Advantages and Disadvantages of Agroforestry Systems and Practices

Trees have always been important part of the countryside. When traveling through rural areas anywhere in the world you will see trees incorporated into the landscape. You will see orchards, scattered fruit trees in gardens, hedgerows separating fields, woodlands, or perhaps majestic oaks forming natural boundaries of meadows.

These trees have been managed by us. They have been planted or left to grow with a purpose. They protect, separate, shield, provide and feed–just like trees in natural ecosystems. We have recognized their priceless services since we have started to settle and cultivate land to produce food.

As the historical records show, already during the Copper Age were original pine forests in Spain replaced by widely spaced oaks and grasses to raise pigs [1]. The combination of grasses for grazing and acorns from oaks to feed on, resulted in a high-quality pork meat. Additionally, large growing oaks supplied plenty of firewood and other products (e.g. cork). It didn’t last long before the majority of Spain’s landscape was covered with these fruitful meadows, as they not only supported livelihoods of local people, but also the biodiversity of wildlife, pollinators and plants [2].

Trees managed this way created such a productive system that it is highly valued until today. In south-west Spain and southern parts of Portugal, thousands of hectares are still managed exactly this way—in a sustainable agroforestry system called Dehesa in Spain or Montado in Portugal [1]. Dehesa is home to a special local pig breed, Ibérico pig, capable of transforming the essential fats from the acorns into its fatty tissues, which gives the meat specific aroma [2].

Freely roaming Ibérico pigs in Dehesa are just one example of how trees can be utilized in farming, the full potential of trees in sustainable agriculture is much greater and gains on popularity in recent years.

Let’s see what is hiding under the term of agroforestry and what benefits and limitations agroforestry comes with.

What is agroforestry system?

Modern agroforestry finds its roots in development issues stemming from the 1970s, as a response to social and environmental downsides of high input agricultural practices on the poor populations.

Simply said, agroforestry is a term used for trees incorporated into farming systems in many diverse ways and for various purposes. Agroforestry can be, for example, scattered trees on pastures, diverse fruit and nut trees in orchards, but even tree plantations like rubber tree forests that resemble jungle or perfectly aligned poplars for biomass production. In this scenario, fruit, fodder and wood products are obtained from trees.

In other cases, agroforestry systems are centered around problems they aim to address. These include shelterbelts and windbreaks that shield crops from fierce weather, strip cropping systems to separate diverse crops, soil protection systems, or riparian buffers that protect creeks from agricultural runoff and deposition of sediments.

Agroforestry systems can be as diverse as natural ecosystems are. They can be found on farmlands but also in public areas, for example, along highways and roads or along rivers. So, next time when you travel through the countryside, pay attention to this wonderful diversity of tree uses in the landscape. You can be sure that trees are there because they have some important functions to perform.

What are the advantages of agroforestry?

Agroforestry belongs to sustainable farming practices. The practice is based on ecological principles that are observed in natural ecosystems. Trees greatly help to mitigate negative effects of modern agriculture on the environment. They offset pollution from farms and make food production systems more resilient.

Despite the beliefs of conservative farmers who claim that this practice cannot sustain needs of the world’s growing population, plenty of evidence suggests otherwise. Agroforestry systems have capacity to simultaneously generate food, biomass and raw products that can be utilized in other economic activities. They encourage diversification of the rural economy and create new socio-economic opportunities for rural populations, especially for the world’s poorest that are often challenged by the lack of jobs and resources.

Agroforestry is identified as a food production system of the future. And here are some good reasons why…

#1 Soil fertility and closed nutrient cycles

Soils are under enormous pressure. Intensive way of farming is based on the principles of economy rather than the principles of ecology, yet, it deals with living organisms that interact in complex ways that we cannot simplify and replicate in a uniform way. The result of us trying to do so are soils depleted of nutrients with destroyed structure, which renders them infertile and vulnerable to degradation.

Agroforestry systems respect structural and ecological diversity of species in ecosystems. When managed well, trees have a great capacity of maintaining soil fertility by building up organic matter and mediating nutrient cycling.

In fact, soils in agroforestry systems are richer in nitrogen, phosphorus, potassium, calcium and organic carbon [4]. As a study of fertile lands in Brazil found out, presence of trees on farms managed by indigenous farmers led up to 20 times higher content of phosphorus and 2.5 higher level of potassium in soils [5]. This is because trees are more efficient at extracting nutrients from deeper soil layers than annual crops with shallow roots.

