Overstory #215 - Agroforestry benefits for tropical organic farming

An agroforestry system approach

Conventional farming focuses on maximizing yields of a specific crop. It is based on a simple presumption: crop yields are increased by nutrient inputs and by controlling pests, diseases and weeds. Organic agriculture is a holistic way of farming: besides production of goods of high quality, an important aim is the conservation of natural resources such as fertile soil, clean water and rich biodiversity. The art of organic farming is to make the best use of ecological principles and processes. Organic farmers can learn a great deal from studying the interactions in natural ecosystems such as forests.

Trees and other plants take up nutrients from the soil and incorporate them in their biomass. The nutrients return to the soil when leaves or branches fall or plants die. Part of the biomass is consumed by various animals (including insects), and their excrement returns the nutrients to the soil. In the soil, a huge number of soil organisms are involved in the decomposition of organic material which makes nutrients available to plant roots again. The dense root system of forest plants collects the released nutrients almost completely. Forests host a high diversity of plant varieties of different size, root systems and requirements. Animals are also part of the system. In a healthy, diverse system, if one organism drops out, it is immediately replaced by another that fills the gap. Thus space, light, water and nutrients are used near-optimally. The result is a very stable system.

Agroforestry is one of the best uses of agro-biodiversity that also generates multiple benefits, including erosion control and moisture retention. Elaborate patterns of vertical striation provide a range of sunny and cooler conditions for different species. In many tropical countries, certified organic products are produced successfully in agroforestry systems. Systems include a diversity of cash and subsistence crops (e.g. bananas, coffee, cocoa, pineapple, yams, beans) as well as livestock. Cattle and pigs are kept in stables ("zero grazing") and the manure is recycled, providing fertility. Home gardens are also designed to maximize diversity.

Tropical rainforests and agro-ecosystems

Tropical rainforests are complex and dynamic ecosystems that are optimally adapted to the prevailing site conditions. The vast diversity of species is important for the stability of the system. Each individual occupies an appropriate niche and thereby fulfils a particular eco-physiological function within the system. The so-called diseases and pests in these systems are nothing but necessary regulation mechanisms that take their turn when there are tensions within the system. The function of the so-called weeds is to occupy niches since natural systems always strive to cover bare soil as quickly as possible.

The more complexly designed an agro-ecosystem is, the fewer interventions required to regulate diseases and pests in the system. Massive problems with pests and diseases point to errors in the system design that should be corrected. Apart from agronomic considerations, the successful development of sustainable systems incorporating cash and subsistence crops requires that further principles of forest dynamics be taken into account.

Forest dynamics

Where clearfelling or the collapse of a giant tree has damaged or removed part of the forest canopy, this gap will quickly be closed under natural conditions. The forest "organism" passes through a number of phases in this process that can be compared to the metamorphosis of an insect that only obtains its final form as an adult "individual" after shedding its skin and changing its exterior form a number of times. Simply speaking, the following phases can be distinguished:

Phase 1 ­ Pioneer Phase

Following the removal of the forest canopy the forest floor is covered by pioneer plants within a few weeks. These pioneer species have a short life cycle of only a few months. The species composition is dependent on site conditions (soil type, slope, solar irradiance, distribution of rainfall etc.).

Phase 2 ­ Secondary Forest Phase (Up to 10 Years)

A multitude of tree species with a variety of life cycles and maximum heights germinates at the same time as the pioneer species. This phase is characterized by fast growing tree species with a life cycle of only a few years. The dynamic of these fast growing species literally drags all the other species in the system along. The resultant high biomass production enhances soil dynamics by rapidly cycling nutrients and organic matter.

Phase 3 (Up to 50 Years) and Phase 4 (Up to 80 Years)

Secondary forest phase ­ medium and long cycle: During these phases the forest formations characteristic of the site develop with tree species that can reach ages of up to 80 years.

Phase 5 ­ Primary Forest

All the preceding phases ultimately lead to the establishment of tree species that characterize the mature primary forest, with species whose life cycle can span centuries up to a thousand years.

Coffee and cocoa production, for example, has gone into crisis because the basic principles outlined above have not been observed. Most of the shade trees for cocoa and coffee belong to the group of secondary forest species with a medium life cycle of between 20 and 50 years (e.g. Inga spp.). If cocoa is grown in the understory of an aging and secondary forest system lacking diversity, the cocoa with its much longer life cycle ages prematurely together with its shade trees and is eliminated by the system's pests and diseases because it can no longer fulfill its function in such a system. Only through understanding and implementing natural forest interconnections will it be possible to breed for resistance and pursue alternative approaches to the control of pests and diseases in such a way that real solutions are developed.

