PROSEA, Introduction to Auxiliary plants
- 1 Definition of auxiliary plants
- 2 Role of auxiliary plants in agriculture and forestry
- 2.1 Major groups and uses
- 2.1.1 Shade and nurse trees
- 2.1.2 Cover crops
- 2.1.3 Green manures
- 2.1.4 Mulches
- 2.1.5 Fallow crops
- 2.1.6 Live fences
- 2.1.7 Wind-breaks and shelter-belts
- 2.1.8 Erosion-controlling plants
- 2.1.9 Land reclamation species
- 2.1.10 Live supports and stakes
- 2.1.11 Fuelwood (firewood and charcoal)
- 2.1.12 Water-clearing agents
- 2.1.13 Importance
- 2.2 Selection of species
- 2.1 Major groups and uses
- 3 Botany
- 3.1 Taxonomy
- 3.2 Growth and development
- 3.3 Atmospheric nitrogen fixation and mycorrhizae
- 4 Ecology
- 5 Management
- 6 Genetic resources and breeding
- 7 Prospects
- 8 Authors
Definition of auxiliary plants
Any plant that forms part of a land-use system and provides a service and/or a product that is secondary to the main outputs of a system, can be classified as an auxiliary plant. The plants covered in this volume are an odd array; what they have in common is their role in agriculture and forestry. Auxiliary plants do not deliver primary products, but assist the farmer or forester to better produce such products. They have a service role. In the terminology of the International Centre for Research in Agroforestry (ICRAF) "service functions" are the production of mulch, shade, shelter, atmospheric nitrogen fixation and erosion control, whereas "production functions" include supply of firewood, stakes, fruits, vegetables and fodder. Fuelwoods as a primary product are included here, since many fuelwoods are planted on farmland and often have a service function as well. They are the main source of energy for many households. On the other hand, the poles produced for building and fencing, such as from Gmelina arborea Roxb., are covered in the volume on Timber Trees, as timber also covers non-sawn wood.
Plants with other primary uses may also have a secondary role as auxiliary crop. However, all the species dealt with in this volume (except fuelwood) have a primary role as a service crop. Multiple uses and dual roles are often difficult to separate quantitatively, and the decision to assign a particular species to a particular commodity group is sometimes arbitrary ('t Mannetje & Jones, 1992). The commodity subgroupings made for the auxiliary plants are: shade and nurse trees, cover crops, green manures, mulches, fallow crops, live fences, wind-breaks and shelter-belts, erosion-controlling plants, land reclamation species, live supports and stakes, and fuelwood (both firewood and the woody species used for charcoal). Of course, many timber species have a secondary use as fuel. Also, many cover crops are good forage. It should be noted that some plants important as water-clearing agents are also dealt with in this volume. Furthermore, in certain production systems a species may have an auxiliary function, whereas in another it may supply primary products. Sometimes, both functions are combined, e.g. grazed cover crops in coconut plantations. Moreover, some important auxiliary plants are also among the most important forages; therefore, a few of the species dealt with in the volume on Forages are highlighted as well.
An early standard reference work on auxiliary plants in South-East Asia was published in the late 1940s (Ossewaarde & Wellensiek, 1946).
Role of auxiliary plants in agriculture and forestry
Major groups and uses
Auxiliary plants can be grouped in several ways. How the species concerned is used provides a practical way of subdivision. Auxiliary plants may also be grouped according to their habit: plants in all major habit groups - erect herbs, herbaceous and woody creepers, or climbers, shrubs and trees - may play a service role. The plant's habit is clearly of prime importance for certain service roles. Not all groups are mutually exclusive, and some plants play many roles. Table 1 gives an indication of the various functions of auxiliary plants. The 12 auxiliary functions discerned are described below.
Shade and nurse trees
Shade trees are primarily used to manipulate the levels of incoming light to the light requirements of associated crops such as cocoa, coffee or lowland tea.
The shade requirements of these perennial crops depend on environmental factors, on cultivars and management practices and, in case of lowland tea, on leaf-quality criteria. More shade is required when crops are still in a single-leaf-layer stage than when a multi-layer canopy has been formed and self-shading occurs.
Shade is sometimes also needed to protect full-grown leaves and branches against sun scorch. As a management tool shade can be used to reduce the photosynthesis-driven growth to levels which can be met by the nutrient supply from the soil. Shade trees are multifunctional. Apart from creating a favourable microclimate, they can increase the organic matter supply through leaf litter and prunings, thereby reinforcing the nutrient cycle of the crop and the cropping system. Ideally, shade trees should form a wide-spreading, horizontal, light-filtering leaf layer, well above the canopy of the crops (National Research Council, 1993). They should be deep-rooting to avoid root competition with the main crop and for uptake of nutrients from the deeper soil layers. Fast-growing and luxuriant trees should tolerate pruning and pollarding.
Commonly used shade trees include well-known agroforestry plants such as various Acacia and Albizia species, Derris microphylla (Miq.) Jackson, Erythrina spp. (E. poeppigiana (Walp.) O.F. Cook., E. subumbrans (Hassk.) Merrill), Gliricidia sepium (Jacq.) Kunth. ex Walp., Grevillea robusta A. Cunn. ex R. Br., Leucaena spp. (L. diversifolia (Schlecht.) Benth., L. leucocephala (Lamk) de Wit), and Paraserianthes falcataria (L.) Nielsen.
During establishment, crops like cocoa and coffee may need lateral shade, also referred to as temporary shade. For this purpose shrubs (Flemingia macrophylla (Willd.) Merrill and Tephrosia spp.) and trees (Leucaena leucocephala) are planted in hedges along single or double rows of crop plants. These hedges have a secondary function: the provision of mulch. This practice, originating from plantation agriculture, has been further developed in the alley-cropping model, in which annual food crops are grown in between nutrient-cycling hedges which are pruned frequently to provide mulch for the inter-row crops. Here the emphasis is on the transfer (recycling) of nutrients from the auxiliary plants to the main crop plants, and not on lateral protection. Hedges of Gliricidia sepium and Leucaena leucocephala are often used for this purpose. On-farm trials are essential to test the shade and alley trees, and to convince farmers of the advantages of the system, especially if rewards are not immediate (IITA, 1995).
In forestry, nurse trees are often planted between the main trees providing timber. Nurse trees are fast-growing and their shade not only improves the microclimate but also the shape of the timber trees and reduces weed growth.
Roadside plants include trees planted along roads, railway lines, canals, rivers and power lines. They provide shade, play a protective role in the landscape and improve comfort during travel; they are dealt with in the volume on Ornamental Plants.
Tree and plantation crops are usually planted directly at final spacing, and therefore the inter-row soil surface needs to be protected while they are establishing. Intercropping with a mixture of cover crops is a common practice to protect the soil against erosion and to prevent soil nutrients from leaching. Cover crops help to maintain the physical properties of the soil by protecting it by lowering the soil temperature and promoting life of microorganisms in the soil, which benefits soil structure. The growth of cover crops usually declines when the tree canopy closes. Sun-loving species disappear first, shade-tolerant species last. Mature oil-palm plantations contain few or no cover crops or weeds on the ground. Rubber plantation canopies allow more light to reach the soil surface (at least periodically), but a luxuriant ground cover is a disadvantage, as it interferes with the daily latex harvest. Unlike green manure crops, cover crops are not ploughed or dug in. Important cover crop species include Calopogonium mucunoides Desv., Centrosema pubescens Benth. and Pueraria phaseoloides (Roxb.) Benth.
