Agroforestry is a land use management system in which trees or shrubs are grown around or among crops or pastureland. This intentional combination of agriculture and forestry has varied benefits, including increased biodiversity and reduced erosion. Agroforestry practices have been successful in sub-Saharan Africa and in parts of the United States.
According to Wojtkowski, the theoretical base for agroforestry lies in ecology, or agroecology. Agroecology encompasses diverse applications such as countering winds, high rainfall, harmful insects, etc. From this perspective, agroforestry is one of the three principal agricultural land-use sciences. The other two are agriculture and forestry.
Agroforestry systems can be advantageous over conventional agricultural and forest production methods. They can offer increased productivity, economic benefits, and more diversity in the ecological goods and services provided.
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. 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 by having a positive influence on other ecosystem services.
Soil and plant growth
Depleted soils 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 soilsurface runoff can be a further advantage of agroforestry. The runoff can be reduced by decreasing its velocity and increasing infiltration into the soil. Compared to row-cropped fields nutrient uptake can be higher and reduce nutrient loss into streams.
Improved human nutrition through more diverse farm outputs
Growing space for medicinal plants e.g., in situations where people have limited access to mainstream medicines
Other environmental goals
Carbon sequestration is an important ecosystem service. 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
Especially in recent years, poor smallholder farmers turned to agroforestry as a mean 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.
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.
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 burnsubsistence 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:
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.
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 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.
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 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.
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.
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 corn fields between rows of walnut trees
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.
Silvopasture over the years (Australia)
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.
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, 4/5 of Swisshochstammobstgärten (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 low.
Similar methods were historically utilized by Native Americans. California Indians periodically burned oak and other habitats to maintain a 'pyrodiversity collecting model'. This method allowed for greater tree health and improved habitat in general.
Although agroforestry systems can be advantageous, they are not widespread in the US as of 2013.
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