Major Agroforestry Systems of the Humid Tropics

Abstract

More than one hundred agroforestry systems (particular land-use systems involving integrating production of trees with crops and/or livestock, which are characterized by the environment, plant species and their arrangement, management, and socio-economic functions) have been recorded yet, with about 30 agroforestry practices (distinct arrangements of agroforestry components in space and time). Agroforestry systems and practices are often used simultaneously. Major agroforestry practices or technologies in the humid tropics include homegardens, perennial crop based systems, shifting cultivation, alley cropping, improved fallows and rotational tree fallows. Other agroforestry systems are valued in the humid tropics, including relay cropping, multilayer tree gardens, multipurpose trees on croplands and plantation-crop combinations. Since the mid-90s, the participatory domestication of high-value and multipurpose indigenous forest species using agroforestry techniques has been gaining momentum in the humid tropics.

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1 Introduction

An agroforestry system is a particular land-use system involving integrated production of trees and crops and/or livestock, characterized by the environment, plant species and their arrangement, management, and socio-economic functions. In contrast, an agroforestry practice reflects a distinct arrangement of components in space and time. Similar practices are found in various systems under different situations. More than one hundred agroforestry systems have been identified in the tropics and temperate regions, together with about 30 agroforestry practices (Table 4.1). The distinction between agroforestry systems and practices is often unclear, and these terms are often used interchangeably, with both referring to forms of land use.

Table 4.1 Major agroforestry practices or technologies and their main characteristics (Adapted from Nair 1993). (Source: Khasa (2001))

One particular agroforestry practice that has gained momentum in the tropics since the mid-1990’s is the participatory domestication of high-value and multipurpose indigenous forest species. These species have provided local communities with food, income , medicine, and shelter (Leakey and Newton 2004; Leakey et al. 2005; Tchoundjeu et al. 2006). This practice is a form of agro-technology (a scientific term for an intervention that changes a practice or an existing system), which modifies the practice of introducing multipurpose trees and grasses on farms . The principles, rationale, and methods of agroforestry systems will be explained in Chap. 7. Several studies have been carried out since the 1980’s to understand the mechanisms underlying the functioning of existing agroforestry systems in order to fine-tune those systems. Systems that have been studied include alley cropping , improved fallows , Taungya , homegardens , windbreaks, and parklands together with cocoa , coffee and tea or rubber farms.

The spatial structure of farm compounds in forest areas in the humid tropics is as follows:

• Houses

• Homegardens

• Cash crops

• Food crops and fallows

• Forests

The most common agroforestry systems in the humid tropics include homegardens, perennial crop based systems, farm woodlots, alley cropping, improved fallows, and rotational tree fallows . Some agroforestry systems are specific to Amazonia (such as small-scale intensive farming systems, which is a form of homegarden) and to Southeast Asia (such as Taungya). Major agroforestry practices or technologies and their main characteristics are given in Table 4.1.

The aforementioned land-use systems that we have listed are described in detail in the sections that follow, starting with homegardens, which are an important component of homesteads in the tropics.

2 Homegardens

A homegarden can be defined as an intimate association of multipurpose trees and shrubs , annual or perennial plants, or livestock within the household compound, with the whole unit being managed by family labor (Fernandes and Nair 1986). Homegardens consist of an assemblage of trees, shrubs, and vines and herbaceous plants that are managed around the home compound (Fig. 4.1) by the household , and the products of which are used primarily for family consumption. This agroforestry system can also provide shade for livestock or serve ornamental purposes. Most homegardens are agrosilvopastoral systems. Kumar and Nair (2004) have suggested that homegardening is a generic concept (i.e., a group of terms), much like agroforestry itself. Homegardens are “structurally and functionally the closest mimics of natural forests yet attained” (Ewel 1999; Table 4.2).

Fig. 4.1

An example of a homegarden in Cameroon. (Source: ICRAF-Cameroon)

Table 4.2 A comparison of the ecologicalattributes of climax forests, homegardens and conventional agricultural systems. (Kumar and Nair 2004)

Several terms have been used to describe agroforestry practices that are undertaken around homes, including mixed-garden horticulture (Terra 1954), home-garden (Ramsay and Wiersum 1974), mixed-garden or house garden (Stoler 1975), Javanese homegarden (Soemarwoto et al. 1976; Soemarwoto 1987), compound farm (Lagemann 1977), kitchen garden (Brierley 1985), household garden (Vasey 1985), and homestead agroforestry (Achuthan and Streedharan 1986; Leuschner and Khalique 1987).

Numerous types of homegardens have been described (Soemarwoto et al. 1976; Lagemann 1977; Bavappa and Jacob 1982; Wiersum 1982; Michon 1983; Fernandes and Nair 1986; Fernandes et al. 1984; Okafor and Fernandes 1986; Reynor and Fownes 1991; Tchatat et al. 1995), indicating that this system is widely distributed in the tropics and has been practiced for millennia. Because the primary function of a homegarden is subsistence, they most often contain vegetables, tuber crops, medicinal plants, multipurpose plants and indigenous fruit trees. Perennial crops such as cocoa , coffee or palms are frequently found in homegardens, but these gardens lack the operational size of cash crop farms. An inventory of the structures and functions of homegardens in the tropics was conducted by Fernandes and Nair (1986). The inventory subsequently was used to classify homegardens by region, depending on biophysical and socio-economic factors, and to describe the different compositions of homegardens (Table 4.3).

Table 4.3 Floristic elements of homegardens in different regions of the world. (Kumar and Nair 2004)

Homegardens contain a wide variety of species, which approximates the range of species encountered in natural forests (Gajaseni and Gajaseni 1999). In one example, 101 plant species were identified in 31 homegardens in Cuba, with each garden containing about 18 to 24 different species (Wezel and Bender 2003; Table 4.4). Similarly, the mean number of woody taxa that are found in homegardens in India can range from 11 to 39, with greater floristic diversity present in the smaller homesteads (Kumar et al. 1994). Homegarden diversity is strongly related to its age and other specific garden characteristics, household socio-economic features, and access to planting material (Coomes and Ban 2004). The average homegarden includes about four canopy strata (Tchatat et al. 1995; Gajaseni and Gajaseni 1999; Figs. 4.2 and 4.3), and their average area is frequently less than one hectare in size (Fernandes and Nair 1986).

