Overstory #261 - Alley Cropping

Alley cropping is broadly defined as the planting of two or more sets of single or multiple rows of trees or shrubs at wide spacings, creating alleys within which agricultural, horticultural, or forage crops are cultivated. The trees or shrubs may include valuable hardwood species, such as nut trees, or trees desirable for wood products. Shrubs can provide nuts, fruit or other products. This approach is sometimes called intercropping and multi-cropping. Currently most of the emphasis and research focuses on pecan, chestnut and eastern black walnut alley cropping applications. However, there are numerous other potential tree, shrub and crop combinations.

Role on the Farm

Alley cropping provides the opportunity to grow wood or other tree products such as nuts or fruit, while providing an annual income through the production of companion crops.

Benefits of Alley Cropping

• Diversify farm enterprise
• Reduce erosion
• Improve water quality
• Protect crops
• Enhance wildlife
• Improve aesthetics

Diversifying farm products and supplementing income

Alley cropping diversifies farm enterprises by providing short-term cash flow from annual crops while also providing medium- to longterm products from the woody components. Timber and non-timber products may contribute to income generation from the farm. In addition to the potential for producing nuts, berries, and fruits, well-managed timber can provide a long-term investment.

Reducing soil erosion from wind and water

Soils with a high erodibility index (>8) are highly susceptible to damage and are difficult to protect when used as crop land. The soil erodibility index provides a numerical expression of the potential for a soil to erode considering the physical and chemical properties of the soil and the climatic conditions where it is located. The higher the index, the greater the investment needed to maintain the sustainability of the soil resource base if intensively cropped.

Alley cropping protects fragile soils through a network of roots produced by the trees and supplemental ground cover resulting from fallen leaves and the companion crop. Rows of trees, shrubs, and/or grasses planted on the contour of a slope will also serve to reduce soil movement down the slope.

Reducing erosion on sloping cropland

The interception of rainfall by the tree canopy and increased infiltration due to tree and herbaceous roots protects the soil; water quality is improved due to interception of sediment by herbaceous cover in tree rows and interception, sequestration, and decomposition of agricultural chemicals by tree and herbaceous root environment. Agricultural chemical (e.g., nitrogen) leached beyond the root zone of the agronomic crops can be absorbed by the deeper root systems of the tree, hence minimizing the leaching of chemical into the ground water resulting in improved water quality.

Microclimate impacts

Trees and shrubs improve crop production by slowing wind speed and reducing wind erosion, modifying the crop microclimate with similar effects to that of windbreaks (see Chapter 6 on windbreaks). Alley cropping can reduce crop evapotranspiration by 15-30 percent and increase water content in the tillage layer by 5-15 percent. Deep tree roots transport soil nutrients to leaves. Leaves contribute organic matter to soil and release nutrients as they decompose.

Protecting crops

Alley cropping reduces damage from insect pests by reducing crop visibility, diluting pest hosts due to plant diversity, interfering with pest movement, and creating habitat more favorable to beneficial insects.

Enhancing wildlife habitat and aesthetics

Linear plantings of trees and/or shrubs in an agricultural landscape increases the habitat diversity for wildlife, both through increased amount of edge and/or as a result of the increased diversity (vertical and horizontal) of vegetation. Increased vertical complexity has been correlated with increased bird numbers. These areas can also serve as protective corridors for wildlife movement and provide a food source.

Limitations to Alley Cropping

Alley cropping, as with other forms of multicropping, requires more intensive technical management skill and marketing knowledge.

The following limitations should be considered:

• Requires a more intensive management system including specialized equipment for the tree management and additional managerial skills and training to manage multiple crops on a given site
• Removes land from annual crop production and may not provide a financial return from the trees for several years
• Requires a marketing infrastructure for the tree products that may not be present in the local area
• Trees may be an obstacle to crop cultivation if not carefully planned and designed
• Trees compete with companion crops for sun, moisture and nutrients
• Companion crops may compete with trees for moisture and nutrients
• Herbicide drift from crops may damage trees

Alley Cropping Functions

There are numerous mechanisms in which alley cropping impacts the landscapes to which it is applied, including water management, nutrient cycling, soil quality, microclimate modification and pest management.

