Agroforestry - a review

Dr Naomi van der Velden,  May 2019 (upated October 2019)

Agroforestry is a combination of agriculture – the cultivation of plants and animals usually for food – and forestry – the cultivation and management of forests often for timber or wood products, but increasingly for other purposes including recreation, food, and ecosystem services.  Agroforestry is an integrated land-use system that looks for both direct (e.g. productivity or income) and wider (e.g. environmental) benefits from combining trees or woody vegetation with livestock (called ‘silvopastoral’) and/or crops (’silvoarable’).  

Brief history

Agroforestry has been practiced for thousands of years.  Across Europe, in the Neolithic (about 11,000 years ago) according to pollen records and archaeological evidence, people began to clear and burn small patches of forest to cultivate food (Iversen, 1956; Roberts et al., 2018). 

Around 4,500 years ago in the Amazonian forests, people were practicing a polyculture agroforestry strategy as shown by the presence of maize and sweet potato and the increase of other edible plant species in pollen records (Maezumi et al., 2019).  This study showed people were likely to be clearing small plots of land to grow crops but the majority of the forest remained intact. 

Today there remain increased frequencies of edible trees and palms in those areas. Ford and Nigh (2009) suggest that people living in southern Mexico into the Yucatan Peninsula over the last 5,000 years practiced not slash-and-burn agriculture within the forest (where plots were abandoned after a few years high production of annual crops) but actually tended the regenerating successional stages of the forest as ‘forest gardens’; favouring, or planting, food-yielding trees. 

In De Re Rustica, a Roman book on agriculture written by Columella ~60AD, description is given on how to plant grape vines supported by poplar, elm or ash trees with branches used to feed livestock (Columella, 1954 (translation) suggesting that mixed systems were common 2000 years ago. Agroforestry, particularly silvoarable, is widely practiced in the humid tropics, where high light levels and abundant moisture support higher levels of plant growth. This review, whilst including relevant tropical examples, focuses on temperate agroforestry particularly across Europe.

Types of agroforestry

Agroforestry is a very flexible approach, and has many different design options for implementation from large-scale farms to forest gardens. 

Silvoarable

• Alley cropping - Alternating strips of crops with rows of trees e.g. wheat and poplar.
• Intercropped orchards - fruit or nut trees, often in a multiple rows, with crops grown between e.g. citrus trees with soybeans.
• Forest farming - high value crops are grown amongst trees, usually in existing woodland e.g. mushrooms, honey, medicinal plants, wood products. 
• Forest gardens / food forests - often small-scale household or community areas combining edible and useful plants across all forest layers (from canopies to ground and climbers).  (term defined by Hart)
• Home gardens - multilayer vegetation around households that supplies fruits and vegetables to owners.

Silvopastoral

• Wood pastures - fodder crops like grasses or legumes are grown between trees (planted as individuals, clusters or rows) and grazed by livestock in situ.
• Grazed orchards - as above, with livestock grazing within an orchard e.g. apple trees and sheep.
• Grazed forests - livestock grazing within forest or woodland e.g. pigs or cattle.
• Woodland chickens - hens (for eggs) and chickens (or meat) free-ranging in woods.

Non-food agroforestry

• Coppice with standards - fast-growing multi-stemmed shrub or tree species grown for wood products (e.g. poles, hurdles) interspersed with a high value timber e.g. hazel with oak.
• Alley coppice - Alternating strips of short rotation coppice with high value timber trees e.g. willow and ash.  (Alley coppice is recently defined - see Morhart et al., 2014).

Woody components of agricultural landscapes

• Hedges and hedgerows - woody vegetation (e.g. blackthorn, hawthorn, hornbeam, privet) in dense rows forming boundaries between fields or gardens. Usually less than two metres high, but sometimes including individual taller trees.
• Windbreaks - rows or strips of trees grown across the prevailing wind direction to provide shelter to up-stream elements.
• Riparian buffer strips - trees grown along the edges of rivers and watercourses, often to prevent soil erosion or protect the water from adjacent land uses e.g. from nitrate runoff.

Locations and extent

In terms of bioclimatic limits to agroforestry, light levels in northern Europe have been considered insufficient to support an economically productive ground crop in silvoarable practices at higher latitudes, with water availability noted as a limitation in Mediterranean areas (Eichhorn et al., 2006).  Despite this, there are many examples of agroforestry in northern Europe.