Fig. 4.2

Vertical physical structure of vegetation in a homegarden system of Sristachanalai, Thailand. (Gajaseni and Gajaseni 1999)

Fig. 4.3

Crown cover projection (A) and profile diagram (B) of homegarden systems in Sristachanalai, Thailand. (Gajaseni and Gajaseni 1999)

Table 4.4 Factors impacting the structure and composition of homegardens with special reference to Indonesian homegardens. (Wiersum 2006)

Homegardens are carefully structured systems. For example, in Nigeria (West Africa), homegardens have a four-strata canopy that is dominated by fruit trees (Okafor and Fernandes 1987). Another example of a homegarden is the intensive small-scale system that is described in sect. 4.1.1. The structure and composition of a homegarden will depend upon its position in the overall farming system and on the livelihood strategies of its inhabitants. Rural transformation results in changes in livelihoods and farming systems , which has further impacts on homegarden function and composition (Wiersum 2006). Factors that affect the structure and composition of homegardens are listed in Table 4.5 .

Table 4.5 Annual litter and nutrient additions through multipurpose trees in homegardens in Kerala, India. (Isaac and Achuthan 2006)

The choice of species to be included in a homegarden depends upon the products that these species provide (Gajaseni and Gajaseni 1999). The choice of species, together with their arrangement and management, can vary within a community or village (Méndez et al. 2001). Tchatat et al. (1995) have described the homegardens of lowland rainforests of Cameroon as following a floristic and structural approach as well as a socio-economic approach. Homegardens in this area consist of a front yard for ornamental plants, and a larger backyard where food crops and fruit trees are grown. Three groups of species characterized the homegardens in this area, depending on the garden’s life history and usage.

The first species group is primarily composed of maize (Zea mays L., Poaceae), which can be combined with other annual crops such as the common bean (Phaseolus vulgaris L., Fabaceae) and groundnut or peanut (Arachis hypogaea L., Fabaceae) . The second group consists of multi-annual food crops such as plantain (Musa spp.), and cassava or manioc (Manihot esculenta Crantz, Euphorbiaceae) . The third group contains mainly fruit species such as safou or African pear (Dacryodes edulis H.J. Lam; Burseraceae) , mango (Mangifera indica L., Anacardiaceae), or citrus (Rutaceae) trees, and other trees with various uses. These homegardens are mainly intended to produce food for the household , whereas Chagga homegardens in Tanzania, for example, tend to be commercial and consist mainly of arabica coffee (Coffe arabica L., Rubiaceae) and bananas (Fernandes et al. 1984). African homegardens that are intended for food production, as is the case in southeastern Nigeria, consist mainly of food crops such as yams (Dioscorea spp., Dioscoreaceae), manioc (Manihot utilissima Pohl, Euphorbiaceae), taro (Colocasia esculenta (L.) Schott, Araceae)), cocoyam or malanga (Xanthosoma sagittifolium (L.) Schott, Araceae), Musa spp., banana (Musa x paradisiaca L.), maize (Zea mays L.), okra (Albemoschus esculentus (L.) Moench = Hibiscus esculentus L., Malvaceae), squashes and pumpkin (Cucurbita pepo L., Cucurbitaceae), Thunberg’s amaranth (Amaranthus thunbergii Moq., Amaranthaceae), and Solanum spp (Solanaceae). These homegardens also harbor trees and shrubs , small ruminants, poultry, and occasionally swine that are kept in pens. Manure from the animals is used as fertilizer for the plants. An analysis of nine fertility properties indicates that the soils in these gardens are healthier than those under fallow or surrounding secondary forests (Tchatat 1996; Tchatat et al. 2004). In central Sulawesi (Indonesia), 149 crop species were recorded in 30 homegardens that had been randomly selected from three villages, and the number of vegetation layers differed depending on the age and size of the homegarden (Kehlenbeck and Maass 2004). There is minimum export of soil nutrients in homegardens (Gajaseni and Gajaseni 1999), and production of nutrients from litter is very high (Fig. 4.4, Table 4.6), which indicates that litter from a homegarden has potential as an agricultural fertilizer.

Fig. 4.4

Litterfall patterns of multipurpose trees in the homegardens of Kerala, India. (Isaac and Achuthan 2006)

Table 4.6 Average species richness and diversity indices of trees per cocoa agroforest in three sub-regions of Southern Cameroon. (Sonwa et al. 2007)

Tropical homegardens have more favorable microenvironments than the surrounding areas, with lower soil and air temperatures and higher relative humidity. They also can be very productive, in India, for example, homegardens produced enough fuelwood to meet societal demands (Kumar et al. 1994) .

2.1 Intensive Small-Scale Farming Systems

Altieri and Farrell (1984) reported on intensive small-scale farming systems that were located close to homegardens and food crop farms. Indeed, these farms are less than 1 ha in area and are located around homesteads. Common inter-cropping (maize and beans; garlic and onions, which are mixed with lettuce and cabbage; maize and potatoes) is practiced in these systems, which can include 5 to 10 tree crops and 10 to 15 annual crops. This system also contains grape arbors to provide shade, and 3 to 5 animal species (chickens, ducks, rabbits and pigs). Such small-scale intensive farming systems can be found in the densely populated Kerala State of India (Achuthan and Sreedharan 1986). These intensive systems contain livestock, poultry, fisheries, and tree and plantation crops in mixtures on the same piece of land. Food crops are mostly arrow root (Maranta arundinacea L., Marantaceae), rice (Oryza sativa L., Poaceae), cassava , Chinese potato (Coleus parviflorus Benth., Lamiaceae), taro (Colocasia spp.), elephant yam (Amorphophallus paeoniifolius (Dennst.) Nicolson, Araceae) and sweet potato (Ipomoea batatas (L.) Lam., Convolvulaceae) (Achuthan and Sreedharan 1986). Pulse crops are cultivated in these systems (for example: cowpea (Vigna unguiculata (L.) Walp., Fabaceae), pigeon pea (Cajanus cajan (L.) Millsp. Fabaceae) , mung bean (Vigna radiata (L.) R. Wilczek, Fabaceae), together with breadfruit (Artocarpus altilis (Parkinson) Fosberg, Moraceae), annona (Annona spp., Annonaceae), banana, kokum (Garcinia indica Choisy, Clusiaceae), gooseberry (Ribes uva-crispa L., Grossulariaceae), guava (Psidium guajava L., Myrtaceae), and jackfruit (Artocarpus heterophyllus Lam., Moraceae)) (Achuthan and Sreedharan 1986). Nath et al. (2005) also reported small-scale homesteading in the densely populated Chittagong Hills Tracts of Bangladesh. Homesteading uses plantings of both horticultural (A. heterophyllus, Citrus reticulata Blanco, Litchi chinensis Sonn. (Sapindaceae), Ananas comosus (L.) Merr. (Bromeliaceae), banana, guava, and mango)) and timber species (Gmelina arborea Roxb. (Lamiaceae), Tectona grandis L.f. (Lamiaceae), Albizia spp. (Fabaceae), Swietenia macrophylla King (Meliaceae), and Acacia spp. (Fabaceae)).