Deeper and stronger root systems of trees reach nutrients that would be unavailable to other plants. Trees incorporate these nutrients into their leaves. Nutrients “locked” in leaves are released during the process of decomposition after leaves have fallen on the ground. This enriches the upper soil layer and enhances the growth of healthy crops.

Additionally, some tree species like Acacia, Alder or Black Locust, fixate atmospheric nitrogen into soil, making it available to crops that do not have this ability.

Tree roots also prevent nutrient leaching from fields. This is the reason why farmers place them on the boundaries of their fields.

The presence of trees reduces soil disturbance which is beneficial for a variety of soil microorganisms as well as allows for a symbiotic formation of mycorrhizal partnerships between plant roots and fungi. Fungi play an important role in plant’s nutrient intake by aiding the decomposition of more complex substances. They also protect plants from toxins and pathogens and increase crop’s resistance to drought [7].

#2 Control of soil salinity

More than 900 million hectares of land worldwide is infertile because of high soil salinity [8]. This land cannot sustain common crops and is often abandoned. But one of the main reasons why the salt content in soils increases is irrigation-dependent agriculture in areas with drier climate.

When perennial plants with thick root system and permanent land cover get replaced by annual crops with shallow roots and wider spacing in between plants, excess irrigation water or even rainwater percolates into the soil and rapidly rises groundwater levels higher than they would be under normal conditions. During this process, groundwater dissolves salt that has naturally accumulated in soils through atmospheric deposition and brings it up to the surface [9].

Crops grown on such soils do not develop properly because soil salinity disrupts nitrogen uptake by plants. Increased salt content further leads to the loss of biological functions of soils and harms microorganisms. Affected soils have difficulties to recover once they have reached this stage. However, as numerous scientists highlighted, trees can restore them and keep salinity levels in check.

Trees are often the first vegetation that is able to grow on salinized lands, as some tree species, like eucalyptus, can withstand fairly high salt concentration. Tree roots immediately start utilizing water from soils around them, while also losing some through their leaves in the process of transpiration. This helps to bring groundwater levels back to their normal state and reduce salinity [9].

Trees meanwhile keep producing wood and other products while performing soil regenerating services. This way, they gradually create suitable conditions for other plants, while maintaining balanced environment underground. 

#3 Prevention of runoff, better water management and cleaner groundwater

Trees do so much for water. Their role in water management is irreplaceable. Incorporating trees into the food production system is in many cases one of the best strategies we can choose to make sure that soils have optimum moisture levels to sustain crops.

Trees are especially helpful for farmers without access to irrigation. By improving soil structure and increasing organic matter content in soils, trees promote water retention capacity of soils. Their crowns shield larger areas of land from direct sunshine and decrease evaporation. Soils then remain moist for longer periods of time, allowing crops to have better access to water from upper soil layers where their roots are.

One of the biggest water problems in agriculture is runoff. Runoff mainly occurs on damaged lands that are stripped of vegetation and eroding away—a common sight on intensively farmed fields. On these lands, 10 to 30 percent of rainwater is instantly lost by running off the surface and flowing into the nearest water bodies along with soil nutrients it has picked up along the way [10].

The result are soils devoid of nutrients that get dry too soon after the rain, and farmers dependent on rainfed agriculture often struggle to grow crops on these soils without irrigation.

Additional damage is done by removed soil particles with nutrients that are carried into the streams, causing sedimentation which alters aquatic environment. Some nutrients like phosphorus and nitrogen from fertilized fields cause algal blooms in water bodies.

A rapid growth of algae in water contaminates drinking water supplies and kills aquatic species. It is a health threatening condition that is also costly to deal with. For this reason, it is important to reduce nutrient runoff from agriculture and the agroforestry practice can be extremely effective in doing so.

Unlike annual crops, trees have roots in the ground all year long. Tree roots and organic debris on the ground slow down runoff and hold back soil particles. This provides time for nutrients to get gradually used up by vegetation or get transformed by soil microbes into soil enriching products rather than immediately changing chemistry of water bodies.

Riparian forest buffers are a commonly used form of agroforestry practice to protect streams from agricultural nutrient leaching. This practice involves planting of trees, shrubs and perennial grasses along streams. According to a study from 2014, 60 meter-wide buffer zone of trees is capable of capturing 99.9 percent of nitrogen and phosphorus runoff.

