The desertification process on rangelands and its reversal
Back to contents - Previous file - Next file
The
Rangeland ecosystem
Decomposers
and microconsumers
The rangeland ecosystem is a complex phenomenon involving a multitude of interrelated functions with each influencing the others. The system's major components are: producers, converters, soils, decomposers and microconsumers, microclimate and manipulators.
Producers
Plants are the producers of the ecosystem. They convert the energy from the sun into energy and other nutrients needed by animals. The producers can be likened to an electric generator. The other wise disturbed, with maximum energy output. They suffer when ther living components of the system thrive, unless the output is reduced and they will eventually die with zero output.
The generator is an aggregation of vascular plants and plant species forming vegetation. The amount of energy produced and made available to the other living components of the ecosystem depends on the characteristic, composition and health of the vegetation. Vegetation can be manipulated and be made either more or less efficient in gross energy production depending on the physiological and ecological responses to the manipulations. Energy outputs below potential resulting in declining productivity is a desertification symptom (Figure 5). In this case, the generator is in need of repair.
Converters
Animals are converters and not producers, contrary to the beliefs of many, including livestockmen. Animals convert the energy and other nutrients produced by plants to products of direct use to humans. Domestic livestock can be managed and the impact that they have on the rest of the ecosystem can be significant, but controlled. The impact can either improve or impair the overall function of the system.
The fact that the impact of domestic livestock on the overall functioning and energy output of the ecosystem can be controlled is highly significant. Here lies the root of the role of domestic livestock on desertification control and rehabilitation. A malfunctioning generator can be repaired by the converters.
Soils
A soil is the supporting component of the ecosystem. It serves as a house for roots and some vegetative reproduction organs and as a warehouse for air, water and minerals. The four soil fractions are mineral materials, organic matter, water and air. The air and water in a soil are variable and their content determines its suitability for plant growth. The ease with which air and water enter the soil depends on soil condition, or degree of porosity, aggregation and granulation. Organic matter, humus and roots play the major role in the formation of these. Organic matter is transitory because it succumbs to the attack of microorganisms. Thus, it requires constant renewal.
Producers must either directly or indirectly supply of organic material for this renewal. Failure to do so results in soil condition deterioration which hinders air and water penetration and increases soil erosion hazard. The overall efficiency of the system is impaired.
Roots can be likened to tubes which transport water and minerals to the generator. This transportation requires energy supplied by the generator. An inadequate energy supply because of an inefficient generator merely adds to its inefficiency. Again, the overall function of the ecosystem is impaired.
Decomposers and microconsumers
These consist of bacteria, fungi, nematodes, protozoa, termites, grasshoppers and other insects. They break down living and dead plant and animal material contributing to the organic and humus composition of soils. They are also a vital component in nutritional cycling. An ecosystem uses the same nutrients over and over again and proper cycling assures an adequate supply. It is appropriate to say here that microconsumers, wildlife and rodents eat large quantities of living forage which influences grazing capacity.
Microclimate
The microclimate is the factor that regulates the speed and efficiency of ecosystem functions. The character of the microclimate is controlled by the functions of the other components.
Manipulators
These are people, beginning with indigenous people. Since the needs of indigenous people were small, their manipulations pertained mainly to hunting. Fire was one of their hunting tools. These plus natural and probably accidental fires were instrumental in the formation and maintenance of pristine grasslands and savannahs.
Modern day people are the great manipulators and domestic stock is their principal tool. They can manipulate grazing to either destroy, improve or maintain a rangeland ecosystem. Improvement with subsequent maintenance must be the objective and it must be achieved if people intend to live and depend on rangelands forever. And, it can be achieved with the application of range management principles and practices.
Range Management
Range management is a relatively new discipline born only a few decades ago in North America by scientists concerned about rangeland deterioration. It is likely the only discipline that originated in the Americas. It is defined as "The science and art of planning and directing rangeland use in order to obtain maximum sustained economic livestock production consistent with the conservation and/or improvement of the related natural resources: soil, water, vegetation, wildlife and recreation" (Huss and Aguirre, 1974).
Range management according to this definition has two objectives; (1) obtaining maximum sustained economic livestock production and (2) conservation and/or improvement of the related natural resource. Scientific range management stands on the premise that the range resources can be improved and grazed perpetually by domestic stock and, at the same time, produce high-quality watershed, wildlife, recreation and, where suitable, forest products. Research and practical application of range management principles and practices have shown this to be true and as solid as the Rock of Gibraltar.
In order to achieve the first objective, the manager must not only plan and direct rangeland use for maximum forage production, he must also see that the forage is efficiently converted by animals on a sustainable basis into consumable products. Plant husbandry and animal husbandry can never be considered separately in range management. This is illustrated as follows:
Poor Producers + Poor Converters = Very Low Production
Poor Producers + Good Converters = Low Production
Good Producers + Poor Converters = Low Production
Good Producers + Good Converters = Maximum Production.