3 Perennial Crop Based Agroforestry Systems

Agroforestry with a cultivated tree crop component is widespread in the tropics. In the humid tropic lowlands, it consists mainly of cocoa (Theobroma cacao L., Sterculiaceae) or robusta coffee (Coffea canephora Pierre ex A.Froehner, Rubiaceae), which is grown under the shade of several trees. In 2000, cocoa agroforests covered between 300,000 and 400,000 hectares in Cameroon (Kotto-Same et al. 2000). In the tropical highlands of Africa or South America, arabica coffee- or tea- (Camellia sinensis (L.) Kuntze, Theaceae) based systems are most common. Other examples of species that are used in perennial agroforestry systems in the tropics are: oil palm (Elaeis guineensis Jacq., Arecaceae), coconut (Cocos nucifera L., Arecaceae), cashew (Anacardium occidentale L., Anacardiaceae), rubber tree (Hevea brasiliensis Müll. Arg., Euphorbiaceae), black pepper (Piper nigrum L., Piperaceae), vanilla bean (Vanilla planifolia Jacks. Ex Andrews, Orchidaceae; in Madagascar), sisal (Agave sisalana Perrine, Asparagaceae), and carnauba palm (Copernicia prunifera (Mill.) H.Moore, Arecaceae; mostly in Latin America). They are mostly cash crops, and although some of these species originate in the tropics, their culture or their introduction coincided with the colonization of the region. Cacao, vanilla, and rubber originated in Latin America; while coffee likely originated in Ethiopia, the arabica species may be indigenous to Central Africa (Lashermes et al. 1999).

Over the last several years, research has been focused on finding methods to increase food crop production, but little has been done pertaining to the introduction of trees and animals into perennial crop farms where cocoa or coffee is grown. Farmers form the bulk of perennial commodity crop producers (cocoa in Ivory Coast, Ghana, Nigeria and Cameroon; coconut in Southeast Asia). Cocoa agroforests mix cacao with other crops and tree species; most of the latter are retained during land clearing. Forest trees that are retained when clearing land for farm establishment provide shade and other environmental services . In Ondo State, Nigeria, 487 non-cocoa trees belonging to 45 species and 24 families were recorded in 21 ha of cocoa agroforests, with 86.8 % of the trees producing edible fruits (Oke and Odebiyi 2007). Each cocoa farm has a multistrata structure (Fig. 3.7), and the flora is very diverse. In the humid forest zone of Cameroon, an inventory of 60 cocoa agroforest stands revealed the presence of 206 tree species, with an average of 21 species per agroforest (Sonwa et al. 2007), thereby demonstrating the high diversity found in the forests (Table 3.8). These figures are consistent with those of Bisseleua et al. (2008), who identified a total of 102 non-cocoa trees and 260 herbaceous species in five traditional cocoa agroforests in Cameroon. Food-producing tree species tend to be more frequently planted than other tree species in cocoa farms, and two-thirds of these food trees are native forest species (Sonwa et al. 2007).

Some fast-growing food crops, such as plantain, are used for shade during cacao establishment. Other food crops like maize, sweet potato, malanga, and cucumber (Cucumis sativus L.) are often associated with cacao in the early years of its growth, to provide for the household’s food needs . This involves optimizing of airspace and soil management for the benefit of the cocoa plants. Plantains provide shade for the seedlings, and as the cocoa plants grow, the soil volume free for cocoa plant root expansion will increase as the food crops are reaped. Species commonly planted to provide shade for cocoa or coffee are typically local fruit trees (e.g., Garcinia kola Heckel (Clusiaceae), Irvingia gabonensis (Aubry-Lecomte ex O’Rorke) Baill. (Irvingiaceae), Cola acuminata Schott & Endl . (Sterculiaceae), Ricinodendron heudelotii (Baill.) Heckel, Euphorbiaceae), and Dacryodes edulis) , timber species (e.g., Baillonella toxisperma Pierre (Sapotaceae), Milicia excelsa (Welw.) C.C. Berg, Terminalia superba Engl. & Diels (Combretaceae), Cedrela spp. (Meliaceae), or multipurpose trees (e.g., Allanblackia floribunda Oliv., Clusiaceae). Cocoa is usually seeded at a density of 1,100 trees per ha, with canopy trees planted at 30 individuals per ha. Shade is beneficial for cocoa, as biomass production is higher in the shade. Isaac et al. (2007a) found that cocoa produced 41.1 Mg ha⁻¹ of standing biomass under Milicia excelsa, compared to 22.8 Mg ha⁻¹ when the plants were not shaded. The study also indicated that soil exchangeable K increased under Newbouldia, and N and P uptake increased under shade. Cocoa agroforests also have beneficial effects on the soil and litter faunal communities. Richness of the fauna is greater in the litter than in the soil (da Silva Moço et al. 2009). In Ghana, Isaac et al. (2007b) reported suppression of K uptake in cocoa foliage by inter-cropping under Terminalia superba and Newbouldia laevis (P. Beauv., Bignoniaceae). The same study revealed that intercropping has no effect on cocoa biomass production in comparison to monoculture cocoa, whereas artificial shading stimulated foliage and root production.

In the tropical highlands, arabica coffee and tea are the most common perennial crops, regardless of the size of the industrial farm, while coconut plantations are most commonly encountered in the Philippines, Indonesia, Sri Lanka, Malaysia, and the Pacific Islands. Woody perennials are grown among the coffee or tea plants for firewood or honey production (e.g., Calliandra calothyrsus Benth.) . Arabica coffee farms typically result in multi-strata agroforests . Examples of crop or livestock integration into coffee farms have been reported in Ethiopia, Colombia, and Kenya. However, in high altitudes, sensory attributes of Arabica coffee are negatively influenced by shade (Bosselmann et al. 2009). Coffee plants in mixed agroforests also have less branch growth and leaf production, and present earlier fruiting than coffee plants in monoculture systems (Campanha et al. 2004). In Costa Rica, Muschler (2001) found that, at low altitudes, an increase in shade results in an increase in fruit mass and bean size of C. arabica. He further postulated that at low altitudes, shade promotes slower and more balanced filling and uniform ripening of berries. Thus, the benefits of shade on coffee quality depend on altitude. Different shade structures can be encountered in coffee systems (Fig. 4.5) and, consequently, shade attributes and soil characteristics can vary according to the shade system (Table 4.7). Shade trees, especially indigenous trees with high leaf tannin concentrations, can also improve soil fertility in coffee systems (Teklay and Malmer 2004). Some timber species, such as silky oak (Grevillea robusta A.Cunn. ex R.Br., Proteaceae), can be inter-cropped with coffee without deleterious effects on coffee production (at densities of 26, 34 and 48 stems ha⁻¹; Baggio et al. 1997). Shade trees that are grown in coffee farms are mostly used for firewood and as timber for local construction, as indicated by an inventory carried out in the Baoulé region of Côte d’Ivoire (Table 4.8).

