Introduction
The Food and Agriculture Organization (FAO) predicts that food production (net of food used for biofuels) must increase by 70% by 2050 to meet the needs of a growing world population1. A major concern is that this will put greater pressure on marginal land and protected habitats, plus intensify existing agricultural practices, with serious implications for the ecological processes that sustain human well-being (‘ecosystem services’2). This food/fuel/biodiversity conflict calls for multifunctional land use, which can meet the multiple demands of food and fuel production, environmental and biodiversity protection, and has the capacity for adaptation or resilience to climate change.
One approach is to design farming systems that mimic the structure and function of natural ecosystems, based on the hypothesis that natural systems are eco-efficient, with internal cycling of nutrients and energy and protection of the resource base. Agroforestry, a land-use system that integrates trees and shrubs with crops and/or livestock production, builds on this idea of ecological design to optimize beneficial interactions between the woody and other components. A central hypothesis in agroforestry is that productivity is higher in agroforestry systems compared to monocropping systems due to complementarity in resource-capture, i.e., trees acquire resources that the crops alone would notReference Cannell, Van Noordwijk and Ong3.
In addition to increased productivity, agroforestry systems are believed to provide a number of other ecosystem services and environmental benefits, including biodiversity conservation, regulation of soil, air and water quality and carbon sequestrationReference Jose4. 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 amenities5. While much of the research has focused on agroforestry in tropical systems as a means of addressing both food security and environmental issues in impoverished rural areas, evidence is now coming to light that supports the promotion of agroforestry in temperate developed countries as a sustainable alternative to the highly industrialized agricultural model with its associated negative environmental externalities. In the following review, we examine this evidence in the context of the ‘ecosystem services’ framework to evaluate agroforestry as part of a multifunctional working landscape in temperate regions.
Definition and classification
While systems integrating trees and agriculture have been practiced for 1000s of years, the term ‘agroforestry’ was first coined in 1977, in a review of research needs, Trees, Food and People, by Bene et al.Reference Bene, Beall and Côté6. In its simplest form, agroforestry can be defined as ‘growing trees on farms’Reference Young7. It is generally accepted, however, that agroforestry systems are deliberately designed and managed to maximize positive interactions between tree and non-tree components. A widely accepted definition was formulated by Lundgren in 1982:
Agroforestry is a collective name for land-use systems in which woody perennials (trees, shrubs, etc.) are grown in association with herbaceous plants (crops, pastures) or livestock, in a spatial arrangement, a rotation, or both; there are usually both ecological and economic interactions between the trees and other components of the systemReference Lundgren8.
This represents a concept of integrated land use that combines elements of agriculture and forestry in a sustainable production system. The emphasis here is on managing rather than reducing complexity. Agroforestry uses the natural woodland ecosystem as a model to create ‘a dynamic, ecologically-based, natural resources management system’Reference Leakey9. Agroforestry systems can be classified according to the components present—trees with crops are referred to as silvoarable, trees and animals as silvopastoral, and trees with crops and animals as agrosilvopastoralReference Nair10.
Ecosystem Services
Agroforestry is valued as a multifunctional approach that balances productive and protective functions, therefore avoiding the trade-off between provisioning and other ecosystem services that occurs in many modern intensive farming systems.
Provisioning services
Agroforestry supports the production of a wide range of products due to the diversity of planned species within the system (Table 1). Higher productivity from agroforestry systems is based on the ecological theory of niche differentiation; different species obtain resources from different parts of the environment. Tree roots generally extend deeper than crop roots, and so access soil nutrients and water unavailable to crops, as well as absorbing nutrients leached from the crop rhizosphereReference Sinclair, Eason, Hooker, Hislop and Claridge11. These nutrients are then recycled via leaf fall onto the soil surface or fine root turnover. This should lead to greater nutrient capture and higher yields by the integrated tree–crop system compared to tree or crop monocultures. In addition to greater resource capture, increased overall productivity of agroforestry systems also occurs as a result of modification of the microclimate by the presence of trees, which benefits crop growth and livestock productionReference Jose, Gillespie and Pallardy12. The establishment of trees on land deemed unsuitable for productive cropping is another way that agroforestry can increase overall productivity from farming systems. Dixon et al.Reference Dixon, Winjum, Andrasko, Lee and Schroeder13 estimated that 140 million ha of marginal or degraded land in North America was available and suitable for agroforestry establishment. In New Zealand, trees have been re-introduced into pasturelands to reduce soil erosion on poor soils; the productive potential of these trees is now being recognized and exploited either for timber production or as a feed resourceReference Mead14.
