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  • Writer's pictureMauricio Sagastuy

Alley cropping: overview and design (Agroforestry)

Updated: May 3

Alley cropping is the planting of rows of trees and/or shrubs to create alleys where agricultural or horticultural crops are produced. Alley cropping systems increase the overall farm productivity while at the same time creating many positive environmental effects, such as increasing biodiversity, carbon sequestration, and reducing soil erosion and nutrient loss.  


In this post, I give you an overview about alley cropping by covering the following topics:


  1. Understanding alley cropping systems

  2. The benefits and limitations

  3. Suitable tree species for alley cropping

  4. Design considerations

  5. How to manage the competition between trees and crops



1. Understanding alley cropping systems


Alley cropping is a type of agroforestry system where the farmer aims at maximizing the overall productivity of the farm. Both the products of the trees and the agricultural crops are important sources of revenue for the farmers. 


This is slightly different compared to other types of agroforestry systems, such as windbreaks or riparian buffers, where generating revenues from the trees is not as important. The simple yet efficient design of straight rows of trees allows for an effective management of the trees and crops. Additionally, just as other types of agroforestry systems, the trees and shrubs provide several environmental benefits to the farm.


Illustration of an alley cropping system

Illustration of an alley cropping system

Source: Illustration from the USDA National Agroforestry Center


Alley cropping systems can look in many different ways and be composed of different tree, shrub and crop species. A common pattern that applies to most alley cropping systems is that the annual crop in the alleys typically generates the most revenues during the first years. However, when trees start producing they tend to become the main source of income, because their products can usually be sold for significantly higher prices compared to cereals or vegetables. This is especially the case if the trees produce high value products such as fruits or nuts.  


When alley cropping systems are designed and managed in the right way they generate yields that are significantly higher than if the trees or agricultural crops would have been cultivated separately. This effect is known as the “Land Equivalent Ratio” (LER). The LER describes how much higher your yields are when cultivating trees and crops in the same field compared to if you would cultivate them in separate fields. The reason why you can generate higher yields is because you maximize the positive effects, such as microclimate improvement and wind protection, while at the same time mitigating the negative side-effects, such as decreasing root competition by root-pruning or decreasing light competition by pruning the lower branches. 


Land Equivalent Ratio (LER) for different tree species in European alley cropping systems

Land Equivalent Ratio for different tree species in European alley cropping systems

Source: Graves, A.R. et. al. (2007) Development and application of bio-economic modelling to compare silvoarable, arable and forestry systems in three European countries, Figure 4 [1].


A study that demonstrated the statement above is the SAFE project, which stands for Silvoarable Agroforestry for Europe (see figure above). This pan-European project analyzed 42 combinations of trees and crops in silvoarable agroforestry, and modeled LERs of at worst 1.0 (equal to monoculture) and at best 1.4 (40% more productive) [2]. Most LERs were in the range of 1.2-1.3, meaning that European alley cropping systems were on average 20-30% more productive than monoculture farming systems.



2. The benefits and limitations

BENEFITS


Increased productivity per hectare: Well designed alley cropping systems take advantage of the synergistic relationship between the trees and the crops, thus increasing the overall productivity of your farm.


Increased economic diversity: The trees produce valuable products, which generates new sources of income and increases the farm´s resilience in case of market changes.


Improved microclimate: The rows of trees create a microclimate that is more favorable for most crops and they help buffer extreme climatic conditions. 


Improved nutrient cycling: The roots of the trees “pump” nutrients from deeper soil layers and they generate more organic matter, which improves the quality of the soils.


Improved wildlife and pollinator habitat: The rows of trees can attract beneficial organisms such as predators to pests or pollinators, which increases the yields of your crops. 


Improvement of several ecosystem services, such as carbon sequestration, increasing the biodiversity, erosion control, reduced nutrient leaching to streams and groundwater.



LIMITATIONS


Competition between the trees and crops: The competition for water, nutrients and sunlight may affect your crops yields. On the other side, there are management measures that can minimize the negative effect that trees can have on your crops, such as pruning and root-pruning.


Increased management complexity: Adding rows of trees into arable land will increase the management complexity of the system. This is especially the case if you are planning on taking extra measures to minimize the competition between the trees and the crops. On the other hand, many farmers that have adopted alley-cropping practices express that rows of trees do not really increase their management time so much. It just takes a bit of time to re-learn the new patterns of machine driving in the fields. 


Additional investment required: Establishing and managing rows of trees will require additional investments. Trees that produce high value products, such as fruits or nuts, tend to be more expensive than timber trees, trees for bioenergy or berry shrubs. 


