Cyrus M. Mckell
Professor of Range Science, Utah State University, Logan, Utah, USA
2.3 Soil fertility maintenance
2.5 Control of desert encroachment
2.6 Medicinal and industrial products
3. Nutritional values of fodder trees and shrubs
4. Biological information on browse species
5. Improved management practices for shrub dominated lands to assure multiple use benefits
5.1 Grazing management to maintain a favourable species balance
5.2 Establishing plants for multiple use
Fodder trees and shrubs the world over have provided many benefits to man and his animals throughout the ages. The uses made of this vegetation are traditional and often taken for granted, even to the point of resource destruction. This destruction may be due to a lack of knowledge regarding limits of plant tolerance to regulated use or to excessive exploitation resulting from over-population of livestock and people.
Destructive exploitation is something justified on the basis of a philosophical belief that man has a God-given right (Box, 1978) to have dominion over the land and animals therein. When tribal or familial rights to overuse fodder trees and shrubs for various purposes are not questioned, problems are created for future generations by narrowing the margin of safety for maintaining this renewable natural resource. Desertization of (Le Houérou, 1976), or a drastically reduced productivity of, many world areas attest to the continual abuse by man of this planet's vegetation resource.
Over the ages, however, practices have been developed that foster wise use of tree and shrub resources and do not destroy them: livestock use can be restricted to seasons when regrowth ability is not destroyed, pruning and harvesting of products can be done on a rotational basis that allows time for regeneration and return of vigour, soil fertility can be maintained by planting or retaining species that are symbionts in nitrogen fixation, and livestock can be herded to prevent their continued overuse of certain plants and simultaneously give them best possible forage choices.
Increased population pressure and cyclic droughts such as occurred in the Sahel in 1974-76 are two major factors forcing drastic changes whenever rational management of arid and semi-arid shrublands is a goal. Failure to institute the needed revisions may be the result of a low appreciation of the tremendous value that trees and shrubs offer to mankind, an inadequate knowledge of their biology and potential responsiveness to management, or an inability to apply appropriate management strategies to protect this natural resource because of economic and demographic pressures. When multiple use can be justified and managed, it should be encouraged because the additional uses help to better justify or support the principal use, which generally is livestock browsing.
Obviously, fodder shrubs and trees have been overlooked in proportion to the research effort devoted to agricultural cropland, pasture grasses and fruit tree crops. Although they are the most visible plant forms in arid lands, shrubs have been neglected in almost all scientific research (McKell, 1974) and land management policies (Le Houérou, 1972). Motivated by a desire to increase livestock forage and reduce the density of low palatablity shrubs, numerous research efforts have concentrated on methods for shrub eradication (Cook, 1958) or control (Scifres et al, 1973). Many examples of shrub control efforts have been cited in a text book on Range Improvements and Development (Vallentine, 1970). The magnitude of these efforts have inclined many students, research workers and land managers toward the biased view that most, if not all, shrubs are of low-value and only by converting shrublands to grasslands can a productive system be obtained. This approach overlooks opportunities for making multiple use of shrubs for wildlife habitat, fuel, maintaining soil fertility and soil stability, and balancing forage nutrient quality during the dormant season of grasses. An international symposium on the biology and utilization of wildland shrubs (McKell et al, 1972) attempted to overcome this bias against useful shrubs. Certainly there must be agreement that worldwide problems in energy supply and cost have implications for the way in which low-value natural resources of arid and semi-arid lands can best be managed in the future and make urgently necessary the implementation of knowledge presently available.
This paper can neither review world literature on multiple use of shrubs nor provide a state-of-the-art discussion because of the magnitude of these tasks and the short time available. Rather, its purpose is to set the stage for the discussions to follow, identify some areas where opportunities for development exist, and highlight available knowledge as well as point out deficencies in knowledge. The paper contains praise for the virtues of shrubs relative to man's needs and suggestions for improving management to realize these virtues. Inasmuch as the distinction between trees and shrubs in many cases is only one of size or form, in this paper the term shrub will include small trees as well.
Vast areas of the earth's surface sustain a vegetative cover whose shrubs and small trees provide feed for domestic livestock and wildlife. By various adaptations, shrubs survive and even thrive under environmental conditions so difficult they preclude highly productive grasses and forbs. Le Houérou (1978) pointed out that nearly one third of the world's land surface is natural grazing land and to varying degrees the shrub component of these grazing lands is an important source of animal feed.