Table 1. Overview of products from agroforestry systems.
Interactions between trees, crops and livestock
Ecological interactions between the tree and crop or animal components in an agroforest occur above ground (microclimate effects) and below ground (root interactions), and involve a number of complex processes related to soil fertility, competition, microclimate, pests and diseases, soil conservation and allelopathyReference Jose, Gillespie and Pallardy12, Reference Young and Young15. Interactions may be direct (e.g., cattle browsing on leaves) or indirect via changes in the environment (e.g., soil compaction caused by cattle gathering in the tree shade, reducing tree growth)Reference Sinclair, Eason, Hooker, Hislop and Claridge11 and occur at multiple scales. The strongest interactions are likely to occur in the zone immediately surrounding the tree (the tree–crop interface), while shelter effects are known to extend up to 30 times the height of the treeReference Tamang, Andreu and Rockwood16, Reference Williams, Gordon, Garrett, Buck, Gordon and Newman17. The influence of trees on soil and hydrological processes may impact watershed/catchment scale systemsReference Bharati, Lee, Isenhart and Schultz18, Reference Seobi, Udawatta, Anderson and Gantzer19, biodiversity benefits may be recorded at the landscape scale, and carbon sequestration effects at a global scaleReference Jose4.
As agroforestry systems are dynamic, these interactions are likely to change over time, so that there may be complementarity between the components in the early stages, which then shifts into competition for resources as the tree component reaches maturityReference Jose, Gillespie and Pallardy12. The nature of interactions also vary depending on different short- and medium-term ecological factors, such as seasonal and annual variations in rainfall and temperature, so that the balance between the tree and the crop can swing between positive interactions (facilitation) and negative interactions (competition)Reference Gea-Izquierdo, Montero and Canellas20. The productivity of a system is a net result of these interactionsReference Jose, Gillespie and Pallardy12.
Measuring productivity
The land equivalent ratio (LER), first proposed by Mead and WilleyReference Mead and Willey21, is a means of comparing productivity of intercropping and monocropping systems. It is calculated as the ratio of the area needed under sole cropping to the area of intercropping at the same management level to obtain a particular yield:
$$\eqalign{\rm LER} =\tab \displaystyle{{{\rm Tree}\,{\rm agroforestry}\,{\rm yield}} \over {{\rm Tree}\,{\rm monoculture}\,{\rm yield}}} + \cr\tab\displaystyle{{{\rm Crop}\,{\rm or}\,{\rm livestock}\,{\rm agroforestry}\,{\rm yield}} \over {{\rm Crop}\,{\rm or}\,{\rm livestock}\,{\rm monoculture}\,{\rm yield}}}.$$If a rotation includes more than one crop, a weighted ratio for each crop can be used, based on its proportion in the rotation. An LER of 1 indicates that there is no yield advantage of the intercrop compared to the monocrop, while an LER of 1.1 indicates a 10% yield advantage, i.e., under monocultures, 10% more land would be needed to match yields from intercroppingReference Dupraz, Newman, Gordon and Newman22. Yields can be expressed in physical units so that the LER refers to the biological efficiency of the mixture, or monetary units where the LER indicates the economical efficiency of the mixture. Biomass yield LER's of agroforestry systems range from 2 in a pear orchard/radish system (Newman, 1986, inReference Dupraz, Newman, Gordon and Newman22) to 1.6 in the early years after establishment of a cherry/fescue system, declining to 1.0 later in the rotation, with an average of 1.2 over the 60-year rotationReference Dupraz, Newman, Gordon and Newman22.