Range science is the organized body of knowledge upon which the practice of range management is based. It is a unique profession. It requires utilization of knowledge in a wide variety of subjects such as soils, plant taxonomy and physiology, ecology, animal production, economics, marketing, agronomy, wildlife and, in some situations, forestry. A specialist in any one of these subjects is not a range scientist because range science is a distinct discipline which requires special study and which has its own principles, practices, concepts and theories. Unfortunately, range management, for all practical purposes, is unknown in the Region except in Mexico and for a few individuals in the other countries who have studied abroad. No country has a range development agency.
Range Plant Nutrition
Nutrition is a word that probably conjures up thoughts of humans or animals. The assumption in fact would be quite right, but there is something else one needs to know. Plants are living organisms and they too have nutritional requirements. In fact, the nutritional requirements of plants and animals are very similar and the general principles of nutrition apply to both. A chief difference between plants and animals is that in animals the walls of the body cells are made mainly of protein while in plants they are composed of cellulose and carbohydrates. In plants, the reserve food is stored as carbohydrates whereas in animals nearly all of the reserve is stored in the form of fat.
A major and most important difference between plants and animals is in their sources of energy. Plants can use energy supplied by the sun to change inorganic matter taken from the earth and from the air into organic compounds. Animals cannot secure directly from the sun the energy necessary for their life. They must live on the organic energy rich compounds built by plants.
With photosynthesis, plants are able to make simple carbohydrates from carbon dioxide and water through the action of sunlight on the green colored chlorophyll in the leaves and other green parts. Carbon dioxide from the air is taken into the leaves through the stomata and water is absorbed from the soil by the roots. From the sugars or other simple carbohydrates first formed, plants can build the more complex organic compounds such as the polysaccharides and pentosans, protein, cellulose, hemicellulose, lignin, vitamins, fats, oils and others upon which all animal life depends.
Plants must be properly nourished in order to carry out photosynthesis and to build the complex organic compounds and they must provide that nourishment themselves. Their role is twofold: first to feed themselves and second to feed man and his animals. It is imperative that plants be properly fed so that they can successfully carry out both roles. This is particularly true in range management and is one of its most important aspects. The objective of obtaining maximum sustained livestock production cannot be achieved with a system of grazing that does not permit the plants to properly feed themselves. Therefore, it is the range manager's responsibility to plan, direct and manipulate grazing in such a manner that the range forage plants can adequately feed themselves and live to produce forage and to protect the environment.
The factors that influence the amount of photosynthesis are: (1) physiological efficiency of the species; (2) amount of carbon dioxide in the air; (3) leaf area; (4) amount and kind of light; (5) amount of water; (6) temperature and (7) soil nutrients. The manipulator can influence all of these except perhaps the amount of carbon dioxide in the air. While leaf area and amount of water made available through root absorption often receive more attention than the other factors, all are important in range management.
It was determined in a classic study that the rate of photosynthesis is between 0.8 and 1.8 grams of sugar per hour per square meter of leaf area (Miller, 1938). If leaf area is reduced, the rate of photosynthesis is correspondingly reduced, which is what happens with grazing because it is in effect defoliation. Considerable research has been conducted to determine how plants can be utilized to assure maximum livestock production and at the same time to assure healthy productive plants and it has been determined that the productivity of plants depends upon the intensity, frequency and season of defoliation.
Defoliation Intensity
Defoliation intensity refers to the amount of the current year's weight production that is cut by man or consumed by livestock. This is also called "degree of utilization" which is often expressed in percent of the produced weight consumed, such as 25, 50 or 75. Research has shown that the proper degree of utilization for most species is around 50 to 60 percent, although some species can withstand heavier degrees of use and some are mortally injured at 50 to 60 percent. The general rule, however, has led the range management technicians to adopt the slogan "take half and leave half", meaning that about one-half of the current year's production can be consumed or destroyed by animals and that the remaining half should be left for the plants in order that they might feed and maintain themselves. With most grasses, 50% use of a plant's weight is not 50% use of its ungrazed height. Normally, two-thirds use of its height is 50% use of its weight. Owing to this or similar relationships, stubble height can be used as one way for estimating degree of utilization on grasslands.
The importance of defoliation intensity on plant nutrition, health and productivity can be demonstrated mathematically. Assume that the ungrazed leaf area of a grass is one square meter, that photosynthesis functions twelve hours per day and the rate is one gram of carbohydrate per hour. This plant will produce 12 grams of carbohydrates per day. Six grams remain for the plant's needs with 50% defoliation, which is enough. Only three grams for the plant remain with 75% defoliation, which is not enough and the plant will begin to suffer from malnutrition. Only 1.2 grams will remain for the plant with 90% defoliation, which is far below its needs and, if continued, the plant will die from starvation. This is also illustrated in Figure 6. Note the effect of excessive defoliation on both leaf and root area and the microenvironment.