Velvet bean (Mucuna pruriens (L.) DC. cv. group Utilis) has become a popular cover crop with farmers in West Africa. Its advantage is that it enriches exhausted soil with nitrogen in only one or two seasons, and takes up whatever phosphate is available (IITA, 1995). The presence of suitable strains of Bradyrhizobium bacteria or of vesicular-arbuscular mycorrhizae (VAM) should be verified if initial growth is slow in a particular area, as successful growth depends on atmospheric nitrogen fixation and efficient nutrient uptake. A good Mucuna crop smothers Imperata grass and shades out unwanted weeds (IITA, 1995; Versteeg & Koudokpon, 1990).
Low-growing cover crops can be grown as live mulch (IITA, 1995). Ideally, they do not compete substantially with the main crop where soil moisture is adequate during the growing season.
Green manure and cover crops are both sown or planted in situ, but the former are ploughed in at an appropriate time. All plants, whether leguminous or not, that enrich the soil with organic matter and nutrients when incorporated in the soil, provide green manure if their residues contain no allelophatic substances. Certain species are particularly useful. These include herbaceous plants which can be worked into the soil before they complete their life cycle, decompose rapidly and release nutrients which become available for the associated crops. Their ability to fix atmospheric nitrogen and their high content of this element make most legumes important green manures. Other desirable characteristics of suitable green manure crops are easy establishment and rapid production of large quantities of green biomass, and a well-developed, deep root system. They should not be host plants for diseases and pests affecting the main crops.
The main function of green manures is chemical improvement of the soil, but beneficial effects on soil biology and soil physical properties are also well documented. Because green manures consist of readily decomposable material, their effects on lowering soil temperature and conserving soil moisture, are of short duration (Tian, 1992).
Green manures include cultivated and wild annual and perennial legumes. Calopogonium mucunoides, Pueraria phaseoloides and Vigna hosei (Craib) Backer are among the most widely used green manures. Other examples of tropical green manures are cluster bean (Cyamopsis tetragonoloba (L.) DC.), cowpea (Vigna unguiculata (L.) Walp.), Crotalaria spp. and Sesbania spp. Many species have been tested in the past, but few have shown promise (Botton & Hallé, 1957, 1958; Keuchenius, 1924).
Green manure can be grown in a rotation with other crops, in a simultaneous association with the main crop (Singh, 1984) or as a relay crop. As rotation crops, they are worked into the soil before sowing the next crop. Green manures may compete with short-duration pulses. When soil moisture is adequate for a short-duration pulse, farmers will plant the latter. When water supply is only assured for less than two months, then green manures are preferred. When farmers lack resources to grow pulses over large acreages as catch crop, green manure legumes may do well (Singh, 1984).
Before being incorporated into the soil green manure may be sown directly in a given field, or grown first on the bunds or nearby wasteland to be cut and transported to the selected field later on ("cut and carry system").
Many species have been reported as being useful as green manure. Several of these just happened to grow as weeds, were ploughed in and found to provide organic matter, without further verification of whether they are better than other species. Some of them are included in this volume.
Plants used as mulches are treated differently from green manures. Their leaves and branches (green or dried) are removed and placed on the soil where physical protection of the surface soil is required, and left to decompose, sometimes eventually to be ploughed in. During decomposition organic matter and nutrients are added to the soil.
Plant material with good mulch properties is characterized by a high C/N ratio and a high lignin content (Tian, 1992). Applied to the soil it decomposes slowly and can thus provide a long-lasting soil cover.
Mulches are mainly used to protect the soil surface. Many plants can be cut or pruned, and the prunings can be spread over the soil in a layer. The benefits include a reduction in soil temperature, and a more optimal soil biology, less evaporation and erosion and prevention of mud splashing on vegetable products. Temperatures in the upper 5 cm of soil mulched with Leucaena leucocephala, Gliricidia sepium, and Flemingia macrophylla, respectively, were lowered by 2.9 °C, 4.6 °C, and 6.6 °C compared to the control (37.1 °C) unmulched soil. The soil moisture under these mulches during a period of 60 days was 7.1%, 8.7% and 9.4% compared to 4.8% observed in unmulched soil (Budelman, 1991).
Some of the worst weeds such as Chromolaena odorata (L.) R.M. King & H. Robinson make good mulch provided plants are cut prior to flowering (Slaats, 1995). Mulching with material containing seed is an effective way of introducing a major weed problem.
The introduction of the agroforestry techniques of alley farming or hedgerow intercropping has renewed interest in the potential of using prunings from fast-growing shrubs and woody species for mulch. The legumes Gliricidia sepium, Leucaena leucocephala and Senna siamea (Lamk) Irwin & Barneby are particularly interesting. Their prunings have more of the characteristics of green manure: they are succulent, rich in nitrogen, decomposing rapidly, and enriching the soil.
Auxiliary crops are used in fallow systems to improve the plant nutrient supply for subsequent crops, to improve the physical conditions of the soil, to suppress weeds (especially grasses) and/or to provide income. Fast-growing leguminous trees have several advantages as a fallow compared with the naturally regenerating vegetation. For example, Sesbania sesban (L.) Merrill is grown for one to two years as a fallow crop in maize cropping systems in southern Africa to accumulate nitrogen in the biomass, to smother weeds and to provide poles and firewood (ICRAF, 1993). However, caution should be observed in selecting planted fallow species. Plants like Sesbania sesban harbour root-knot nematodes which may be harmful to certain crops, in particular root and tuber crops.
In alley-cropping trials with a one year fallow period in which the trees are left unpruned, shading has proved to be effective in weed control. Alley cropping with Leucaena leucocephala resulted in a shift from fast-growing annual weeds characteristic for frequently cropped fields to shade-tolerant and less competitive weeds (Akobundu et al., 1995).
In areas in which weed control is an important consideration (e.g. in Imperata grasslands) Peltophorum dasyrhachis (Miquel) Kurz, with its dense umbrella-shaped canopy, appears to be promising for low-cost reclamation of weed-infested land and also as a woody component of alley cropping (van Noordwijk et al., 1992). In long-term trials in farmers' fields in Benin, West Africa, short velvet bean (Mucuna pruriens cv. group Utilis) fallows have proved to be very effective in smothering Imperata and improving soil fertility (Versteeg & Koudokpon, 1994). An interesting research finding in the 1920s and 1930s in North Sumatra, Indonesia was that a fallow crop of Mimosa diplotricha C. Wright ex Sauvalle was rather effective in reducing bacterial wilt disease caused by Pseudomonas solanacearum in a subsequent tobacco crop (Wiersum, 1983). However, the effect appeared to be only modest when the fallow period was not longer than about 8 years (Van der Laan, 1949).
Around houses, farms or fields and along roads, live fences serve the useful role of physically separating areas. These fences are sometimes managed as hedges, i.e. are pruned and pollarded. Live fences mark boundaries between properties, contain or exclude livestock, and provide shade, fuelwood, fodder, mulch material or green manure. Suitable species for live fences include Acacia spp., Casuarina spp., Leucaena spp., and Tithonia diversifolia (Hemsley) A. Gray. Ornamental species are often used in hedges near houses. These are mainly dealt with in the volume on Ornamental Plants, however. If properly established, hedges can avoid high costs of erecting non-living fences, and may prove very economical in maintenance. Their role as fodder bank may be considerable, especially during the dry season. An inedible hedge, e.g. of Euphorbia tirucalli L., provides protection from browsing livestock. Hedges should be pruned regularly, to keep them at a manageable size.
