Introduction
One of the key questions of primary importance to global agriculture and food security is how to optimize sustainable intensification to balance competing demands on land for food and energy production while ensuring the provision of ecosystem services and maintaining or increasing yieldsReference Pretty, Sutherland, Ashby, Auburn, Balucombe, Bell, Bentley, Bickersteth, Brown, Burke, Campbell, Chen, Crowley, Crute, Dobbelaere, Edwards-Jones, Funes-Monzote, Godfray, Griffon, Gypmantisiri, Haddad, Halavatau, Herren, Holderness, Izac, Jones, Koohafkan, Lal, Lang, McNeely, Mueller, Nisbett, Noble, Pingali, Pinto, Rabbinge, Ravindranath, Rola, Roling, Sage, Settle, Sha, Shiming, Simons, Smith, Strzepeck, Swaine, Terry, Tomich, Toulmin, Trigo, Twomlow, Vis, Wilson and Pilgrim1. Agroforestry has been identified by the International Assessment of Agricultural Knowledge, Science and Technology for Development (IAASTD) as a ‘win–win’ multifunctional land use approach that balances the production of commodities (food, feed, fuel, fiber, etc.) with non-commodity outputs such as environmental protection and cultural and landscape amenities2. Agroforestry land-use systems integrate trees with crops and/or livestock, with ecological and economic interactions between the trees and other componentsReference Lundgren3. Evidence is now coming to light that shows agroforestry systems can deliver a range of ecosystem services and environmental benefits, including biodiversity conservation, regulation of soil, air and water quality, and carbon sequestrationReference Jose4, Reference Smith, Pearce and Wolfe5.
Trees have traditionally been important elements of temperate agricultural systems around the world, evolving from systems of shifting cultivation toward more settled systems involving agriculture, woodland grazing and silvopasture, with fertility transfer from woodlands to cultivated land via manureReference Eichhorn, Paris, Herzog, Incoll, Liagre, Mantzanas, Mayus, Moreno, Papanastasis, Pilbeam, Pisanelli and Dupraz6, Reference Von Maydell7. These early systems integrating trees and agricultural production began to decline during the Middle Ages, when crop rotations were developed to maintain soil fertility rather than relying on transfer of nutrients from woodlands to croplands. This separation was further enhanced by the introduction of chemical fertilizers during the 19th century and the development of separate policy regimes for forestry, agriculture and nature conservation over the past 60 yearsReference Eichhorn, Paris, Herzog, Incoll, Liagre, Mantzanas, Mayus, Moreno, Papanastasis, Pilbeam, Pisanelli and Dupraz6. This paper discusses what we can learn from these traditional agroforestry systems to help develop modern systems that integrate ecological farming and agroecological advances to achieve sustainable intensificationReference Pretty8. We also discuss the existing barriers to wider adoption of agroforestry, and identify how these barriers can be overcome to promote agroforestry as a mainstream land-use system.
Traditional Agroforestry Systems
Fruit tree systems
Up until the 20th century, fruit and nut silvoarable (trees with crops), and silvopastoral (trees with pasture/livestock) systems covered large areas in central Europe. These systems are still widespread in certain countries; in Sicily almond trees with cereals or fodder cover 18,000 ha and fig trees with cereals cover over 10,200 ha in Crete and the Aegean islandsReference Eichhorn, Paris, Herzog, Incoll, Liagre, Mantzanas, Mayus, Moreno, Papanastasis, Pilbeam, Pisanelli and Dupraz6. However, in large parts of Europe, other traditional agroforestry fruit tree systems have been declining. The pomeradas, apple trees planted in lines or scattered throughout meadows and croplands, in humid areas of northern Spain have existed since the 13th century but have been declining dramatically over the past 35 yearsReference Eichhorn, Paris, Herzog, Incoll, Liagre, Mantzanas, Mayus, Moreno, Papanastasis, Pilbeam, Pisanelli and Dupraz6. In southern Europe, mixed vineyards that incorporate trees as mechanical support for the grapevines, with the added bonus of diversified economic return, have also greatly declined due to intensification and increased mechanizationReference Eichhorn, Paris, Herzog, Incoll, Liagre, Mantzanas, Mayus, Moreno, Papanastasis, Pilbeam, Pisanelli and Dupraz6.