Economics
Economic studies of agroforestry systems have shown that financial benefits are a consequence of increasing the diversity and productivity of the systems, influenced by market and price fluctuations of timber, livestock and cropsReference Benjamin, Hoover, Seifert and Gillespie23–Reference Yates, Dorward, Hemery and Cook27. Incorporating trees into an agricultural system produces higher long-term returns (Thorrold et al., 1997, inReference Benavides, Douglas and Osoro28), while agroforestry practices are able to recoup initial costs more quickly than forestry-only land use due to the income generated from the agricultural componentReference Rigueiro-Rodríguez, Fernández-Núnez, Gonzalez-Hernandez, McAdam, Mosquera-Losada and Rigueiro-Rodríguez26. Fernández-Núnez et al. (2007, inReference Rigueiro-Rodríguez, Fernández-Núnez, Gonzalez-Hernandez, McAdam, Mosquera-Losada and Rigueiro-Rodríguez26) carried out an assessment of initial investments and establishment costs of forestry, agriculture and agroforestry in the Atlantic area of Spain. They found that establishing agroforestry required higher initial investment than the agricultural and forestry systems due to higher initial inputs, but over a 30-year period, profitability per hectare was higher in the agroforestry system than in the exclusively livestock (17%) or forestry (53%) systems.
Modeling of economic returns from a black walnut alley cropping system in Midwestern USA highlighted the importance of system design and management for maximizing productivityReference Benjamin, Hoover, Seifert and Gillespie23. Systems with widely spaced tree rows (12.2 m between tree rows) were predicted to be more profitable than both closely spaced (8.5 m between tree rows) designs and walnut plantations, while root-pruning increased economic returns by extending the period of profitable crop production within the rotation. All agroforestry systems were modeled as having higher rates of returns than monocropping systemsReference Benjamin, Hoover, Seifert and Gillespie23.
Regulating services
Air quality regulation
Trees have long been used in urban settings to take advantage of their capacity to ameliorate air pollution from vehicles, buildings and industry. The use of agroforestry in mitigating livestock odor has recently been explored by Tyndall and CollettiReference Tyndall and Colletti29. Intensive livestock production units in the American Midwest emit odors that have caused significant social problems with implications for rural and state economies, human health and the quality of rural life. The situation is exacerbated by the highly modified landscapes that lack natural barriers that surround these units, which allow emissions to travel further. Tyndall and CollettiReference Tyndall and Colletti29 recommend the use of shelterbelts to biophysically mitigate odors, with shelterbelts of modest heights ideal for intercepting the movement of particulates which form the majority of odors.
Climate regulation
Integrating trees into the agricultural landscape can regulate the climate at the local scale through modification of the microclimate and at the global scale through reductions in atmospheric carbon by sequestering, conserving and substituting carbon.
Microclimate modification
As ‘ecosystem engineers’ (i.e., they directly and indirectly influence ecosystem dynamics by changing the availability of abiotic and biotic resources to other species in an ecosystemReference Jones, Lawton and Shachak30, Reference Jones, Lawton and Shachak31), trees modify microclimatic conditions, including temperature, humidity and wind speed, which can have beneficial effects on crop growth or animal welfareReference Jose, Gillespie and Pallardy12.
Trees provide an important service by reducing wind speed with reductions extending up to 30 times the height of the windbreak on the leeward side, and between two and five times the shelterbelt height on the windward sideReference Tamang, Andreu and Rockwood16, Reference Williams, Gordon, Garrett, Buck, Gordon and Newman17. This can have multiple benefits for crops; increased growth rate and quality, protection from windblown soil, moisture management and soil protectionReference Jose, Gillespie and Pallardy12, Reference Tamang, Andreu and Rockwood16, Reference Benavides, Douglas and Osoro28, Reference Bird32–Reference Wang and Shogren35. Higher air and soil temperatures in the lee of a shelterbelt can extend the growing season, with earlier germination and more growth at the start of the seasonReference Brandle, Hodges and Zhou33. Fruit and vegetable crops are particularly sensitive to wind stress and suffer lower yields and poorer quality at lower wind speeds than agronomic cropsReference Tamang, Andreu and Rockwood16, Reference Brandle, Hodges and Zhou33. Horticultural crops grown in the shelter of a wind break benefit from moderation of temperature extremes, warmer soil and air temperatures, and improved soil water conditions, all of which contribute to increasing total marketable yields and individual fruit weightReference Brandle, Hodges and Zhou33. Sheltered conditions increase flowering periods and bee activity, leading to increased fruit set and earlier maturityReference Norton36.
