A large number of traditional agroforestry systems have been recognized from different parts of the world. Each is a specific local example of the association or combination of the components, characterized by the plant species and their arrangement and management, and environmental and socioeconomic factors; thus, location specificity is an important characteristic of these systems. The major practices that constitute the multitude of location-specific systems in the tropical and temperate regions are listed in Table 1. In a time sequence (temporal) pattern of
Table 1 Major agroforestry practices in the tropics and the temperate regions
Tropical agroforestry Alley cropping (hedgerow intercropping)
Multipurpose trees (MPTs) on farms and rangelands
Silvopasture: Grazing systems Cut and carry system (Protein banks)
Shaded perennial-crop systems
Shelterbelts and windbreaks Taungya
Temperate agroforestry Alley cropping
Riparian buffer strips
Fast-growing, preferably leguminous, woody species grown in crop fields; the woody species pruned periodically at low height (<1.0 m) to reduce shading of crops; the prunings applied as mulch into the alleys as a source of organic matter and nutrients, or used as animal fodder.
Intimate multistory combinations of a large number of various trees and crops in homesteads; livestock may or may not be present.
Fast-growing, preferably leguminous, woody species planted and left to grow during fallow phases between cropping years for site improvement; woody species may yield economic products.
Fruit trees and other MPTs scattered haphazardly or planted in some systematic arrangements in crop or animal production fields; trees provide fruits, fuelwood, fodder, timber, etc.
Intergrating trees in animal production systems: Cattle grazing on pasture under widely spaced or scattered trees. Stall-feeding of animals with high-quality fodder from trees grown in blocks on farms.
Growing shade-tolerant species such as cacao and coffee under or in between overstory shade-, timber-, or other commercial tree crops.
Use of trees to protect fields from wind damage, sea encroachment, floods, etc.
Growing agricultural crops during the early stages of establishment of forestry (timber) plantations.
Trees planted in single or grouped rows in herbaceous (agricultural or horticultural) crops in the wide alleys between the tree rows.
Utilizing forested areas for producing specialty crops that are sold for medicinal, ornamental, or culinary uses.
Strips of perennial vegetation (tree/shrub/grass) planted between croplands/pastures and streams, lakes, wetlands, ponds, etc.
Combining trees with forage (pasture or hay) and livestock production.
Row trees around farms and fields, planted and managed as part of crop or livestock operation to protect crops, animals, and soil from wind hazards.
component arrangement, these practices may be simultaneous (trees intercropped with crops or livestock), or sequential (trees and crops rotated in sequence). The system complexity varies considerably depending on the number and arrangement of component species ranging from relatively 'simple' two-species combinations as in tree + crop intercropping (Figures 1a and 1b) to multi-strata homegardens (Figure 2), and intensive shaded perennial systems (Figure 3) to extensive animal grazing under scattered trees such as the dehesa system of Southern Europe (Figure 4).
The nature, complexity, and objectives of agroforestry vary considerably between the tropics and the temperate region. In many parts of the tropics, climatic conditions favor longer production cycles within a year (than in the temperate regions); thus, the tropics have a large diversity of species facilitating the existence of numerous and diverse agroforestry systems. Besides, socioeconomic factors such as human population pressure, more availability of labor, smaller land-holding size, complex land tenure, and less proximity to markets often support more and diverse agroforestry systems in the tropics than in the temperate regions. In general, small family farms, subsistence food crops, and emphasis on the role of trees in improving soil quality of agricultural lands are characteristic of tropical agroforestry systems. On the other hand, environmental protection is the main motivation for agro-forestry in the industrialized nations, where monocultural production systems of agriculture and forestry have contributed to reduced biodiversity and loss of forest resources and wildlife habitat, and increased environmental hazards such as erosion, nonpoint source pollution of ground water and rivers, and greenhouse-gas emission.
The complexity of agroforestry systems is intense in the lowland humid and subhumid tropics, where the climatic conditions generally favor rapid growth of a large number of plant species. Homegardens, shaded perennial systems (or plantation-crop combinations), and multilayer tree gardens are common in regions with high human population, and less intensive systems such as taungya and shifting cultivation are common in areas with less population density. In the semiarid tropics also, the nature of agrofor-estry systems is influenced by population pressure: homegardens and multilayer tree gardens are found in the relatively wetter areas, windbreaks, and shelterbelts, and multipurpose trees on croplands are found in the drier regions. In regions of highland tropics that have favorable rainfall regimes, sloping lands and rolling topography make soil erosion an issue of major concern; consequently, soil conservation is one of the main objectives of agroforestry in these regions. Shaded perennial systems, use of woody perennials in soil conservation, improved fallows, and sil-vopastoral systems are the major forms of agroforestry in such tropical highlands. Several other specific systems also exist in the tropics, for example, apiculture with trees, aquaculture involving trees and shrubs, and woodlots of 'multipurpose trees' (defined in the next section).
Alley cropping, forest farming, riparian buffer strips, silvopasture, and windbreaks are the five major agroforestry practices recognized in North America. Other temperate agroforestry systems include ancient tree-based agriculture involving a large number of multipurpose trees such as chestnuts (Castanea spp.), oaks (Quercus spp.), carob (Ceratonia siliqua), olive (Olea europa), and figs (Ficus spp.) in the Mediterranean region. The 'dehesa' system (called 'montado' in Portugal), which involves grazing under oak trees and has strong linkages to recurrent cereal cropping in rangelands (Figure 4), is also a very old European practice.
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