Across the European Union (EU) there are 173 million hectares (m ha) are used for agricultural production, or about 39% of the total land area (EUROSTAT, 2018). In Europe, agroforestry has been calculated to occupy about 15.4 (den Herder et al., 2017) to 19.8 m ha (Mosquera-Losada et al., 2018).  Mosquera-Losada et al. (2018) found that, of that total, 90% (17.8 m ha) is silvopastoral including 4.3% (850,000ha) grazing under permanent crops or fruit trees. 

Home gardens account for the next greatest area, covering 8.4% of land (1.6 m ha), with countries in Eastern Europe having a higher proportion of land allocated to home gardens.  Silvoarable agroforestry accounts for less than 1% of EU land occupied by agroforestry (360 000 ha), half of which is under permanent crops.  Riparian buffer strips and hedgerows cover 1.8 m ha.  No data were available for forest farming in this study.  The calculations by den Herder et al. (2017) were generally slightly lower, but included high value tree agroforestry covering about 1.1 m ha. 

According to den Herder et al. (2017), the main areas for agroforestry are around the Mediterranean and include Spain (5.6 m ha), Greece and France (both 1.6 m ha), Italy (1.4 m ha), Portugal (1.2 m ha) and Romania (888 200 ha).  In terms of proportional agricultural area, Cyprus (40%), Portugal (32%) and Greece (31%) have a strong culture of agroforestry.  In comparison, the UK estimates are 551 700 ha, and 3.3% of the usable agricultural area.  The map below indicates where agroforestry is likely to be practiced.

 

Agroforestry clusters in Europe.  From: den Herder et al. (2017)

Home gardens and forest gardens have ancient origins, as seen above.  Although originally used to describe the ornamental plantations of trees in large gardens in the 1800s (e.g. Brandis, 1890), the term “forest garden” was coined by Robert Hart in the 1980s (e.g. Hart, 1988) to describe a seven-storey design of perennial edible and medicinal plants with fruit trees as the top canopy layer (Hart, 1996). 

These are highly productive examples of agroforestry (Carruthers, 1990; Mosquera-Losada et al., 2018).  The maps of home gardens across Europe below are from the latter study and illustrate their importance particularly in Eastern Europe.

 

Importance of home gardens in Europe. Left by area, right by proportion of agricultural land.  From: Mosquera-Losada et al. (2018)

Agroforestry, farming and policy

Agriculture and forestry have become separate enterprises each requiring specific skill sets and using specialist machinery.  Until recently lack of defined need and policy support has endangered the prospects for agroforestry (Carruthers, 1990; Eichhorn et al., 2006), but a surge of interest in the environmental and climate change potential of wooded farm systems increases the likelihood of future support. 

In the UK the oversight of agriculture and of forestry belongs to separate government agencies (DEFRA and the Forestry Commission, respectively), each with different agendas and policies.  Across Europe, policy and support for agriculture is also separate to, and greater than, that for forestry.

The European Common Agricultural Policy (CAP) is responsible for about 40% of the European Union (EU) budget and supports farmers and food producers.  Initially it focused on ensuring plentiful and affordable food production, but in more recent decades has shifted to ensure food production is within the sustainable principles outlined by the Food and Agriculture Organisation of the United Nations (FAO). 

Funding within the CAP is split into Pillar I, focused directly on land productivity, and Pillar II, which supports rural development, the environment and climate change. Agroforestry was first included in Pillar 2 in revisions 2007 and updated in 2013 (EU, 2013). 

Arable land with tree density of greater than 100 trees per hectare (e.g. trees are 10 metres apart) is not considered as arable and so not eligible for direct farm payments. It is not clear whether this refers to seedlings or mature trees only and clarification on this is recommended for future (Lawson et al., 2016).  Usual forestry planting densities for tree seedlings would be 1000-2500 trees per hectare.  A clear exception to the CAP tree densities is where the trees themselves are permanent crops (e.g. apple, pear, orange, lemon, olive). 

In addition, member states (‘countries’ in the EU) with an established local practice of trees in farmed landscapes can decide whether or not to include agroforestry in their national adoption of CAP and thus whether or not to encourage the uptake of agroforestry.  Increasingly, it is recognised that agroforestry approaches can help meet the broader requirements of the new CAP proposals for 2020 and beyond (Hernández-Morcillo et al., 2018).  These include climate change action, environmental care, and the preservation of landscapes and biodiversity. 

Does it work?

