HAROLD F. HEADY *
(*) Harold F. Heady: Associate Dean, College of Natural Resources, University of California, Berkeley, California, U.S.A.
Range condition defined
Range trend defined
Types of ecosystem changes
Factors used to measure condition
Factors used to indicate range trend
Range condition and range trend are concepts with numerous shades of definition, means of application to land, and complex ecological foundations.
Range condition measures the present state of health of the range in relation to what it could be within a given set of environmental and managerial factors.
Range trend states the direction of change in range condition.
The first step in assessment of range condition and range trend requires delineation of range sites.
The manager is faced with assessing condition and trend of range ecosystems that continually change in response to weather and whatever he does.
The most important factor used to measure range condition is botanical composition; others are soil cover, erosion, and productivity.
Reproduction or change in species composition and vigor of plants indicates trend of range condition.
Range condition and range trend are long-time concepts, each with several definitions, numerous procedures for application, and as many shades of meaning as individuals who use them. This paper will not survey the definitions but will concentrate on my interpretation of the concepts as useful range management tools. Discussion of surveying and sampling procedures will be minimized in favor of examinations of the parameters which yield the most applicable data. This approach will emphasize the kinds of data to be collected not how to collect and analyze data. Choice of criteria for field determination of range condition and trend will not be emphasized. My purpose is to present the ecological foundations of range condition and trend in an organizational arrangement that will promote further discussion and understanding of these highly valuable but often misunderstood concepts. In fact, I believe them to be the central tenets in the evaluation of grazing effects on rangeland.
Range condition is the present state of health of the range in relation to what it could be with a given set of environmental and managerial factors. Range condition measures range deterioration and improvement. Some persons have likened range condition to the inventory of a store. If the shelves are full and the display of products complete, the customer has a wide choice and an excellent store condition exists. If the shelves contain few goods, bare spaces spoil the display, the best products are gone, and only the poorest are left for late customers, then a poor store condition exists. Thus excellent, good, fair, and poor range conditions suggest evaluation of the present range ecosystem in terms of a defined standard of excellence. Descriptions of excellent range rely upon evaluation of moderately grazed areas, relicts, enclosures, historical accounts, and improvements following relief from overgrazing.
The nature and definition of excellent range conditions presents major difficulties in application to many range sites. Widespread understanding of rangeland deterioration requires that excellent range condition must be reconstructed deductively. To many persons this suggests reconstructing climax vegetation. Similarities between excellent condition grasslands and climax grasslands in the central United States do exist but only in an approximate fashion. Excellent range conditions in desert, chaparral, woodland, and forest types may have little relationship to climax; yet only recently I have read that Americans use climax and excellent conditions synonymously. They are not synonyms, and the two concepts should not be confused by anyone.
Range condition takes for granted the fact that a range site has a set of environmental factors that are characterized by such terms as soil type, mean annual precipitation, and the like. These ecological parameters may or may not be well defined. Range condition also includes managerial factors: such items as control over seasonal grazing, stocking rates, kinds of animals, seeding, noxious plant control, and many others. The effects of managerial decisions, especially the selection of products and the levels of production, must be parts of the definition of excellent range condition. Differences in managerial objectives and in interpretation of natural ecological phenomena continue to cause disputes over the usefulness of range condition as a criterion for range evaluation. Ecological characteristics insufficiently define range condition, although they are necessary to the understanding of range condition.
Range trend states the direction of change in range condition. In the analogy with the store inventory used to illustrate range condition, continued disappearance of goods from the shelves defines downward trend. Fewer and fewer desirable forage plants, increasing erosion or bare soil, and other changes characterize downward trend in range condition. Little argument exists concerning the general principle in this definition; however, difficulty in measurement and quantification of ecosystem changes cause arguments. Repeated measurements with a year or more time interval between them can quantify trend; but with many problems, the range professional must assess trend immediately or during the first examination. In those situations his criteria become qualitative, especially subject to uncontrollable environmental variation. Less accurate indicators than repeated measurements must be used. Quite obviously, measurement of range trend and its evaluation have much in common with the study of plant succession.
The first step in assessment of range condition requires delineation of range sites. Each range site should ideally respond homogeneously to climatic variations, have uniform topography and productivity throughout, maintain closely similar botanical compositions in the range condition classes, be grazed in the same manner throughout, and respond uniformly to cultural treatment. These implied criteria for uniformity are applied with the degree of rigor demanded by managerial decision-making. Scale, then, of range sites needs to be reckoned at sizes between hectares and square kilometers for practical purposes, in contrast to research, which requires uniformity attainable, perhaps, only in square meters. The ideal scale of range sites promotes wise management and correct treatment of all the land.