In West Africa, Boudet (1970) pointed out the valuable role of shrubs in supplementing the low nutritional value of standing grasses during the dry season. Increasing pressures on the land could be ameliorated by planting Acacia albida and bamboo to serve also as windbreaks.
In North America, shrubs of the Great Basin desert rangelands provide 50 to 70% of the diet of sheep and 40% of cattle that graze these lands during the winter (Cook and Harris, 1968). At the Great Basin Experimental Range, Holmgren and Hutchins (1972) found this salt desert shrub range type to be an adequate "holding area" for maintenance of breeding or gestating livestock that utilized the living twigs of dormant browse plants in the winter. Our recent work (Otsyina et al, 1980) suggests proportions for interplanting palatable shrubs of high nutritional value in rangelands seeded to crested wheatgrass (Agropyron desertorum). Such interplanting would enhance late fall-early winter grazing potentials at a time when wheat grass alone is only about 25 percent adequate in digestible protein.
Forti (1971) reported that introduction of shrubs to the northwestern Negev could create a new ecosystem to provide high palatability browse but also interact with certain low-growing introduced species to increase their productivity. He also pointed out an often-recognized fact that shrubs could withstand and survive under intense grazing. Gasto (1971) reported using Atriple repanda in an artificial forage plant community structure which optimized space by stratifying levels of plant overstory and thus increase the potential productivity. A report by the US National Academy of Sciences (1979) declares that tropical legumes are important resources for the future. Information assembled on Cassia sturtii, Desmanthus virgatus, Desmodium species, Honey locust and Leucaena describes the opportunities for use and some management practices presently employed. Names of references who can be reached for specific information are listed. In addition, the report describes how Acacia and Prosopis species in several world locations are managed and used for multiple purposes. Le Houérou (1978) summarized data from various world locations indicating a high dependence of grazing animals on shrubs to satisfy their protein requirements, especially during seasonal dry periods. Without such shrub grazing in dry seasons to complement the forage available during moist seasons the entire cycle of livestock use would be in jeopardy.
Fuel from shrubs has been minimally discussed in the literature. Nevertheless, cutting or grubbing out shrubs to be burned as a fuel for cooking and heating constitutes an important use. More important, this use seriously depletes the vegetation resource. Desperation or ignorance may lead to the removal of all shrub and tree species regardless of their values if left in place. In his research on the ecology of desert shrub rangelands in Iraq, Thalen (1979) pointed out the destructive nature of fuel cutting on shrublands bordering settlements. He estimated 25 kg per day to be a family requirement for cooking and heating in cold seasons. Van Der Veen (1964) estimates that 40,000,000 Haloxylon shrubs, equal to a thousand hectares of dense shrub vegetation, are burnt yearly in Syria to prepare tea!
Woody fuel gathering and transport back to villages is a common sight in developing countries where petroleum-based fuel sources are unavailable or too expensive. Unfortunately, as pressure mounts for an increase in production of charcoal (Kabagambe, 1976) greater emphasis shifts from woodlots to the use of trees and shrubs of rangeland areas. In some areas where the shrub resource is already depleted by removal for fuel or by overgrazing such additional pressure is disastrous. Changing long-standing cutting practices such as restricting cutting to top growth so that the plants can regrow may be difficult.
Combined and unrestricted use of shrubs for both grazing and cutting for fuel is a destructive practice. Thalen (1979) pointed out that grazing alone will not destroy shrubs because livestock will move on before all vegetation has disappeared leaving the woody stems to regrow. Fuel gathering, although more restrictive in area, destroys the remaining crown at the surface level.
A more expensive alternative is to plant desirable species that may satisfy fuel as well as other needs such as browse or soil fertility maintenance. Where dense shrub growth exists removal of the less desirable species for making charcoal would at the same time release good browse and forage grass species from competition and stimulate their growth.
Energy from woody biomass is being investigated as an alternative fuel for industrial use through pyrolysis or bioconversion to a liquid or gas fuel. Van Epps et al (1980) have found that selected shrub populations growing in favourable sites of the Great Basin can produce as much as 458,000,000 Kcal/ha of energy. Shrubs are generally equal in heat value to low quality bituminous coal (7500 BTU/lb). Other indices of biomass yield from arid lands range from 10,000 kg/ha/yr for Atriplex canescens (Goodin and McKell, 1971) to 28,160 kg/ha for Prosopis chilensis in the Imperial Valley of California under irrigation (Felker et al, 1979). However, under less favourable conditions an annual yield of 4,000 kg/ha was considered a reasonable production.