#4 Stabilization of soils and microclimate 

Standing tall and strong in the landscape, well-rooted in the ground, with branches swinging in the wind, trees are one of the most stabilizing elements in the landscape. Wildlife and people have instinctively been drawn to trees for safety in times of discomfort and it has always worked in our favor.

You may have noticed this yourself: trees have a great ability to modify the microclimate. Air surrounding them feels cooler on a hot day; they shield from rain; they slow down wind. Trees help smaller plants to prosper and it works amazingly well for agroforestry farmers with their crops and animals too.

According to a study in Brazil, trees planted as windbreaks have the capacity to reduce wind speed by nearly 50 percent [11]. This protects crops from breaking or falling down–especially during the ripening stage when they are heavier.

By reducing wind velocity, trees also protect agricultural lands from wind erosion.

But these are not all the benefits. Windbreaks and shelterbelts encourage even distribution of snow pack across the land and mitigate impacts of storms. These measures prevent damage to crops and help to spread water more evenly across the surface.   

Trees scattered across the pastures promote wellbeing of livestock by creating a natural shelter for animals and improving their thermal comfort. Many livestock farmers can confirm that this leads to better animal health and performance [11]. For example, windbreaks can reduce energy needs of cattle to keep warm during the freezing winter days by 14 percent. It may not seem like much, but the numbers add up when considering that each animal would need to get these 14 percent of extra energy from food [12].

#5 Lower input of agrochemicals

Agroforestry improves soil quality, provides nutrients and modifies microclimate in a way that naturally supports many crop varieties and livestock. Tree crowns casting shade over land suppress the growth of weeds. Leaves, branches and bark create protective mulch in intercropping systems. This encourages recycling of nutrients [13].

On top of that some tree species (e.g. Mesquite or Redbud) can fixate nitrogen from the atmosphere and convert it into a form that can be utilized by other plants. In some agroforestry systems, farmers plant these trees in alleys between crop rows or on contours to enrich soils of nitrogen [14].

Agroforestry helps this way to minimize the need for synthetic fertilizers, which reduces the concentration of chemicals in agricultural runoff and prevents environmental pollution.

Diversifying agricultural land by planting perennial trees provides long-term habitat for insects and small wildlife like birds. Their presence works like magic in controlling pest populations. According to estimates, birds eat around 500 million tons of plant-eating insects each year [17].

Tom Staton mentions in his PhD work that the population of insect pests has been reduced by 25 percent in silvoarable agroforestry system (trees planted in arable fields). And numbers of insect predators increased by nearly 30 percent on these lands compared to conventional arable fields [15].

Farmers also see the decrease of pests in silvopasture systems, where natural behavior of grazing livestock actually helps to protect fruit and nut trees from pests and pathogens.

You can watch this nice interview with a silvopasture farmer from Wisconsin to learn more.

Agroforestry is a sustainable farming method with a good reason. Unlike conventional agriculture, it has the potential to reduce the application of chemicals on our food without compromising its quality.

#6 Improvement of wildlife and pollinator habitat

Habitat loss due to the modern development and intensive agriculture with hectares of monoculture fields drives many species of birds, amphibians, insects and even mammals away from large areas of land. This is a problem because we cannot substitute for services these little creatures perform for the health of ecosystems where we live. In fact, in many cases we are not even aware of the function of some organism until it disappears and adverse effects following its demise take over.

For example, in some areas of Czech Republic, farmers suffer from great crop losses due to the overpopulation of a small rodent called common vole. The situation got critical this year with rodents slowly invading even cities and gardens, destroying vegetation and harvests. But the problem could have been easily prevented if farmers incorporated more trees and shrubs between their fields to create islands of habitats for birds of prey and foxes which are scarce in affected areas [16].

Agroforestry practices like riparian buffers, windbreaks, or silvopasture, provide shelter, sources of food and space to many beneficial species. Trees often serve as corridors, connecting different habitats and supporting free movement of animals. For example, around 40 percent of forest-dwelling birds were observed during the day in adjacent orchards [18].

Wildlife corridors are important for migratory birds and insect pollinators, such as Monarchs butterflies that annually fly thousands of miles to their overwintering grounds and need spaces to rest along their routes.

#7 Remediation of polluted soils

Trees have the ability to absorb pollutants from deeper soil layers and immobilize them in their woody parts. Through their roots creating a rich network that can reach as low as water table, trees prevent groundwater pollution from nutrient leaching and effectively remove soil contaminants. This includes agricultural pesticides and their degradation products as well as heavy metals (e.g. cadmium, lead, mercury), but also persistent organic pollutants [19].