Fig. 4.5

The vertical structure of shade of two coffee systems in Chiapas, Mexico. a Most common species in an Inga system (IS): 1. Coffea arabica, 2. Inga latibracteata, 3. Heliocarpus appendiculatus. b Most common species in a rustic shade (RS) system: 1. Coffea arabica, 2. Inga latibracteata, 3. Heliocarpus appendiculatus, 4. Beliotia mexicana, 5. Croton draco. (Romero-Alvarado et al. 2002)

Table 4.7 Mean values for shade attributes, soil variables, coffee-shrub characteristics and yields, in an Inga-coffee system (IS) and a Rustic-shade coffee system (RS) in Chiapas, Mexico. (Romero-Alvarado et al. 2002)

Table 4.8 Tree species, according to their overall frequency in coffee and cocoa plantations. (Modified from Herzog 1994)

Perennial crop farming is often associated with pastoral activities. In such cases, animal manure is used as fertilizer. For instance, in Malaysia, poultry farming is often practiced in coffee farms (Ismail 1986).

The choice of species to be introduced into a perennial crop farm depends on the canopy diameter of the trees and the rooting volume of the perennial crop during its growth phase, light levels (if the associated species is shade-tolerant), the possible interaction between the perennial species and the associated species, the life history of the perennial species, and the value (food, commercial, medicinal) of the associated species. Other factors such as parasites that are hosted by the associated crop are also considered.

Fruit tree-based agroforestry systems are being more commonly adopted within the Congo Basin . In southern Cameroon, the majority of farmers grow safou or African pear (Dacryodes edulis) on their lands (Ayuk et al. 1999; Schreckenberg et al. 2002). Safou is a high value, indigenous fruit tree that produces a widely traded fruit. In the Makenene and Kekem areas of Cameroon, most farmers maintain orchards of safou. These orchards harbor other valuable timber and edible fruit trees, but safou is considered one of the main cash and staple food crops in these regions. Other valuable indigenous fruit trees, including Irvingia species, are common in orchards of lowland humid tropics of Africa. Agroforests that are based on bush mango (Irvingia wombolu Vermoesen = Irvingia gabonensis var. excelsa; also known as dika or ogbono) are found in the Ejagham region of Cameroon, as their seeds (dika nuts) are widely traded and consumed in the region. Fruit tree-growing strategies in the humid forest zone of Cameroon are strongly influenced by accessibility of markets to farmers (Degrande et al. 2006).

Trees with important food, medicinal, fodder , or timber values are retained when clearing land for food crop establishment in the tropics. This selective retention of high-value indigenous fruit trees is common in humid forest zones and tropical savannas. Therefore, croplands in the tropics most often harbor scattered tree species with food and commercial value. In the humid forest zone of Africa, indigenous tree species that are most frequently encountered in agricultural landscapes include Irvingia wombolu, Dacryodes edulis , Baillonella toxisperma, Garcinia kola, Ricinodendron heudelotii and Cola spp (mainly C. acuminata, C. anomala and C. nitida). In Southern African savannas, the most common indigenous fruit trees in farmlands include Adansonia digitata L. (Malvaceae), Azanza garckeana (F. Hoffm.) Exell et Hillc. (Malvaceae), Ficus spp. L. (Moraceae), Diospyros mespiliformis Hochst. Ex A. DC. (Ebeneceae), Strychnos cocculoides L. (Strychnaceae), Strychnos madagascariensis L. (Strychnaceae), Strychnos spinosa L. (Strychnaceae) and Sclerocarya birrea (A. Rich.) Hochst. (Anacardiaceae), whereas Combretum imberbe Wawra (Combretaceae), Pericopsis angolensis (Baker) Meeuwen (Fabaceae) and Swartzia madagascariensis Desv. (Fabaceae) are valued for timber (Campbell et al. 1991). Gradually, these tree-crop ecosystems are being transformed into tree-based agroforestry systems when food crops are harvested. Indeed, when the plot is left to fallow, the previous tree crop system becomes a tree fallow system, followed by a secondary forest system that is managed by farmers for the collection of non-timber or timber forest products. Such secondary forests with important fruit, nut, medicinal and timber trees are considered tree-based agroforestry systems.

Another system involving the association of annual crops and forest species during the early years of establishment of the plantation forestry is the Taungya system, the development of which was described by Nair (1993). The practice of ‘Taungya’ (from the Burmese taung meaning hill and ya meaning cultivation) originated in Myanmar (Burma) and dates back to the early 20th century (Blanford 1958; Nair 1993). As Nair (1993) noted: “Originally this term designated shifting cultivation , and was subsequently used to describe afforestation methods. The land belongs to the state, which allows farmers to cultivate their species of interest (annual crops) in the plots , while dealing with forest tree seedlings such as T. grandis”.

The agreement between government and the farmers would last two to three years, during which time the tree species would expand the canopy, soil fertility decreased, some surface soil was lost through erosion , and weeds infested the area, making the land less productive. Although wood is the ultimate product of a Taungya system, this system is an example of sequential culturing of woody plants and annual crops. The cultivation of annual crops in this system is dependent upon the availability of space and light, based on the spatial arrangement of trees. This system, which is native to southeast Asia, is basically an alternative to shifting cultivation , and is widespread throughout the tropics of Asia , Africa , and America , where it is known under different names, such as: Tumpangsari (Bahasa Indonesia) in Indonesia, Kaingining in the Philippines, Ladang (Bahasa Malaysia) in Malaysia, Chena in Sri-Lanka (Sinhalese), Khumri, Jhooming (or shifting cultivation Jhoom as practiced in northeastern India and Bangladesh), Ponam, Taila and Tuckle in India, Shamba in Kenya (meaning small farm in Swahili), Parcelero in Puerto-Rico, and Consorciacao (meaning intercropping in Portuguese) in Brazil (King 1968). Taungya systems can be classified as “partial” (participants’ interests in crop establishment are primarily economic) or “full” (a more traditional system based on the lifestyle of the farmers). Forest plantations in the Congo Basin owe their origins to Taungya . The most common crops in a Taungya system are rice (Oryza sativa) in Asia , yams (Dioscorea spp.) and bananas in Africa, and maize (Zea mays) in the Americas. The major drawback to the system is the erosion of soil during early growth of forest species, which is during the years in which food crops are grown. However, there is potential for various combinations of species to sequester carbon through tree growth, such as coffee associated with food crops (Soto-Pinto et al. 2010).