Reduced farm flexibility: Rows of trees become semi-permanent structures on the farm, which reduces the flexibility and adaptability of your farm. For example, some crops may be incompatible with light shade. Additionally, certain herbicides for your crops may damage your trees, which may limit the number of crops or pesticides you can use in your farm.


Less efficient tree management: Managing rows of trees that are widely spread from each other throughout a large field is less efficient than managing closely spaced trees, such as orchards, forests or bioenergy fields. Having widely spaced trees means that you would need to take extra time to move from one place to another to apply herbicides, pesticides, prune them, harvest the products and do other management tasks. On the other hand, closely spaced trees create a favorable environment for pests, fungi and diseases to spread and multiply.


More expensive and time consuming tree protection measures: Certain tree-protection tasks such as establishing fences or windbreaks become more expensive and time consuming, since you need to cover longer distances to protect your widely spaced trees. However, not all alley cropping systems need fences against wild animals or wind protection. In those cases, tree protection measures don't become more expensive or time consuming. 



3. Suitable tree species for alley cropping


Alley cropping systems can be composed of many different tree species. Which tree species you choose will depend on the outcomes you want to achieve. 


Most farmers plant trees that can generate additional income, such as timber trees, trees for bioenergy production, fruit trees, nut trees, or berry shrubs. The trend seems to be that farmers prefer to establish trees that produce high valuable crops, such as fruits or nuts, rather than trees that produce less valuable crops, such as timber trees or berry shrubs. Establishing rows of fast-growing trees is another trend that allows farmers to establish the system quickly and manage it efficiently.


Alley cropping system with several rows of poplar in Forst, Germany

Alley cropping system with several rows of poplar in Forst, Germany

Source: AGFORWARD project


Planting trees only for their value to the ecosystem is also possible, however from an efficiency point of view it is not advisable to offer productive land for planting trees that won't generate an additional income. Trees that do not produce are most recommended for other types of agroforestry systems, such as windbreaks or riparian buffers.


Some of the most important factors that should influence your choice of tree species are: 


  1. The soil and climatic conditions of your field

  2. The management requirements of the trees

  3. What product do you want to produce (if any)?

  4. Do you want to prioritize your agricultural crops, your trees or both? Will this prioritization change over time?

  5. How will the shade and root competition affect your yields?

  6. What beneficial insects or pests could your trees or shrubs attract?

  7. Is this tree species compatible with your crops in terms of light absorption (see more below), chemical compounds in the soil, and the timing of the different management tasks, such as harvesting or pruning.


Examples of tree and shrub species and how they influence the system

Examples of tree and shrub species and how they influence the system

Source: Adapted from Kate MacFarland (2017) [3].



4. Design considerations


The most important decisions you would need to make when designing your alley cropping system are: 1) In which direction will you place the tree rows? 2) What will be the width of your alleys? 3) What will be the distance between the trees in the rows? 4) What will be the width of your tree rows (the non-cultivated strip)?


As usual, there is no one exact design pattern or distance that fits best for all farms. How you design your system will depend both on the current circumstances of your farm and the goals you want to achieve. Below, I explain each of the design considerations in more detail:


1. The direction of your tree rows 


In most temperate or colder climates it is advisable to plant your trees in a north-south direction, in order to allow the most sunlight into the alleys (strongest sunlight comes from the south in the Northern Hemisphere). Besides the sunlight, there are other factors that you should take into consideration when deciding in which direction to plant your tree rows. Such factors can be:

  • Wind, where you would want to plant your trees in a perpendicular direction against the most damaging winds.

  • Water, where you would want to plant your tree rows parallel to the contour lines in order to increase the water intake from the trees, increase soil humidity and decrease soil erosion. This could be especially important in dry climates, in soils that experience a lot of water erosion or in hilly terrain.

  • Drainage system, where you would want to plant your trees parallel to the drainage pipes and ditches. 


Alley cropping systems with apple trees and pears in Uppland, Sweden

Alley cropping systems with apple trees and pears in Uppland, Sweden

Source: Agroforestry in Sweden, the Swedish ministry of agriculture (Jordbruksverket)


2. The width of your alleys


How wide your alleys should be will depend mostly on the 3 following factors: 


  • The height of your trees: The taller your trees are, the more they will negatively impact the alleys with their roots and shade. However, from a wind protection stand-point, the taller your trees are the longer their shelter-effect will be on your crop. A good rule of thumb to calculate the wind protection effect of your trees is that for every meter of height of your trees you will get approximately 10 meters of shelter on the lee-side of your trees (learn more about this in the blog post on windbreaks). Thus, if you want to maximize the microclimatic benefits of your trees, then the maximum width of your alleys should be 10 times the height of your trees. For example, a row of fruit trees that are 3 meters tall should have alleys that are 30 meters wide or less. 