According to the latest measurements, the capacity of trees to capture pesticide residues from soils varies between 40 to 100 percent. The range is fairly broad because it is affected by the tree type, soil characteristics and environmental conditions.

Trees like poplars and willows are generally known for being very efficient in pesticide uptake [20]. That is why you can see willows planted along streamsides or on the lakeshores as protective buffers between water and agricultural land.

#8 Provision of diverse products and poverty reduction

Despite the arguments of farmers advocating for monocultures, properly managed agroforestry systems diversify farm’s production and can produce bigger quantity from the land. When suitable tree species are selected, they do not compete with crops for resources. Instead, they provide additional benefits that support higher crop yields.

One such crop is cocoa. Cocoa originates from the dense jungle of the Amazon rainforest. This perennial crop has developed in shaded conditions and doesn’t do well in the direct sun. That is why agroforestry systems where cocoa plants grow under native trees have been used by Brazilian farmers for more than 200 years. Cocoa plants achieve higher yields when grown close to their preferred conditions, and farmers get harvest even from other trees grown on the same plot [21].

In sustainable agroforestry systems, farmers profit from a wide range of products. This includes timber and firewood, fruits, nuts, medicinal products (e.g. witch hazel) and complementary fodder for animals, but also less obvious produce like mushrooms, leaves, and bark that can be marketed to gardeners as a premium mulching material.

Trees are especially important for subsistence farmers in developing countries. For example, many families in Africa rely on their trees for fruits as an important source of nutrition and supply firewood for cooking and keeping warm [22]. According to FAO, around two billion people still need firewood for these basic house chores, and one great advantage of cultivating trees is that the wood is available throughout the year without the need to fell a whole tree. 

In other cases, trees contribute to farmers’ income. In Ethiopia and Sudan, farmers obtain around 20 percent of their yearly income from gum yielding trees like Acacia senegal and Boswellia serrata [23]. The harvest of gum and resins is an important part of people’s livelihood during the dry season when other sources of income, such as livestock products, decline. Agroforestry helps to alleviate financial stress of these vulnerable communities [24].

#9 Prevention of damage to forests

Through the provision of a wide range of products, agroforestry practices help to prevent deforestation in places where wood is needed for cooking and as a fuel for other activities. For example, Malawi suffers from great rates of deforestation because large amounts of firewood are needed to make bricks that are traditionally used for building houses. Other activities that lead to a quick loss of forest cover are charcoal burning and expansion of agriculture.

The need for wood as fuel is large in countries like Malawi. At the same time, even the need for land to grow food increases. This puts extreme pressure on native forests, as trees are cut to make space for farmland and to supply fuelwood. Deforestation happens often in an uncontrolled manner and comes with negative effects that outweigh short-term benefits of utilizing forest’s resources.

In such a scenario, agroforestry comes with many benefits. Trees on farms produce wood and non-wood products sustainably, while allowing for cultivation of crops on the same plot. Besides previously mentioned advantages of agroforestry like improved soil fertility, crop yields and possibility of income generation, farmers also save time and money when having firewood directly available from their own land. They do not have to buy fuelwood, neither have to spend time collecting wood from forests [22].

#10 Climate change mitigation (carbon sequestration)

In recent years, the role of agroforestry in climate change mitigation and adaptation received particularly strong attention. The United Nations Framework Convention on Climate Change (UNFCCC) identifies agroforestry as one of the main climate mitigation practices used in agriculture, and 40 percent of developing countries are considering agroforestry as an important part of their climate change mitigation plans [25].

There are two ways how agroforestry helps to address problems that are contributing to climate change. Agroforestry systems:

• sequester atmospheric carbon;
• reduce deforestation, restore degraded lands and biodiversity.

According to the Climate Institute, agroforestry lands contain equivalent or even higher amounts of organic soil carbon than some natural woodlots. Trees are extremely efficient at absorbing atmospheric carbon and utilizing it to form their bodies, but they also improve soil quality, making soils rich in organic matter and capable of storing higher amounts of carbon than other cultivated soils [26]. For example, a multistrata agroforestry system that mimics the structural diversity and plant layering of a real forest, has the capacity of sequestering 2.8 tons of carbon per acre each year.

Equally as important is the prevention of deforestation and restoration of degraded lands that can be achieved by applying this sustainable farming method. When deforestation happens, carbon that has been stored in trees and soils is immediately released as a result of the disturbance. This increases carbon dioxide emissions and exacerbates the effects of climate change.