The main claimed benefits of agroforestry are:  

• Shelter and support livestock and animals
• Stabilise slopes and reduce erosion
• Land use efficiency
• Carbon sequestration
• Diversification and additional products
• Higher biodiversity
• Contribute to climate resilience and food security"       

Shelter and support productive animals

 

Trees can be planted in pasture for direct value (e.g. timber, wood products) and/or a service value (e.g. to shelter animals, recycle nutrients).  This topic was well researched in the 1970s-1990s, and there is strong evidence that integrating livestock with trees offers benefits to animals, farmers and the environment.  

Sheep actively seek out and make high use of shelter, including rows of trees and individual trees (Taylor et al., 2011).  The inclusion of trees with sheep in Scotland offers animals protection from wind exposure and heat loss, particularly during winter (McArthur, 1991).  Sheltering animals from wind and rain can reduce their metabolic stress, especially for young animals or after shearing (Bhardwaj et al., 2017), and reduce mortality rates (Alexander et al., 1980).

Animals in hotter conditions, e.g. as experienced in Australia, can benefit from the shading effect of trees, and this can be particularly important to calves and lambs, and to shorn sheep (McArthur, 1991). Plantations in New Zealand (usually pine) are frequently grazed by cattle and sheep which can offer additional food to nearby farms in winter but also makes it easier for foresters to work (Bhardwaj et al., 2017).  Pine-sheep agroforestry systems have been shown to provide adequate nutrition to adults (but always not growing lambs), particularly when pine needles fall in winter and spring (Hawke, 1991).  Some cautions are advised with pregnant livestock in agroforestry systems.

Many trees are directly suitable as supplementary fodder - with branches or leaves being cut and fed to animals (e.g. Torres, 1983; Vandermeulen et al., 2018). Rows of trees can also improve the growth of forage crops e.g. by reducing water loss from soil and can give longer-term yields 35% greater than both pure systems (Anderson et al., 1988).  

Wind breaks can also be grown in a successional rotation and using tree species with additional value when harvested (Bagley, 1988).  Denser tree levels across a site, as usually required for high value straight timber, can limit understory productivity and thus the amount and quality of forage available, so more open forests are preferable for grazing (Hawke, 1991). 

A study in North Wales UK, showed no impact on livestock productivity of trees planted either in forestry blocks (2500 stems per ha), dense clumps (400 stems per ha) or more dispersed (100 stems per ha) in the first 6 years of establishment, or compared with pasture (Teklehaimanot et al., 2002). Canopy closure as trees mature can limit the amount and quality of forage available and the number of livestock that can be supported over time (McElwee & Knowles, 2000).   

The introduction of cattle to forest edges can also have benefits to the forest, and cattle (which are less selective grazers than sheep, which preferentially eat broadleaved trees) are being used in conservation management to increase tree seedling recruitment and the expansion of desirable understory species (Hancock et al., 2010).  

It is important to note that many animals like to eat trees, especially young broadleaf trees.  Therefore newly planted trees will need protection where there are grazers, including livestock and wild animals like rabbits or deer (Smith et al., 2015).  Livestock can also damage tree protection measures e.g. stakes and protective shelters (McAdam, 1991). 

Overall, agroforestry can benefit animals and animal production.  Careful selection of trees and animals is required for each site.  Animals and trees require specialised knowledge and investments and on-going care to ensure optimum health and productivity.

Stabilise slopes and reduce erosion

 

Agroforestry is more effective at controlling soil erosion than arable and pastoral systems but less effective than forests.

It is widely accepted that trees can reduce rates of soil erosion by wind and water and contribute to slope stability and reduced sediment and nutrient loss to streams (e.g. Nadal-Romero et al., 2016).  Rows of trees (e.g. as windbreaks or margins) can significantly reduce wind speeds and the wind erosion potential on soils, with the actual effect dependent on tree height and density and wind speed and direction (Bird et al., 1992).

A meta-analysis of 53 studies by Torralba et al. (2016) found that agroforestry showed significant soil erosion and soil fertility benefits (organic matter content and nutrient concentrations) even compared to forestry, and that silvoarable had a greater effect than silvopastoral systems.  A direct study of run-off and soil erosion in Germany found a 90% reduction in soil loss and ~80% less nutrient loss under agroforestry (high value trees with short-rotation coppice / barley / oats) compared to adjacent arable (barley / oats) system (Nerlich et al., 2013). 

Water stable soil aggregates (small clumps of soil that are resistant to erosion) have been found to be higher under agroforestry and perennial grass around watercourses than under arable crops (wheat and maize) or grass contour buffers (Weerasekara et al., 2016). In contrast, no difference in soil loss between agroforestry and agriculture (mainly arable) was found in Swiss orchards (Kay et al., 2018). 

