Agroforestry is a form of multiple cropping in which annual herbaceous crops are grown interspersed with perennial trees or shrubs. The deeper-rooted trees can often exploit water and nutrients otherwise unavailable to the crops. The trees may also provide shade and mulch, creating a microenvironment, whilst the ground cover of crops reduces weeds and prevents erosion. One main advantage of agroforestry is that it provides vegetative material that can be used as mulch, and protect the soil from erosion, desiccation and heat. Agroforestry – land with greater than 10% tree cover – makes up 43% of more than 1 billion hectares of global agricultural land, 190 million of which are located in sub-Saharan Africa.
Agroforestry systems can be classified in a variety of ways, most commonly by their structural characteristics, such as for the types of trees that are grown or the crops and animals with which they are integrated. Some of the most commonly used practices included alley cropping, forest farming, erecting buffer strips, windbreaks or shelterbelts. The type of agroforestry system used and trees grown depends on factors such as the location, soil type and crops native to the region. If an agroforestry system is to be successful, the tree species must be chosen carefully, depending on both environmental and social suitability factors.Contribution to Sustainable Intensification
The integration of trees on farms can result in impressive benefits for yields, resource use and conservation, bringing both environmental and economic benefits. Agroforestry systems can improve farm profitability by increasing productivity (when the tree and crop or animal combination is greater than a single component alone); increasing the productivity of crops and livestock by providing shelter and nutrients; and increasing the financial diversity of the farm and its ability to withstand and adapt to new conditions. Trees themselves can also be a long-term investment. Agroforestry also helps to conserve and protect natural capital by limiting soil erosion and creating wildlife habitat. Care must be taken, however, to ensure that the trees do not overpower the crops leading to crop damage or stunted growth that results in economic losses.
The presence of trees and shrubs can aid crops in making better use of soil nutrients and light or provide new sources of nutrients as the tree roots reach deeper into the soil horizon. Nutrients absorbed by the tree are returned to the soil in leaf litter, resulting in better production in comparison to a single crop. Where trees are leguminous, soil fertility and crop yields improve due to additional nitrogen made available.
Beyond the soil, trees can have multiple benefits that provide a better growing environment for crops and animals, allowing them to become more productive. Shrubs and trees can act as wind barriers protecting crops and livestock from weather extremes, harsh climatic conditions, and soil and water erosion. Trees can also function as “bio-filters” of dust, noise and odours, as well as provide a food source and shelter for livestock. Agroforestry systems can also be managed to provide vegetative material that can be used as mulch, protecting the soil from erosion, desiccation and heat.
Trees planted in a field can lower yields of food crops by reducing the amount of space available to grow crops. However, if both crop and tree products are taken into account, a higher total yield from unit of land will be achieved than in a monoculture. Agroforestry also carries the risk of introducing new species such as eucalypts that in certain circumstances may harm the growth, survival and reproduction of crop species.
According to the Intergovernmental Panel on Climate Change (IPCC), most of agriculture’s potential to mitigate climate change lies in improving the soils’ ability to sequester and store carbon. Trees are well known for their ability to sequester carbon from the atmosphere. In Africa, although measures of carbon stocks and carbon sequestration vary widely across the continent, agroforestry systems have been found to be the third largest carbon sink after primary forests and long term fallows. Agroforestry systems in general have 3-4 times more biomass than traditional treeless cropping systems. Whilst field measurements to validate agroforestry’s potential in mitigating climate change are limited, agroforestry systems can sequester more carbon both above- and below-ground than treeless systems as well as to store more stable carbon in the soil.
Agroforestry systems can also play a part in reducing pressure on natural forests for products such as timber and fuelwood and offer sustainable energy options. In sub-Saharan Africa, 15% of farms have tree cover of at least 30%. Across the whole continent, approximately 1.55 billion hectares are suitable for some type of agroforestry. This indicates that there is significant potential in Africa for sequestering carbon whilst maintaining, or even boosting, production on farms.
Despite impressive results achieved at experiment stations and demonstration farms, adoption of agroforestry as a type of multiple cropping has been poor. This is partly because it has been developed as a package, whereas farmers tend to be more willing to adopt various components, gradually modifying their farms. More labour and time is required for agroforestry than for conventional agriculture, as trees need to be watered regularly and require more care when young. However, row planting trees allows weeding for trees and crops to take place at the same time, reducing the amount of labour.