Shading effects of trees may result in higher levels of soil water content beneath the canopy compared to open cropland or pasture due to tree shade reducing evapotranspiration, especially in semi-arid regionsReference Benavides, Douglas and Osoro28. In Spanish dehesas, shade from the cork oaks significantly improved the microclimate of a fodder crop (oats), with a reduction in daily and seasonal variation in soil and air temperatures in the proximity of the trees having implications for understorey phenology and physiology and lengthening the growing seasonReference Moreno, Obrador, Garcia, Cubera, Montero, Pulido and Dupraz34.
However, where the tree and crop or livestock components overlap in their use of resources, competition may lead to reduced productivity compared to a monoculture system. Within northern temperate regions, the main limiting resource for plants is usually light, and studies have shown that shading has reduced yields in temperate agroforestry systemsReference Benavides, Douglas and Osoro28, Reference Chirko, Gold, Nguyen and Jiang37, Reference Reynolds, Simpson, Thevathasan and Gordon38. Competition for water between tree and crop components is likely to limit productivity in semiarid regions such as the Mediterranean, although it is difficult to separate competition for water from that for nutrientsReference Jose, Gillespie and Pallardy12 and, indeed, reduced evapotranspiration due to tree shade effects on understorey plants may increase soil water content compared to open pasturesReference Joffre and Rambal39. The complex relationship between soil water content, rainfall, water uptake by plants and evapotranspiration throughout the seasons makes it extremely difficult to fully understand water dynamics within an agroforestry system. In addition to competing for resources, some species of plants and fungi can have a direct negative impact on others through the production of biochemicals called allelochemicals that influence germination, growth, development, reproduction and distribution of other organisms. These allelochemicals can be released into the rhizosphere as plant residues decompose or via root exudatesReference Jose, Gillespie and Pallardy12. For example, walnut and pecan trees produce juglone, a phenolic compound that has been shown to inhibit survival and growth of several herbaceous and woody plants in pot experimentsReference Jose and Gillespie40.
Shelter for livestock
Modification of the microclimate by trees provides many benefits for livestock; trees provide shelter from rain and wind, shade from the sun, and cover from predators. Cattle are particularly sensitive to heat stress and evaporative cooling is the primary mechanism by which cattle reduce their temperature. By providing shade, trees can reduce the energy needed for regulating body temperatures, and so result in higher feed conversion and weight gain. Research in the southern United States found that cattle that had been provided with shade reached their target weight 20 days before those with no shadeReference Mitlohner, Morrow, Dailey, Wilson, Galyean, Miller and McGlone41. Higher respiration rates and lower activity rates of those cattle without shade were thought to reduce productivityReference Karki and Goodman42. Cattle are more evenly distributed in the silvopasture compared to open pasture, which has important implications for optimizing forage use and uniform nutrient recyclingReference Karki and Goodman42.
During cooler months, windbreaks and shelterbelts provide valuable protection from the wind for livestock, particularly for new-born lambs and freshly shorn sheep. When livestock have been protected from winter storms by windbreaks, significant savings in feed costs, survival and milk production have been reported by producers in Dakota, US (Anderson and Bird, 1993, inReference Brandle, Hodges and Zhou33).
For livestock species such as pigs and poultry that have evolved from forest-dwelling ancestors, including tree cover within their range can influence ranging behavior and enhance animal welfare. For poultry, trees offer protection from aerial predators in particular, and can provide an escape from aggressive behavior within the flock, as well as reducing visual stimulation that can provoke aggressionReference Yates, Dorward, Hemery and Cook27. Dawkins et al.Reference Dawkins, Cook, Whittingham, Mansell and Harper43 observed ranging behavior in commercial free-range broiler (table bird) systems and found that the number of birds ranging outside was correlated with the percentage tree cover on the range, with behavioral studies showing that trees and bushes were the preferred habitatReference Dawkins, Cook, Whittingham, Mansell and Harper43. Like chickens, pigs have a forest-dwelling ancestor, the Eurasian wild boar (Sus scrofa scrofa), which is found primarily in mixed, predominantly deciduous woodland. Behavioral studies of domestic pigs have shown that trees encourage expression of normal behavioral patternsReference Stolba and Woodgush44.