Olives
Dating back to pre-Roman times, when rows of olive trees were intercropped with wheat in alternate years to improve the olive yield the following year, olive groves still cover an estimated 650,000 ha in Greece and 20,000 ha in ItalyReference Eichhorn, Paris, Herzog, Incoll, Liagre, Mantzanas, Mayus, Moreno, Papanastasis, Pilbeam, Pisanelli and Dupraz6. Grown in rows or as scattered trees, the olive trees are intercropped with cereals, vegetables and fodder crops, and systems combining olives with grape vines are still found in Spain (46,600 ha) and GreeceReference Eichhorn, Paris, Herzog, Incoll, Liagre, Mantzanas, Mayus, Moreno, Papanastasis, Pilbeam, Pisanelli and Dupraz6.
Shelterwoods and woodland grazing
The practice of pasturing in woodland is one of the oldest land-use practices in human history. In northern Europe, mature woodland provided shelter to cattle and sheep during the winter months, while in Mediterranean regions woodland provided browse, forage and shade for livestock during early summer drought periods, and in turn grazing controlled the understorey, reducing the fire riskReference Sheldrick, Auclair, Hislop and Claridge9, Reference Casals, Baiges, Bota, Chocarro, de Bello, Fanlo, Sebastia, Taull, Rigueiro-Rodríguez, McAdam, Mosquera-Losada and Rosa10. Developed over centuries, transhumance systems (extensive animal husbandry involving seasonal relocation of livestock) are still found in mountain areas throughout Europe from Norway to Switzerland, Italy and SpainReference Bunce, Perez-Soba, Smith, Rigueiro-Rodríguez, McAdam, Mosquera-Losada and Rosa11–Reference Luick, Rigueiro-Rodríguez, McAdam, Mosquera-Losada and Rosa13.
Wood–pasture remnants in England feature some of the oldest and widest trees in Europe, providing valuable resources for a wide range of associated biodiversity, as well as having historical and cultural valueReference Isted, Mosquera-Losada, McAdam and Rigueiro-Rodríguez14, Reference Butler15. The New Forest in southern England is one of the largest remaining areas of wood–pasture in temperate Europe, with over 3000 ha of woodland grazed by ponies, deer, cattle and pigs. Recently formed into a National Park, the New Forest pasture woodland has high biological and cultural value and must be grazed to maintain its unique nature.
Pollards
Pollarding, cutting branches from trees 2–3 m above ground, is an ancient practice that provided fodder for livestock and/or wood for fuel or other uses. Archaeological excavations have uncovered pollards dating back to the Iron AgeReference Austad and Hauge16, and a fossil oak pollard found during gravel extraction in the UK has been carbon dated as 3400 years oldReference Butler15. Fodder pollards were often established in wooded meadows where a hay crop was cut from under the trees and grazed subsequently, while wood pollards were more widely found in wood pasture where grazing occurs for most of the yearReference Read17.
Pollarding for fodder was practiced across Europe, and was particularly common in northern Europe and mountainous areas such as the Pyrenees, Alps and high pasture areas of the Basque countryReference Read17. Leaf-bearing branches from pollards were cut on a 2–6-year rotation in the summer, dried and stored for use as livestock feed during the winter. Leaves were also plucked by hand from the trees and dried as a livestock fodder, particularly for dairy cows, and fallen leaves raked and collected in the autumn, primarily for beddingReference Austad and Hauge16. Fodder tree systems are still found in the Mediterranean—deciduous oak leaves are shredded and dried for sheep fodder in Greece, while in Crete and Sicily, carob pods are stored for fodderReference Eichhorn, Paris, Herzog, Incoll, Liagre, Mantzanas, Mayus, Moreno, Papanastasis, Pilbeam, Pisanelli and Dupraz6. Pollarding for wood was also widespread across Europe, though often concentrated in specific areas to meet special demands, such as in Epping Forest, England, where industrial-scale pollarding was used to supply fuel for London. Wood pollards were also important features for farming communities and were often located close to buildings or on rough grazing common land belonging to a villageReference Read17.