A recent publication by ICRAF, the International Council for Research in Agroforestry, (Mongi and Huxley, 1979) focuses on the role of trees and shrubs in maintaining soil fertility in forest/agricultural ecosystems. Legume trees and shrubs offer a low cost alternative to maintenance of soil fertility in developing countries and areas where the expense of artificial fertilization would not be economically feasible. According to ICRAF the focus of soils research should be on studies in marginal lands, rehabilitation and protection of disturbed ecosystems, improving soil and water conservation, investigating the efficiency of traditional agroforestry practices, evaluating the efficiency of N2 fixation and ways to exploit the process, study "nutrient pumping" by deep rooted perennials, determining optimum fallow period, and developing ways to use woody perennials to stabilize agricultural systems.
Charley (1972) reviewed various aspects of how favourable soil fertility resulted from the presence of shrubs. He pointed out that soil nutrient information on arid ecosystems was both geographically and biologically diverse. Most of the soil nutrient capital is near the surface and not in the sub-soil except in relation to rooting pattern. Le Houérou (1979) gave several examples using agroforestry techinques to enhance pasture and crop production. Felker and Bandurski (1979) characterized an ideal food crop tree as one that would concurrently enhance soil nitrogen levels.
Shrubs and trees have been used in various cropping practices where they were deemed critical to the maintenance of soil nitrogen and other elements essential to plant growth. This application is limited in value to specific areas but generally advocates of high intensity agriculture have ignored any potentials for contribution of legume trees and shrubs to soil fertility.
Desert fertility is intimately tied to the shrub species that make up its vegetative cover. Garcia-Moya and McKell (1970) showed that nitrogen was concentrated in the soil under the canopy of shrubs and in shrub tissues, thus constituting "islands of fertility" in the desert ecosystem. West and Skujins (1979) reviewed sources of nitrogen in desert ecosystems and the processes by which it is cycled. Shrubs play a vital role in this cycling process. Destruction of shrubs, especially those that promote symbiotic N-fixation can cause a significant reduction in ecosystem productivity.
Numerous reports have been made of the favourable soil productivity impact of Prosopis cineraria in agropastoral systems in Pakistan. Shankar et al (1976) reported that range grass productivity was found to be nearly doubled under a P. cineraria canopy relative to other legume trees. In addition to an increase in soil nitrogen, other soil characteristics such as pH, organic matter, P205 are also more favourable under a tree of shrub cover (Charley 1972, Ebersohn and Lucas 1965, Charreau and Vidal 1965).
The Dehesa live oak woodland of the Spanish Estremadura is managed in a mixed farming system of winter cereals and cultivated fallow, which are followed by several years of pasture in which diminutive legumes add modest amounts of nitrogen. The natural populations of trees and shrubs serve as a stabilizing agent in the system although 2,000 plus years of taking off more soil nutrients in crops than are cycled back home left this system producing at a bare fraction of the original potential (Young and McKell, 1976).
Increased productivity as well as a shift in species composition from Bromus mollis to Bromus rigidus is attributed to the presence of blue oak (Quercus Douglasii) in the annual range type of California (Gordon and Sampson, 1939) whether elimination of blue oak as a range improvement practice would ultimately reduce the level of forage production is not known but may be expected based on observations from other ecosystems.
In the areas of the Sahel where annual rainfall permits marginal cropping, the increased nutritional status of the soil under canopies of Acacia albida makes possible a two and one half times better production (Wickens, 1976, quoting from Charreau and Vidal, 1965).
One of the most striking examples of potential soil nutrition enrichment by a shrub/tree species is that of Leucaena leucocephala. Reports from field trials in Hawaii indicate a benefit of 500–600 kg/ha/yr of N if the foliage harvested at 3-month intervals from this plant were mulched into the soil (U.S. National Academy of Science, 1977). It may be possible to develop drought resistant strains of this species as indicated by survival of one-metre tall plants introduced at the Kiboko Experiment Station in Kenya by an FAO project on genetic resources. However, soil nitrogen contributions under arid conditions by Leucaena are not known.
