droughts and intense rainstorms.
In other agricultural ecosystems, such as intensively managed crop-lands, the capacity to produce resources and satisfy values is often augmented by using high levels of external inputs such as irrigation water or fertilizer, the physical environment is modified by tillage or terracing of the land, and pests are controlled by applying chemical pesticides. Rangelands, for the most part, do not receive such inputs. The capacity of rangelands to produce commodities and satisfy values depends on the integrity of nutrient cycles, energy flows, plant community dynamics, an intact soil profile, and stores of nutrients and water.
Overgrazing, harmful recreational activities, disease and insect outbreaks, drought, and other factors can degrade rangeland health and, hence, the quantity and quality of the values and commodities that are provided. Rangeland degradation can result in an irreversible loss of the capacity to produce commodities and satisfy values and the loss of future options to use and manage rangelands.
The importance of protecting and sustaining the capacity of rangeland ecosystems to provide the values and commodities desired by society has been repeatedly recognized in national legislation. (See Chapter 5 for a more complete discussion of national legislation pertaining to rangeland assessments.) The Soil Conservation Service (SCS), the U.S. Forest Service (USFS), and the Bureau of Land Management (BLM) have been mandated to provide the assessments of rangeland ecosystems needed to protect the quality and sustained yield of renewable resources. The Environmental Protection Agency is developing the Environmental Monitoring and Assessment Program (EMAP) to monitor changes in U.S. ecosystems, including rangelands. Providing policymakers and the public with the information needed to determine whether the capacity of rangelands to satisfy values and produce commodities is being sustained, improved, or degraded should be the primary goal of national assessments of rangelands.
STANDARDS FOR RANGELAND ASSESSMENTS
The long-running debate over the use and management of rangelands has intensified recently. The debate largely centers on whether grazing is an appropriate use of federal rangelands, whether grazing is being prop-
erly managed by ranchers and the agencies responsible for managing federal rangelands (BLM and USFS), and whether grazing is degrading federal and nonfederal rangelands (Royte, 1990; Shaw, 1990; Wuerthner, 1991).
At the same time that public debate over the use of the nation's rangelands has grown, a scientific debate over the use of current range condition (SCS) and ecological status (USFS and BLM) ratings as broad assessments of the ecological condition of rangelands has emerged and intensified. As recently as 1989, range condition (SCS) and ecological status (USFS and BLM) ratings have been interpreted as measures of rangeland health (U.S. Department of Agriculture, Soil Conservation Service, 1989a; Society for Range Management, 1989). Now, however, the scientific debate over the utility of current range condition (SCS) and ecological status (USFS and BLM) ratings has intensified, leading to disagreements over the proper interpretation of past and ongoing range condition (SCS) and ecological status (USFS and BLM) ratings.
Pendleton (1989), for example, has asserted that there is a direct connection between the assessment methods currently used by SCS (referred to as range condition classes) and many important characteristics of rangeland ecosystems.
There is a direct relationship between secondary succession [succession occurring on land where the original vegetation was disturbed], range condition [as determined by current methods], and the conservation of soil, water, and related range resources. Although the relationships are not exact and precise, it can reasonably be inferred that depleted ranges, those in the lower condition classes, are producing less forage than they are capable of, that erosion is higher than is normal or proper, that wildlife habitat values are less than optimal, and that range hydrology is impaired. (Pendleton, 1989:30-31)
In contrast, Smith (1989) questioned whether there is any connection between range condition classes and rangeland health.
Current methods do not distinguish between areas where site deterioration is occurring and those where it is not .... The general public assumes that (1) ranges in poor to fair condition [classes] got that way due to over-grazing by livestock, (2) these ranges are deteriorating, and (3) reduction or removal of livestock will improve the range condition (Comptroller General, 1977; Sharpe, 1979; U.S. Department of the Interior, 1979). Economists often assume that range condition [range classes] is related to livestock production or wildlife values and that the greatest return will come from improving poor condition ranges (Martin, 1984). All these assumptions are logical, but incorrect. (Smith, 1989:125)
Such divergent views on the proper interpretation of current rangeland classification and inventory methods have confused the debate over
decrease the black grama grass ground cover. Bare patches, which may have developed during the drought, become worse. Topsoil is compacted by trampling, less rainfall infiltrates the topsoil, and erosion accelerates. At this point, the rangeland is at risk of shifting to desert shrub-land. If grazing pressure is reduced or the drought breaks, however, black grama would be expected to cover the bare patches without any further human intervention and the transition to desert shrubland would be prevented.