The inclusion of a diverse range of woody species within livestock range has potential for enhancing animal health and welfare by providing opportunities for self-medication. For example, willow has been traditionally used as a feed resource for its anti-helminthic propertiesReference Waller, Bernes, Thamsborg, Sukura, Richter, Ingebrigsten and Höglund45. However, there has been little recent research exploring the full potential of this approach.
Global climate
There has been an increase in research over the past 20 years investigating the potential of agroforestry as a tool for addressing the issues of climate change through mitigation and adaptationReference Schroeder46–Reference Schoeneberger51. Agroforestry has the potential to reduce atmospheric carbon, by increasing afforestation of agricultural lands (C sequestration), by reducing resource use pressure on existing forests (C conservation) and by producing both durable wood products and renewable energy resources (C substitution). However, agroforestry systems can also be sources of C and other greenhouse gases, where tillage, burning, manuring, chemical fertilization and frequent disturbance can result in emissions of CO2, CH4 and N2O into the atmosphereReference Dixon52. Establishing agroforestry systems on previously natural primary or secondary forests will reduce C sequestration potential, whereas planting trees on degraded or treeless agricultural land will increase their C sequestration valueReference Montagnini and Nair49.
C sequestration
Agroforestry can increase the amount of carbon sequestered compared to monocultures of crops or pasture due to the incorporation of trees and shrubsReference Jose4. The potential for agroforestry systems to sequester carbon depends on a number of factors, including system design, species composition and age, environmental factors such as climate, management and the end product. As a result, estimations for carbon sequestration of agroforestry systems vary from 0.29 Mg ha−1 yr−1 in a fodder bank system in West Africa to 15.21 Mg ha−1 yr−1 in mixed species stands in Puerto RicoReference Nair, Kumar and Nair53. SchroederReference Schroeder46 estimated average carbon storage by agroforestry systems as 9, 21, 50 and 63 Mg C ha−1 yr−1 in semiarid, sub-humid, humid and temperate regions, respectively, with higher rates in temperate regions reflecting longer rotations and longer-term storage.
Carbon substitution
Biomass energy from short rotation coppice (SRC) is a carbon-neutral source of energy that does not contribute to CO2 enrichment of the atmosphere. SRC woody crops such as willow produce between 11 and 16 units of useable energy per unit of non-renewable fossil fuel energy used to grow, harvest and deliver SRCReference Volk, Abrahamson, Nowak, Smart, Tharakan and White54. However, there have been concerns that widespread adoption of biomass crops such as Miscanthus and SRC willow will compete with food production and impact biodiversityReference Hall and House55, Reference Rowe, Street and Taylor56. Incorporating SRC into an agroforestry system is one approach to reconciling these conflicting demands. In temperate regions, species with potential as SRC's include poplar (Populus spp.), willow (Salix spp.) and black locust (Robinia pseudoacacia). SRC species such as willow are often suitable for unproductive land such as erodible land or waterlogged sites, thus providing income from marginal land, and as SRC is usually harvested on a 3-year cycle, it provides a more regular income than other forms of agroforestry (e.g., timber).
Greenhouse gas abatement
The role of temperate agroforestry in mitigating greenhouse gases has not yet been investigated fully, although a review of tropical systems highlights the potential of agroforestry for mitigating CO2 and N2O and increasing the CH4 sink strength compared to monoculture systemsReference Mutuo, Cadisch, Albrecht, Palm and Verchot57. In the UK, current work by the Centre for Ecology and Hydrology in Edinburgh is exploring the potential of farm woodlands for ammonia abatement using targeted field measurements and mechanistic and atmospheric emission modelingReference Braban, Famulari, Twigg, Robertson, Quinn, Theobold, Nemitz, Dragosits, Bealey, Dore and Sutton58. In agroforestry systems, there is a reduced need for supplementary nitrogen applications, and recycled nitrogen from leaf litter provides a quantifiable contribution to adjacent crops that can replace inorganic N additions and thus reduce N2O emissionsReference Thevathasan and Gordon59. A decrease in nitrogen leaching out of the rooting zone will reduce NOx emissions as a result of denitrification in surface water resourcesReference Thevathasan and Gordon59. Models estimate that nitrates leaving a tree-based intercropping system can be reduced by 50% compared to a monoculture controlReference Thevathasan and Gordon59.