Defoliation Frequency
Defoliation frequency refers to the interval between defoliation intensities such as days, weeks, months or years. As a general rule, damage to the plant increases with increased frequency of defoliation. For example, a study in Mexico revealed that buffelgrass (Cenchrus ciliaris) was most productive when defoliated to a 15 cm stubble height (the basal portion of herbaceous plants remaining after the top has been harvested). However, weekly defoliation to 15 cm caused a considerable reduction in forage production compared with bi-weekly or monthly defoliation to the same stubble height (Barbaroux, 1969). Clearly, in this case, a properly stocked rotation grazing system that would result in defoliation to a 15 cm stubble height every other month would be superior in forage production, grazing capacity and livestock production than a continuous grazing system that resulted in weekly defoliation to the same stubble height.
This example refers to defoliation frequency in terms of weeks or one month. Frequency can also be thought of in terms of months and years. While excessive defoliation for several months is harmful, it is not necessarily destructive, especially if it is followed by a period of proper use and rest. The same is also true in respect to years. It is year after year of excessive defoliation that causes destruction to perennial vegetations. These are most important points and deferred rotation grazing systems have been designed around them with the objective of reducing defoliation frequency in terms of months and years.
Seasonal Defoliation
Seasonal defoliation refers to the time of defoliation in respect to a plant's physiological activities. The two most critical times in a plant's growth cycle are: (1) the season when emergence from dormancy occurs; and (2) the season when it produces seed and enters dormancy. These critical periods are related to carbohydrate production and storage which are illustrated in Figure 7. When a perennial plant enters dormancy the carbohydrates located in the leaves and stems are translocated to the roots and buds where they are placed in reserve to initiate the following year's growth. Excessive defoliation during this period reduces carbohydrate reserves which can adversely affect subsequent year's growth, and if this is repeated over a period of successive years, it will eventually result in plant death.
The same relationship is true of annual species except that their carbohydrates are reserved in seeds. Heavy defoliation during time of flowering and seed set reduces or prevents seed production, meaning that there will be less plants and forage the following year.
The most critical time in a perennial plant's growth cycle is the period when it emerges from dormancy. About 90% of the reserved carbohydrates are required to initiate new leaf and stem growth and all of the carbohydrates manufactured by the new leaves are used to make more leaves and stems. The plant rebuilds its carbohydrate reserves only when it has sufficient leaf area to manufacture the amount of carbohydrates required to carry on its normal activities (see Figure 7). Continual and excessive defoliation during this period causes serious damage to the plant's health and nutrition, thus reducing the current year's productivity. If such use continues over a succession of years, the plant will eventually die. Wilson, et al. (1966) found that three or more years of severe defoliation during spring would eliminate Agropyron spicatum, a valuable grass in the Western United States. Cook (1971) found that 50% defoliation was too severe for spring harvesting of desert shrubs and grasses and 25% defoliation was more reasonable.
It has been incorrectly postulated that 100% defoliation during the dormant season will not injure plants. Many of the herbaceous species, especially the grasses, are characterized by basal buds where carbohydrates are also stored (Figure 7) and if these buds are consumed during dormancy, a large percent of the stored carbohydrates are consumed as well. In addition to roots, shrubs also store carbohydrates in terminal and axillary buds and their consumption will adversely affect production and health. Besides, the unused mantel of vegetation is not wasted. It keeps the soil safe from washing and blowing, it soaks up rain when it falls and saves soil moisture from evaporation. And leftover vegetation sustains the microscopic life in a soil which constantly renews the natural fertility of the land.
Defoliation and Roots
The effects of defoliation upon the too often forgotten roots are of extreme importance. A healthy and productive forage plant normally requires a root system several times greater than its above ground parts. The magnitude of the root system of a single plant is often astounding. Povlychenko (1937) found that the total root length of a 3 year old crested wheat grass was over 600 km and Dittmar (1937) discovered that the combined surface of roots and root hairs of a winter rye plant was 103 times that of the above ground parts. Some shrubs have roots that penetrate up to 8 m along with lateral roots of 15 m or more in length. The amount of water available for photosynthesis depends upon the amount of soil moisture absorbed by the roots and it is axiomatic that large root systems can absorb more soil moisture than small systems. Some species have the genetical ability to produce larger root systems than others, which gives them an ecological advantage.
Studies regarding the measurable responses of roots to defoliation are limited because roots cannot be seen and measurements are difficult. It is known that a large percent of grass roots die each year, requiring replacement, although the percent varies between plants of the same species and between species. This is one reason why grasses are great soil builders. It is also known that renewal is essential to overall plant health and productivity. If the nutrients required for renewed growth are inadequate, the size of the root system dwindles which, in turn, reduces photosynthesis efficiency (see Figure 6). Obviously, root growth and function are related to defoliation intensity, frequency and season.
