Agroforestry systems and their classification
What agroforestry is, how its systems are classified, the main practices from alley cropping to parklands, and how trees and crops interact.
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Agroforestry is the deliberate growing of trees and shrubs together with crops or livestock on the same land, in a way that the components interact. It is not new: home gardens, parkland trees in cereal fields and shade trees over coffee are centuries old. What changed in the 1970s was that agroforestry became a science, with its own research centre (now CIFOR-ICRAF, formerly the World Agroforestry Centre) and a growing body of evidence.
Studies of satellite imagery suggest that more than 40 per cent of the world's agricultural land has at least 10 per cent tree cover, so agroforestry in some form is part of the lives of hundreds of millions of farmers. It is a core topic in forestry, agronomy and sustainability syllabi, usually starting with its classification.
What counts as agroforestry
The definition widely used by the World Agroforestry Centre describes agroforestry as a collective name for land-use systems and technologies where woody perennials (trees, shrubs, palms, bamboos) are deliberately used on the same land as agricultural crops and/or animals, in some form of spatial arrangement or temporal sequence, with both ecological and economic interactions between the components.
Two ideas are essential:
- Deliberate combination on the same unit of land, at the same time or in sequence.
- Interaction: the trees affect the crops or animals, and the other way round. A woodlot next to a field is not agroforestry; trees planted to shelter, feed or fertilise it are.
Classifying agroforestry systems
The classification by P. K. R. Nair (1985) is the one most textbooks and exams use. It recognises four bases.
1. Structural: what the components are and how they are arranged
- Agrisilvicultural: trees with crops (including shrubs and vines).
- Silvopastoral: trees with pasture and/or animals.
- Agrosilvopastoral: trees with crops and animals.
- Others: trees with bees (apiculture), trees with fish ponds (aquaforestry), multipurpose woodlots.
Arrangement in space may be mixed (dense or sparse), in strips, or on boundaries; in time, components may coincide, overlap, or follow each other (as in rotational fallows and taungya).
2. Functional: what the system mainly does
- Productive: food, fodder, fuelwood, timber, fruit, fibre, gums and medicines.
- Protective (service): windbreaks, shelterbelts, soil conservation, soil fertility improvement, shade, live fences, watershed and biodiversity protection.
Most systems do both.
3. Socio-economic: scale and purpose
- Commercial: large-scale, market-oriented (rubber or oil palm with cover crops and grazing; timber over pasture).
- Intermediate: medium farms, partly market-oriented.
- Subsistence: mainly for the household's own needs.
4. Ecological: the environment
Systems are grouped by agro-ecological zone (humid and subhumid tropics, semi-arid tropics, highlands, temperate zones), since what works in one rarely works unchanged in another.
The main practices
| Practice | Description | Where it is common |
|---|---|---|
| Alley cropping (hedgerow intercropping) | crops grown between rows of fast-growing, often nitrogen-fixing shrubs that are pruned regularly; prunings used as mulch or fodder | humid and subhumid tropics; widely researched, adoption limited by labour |
| Taungya | farmers grow food crops between young forest trees for the first years of a plantation, then leave as the canopy closes | originated with teak plantations in Myanmar in the nineteenth century; used across Asia and Africa |
| Home gardens | multistorey mixtures of trees, shrubs, vegetables and often animals around the house | South and South-East Asia, East Africa (Kilimanjaro), Central America, the Pacific |
| Parklands | scattered trees retained or encouraged in cropped fields | the West African Sahel (shea, néré, Faidherbia albida), drylands elsewhere |
| Shaded perennial crops (multistrata) | coffee, cocoa or tea under shade trees | Latin America, West and East Africa, South and South-East Asia |
| Improved fallows | fast-growing leguminous shrubs planted to restore soil fertility in a fallow period | southern and eastern Africa |
| Windbreaks and shelterbelts | rows of trees across the prevailing wind | dry plains on every continent |
| Silvopasture | trees deliberately managed with forage and grazing | Latin America, temperate and tropical grazing lands |
| Fodder banks, live fences, boundary planting | trees as feed, fences and property markers | smallholdings everywhere |
| Riparian buffers | tree strips along streams | intensively farmed landscapes |
Faidherbia albida is a favourite exam example because of its reverse phenology: it sheds its leaves in the rainy season, when crops grow beneath it and need light, and grows them in the dry season, when it gives shade and fodder. Crops under it often yield more than in the open.
How trees and crops interact
The success of a system depends on the balance between competition and complementarity.
Competition
- Light: tree shade reduces crop photosynthesis, especially for sun-loving C4 cereals.
- Water: tree roots near the surface can dry the soil for crops, a serious issue in drylands.
- Nutrients: trees take up nutrients the crop needs, especially close to the trunk.
Complementarity and facilitation
- Nitrogen fixation by leguminous trees, and nutrient cycling through leaf litter and prunings.
- Safety-net roots: deep tree roots catch nutrients leached below crop roots and return them via litter.
