|The Improvement of Tropical and Subtropical Rangelands (BOSTID)|
|Criteria for plant selection|
Revegetation for forage production is quite common either by range seeding (with or without water conservation techniques) or planting fodder shrubs and trees for fodder reserves. Sown pastures, also called "tame pastures," may be included in rangeland systems either for continuous grazing, for seasonal grazing, or for deferred standing hay to be used in periods of shortage. Range seedings are commonplace, and suitable species are well documented for use in a number of countries.
The use of rhizobial inoculants with annual or perennial legumes for agricultural areas is well known. The use of well-adapted trees that can serve both as forage and to enhance nitrogen in the soil is an obvious need. Various shrubs and trees are not only excellent sources of nitrogen but also serve larger roles in integrated land-use systems (National Research Council, 1979).
Fodder Trees and Shrubs
Fodder tree and shrub plantations probably occupy over one million hectares in the arid and semiarid zones. Some particularly useful trees and shrubs include acacias (such as Acacia saligna, A. Senegal, A. tortilis, A. albida, cacti (Opuntia ficus-indica), salt bushes (Atriplex nummularia, A. canescens, A. halimus), saksauls (Haloxylon persicum, H. aphyllum), and mesquites (such as Prosopis juliflora, P. glandulosa, P. chilensis, P. alba, P. cineraria). Some are managed as wooded grazing lands with evergreen oaks, such as Quercus rotundifolia, Q. ilex, Q. suber, and Q. lusitanica (for example, the circum-Mediterranean region and parts of California), with mesquites (India, Chile, Mexico), with olive trees (the Mediterranean Basin), and with acacias (East and West Africa).
Plantations of fodder shrubs or fodder trees may be integrated either into pastoral systems or into farming systems. They may be used as browse either for seasonal, deferred grazing or as buffer fodder reserves for periods of drought (Le Houu, 1980). They may be directly browsed, or cut and carried to livestock. Fruits, such as the pods of Acacia or Prosopis, may be collected, stored and fed, or sold as concentrated feed. Some of these plantations may be intensively managed and fertilized for high productivity, such as Leucaena leucocephala, which can produce as much as 5,000-10,000 kg of dry matter per ha per year of high-protein feed. Opuntia ficusindica plantations in Mexico, Brazil, South Africa, and North Africa may produce 5,000 15,000 kg of dry matter per ha per year under arid and semiarid conditions. Atriplex nummularia plantations may produce 2,000 5,000 kg of protein-rich dry fodder per ha per year in the arid zones of North Africa, the Near East, and South Africa with or without complementary irrigation, while 10,000 kg of dry matter per ha per year under brackish water irrigation have been obtained in Israel (Pasternak et al., 1979; Le Houu, 1986). Two-year-old Atriplex canescens plantations in southern California produced 9,189 kg per ha (Goodin and McKell, 1971). Plantations of tree lucerne (Cytisus proliferus) and tree medic (Medicago arborea) are also capable of high production of quality browse under relatively intense management systems.
Fuelwood is in short supply in many countries, particularly around towns and cities. One solution to the fuelwood problem is to plant woodlots of fast-growing species of genera such as Eucalyptis, Pinus, Populus, Casuarina, Azadirachta, Cassia, Albizia, and Gmelina. Such woodlots may prove to be excellent investments when the site and species are appropriately chosen (National Research Council, 1980).
However, particular care should be taken in establishing woodlots of fast-growing trees, as such species are frequently heavy consumers of soil nutrients, and the soils of woodlots can become seriously depleted with the loss of the nutrients contained in the exported fuelwood. Further, in many species of fast-growing trees, the greater exposed juvenile growth increases their susceptiblity to predators, disease, and desiccation. Hence, solutions to fuelwood scarcity are increasingly being sought in diversified strategies, possibly including the establishment of woodlots, fuelwood production in agroforestry systems, improved natural forest management, and more efficient fuel utilization.
Windbreaks, Shelterbelts, and Living Hedges
Although windbreaks and shelterbelts cannot alter regional climatic conditions, they do change microclimatic conditions at the ground surface by reducing wind speed and advective energy inputs (the oasis effect). Shelter plants are thus able to reduce temperature and potential evapotranspiration, which in turn reduces the water demand of crops, natural vegetation, and animals. An added benefit is the production of fuelwood and timber.
Traditionally, cut branches from thorn trees and shrubs are used for fencing and corrals. In the tropics, these structures need to be renewed frequently because of the destruction of the cut wood by termites, thus resulting in substantial losses of native trees and shrubs. The establishment of permanent live hedges composed of local thorn shrubs could help solve this problem.
Maintenance and improvement of Soil Fertility
The role of trees and shrubs, particularly legumes (such as Acacia, Prosopis, Albizia, Gliricidia, and Leucaena), in maintaining or restoring soil fertility has been documented for a number of agroforestry systems: for example, with Acacia albida in the semiarid tropics of Africa, with Prosopis cineraria in the Rajasthan Desert of India, with Leucaena leucocephala in various countries of the humid tropics, and with desert shrubs throughout the drylands of the tropics and the subtropics.
Leaf litter, combined with the shading effect of the canopy, produces a recycling of nutrients (N. P. Ca, K, Mg) brought from deep, subsurface layers to the topsoil. As a result, yields of millet under Acacia albida, Prosopis cineraria, or other legumes are up to two-and-one-half times greater than in open fields in otherwise similar conditions (Charreau and Vidal, 1965; Garcia-Moya and McKell, 1970; Mann and Sazena, 1980).
Conservation and Rehabilitation
Anti-erosion revegetation projects have been successfully carried out in various countries under extremely arid conditions. For instance, saksauls (Haloxylon persicum, N. ammodendron, and Calligonum spp.) have been used in the Near East.
Sand dunes can be stabilized by planting short shrubs, such as Buphorbia spp., perpendicular to the direction of the wind. Commonly called palisades, this method requires a great deal of labor since the plants must be closely spaced - from 10 cm intervals at the base of the dunes to as close as 2 cm intervals near the tops of dunes. Lateral rows also need to be created so that the effects of lateral sand transport will be diminished. This may result in a grid or checkerboard pattern. However, there must be enough space between shrubs so that small quantities of sand can pass through. Otherwise, the formation of dunes superimposed over the old dunes could result. Local demonstration areas might be required to determine palisade orientation, spacing, and height.
Whatever the species chosen, a program of protection and managed use must be established and maintained to ensure the longevity of the effort. Regardless of the method used, the major obstacle is always the provision of appropriate management strategies and an alternative livelihood for the populations concerned until the revegetation projects become self-supporting. Ways and means to accomplish this should be spelled out in the feasibility studies.
Revegetation projects may consider adjacent plantations that can sustain multiple uses for one or more of the reasons previously mentioned, such as using cacti (Opuntia spp.) for its fruits, fleshy pads, and joints in northern Africa; mulberry (Mows alba) for fruits and wine making, feed for livestock and wildlife, and silkworm forage; cashews (Anacardium occidentale, European filberts (Corylus avellana) and pistachio-nuts (Pistacia vera) for fruits; Eucalyptus spp. and Prosopis spp. for honey. Other uses include chemicals, gums, and tannins from species such as carob (Ceratonia siliqua) and Acacia spp., and fibers from Agave spp.