Similar situation happens with degraded soils, as they get depleted to the point when they lose the ability to store carbon and support nutrient cycles. Planting trees on degraded lands is often the key solution to reviving soils and restarting processes that normally take place in these ecosystems [27].

What are the disadvantages of agroforestry?

Agroforestry systems can be as diverse as natural ecosystems are. What works on one farm, may not work on another farm. The combination of crops, animals and tree species interacts in complex ways that can be difficult to predict.

Before getting all excited about the numerous advantages of this farming method, it is good to learn about disadvantages of agroforestry as well. Being well informed is the most effective tool in preventing unwanted consequences.

#1 Labor intensive system

A successful agroforestry system requires adequate knowledge, planning and periodic tree maintenance. Having trees or shrubs among the crops doesn’t allow complete mechanization of the farm’s production, which can be a nuisance for some farmers.

Spaces between the trees have to be individually maintained to control weed growth and to make sure that each tree has enough space to develop according to its purpose. For example, trees grown for timber can be grown closer together because this will encourage the development of a straight trunk. On the other hand, fruit or nut trees should have larger spacing to allow full crown formation.  

This requires regular monitoring and systematic work that differs during tree growth stages. Small trees may require fertilization and irrigation in their first years. When they grow bigger, regular pruning and thinning is needed to ensure their healthy development and good yield. Individual trees should also be checked for pests and diseases throughout the season.   

#2 Long waiting time for payback

There are very few downsides to agroforestry. However, the main disadvantage for those trying to grow trees and shrubs for profit is time. Agroforestry is never a quick “fix” because trees, unlike crops, take a long time to mature before they can really fulfill their purpose in the system.

Take for example a pecan tree. The tree usually reaches its full production when ten years old. This is a long time for a single farmer to wait for the potential profit. But if farmers plant trees with a greater vision, for future generations, agroforestry is definitely worth the effort and investment. A single pecan tree can keep producing nuts for more than 100 years, not even mentioning other services it will perform for the soil and adjacent crops [28].

In some situations, such a long waiting time is a serious obstacle. This is often the case of small farmers from developing countries whose livelihood depends on their annual harvest. Due to the lack of money, these farmers have to plan their activities carefully. They cannot afford to spend time caring for trees, which will not earn money for the season. They rather spend their time cultivating cash crops or performing other crafts that will bring them money instantly.

Additional limitation for subsistence farmers is the uncertainty of market prices. The price for agroforestry products may be high at the moment, but since it involves a few years waiting time, farmers cannot be certain that the price will not drop when they want to sell their products, which would render their hard work disappointing [29].

#3 Limited possibilities to sell products

Agroforestry is seriously underestimated and overlooked from many perspectives. Unfortunately, one reason why farmers are reluctant to switch to agroforestry are poorly structured markets for many tree products. A part of the problem is that many agroforestry products are not commonly traded goods. They are rare and it is difficult for farmers to market them or to access information about the market development. This results in a lot of uncertainty.

Farmers have to face price fluctuations, or a refusal of their products and an inability to find promptly a new buyer. For example, a buyer may refuse the products if they do not look according to expectations or if the harvest was lower that year and a farmer cannot supply agreed amount.

Additional problem arises from the diverse nature of agroforestry products. As a study of marketing limitations in agroforestry in India mentions, farmers do not have a problem to access market for crops, fruits and vegetables. Even the information about prices is transparent in case of these products [30].

But when it comes to wood products, the situation is different. Wood product sales fall under the forest and environmental laws that do not fully recognize agroforestry as one of its branches. When agroforestry farmers want to market their wood products, they need to go through a lengthy and difficult process of obtaining special permits [30].

Similar issue happens in Philippines. Tree farming is more profitable in the long term than crop production, but uncertain marketing conditions discourage small farmers from taking the risk of tree planting for their livelihoods.      

#4 Lack of legal support for agroforestry farmers

After the Second World War, intensive agriculture quickly replaced other forms of farming. Monocultures prevailed because they were seen as the most productive systems, allowing for mechanization and efficiency of farming operations. That is when agricultural policies and incentives started to favor these farming methods. And this also marked the time when many trees on farms were removed to make space for subsidized cash crops [31].

Despite the research on the numerous benefits of agroforestry for sustainable agricultural production, supportive policies for this farming method are still insufficient. Since this form of land management combines forestry and agriculture, it often fails to qualify for subsidies of either sector. Agroforestry doesn’t have characteristics of a typical forest or agricultural land, it is a complex combination of both land use systems.