With the continuation of overgrazing during the drought, however, fundamental changes in the distribution of soil nutrients and moisture occur. These changes affect the soil and vegetation of the rangeland. Erosion becomes a significant problem as rain and nutrients are carried from the spreading bare spots and are deposited in depressions on the landscape. Creosote and mesquite shrubs begin to appear in the depressions where soil and nutrient deposits accumulate throughout the soil profile at a greater than average density. Pedestaling occurs around established, deeply rooted plants.
These changes are self-reinforcing. Soil around the grasses loses more nutrients and moisture, restricting the regeneration of grasses. Bare patches of soil release their available ammonia into the atmosphere, and denitrification of the soil intensifies after rainfalls. Runoff deposits nutrients to nurture young bushes and trees. Significant fire cannot be sustained on the patchy bare areas because of the lack of flammable organic material. Without natural burning, saplings and shrubs mature and proliferate. The black grama grassland has crossed a threshold to a desert shrubland. Even with human intervention, it may not be possible for the land to return to a black grama grassland, since soil nutrients and moisture essential to the grasses are limited to isolated pockets in which shrubs are firmly established.
At the Jornada Experimental Range, a range scientist inspects Lehmann lovegrass. Researchers removed invading brush and shrubs, replacing them with native and forage grasses. Credit: USDA Agricultural Research Service.
proper rangeland management. An agreed-to standard that can be used to determine whether the capacity of these rangelands to satisfy values and produce commodities is being conserved, degraded, or improved is needed. The lack of a consistently defined standard for acceptable conditions of rangeland ecosystems is the most significant limitation to current efforts to assess rangelands. The lack of such agreed-to standards has and continues to confuse the public, the U.S. Congress, ranchers, and range scientists themselves.
Rangeland health should be defined as the degree to which the integrity of the soil and ecological processes of rangeland ecosystems are sustained.
The capacity of rangelands to produce commodities and satisfy societal values depends on the interactions of climate, plants, and animals in a given physical landscape over time. These interactions are mediated by the soil and by internal ecological processes such as nutrient cycles, energy flows, and plant community dynamics. The integrity of the soil and ecological processes determines the vegetation, habitat, aesthetics, and other commodities and values that rangelands can provide and determines how well rangelands are able to resist the destructive effects of mismanagement or natural disturbances.
Rangelands are ecosystems not individual organisms and the use of the term ''health" should not imply that simple analogies can be made between the health of an organism and the health of an ecosystem. Health, however, has been used to indicate the proper functioning of complex systems and is increasingly applied to ecosystems to indicate a condition in which ecological processes are functioning properly to maintain the structure, organization, and activity of the system over time. Recently, for example, Haskell et al. (1993) defined ecosystem health in terms of sustainability and stability, suggesting that an ecological system should be considered healthy if the system is "active and maintains its organization and autonomy over time and is resilient to stress" (Haskell et al., 1993:9).
The concept of forest health has frequently been used to refer to the effect of pests, pathogens and toxic compounds on the growth, development, and reproduction of forest communities (Brooks, 1992; Johnson et al., 1992). Use of the term "forest health," however, increasingly refers to a broader conception of ecosystem health that recognizes the importance of changes in nutrient cycles, soil attributes, air quality, and other structural and functional characteristics of forest ecosystems (Burkman and Hertel, 1992; Commission of the European Communities, Directorate General for Agriculture, 1990; Gray and Clark, 1992). Similarly, the concept of crop health is increasingly being expanded to include the health of the
agroecosystem as a whole (Cook and Veseth, 1991). Efforts to develop measurable indicators of change in ecosystems as part of ecological risk assessments has also increased the use and development of the concept of ecological health as a measure of the integrity of the structure and function of ecosystems (International Joint Commission, 1991; National Research Council of Canada, 1985; Schaeffer et al., 1988).