In comparison to forests, agroforestry systems may not always reduce erosion; in Spain and Portugal, soil erosion rates (based on rankings of various factors) of agrosilvopastoral systems (“dehesas”), were found to be higher than in established forest (oak and olives), particularly where there was heavy grazing, but lower than eucalyptus forest (Shakesby et al., 2002). 

Soil erosion is dependent on a number of factors, including the length and steepness of slopes, canopy cover, soil type and precipitation ratesand land management (Panagos et al., 2015), and can also be affected by ground vegetation cover (Ghahramani et al., 2011). 

Agroforestry systems planted as part of swales or in contour lines and designed to reduce slope length and steepness and increase infiltration rates could further reduce soil erosion and water and nutrient loss from sloped landscapes (Knight et al., 2013; NWRM, 2015). However,  despite their wide uptake in permaculture systems, very little academic research has been done  on the performance of swales to date.

Ecosystem provisioning services including land use efficiency, food security, diversification and additional products

 

In terms of food and biomass production, it is not clear whether yields from agroforestry are higher, the same or lower than agriculture or forestry.  Few studies compare the full suite of production yields from theses systems and instead focus on a single element (e.g. food or biomass). 

Although the yields from individual components (arable crops and trees) were found to be lower, higher combined yields were obtained from agroforestry in Spain, France, and the Netherlands from field experiments and modelling exercises.  Total yields were up to 40% greater relative to monoculture arable and woodland systems, and always at least as productive as the monoculture components (Graves et al., 2007).  

Several studies determine that even where production yields are not as high, the gains to the environment are more than worth the slight loss of production (Smith et al., 2012; Torralba et al., 2016; Kay et al., 2018). 

Agroforestry systems offer the benefits of diversification and could help farmers to mitigate against a poor year for one element of the system (e.g. wheat) by gains in the other (e.g. fruit).  Practiced at the small scale, higher diversity systems and those that support pollinators can offer better nutritional security to subsistence households (Jamnadass et al., 2013). 

Where multi-functional indigenous trees are included, food security as well as provision of fuel can be achieved (Mokgolodiet al., 2011). They also provide high value traditional products e.g. Iberian ham, Pecorino Romano PDO cheese, or oak (Moreno et al., 2018).  Further, they often have high cultural and landscape values (e.g. Molnár et al., 2016). 

Where systems are designed for tree biomass or timber production, however, the loss of regular annual income could decrease farmer uptake without initial financial investment support for the longer-term timber crop.  In Europe, changing focuses for the CAP financial support may lead to better support for agroforestry systems.

Ecosystem regulating services e.g. carbon sequestration, climate resilience, and biodiversity

The evidence here is very clear.  Agroforestry is considered to improve ecosystem services and biodiversity (Jose, 2009; Mosquera-Losada et al. 2016; Torralba et al., 2016; den Herder et al., 2017).  In agroforestry landscapes there were reduced nitrate losses, higher carbon sequestration, reduced soil losses, higher functional biodiversity focussed on pollination and greater habitat diversity reflected in a high proportion of semi-natural habitats (Kay et al., 2018). 

Agroforestry has been found to enhance biodiversity and ecosystem service provision relative to conventional agriculture and forestry in Europe (Torralba et al., 2016).  According to the IIASTD report of 2009, agroforestry offers a win-win to climate change mitigation and sustainable development (McIntyre et al., 2009). 

The ability of agroecosystems to sequester carbon depends on whether it is practiced on fertile or marginal soils and in which climate, but the potential to improve degraded lands is enormous (Jose, 2009).  There is high potential to include more agroforestry across Europe, and policy and financial support is shifting towards this. High diversity home garden systems with dense trees are highly effective at storing carbon (Saha et al., 2009). Home gardens, including forest gardens, are considered a key tool to mitigate climate change within a shift to local production and a circular economy (Mosquera-Losada et al., 2018).

“Activities related to permaculture, agroecology and also agroforestry as part of them (when the woody component is present) should be better enhanced to deliver more healthy foods as the biodiversity will be the baseline supporting these systems.”

Mosquera-Losada et al. (2018) 

Overall, it is clear that agroforestry provides many benefits and may be particularly valuable when practiced on a small-scale.

Acknolwedgements

This academic literature review is by Naomi van der Velden as part of our collaborative GROW Observatory project.

The GROW Observatory has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 690199. 

 

References