Habitat that provided suitable cover, food and behavioural activities is critical to maintaining wildlife populations (Talbot and Talbot, 1961). Many countries in Africa depend heavily on a tourist industry that is based on the viewing of unique wildlife species. Big game species and upland birds are constrained by the size and quality of habitat that is under pressure from agriculture and community encroachment. Areas degraded by over use may need to be restored (Plummer et al, 1968). Some animals have very specific habitat requirements involving certain plants and plant densities without which their populations would decline. A recent compilation of habitat requirements in the arid and semi-arid United States to guide habitat rehabilitation indicated that some animals are restricted to few vegetation types while others use a broad range of types (Institute for Land Rehabilitation, 1978). Competition between wildlife and livestock for forage and browse has caused serious problems in many areas of Africa, especially in areas close to National Game Parks. Vegetation management such as clearing shrubs and trees in a corridor around game parks to deter migration or better control of fire to increase shrub density may be a means of reducing the areas of conflict. However it is less likely that increasing shrub density would deter big game species because shrubs are generally important to their cover and escape requirements. On the other hand, continued herding livestock into bomas (night holding corrals) for protection is often a necessity in many areas where carnivorous wildlife occur (Pratt and Gwynne, 1977).
Opportunities for complementary browsing should not be overlooked. For example, eland thrive on browse (Pratt and Gwynne,1977) and, whether domesticated or not, this species could exercise some control over woody vegetation not preferred by other animals. Jensen et al (1972) have shown that grazing by sheep or cattle for a short period in the early growing season on areas reserved for wildlife can provide additional grazing units without harming browse-dominated vegetation. Studies of this type are needed when conflict for livestock use and wildlife habit-use occurs.
The International Conference on Desertization in 1977 brought to the fore some of the critical locations where desert encroachment threatens villages, roads, structures or agricultural areas with drifting sand and soil particles (Rape et al, 1976). Equally important was the discussion of physical processes and inappropriate resource management practices that accelerate the processes of desertification to marginal lands in many world locations. Shrubs can be important in the restoration of soil stability and vegetation cover of lands destabilized by improper use. Shrubs interfere with wind currents near the soil surface thereby increasing turbulence and cause wind born soil particles to be deposited. In addition, shrubs stabilize the surface soil and retard aeolian soil movement.
Plantings of saltbush and other drought and salinity-tolerant shrubs effectively stabilized an extensive area of wind erosion in Iran (Shaidee and Nickxnan, 1974). Substantial plantings of Acacia albida and other species in the Sahel hold promise for future land stability (Weber and Dulansey, 1978). The selection criteria and establishment methods described by Van Epps and McKell (1978) may provide some guidance to land stabilization practices.
Of all the multiple uses of browse species in arid lands, provision of livestock feed appears to have the highest priority because of the animal products that are subsequently produced. Wise management of livestock grazing use can facilitate additional uses.
Numerous medicinal uses are made of arid land trees and shrubs. Some properties are of documented values while others may be mythical and legendary. Krockmal (1972) touched on the subject of medicinal uses in his shrub symposium chapter. McKell et al (1968) described some medicinal and industrial uses of range plants in a symposium in Monterrey, Mexico. Some of the uses they discussed varied from gum arabic obtained from Acacia senegal and used in the cosmetics and pharmaceutical industry as adhesives and carriers, to latex obtained from Guayule (Parthenium argentium). Interest is now high in the potential for obtaining a quality lubricating oil from Jojoba (Simmondsia chinensis) which could replace scarce sperm oil from whales. Gooding and Northington (1979) include many examples of new products from arid land plant resources in their symposium proceedings. Another review of new plant products from arid lands was published by the US National Science Foundation (Theism et al, 1978) and included some interesting products from shrubs such as berries from Sheperdia argentea, latex from Guayule (Parthenium argentatum. pine nuts (seeds) from Pinus edulis, herb leaves and grain seeds from Atriplex hortensis (also known as A. patula).
Although not generally considered as a suitable building material for large structures, shrubs and small trees are used considerably by people in rural areas for houses, shelters, animal pens, and fences. Without shrubs mud-plastered walls of many structures would lack the strength provided by woody stems woven among the supporting poles. Many useful implements and household tools are made from sticks and stems from trees and shrubs. Bark and fibres taken from woody plants also find use in construction as a framework and matrix for baskets, shelters and chinking material in walls made from poles or boards.
Large shrubs and small trees offer protection from the erosive force of wind by serving as windbreaks. Even in areas where trees are less adapted the wind-breaking force of shrubs is important to prevent the destructive action of windblown sand particles (Bagnold, 1941).
Planting of windbreaks on the margins of fields, roads, livestock holding areas and settlements opens up several opportunities of multiple uses such as browse, food, and fuel. In such areas there is a good likelihood of favourable plant growth as a result of increased soil fertility associated with the deposit of fine soil particles as well as water harvested from adjacent areas where plant growth is restricted by traffic or use.