3.1 Jungle Rubbers (Rubber Agroforests)

Noble and Dirzo (1997) defined jungle rubbers as “an enhancement of traditional slash-and-burn practices in which rubber trees, fruit and occasionally timber species are planted during the garden phase. Natural regeneration occurs, leading to an ‘enriched’ secondary forest.” These perennial crop-based agroforestry systems (agroforests) are found in Southeast Asia , and produce fruits and rubber . However, rubber-based agroforestry systems are also encountered in Latin America and in the Congo Basin .

Private Dutch colonial estates introduced rubber trees (originally from Brazil, smuggled to Kew Gardens in the 18^th century, distributed to India and other British colonies in the 19^th century, and finally to Buitenzorg Botanical Gardens on Java in the early 1880’s) into agricultural landscapes in Southeast Asia in the early 1900’s, and farmers enriched their fallows with rubber trees. This cropping system became complex when farmers innovated with improved rubber farming practices and germplasm . Jungle rubbers are essentially rubber-based secondary forests, as their structure consists of a more or less closed canopy that is 20 m to 25 m in height and which is dominated by rubber trees and a dense undergrowth layer (Gouyon et al. 1993). These systems are very complex and high in terms of their biodiversity . Of the 268 plant species (other than rubber trees) that were recorded in a jungle rubber plot in Indonesia, 91 were tree species belonging to 22 families, including Anacardiaceae, Apocynaceae, Bombacaceae, Dilleniaceae, Euphorbiaceae, Fagaceae, Flacourtiaceae, Guttiferae, Lauraceae, Melastomaceae, Mimosaceae, Moraceae, Myrtaceae, Orchidaceae, Palmae, Papilionaceae, Proteaceae, Rubiaceae, Sapindaceae, Styracaceae and Theaceae (Gouyon et al. 1993). Jungle rubbers require low inputs, as tree species protect rubber from grasses.

Table 4.9 Planting periods of rubber (Hevea brasiliensis) groves on the sandy river bank of the more fertile slope and plateau soils of the Eastern side of the Tapajós river, Brazilian Amazon. (Schroth et al. 2003)

Jungle rubbers, which are also known as rubber agroforests , are not specific to Southeast Asia . These agroecosystems were also reported in Amazonia (Schroth et al. 2003). Indeed, rubber plantations date back to the early 1900’s in the Brazilian Amazon (Table 4.9); these rubber plantations, which resemble secondary forests, were planted on sandy riverbanks, or on humus or clay-rich soils. Cultivation in Amazonia is problematic though because of leaf blight (Dean 1987; Lieberei 2007).

4 Farm Woodlots

Woodlots are stands of trees that provide environmental services , including soil rehabilitation or fertilization , and wood for households , thereby replacing wood collected from off-farm stands or forests. Rotational woodlots are sequential agroforestry systems, as they involve three phases (Kwesiga et al. 2003):

• An establishment phase (trees are inter-cropped with annual food crops, i.e., maize, sorghum, millet, rice in tropical savannas or groundnut and cassava in humid forest zones);

• A tree fallow phase (no cropping);

• A cropping phase after harvest of trees.

The woody species used in rotational woodlots in the tropics of Africa include Acacia auriculiformis, A. crassicarpa, A. julifera, A. leptocarpa, A. mangium, A. polyacantha, A. nilotica, Gliricidia sepium , Leucaena leucocephala , Senna siamea and Sesbania sesban (Kwesiga et al. 2003; Nyadzi et al. 2003; Kimaro et al. 2007; Akinnifesi et al. 2008). Other examples of this successful sequential agroforesy system include the agroforestry Mampu village or the IBI clean development mechanism carbon sink project on the Bateke plateau in the Democratic Republic of the Congo (http://cdm.unfccc.int/filestorage/L/U/G/LUGSF92NSFDYLAC5AWRWQ2IO1RJTW5/PDD.pdf?t=NTh8bWxuNHl0fDAI_g_yoXMpOCpFsGDFyf07, http://www.mampu.org/historique_en.html, Bolaluembe Boliale 2009; Peltier et al. 2010). The selection of woody species to be used in rotational woodlots should be made on the basis of many criteria , including wood production and crop yield (Nyadzi et al. 2003). For instance, L. leucocephala produced more wood than the other studied tree species in a rotational woodlot experiment in Northwestern Tanzania (Table 4.10); L. leucocephala also increased the subsequent maize yield over a three-year period in the same experiment (Fig. 4.6).

Fig. 4.6

Grain yield of maize intercropped between trees during the first three years of woodlots and after clearing woodlots at Shinyanga, Tanzania. Vertical bars are the standard errors of differences (SED) between treatment means in the respective years. (Nyadzi et al. 2003)

Table 4.10 Growth and biomass of three tree species planted as woodlots at the age of seven years at Shinyanga, Tanzania. (Nyadzi et al. 2003)

5 Annual or Biennial Food Crop Farms: Slash-and-Burn Agriculture

Shifting cultivation is widespread throughout the tropics (Grandstaff 1980; Padoch and De Jong 1987). The practice is described by various terms, depending on the locality (Nair 1993 page 56). Shifting cultivation consists of growing two or three seasons of food crops on a plot of land, and then leaving the plot in fallow for several (7-10) years to restore land fertility, while other plots are being cultivated (crop rotation). Vegetation is removed by clearing and burning before food crop plantation. Crutzen and Andreae (1990) estimated that this method was being practiced by nearly 300-500 million people on 300 to 500 million ha of land in the tropics. Several crops are grown using this method in the tropics, the most important being cassava (Manihot esculenta) , yams (Dioscorea spp.), rice (Oryza sativa and O. glaberrima), maize (Zea mays), groundnuts (Arachis hypogaea) , plantains and bananas (Musa spp.), and cucumbers (Cucumis sativus L., Cucurbitaceae), all of which are primarily intended for consumption by farmers, and secondarily for sale. To this list, we could add tomatoes and other fruits (such as pineapple). Rice cultivation is predominant in Madagascar, as well as in the tropics of Asia and the Pacific Islands. Rice is also cultivated in West and Central Africa, whereas cultivation of cassava is widespread in Latin America and in the Congo Basin . Sometimes crop species are associated on farms. There is a spatio-temporal association of cassava, groundnut, and maize, which are grown at the same time on the same land in the rainforest zone of Africa. The groundnut fixes nitrogen in the soil, and has a short life cycle, allowing maize and groundnuts to be harvested after 3-4 months, while cassava is allowed to grow, and the leaves and tubers are harvested one year later. Very little shade is left intact, and previous vegetation is completely removed by the shifting cultivation. This technique is widespread in the tropics. The land previously occupied by dense vegetation, and containing large trees is cleared to make room for crops, and the wood ash from burning the cleared vegetation is used as fertilizer. However, this technique is destructive. Soil microfauna are destroyed, as is the humus. In addition, wood ash only acts as fertilizer for the first crop cycle, and a fallow period of at least 10 years is required to rebuild the original vegetation and restore soil fertility. This explains roaming techniques, whereby farmers cultivate on the same plot for 2-3 years, then move to an area that has been previously set-aside in primary or secondary forest.

