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Agroforestry is a land use management system in which trees or shrubs are grown around or among crops or pastureland. This diversification of the farming system initiates an agroecological succession, like that in natural ecosystems, and so starts a chain of events that enhance the functionality and sustainability of the farming system. Trees also produce a wide range of useful and marketable products from fruits/nuts, medicines, wood products, etc. This intentional combination of agriculture and forestry has multiple benefits, such as greatly enhanced yields from staple food crops, enhanced farmer livelihoods from income generation, increased biodiversity, improved soil structure and health, reduced erosion, and carbon sequestration. Agroforestry practices are highly beneficial in the tropics, especially in subsistence smallholdings in sub-Saharan Africa and have been found to be beneficial in Europe and the United States.
Agroforestry shares principles with intercropping but can also involve much more complex multi-strata agroforests containing hundreds of species. Agroforestry can also utilise nitrogen-fixing plants such as legumes to restore soil nitrogen fertility. The nitrogen-fixing plants can be planted either sequentially or simultaneously.
As a science
According to Paul Wojtkowski, the theoretical base for agroforestry lies in ecology, or agroecology. Agroecology encompasses diverse applications such as: improved nutrient and carbon cycling; water retention of soils; biodiverse habitats; protection from pest, disease and weed outbreaks; protection of soils from water and wind erosion, etc. From this perspective, agroforestry is one of the three principal agricultural land-use sciences. The other two are agriculture and forestry.
Benefits
Agroforestry systems can be advantageous over conventional agricultural and forest production methods. They can offer increased productivity; social, economic and environmental benefits, as well as greater diversity in the ecological goods and services provided. It is essential to note that these benefits are conditional on good farm management. This includes choosing the right trees, as well as pruning them regularly etc.
Biodiversity
Biodiversity in agroforestry systems is typically higher than in conventional agricultural systems. Two or more interacting plant species in a given area create a more complex habitat that can support a wider variety of fauna.
Agroforestry is important for biodiversity for different reasons. It provides a more diverse habitat than a conventional agricultural system in which the tree component creates ecological niches for a wide range of organisms both above and below ground. The life cycles and food chains associated with this diversification initiates an agroecological succession that creates functional agroecosystems that confer sustainability. Tropical bat and bird diversity for instance can be comparable to the diversity in natural forests. Although agroforestry systems do not provide as many floristic species as forests and do not show the same canopy height, they do provide food and nesting possibilities. A further contribution to biodiversity is that the germplasm of sensitive species can be preserved. As agroforests have no natural clear areas, habitats are more uniform. Furthermore, agroforests can serve as corridors between habitats. Agroforestry can help to conserve biodiversity having a positive influence on other ecosystem services.
Soil and plant growth
Depleted soil can be protected from soil erosion by groundcover plants such as naturally growing grasses in agroforestry systems. These help to stabilise the soil as they increase cover compared to short-cycle cropping systems. Soil cover is a crucial factor in preventing erosion. Cleaner water through reduced nutrient and soil surface runoff can be a further advantage of agroforestry. Trees can help reduce water runoff by decreasing water flow and evaporation and thereby allowing for increased soil infiltration. Compared to row-cropped fields nutrient uptake can be higher and reduce nutrient loss into streams.
Further advantages concerning plant growth:
- Bioremediation
- Drought resistance
- Increased crop stability
Contribution to sustainable agricultural systems
Agroforestry systems can provide a number of ecosystem services which can contribute to sustainable agriculture in the following ways;
- Diversification of agricultural products, such as fuelwood, medicinal plants, and multiple crops, increases income security
- Increased food security and nutrition by restored soil fertility, crop diversity and resilience to weather shocks for food crops
- Land restoration through reducing soil erosion and regulating water availability
- Multifunctional site use, e.g., crop production and animal grazing
- Reduced deforestation and pressure on woodlands by providing farm-grown fuelwood
- Possibility of reduced chemicals inputs, e.g. due to improved use of fertiliser, increased resilience against pests, and increased ground cover which reduces weeds
- Growing space for medicinal plants e.g., in situations where people have limited access to mainstream medicines
According to FAO's The State of the World’s Forests 2020, adopting agroforestry and sustainable production practices, restoring the productivity of degraded agricultural lands, embracing healthier diets and reducing food loss and waste are all actions that urgently need to be scaled up. Agribusinesses must meet their commitments to deforestation-free commodity chains and companies that have not made zero-deforestation commitments should do so.
Other environmental goals
Carbon sequestration is an important ecosystem service. Agroforestry practices can increase carbon stocks in soil and woody biomass. Trees in agroforestry systems, like in new forests, can recapture some of the carbon that was lost by cutting existing forests. They also provide additional food and products. The rotation age and the use of the resulting products are important factors controlling the amount of carbon sequestered. Agroforests can reduce pressure on primary forests by providing forest products.