Wind-breaks and shelter-belts
Wind-breaks are strips or rows of trees and shrubs that are planted very closely together along the edges of a field or garden to protect crops and soil from the detrimental effects of wind. Sometimes they are planted on a much larger scale in semi-arid regions to protect areas against desertification. Strong, or hot and dry winds blowing from a predominant direction, are alleviated by fences planted windward to the field.
Very few species are grown solely as a wind-break. Usually this role is combined with other uses such as providing shade, fuelwood, fodder, or green manure. Live fences withstand pruning, and the prunings can be fed to livestock if palatable, or used as green manure or mulch. The species most used, combined with the latter two purposes, is probably Leucaena leucocephala. Other species commonly used for wind-breaks and shelter-belts in South-East Asia are Casuarina equisetifolia L., C. junghuhniana Miquel, Erythrina variegata L. and Gliricidia sepium. Flemingia macrophylla is a successful wind-break on steep slopes in the Philippines.
The areas rendered unpractical for cultivation for reasons of erosion, or those prone to erosion due to the slope of the terrain, can be protected by species that have good rooting properties to fix the soil. To cover waste areas a pioneering habit forming a dense ground cover is required. This may be a disadvantage as plants with these characteristics, such as Mimosa diplotricha, run the risk of becoming noxious weeds. Tephrosia purpurea (L.) Pers. is less invasive. Cyperus spp. produce many seeds and some species have long stolons that form an underground mat which holds the soil. The well-known beach plant Ipomoea pes-caprae (L.) R. Br. binds sand and helps combat wind erosion. Vetiveria zizanioides (L.) Nash, a coarse perennial grass, is now widely used in the tropics to protect contours. Harvesting its roots containing an essential oil, however, may be a major cause of erosion. Therefore, its cultivation for erosion control was prohibited in Java. Wet or dry, sandy or compacted wastelands can be seen covered with various plants, such as Cyperus spp., but this is most undesirable with nutsedge (Cyperus rotundus L.), the most notorious weed in the world (Holm et al., 1977). When the pioneer role has been fulfilled, these sedges can be replaced by other plants.
Land reclamation species
Vast areas of wastelands such as abandoned open mines and mine spoils and eroded land have been used with limited success for agriculture in South-East Asia, as they require high inputs of capital and labour for rehabilitation. Auxiliary plants have been shown to improve the properties of soils on these difficult sites. Chemically they will increase the organic carbon and nutrient contents of the soils and improve the pH. The physical properties of the soil, such as its water-holding capacity, are improved by the good soil cover and accumulation of leaf litter provided by fast-growing auxiliary plants. Furthermore, their usually dense and shallow root system make such plants suitable for stabilizing eroding land.
Acacia crassicarpa A. Cunn. ex Benth. has shown outstanding potential in land reclamation and soil improvement in a wide range of degraded sites in the sub-humid and humid tropics, as do A. auriculiformis A. Cunn. ex Benth. and A. mangium Willd. in the rehabilitation of tin tailings in Malaysia and Imperata grassland in Indonesia. These species and also A. aulacocarpa A. Cunn. ex Benth. produce numerous root nodules, survive on land low in organic matter and nitrogen where most other species fail, and are able to suppress Imperata grass, thus making them useful reclaimers.
Live supports and stakes
Many food and spice plants are climbers that need support from poles or trellis to economize space and induce flowering. Pepper (Piper nigrum L.) and betelvine (P. betle L.) cultivation is unthinkable without plant support. Erythrina spp. are good examples for live supports. Gliricidia sepium and Leucaena leucocephala are other examples of live stakes for yam cultivation in Africa (Budelman, 1991).
Maize and sorghum serve as live support for beans, but obviously that is not their primary role. Bamboo poles may be used to stake vegetables. The material used to make trellis is rarely specified.
Fuelwood (firewood and charcoal)
From many studies on forecasting the demand and supply of fuel sources it appears that fuelwood will be a necessary commodity in South-East Asia, at least for the rural economies, for many decades to come. Fuelwood plantations will play an increasingly important role in this, if forests are to be managed in a sustainable way. Crop residues, particularly from woody species (e.g. Cajanus cajan (L.) Millsp., even stems of cassava (Manihot esculenta Crantz) are becoming more valuable in the provision of firewood.
Fuelwoods are often planted in marginal areas, where they also protect the land against degradation and erosion. Those woody species that can be grown in agroforestry or forestry systems to produce firewood have a definite service role as well. Timber species from which fuelwood is obtained by pruning of branches or thinning, are not included in this volume, however, and roadside trees have been included only if their firewood or protective role are important. Mangrove species of Avicennia, Bruguiera and Rhizophora are important as fuel; because of excessive cutting, the protection afforded by mangroves to many low-lying coastal areas in the tropics is threatened. The mangrove swamps are economically important sources of crustaceans (shrimps, crabs and lobsters), are nursery grounds for many species of fish and house many sea birds and other wildlife. Conservation of the mangrove forests is urgent; the mangrove vegetation near centres of population has sometimes been entirely consumed for firewood.
Fast-growing trees for communal fuel plantations are becoming increasingly important (e.g. in Thailand, Nepal), but are not a universal panacea. The private cultivation of trees for firewood is also increasing. Species commonly used for fuelwood include Acacia auriculiformis, Calliandra calothyrsus Meissner, Casuarina equisetifolia, Eucalyptus camaldulensis Dehnh., Gliricidia sepium and Prosopis spp.
Firewood is used not only for the domestic purposes of cooking, and in hilly and mountain areas for heating, but also supplies energy for rural industries (Bhattacharya, 1986; Smiet, 1990), even when fossil fuels are available.
In Java, 90% of all fuelwood is used for domestic purposes. This island has a centuries-old tradition of agroforestry. About 3 million ha are under agroforestry, which provides fuel for more than 100 million people. Only 10% of the fuelwood is supplied by natural forest (Smiet, 1990).
In the Philippines, 60 000 ha of wood-energy plantations have been established since 1979. It is government policy to substitute wood for imported fuel for some industrial processes, in order to reserve more fuelwood for the growing population, but it is planned to generate power for rural electricity grids in 60-70 plants using firewood. Tall Leucaena leucocephala cultivars are used in 90% of the total area planted. Reported annual growth rates range from less than 20 m3 (8 t) to 90 m3 (36 t) per ha, less for coppice crops. Harvests are taken after 3-5 years (Perlack et al., 1995).
Although the importance of domestic fuel in Malaysia has declined due to the considerable increase in the standard of living, rural people still use firewood and charcoal as a source of energy. These fuels are also consumed in substantial amounts in the manufacture of bricks, roof tiles, pottery, for the curing of tobacco, and in sawmills with kiln-drying facilities. Most of the charcoal is used in steel mills. In Peninsular Malaysia alone, the average annual consumption from 1972 to 1978 was 34 008 m3 firewood and 163 569 m3 charcoal (average of 1972, 1974, 1976 and 1978) (Wong & Kader, 1980). The most popular Malaysian fuelwoods are Bruguiera spp. and Rhizophora apiculata Blume. Three large areas in Malaysia are managed sustainably for the production of charcoal and firewood: Matang (Perak), Kelang (Selangor) and South Johor. The Matang mangroves have the reputation of being the best managed in the world (Frisk, 1984).