Webster's Third New International Dictionary defines healthy as "(1) functioning properly or normally in its vital functions, (2) free from malfunctioning of any kind, and (3) productive of good of any kind." The terms "healthy" or ''unhealthy'' are most properly applied to ecosystems as an indication of proper or normal functioning of ecological processes resulting in the production of good, that is commodities or values, that are important to private landowners and the public at large.
The term "health," then, as used by the committee, is an indication of the ecological integrity of rangeland ecosystems. The term "ecological integrity" has recently been defined as "maintenance of the structure and functional attributes characteristic of a particular locale, including normal variability'' (National Research Council, 1992:520). More specifically, the committee recommends the term "rangeland health" be used to indicate the degree of integrity of the soil and ecological processes of rangeland ecosystems that are most important in sustaining the capacity of rangelands to satisfy values and produce the commodities.
Determining whether the capacity of a rangeland to satisfy values and produce commodities is being sustained will not resolve the debate over the proper use and management of that rangeland. A separate system is needed to evaluate use of a particular rangeland and the kind and amounts of vegetation needed to support that use (Ellison, 1949; Friedel, 1991; Humphrey, 1947; Lauenroth, 1985; Laycock, 1989; Shiflet, 1973; Society for Range Management, Range Inventory Standardization Committee, 1983; Tueller, 1973; West, 1985; Wilson, 1989). If, however, the public, policymakers, ranchers, and range managers can be assured that rangeland health is conserved, the debate can profitably shift to whether rangelands are best used for the production of livestock, wildlife, or recreation or some combination of these. These decisions will be contentious, but they can at least be made in the context of conserving the health, and therefore the capacity, of rangelands to produce commodities and satisfy values, regardless of their use.
Categories for Rangeland Assessments
Rangeland ecosystems are dynamic systems, and fitting rangelands into categories based on ecological criteria is a difficult but essential task for national assessments of rangelands. The capacity of rangelands to
produce commodities and satisfy values depends on the integrity of soils and ecological processes, that is, on their health. Range managers, policymakers, and the public need to know whether the health of federal and nonfederal rangelands is being sustained, improved, or degraded. This requires defining boundaries between states of rangelands depending on the degree to which the integrity of the soil and internal ecological processes are protected.
The principal purpose of the rangeland inventories completed by SCS, BLM, and USFS should be to determine the proportion and location of rangelands that are healthy, at risk, or unhealthy.
The categories defined for purposes of national rangeland assessments should facilitate the interpretation of the results of those assessments for policymakers, range managers, ranchers, and the public. The categories used for national assessments should signal where rangeland management or technical assistance are needed to prevent degradation or to improve damaged rangelands.
The committee recommends that rangelands be placed in three broad categories based on an evaluation of ecological health. Rangelands should be considered (1) healthy if an evaluation of the soil and ecological processes indicates that the capacity to satisfy values and produce commodities is being sustained, (2) at risk if the assessment indicates an increased vulnerability to degradation, and (3) unhealthy if the assessment indicates that degradation has resulted in an irreversible loss of capacity to provide values and commodities.
Categorizing rangelands as healthy, at risk, or unhealthy requires defining two boundaries: the boundary distinguishing healthy from at-risk rangelands and the boundary distinguishing at-risk from unhealthy rangelands. Rangelands, however, are constantly adapting in response to changes in physical environment, use, and management and to episodic events such as fires, droughts, and intense rainstorms. These constant adaptations are reflected in changes in many characteristics of the rangeland ecosystem such as plant composition, the amount of plant biomass produced, the amount of nutrients and the rate at which they are cycled, and the amount and composition of soil organic matter. The ecological state of a rangeland at a given point in time is the sum of these characteristics. The rangeland ecosystem shifts between different ecological states over time in response to natural or human-induced factors. Such changes can be sudden or they may occur gradually.