Pannage
Since Roman times, pigs were released into beech and oak woodlands to feed on the acorn and beech mast, and into fruit orchards to eat fallen fruit. During the Middle Ages, most forests in central Europe were valued by surveyors according to the number of pigs that could be supported by the acorn and beech mast, with woods designated as one-hog to hundred-hog woodsReference Shaw18. Pannage was a legal term from Norman times describing the right to release swine into a woodland during a specified seasonReference Brownlow19. It often involved long-distance movement (droves) of pigs from areas around larger forest complexes. For example, in 17th-century Denmark, animals were driven from the treeless western Jylland to eastern forestsReference Bruun and Fritzboger20. Although pannage in a legal sense had mostly disappeared by the 1800s, it continues today in the New Forest, southern England. In addition to fattening up for the winter, pigs provide a useful service in reducing the density of acorns which are poisonous to the cattle and ponies that graze in the forests21.
Dehesas (Spain)/montados (Portugal)
Dehesa/montado is a Mediterranean land-use system based on widely spaced oaks (Quercus rotundifolia, Quercus ilex, Quercus pyrenaica and Quercus suber) on shallow stony soils, which produce acorns, wood, charcoal and cork, with sheep, goats, pigs and cattle grazing and browsing beneath the trees (Fig. 1)Reference Moreno, Pulido, Rigueiro-Rodríguez, McAdam, Mosquera-Losada and Rosa22, Reference Castro, Rigueiro-Rodríguez, McAdam, Mosquera-Losada and Rosa23. The system developed primarily to produce fine hams; scattered oaks produce substantially more acorns than woodland trees, which, combined with understorey grazing of grasses and herbs, produce high-quality pigsReference Eichhorn, Paris, Herzog, Incoll, Liagre, Mantzanas, Mayus, Moreno, Papanastasis, Pilbeam, Pisanelli and Dupraz6. The production of cork from the bark of Q. suber L. in these systems is of international importance; about 340,000 t of cork are produced annually with Portugal (54%) and Spain (26%) producing most of thisReference McAdam, Burgess, Graves, Rigueiro-Rodríguez, Mosquera-Losada, Rigueiro-Rodríguez, McAdam, Mosquera-Losada and Rosa24.
Figure 1. Dehesa system in Spain (© ORC 2010).
Erosion control and shelter belts
The value of tree planting for controlling soil erosion was recognized back in the 19th century, when the French forestry department planted pine trees on overgrazed, steep slopesReference Sheldrick, Auclair, Hislop and Claridge9. Some traditional European landscapes are shaped by the establishment of shelterbelts centuries ago, such as in the Rhône valley where trees were planted to protect against the mistral winds. In the 1950s and 1960s in the ‘bocage’ regions of Normandy and Brittany, the removal of hedgerows led to severe wind erosionReference Sheldrick, Auclair, Hislop and Claridge9.
Hedgerows
Traditional hedgerows provided many benefits; in addition to the provision of shelter, hedges provided stock-proof barriers, forage and browse for livestock, as well as food and medicinal plants for rural populations. During the second half of the 20th century, hedgerows were removed to create larger fields for more efficient use of farm machinery. Within the UK, over 50% of hedgerows have disappeared since 194725. Despite recent agri-environment schemes that have helped to slow this decline by introducing options that encourage hedgerow management, re-creation and restoration26, 27, the Countryside Survey reported that the length of ‘managed’ hedgerows decreased by 6.2% between 1998 and 2007, primarily due to lack of management causing hedgerows to become neglectedReference Carey, Wallis, Chamberlain, Cooper, Emmett, Maskell, McCann, Murphy, Norton, Reynolds, Scott, Simpson, Smart and Ullyett28.
Timber tree systems
Intercropping poplar with cereals became fashionable in France in the 18th century, and still covers about 6000 ha in well-irrigated alluvial regionsReference Eichhorn, Paris, Herzog, Incoll, Liagre, Mantzanas, Mayus, Moreno, Papanastasis, Pilbeam, Pisanelli and Dupraz6. In the province of Noord-Brabant, in The Netherlands, poplar was integrated with livestock over an area of 3000 ha, for the production of veneer for matchsticksReference Oosterbaan, Kuiters, Rigueiro-Rodríguez, McAdam and Mosquera-Losada29.