Each site has its particular botanical composition, group of soil series, water relations, physiographic uniformity, range of precipitation, and productivity. Therefore excellent, good, fair, and poor range conditions occur on each site. The productivity of one site in poor condition may be higher than that of another in excellent condition. The importance of range site delineation can hardly be overemphasized in discussion of range condition and trend. Range sites are the basic land units of practical size.
Vegetation everywhere changes in response to many factors. Water content of leaves, concentration of photosynthates, position of leaves, flower opening, and many other responses occur on a daily basis. Animal rhythms peak and decline in correlation with day, twilight, and night. Longer cycles characterize responses to annual weather patterns such as seasonal bird migrations and alternating dominance of cool-season and warm-season plants. On a still longer time scale, drought and wet cycles characterize grazing lands throughout the world. Rodents and insects exhibit demographic cycles which span several years. On geologic time scales new species evolve, others become extinct, and earth movements gradually change habitats for plants and animals.
These changes in organisms and habitats cannot be controlled by man and they are superimposed on each other. Therefore the manager is faced with assessing condition and trend of ecosystems which are continually changing in many different directions, whatever he does. About all he can do is to understand these changes, measure them, and separate their effects from the changes that he can control.
The controllable (at least to some degree) ecosystem changes include plant and animal successions, immigration and introduction of species, mechanical and chemical control of species, and addition of minerals to the soil. These influences are superimposed upon the uncontrolled changes mentioned earlier, resulting in highly complex systems in which the land user frequently understands neither what is happening nor his role in causing the changes. For example, a drought is an expected but uncontrolled weather phenomenon resulting in range deterioration. The result is very poor range condition, disaster, and confusion. Some persons blame the drought and others the livestock.
These many ecosystem changes cause misunderstanding of range condition and range trend. Should range condition terms and assessment account for all types of ecosystem change? Or should condition be limited to those effects which the manager might control? I prefer a positive answer to the first question and a negative one to the second. However, the results of overgrazing must be separated from the results of variable environments, if the range manager and user are to understand that which they can correct and that which they cannot.
For whatever cause, range condition is down when desirable species are replaced by poor species; when reduced soil cover exposes excessive bare surfaces; when erosion accelerates; when production of forage and animals drops; or when any combination of these effects occurs. Briefly, one or more of the ecological parameters - species composition, cover, erosion, and production - are used to measure range condition.
For various reasons different inventory systems have used these parameters in various formulae to determine range condition. The most widespread usage places major emphasis on species composition, in the belief that ideal combinations of species also indicate highest density of plants, stable soil, and high production of both forage and animals. In other words, species composition is independent and the others are dependent. The major exception to this relationship occurs on steep slopes and unstable soils which may erode after slight decrease in cover but little change in the species mix. Strictly speaking, all the factors are inter-dependent. Wherever a single independent factor can be used to evaluate range condition for management purposes, other factors should be considered as dependent. The gain in simplicity of sampling and interpretation of single factors outweighs loss of accuracy in measuring the whole system, especially where managerial decision-making cannot take advantage of small differences. Multiple factor analysis unnecessarily complicates analysis beyond practical necessity, except where operators can use computerized systems.
Except on a research basis and where management of land continues on a stable basis allowing repeated measurement, range trend assessments depend upon evaluation of the general health of individual plants, the vegetation, and the soil. Health in these contexts is difficult to measure, and no completely satisfactory procedures have been found to determine range trend on the basis of one field examination.
Perhaps the most useful factor is reproduction of the desirable species. Presence of young, medium-sized, and large grass plants indicates that the species is regenerating and that the stand is increasing in density. Presumably, then, the trend is improving. On the other hand, dead centers in bunch grasses, dead stubble, and lack of tillering suggest that plants are dying and the trend is downward. Plant parts may be shorter, narrower, and fewer than expected, also indicating downward trend.
Measurements of size, number, and dead areas in plants are relatively easy, but interpretation of general health requires experience and comparison of areas that have been subjected to various grazing pressures. In developing regions the background information for these interpretations is difficult to obtain. Determination of trend necessitates evaluation of plant establishment, vigor of growth, changes in plant residue, directional change in botanical composition, and soil surface conditions. It is well to keep in mind that numbers and sizes of plant parts may be as much a result of a single rain as they are the result of past grazing pressure. However, vigor must be good, regardless of cause, before range condition can change. Poor condition ranges must have some plants in good vigor in order to improve.
Other factors such as livestock production, forage utilization, livestock condition, populations of rodents and insects, prosperity of the operator, and ground cover only indirectly suggest range condition and range trend. Properly managed low condition ranges will sustain animals making satisfactory gains in weight.