There are important differences between processes of change, howev-
er, that can be used to identify boundaries between healthy, at-risk, and unhealthy rangelands for the purposes of national assessments. Some changes in ecological state may have no long-term effect on the capacity of the rangeland to produce commodities or satisfy values. A change in the relative abundance of the dominant plant species, for example, may reflect seasonal variation in rainfall rather than a change in the capacity of the rangeland to produce wildlife habitat or forage. Other changes can be destructive, but their destructive effects can be reversed by changes in use and management or as natural conditions improve if the integrity of the soil and ecological processes has been conserved. Still other changes—soil degradation, the interruption of nutrient cycles, and the loss of important species or seed sources, for example—can lead to irreversible changes that reduce the amount and diversity of vegetation, habitat, aesthetics, and other commodities and values. Once these changes have occurred, external inputs, reseeding, or soil reclamation, for example, will be required to restore the rangeland to a healthy state. Even with restoration, however, some loss of capacity to produce commodities and satisfy values may be permanent.
The boundaries between healthy, at-risk, and unhealthy states of a rangeland should be distinguished based on changes in the soil and ecological processes that determine (1) the capacity of the rangeland to produce commodities and satisfy values and (2) the reversibility of the changes between states and can be illustrated by a model (Figure 2-1). The boundary between at-risk and unhealthy states should indicate a reduction in capacity to satisfy values and produce commodities that is difficult to reverse without substantial external inputs. The boundary between healthy and at-risk states should indicate a reduction in capacity to provide values and commodities that is likely to be reversed through changes in use and management or as natural conditions improve. These boundaries are referred to in Figure 2-1 as the threshold of rangeland health and the early warning line. A brief discussion of the processes leading to changes in ecological state will help clarify the distinctions between healthy, at-risk, and unhealthy rangelands.
THRESHOLDS BETWEEN ECOLOGICAL STATES
A threshold can be defined as a boundary in space and time between two ecological states. Ecologists have recognized and studied how ecosystems change from one state to another across thresholds (Holling, 1973; Wissel, 1984). Threshold changes involve shifts in plant composition; changes in the physical, chemical, or biological properties of soils; or changes in basic ecological processes such as nutrient cycles. They are different from other changes from one state to another because they are
not reversible on a practical time scale without human intervention. In some cases, human intervention may not be sufficient to reverse these changes (Friedel et al., 1990), for example, severe soil erosion.
Interaction between factors often accelerates changes in rangelands. Changes in grazing management can result in rapid positive changes in the composition of plant communities and the amount of annual biomass that is produced if the changes in use and management are accompanied by a series of years with above-average precipitation. Changes in the vegetation will occur more slowly or may not occur at all under climatic conditions that are not favorable for seedling establishment and growth. Similarly, overgrazing that coincides with drought years can result in the rapid degradation of rangeland health. A high-intensity rainstorm that occurs at a time when plant cover has been reduced by overgrazing, fires, or droughts can result in accelerated rates of soil erosion.
PROCESSES OF CHANGE
Ellison (1949) distinguished two types of rangeland change—secondary succession and destructive change. (Primary succession is a series of changes in the composition of the plant and animal life of a particular area. Secondary succession occurs in places where the original vegetation
has been disturbed, for example, on land affected by fire or drought.) According to Ellison, secondary succession entailed changes in plant composition, such as a decline in the number of plant species that were palatable to grazing animals or an increase in the shrub component because of grazing pressure. These changes in plant composition were considered normal adjustments as a result of grazing.
Ellison considered destructive change to be beyond the limits of normal change and to be induced by accelerated erosion, which was evidence of a basic change in the relationship between components of the rangeland ecosystem—a change of drastic proportions over and above the normal range of environmental stresses. Furthermore, once such change was initiated, it could not easily be reversed, even with the discontinuation of grazing. Destructive change represented a new process of change that is not comparable to the process of soil development and that results in the permanent loss of productive capacity. The process of destructive change, described by Ellison can be thought of as leading to a threshold shift between two ecological states. In this case, soil degradation leads to a reduction in the productive capacity of the rangeland that is difficult or impossible to reverse.
Friedel (1991) suggests that rangeland plant communities change in response to various combinations of different factors such as grazing season, the animal species present on the rangeland site, and variables related to the specific site. These changes do not preclude shifts to other short-lived plant communities if the grazing season and grazing pressures change. Various combinations of climatic and grazing conditions may, however, induce a change in plant species composition that is not readily reversible. In such cases, the system has crossed a threshold.