The Decline of Traditional Agroforestry
Eichhorn et al.Reference Eichhorn, Paris, Herzog, Incoll, Liagre, Mantzanas, Mayus, Moreno, Papanastasis, Pilbeam, Pisanelli and Dupraz6 identify seven basic causes responsible for the decline of traditional agroforestry systems in Europe:
• Increasing mechanization leading to the removal of scattered trees to facilitate cultivations.
• The post-war demand for increased productivity through monocultures.
• A reduction in the agricultural work force, prohibiting labor-intensive systems such as full-stature fruit orchards.
• A shift from small, fragmented land holdings to larger single farms, with an increase in field sizes, the removal of boundary trees and landscape simplification.
• Policy regimes that favored single crop systems over crop associations.
• For many years, wooded areas were ineligible for subsidy payments, and so trees were removed to maximize subsidy income.
• Stricter quality regulations for dessert fruit leading to intensification of orchard productionReference Eichhorn, Paris, Herzog, Incoll, Liagre, Mantzanas, Mayus, Moreno, Papanastasis, Pilbeam, Pisanelli and Dupraz6.
Developing Modern Agroforestry
Agroforestry was reintroduced as a modern concept in the late 1970sReference Bene, Beall and Côté30; since then the emphasis has been on the development of new systems designed to fulfil the potential benefits of increased productivity balanced with resource and environmental conservation. Four main features of traditional agroforestry systems can be embraced in the development of new systems to achieve these aims: resource conservation, energy conservation, multifunctionality and diversity.
Resource conservation
Traditional systems harnessed the efficiency of trees to ensure conservation of the key resources, soil, nutrients, water and biodiversity, and avoided the need for external inputs to maintain productivity. This was of particular value on marginal land where soil type, climate and/or topography limited the adoption of more intensive forms of agriculture, such as in the Mediterranean dehesas. Modern systems can build on this knowledge both to restore degraded land, for example, in areas prone to soil erosion or salinization, as well as to increase the productivity of marginal landReference Bari and Schofield31, Reference Wilkinson32. By reducing the need for external inputs such as fertilizer and pesticides, these systems should be of particular relevance to low-input and organic farmers. Furthermore, given the acute problem of greenhouse gas emissions and global climate change, it is clear that the incorporation of trees can also provide a major boost to carbon sequestration, conservation and substitutionReference Albrecht and Kandji33–Reference Schroeder35.
Energy conservation
Trees were once valued for providing shelter for animals, crops and humans, as well as a source of wood fuel; this is becoming particularly relevant once again as we look to conserve energy and develop renewable energy resources. Establishing shelter belts around domestic and livestock housing can reduce energy use for heating or cooling by 30%Reference DeWalle and Heisler36, while providing tree cover on livestock range can reduce the energy needed for regulating body temperatures, and so result in higher feed conversion and weight gainReference Mitlohner, Morrow, Dailey, Wilson, Galyean, Miller and McGlone37. Traditional agroforestry systems such as wood pollarding provided fuel as its primary woody product. New systems incorporating biomass crops such as short-rotation willow coppice could meet the energy needs beyond the farmgate, while other systems could provide wood fuel for domestic wood burners as a by-product of tree management practices including thinning and pruning operations.
Multifunctional
Trees are multifunctional; in addition to primary outputs of food, fuel and timber, trees can provide a range of other resources and functions for use on farms, such as wood products (e.g., fence posts), seasonal fodder production during shortages of forage, stockproof barriers and wood fuel, as well as protecting the environmentReference Jose4, Reference Smith, Pearce and Wolfe5. Traditional systems exploited the full range of functions to maximize total productivity and minimize inputs; novel systems should look beyond the primary products to identify and exploit these additional benefits through careful design and management. This may involve more complex systems of management than currently employed, for example to take advantage of seasonal resources such as acorns and beech mast by running pigs through farm woodlands.
Diverse systems
In contrast to current mainstream agriculture with its focus on a limited range of crops, diversity was inherent to traditional agroforestry systems. This diversity had both economic and ecological benefits; a diversity of products gave both short-term (from crops, livestock, fruits/nuts and wood fuel) and long-term (timber) returns, provided income stability and diversified local economies, while creating a structurally and temporally diverse agroecosystem which benefited associated biodiversity and ecosystem services. It is now becoming generally accepted that, even if the processes are highly complex, there is a general positive relationship in ecosystems between diversity and stability and also between diversity and productivityReference Loreau38, Reference McCann39. Modern agroforestry should aim to incorporate diversity within the planned system, both within the tree and crop components to maximize these economic and ecological benefits.