In Vietnam, about 75% of the total energy consumed is contributed by biomass fuel. The proportion in the domestic sector is nearly 100%. This includes about 45% fuelwood and 55% residues (Ministry of Forestry, 1992).
Burma (Myanmar) also relies heavily on biomass-based fuel: about 85% of the total amount of energy is provided by firewood and charcoal. As in Thailand, bamboo provides good fuel. As the rotation cycle of bamboo is usually shorter than for other fuelwood, the use of bamboo will diminish pressure on other sources of biomass fuel (Bhattacharya, 1986; U Saw Thun Khiang, 1993).
Other more recent estimates on the share of biomass in the total energy consumption for South-East Asia are presented in Table 2, some differing from the values given above.
Some wood is particularly suitable for charcoal. Casuarina equisetifolia and C. junghuhniana, for example, are widely used in Thailand. Rhizophora mucronata Poiret produces excellent charcoal too. Charcoal is preferred in many households in South-East Asia particularly in the slightly more affluent urban ones. It has a higher energy value than plain firewood, provides higher temperatures for industrial purposes (such as for smithies), is cheaper and easier to transport and provides cleaner cooking (less smoke).
Domestic and industrial waste water may be treated by biological means: many microbial agents and water plants purify water by producing oxygen and taking up inorganic substances, even including heavy metals.
Small amounts of drinking water can be purified by adding vegetal products. These products are traditionally used in Africa and on the Indian subcontinent; their use is limited in South-East Asia. Flocculating agents or coagulants that clean water include crushed seeds of Moringa oleifera Lamk (Folkard & Sutherland, 1996), dried leaves of Strychnos colubrina L., and gum of Anacardium occidentale L. or Lannea coromandelica (Houtt.) Merrill that clarifies cane sugar or palm sugar juice e.g. in Indonesia (Jahn, 1988). Larger amounts of domestic or industrial waste water can be treated by channelling it into basins, water courses or lakes where water plants act as biological clearing agents (National Academy of Sciences, 1976). Common aquatic weeds in swamps can clean waste water. Situations where water moves slowly are advantageous for biological cleaning. Reeds (e.g. Phragmites australis (Cav.) Trin. ex Steudel), rushes (e.g. Scirpus lacustris L.), water hyacinth (Eichhornia crassipes (Martius) Solms), Nile cabbage (Pistia stratiotes L.) and submerged plants such as Ceratophyllum demersum L. take up nitrogenous and phosphorous compounds, harbour active microbial organisms and render the water safe for release to rivers or for use (or re-use) as domestic water. Anaerobic conditions should be avoided, by maintaining a partially free water surface or feeding oxygenated pretreated water to the weed-filled treatment ponds. Plants that have treated raw sewage must be disposed of safely. Water hyacinth can be fermented to methane gas.
In situ water clearing, even if not yet practised deliberately, occurs in all water courses with abundant plant life. In Belgium, dirty water from the Schelde river is treated in shallowly flooded reed beds. In the Netherlands, long ditches in which reeds grow reduce the investments and operating costs of activated sludge installations by 75% or more (de Ridder, 1996).
Water purification should only be tested with species present in the area, and never with newly introduced species, as water weeds are among the most dangerous weeds (National Academy of Sciences, 1976).
It is difficult to make sweeping statements about the actual role of auxiliary plants in terms of economic benefit, as statistical information is lacking. Only the value or tonnage of fuelwood can be estimated to a certain degree, using data from experiments (National Academy of Sciences, 1980). The fact that more than half of the world population depends on fuelwood for cooking, and that the consumption of fuelwood is still rising (FAO, 1994), illustrates the importance of this commodity.
The actual role of soil-protecting and soil-improving auxiliary plants is quantified in field experiments where the soil nutrient status and yield before and after the introduction of auxiliary plants have been measured. In some cases the effects are negative e.g. as a result of competition for resources between auxiliary and main crop plants and the overexploitation of the production function of the auxiliary plants. Possible yield losses of the main crop might, however, be compensated by these secondary products. Auxiliary plants are important in maintaining soil fertility in the plantation agriculture of South-East Asia. This effect is such that soils are still highly productive even a hundred years after the forest vegetation has been cleared, even in areas with high rainfall and sloping land. However, over-exploitation and removal of products may result in nutrient mining and degradation of the soil.
Establishing contour strips of auxiliary shrubs and trees on slopes and then cultivating along the contours is an effective technique for erosion control. In the Philippines, this concept has been developed for farmers' use in the so-called "Sloping Agricultural Land Technology" (PCARRD, 1986). As long as land resources are adequate, improved fallows permit the fallow period to be shortened without much loss of yield and so contribute to intensification of production.
Atmospheric nitrogen fixation is an prominent attribute of the important category of leguminous auxiliary species. In many cropping systems the amount of N supplied by legumes corresponds to at least 50 kg/ha per year. Generally speaking, farmers are only willing to adopt auxiliary crops if the rewards of the new practice appear substantial enough to make it worthwhile relinquishing traditional cropping practices. It is difficult to motivate farmers, particularly if the rewards are delayed, and show up in the productivity of subsequent primary crops (IITA, 1995). Participatory experimentation in these technologies as part of on-farm testing is essential, but many practical difficulties are involved (Versteeg & Koudokpon, 1994).
Selection of species
Plants having a service role as their primary use have been selected for inclusion as major species in this volume (Chapter 2); others are dealt with as minor species (Chapter 3). Mention has been made of the auxiliary role of plants in other commodity groups (Chapter 4), but only occasionally is such a species given a full treatment focusing on the service task (e.g. some forages).
A large majority of the species dealt with in this volume are Dicotyledons. A few Gymnosperms (Pinus caribaea Morelet and other spp.), which are potentially of interest for reclamation purposes, are covered in the volume on Timber Trees. The major family harbouring plants with an auxiliary role is the Leguminosae. Table 3 shows that about 56% of the species treated in this volume as auxiliary plants are Leguminosae.
The most recent taxonomic classification has been followed, in accordance with international usage. Therefore several well-known Cassia species now have to be referred to as Senna or Chamaecrista species. The former subgenera in the large genus Cassia L. sensu lato have been elevated to genera, a taxonomic decision backed not only by morphology, but also by molecular and microbial data. However, it has been decided not to follow the split in Acacia L. in this volume, although this very large genus may well merit subdivision into several genera. This new classification of Acacia has only been carried out for the Australian species (Pedley, 1986), and this drastic step has met with some opposition. The genus Racosperma C. Martius has been reinstated, and the new combinations are listed in the relevant treatments as synonyms.
Growth and development
Certain characteristics of plants with different service functions are known; for instance, unbranched, rapidly growing stems are useful for trees producing stakes, prolific branching is useful for hedges, multiple stems produce firewood easy to fell, and rooting habit is important for cover crops. Fast growth is generally required. Good regeneration and coppicing abilities are major attributes for many plants in this commodity group. However, little is known about the growth and development of most auxiliary plants. Whatever information has become available is presented in the species treatments.
Atmospheric nitrogen fixation and mycorrhizae
Atmospheric nitrogen-fixing organisms
Many of the auxiliary plants are associated with atmospheric nitrogen fixation, the reason why many cropping systems persisted to support reasonable levels of food production over the centuries without the addition of much fertilizer. Four groups of nitrogen-fixing organisms useful to plants can be distinguished (see also Table 4): root and stem-nodulating rhizobia, cyanobacteria (blue-green algae), Frankia species, and free-living nitrogen-fixing agents.