Friedel recognized two such changes in threshold—from grassland to woodland and from stable to degraded soil—on Australian rangelands. The first threshold change from grass to woody vegetation—results when grazing reduces the density of the grass layer. Germinating woody plants displace grasses because their deeper roots are able to reach the water found in increasingly deeper subsoil layers. This change results in a transition across a threshold to woody vegetation that is difficult to reverse (Friedel, 1991).
The second threshold occurs when soil erosion irreversibly alters the physical, chemical, and biological properties of the soil. This results in reduced infiltration of rainfall into the soil. The land becomes too xeric (dry) for the establishment of grasses or woody plants. Well-established plants may remain and new plants may become established during infrequent periods when climatic conditions are particularly favorable (Friedel, 1991). Soil erosion thereby irreversibly changes the kind and amount of vegetation the site can produce.
Rangelands in Transition: South African Tall Grassveld
Tall grassvelds in South Africa can shift from a mix of palatable grasses to a mix of unpalatable perennial grasses and annual grasses, depending on grazing management (Westoby et al., 1989). This particular transition can be reversed with minimal human intervention. Prolonged overgrazing, however, in combination with factors such as increased erosion and reduced soft cover can cause a shift to a patchy mix of annual grasses and bare spots. Once this transition occurs, a return to the original state is likely to be difficult, if not impossible, without significant human intervention.
Light grazing decreases the cover of palatable perennial grasses, including Themeda triandra and Eragrostis racemosa, and increases the cover of unpalatable perennial or annual grasses, such as Cymbopogon excavatus. These changes signal that the rangeland is at risk of shifting across a threshold to a different combination of spe-
cies. A timely decrease in grazing pressure, however, allows a resurgence of the palatable grasses and a return to the original state.
If overgrazing continues, however, problems such as erosion and soil runoff combine to force the rangeland over a threshold. Palatable and unpalatable perennial grasses are reduced and bare patches appear. Annual grasses begin to replace perennial grasses. As erosion becomes more severe, the seed supply from perennial grasses is greatly reduced or even lost. The seeds that are still available cannot germinate because of
insufficient nutrient-rich soil and lack of suitable seedbeds. These changes signal that the rangeland has crossed a threshold to a new ecological state. Simply reducing grazing pressure, once the threshold is crossed, is no longer sufficient to regain the initial state. Management techniques such as soil reclamation and reseeding with perennial grasses would be required, but they may not necessarily lead to a recovery to the initial state.
Episodic events can be significant causes of threshold changes. A period of above-average rainfall, for example, may facilitate the germination and growth of seedlings of woody plants. These woody plants may eventually dominate the rangeland unless fires occur before the seedlings become well established. Similarly, if rainfall is episodic, plants may have only infrequent opportunities to regenerate, and these opportunities may be decades apart. The season in which the rainfall occurs will influence which plant species produce seed and which seeds germinate and become established. The timing of an episodic rainfall event, then, may determine the plant composition of a rangeland for many years. Finally, a single storm producing large amounts of rain can also initiate the erosion of susceptible soils and alter the productivities of entire landscapes (Friedel et al., 1990).
Risser (1989) suggested that recovery of species composition following a disturbance may be slow either because the species that must increase have slow dispersal rates or because their seeds may germinate infrequently and their seedlings may have strict requirements for water, nutrients, or other factors that must be met if they are to grow. Succession may become suspended or static for long periods because of a lack of seeds or seed dispersal, dominance of a life-form that does not allow other species to increase or invade the area, specific physiological requirements that limit seedling establishment except, climatic changes, restriction of natural fires, or other factors (Laycock, 1989).
THRESHOLD OF RANGELAND HEALTH
The threshold of rangeland health should be defined as a boundary between ecological states of a rangeland ecosystem that, once crossed, is not easily reversible and results in the loss of capacity to produce commodities and satisfy values.
The threshold of rangeland health is distinguished from other boundaries between the ecological states of a rangeland ecosystem by two key factors. First, as the use of the term "threshold" suggests, the shift from one ecological state to another across the boundary is not easily reversed. Second, as the use of the term "health" suggests, changes in the soil or ecological processes result in a change in the capacity of the rangeland to satisfy, values or produce commodities.