- Hydraulic lift: some deep roots move water up into dry topsoil at night.
- Microclimate: shelter from wind, lower soil temperature and evaporation, reduced crop heat stress.
- Soil conservation: roots, litter and hedgerows reduce erosion and improve infiltration.
Managing the balance is the craft of agroforestry: choosing tree species with deep rather than shallow roots and light canopies, spacing, pruning the canopy and the roots, orienting rows east–west to reduce shading, and timing tree leaf-fall to the crop season.
Land equivalent ratio of a tree–crop system
A farmer intercrops maize between rows of timber trees. Over the trees' rotation, the maize yields 70 per cent of what a sole maize crop would have produced on the same land, and the timber reaches 60 per cent of the volume of a sole plantation.
LER = 0.70 + 0.60 = 1.30: the agroforestry system produces on one hectare what would need 1.3 hectares of separate maize and plantation. As with any intercrop, the comparison is only valid against well-managed sole crops, and money values, timing and risk also matter.
How far does a windbreak protect?
A windbreak reduces wind speed for a distance downwind commonly given as 10 to 20 times its height (H), with the strongest effect within about 10H, and a smaller effect upwind (about 2–5H). Moderately porous windbreaks (around 40–50 per cent porosity) protect a longer distance than dense ones, which create turbulence behind them.
A shelterbelt 12 m tall therefore shelters roughly 120 to 240 m of field downwind. To protect a field 600 m deep, belts would be planted every 150 to 200 m.
What agroforestry offers, and its limits
Benefits reported across many studies include diversified income and food, fuelwood and fodder close to home, better soil fertility and less erosion, more biodiversity on farms, resilience to drought and price shocks, and carbon storage in tree biomass and soils, making agroforestry a recognised practice in climate-change mitigation and adaptation plans.
Limits: trees take years to pay back; land and tree tenure rules can discourage planting when farmers cannot be sure of owning the trees they plant; seedlings and know-how can be scarce; markets for tree products may be poor; and competition can reduce crop yields if the system is badly designed. Policies that secure tree ownership, supply quality planting material and support farmer-managed natural regeneration (protecting and pruning tree stumps that resprout in fields, which has re-greened large areas of the Sahel) have often mattered more than new technology.
Key points
- Agroforestry: deliberate combination of woody perennials with crops and/or animals on the same land, with ecological and economic interaction.
- Nair's classification: structural (agrisilvicultural, silvopastoral, agrosilvopastoral, others), functional (productive, protective), socio-economic and ecological.
- Key practices: alley cropping, taungya, home gardens, parklands, shaded perennials, improved fallows, windbreaks, silvopasture.
- Faidherbia albida shows reverse phenology: leafless in the rainy season, leafy in the dry season.
- Success depends on balancing competition (light, water, nutrients) with complementarity (nitrogen fixation, nutrient cycling, microclimate, soil protection).
- A windbreak protects roughly 10–20 tree heights downwind.
What the exams ask
- School-leaving and entrance papers: the definition of agroforestry and examples of systems and multipurpose trees.
- Degree and research-fellowship entrances in forestry and agronomy: Nair's classification, alley cropping, taungya, home gardens, tree–crop interactions, and the LER of agroforestry systems.
- Professional and civil-service forestry examinations: designing agroforestry for degraded or dry land, windbreak design, policy and tenure issues, and agroforestry's role in climate strategies.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Do trees always reduce crop yields?
Not always. Close to trees, competition often reduces yields; further away, shelter and better soil can raise them. Species such as Faidherbia albida can raise yields beneath them. The overall result depends on species, spacing and management.
Is agroforestry the same as social forestry?
No. Social forestry is a broader policy approach that involves communities in growing and managing trees, on farms, village land or degraded public land. Agroforestry is a set of land-use practices that combine trees with farming, and it is often one component of social forestry programmes.
Which trees are best for agroforestry?
Multipurpose trees suited to the local climate and the farmer's needs: leguminous trees for fertility and fodder, fruit and nut trees for income, timber trees for long-term value. Deep roots, a light canopy and tolerance of pruning are valued.
Sources
- Nair, P. K. R. "Classification of agroforestry systems." Agroforestry Systems 3, 97–128 (1985).
- Nair, P. K. R., Kumar, B. M. and Nair, V. D. An Introduction to Agroforestry: Four Decades of Scientific Developments, 2nd edition. Springer, 2021.
- Zomer, R. J. et al. "Global tree cover and biomass carbon on agricultural land." Scientific Reports 6, 29987 (2016).
- FAO. Advancing Agroforestry on the Policy Agenda. Agroforestry Working Paper 1, FAO, 2013.
- Brandle, J. R., Hodges, L. and Zhou, X. H. "Windbreaks in North American agricultural systems." Agroforestry Systems 61, 65–78 (2004).
- Reij, C., Tappan, G. and Smale, M. Agroenvironmental Transformation in the Sahel. IFPRI Discussion Paper 914, 2009.
Discussion
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