Root and stem-nodulating rhizobia
Rhizobia, the root-nodulating (Bradyrhizobium and Rhizobium spp.) and the stem and root-nodulating (Azorhizobium caulinodans) bacteria, that form symbiotic associations with Leguminosae and Ulmaceae, are the main group of nitrogen-fixing species. In waterlogged conditions the associations between Aeschynomene spp. and Bradyrhizobium and between Sesbania rostrata Bremek. & Oberm. and Azorhizobium caulinodans and Bradyrhizobium are particularly interesting. Classification of these bacteria has made great strides in the last decade (Holt et al., 1994; Sprent, 1994; Sprent & Sprent, 1990; Woese, 1987).
Sole crops of grain legumes in the tropics may fix from a few kg up to 200 kg N/ha annually in widely different cropping systems (Giller & Wilson, 1991). Trees and shrubs may fix up to about 270 kg N/ha per year, but here assessment of restricted root systems may not reveal the entire picture. Densely-planted saplings of Leucaena leucocephala may accumulate 500-600 kg N/ha in the aboveground mass annually, but these amounts are not due to nitrogen fixation alone. It should be recalled that it is notoriously difficult to quantitatively record and standardize atmospheric nitrogen fixation. Annual or short-lived perennial legumes are important for their role in rotations if crop residues are left in or returned to the field, or at least the root mass is left behind, or when these species are deliberately grown as green manure. In agroforestry the benefits are usually from leaf litter and ecological protection (shade, wind-break) but they may be offset by competition for light, water and nutrients. In that case, pruning treatments are called for (see 1.5.3).
Tropical pasture legumes have proven their value for nitrogen fixation, carbon storage and animal production ('t Mannetje, 1997). The use of nitrogen-fixing legumes is well established in Australia and there is reasonable adoption in parts of South-East Asia and Latin America. However, many countries, particularly in Africa, have problems related to land tenure, infrastructure and social justice that need to be solved first before improvements in agricultural production can take place.
Symbiotic nitrogen fixation by legumes is also important for phosphate nutrition. As a result of the availability of symbiotically fixed nitrogen, the plants will absorb more cationic than anionic nutrients. This uptake pattern causes the vicinity of the absorbing roots to acidify, and "unavailable" soil phosphates and added rock phosphates may partially solubilize. As a result, leguminous plants are more efficient in phosphate uptake than, for example, cereals (Aguilar Santelises, 1981). Auxiliary leguminous plants can thus indirectly contribute to the phosphate nutrition of the main crop through leaf litter decomposition. There is also experimental evidence that P absorption of legumes can be enhanced further when vesicular-arbuscular mycorrhizae are active in addition to rhizobium bacteria (Aguilar Santelises, 1981).
Cyanobacteria (blue-green algae)
The second most important group of atmospheric nitrogen fixers covers the cyanobacteria (the blue-green algae), both the free-living ones and those associated with plant species, notably the aquatic fern Azolla, that harbours Anabaena azollae (Whitton, 1993). Free-living cyanobacteria can be cultivated and added to soils, the so-called algalization of rice paddies. In India, some two million ha are treated with mixtures of Anabaena, Aulosira, Nostoc, Plectonema, Scytonema and Tolypothrix. The inoculum is prepared in shallow open-air trays in which farm soil, superphosphate, starter inoculum and some insecticide are mixed. Lime is added to adjust the soil pH to 7.0-7.5. The cyanobacterial mat develops within 20 days and is then allowed to dry, producing flakes that can be harvested to inoculate the rice fields, typically about one week after transplantation of the seedlings. A tray of 1.6 m2 provides sufficient inoculum for 1 ha. Preliminary results from Taiwan and Japan have not been as promising as in India, suggesting that prospects appear to be better in warmer regions. Local inocula may be more suitable than those introduced from other regions. The regional specificity of cyanobacterial strains needs to be verified by research (Whitton, 1993).
Of the six species of the microphyllous ferns, the only Azolla species used for agricultural purposes to date is A. pinnata R. Br. It is currently used in Thailand, North Vietnam, and China. In rice paddies, Azolla can accumulate 25-170 kg N/ha annually (Kikuchi et al., 1984), an average of 30 kg N per rice crop (Watanabe et al., 1977). Azolla requires particular conditions, which often restrict its applicability to certain areas. Permanent ponds are needed to maintain inocula, and high temperatures (30 °C is optimum) and light intensities are generally not tolerated. Under proper conditions, Azolla spp. multiply rapidly: the population may double in two up to ten days in the field.
Azolla-Anabaena combinations suitable for tropical conditions have to be selected; in China Azolla-rice cultivation occurs in 2% of the total 34 million ha used for rice. Competition with free-living bacteria, grazing and increased disease incidence under high temperatures are explanations for suboptimal performance. Rice cultivars less demanding of nitrogen fertilizer are often successfully grown with Azolla alone without fertilizer input (Whitton, 1993).
In the Philippines, a Bangkok strain of Azolla pinnata has been tested in the South Cotabato area and found to be successful: analysis after some years showed that up to 50 kg of fertilizer per ha was saved, and less labour was required to apply Azolla than to apply fertilizer (Watanabe et al., 1977). Azolla will be dealt with in the volume on Cryptogams.
A third important group in nitrogen fixation comprises Frankia species (Actinomycetes) grown in association with several woody species of some families like the Casuarinaceae, Myricaceae and the Rhamnaceae, which include perennials particularly important in agroforestry. This group has attracted interest relatively recently, and research in this field is increasing. Young Casuarina trees in symbiosis with Frankia have been found to fix 40-60 kg N/ha annually, but lower figures are also occasionally reported.
Free-living nitrogen-fixing agents
It is extremely difficult, if not impossible, to estimate the amounts of atmospheric nitrogen fixed by free-living N-fixing agents such as Acetobacter and Azospirillum present in the rhizosphere of plants. The amounts do not seem to surpass 5 kg N/ha per year; this is mainly observed in grassland.
For a review of the last hundred years of atmospheric nitrogen fixation research, see Bothe et al. (1988).
The specific mycorrhizal associations of auxiliary crops are still largely unknown. Potentially, mycorrhizae can increase the efficiency of nutrient uptake of the auxiliary host plants. An interesting observation is that the P uptake of leguminous plants growing on soils with low P levels or on soils to which barely soluble rock phosphate has been applied can be improved by vesicular-arbuscular mycorrhizae (Aguilar Santelises, 1981). This implies that the often yield-limiting plant nutrient phosphate can be given in the cheap form of rock phosphate, and that when auxiliary crops are used, the crop-mycorrhiza association will not deplete assimilates at the expense of the main crop. For a recent review of the mycorrhizal associations in agriculture, forestry and horticulture, see Mitchel (1993). The finding that most indigenous trees in the tropical rain forest have vesicular-arbuscular mycorrhizae suggests that research on mycorrhizal associations in auxiliary tropical tree species is worthwhile.
Climatic and soil conditions are important not only for the growth and development of the auxiliary crop in question, but also for the feasibility of its specific service function. The situations in which auxiliary plants are used vary widely, and hence the microclimate also differs accordingly. Auxiliary plants may influence microclimate by moderating the climatic factors, when grown as hedges, wind-breaks, shade trees and cover crops.