Significant external inputs, such as soil stabilization or reclamation, reseeding, or control of unwanted vegetation, are usually required for a rangeland to regain a healthy state once the threshold has been crossed. Simple management changes such as improved grazing or the reintroduction of fire will not restore rangeland health within a practical time frame when soil degradation, loss of seed sources, changes in vegetative struc-
ture of the plant community, disruption of nutrient cycles, or a combination of these and other factors are also involved.
Degradation of the soil and of ecological function, which leads to the transition from an at-risk to an unhealthy state, causes a reduction of the capacity to produce commodities and satisfy values. Given soil reclamation or reseeding efforts or other external inputs, transition across the threshold of rangeland health from unhealthy to healthy is possible. Even though health is restored, the rangeland may not produce the same mix and amount of resources and satisfy the same values as it did in the original healthy state.
EARLY WARNING LINE
The rangeland inventories and routine monitoring completed by SCS, BLM, and USFS should provide an early warning of rangelands that are vulnerable to a shift across the threshold of rangeland health.
An early warning of changes in soil or ecological processes that increase the vulnerability of rangelands to a shift across the threshold of rangeland health is essential to preventing loss of health. Degradation of soils or ecological function results in some loss of rangeland health and, therefore, the capacity to produce commodities and satisfy values as a rangeland changes from a healthy (state A) to an at-risk (state B) state (Figure 2-1). The transition from a healthy to an at-risk state is thought to be reversible, however, if the human-induced or natural factors that caused degradation are alleviated. A change from an at-risk to a healthy state, however, does not necessarily entail a return to the original plant community composition of the site.
The boundary between healthy and at risk can be thought of as an early warning and signals the need to take corrective action or further investigate the site. Identification of at-risk rangelands would enable range managers to take appropriate action before the health of the rangeland and the capacity of the rangeland to produce commodities and satisfy values is impaired.
The transition from a healthy rangeland (state A) to one at risk (state B) involves changes in the physical environment and biological systems that make the area more susceptible to near-permanent changes resulting from extreme climatic events, improper use or management, or other stresses. This risk may be due to an increased vulnerability to extreme events that cause a sudden transition across the threshold of health or to the cumulative effects of one or more ecological stresses. A rangeland with compacted soil or reduced plant cover, for example, may be vulnerable to rapid and irreversible gully formation during a torrential rainfall. Alternatively, a rangeland may approach the threshold of rangeland
Rangelands in Transition: The Rio Grande Plains
The historical change in vegetation on the Rio Grande Plains near Alice, Texas—from a savannah with only scattered trees to a subtropical thorny woodland—illustrates how several factors can interact to cause a rangeland to shift across a threshold. For example, a change in climate, such as a shift in rainfall pattern from frequent showers limited to small areas to infrequent storms across the rangeland; overgrazing, which reduces grass cover; a decrease in fire frequency; and changes in soil moisture can combine to have dramatic, far-reaching effects. Research indicates that the interactions of those factors are causing large-scale changes in vegetation in southern Texas (Archer, 1989).
The climate of the subject site at the Texas Agricultural Experimental Station in La Copita Research Area is subtropical, with mean annual rainfall of 68 centimeters (27 inches) and a mean annual temperature of 22.4ºC (72.3ºF). The soils are fine, sandy loams from sandstone. Initially classified as a Prosopis-Acacia-Andropogon-Setaria savannah, the site has been grazed by cattle since the late 1800s and has experienced a 23 percent increase in woody cover since 1941. At issue is the reason for the increase in the number of mesquite (Prosopis glandulosa ) trees and the shrubs that surround them. Mesquite is becoming common and more dominant in ecosystems throughout the southwestern United States, although historically it was a minor component of rangeland ecosystems. This change to more mesquite trees and the corresponding shrub clusters continues, but the shift accelerated between 1960 and 1983.
Archer's (1989) research uncovered four major reasons for the change to a woodland: a change in rainfall patterns over the past 100 to 200 years,
reduced grass cover, fewer fires, and a reduction in the available moisture in the topsoil. Any one factor alone would be insufficient to so dramatically alter the vegetation of the Rio Grande Plains; a combination of two or more of the suggested factors is probably required, according to Archer.