Landscaping with native shrubs can provide an improvement in natural beauty and only requires minimal maintenance. Although landscaping gets little if any attention from people under poverty conditions, the presence of multiple use shrubs around a dwelling or livestock pens can shade the structures from the direct rays of the sun and also provide food and some protection. Opuntia species serve well in these conditions (Natural Vegetation Committee, 1973).
Various parts of browse species have been chemically analyzed at different development stages. A major contribution to understanding browse chemical composition was published thirty three years ago (Imperial Agricultural Bureau, 1947). Subsequently, browse data of a wide variety have been published (National Research Council, 1959, Everist, 1969, Wilson, 1974, Soikes et al, 1970). Additionally, many reports of chemical constituents have been included in descriptions of browse species from various regions. Some examples are: Wilson (1977) on Australian trees and shrubs, Cook (1954) for the Great Basin of the US, Dougall et al (1964) for Kenya, Rose-Innes and Mabey (1964) on browse plants in Ghana, and Sokies et al (1970) for the Altiplano of Peru. Many additional references could be added to this list. Our symposium organizer, Dr. Le Houérou, has assembled an impressive array of references on chemical composition of browse with an emphasis on tropical West Africa.
Given the impressive list of publications dealing with chemical composition of browse plant tissues, an important goal is to determine the significance of such values in light of large differences in free-choice preference of animals for individual browse species and plant parts. One important problem is to determine the nutritional relevance for each livestock species of such measurements as total nitrogen, nitrogen free extract, ether extract and others. Extensive feeding and digestion trials make it possible through regression equations to determine the nutritional significance of certain types of analytical data (Harris 1970) when unique sources of feed such as browse are being considered. Clearly, much work needs to be done to determine the true nutritional value of browse species that are used at various seasons by particular animals. For example, sagebrush (Artemisia tridentata) has a relatively high energy content but not all of the indicated energy value is available to an animal because a portion of the energy-yielding base is from volatile oils which may not be easily metabolized by the animals but more importantly, their presence reduces plant palatability.
For various reasons, animals eat some shrubs in preference to others, as has been shown by Dougall and Bogdan (1958) and Dougall et al (1964). Spines, rough surfaces, stout stems, salt content, volatile oils or various other characteristics may be responsible for such palatability preferences as exercised by various types of animals. Hanks et al (1971) have shown that sagebrush plants with a high phenolic content were often the least browsed by deer in winter. Other substances and/or characteristics need to be better understood if valid selections are to be made of populations within species for multiplication to be used in seeding or planting. Intensely grazed plants, however are a good indication of high palatibility — unless animal numbers are much in excess of the carrying capacity of the range and forced grazing is occurring. Species or plants within species that can sustain intense browsing should be emphasized in grazing land plantings.
Shrubs provide their greatest benefits to grazing animals during periods of nutritional stress when the nutritional value of dormant grasses and forbs is low (Cook, 1972, Draz, 1974, Novikoff, 1977, Van Epps and McKell, 1977) or when there is little forage except shrubs available because of drought (Le Houérou, 1976, Everist, 1969). The value of browse shrubs was central in Bulletin 10, "The Use and Misuse of Trees and Shrubs as Fodder" (Imperial Agricultural Bureau, 1947). Authors of that publication recognized the widespread value of shrubs as a stability factor in grazing land productivity but suggested that areas of high potential or where bush encroachment had occurred could be made more productive by substituting a grass forage. Millions of hectares, however, are stable only if they support a combination of shrubs, small trees, and forb-grass understory and should be utilized only under a programme of ecological management. Some of the gaps in knowledge mentioned in Bulletin 10 have been filled, but the hope for improved range conditions which would include shrubs has not materialised due to drought and increased animal grazing pressure. This challenge is still with us but to meet it will require palatability evaluations and management opportunities. For example, Fort (1971) concluded that sheep preferred groups of species rather than single species. Further, he showed that the 14 shrub species he studied had excellent ability to recover after severe browsing. Progress is being made as evidenced by plantings of large blocks of land to fodder shrubs in North Africa (Ionesco, 1979), Syria (Draz, 1977), Iran (Shaidaee and Nicknam,1974), and other locations.
