11.10: Agroforestry

Agroforestry is the integration of trees and shrubs into crop and animal farming systems. The idea is that environmental, economic and social benefits are gained by intensively managing an integrated and interactive system. Here, trees do not just exist as an unmanaged plot of woods but rather benefit the crops and animals on the farm either directly or indirectly. In most cases, agroforestry benefits the farm through income diversification, a more favorable microclimate (shade or shelter from strong wind), and by providing wildlife habitat. Also, in many cases it can improve marginal lands that are not suitable for crop production. Agroforestry, however, requires a long-term commitment because the trees often don’t produce income for several years, or even for decades in the case of timber species.

Alley Cropping

Figure 11.3. Alley cropping involving walnut trees and wheat. Photo by USDA NRCS.

Alley cropping involves planting rows of trees at wide spacings with a companion crop grown in the alleyways between the tree rows. It is often done to diversify farm income, but it can also improve crop production and provide protection and conservation benefits to crops. In the United States, these systems often include cereals, row crops, hay or vegetable crops planted in the alleys between rows of high-value timber, fruit or nut trees (Figure 11.3). High-value hardwoods like walnut and oak trees, or even ornamental trees like woody decorative florals or Christmas trees, are good species and can potentially provide long-term income while short-term proceeds are derived from a companion crop planted in the alleyways. Pecan and chestnut trees are good species for nut production, if that is desired from the tree rows.

Light interception by the trees is a concern when you grow crops in the alleys, especially at higher latitudes. (This is less a concern when the alley crop is shade tolerant, like certain herbs and forages). There are several ways to reduce this effect:

• Space the tree rows more widely.
• Orient tree rows in an east-west direction, which maximizes light interception because the tree obstruction mostly occurs when the sun is at a high angle. This may need to be balanced with other objectives like intercepting wind, which often requires north-south orientation.
• Use trees with fine leaves and less dense canopies that allow for more light penetration for the companion crop.
• Use tree species that leaf-out late or drop leaves early. For example, a late-leafing tree will not intercept light for winter wheat in the early season.
• Thin and prune (coppice) to control large tree canopies and enhance timber quality.

Farmers should tailor the tree layout to the type of species and product. Trees in single rows that are spaced farther apart within the row tend to take longer to close the canopy but also develop more branched crowns, which is desirable for some tree crops, like nut trees. Closely spaced trees in single or double rows encourage more self-pruning and straight trunk development, which is favorable for timber. Sometimes, a taller and shorter tree type can be grown together.

In tropical environments, alley cropping raises fewer concerns related to light interception because the sun is generally more intense and higher in the sky, and there are longer growing seasons. Also, in many tropical countries crop input costs, including fertilizers, are higher while labor costs and mechanization are lower. This creates a greater opportunity to use tropical leguminous trees interspersed with crops to increase the availability of organic nitrogen for the crops, fodder for animals, and firewood for cooking and heating (Figure 11.4).

Figure 11.4. Examples of alley cropping in tropics: Left: Moringa trees grown for vegetable seed pods and herbal medicine together with sorghum-sudangrass grown for forage or soil improvement. Photo by Stuart Weiss. Right: Gliricidia legume trees where new growth is suppressed by regular harvesting of shoots, which are used for animal feed or organic fertilizer for corn (note: corn was not yet planted on ridges).

Figure 11.4. Examples of alley cropping in tropics: Left: Moringa trees grown for vegetable seed pods and herbal medicine together with sorghum-sudangrass grown for forage or soil improvement. Photo by Stuart Weiss. Right: Gliricidia legume trees where new growth is suppressed by regular harvesting of shoots, which are used for animal feed or organic fertilizer for corn (note: corn was not yet planted on ridges).

Figure 11.4. Examples of alley cropping in tropics: Left: Moringa trees grown for vegetable seed pods and herbal medicine together with sorghum-sudangrass grown for forage or soil improvement. Photo by Stuart Weiss. Right: Gliricidia legume trees where new growth is suppressed by regular harvesting of shoots, which are used for animal feed or organic fertilizer for corn (note: corn was not yet planted on ridges).

Although alley cropping can offer advantages, there are some challenges that should be understood. As with other forms of multi-cropping, alley cropping requires more intensive technical management skill and marketing knowledge, and also may demand specialized equipment for tree management. It additionally removes land from annual crop production that may not provide a financial return for several years. Trees may be an obstacle to crop cultivation if their arrangement is not carefully planned and designed. The trees may also result in yield losses for the companion crops grown in the alleys by competing for sun, moisture and nutrients, and in some cases herbicide drift from crops may damage trees.

Other Agroforestry Practices

Forest farming does not separate the land into distinct growing zones like alley cropping but grows understory crops within an established forest, either a natural forest or a timber planting. In this system, the shade from the trees is actually a desired quality because the planted or wild understory crops thrive in such an environment. Typical examples are medicinal herbs like ginseng, certain types of mushrooms, fruits like elderberries, and ornamentals like rhododendrons and moss. Many of these understory crops can be quite profitable.

Figure 11.5. Animals grazing in a silvopasture system. Photo by USDA National Agroforestry Center.

Silvopasture systems involve the integration of trees and grazing livestock operations on the same land (Figure 11.5). They provide both harvestable forest products and animal forage, offering both short- and long-term income sources. In temperate climates, cool-season grasses may grow better during the hotter times of the year with partial shade provided by the trees (while critical early growth is not affected until leaf-out). In hotter climates, the trees help keep the grazing animals cool. Silvopasture systems still require the use of agronomic principles, like appropriate selection of forages, fertilization and rotational grazing systems that maximize vegetative plant growth and harvest. As discussed in Chapter 14, silvopasture systems may also be beneficial on landslide-prone slopes by stabilizing the soil (see Figure 14.12). Faidherbia albida is a tropical legume tree that thrives in seasonally dry climates and can be used both in silvopasture systems and in alley cropping systems. Its leaves are feather-like, and its canopy is therefore not overly dense and permits light penetration for crops like corn or pasture grasses. Also, it has a deep taproot and grows foliage in the dry season when other forage sources are limited. Faidherbia blooms at the end of the dry season and thereby provides food for bees. Its seed pods are feed for livestock or wild game, and the woody parts make good fuel.

Riparian buffer systems involve trees or shrubs that are planted along streams, rivers, lakes and estuaries to help filter runoff from upstream agricultural or urban lands. They also stabilize stream banks and provide habitat and shade for aquatic animals. Although mostly used as a conservation practice, interest has recently developed in using buffer zones for income production, including bioenergy crops by planting willows, decorative woody floral crops, and fruit and nut crops. Similarly, windbreaks and shelterbelts are generally planted for conservation purposes like reducing wind erosion, enhancing microclimates and promoting landscape biodiversity (see also Chapter 14), but they are increasingly valued for potential income from the trees themselves.

Transitional systems take advantage of the increased shading and changed microclimate as trees mature. For example, landowners may initially use an alley cropping system where annual crops are grown between young trees, which is then transitioned into silvopasture, forest forming or an orchard. Alternatively they may decide to trim the trees and continue alley cropping.