Modern systems in practice
Agroforestry can be established for the production of food, fuel and timber, and for environmental protection such as riparian (riverside) buffers, shelterbelts and soil protection. Examples of modern agroforestry can be found throughout temperate regions of the world.
In North America, there are many current farm practices that fall under the description of agroforestry. These include windbreak systems (shelterbelts), silvopastoral systems, alley–cropping systems, integrated riparian management systems and forest farmingReference Williams, Gordon, Garrett, Buck, Gordon and Newman40–Reference Rule, Colletti, Liu, Jungst, Mize and Schultz43. Silvopasture is the most common agroforestry practice in the southern United States, most of which is pine-based (Pinus spp.)Reference Nair, Bannister, Nair, Alavapati, Ellis, Jose, Long, Mosquera-Losada, McAdam and Rigueiro-Rodríguez44. Much of the research and establishment has utilized black walnut as the tree species of choice as this is the most valuable wood grown in North AmericaReference Williams, Gordon, Garrett, Buck, Gordon and Newman40.
In New Zealand and temperate regions of Australia, agroforestry systems have been developed over the past 30 years to address the problems of land degradation including salinization and soil erosionReference Moore, Bird, Gordon and Newman45. Agroforestry systems in Australia include scattered trees in pastures, tree belts and woodlotsReference Moore, Bird, Gordon and Newman45. In New Zealand, systems that integrate the production of high-grade Pinus radiata saw-logs with cattle and sheep grazing the understorey to reduce fire risk and provide early returns from the land have been established in degraded areas to minimize soil erosionReference Sheldrick, Auclair, Hislop and Claridge9, Reference Hawke, Knowles, Gordon and Newman46.
Modern agroforestry systems in China have been developed to address three serious issues—environmental degradation, population growth and resource depletionReference Wu, Zhu, Gordon and Newman47, Reference Dazhong, Yingxin, Xunhua and Yungzhen48. Farmland is limited, with an average of 0.1 ha of land available to a farmer, so agroforestry is seen as a means of maximizing productivity from limited resources as well as bringing marginal land into productionReference Wu, Zhu, Gordon and Newman47. Agroforestry systems are implemented both at the small scale on small land parcels by individual farmers, and at the large scale through government projects. One such project is the ‘Three North’ project in north-eastern China which aimed to provide environmental services through afforestation using long- and short-term combinations of multi purpose, multiple-storey species on an area covering 4.1 million km2.Reference Wu, Zhu, Gordon and Newman47
In Europe, Mosquera-Losada et al.Reference Mosquera-Losada, McAdam, Romero-Franco, Santiago-Freijanes, Rigueiro-Rodríguez, Rigueiro-Rodríguez, McAdam and Mosquera-Losada49 identified six basic types of agroforestry: silvoarable, silvopasture, forest farming, riparian buffers, improved fallow and multipurpose trees. Several novel agroforestry systems have recently been developed to investigate the potential of agroforestry to address recent issues such as renewable energy supplies (Fig. 2). A ‘combined food and energy’ system that integrated bioenergy production from belts of alder, willow and hazel with crop and pasture production has been established in Taastrup, Denmark in 1995Reference Porter, Costanza, Sandhu, Sigsgaard and Wratten50, Reference Kuemmel, Langer, Magid, De Neergaard and Porter51. At Aarhus University, Denmark, pigs have been released into energy crop systems to provide benefits for both the pigs and crops52.
Figure 2. Intercropped willow coppice system at Wakelyns Agroforestry, England (© Martin Wolfe 2010).