Agroforestry practices may realize a number of environmental goals, such as:
- Odour, dust and noise reduction
- Green space and visual aesthetics
- Enhancement or maintenance of wildlife habitat
Adaptation to climate change
Agroforestry can significantly contribute to climate change mitigation along with adaptation benefits. A case study in Kenya found that the adoption of agroforestry drove carbon storage and increased livelihoods simultaneously among small-scale farmers. In this case, maintaining the diversity of tree species, especially land use and farm size are important factors.
Especially in recent years, poor smallholder farmers turned to agroforestry as a means to adapt to climate change. A study from the CGIAR research program on Climate Change, Agriculture and Food Security (CCAFS) found from a survey of over 700 households in East Africa that at least 50% of those households had begun planting trees in a change from earlier practices. The trees were planted with fruit, tea, coffee, oil, fodder and medicinal products in addition to their usual harvest. Agroforestry was one of the most widespread adaptation strategies, along with the use of improved crop varieties and intercropping.
Applications
Tropical Agroforestry
Research with Faidherbia albida in Zambia showed maximum maize yields of 4.0 tonnes per hectare using fertilizer and inter-cropped with these trees at densities of 25 to 100 trees per hectare, compared to average maize yields in Zimbabwe of 1.1 tonnes per hectare.
Hillside systems
A well-studied example of an agroforestry hillside system is the Quesungual Slash and Mulch Agroforestry System (QSMAS) in Lempira Department, Honduras. This region was historically used for slash and burn subsistence agriculture. Due to heavy seasonal floods, the exposed soil was washed away, leaving infertile barren soil exposed to the dry season. Farmed hillside sites had to be abandoned after a few years and new forest was burned. The Food and Agriculture Organization of the United Nations (FAO) helped introduce a system incorporating local knowledge consisting of the following steps:
- Thin and prune Hillside secondary forest, leaving individual beneficial trees, especially nitrogen-fixing trees. They help reduce soil erosion, maintain soil moisture, provide shade and provide an input of nitrogen-rich organic matter in the form of litter.
- Plant maize in rows. This is a traditional local crop.
- Harvest from the dried plant and plant beans. The maize stalks provide an ideal structure for the climbing bean plants. Bean is a nitrogen-fixing plant and therefore helps introduce more nitrogen.
- Pumpkin can be planted during this time. Its large leaves and horizontal growth provide additional shade and moisture retention. It does not compete with the beans for sunlight since the latter grow vertically on the stalks.
- Every few seasons, rotate the crop by grazing cattle, allowing grass to grow and adding soil organic matter and nutrients (manure). The cattle prevent total reforestation by grazing around the trees.
- Repeat.
Shade crops
With shade applications, crops are purposely raised under tree canopies within the shady environment. The understory crops are shade tolerant or the overstory trees have fairly open canopies. A conspicuous example is shade-grown coffee. This practice reduces weeding costs and improves coffee quality and taste.
Crop-over-tree systems
Crop-over-tree systems employ woody perennials in the role of a cover crop. For this, small shrubs or trees pruned to near ground level are utilized. The purpose is to increase in-soil nutrients and/or to reduce soil erosion.
Intercropping and alley cropping
With alley cropping, crop strips alternate with rows of closely spaced tree or hedge species. Normally, the trees are pruned before planting the crop. The cut leafy material - for example, from Alchornea cordifolia and Acioa barteri - is spread over the crop area to provide nutrients. In addition to nutrients, the hedges serve as windbreaks and reduce erosion.
In tropical areas of North and South America, various species of Inga such as I. edulis and I. oerstediana have been used for alley cropping.
Intercropping is advantageous in Africa, particularly in relation to improving maize yields in the sub-Saharan region. Use relies upon the nitrogen-fixing tree species Sesbania sesban, Tephrosia vogelii, Gliricidia sepium and Faidherbia albida. In one example, a ten-year experiment in Malawi showed that, by using the fertilizer tree Gliricidia (G. sepium) on land on which no mineral fertilizer was applied, maize/corn yields averaged 3.3 metric tons per hectare (1.5 short ton/acre) as compared to 1 metric ton per hectare (0.45 short ton/acre) in plots without fertilizer trees or mineral fertilizers.
Weed control is inherent to alley cropping, by providing mulch and shade.
Taungya
Taungya is a system originating in Burma. In the initial stages of an orchard or tree plantation, trees are small and widely spaced. The free space between the newly planted trees accommodates a seasonal crop. Instead of costly weeding, the underutilized area provides an additional output and income. More complex taungyas use between-tree space for multiple crops. The crops become more shade tolerant as the tree canopies grow and the amount of sunlight reaching the ground declines. Thinning can maintain sunlight levels.