A shift from light and frequent to heavy and less frequent rainstorms or an increase in winter rainfall over that in summer increases the proportion of rain that infiltrates to deeper soil layers. This change in the ratio of topsoil to subsoil moisture is also caused by overgrazing, which reduces grass cover. Shallow-rooted grasses that capture most of the rainfall, keeping it in the topsoil, are reduced, allowing more rainfall to reach the subsoil. Mesquite, which can reach the subsoil moisture, therefore gains a competitive advantage over grasses, and individual shrubs become established. These changes signal that the rangeland is at risk of shifting to a woody shrubland if the trends continue.
At this point, reduced grazing pressure or fire may be sufficient to reverse the increase in shrubs. If no change in management or climate occurs, however, shrubs and trees become established in clusters around the mesquite trees. These clusters capture more and more of the available water and nutrients. The development of clustered woody vegetation reduces the chance of fire. Historically, fire sweeps across the grassland, burning seedlings in its path and permitting the regeneration of grasses, which grow more quickly after a fire. Because of the patchiness in the grassland, however, fire is extinguished in the bare spots because of the lack of flammable organic matter. Without fire, mesquite trees and their associated shrubs proliferate. Grazing contributes to this trend as cattle eat mesquite seeds, depositing them in dung, a nutrient-rich environment ideally suited to germination. The rangeland crosses a threshold, becoming a subtropical thorny woodland.
Grazing contributes to the proliferation of mesquite trees and their associated shrubs as cattle eat mesquite seeds, depositing them in dung, a nutrient-rich environment ideally suited to germination. Credit: USDA Agricultural Research Service.
health gradually as important seed sources are lost as a result of the combined effects of drought and poorly managed grazing. Lands considered at risk can become healthy once the climate has returned to a more average state or once use and management are appropriately changed.
MULTIPLE STATES AND TRANSITIONS
The model shown in Figure 2-1 suggests that pathways of loss and recovery of rangeland health are linear and simple. This is not necessarily the case. States A, B, C, and D may represent a complex of related plant communities rather than a single stable community (Figure 2-2). Change from one plant community to another within the complex may be caused by predictable successional processes or by climatic variability, insect and disease outbreaks, the grazing system used, or other factors. The plant communities within a complex may not produce the same mix of commodities or satisfy the same values. A short-lived drought or temporary heavy grazing, for example, may result in a reduction in biomass production. Increased amounts of rainfall or improved grazing management may produce a shift to a different plant community within a complex that produces a different mix of commodities and values. Changes within a
complex are reversible, as are transitions between complexes in states B and A.
In Figure 2-2, transition from state B to state C indicates an irreversible change that results in loss of capacity to produce commodities and satisfy values. There may be shifts from one plant community to another within complex C, but the transition from state C to state D will not occur without human intervention.
Transition from state B to state C in Figure 2-2 entails the loss of health. Continued soil degradation, disruption of nutrient cycles and energy flow, or the loss of species that are important functional components of the complex results in a transition that is difficult to reverse. The composition of the rangeland may continue to change within the new complex, but reversal of the degradation will require external inputs, such as soil reclamation and reseeding of vegetation. Even with such corrective action there may be permanent loss of capacity to produce commodities and satisfy values, at least within practical time frames and costs. Future options to use and manage the site may be lost as well.
The elements of the simple (Figure 2-1) or the expanded (Figure 2-2) model are the same. Both incorporate the idea of a transition across a threshold of rangeland health that is not easily reversible and that entails the permanent loss of capacity to produce commodities and satisfy values, even if corrective action is taken.
ROLE OF RANGELAND HEALTH IN RANGELAND MANAGEMENT
Although the concept of changes across ecological thresholds has long been recognized, the concept has not explicitly been included in assessments of rangelands. It is essential to understand the role that rangeland health assessments should play in the larger effort to classify, inventory, monitor, and manage the nation's rangelands. The concept of rangeland health should be only one part of a complete system for managing rangelands. No single index will meet the informational needs of managers, ranchers, policymakers, and the public.
Goal of Range Management
The minimum standard for rangeland management should be the prevention of human-induced loss of rangeland health.