Designing and managing agroforestry for maximum, sustainable, production
Interactions between woody and non-woody components in agroforestry can be positive, negative or neutral, and the productivity of a system is a net result of these interactions53. Agroforestry systems should be designed to optimize resource capture by maximizing positive interactions and minimizing negative ones. Appropriate selection of the woody and crop or livestock species of the system to meet site and farm business requirements is necessary, as well as careful consideration of the potential interactions between the different speciesReference Oltenacu and Broom54. Ideal tree species for agroforestry systems should maximize niche differentiation between the tree and crop; deep roots are keys to access nutrients and water unavailable to the crop and either a crown that is in leaf outside the crop's main growing period or that casts a light, even shade. The spatial design of the system will also influence productivity by determining the zone of interactions between the trees and crops, and therefore, the relative potential benefits. For example, trees distributed evenly will have a larger zone of interaction with the adjacent crop or pasture compared to a clumped distributionReference Oltenacu and Broom54 and in temperate regions, orientating tree rows in a north–south direction is generally accepted as the most efficient orientation to optimize direct sunlight penetration to the crop/pasture.
Within agroforestry systems, the productivity of each component can be manipulated by management practices including pruning, weed control and protection from animal damageReference Peoples, Brockwell, Herridge, Rochester, Alves, Urquiaga, Boddey, Dakora, Bhattarai, Maskey, Sampet, Rerkasem, Hauggaard-Nielsen and Jensen55, Reference Jose, Gillespie and Pallardy56. Controlling the density of the tree canopy through pruning will determine the amount of sunlight reaching the crop or pasture, and is particularly important in hardwood systems to ensure good quality timber. Below-ground pruning of tree roots through management practices such as trenching, knifing, disking or subsoiling aims to minimize belowground competition and so prolong profitable crop productionReference Walther, Post, Convey, Menzel, Parmesan, Beebee, Fromentin, Hoegh-Guldberg and Bairlein57. Weed control is important in the early years after tree planting to reduce competition, and plastic mulching is often used to reduce weed pressure on newly planted treesReference Iverson and Prasad58. Alternatively, ground cover plants such as clover and lucerne can be established within the tree rows to prevent weed invasion, as long as they have minimum competitive pressure on the treesReference Millar, Stephenson and Stephens59.
Future Developments to Optimize Sustainable Intensification
Recent agroecological advances in sustainable production systems can be adopted to optimize temperate agroforestry systems. Composite cross populations (CCP) of cereal and vegetable crops have been created as an alternative to pedigree lines, by the recombination of diverse seed stocks through hybridization53, Reference Phillips and Wolfe60. This results in a highly heterogeneous crop that provides a dynamic gene pool with the potential to adapt to the local cropping environment and be more resilient to a changing climate. In an agroforestry setting, the use of a CCP may support the development of a crop that is well adapted to the unique abiotic (e.g., shade and water stress) and biotic (competition and allelopathy) conditions within the system (Fig. 3).
Figure 3. CCP of winter wheat in a mixed hardwood system at Wakelyns Agroforestry, England (© Martin Wolfe 2010).
Livestock breeding programs over the past 60 years have focused on the genetic improvement of production traits, such as milk yield, growth rate and number of eggs. This has led to specialized high productivity breeds reliant on high nutrient and energy input systems based on feeding of concentrates that could have been used by humansReference Oltenacu and Broom54. Concerns about associated negative impacts on animal welfare and the environment have led to new breeding programs (e.g., LowInputBreeds: http://www.lowinputbreeds.org) that aim to improve animal health and efficiencies of organic and low-input production systems by reducing concentrate rations and increasing the use of on-farm forage and fodder. This research will be useful in optimizing livestock production within an agroforestry system, by informing selection of breeds that can effectively use a variety of forage and fodder resources.
The use of green manures and nitrogen-fixing legumes is an essential part of organic farming systems, and research is increasing our understanding of the dynamics and processes of nutrient cycling, release and availability for the subsequent cropsReference Peoples, Brockwell, Herridge, Rochester, Alves, Urquiaga, Boddey, Dakora, Bhattarai, Maskey, Sampet, Rerkasem, Hauggaard-Nielsen and Jensen55. Many tropical agroforestry systems include leguminous shrubs and trees as a green manure resource, but this practice is less developed in temperate systems. Relatively few of the 650 woody species that are able to fix atmospheric nitrogen occur in temperate regions; of these Robinia, Prosopis, Alnus and Eleagnus have been investigated for their nitrogen-fixing potential in agroforestry systemsReference Jose, Gillespie and Pallardy56. Significant transfer of fixed nitrogen to crops has been observed in a study that showed that 32–58% of the total nitrogen in alley-cropped maize came from nitrogen fixed by the adjacent Alnus rubra (Seiter et al. 1995, inReference Jose, Gillespie and Pallardy56). Incorporating these species into an agroforestry design may provide a continuous source of nitrogen that reduces the need for a fertility-building ley within the crop rotation.