An overview of the ecological zones in which the most important auxiliary plants are grown is presented in Table 1.
The degree and type of interaction depend on the proximity of auxiliary plants and main crops in time and space. Sequential and simultaneous associations of auxiliary plants and main crop plants can be distinguished (Sanchez, 1995).
In sequential associations the growth of auxiliary plants and main crops peaks at different times. In this case auxiliary plants usually enhance the growth of the main crop by improving the soil conditions. Herbaceous plants, shrubs and trees established as an improved fallow accumulate nutrients (and in the case of some leguminous species may also fix atmospheric nitrogen) which are released to subsequent crops through clearing and burning and the decomposition of non-burnt material.
Important attributes of fallow crops are easy establishment, rapid soil coverage, vigorous growth, deep rooting, no pest problems and, preferably, atmospheric nitrogen fixation. Easy removal and limited or no regrowth as a weed are also desired.
Improved fallows are only a relevant option if farmers own their land or have secure land tenure and can afford to stop cropping on part of it. Compared to natural fallows, improved fallows should either allow the fallow period to be shortened without reducing subsequent crop yields, or result in higher yields when fallowing for the same period. Compared to continuous cropping, yields after fallowing should be high enough to more than compensate for the yields forgone in the non-cropping period and for the costs of establishing and clearing the fallow vegetation. Improved fallows are especially attractive for farmers if they produce valuable products. However, appreciable removal of these products may interfere with the nutrient accumulation in the vegetation and topsoil.
The use of Leucaena leucocephala by farmers in the Philippines is an example of an improved fallow with auxiliary plants. This leguminous tree enables fallow periods to be reduced from 6-8 to 2-4 years without depressing subsequent yields of crops like rice and maize (MacDicken, 1991).
A transition between sequential and simultaneous associations is the use of cover crops in oil palm and rubber plantations. Cover crops are usually established 1-2 years before the main crop is planted. A few years later, by the time that competition for water and nutrients is becoming a serious problem, the cover plants phase out because of lack of light. The 4-5 year cover crop period, however, is long enough to enrich the soil with nitrogen, to recycle substantial amounts of nutrients for later use by the plantation crop and to provide favourable conditions for root development. In Malaysia, the combined action of these factors has been found to have a long-lasting positive effect on rubber yields (Broughton, 1977).
Another transitional type of association occurs when during the cropping season annual crops are interplanted with auxiliary fallow plants which are removed at the beginning of the next cropping season the following year. Research in Zambia (Sanchez, 1995) has demonstrated that relay intercropping maize with Sesbania sesban gives yields double those obtained when maize is the sole crop, whether or not fertilizer is applied. In this system Sesbania sesban reseeds itself. Initially, it grows slowly and hardly competes with the maize plants but later it grows fast and ultimately yields about 1.8 t fuelwood per ha annually. A two-year Sesbania fallow gave double maize yields over a six-year period in comparison with continuous maize production without fertilizers.
In south-western Ivory Coast, similar use is made of Chromolaena odorata, which has replaced the natural forest fallow vegetation on many sites. After a 2-4-year-old Chromolaena vegetation is slashed and burnt, one crop of maize is grown, while Chromolaena seedlings and sprouts from Chromolaena stumps quickly re-establish the new fallow vegetation (Slaats, 1995).
It can be concluded that in sequential associations the interactions between the components of the system are mainly positive.
In all simultaneous associations, sharing of space and of the resources light, nutrients and water occurs. If one or more of these resources is in short supply, species compete unless they can occupy a different part of the same niche. This niche differentiation occurs, for example, in combinations of shade-tolerant crop plants and taller auxiliary plants. In this case, sharing of light is, in principle, beneficial for the output of the system, provided that nutrients and water are not limiting. The effect of shade on companion crops is very complex. It involves the reduction in light intensity, temperature and air movement and it affects relative humidity and soil moisture. Reduction in light is a very important effect, as radiation is one of the main factors governing photosynthesis. In a crop like cocoa in which the photosynthetic rate of individual leaves declines at light intensitites greater than 30% full sunlight, shade is needed, especially when young trees still have a single layer of leaves. The light requirements increase as trees start to develop a closed canopy with several layers of leaves. With higher light intensities the demand for nutrients also increases. This relationship between light and nutrition also means that shade can be used to balance nutrient demand and supply on less fertile soils. In addition to improving microclimate and light utilization, shade trees can contribute to improved nutrient cycling and to the supply of organic matter and nitrogen (see 1.4.2). In a crop like cocoa, where the presence of shade trees generally improves the output of the system, the interaction between the components can be described as complementary.
Interactions in associations with species that do not tolerate shade have especially been investigated in alley cropping. In these agroforestry systems, food crops are grown between hedges of nutrient-cycling trees or shrubs which are periodically pruned during the cropping season to reduce shading and to provide green manure or mulch for the food crops. Fast-growing leguminous auxiliary species such as Gliricidia sepium and Leuceana leucocephala are often tested in experiments, using alleys about 4 m wide. As a rule, the effects of prunings on crop yields, including the effects of nutrients and mulch, have been found to be positive and any negative shading effects have been minor when hedge plants are timely pruned. The subterranean sharing of resources, however, has often adversely affected crop growth, especially in semi-arid areas where soil moisture is limiting (Ong, 1994).
On the basis of two decades of research it can be concluded that alley cropping is most likely to work well only on moderately fertile soils without nutrient limitations and when rainfall is adequate during the cropping season (Sanchez, 1995). On sloping land the prospects for contour alley cropping in drier areas are more favourable. In long-term experiments in the semi-arid Machakos area of Kenya the soil cover of prunings and the Senna siamea hedges themselves greatly reduced erosion, while increased infiltration rates in the soil under the hedges trapped runoff water. This accumulation of water under the hedges was such that no competition for soil moisture occurred (Kiepe, 1995).
It can be concluded that although in simultaneous associations the interactions between the components of the system may be positive, they are often negative.
Aspects of soil fertility
One of the most important service functions of auxiliary plants is the maintenance and improvement of soil fertility. Auxiliary plants can fulfil this function by reducing and/or preventing losses from the soil, improving the chemical and physical conditions of the soil and promoting soil biological processes on account of increasing inputs (e.g. nitrogen, organic matter).
Through carbon fixation in photosynthesis and its transfer via litter and prunings and subsequent decomposition and via root decay, auxiliary plants can help in maintaining and sometimes in improving soil organic matter levels. It has been estimated (Young, 1989) that to maintain soil organic matter in the humid, sub-humid and semi-arid tropics annual supplies of organic matter in the order of 8 t, 4 t and 2 t dry matter per ha respectively, are needed to compensate for decomposition and minor erosion losses. There is clear experimental evidence that these supply levels can be achieved in cropping systems with auxiliary plants. Some data are given in Table 5.
The use of leguminous crops to augment nitrogen in the cropping system is a well-known agricultural practice. Rates of biological nitrogen fixation in herbaceous legume crops in fallows and plantation agriculture range annually from a few to 50 kg and up to 200 kg per ha (Giller & Wilson, 1991). Low levels of available phosphate - a common feature of tropical soils - and a lack of soil moisture in drier areas are well-known factors limiting nitrogen fixation. Very few data are available on nitrogen fixation by legume trees and shrubs. Studies on Leucaena leucocephala, however, suggest that shrubs and trees which will yield 50-100 kg biologically fixed N per ha per year in agroforestry systems can be identified. Since most of the nitrogen from leguminous crops is taken up as mineral N from the soil, the N yield of leaf fall and prunings is at least twice as high (see Table 5).