Many shrub species have been recognized for their values as a green feed during periods of forage deficiency as well as other uses. Le Houérou (1978) reviewed potentials of fodder trees and shrub species from a world-wide forestry perspective. Earlier, papers presented at the International Shrub Symposium outlined some of the uses and potentials, in Australia (Everest, 1972), the Mediterranean region of Africa (Le Houérou, 1972), Asia (Petrov, 1972), South America (Soriano, 1972), North America (McGinnies, 1972), and India (Dakshini, 1972). In North America, fourwing saltbush (Plummer et al, 1966), antelope bitterbrush (Guinta et a1,1978), and winterfat (Stevens et al, 1977), are highly favoured for livestock and wildlife feed, and are utilized primarily in mixed rangeland stands.
Improvement programmes for multiple use grazing lands should include plants of the highest possible biological potential. Information on shrub biology is deficient in many areas, however, and difficult to obtain from numerous worldwide sources. Computer-based data retrieval systems often fail to yield desired information because of inappropriate or inadequate designation of key words related to the nature of the research. Authors preparing key-words for their manuscripts should take care to properly identify the major points of their work and include appropriate designations of environment and use such as "arid land", and "grazing use". One desired outcome for scientists attending this symposium is their greater awareness of information sources and access to additional data about the biology of high potential shrubs. Field observations have provided many data about animal use as it occurs under certain ecological conditions. From this information we have learned to predict a potential range of adaptation and possibilities for expanded use. Thus, we can describe what species exist in an ecological niche with some degree of success and to a lesser degree what we must do to put the biological potential of specific browse species to good use(s). Because of our persistently inadequate knowledge on the biology of shrubs, however, we know very little as to how certain management practices function or how to make them more effective.
This lack of background information prompted the publication of a fact-filled applied bulletin entitled, "Underexploited tropical plants with promising economic value" (National Academy of Science, 1975) which included data on several shrubs that have multiple use possibilities.
Valuable information on shrub biology was assembled in the Proceedings of the 1971 International Shrub Symposium and in the comprehensive treatment of shrub radio-ecology and ecophysiology by Wallace and Romney (1972). The extensive reports of research from the International Biological Program (IBP) Desert Biome represent another source of basic information. Several synthesis volumes from the IBP Desert Biome project are available or in press, and provide new information on deserts and their ecological functions. The general insufficiency of data for many geographical areas and subject matter topics represents a challenge to the present symposium. Dr. Le Houérou has identified a list of knowledge gaps that should receive a high priority for investigation. Gaining knowledge in these priority areas should further assist the development of multiple use management practices.
A useful conceptual approach for researchers seeking ways to develop browse shrubs for multiple uses is to keep in sight the relationship of the specific research to the biology of the species and how derived information can be put to use. Because of the opportunities, if not the necessity, for multiple use, the implications for related applications need to be borne in mind. For example, this interrelationship is the guiding force in a new research project at Utah State University which concentrates on "ecosystem reconstruction" following drastic disturbance. In this project soil physical-chemical processes are seen as being of direct concern to the establishment of shrubs and grasses individually and in early plant community distribution patterns. Plant survival is conditioned by a host of interacting factors ranging from soil processes of moisture supply, mycorrizal inoculation, and nutrient availability to aerial factors of temperature, insect parasites, and seedling competition. Because they must plan for post disturbance land uses of watershed, livestock grazing, and wildlife habitat after achieving a stabilized hydrologic system the research team is not only given rigour but also meaning to their pursuit of the project. The intent of all browse research should include: to make the browse system work better, produce more effectively and be a stabilizing influence on the ecosystem.
Few arid and semi-arid lands used for grazing and other purposes are currently producing at optimum levels. With improved management these lands could be more productive and stable. However increased understanding of available management practices and the biology of plants and animals is needed. Undoubtedly social changes must also be initiated but these must be worked out in conformity with local customs and values (Gonzalez, 1978).
Five major questions must be addressed to ensure the optimum use of shrub-dominated grazing lands. These problems are:
1) How much intensity of use or kinds of use are possible within the capacity of plants to maintain productivity?
2) How can undesirable plants be kept from invading or increasing in areas where desired species are under intense use?
3) What measures are available to control poisonous plants?
4) How can a desired balance of species be maintained to provide optimum productivity and ecosystem stability?
4) What are the best methods of establishing or replacing desired species to meet multiple use requirements?
Capacity to sustain use depends to a great degree on the inherent biological constitution of the plants involved. Critical periods of phenological/physiological development should be understood and grazing avoided at those times. Replenishment of carbohydrate reserves must be considered in determining the intensity and timing of defoliation. George (1976) showed that continued removal of new growth of snowberry (Symphoricarpos oriophilus) depleted carbon reserves to the point that plants died even though a portion of stored carbohydrate was still present, but apparently not available for translocation from storage tissues. Various experiments in clipping browse at various times of the year have shown that regrowth can be stimulated as long as growing points are not destroyed. Willard (1972) found that the highest number of new shoots and the most favorable shoot survival was associated with a simulated management/utilization schedule resembling rest-rotation grazing.