It is essential to use tree material with good provenance when establishing a perennial system with trees, but tree breeding programs have so far focused on a limited range of species and good seed stocks for many species are difficult to source. Programs such as the British and Irish Hardwood Improvement Program (http://www.bihip.org) have made some progress toward improving the quality of home-grown hardwoods. However, it is now necessary to also consider the implications of climate change for tree growth and productivity. Modeling of species responses to changing environmental conditions predicts a poleward and upward shift in ranges for many speciesReference Walther, Post, Convey, Menzel, Parmesan, Beebee, Fromentin, Hoegh-Guldberg and Bairlein57, Reference Iverson and Prasad58 and suggested adaptation strategies include assisted migration that establishes species into areas predicted to be their future habitatsReference Millar, Stephenson and Stephens59. This advocates careful selection of tree stock provenances, with a consideration of including stock from the southern part of the species' range in the northern hemisphere.
Ecological theories such as island biogeography and metapopulation dynamics lend support to a landscape-scale approach to enhancing and supporting biodiversity and have resulted in the development and implementation of a range of agri-environment scheme prescriptionsReference Donald and Evans61. A heterogeneous farm landscape, with a variety of habitats and structural diversity of vegetation, provides food, nesting and overwintering resources for farmland inhabitantsReference Tscharntke, Klein, Kruess, Steffan-Dewenter and Thies62. Additionally, diverse farmland can support biodiversity in otherwise open landscapes and allow movement of species between isolated patches of natural habitats, as well as buffer protected areas from the impacts of more intensive systemsReference Bhagwat, Willis, Birks and Whittaker63. Agroforestry systems should be established and managed taking these principles into account by designing a system that provides a range of habitats and resources and acts as a corridor between natural habitat patches.
Barriers to Wider Adoption
Reisner et al.Reference Reisner, de Filippi and Herzog64 used a modeling approach to identify the potential for silvoarable agroforestry within 32 European countries and concluded that one of five commercial tree species (Prunus avium, Juglans sp., Populus sp., Pinus pinea and Q. ilex) could grow productively in an agroforestry system on 56% of utilized arable land, while providing ecosystem services such as reducing soil erosion and N leaching on 6 and 30 million ha respectively. However, this potential has not yet been fully realized in temperate regions. Three key areas of activity essential for promoting agroforestry into the mainstream are research, dissemination of information and policy.
Research
Scientific research on agroforestry systems started in the late 1970s, and focused on tropical systems; studies on temperate systems only starting to appear in the literature from the early 1990sReference Young and Young65, Reference Young66. The long time scale needed for such research is a limiting factor, with very few examples yet available of complete cycles of the systems through to tree harvest. The complexity of the system requires a multidisciplinary approach to research, across a range of temporal and spatial scales. Research needs range from studies at the fine-scale (species interactions), the farm-scale (economic as well as environmental benefits) right up to the landscape-scale (e.g., watershed impacts on nitrate leaching and biodiversity enhancement), national-scale (e.g., home-grown timber and fuel to reduce imports and increase renewable energy production) and global-scale (climate change mitigation and adaptation).