As pruning prevents nutrients being translocated from the leaves to perennial organs (the normal process preceding natural leaf fall) it is an effective practice to ensure that nutrients are re-allocated from the auxiliary crop to the main crop at the proper time (see also 1.5.3). This re-allocation involves the decomposition of residues of natural litter and prunings. The favourable effects of this re-allocation on the nitrogen nutrition of the main crop are well known in rubber. Results presented in Table 6 show that young rubber trees with a leguminous cover return twice as much nitrogen to the soil by annual leaf fall than with a grass ground cover (Watson, 1988).
The question of direct N transfer from the roots of auxiliary plants to companion crops is still controversial. Nitrogen-fixing plants in general acidify the rhizosphere and there is evidence that this increases the P uptake from insoluble phosphate (see 1.3.3). This would be an additional benefit of growing leguminous auxiliary crops (Aguilar Santelises, 1981).
Uptake of nutrients from deeper soil layers is another mechanism by which deep-rooting auxiliary crops can increase the input supply. There is still a lack of quantitative data on this. It is clear, however, that a substantial input of nutrients can only be expected if subsoils are chemically rich, which is rarely the case.
Reduction of losses from the soil
Auxiliary plants can play an important role in reducing or even preventing carbon and nutrient losses and erosion. Improved fallow crops and cover crops in plantation agriculture intercept and transpire a significant part of the rainfall which would otherwise have caused leaching of nutrients. Deep-rooting auxiliary plants can potentially recover and recycle nutrients leached into deeper soil layers, but the importance of this process is still unknown. The vegetation cover lowers the soil temperature and this slows down the decomposition rates of soil organic matter. Shade trees prevent and reduce losses in a similar way by providing an extra canopy layer.
A ground cover of auxiliary plants also reduces nutrient losses in runoff and erosion. Hedges along the contours of slopes have been found to be very effective in erosion control. They check erosion and runoff through the cover effects of prunings and reduce soil losses through a barrier effect. Moreover, they contribute to the development of terraces through soil accumulation upslope of the hedges. Recent results of a long-term experiment in Kenya confirm earlier findings in Nigeria that contour hedges on sloping land greatly reduce erosion, especially when prunings are applied to the soil surface in the inter-rows (Kiepe, 1995). A summary of the Kenyan results is presented in Table 7.
Improvement of physical soil conditions and biological processes
By providing a permanent well-developed living or mulch cover, soils under auxiliary crops exhibit a better structure, porosity and moisture characteristics than most soils under arable crops. The favourable effects of improved physical soil conditions on crop growth are well documented and are illustrated in Table 6. Improved soil structure and porosity are linked to the decomposition of plant residues by microorganisms and fauna. Their activity is stimulated by the extra supply of litter and prunings, but depends greatly on the source of the plant biomass.
Plant residues can be classed as being of high or low quality in terms of decomposition rates. The first category has a low C/N ratio and low lignin and polyphenol contents, and decomposes and releases nutrients rapidly, mainly by microbial and fauna processes. Gliricidia and Leucaena species are among the plants with high-quality residues, hence these residues are a good source of nutrients for fast-growing crops. Low-quality residues with a high C/N ratio and high lignin and polyphenol contents decompose and release nutrients slowly. Mulching with plant residues improves soil conditions by lowering soil temperature and maintaining soil moisture, and slowly decomposing mulches have an advantage for this purpose (Tian, 1992).
The foregoing observations imply that auxiliary plants can be specifically chosen for their nutritional effects or as mulch, and that the timing of pruning is important to synchronize soil nutrient supply and crop nutrient demand.
Farmers pay most attention to their primary food and cash crops. They are more interested in production functions than in service functions. Auxiliary plants will therefore probably not receive inputs such as fertilizer, irrigation and pest control. Management is needed in the form of assistance in establishing the plant stand, pruning and harvesting. Therefore, the introduction of auxiliary crops in existing cropping systems is often not attractive, as more labour is involved, and the system becomes more complicated. A green manure that produces an edible by-product is likely to be accepted more readily by farmers.
It is likely that in the case of auxiliary crops, the removal of a product will result in depletion of soil fertility if not compensated. Auxiliary crops only increase the ecological sustainability of cropping systems if the emphasis is on their service functions. The production functions, however, can be vital in strengthening the economic basis of these systems.
Most auxiliary species are established from seed. For seeds with very hard impervious testa, as found in many leguminous crops (such as Centrosema pubescens), mechanical or chemical scarification or hot water treatment is used to ensure a quick and even germination. Sometimes seeds are dusted or coated with phosphate fertilizer to improve early growth. Inoculation of seeds with Rhizobium strains to improve nodulation is still in an experimental phase but has given promising results in Brazil with Centrosema macrocarpum Benth. and Pueraria phaseoloides (Sylvester-Bradley & Mosquera, 1985). Some species with poor seed production, for example Vigna hosei, are established from cuttings. Sometimes cuttings are used for economic reasons. Kudzu (Pueraria lobata (Willd.) Ohwi) cuttings, for example, can be planted at 1 m spacing; only the planting spot has to be weeded, as the rapidly growing sprouts quickly overgrow the weeds in the surrounding area.
Some woody species like Gliricidia sepium are successfully and cheaply planted as cuttings. The seedless cross between the varieties glabrata (Rose) Zarate and leucocephala of Leucaena leucocephala is budded on one of the parent rootstocks in Indonesian plantation agriculture, to avoid weed problems caused by profuse seed production.
Azolla is propagated vegetatively by means of older secondary stems which detach themselves from the main stem after an abscission layer has been formed. As Azolla cannot stand desiccation, its application depends on the presence of irrigation or perennial ponds. Another condition for its use is the availability and conservation of inoculum (Cagauan & Pullin, 1994).
The growing of cover crops in admixtures is often practised in oil palm and rubber estates in South-East Asia to quickly achieve a complete soil cover, a gradual phasing out when the canopy closes and to diminish the effects of diseases and pests. A common mixture is Pueraria phaseoloides, a vigorous grower which provides a thick cover and suppresses weeds, Calopogonium mucunoides which shows rapid initial growth but does not persist and is susceptible to pests, and Centrosema pubescens which forms a good cover after a slow initial growth and has some tolerance of shade. Recently, Calopogonium caeruleum (Benth.) Sauv. gained some prominence.
In simultaneous systems, planting patterns are important in view of competitive interactions. If crops are mutually non-competitive or beneficial, intimate spatial mixtures can be used. If there is strong mutual competition, auxiliary crops can best be grown in separate blocks or along field boundaries. Alley cropping in which auxiliary plants are arranged in rows or strips represents an intermediate degree of mixing. Distances between hedges depend on the auxiliary species, their possible secondary economic value, and the terrain (slope angle). In most experiments single hedgerows and alley widths of about 4 m have been used, which corresponds with a tree cover of 15-20%. Much greater distances are used for shelter-belts and wind-breaks.
During early establishment cover crops have to be weeded, unless they are on newly cleared forest soils. This is one of the reasons that farmers may prefer vigorous and self-seeding fallow crops such as Chromolaena odorata and Sesbania sesban or fast-growing large-seeded species such as velvet bean, rather than the well-known cover crops used in plantation agriculture.