Due to its specific character, agroforestry needs policies that are specifically targeted on the functioning of this system. Policies that will coordinate and define various elements involved in agroforestry development. There is also a need to simplify regulations to allow easier access of farmers to the market. Unfortunately, this is not an easy task and the whole process may take time, hindering some farmers from adopting this agricultural method.

While this may sound disappointing, some countries have made the first steps towards creating a favorable environment. The first country that adopted an agroforestry policy was India in 2014. India’s National Agroforestry Policy addresses problems and risks of agroforestry farmers and aims to encourage integration of trees into rural landscapes.

#5 Knowledge and technology intensive method

Successful agroforestry systems require proper knowledge and evaluation of the complexities of such a multi-dimensional production. Farmers need to master the methods of combining different plants, considering their compatibility and long-term effects on each other. They also need to think of their main objective. What is the main purpose of tree integration into their farming system–is it products or services such as erosion control?

Agroforestry may fail miserably when applied to the wrong situation (see the disadvantages below to learn more). It is, therefore, recommended to seek expert advise or do a thorough research that will take into consideration local conditions, market situation and government regulations for land management. In some cases, this can be rather difficult since agroforestry is a new concept and some of this important information is missing.

Another barrier to the process of determining the right system for the desired purpose is the long time scale. Many farmers are able to harvest trees only once in their lifetime. This means that they lack the experience and knowledge of the best management practices, which leaves many of them working on the basis of trial and error.

Due to the lack of information combined with poor understanding of how agroforestry could improve production on small farms, poorer subsistence farmers, who could have benefited from this practice the most, are often reluctant to trying [32]. More research and awareness raising is needed if we want to see more trees on farms.

#6 Competition for resources

When not selected to complement each other, trees may compete with crops or livestock for resources. If farmers plant trees in narrow alleys, it is likely that when they will grow bigger, their crowns will shade most of the land below. In this situation, farmers need to switch to shade tolerant crops, otherwise they will experience poor harvest and will be forced to cut the trees down without getting full benefits from them.

In semiarid regions, trees can compete with crops for water, making soils drier and exacerbating problems with available water content in soils. This is usually a result of having too many trees in an area that cannot support rich vegetation or from planting unsuitable tree species that require larger amounts of water than native species.

Similar problem arises when trees are grown on soils with low nutrient content. In areas where tree roots and crop roots overlap, trees could compete with crops for available nutrients. To minimize this competition, experts recommend adding fertilizer to crops grown closest to the trees and planting of deep-rooted trees rather than shallow rooted varieties with lateral root branching [33].

On the other hand, nutrient content in soils increases farther away from tree roots. It is thanks to the decomposition of tree litter. When maintained properly, suitable tree species do support better crop growth, but they have to be chosen wisely.

#7 Invasive species and alternate hosts of pests

The choice of the right tree species determines the success of the whole system. Trees impact their surrounding environment and their impact doesn’t have to be only a positive one. In some cases, trees can harbor pests of crops or provide nesting habitats to birds and rodents that damage crops.

For example, in Côte d’Ivoire, scientists observed that maize harvest was reduced by 25 percent in the closest distance to hedgerows due to the damage caused by rodents which live in them [34].

There have also been cases when introduced trees turn into invasive species, causing more harm than benefit.

One such example is Leucaena leucocephala, a tree that is very versatile as a source of firewood, animal fodder and even for human consumption of its pods. At the same time, this tree is one of the worst invasive species, spreading quickly and forming dense thickets that destroy other vegetation when not controlled [35].

#8 Allelopathy

Some trees produce chemicals that inhibit growth of other plants. This effect is called allelopathy and can be very profound in some tree species. For example, Eucalyptus trees, that were once favored in agroforestry, suppress vegetation (including crops) up to a distance of 36 feet (11 meters) away from trees.

Eucalyptus releases highly toxic volatile terpenes that inhibit germination of other seeds [36]. For this reason, it is not recommended to use these trees in agroforestry anymore.

Another example is neem. A tree used in cosmetics, medicine, or pest control. The tree releases chemicals that affect root growth of common crops such as oats, wheat, maize or soybean. Nearly one quarter of oats harvest has been lost in the presence of neem trees on the field boundary [36].

What makes the situation even more tricky is that the interaction between trees and different crops is not yet fully understood. More research has to be done to determine how to eliminate negative influences and encourage positive effects of trees on crop plants.