• Alley cropping impacts water management by altering the hydrologic cycle through increased water infiltration via disruption of overland flow by the tree/grass strip. Water cycled through the system is more thoroughly filtered and any excess is gradually released.
• Nutrient cycling and soil quality are impacted as deeply rooted trees exploit lower soil horizons and cycle the nutrients to the surface through litterfall. Additional nitrogen is added to the nutrient pool if a nitrogen-fixing tree or shrub is used. Reduced soil erosion by wind and water help maintain soil quality. Additional moisture is added to the site through interception of rainfall by the tree canopy.
• Microclimates are modified due to reductions in wind velocity which reduces air temperatures and evapotranspiration of intercropped plants and soil.
• Pest management can be strengthened through the structural diversity in the landscape developed by the intentional association of trees and crops. Alley cropping creates habitat to build up biodiversity and associated populations of natural enemies of insects, diseases, or weed pests and can interrupt pest cycles.
• Similar to the function of riparian forest buffers, alley cropping practices may help intercept, fix and biodegrade sediments, nutrients, pesticides, and other biological pollutants present on the site.
• Similar to the establishment of windbreaks, alley cropping may improve wildlife habitat by providing food, cover, nesting sites, and travel lanes for a variety of wildlife species.
• Incorporation of trees and shrubs add opportunity for additional products which are derived from the tree/shrub component (wood, nuts, fruit, foliage) as well as the option to plant sensitive crops which can be grown due to the protection from the trees.

Not all of these functions may exist with each application of alley cropping. The function is dependent upon the way the plant components are manipulated in the design process. There is also a lack of understanding of all the different interactions that can occur with the different combinations of tree/shrub/herbaceous (annual and perennial) plants. For a given design, we do not have enough information to evaluate all the different pest interactions to definitively state that beneficial insects will be favored and the negative pests will be reduced, although there are examples of this.

Tree arrangement

The tree and/or shrub row(s) are placed at intervals across the crop field, depending on the purpose, either on the contour or perhaps even perpendicular to prevailing troublesome winds. Several factors are used to determine the interval between the row(s) of trees or shrubs including slope length, field width, crop light requirements and equipment width.

As mentioned earlier, landowner objectives will determine the products to be harvested from the alley cropping practice. These objectives also determine the arrangement of trees/ shrubs and crops and the set of management practices needed to obtain those products. Alley cropping practices are highly diverse and range from simple to complex. Plantings can consists of a single tree species or a number of species. Similarly, single tree rows or multiple rows may be used.

There are several key factors to consider when planning and establishing the practice on a given site:

Layout: Tree Arrangement

• Single or mixed species
• Number of tree rows - single vs. multiple
• Alley width: Between row spacing
• Within row spacing

With a conservation perspective in mind, slope length relates to the spacing needed to reduce water erosion. The light requirement for the crop or forage to be grown in the alleyway must be considered prior to tree establishment. Finally, alley width must be set as multiples of the widest field equipment width.

Single vs. Mixed Species - The row(s) of trees can have either a single species in the row or mixed species. A single species is the easiest to plant but a mixed species planting with similar growth rates and site requirements may provide greater economic and environmental diversity.

Factors to consider when deciding how many rows to establish and the arrangement of the trees within the rows may be based on a number of potential benefits including:

• Annual crop being produced and area removed from production by tree/shrub rows
• Desired tree/shrub crops and management needed to enhance production (such as weed control and pruning)
• Erosion concerns that multiple rows and combinations of trees/shrubs/grasses can better address
• Wildlife habitat created through multiple rows of combined trees/shrubs/grasses

Single vs. Multiple Row Sets of Trees - The single row takes up the least amount of space but the trees will probably require pruning to enhance the quality of the future wood product. Multiple rows, however, will result in self pruning of the interior row(s). Conifers are a good choice as the “trainer” trees in the outside rows since hardwood species will tend to bend toward the light in the alleyway thus reducing their wood value except for chips. Nitrogen-fixing “nurse trees” can also be used.