  • The machinery you own: You will want to drive as efficiently as possible in the alleys, and avoid having to turn back to drive in the pieces of land that your machines missed. In order to do this, you should have alleys that are as wide as the multiple of the widths of your different machines. For example, if you have a plough that is 4 meters wide, a harrow that is 6 meters wide and a harvester that is 8 meters wide, then you would want the width of your alleys to be 24, 48 or 72 meters wide. This would allow you to drive efficiently with each machine without having to do “double work” with any machines.


  • The climate of your farm: Crops growing in a colder climate will want to have more warmth and sunlight to grow well. In these climates you should have wider alleys where the trees do not shade large parts of the alleys. In warmer climates, you may even want to have more shadow and coolness for your crops. In these climates it is possible to have more narrow alleys or plant larger trees that will shade big parts of the alleys.



3. The distance between the trees in the rows 


The distance between your trees in the rows will depend primarily on the tree species you chose to plant. Some trees grow naturally large, while others are smaller in nature. The bigger trees will need a longer distance between each other than the smaller trees. Another aspect that can affect the distance between your trees is how hard you choose to prune them. When pruning, you can influence the size and width of your trees. 



4. The width of your tree rows (the non-cultivated strip)


It is advisable to leave a non-cultivated strip on both sides of your tree-line in order to avoid damage from machinery and to leave some space for the roots of the trees to develop. I would recommend you to leave a 2-4 meter wide non-cultivated strip, where the tree line is situated in the middle of that strip. This is the width of the non-cultivated strip that most commercial alley cropping systems have in the UK [4]. If you are planning on planting several rows of trees in your “tree row”, for example when planting fast-growing tree species, then you should adapt the width of your non-cultivated strips accordingly. 



Aerial view of the non-cultivated strips in the tree rows of the alley cropping system in Wakelyns farm, UK

Aerial view of the non-cultivated strips in the tree rows of the alley cropping system in Wakelyns farm, UK

Source: AGFORWARD project


5. How to manage the competition between trees and crops


As mentioned earlier in this post, one of the greatest advantages of alley cropping systems is that you can maximize the positive interactions between the trees and the crops in order to produce higher overall yields. In other words, both the trees and the crops can benefit from each other and thus they can produce higher yields than if they would have been cultivated separately.


The following management measures will help you minimize the competition between the trees and the crops, thus maximizing the total yields of your farm: 


1.Pruning: You can influence how much light reaches the alleys by how you prune your trees. The more you prune your trees, the more light will come into the alleys. At the same time you don't want to prune your trees too hard just to let more sunlight reach the alleys. Your first priority should always be your tree's health and your production objectives. And whenever necessary you can prune your trees harder to let more sunlight into the alleys. 


A good general recommendation is to prune the lowest branches of your trees in order to increase the amount of sunlight that reaches the alleys and decrease the likelihood of machinery damaging the branches of your trees, for example when plowing, harrowing, or harvesting. 


2. Root pruning: Root pruning can minimize the competition of your trees-roots with your crops. The best and easiest way to do this is by using a subsoiler and driving it 1-2 meters away from the tree lines. Additionally, whenever you plow your field you will naturally do some root pruning as well. However, plowing won´t prune the roots as deep as the subsoiler.



Complementary light interception of poplar hybrid Gibecq and winter wheat

Complementary light interception of poplar hybrid Gibecq and winter wheat

Source: The Agroforestry Handbook UK [4]. 


3. Use complimentary tree and crop species: There are different ways how the trees and crops can complement each other. One of the most evident ones is by having trees and crops that absorb most of their sunlight (photosynthesize) during different times of the year. For example, crops sown during autumn, like winter wheat, can absorb sunlight when leafy trees are dormant from December to May, and the leafy trees can use the light when the crop is harvested (see Figure above).  

 


References:


[1] Graves, A.R. et. al. (2007) Development and application of bio-economic modeling to compare silvoarable, arable and forestry systems in three European countries, Figure 4

[2] Dupraz, C. et. Al. (2005) Synthesis of the Silvoarable Agroforestry For Europe project

[3] Kate MacFarland (2017) Alley Cropping: An agroforestry practice, USDA National Agroforestry Center

[4] The Agroforestry Handbook: Agroforestry for the UK 1st Edition (2019)


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