Removal of the bark (Phloem) of cork oak (Quercus suber) is an example of timing the harvest of a product that cannot be repeated too frequently or both a low quality product and damage to the plant would occur. Lopping off shrub or tree stems for emergency feed in Australia has its limits (Everist, 1969). The frequency of severe pruning of oak trees in Spain must allow time for recovery in order to assume production of acorn crops. Proper use factors were developed for certain grasses, forbs and shrubs as a guide to grazing management and to estimating the number of animals that could be allowed to graze a rangeland unit (Stoddard et al, 1975). Such proper use factors were often developed from observation rather than experimentation but the principle remains the same. Standards must be developed and used in relation to any management plan that seeks to optimize a multiple use plan.
Control of undesirable plants is a necessity where differential use within a plant community places an extra burden on the desired species. Uncontrolled plant competition plus grazing pressure will result in an increase in the less palatable species or those that are not grazed. Control measures may be drastic such as selective herbicides or mechanical means as reviewed by Vallentine (1969), or more subtle methods such as changing the season of use or livestock. For example West et al (1979) found that in rangelands grazed by sheep in the fall the average longevities of major grasses were enhanced, whereas the longevity of three-tip sagebrush (Artemisia tripartita) was reduced. Where shrub species of low value tend to dominate their abundance can be reduced by concentrating goat use on target areas as reported by Provenza (1978) to have occurred in southern Utah in controlling blackbrush (Coleogyne ramosissima).
Control of poisonous plants can be a serious problem because of the lack of selectivity in available chemical and mechanical methods. Control by hand labour is a good possibility in areas of low cost labour but the gained security value for grazing use must be worth the control costs. Selectivity by animal preference is a possibility if based upon careful observation. For example, Solanum incanum is known as a poisonous plant to cattle in East Africa but it is palatable to goats (Kloetzii, 1978) and is thus amenable to control through management if conditions warrant. Sheep grazing desert shrublands in the Great Basin of the western USA must be carefully herded and managed so as to avoid their over-consumption of the poisonous weed Halogeton glomeratus, which contains oxalic acid. Low amounts of ingested Halogeton can be tolerated if taken in with other forage species. Chemical control of the weed is impractical because of its widespread occurrence but close relationship to Atriplex species which are also members of the family Chenopodiaceae.
Shrub-dominated grazing lands can easily shift in botanical composition to a less-productive state if proper management of some degree is not applied. Differential grazing preferences will reduce the vigour and/or numbers of palatable species and thus allow less palatable plants to exert undue competition. In a grazing study extending over 30 years on the desirable species, Holmgren and Hutchins (1972) found that plant composition shifted in response to season of use, intensity of use and type of livestock use. Grazing by sheep tended to encourage grass dominance whereas cattle grazing encouraged shrub dominance. Late winter grazing resulted in the loss of valued spring forage species. A highly palatable species, Artemisia nova which was used under heavy winter grazing, has been replaced by Chrysothamnus stenophyllus, a species of low palatability. Even within a species, the different strains-or biotypes that are more palatable will tend to decrease in proportion. Parker (1978) theorized that old shrub plants in a rangeland site were the least palatable ones within the population and represented the end product of palatability selection. He further recommended a fast-chaining as a moderate range improvement practice to uproot the old brittle plants but leave the younger, lower, pliable plants that have had less selective pressure for palatability.
Grazing management practices that reduce the selectivity of animals for a given species while leaving others free to compete with the grazed plants can help maintain a favourable species balance. Corbett (1978) allows livestock to graze for a short duration and does not return them to the same area until it has recovered. Some adaptation of a high intensity-low frequency or rest scheme may help to minimize grazing induced plant succession (McKell and Norton, 1980).
Many areas where shrubs are needed for various uses have been depleted of their woody cover. In spite of the aggressive nature of shrubs and their high adaptability to arid conditions they cannot survive excessive use. Reestablishment of an effective shrub population is thus necessary even though seedling establishment is difficult because of drought, disease and destruction by insects and small mammals. Considerable information is needed about shrub seedling biology to sustain a revegetation effort. The woody plant seed manual (USDA, 1974) provides data on seed characteristics, areas of adaptation and germination requirements on many useful shrubs and trees of North America. Such data would be helpful if available for other continents.