Advantages to single rows

• Environment maintained
• Less tree to tree competition
• Reduced competition between components
• Wildlife habitat enhanced
• Plant-insect relationships increased
• Economics improved

Single rows create the proper environment for nut trees to develop full crowns. Trees in single rows which are spaced further apart develop wider, more branched crowns. Conversely, if high value tree form is important, then closely spaced trees may encourage self pruning and straight bole development. In single rows, trees are open on at least two sides, and therefore have less competition between trees within each row, when compared to multiple row configurations. Single tree rows add diversity to a typical row crop field. Researchers think the greatest value to wildlife of woodyherbaceous buffers are the benefits created through breaking-up the traditional mono-culture setting associated with agriculture. Vegetation change and structural diversity is an important tool in controlling agricultural pests. Products coming from the farm are diversified through the addition of trees and their products.

Advantages to double rows

• Environment maintained
• Reduced competition
• Wildlife habitat
• Economics

When rows are offset, double rows of trees maintain similar advantages to that of single row plantings while improving the potential to realize environmental benefits, such as soil and water protection. Compared to rectangular grid patterns of tree planting, double rows allow maximum utilization of space for companion crops.

Competition for light between trees can be reduced through offset row configurations. Multiple rows of trees planted in offset configurations maintain exposure of a majority of each trees crown/canopy to sunlight. Double rows provide the same benefits as a single row alley crop setting, but allow structure (vertical and horizontal vegetative layering and density components) and diversity (variety of species planted) to be increased. This creates an environment for greater utilization of the tree row by increased numbers of wildlife species. Finally, products coming from the farm are further diversified. It is also possible that thinned trees can provide early economic gain (prior to final crop tree maturation).

As mentioned, caution should be observed since deciduous hardwood trees will exhibit a tendency to grow towards light. If an environment of unequal lighting is created (more light to one side of a trees’ crown), most hardwood species will grow towards sunlight, and away from competition. This can cause devaluing of the tree for wood products due to sweep (stem curvature).

Advantages to multiple rows

• Benefit from competition between tree rows
• Wildlife habitat
• Plant-Insect relationships
• Economics

Certain trees will benefit from some light competition. For high value wood, it is desirable to grow a single, straight stem. Trees and /or shrubs planted on either side of a high value tree species, can be used to train the stem of that tree. By using ‘trainer’ trees on either side of the high value tree, natural pruning and straight stem growth can be encouraged. Ultimately, this starts the process (may also require pruning) of producing a straight, clear (small or no knots) log of higher quality than might be grown in an open setting. However, choosing the correct ‘trainer’ species is important because you do not want the outside trees to outgrow your center tree. If they do, they will provide too much shade.

Wildlife habitat potential increases greatly with a wider row of trees. Increased numbers of animals will use this area for travel lanes and the interior creates protective cover opportunities for birds and small mammals.

Plant-insect benefits are the same for single and double row configurations, though some additional advantages may be realized by diversifying the species planted.

Economic benefits are similar to double row configurations. As with double rows, additional trees per acre in multiple row configurations may also qualify these plantings for cost-share assistance and create opportunities for medium- term tree crops to be removed for cash flow (e.g., trees grown for landscaping).

Summary

Alley Cropping needs to be part of an overall management system including crop rotation, crop residue management, combinations of buffer practices, pest management and nutrient management. Alley Cropping can help diversify farm enterprises, protect soil, improve air and water quality, enhance fish and wildlife habitat, conserve biodiversity, and beautify the landscape.

To learn more

Read the full, detailed, unabridged article at http://centerforagroforestry.org/pubs/training/index.php.

An introductory video by the University of Missouri Center for Agroforestry provides a well-organized overview of alley cropping. The images are from temperate areas but most of the practices covered can be applied to alley cropping in tropical environments.

Can Vegetables Be More Productive Under Tree-Based Systems (PowerPoint Presentation) by Dr. Manuel C. Palada of The World Vegetable Center. Describes how vegetables can be more productive under a tree-based multistory system like alley cropping.