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 Claridge^9, 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 Rosa^11–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-Rodríguez^14, 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 Rosa^22, 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 restoration^{26, 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.

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 Schofield^31, 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 Kandji^33–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 Jose^4, 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 Loreau^38, 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 Newman^40–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 Claridge^9, 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 Newman^47, 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 Wratten^50, 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 Jensen^55, 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.

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 Young^65, 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 Pont^67, 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.