Slash-and-burn agriculture is also a sequential rotation of trees and crops on the same plot. However, slash-and-burn techniques differ from Taungya because food crops are the primary objective of the system, and trees are not planted. A large number of cases (136) of slash-and burn were reported as part of a project on “alternatives to slash-and-burn”, (Fujisaka et al. 1996). These cases were classified according to country and fallow length (Tables 4.11 and 4.12). Slash-and-burn agriculture is known under different names depending on the country: Chitimene in Zambia, Tavy in Madagascar, Masole in Central Africa, Milpa in Mexico and Central America, Conuco in Venezuela, Roca in Brasil, Ladang in Indonesia, etc .

Table 4.11 Coding cases of slash-and-burn agriculture for Table 4.12. (Fujisaka et al. 1996)

Table 4.12 Classification of 107 cases of slash-and-burn agriculture from secondary data according to initial vegetative cover, user, “final” vegetative cover, and fallow length (Fujisaka et al. 1996). For coding, see Table 4.11

Fallow, a period during which a plot is left to stand between two phases of culture , is often associated with food crops. Fallow periods may extend for up to 10 years or more. Depending on the duration of the fallow period, it can be considered either as a fallow or a secondary forest if the duration is long enough. Fallow allows the rebuilding of physio-chemical soil properties related to fertility for a new crop-growing cycle. In the rainforest region of Madagascar, fallow periods decreased over 30 years (between 1969 and 1999) from between 8 and 15 years to 3 and 5 years. Fallow vegetation changed within 5-7 fallow/cropping cycles after deforestation from trees and shrubs , to herbaceous fallows with Imperata cylindrica and Aristida spp. L. (Poaceae) grasslands (Styger et al. 2007). Frequent use of fire when converting fallow lands to crop production also encourages the replacement of native species with exotic ones and favors grasses over woody species .

5.1 Alley Cropping/Intercropping Systems

Alley cropping was developed as an alternative to bush fallowing in the tropics (Kang et al. 1981). It was popularized during the 1980’s and 1990’s by ICRAF . Alley cropping is an agricultural system in which agricultural crops are grown in alleyways formed by hedgerows of various leguminous plants including trees and shrubs, and grasses. These plants are usually trees and shrubs, and can include legumes (Acacia spp., Leucaena leucocephala, Gliricidia sepium (Jacq.) Kunth ex Walp. (Fabaceae), Sesbania sesban, Sesbania grandiflora (L.) Poiret (Fabaceae), and Calliandra calothyrsus Benth. (Fabaceae)) and actinorhizal plants (Alnus nepalensis D.Don (Betulaceae), Alnus acuminata Kunth (Betulaceae), Casuarina equisetifolia L. (Casuarinaceae), Asteraceae family (e.g., Tithonia diversifolia (Hemsl.) A.Gray). The plants are pruned during a crop’s growth to prevent shade, and to reduce competition for light, nutrients , and soil moisture between crops and legumes. A distinction is sometimes made between alley farming, which involves livestock production, and alley cropping, which does not involve animals. In the intercropping system, coppicing whereby trees are cut down, allowing the stumps to regenerate for a number of years, is not allowed. Commercial trees are managed through pruning and thinning in order to obtain a good quantity and quality of wood at the rotation age (e.g. winter wheat-Paulownia in China, beans-Eucalyptus grandis in Brazil).

The principle of alley cropping encourages nitrogen fixation by the legumes or actinorhizal plants to maintain soil fertility during cultivation or to replace nutrients exported by crops, while avoiding or mitigating competition between the legumes/actinorhizal plants and crops. Pruning twigs and leaves can enrich the soil when added as mulch, or are used as a source of fuelwood for the household . The pioneers of alley cropping in sub-Saharan Africa are B.T. Kang and D.U.U. Okali (International Institute of Tropical Agriculture, Ibadan, Nigeria), Bahiru Duguma (ICRAF, Cameroon), and Freddy Kwesiga and Bashir Jama (ICRAF East and Southern Africa). The design and implementation of an alley cropping system in a given region should be determined by the following conditions:

• Identify the legume/actinorhizal plant suitable for the ecological area of interest. Cassia siamea Lamk (Syn. Senna siamea Irwin et Barneby, Fabaceae) is more appropriate than Leucaena leucocephala in a semi-arid climate (Jama-Adan et al. 1993). Similar studies found that Calliandra calothyrsus was a more appropriate choice than Leucaena leucocephala in the humid lowland forest zone of Africa , where the latter species became an invasive weed (Kanmegne and Degrande 2002).

• Determine the appropriate number of hedgerows per hectare and quantify the “loss of farm space” due to these hedges. Different arrangements are possible; the most common being 4 × 0.25 m, which gives 10,000 trees ha⁻¹, which seems quite optimal. Double rows are also possible. In that case they are spaced more widely apart. It is possible to establish in one ha 20 rows of L. leucocephala, 100 m in length, separated by 5 m (Duguma et al. 1988).

• Determine the optimal fallow-cultivation period, and the time required before the first pruning on leguminous species in the system.

• Determine the intensity and frequency of pruning (Duguma et al. 1988).

• Consider the slope in the region, as hedges are effective against erosion.

• Consider the labor intensity provided by households for the establishment and maintenance of the system.

Establishment of alley cropping is illustrated in Fig. 4.7.

Fig. 4.7

Establishment of alley cropping. a Tree establishment: Calliandra calothyrsus or calliandra is planted alongside maize (Zea mays); b After harvest of maize, calliandra is left to grow for about 2 years; c During the fallow period, beehives are placed in the calliandra tree plot; d After the fallow phase, calliandra is cut back; e The best branches of calliandra are removed and used for staking of yams, tomatoes, etc; f. After the cutting back the calliandra trees, crops are planted in the alleys (Source: ICRAF-Cameroon)

In general, legumes are initially pruned one year after the system has been implemented and crops are grown in the corridors. Leguminous trees fix nitrogen to the soil, and studies have shown that it increases soil concentrations of P, K, Ca, and Mg (Nair 1993, page 126). Alley cropping also helped to increase plant production by over 8 tons of dry matter per hectare per year (Nair 1993, page 125). The dry matter can be used as mulch to increase soil organic matter content and improve soil chemical properties (Nair 1993, page 127). Species such as C. siamea and Inga edulis can be used to control erosion and to increase soil organic matter, due to the slow rate of decomposition of their leaves. The more rapid the decomposition of the dry matter that has been used as mulch, the faster the nutrients are released into the soil. The loss of soil nutrients in alley cropping is less marked than in soils that have been planted with conventional crops (Nair 1993, page 128). Some legumes, such as Flemingia macrophylla (Willd.) Merr. (Fabaceae), have a positive effect on soil temperature and moisture conservation (Nair 1993, p. 129). Various studies have observed increasing crop production through the practice of alley cropping (Nair 1993, p. 130; Kang et al. 1989, 1990; Kang and Duguma 1985). Therefore, alley cropping appears to be an alternative solution to four problems of natural resource management in the tropics: land tenure; reduced soil fertility due to short fallows ; soil erosion; and lack of fuel wood. In addition, alley cropping provides nutrients to the soil.