Large investments of time, money, and energy are required to restore unhealthy rangelands. Even with restoration, there may be permanent loss of capacity to produce commodities and satisfy values or loss of options to use and manage those rangelands in the future. Any human-induced loss of rangeland health should be prevented.
The health of rangeland ecosystems may be affected by natural as well as human-induced factors. The ability of rangelands to produce commodities and satisfy values, for example, may be altered by long-term climate changes. Management of rangeland ecosystems must be sensitive to such changes, particularly changes that can threaten rangeland health, such as prolonged drought. So that human-induced loss of rangeland health is prevented, management and use of rangeland ecosystems will have to be adjusted if climate changes increase the vulnerability of rangelands.
Because rangelands are managed and used in ways that depend on the integrity of their soils and ecological processes, the fundamental aim of an assessment of the status of rangelands should be to determine whether a rangeland is healthy, at risk, or unhealthy. Rangelands found to be at risk should receive special attention. Management and monitoring of at-risk rangelands should be more intense than management and monitoring of healthy rangelands. Corrective action must be taken to prevent human-induced loss of rangeland health.
Additional Information Needed to Determine Appropriate Management
Rangeland health inventories and monitoring systems should be one part of a larger system of data gathering and analysis to inform range managers, policymakers, and the public.
No single method of evaluating rangelands will provide all the information needed by range managers, ranchers, policymakers, and the public. Rangeland health is a measure of the integrity of the soil and ecological processes. Other information will be needed to determine what causes the loss of health, what needs to be done to improve health, and how a particular rangeland should be used.
Rangeland health is a measure of whether the capacity of rangelands to produce commodities and satisfy values is being conserved. An assessment of rangeland health is not intended to quantify the suitability of particular rangelands for particular purposes. (The relationship between rangeland health and these uses is discussed in more detail in Chapter 3.) Quantification of the capacity of rangelands to produce specific commodities and satisfy particular values may require a separate assessment. The adoption of such an assessment system has been recommended by several experts and committees, most recently, the Society for Range Management's Task Group on Unity in Concepts and Terminology (1991).
Although rangeland health is related to the capacity of rangelands to produce commodities and satisfy values, the two concepts are different and are represented by different axes in Figures 2-1 and 2-2. One range-
land, because of its soil, climate, and topography, may be able to produce more total biomass annually than another rangeland. But both can be healthy. Differences between the capacities of different rangelands to produce commodities and satisfy values do not necessarily imply differences in health as defined here.
Rangeland health estimates the risk of the loss of the capacity to produce commodities and satisfy values by evaluating the integrity of the site's ecological processes and soils. Such an evaluation does not determine conclusively the processes that are responsible for the current state of health or determine what changes in management are required. Assessment of rangeland health may not be sufficient for a full understanding and treatment of many problems. It should serve as an early warning of problems and help the range manager decide whether detailed measurements are needed and, if so, what additional measurements need to be made.
Rangeland in Extreme Environments
There are landscapes where the prevailing environmental conditions constrain the development and conservation of the soil and ecological
processes that indicate healthy conditions. Soils on these landscapes are unstable or absent, and nutrient cycles, energy flows, and other ecological processes are not established or are only poorly established. In such landscapes, a relatively stable biotic community has not gained a foothold and abiotic processes are more important than biotic processes. Rates of erosion are too great or rates of nutrient enrichment and organic matter accumulation are too slow to allow the development of soils. The Badlands of South Dakota and the Mancos-shale regions of Utah are examples. These landscapes are characterized by a lack of developed soils, by extreme climates, or both. The lack of developed soils may be due to the youth of these sites in geological terms, climates that are unfavorable to soil formation, or geological rates of erosion that preclude the development of soils.
Such sites would be considered unhealthy given the definitions proposed in this report. This unhealthy state, however, has not been induced by humans, and even the best management of these landscapes may not be sufficient to achieve healthy conditions. The primary concern of range management should be preventing loss of health on rangelands where human use and management can improve or degrade rangeland health. Even though sites such as the Badlands o£ South Dakota could be considered naturally unhealthy, they often satisfy important aesthetic and recreational values, and recreational use of these areas is often an important source of economic activity in the local area. Careful management may be required to protect their aesthetic and recreational values.