Selection of shade trees

A challenge is the selection of shade trees that originate from other ecosystems or which require different site conditions. Each crop grows best under different shade conditions and shade tree species. Trees of the family Fabaceae such as Leucaena, Gliricidia, and Inga are of great interest since they fix nitrogen from the air.

Cocoa farmers in the eastern part of Cuba usually use a shade density of around 40% in cocoa plantations. Generally suitable shade trees are:

Leguminous trees: Samanea saman (Algarrobo), Gliricidia sepium (Júpiter, Piñon Florido), Erythrina poeppigiana (Búcaro), Guazuma tomentosa (Guasima), Leucaena spp., Spondias mombin (Jobo)

Palms: Roystonea regia (Palma real)

Fruit trees: mangos, zapote, citrus, avocadoes, guapén, breadfruit (Fruta de Pan)

The selection of companion crops

When selecting the companion crops and native forest tree species to be planted in a cocoa, coffee or banana plantation, it is important to select species from each of the guilds that allow for a multi-tiered vertically diverse forest system. There will only be competition between individual plants if within the same guild more than one species occupies the same stratum (grows to the same height). Depending on the crops a maximum number of trees per square meter are planted. The more densely planted the system is, the less the maintenance work will be. The continuous thinning of maturing individual plants as well as harvesting open up the canopy and at the same time accelerate the addition of organic matter and woody material to it.

Example crop progression over time

Year 1 maize/beans, pigeon pea (Cajanus cajan), papaya, pineapple, bananas, cocoa, forest trees/rubber/fruit trees, palms

Year 2 pigeon pea, papaya, pineapple, bananas, cocoa, forest trees/rubber/fruit trees, palms

Year 3 pineapple, bananas, cocoa, forest trees/rubber/fruit trees, palms

Years 5-10 bananas, cocoa, forest trees/rubber/fruit trees, palms

Year 11+ cocoa, forest trees/rubber/fruit trees, palms

The example above shows that with such systems the first harvests can be take place after only a few months. Cultivation and maintenance measures should always be combined with harvesting operations and thus be economically supported by the latter. Combinations consisting of a mix of fruiting trees such as avocado, carambola, mango and jackfruit (higher understory) and a density of 150 trees per hectare enhance cocoa production. Additionally, fruit trees such as sapote (overstory) and other productive trees such as para rubber trees (Hevea brasiliensis) can be interspersed. For the overstory, trees which shed their leaves should be planted (e.g. Ceiba pentandra).

Improvement and conversion of established plantations into agroforestry systems

Existing coffee or cocoa plantations can be converted into agroforestry systems in a number of ways. This question often arises concurrent with the conversion of organic coffee or cocoa. The approach taken depends primarily on the existing situation of the plantation. It is not possible to simply plant extra trees into an existing plantation with established shade trees (Inga spp., Erythrina spp.).

One possibility of improving the system is to create small islands of more complex plantings within the plantation. Cocoa trees, for example, are identified as deficient or unproductive, or gaps are identified. The unproductive trees are felled and adjoining cocoa trees are heavily pruned. All the shade trees in the sphere of influence of the island are pruned back to the remaining crown and the prunings are evenly shredded and dispersed on the ground. All the guild members are planted into this gap (if the area is big enough pioneer plants such as maize can also be planted). In this case it is better to use seedlings started in a nursery. Bananas and palms should also be included. The plants of the different guilds including those that will reach different heights can be planted at distances of 0.5-1m. A number of these "agroforestry islands" will have a positive influence on the dynamics of the entire plantation.

As long as such plantations are of good productivity and do not have pest or disease problems, no major interventions should be undertaken. Such plantations can be converted to organic cocoa plantations with the normal conversion processes, i.e. by abandoning the use of all synthetic chemicals and by correctly carrying out maintenance operations.

Original source

This article was excerpted with the kind permission of the publisher from:

United Nations. 2003. Organic Fruit and Vegetables from the Tropics:
Market, Certification and Production Information for Producers and
International Trading Companies. UNCTAD/DITC/COM/2003/2. United Nations,
New York and Geneva, Switzerland.
Web: <http://www.unctad.org/en/docs/ditccom20032_en.pdf>

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