In Cameroon, Calliandra calothyrsus is the shrub most commonly used in alley cropping (refer to the work of Bahiru Duguma and colleagues in 1988-1998). This species was selected after screening 10 species suitable for agroforestry common in the humid lowland rainforest of Cameroon (Duguma and Tonye 1994). Calliandra was planted in 4 × 0.25 m spacing, and then pruned down to 0.50 m height one year after implementation, with the prunings used as mulch. Early results were disappointing. Problems included poor growth of shrubs , low biomass production due to pruning being done too early, low impact on weed control, and high demand for labor (Degrande et al. 2007). Changes to the original method led to improvements; such changes include (1) delaying the first pruning until after 2 years, (2) alternating the cropping period with a year of fallow, and (3) pruning Calliandra down to only 0.05 m height. These modifications led to the evolution from the original alley cropping to a rotational tree fallow system (Kanmegne and Degrande 2002). Tests conducted on-station in Yaoundé showed that this system maintains high maize production (Table 4.12). However, performance in a farm setting was not as good as what had been observed on-station (Degrande et al. 2007), resulting in a low rate of adoption of the system by farmers (Degrande and Duguma 2000; Degrande et al. 2007). About 52 % of households involved in testing alley cropping and rotational tree fallows indicated that more than half of their soils were fertile, and respondents did not perceive the decline in fertility as a problem. About 73 % of households reported that they had enough land to meet their households’ needs (Degrande and Duguma 2000). In reality, numerous problems arose with the adoption of alley cropping and rotational tree fallow in the area, especially in terms of labor demand.

The system is labor-intensive, with high labor requirements , especially for tasks such as filling polyethylene bags for seedlings, watering plants, especially during the dry season, and pruning. In addition, erosion is not a problem in humid lowland zones of Cameroon, where there are very low slopes and many trees. Livestock production is also not popular in the region due to the presence of the tsetse fly (Glossina palpalis Wiedemann, Glossinidae), which limits the usefulness of Calliandra leaves as fodder , one of the major benefits associated with this plant. Constraints on adoption of alley cropping and tree fallows are shown in Table 4.13. Additionally, there are constraints imposed by land rights . Poor farmers, who do not have long-term tenure, are unlikely to invest in improved fallows , because benefits are obtained only after a longer period of time. Wood production for fuel is also not a constraint in the humid tropic lowlands, due to the presence of a large amount of dead wood in farmers’ fields. To overcome these difficulties, an alternative land-use system was suggested to farmers: improved fallows with legumes shrubs, such as pigeon pea (Cajanus cajan) .

Table 4.13 Fallow cropping cycle and maize grain yields (Mg ha⁻¹) on station, Yaoundé. (Degrande et al. 2007)

Table 4.14 Land-use constraints, farm conditions, and potential agroforestry solutions in West province, Cameroon. (Degrande et al. 2007)

In Southern Africa, herbs or leguminous trees like Sesbania sesban , and Tephrosia vogelii have been used to restore soil fertility (Table 4.14). A fallow of 2 to 3 years with S. sesban planted at a spacing of 0.5 × 0.5 m proved effective in the maintenance of soil fertility in Southern Africa (Kwesiga and Coe 1994). This agroforestry system accumulates more nitrogen than do herbaceous fallows . It has proven to be successful in Zambia, and the number of farmers that were reported as using this practice increased from 200 in 1994 to 3,000 in 1997 (Kwesiga et al. 1999). Sesbania sesban not only restores soil fertility, but also provides fodder for cattle and wood for fuel. In Malawi, the adoption of this practice is low (Mafongoya et al. 2007), perhaps due to problems related to land tenure, labor shortages, and a long waiting time for benefits (2–3 years). The Sesbania fallow should be performed in rich soil (Goma 2003). Other species that provide organic inputs to the soil, such as Gliricidia sepium , Leucaena leucocephala , and Calliandra calothyrsus , are better suited for alley cropping in the region (Fig. 4.10).

Alley cropping with Leucaena leucocephala and Calliandra calothyrsus at densities of 6,680 trees per ha has been implemented successfully by farmers in East Africa (Shepherd et al. 1997). Leucaena , in association with maize (six rows of maize spaced at 75 ´ 25 cm between two rows of Leucaena spaced at 4 ´ 0.5m) gave better results than Calliandra planted at the same density as Leucaena (Mugendi et al. 1999a, b) on the humid highland slopes of Mount Kenya. Leucaena has also been used for alley cropping in the wet uplands of Western Kenya (Imo and Timmer 2000). This technique seems appropriate for the African highlands (Kang 1993), where problems include erosion and lack of suitable firewood , and the agricultural system is characterized by the cultivation of cereals (mainly maize) in combination with cattle grazing.

Mulching twigs from pruning provides excellent fertilizer for tropical soils (Mureithi et al. 1994). However, mulch from Leucaena pruning did not increase production of crops in alley cropping in the semi-arid tropics of India, where the primary benefit of alley cropping was fodder production during the dry season (Singh et al. 1988). This practice increases competition for soil water between Leucaena and crops, which negatively affects crop production (Singh et al. 1989). Soil moisture availability is a very important factor to consider in the implementation of alley cropping with shrubs in semi-arid tropics. The number of prunings conducted on the legume should be limited to three times, implying that nitrogen harvest would be sufficient to achieve substantial benefits for the crop (Nair 1993). The relationship between rainfall and alley cropping is illustrated by Nair (1993, p. 136). Alley cropping studies with Faidherbia albida , in association with maize and common bean, were also tested in coastal Kenya, with inconsistent results (Jama and Getahun 1991). Alley cropping would be appropriate in the highlands of the humid tropics, where agriculture and husbandry are practiced in rural areas, but inappropriate for the lowlands.

Table 4.15 N₂ fixed on smallholder farms in Southern Africa by various legumes (Mafongoya et al. 2007)

Other soil-improving agroforestry technologies that are well-suited to situations of poverty and other demographic pressures on the land include, simultaneous inter-cropping or coppiced fallows (trees are planted in between maize, and once properly established, the trees are cut back and the biomass is incorporated into the soil), annual relay (fallow) cropping of trees (i.e., fast-growing trees or shrubs are planted after the food crop is established), and biomass transfer (cut-and-carry system requiring separate areas where shrubs and trees are planted) (de Wolf 2010).