Itteri Agroforestry
Itteri agroforestry systems have been used in Tamil Nadu since time immemorial. They involve the deliberate management of multipurpose trees and shrubs grown in intimate association with herbaceous species. They are often found along village and farm roads, small gullies, and boundaries of fields.
Bamboo-based agroforestry systems (Dendrocalamus strictus + sesame–chickpea) have been studied for enhancing productivity in semi-arid tropics of central India.
Agroforestry in Africa
A project to mitigate climate change with agriculture was launched in 2019 by the "Global EverGreening Alliance". The target is to sequester carbon from the atmosphere. By 2050 the restored land should sequestrate 20 billion tons of carbon annually
Temperate Agroforestry
Although originally a concept used in tropical agronomy, the USDA distinguishes five applications of agroforestry for temperate climates.
Alley cropping and Strip cropping
Alley cropping (see above) can also be used in temperate climates. Strip cropping is similar to alley cropping in that trees alternate with crops. The difference is that, with alley cropping, the trees are in single row. With strip cropping, the trees or shrubs are planted in wide strip. The purpose can be, as with alley cropping, to provide nutrients, in leaf form, to the crop. With strip cropping, the trees can have a purely productive role, providing fruits, nuts, etc. while, at the same time, protecting nearby crops from soil erosion and harmful winds.
Fauna-based systems
Trees can benefit fauna. The most common examples are silvopasture where cattle, goats, or sheep browse on grasses grown under trees. In hot climates, the animals are less stressed and put on weight faster when grazing in a cooler, shaded environment. The leaves of trees or shrubs can also serve as fodder.
Similar systems support other fauna. Deer and hogs gain when living and feeding in a forest ecosystem, especially when the tree forage nourishes them. In aquaforestry, trees shade fish ponds. In many cases, the fish eat the leaves or fruit from the trees.
The dehesa or montado system of silviculture are an example of pigs and bulls being held extensively in Spain and Portugal.
Boundary systems
- A living fence can be a thick hedge or fence wire strung on living trees. In addition to restricting the movement of people and animals, living fences offer habitat to insect-eating birds and, in the case of a boundary hedge, slow soil erosion.
- Riparian buffers are strips of permanent vegetation located along or near active watercourses or in ditches where water runoff concentrates. The purpose is to keep nutrients and soil from contaminating the water.
- Windbreaks reduce wind velocity over and around crops. This increases yields through reduced drying of the crop and/or by preventing the crop from toppling in strong wind gusts.
Agroforestry in Switzerland
Since the 1950s, four-fifths of Swiss hochstammobstgärten (traditional orchards with tall trees) have disappeared. An agroforestry scheme was tested here with hochstamm trees together with annual crops. Trees tested were walnut tree (Juglans regia) and cherry tree (Prunus avium). Forty to seventy trees per hectare were recommended, yields were somewhat decreasing with increasing tree height and foliage. However, the total yield per area is shown to be up to 30 percent higher than for monocultural systems.
Another set of tests involve growing Populus tremula for biofuel at 52 trees a hectare and with grazing pasture alternated every two to three years with maize or sorghum, wheat, strawberries and fallowing between rows of modern short-pruned & grafted apple cultivars ('Boskoop' & 'Spartan') and growing modern sour cherry cultivars ('Morina', 'Coraline' and 'Achat') and apples, with bushes in the rows with tree (dogrose, Cornus mas, Hippophae rhamnoides) intercropped with various vegetables.
Historical use
Similar methods were historically utilized by Native Americans. Indigenous peoples of California periodically burned oak and other habitats to maintain a 'pyrodiversity collecting model'. This method allowed for greater tree health and improved habitat in general.
Challenges
Although agroforestry systems can be advantageous, they are not widespread in the US as of 2013.
As suggested by a survey of extension programs in the United States, obstacles (ordered most critical to least critical) to agroforestry adoption include:
- Lack of developed markets
- Unfamiliarity with technologies
- Lack of awareness
- Competition between trees, crops and animals
- Lack of financial assistance
- Lack of apparent profit potential
- Lack of demonstration sites
- Expense of additional management
- Lack of training or expertise
- Lack of knowledge about where to market products
- Lack of technical assistance
- Adoption/start up costs, including costs of time
- Unfamiliarity with alternative marketing approaches (e.g. web)
- Unavailability of information about agroforestry
- Apparent inconvenience
- Lack of infrastructure (e.g. buildings, equipment)
- Lack of equipment
- Insufficient land
- Lack of seed/seedling sources
- Lack of scientific research
Some solutions to these obstacles have been suggested.