Dissemination
Another primary barrier to wider adoption of agroforestry is limited awareness among farmers and landowners of agroforestry practices and this has been identified by a number of studiesReference McAdam, Gazeau and Pont67, Reference Graves, Burgess, Liagre, Pisanelli, Paris, Moreno, Bellido, Mayus, Postma, Schindler, Mantzanas, Papanastasis, Dupraz, Rigueiro-Rodríguez, McAdam, Mosquera-Losada and Rosa68 (Thomas and Willis, 1997, inReference Doyle, Thomas, Hislop and Claridge69). For example, in a recent study only 33% of farmers correctly defined agroforestry as the integration of trees with crops or livestock systemsReference Graves, Burgess, Liagre, Pisanelli, Paris, Moreno, Bellido, Mayus, Postma, Schindler, Mantzanas, Papanastasis, Dupraz, Rigueiro-Rodríguez, McAdam, Mosquera-Losada and Rosa68. These studies showed, however, that where farmers were shown agroforestry systems, their level of interest increased. In a survey of farmers in seven European countries, once the concept of agroforestry was explained and demonstrated with photos, half of all farmers interviewed would be interested in attempting silvoarable agroforestry on their farmReference Graves, Burgess, Liagre, Pisanelli, Paris, Moreno, Bellido, Mayus, Postma, Schindler, Mantzanas, Papanastasis, Dupraz, Rigueiro-Rodríguez, McAdam, Mosquera-Losada and Rosa68. This varied according to region: from 18 to 20% in Bedfordshire (UK), Centre (Fr) and Franche Comté (Fr) to 90% in northern Italy. Perceived benefits and constraints of agroforestry also differed regionally. In northern Europe, the principal benefit was considered to be environmental, while in Mediterranean Europe, increased profitability was perceived to be the main benefit. The complexity of work, and mechanization, were identified as the principal constraints in northern Europe, and crop yield decline as the main constraint in the MediterraneanReference Graves, Burgess, Liagre, Pisanelli, Paris, Moreno, Bellido, Mayus, Postma, Schindler, Mantzanas, Papanastasis, Dupraz, Rigueiro-Rodríguez, McAdam, Mosquera-Losada and Rosa68. Other studies indicate that a lack of financial incentives also limits agroforestry adoption (Willis et al., 1993 and Bullock et al., 1994, inReference Doyle, Thomas, Hislop and Claridge69). For agroforestry to be adopted on a wider scale, economic viability and practical management skills need to be demonstrated to farmers and landowners. This relies crucially on effective dissemination and therefore outreach support and extension projects are essentialReference Current, Brooks, Ffolliott and Keefe70.
Policy
A lack of policy support is seen as one of the main barriers to wider adoption of agroforestry. The integration of trees at a low density into agricultural land challenges the conventional specialization of forestry and agricultural policy mechanismsReference Dupraz, Liagre, Manchon and Lawson71. Within Europe, agricultural support is provided through direct aids and market support (Pillar 1) and Rural Development Policy (RDP) (Pillar 2) including forestry policy schemes for farm woodlands. Within Pillar 1, direct payments are made based on the area of eligible agricultural land. Agroforestry is not currently recognized as a valid land use class, and eligibility of a particular agroforestry system depends to a great extent on the nature of the woody component in the system. Timber or wood fuel trees are ineligible for payments unless the area can be grazed or agricultural activities can carry on in the same way as if trees were not present, while systems that include permanent crops such as top fruit, hardy perennial soft fruits such as blackberries and raspberries, nuts and vines are eligible for payments, as are short rotation coppice systems.
Under Pillar 2, agri-environment schemes are designed to protect the natural and historic environment, promote public access and protect natural resources. While agroforestry systems provide a means of improving ecosystem service delivery on farmland, the management needed to maintain productivity often conflicts with management requirements specified by the schemes’ traditional agroforestry methods. For example, parklands, hedgerows, wood pastures and traditional orchards are particularly valued for their cultural heritage, and several options in RDP support the restoration and maintenance of these systems. However, within these options there is little emphasis on managing these systems for productivity.
Also under Pillar 2, there is direct support available for agroforestry through Article 44, which supports the first establishment of agroforestry. In 2009, a review of implementation of forestry measures under the RDP by member states found that only 17 Regions or States had adopted this measure72 although France has recently incorporated Article 44 into its new RDP ‘Objectif Terres 2020’. Raising awareness of the potential of agroforestry among policy-makers essential for promoting agroforestry as a mainstream land-use system.
Conclusion
In temperate systems, the general belief seems to be that the high cost of manual labor in Europe necessitates a greater reliance on agrochemical input and intensive management, particularly in the industrialized northern countries. However, we can develop sustainable agroforestry systems by building on successful aspects of traditional systems and assimilating these with recent advances in agroecological approaches to production. This should allow the full potential of agroforestry as a low-input, biodiverse approach to sustainable production and ecosystem service delivery to be realized.
Acknowledgements
We gratefully acknowledge the support of the Ashden Trust that allowed us to carry out this literature review. Thanks also to Sophie Lewis for valuable comments on an earlier draft.