6 Improved Fallows and Rotational Tree Fallows

Improved fallows , which are sometimes called a sequential agroforestry system (Rae et al. 1998), consist of planting selected species or retaining them from natural regeneration. Short-duration fallows are characterized by fast-growing leguminous trees or shrubs for replenishment of soil fertility to support food crop production. Medium- to long-duration fallows harbor diverse species that have been established for amelioration of degraded and abandoned lands as well as for the use of tree products (Rao et al. 1998). Improved fallows tend to attain the objectives of natural fallows in a shorter time through the choice of tree species, spacing, density, pruning, and establishment. Fallow systems overcome constraints on crop production through maintenance of soil fertility during the cropping period by recycling and conserving nutrients, restoring the soil’s physical properties, and controlling soil-borne pests and weeds (Buresh and Cooper 1999). Fallow processes for overcoming constraints to crop production that are used in the tropics were ranked by Buresh and Cooper (1999; Table 4.15). These processes can be achieved through the choice of an appropriate fallow system. Rotational tree fallow and short-rotation fallow are the most popular improved fallows in the tropics. Improved fallows should be distinguished from enriched fallows , which consist of planting certain tree species at low density into natural fallows in an effort to produce high-value products such as fruits, medicines, or high-grade timber that provide economic benefits to households during the fallow period (Brookfield and Padoch 1994). A summary of case studies of improved fallows in the tropics is illustrated in Table 4.16.

Table 4.16 A ranking of fallow processes, listed in decreasing order of importance, for overcoming constraints to crop production in the tropics. (Buresh and Cooper 1999)

Fig. 4.8

Grain yield (Mg ha⁻¹) obtained from various fallow species for ten seasons at Msekera Research Station, Zambia. (Mafongoya et al. 2007)

It is important to note that, in certain cases, alley cropping is classified within improved fallows. This is the case when trees are allowed to grow, and a fallow period occurs between crops. Alley cropping is then referred to as rotational hedgerow inter-cropping or tree-based improved fallow if the trees are not in hedges but are planted in spacing of 1 × 1 m or 2 × 2 m. In the humid lowland forest zone of Cameroon, evaluation of alley cropping revealed several difficulties encountered by farmers. The system evolved into a rotational tree fallow, following the introduction of a fallow phase of at least one year (Kanmegne and Degrande 2002) .

6.1 Improved Fallows with Herbaceous Legumes: the Case of Cajanus Cajan

Improved fallows agroforestry using Cajanus cajan shares similarities with the widespread groundnut (Arachis hypogaea, or peanut) farms in the humid tropic lowlands of Africa . A groundnut (or peanut) farm is a food crop farm where peanuts are grown in combination with cassava , maize, and a few other food crops. The primary crop consists of groundnuts , which are harvested after 3-4 months. The groundnut farm is a phase in the shifting cultivation system. The system cycle is (i) fallow or forest, (ii) farm establishment after clearing, logging , and burning, (iii) groundnut farm, in combination with cassava and maize, (iv) harvest of groundnut and maize, (v) cassava farm maintenance until the next year, (vi) harvest of cassava tubers and leaves, (vii) fallow. Sometimes, a second crop cycle is planted before the plot is left to fallow. The peanut, which is a legume, enriches the nitrogen in the soil.

Table 4.17 A summary of case studies of short-duration, improved fallows in the tropics in 1999. (Buresh and Cooper 1999)

Improved fallow with Cajanus cajan (pigeon peas) has pigeon peas planted at 1 × 0.40 m spacing, and then maize is planted between rows of pigeon peas at the same spacing. After harvesting the maize, the pigeon peas are left on the plot for a second year. The pigeon peas are harvested the next year, the residues are burned or incorporated in the soil, and food crops (e.g., cassava, maize, peanut) grown. In the third year, the cycle restarts with the cultivation of pigeon peas being inter-cropped with maize. Cajanus fallow is illustrated in Fig. 4.9. Farmers in Edo State, Nigeria, combine pigeon peas with Dioscorea, maize or cassava in their homegardens and farms, and the occurence of Cajanus/cassava combinations can go up to 35 % in farms (Table 4.17). Pigeon pea is advantageous because it does not lower crop production. There is even an increase in crop production (80 % for maize and 97 % for peanut) after a Cajanus fallow. This increase has had a positive effect of the adoption of this technology (Degrande et al. 2007). Other reasons for adoption are soil fertility improvement and weed suppression (Degrande et al. 2007). Advantages listed by farmers include the reduction of the fallow period, the availability of pigeon pea beans for consumption, the ease of clearing of a Cajanus fallow, especially for the women, the ease of planting peanuts on a plot where Cajanus had previously been cultivated, and the direct seeding of Cajanus,that requires less physical effort than alley cropping establishment (Degrande et al. 2007). In addition, the increased crop production from the practice occurs quickly, and its profitability has been demonstrated (Degrande 2001) .

Fig. 4.9

Cajanus cajan fallow, one year after the pigeon peas were planted. (Source: ICRAF-Cameroon)

Table 4.18 Frequency of crop combinations in different farming systems in Edo State, Nigeria. (Ogbe and Bamidele 2007)

In Nigeria, Cajanus fallows increased maize production by 200 % and that of groundnut by 350 % over 6 years. A Cajanus fallow, pruned at 60 cm, was also found to be suitable for livestock production in savanna zones (Agyare et al. 2002). In the same region, Cajanus fallows were found to increase maize grain yield between 0.43 and 2.39 Mg per ha in the first year after fallow, but yield decreases in the second year by 17.6–50 % (Abunyewa and Karbo 2005). The same study revealed that after two years of a fallow period, there was an increase in organic carbon in the soil, as well as an improvement of total nitrogen by 48.5 %, and CEC (Cation Exchange Capacity) by 17.8 % (Abunyewa and Karbo 2005). There are two major constraints upon the adoption of this technique: seed supply, and storage of Cajanus seeds (Degrande et al. 2007). Cajanus fallow, along with other rotational fallows, has also been found to increase soil infestation of snout beetle (weevil, Curculionidea) in maize farms in Eastern Zambia (Sileshi and Mafongoya 2003). Snout beetle is a major pest for maize production; therefore, some landowners are likely to be discouraged from adopting Cajanus fallows because of this negative factor.

Other agroforestry practices used in the tropics include shelterbelts or windbreaks (rows of trees planted around farms to protect crops, animals and soil from natural hazards), silvopasture (Alavalapati et al. 2005) and contour tree buffer strips . For more details, please refer to Nair (1993).