Once established, plantation cover crops are regularly removed from the tree weeding circles and slashed, especially at the beginning of dry periods, to reduce competition. In rotation systems the cover crops are ploughed into the soil before the next crop is planted. Shade trees are usually planted at a close spacing one or two years ahead of the main crop. Later they are thinned to a final stand, depending on the species and the shade requirements of the crop. Afterwards, shade levels are managed by pruning. Some shade trees such as Erythrina poeppigiana are regularly pollarded or pruned for shade management and the production of mulch.
Hedgerow trees and shrubs are periodically pruned to reduce shading and to provide prunings for mulch or fodder. Pruning at the beginning of the cropping season is essential for crop development. Pruning at this time is a major problem for most farmers, however, because it coincides with land preparation and weeding. Both the frequency and the height of pruning affect the biomass production and its N content. An experiment at Ibadan, Nigeria showed that a higher pruning height and less frequent pruning resulted in a higher N yield in a Leucaena hedgerow system, but reduced maize yields due to shading (Duguma et al., 1988). In farming practice a compromise has to be found between mulch production and maize yields.
The decomposition rates of prunings depend on the nature of tree and shrub species but also on the mode of application. Leucaena prunings, for example, usually decompose quickly (within 40 days), more rapidly if applied fresh than dried. The direct nutritional effect of prunings is better when they are buried in the soil, because they decompose faster (Young, 1989). Similar results have been obtained with Gliricidia sepium. For practical reasons, surface application is the normal practice. Dried material is easier to transport (less bulky) than fresh material.
As to the timing of pruning, additions of organic inputs should be directed at a nutrient release in synchrony with the crop's uptake pattern, not only to promote crop growth but also to reduce losses of released nutrients by leaching and denitrification.
The most important cultivation practice with Azolla in rice fields is to incorporate it into the soil. If grown as a sole crop before transplanting, a permanent water layer is needed. If grown as an intercrop, it is incorporated once or several times during the first month after transplanting. Incorporation at a later stage makes nitrogen available during the maturation period. In the intercropping system incorporation is very labour intensive. If Azolla is grown outside the rice field it can continuously produce a biomass which can be incorporated fresh or after composting before transplanting.
Genetic resources and breeding
There is such a diversity of species useful as auxiliary plants that it is complicated to involve institutions to include these genetic resources in their mandate. In addition to the occasional samples in certain botanic gardens, the agricultural research institutes have also gathered germplasm of auxiliary plants. The major international research centres focus on food and technical crops. The International Centre for Research in Agroforestry (ICRAF, Kenya) specializes in agroforestry, and maintains a large database of a multipurpose tree and shrub seed directory (von Carlowitz, 1986). International cooperation in multipurpose tree germplasm is well documented (Burley & von Carlowitz, 1984). The whereabouts of genetic resources of many auxiliary plants can be found in the Food Legumes and Forages volumes of the IBPGR (now IPGRI) Directories of Germplasm Collections (Bettencourt et al., 1992), including most relevant legume species and many browse plants (including legumes too). The various institutions are listed countrywise, and usually more than 80% of the samples are available without restrictions. In the South-East Asian region, the Malaysian Agricultural Research and Development Institute (MARDI) in Serdang, and the Philippine Institute of Plant Breeding (IPB), College of Agriculture in Laguna are listed as having limited collections of forages. These institutes also have browse legumes in their collections. The Commonwealth Scientific and Industrial Research Organization (CSIRO) of Australia has also accumulated large germplasm collections of forage species with a potential auxiliary role.
The Centro Internacional de Agricultura Tropical (CIAT, Colombia) has included South-East Asia in its worldwide search for potential fodder crops, and amongst these several species have auxiliary roles. Although several catalogues are available, they provide few data below the genus level (Schultze-Kraft, 1990, 1991a, 1991b). The Kew seed bank at Wakehurst, England, maintains many species for the semi-arid and arid tropics. It provides samples, but in small quantities for research purposes only and in small numbers per species.
Unlike many of the food crops, auxiliary species do not have numerous cultivars. Where large amounts of seed are required, it is difficult to obtain the proper ecologically adapted seed source. Information as obtained from the species treatments in this volume points to CIAT and CSIRO as organizations that have the occasional selection of improved fodder crop cultivars, but few particular cultivars have been specially bred for e.g. green manure. An exception is the attention Leucaena has received; many selections of this crop have been released. In several countries with a long-established practice of plantation cropping, such as Malaysia and India, seed of clover and other auxiliary plants can be obtained from commercial companies.
Until the 1970s auxiliary crops mainly received attention in plantation agriculture. Suitable species were selected, their service functions and cultivation practices studied and documented, and this resulted in a general adoption in plantation management. Auxiliary crops will continue to play an important role in commercial tree crop cultivation but no major breakthroughs can be expected in this field. During the last 30 years, when degradation of natural resources became an important issue, the need to develop production systems which would integrate growing of trees or shrubs, arable crop production and/or animal husbandry in order to optimize tropical land-use systems became apparent. The study and development of these so-called agroforestry systems has renewed interest in auxiliary crops and put them back on the agenda of international and national research and development organizations. By delivering services and products and by spreading the risk of crop failure, agroforestry systems have the potential to strengthen the ecological and economic basis of agricultural production systems. Ethnobotanical surveys indicate that many multipurpose plants with auxiliary functions are used in traditional agriculture.
In the transition from semi-permanent arable cropping to more intensive land-use systems, the role of auxiliary species is likely to increase. However, when the stage of permanent arable cropping is reached, only short-duration fallows with auxiliary crops will be used. Farmers will only adopt this strategy if soil moisture is inadequate for other short-duration crops such as pulse crops. Strip cropping with auxiliary shrubs and woody species has been proposed in permanent arable cropping systems, but multi-locational experiments have clearly shown that these so-called hedgerow intercropping systems are only suitable when soil moisture and soil fertility are not limiting.
In some parts of South-East Asia where large areas of wasteland need to be reclaimed for agriculture and forestry, the role of auxiliary plants is becoming very important. Finally, the need to take marginal land into cultivation may also imply an increase in use of auxiliary crops as well. Contour planting with auxiliary trees and shrubs has shown promise, especially on sloping land.
Being of less direct importance than the primary crops, research on the role and performance of auxiliary plants has until recently received little attention. Their large potential for fodder and fuelwood production and soil conservation, and their specific, often customized role in traditional cropping systems merits further research.
Research on the lesser-known auxiliary plants covered in this volume will prove useful, especially to corroborate the few studies done sofar. The adoption of cropping systems that include auxiliary plants would be facilitated if seed of auxiliary species were more readily available from national and international research centres and from commercial suppliers.
- M. Wessel & L.J.G. van der Maesen
Table 1. Major auxiliary plants, their functions and occurrence in different ecological zones.
Table 2. Share of biomass in the total energy consumption (%).
Table 3. Plant families contributing auxiliary plants.
Table 4. Nitrogen-fixing organisms useful in plants.
Table 5. Annual return of dry matter and nutrients by auxiliary crops in different cropping systems (kg/ha).
Table 6. Effect of different soil covers on nitrogen cycles and root development of young rubber trees.
Table 7. Mean annual soil loss in t ha-1 over 6 seasons on a 14% slope at Machakos Research Station, Kenya (Cassia siamea hedges planted at a distance of 4 m across the slope).