Agroforestry

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. Tropical bat and bird diversity for instance can be comparable to the diversity in natural forests.^[8] 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.^[9] 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.^[9]

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.^[10] Soil cover is a crucial factor in preventing erosion.^[11] Cleaner water through reduced nutrient and soil surface 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.^[12][13]

Further advantages concerning plant growth:

• Bioremediation
• Drought resistance
• Increased crop stability

Contribution to sustainable agricultural systems

• Reduced poverty through increased production of wood and other products
• Increased food security by restored soil fertility for food crops
• Multifunctional site use, e.g., crop production and animal grazing
• Reduced global warming and hunger risk by increasing the number of drought-resistant trees and the subsequent production of fruits, nuts and edible oils
• Reduced deforestation and pressure on woodlands by providing farm-grown fuelwood
• Reduced need for toxic chemicals (insecticides, herbicides, etc.)
• 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.^[9] 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.^[14]

Agroforestry practices may realize a number of associated 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.^[15]

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.^[16] 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:^[17][18]

1. 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.
2. Plant maize in rows. This is a traditional local crop.
3. 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.
4. 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.
5. 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.
6. Repeat.

Parkland

Parklands are visually defined by the presence of widely scattered trees . The trees are usually of a single species with regional favorites. The trees offer shade to grazing animals, protect crops against wind bursts, provide tree prunings for firewood, and host insect or rodent-eating birds.

Research with Faidherbia albida in Zambia showed that mature trees can sustain maize yields of 4.1 tonnes per hectare compared to 1.3 tonnes per hectare without these trees. Unlike other trees, Faidherbia sheds its nitrogen-rich leaves during the rainy crop-growing season so it does not compete with the crop. The leaves regrow during the dry season and provide land cover and shade for crops.^[19]

Shade systems

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.^[20][21] The shade does not necessarily translate into lost or reduced yields. This is because the rate of photosynthesis hardly increases once the light intensity is over about one tenth that of direct overhead sunlight and declines with increasing light intensity. This means that shaded plants can grow well. Planting more than one level of vegetation can increase total photosynthesis and overall yields.

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.

Alley cropping

Alley cropping corn fields between rows of walnut trees

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.

Alley cropping 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, Euphorbia tricalii, Tephrosia vogelii, Gliricidia sepium and Faidherbia albida. In one example, a ten-year experiment in Malawi showed that, by using the fertilizer tree Gliricidia (Gliricidia sepium) on land on which no mineral fertilizer was applied, maize yields averaged 3.3 tonnes per hectare as compared to one tonne per hectare in plots without fertilizer trees or mineral fertilizers.^[22]

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.^[23]

Strip cropping

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

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.^[24] 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.

Boundary systems

A riparian buffer bordering a river in Iowa

Multiple applications use a boundary system. These include living fences, riparian buffer and windbreaks.

• 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.

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.^[25] 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.

Physical support systems

Trellises are a relatively recent addition to agriculture. Earlier, grapes and other vine crops were grown atop pruned trees. Variations of the physical support theme depend upon the type of vine. The advantage is greater in-field biodiversity and control of weeds, diseases and insect pests.

Agroforestry in Switzerland

Since the 1950s, 3/4 of Swiss orchards have disappeared. Increasing costs drove conversion to more intensive agriculture. Agroforestry there plants trees together with annual crops.

Example trees include the maple tree (Acer), apple tree (Malus domestica), pear tree (Pyrus domestica), Sorbus torminalis, oak tree (Quercus), ash (Fraxinus excelsior), Sorbus domestica, walnut tree (Juglans regia), and cherry tree (Prunus avium).

Crops include wheat, barley and grass.^[26]