Addition of new shrubs to a plant community can be obtained by natural regeneration only under rare circumstances when ecological conditions are favourable. At such times the competition between perennial and ephemeral species is great and unless the favourable moisture and temperature regime is sufficient the perennial species perish.
Two alternatives to waiting for natural regeneration are seeding and transplanting (Vallentine, 1969). Seeding may be accomplished by broadcasting seed by hand or from a vehicle but is of minimal effectiveness and wastes seeds unless they are covered by soil. Seeding with a drill implement is more effective than broadcasting but is limited to relatively level topography. Plummer et al (1968) have tested and recommended 10 principles for restoring big game range. These principles included developing a rational seeding plan for suitable sites of adequate precipitation and for using a sufficient quality and quantity of adapted species in a season of highest suitability. Even with drill seeding, the chances for success can be low in areas of low and erratic rainfall. Bleak et al (1965) reported numerous failures in seeding grasses in the salt desert shrub vegetation type of the Great Basin and recommend native shrubs as an alternative to grasses. Better techniques for drill seeding are needed to make this relatively low-cost method effective for arid and semiarid lands.
Transplanting of container-grown seedlings or cuttings is a high labour alternative to drill seeding. Because older seedlings are used the hazards normally eliminating tender young seedlings are not as critical. Even so, care must be taken in propagating container-grown plants and in planting them sufficiently deep in moist soil to assure survival (Institute for Land Rehabilitation, 1979). Planting large blocks of fodder shrubs requires an organized programme of seed preparation, planting in containers, nursery or greenhouse growth of seedlings and field planting. Cooperation of a forestry nursery unit to provide field-ready seedlings as reported by Draz (1977) in Syria can help to increase chances for success. Involvement of local people in planting the seedlings gives them a sense of project participation and concern for the success of the project. Limited information or seed sources may result in a decision to plant a single species but this should be avoided if possible. A mixture of ecologically adapted species is generally more desirable than a single species — especially if that species is from a single collection or location.
Protection from use for a suitable period of time is essential to allow plants to gain an adequate root system and plant vigour. Subsequently, the intensity of use should not exceed the capacity of the planted species to sustain use. The attitudes of local people need to be considered in planning the development programme (Gonzalez, 1978). This caution would hardly seem necessary but without a management plan that prevents overuse, shrub and tree plantings cannot survive as has been seen in a number of locations. Involvement of local farmers and livestock growers has the advantage of training them in the management of shrubs and gives them an appreciation for protecting seedlings until they are ready for use (Weber and Dulansey, 1978).
Shrubs have been universally used for many purposes by grazing societies through the ages in the world's arid and semi-arid lands. Use and management practices have evolved to deal with the different animal habits and plant characteristics.
Recent efforts to improve grazing land productivity have emphasized grasses rather than shrubs and have helped increase the dominance of less palatable or even noxious shrubs while ignoring the values of shrubs except as an emergency feed.
Identified values of palatable shrubs indicate they are a highly favourable source of protein and other valuable animal nutrients. Actual nutritional values are not clearly known however because of characteristics that may limit palatability and utilization, ie. volatile oils, spines and woody "hedged" growth habit. Provision of green feed for animals in periods of low nutritional value of grasses and forbs is a major advantage offered by shrubs. Equally important is their importance in providing a drought reserve when other forage sources are in drastic short supply.
Biological information on the various shrub genera and species is inadequate to serve as a basis for developing the full potential of browse in increasing forage, stabilizing watershed areas, improving wildlife habitat, combating desertization and improving soil fertility and ecosystem functions. Particularly necessary is information on seed biology, establishment, factors influencing growth, insect and disease resistance, response to defoliation and management, reproduction and longevity.
Management practices need to be improved to optimize the advantages shrubs offer for multiple rangeland uses. Better methods for direct seeding or transplanting seedlings under stressful conditions are needed. Selective control of noxious plants and unwanted plant competitors could improve browse productivity. Improved livestock management methods are needed to enhance the use of browse in block plantings of shrubs or on manipulated rangelands. Our goals should be the sustained use of shrubs and avoidance of undesirable plant successions which would result in the loss of desired species or a regained dominance by undesirable ones.
Greatest returns to the communities that depend on shrubs for a livelihood can be realized if multiple use management is practised on a sustained yield basis.
Land managers and users should avoid destructive use of shrubs, seek ways to control degradation/ desertification and utilize shrubs for the multiple values they can provide in a rational grazing/use system in which energy inputs are minimized.
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