Water quality and regulation
Research has demonstrated that agroforestry can reduce pollution from crops and grazed pasturesReference Dougherty, Thevathasan, Gordon, Lee and Kort60–Reference Udawatta, Garrett and Kallenbach64. Riparian (riverside) buffers in particular, can reduce non-point source water pollution from agricultural land in five general waysReference Dosskey65:
1. Reducing surface runoff from fields.
2. Filter surface runoff.
3. Filter groundwater runoff.
4. Reduce bank erosion.
5. Filter stream water.
The ‘safety net hypothesis’ is based on the belief that the deeper-rooting tree component of an agroforestry system will be able to intercept nutrients leached out of the crop-rooting zone, thus reducing pollution and, by recycling nutrients as leaf litter and root decomposition, increasing nutrient use efficienciesReference Jose, Gillespie and Pallardy12. Greater permanence of tree roots means that nutrients are captured before a field crop has been planted and following harvest, when leaching may be greater from bare soil.
Buffer strips can significantly decrease pollution run-off, with reductions of 70–97% reported for suspended solids, 60–98% for phosphorus and 70–95% for nitrogenReference Lee, Isenhart and Schultz62, Reference Borin, Passoni, Thiene and Tempesta66. Agroforestry systems also have the potential to mitigate movement of harmful bacteria such as Escherichia coli into water sourcesReference Dougherty, Thevathasan, Gordon, Lee and Kort60 and reduce the transport of veterinary antibiotics from manure-treated agroecosystems to surface water resourcesReference Chu, Goyne, Anderson, Lin and Udawatta67. Agroforestry has been used to address issues of soil salinization in Australia where a study recorded a lowering of the saline groundwater table by 2.0 m over a 7-year period under a Eucalyptus–pasture system, relative to nearby pasture-only sitesReference Bari and Schofield68.
A principal cause of non-point source pollution and soil erosion is excessive surface water runoff. Riparian (river bank) buffers and other agroforestry systems can help reduce runoff and increase infiltrationReference Bharati, Lee, Isenhart and Schultz18, Reference Seobi, Udawatta, Anderson and Gantzer19, Reference Udawatta, Krstansky, Henderson and Garrett61. In Midwestern USA, a multispecies buffer that included woody perennials increased infiltration rates to five times that of cultivated and grazed fieldsReference Seobi, Udawatta, Anderson and Gantzer19. Agroforestry can reduce soil water content during critical times such as fallow periods and increase water infiltration and water storage. Furthermore, above ground, stems, leaf litter and pruning debris in agroforestry systems can reduce runoff flow rates, thereby enhancing sedimentation within the agroforestry strip and increasing infiltrationReference Seobi, Udawatta, Anderson and Gantzer19.
The role of agroforestry in rehabilitating polluted soils has been investigated, through exploiting the ability of trees to capture nutrients and pollutants. For example, research has shown that willows can take up heavy metals from soil into their biomass, help break down pollutants to non-toxic compounds and control water dynamics, including contaminated groundwater flow and water penetration into soils via evapotranspirationReference Volk, Abrahamson, Nowak, Smart, Tharakan and White54. Agroforestry systems have been used to recycle urban and agricultural organic waste with the added benefit of increased biomass productivity from the additional nutrientsReference Aronsson and Perttu69, Reference Rockwood, Naidu, Carter, Rahmani, Spriggs, Lin, Alker, Isebrands and Segrest70. Previously a burden to society, these waste products can be viewed as a valuable resource to maximize biomass productionReference Mirck, Isebrands, Verwijst and Ledin71.
Erosion regulation
The conversion of natural forest and scrublands to grasslands devoid of trees by European settlers on hill country with steep slopes and erodible soils in New Zealand resulted in increased run-off and accelerated erosionReference Wilkinson72, Reference Blaschke, Trustrum and DeRose73. Soil conservation measures involving farm woodlots and wide-spaced tree plantings are needed to support sustainable pastoral and cropping uses on 32% of the North Island and 25% of the South Island (Eyles et al., 1992, inReference Wilkinson72). New Zealand government-subsidized erosion control schemes in the 1960s and 1970s planted over 2 million poplars; during the late 1980s and 1990s there was a shift to planting schemes using radiata pine (Pinus radiata), poplar silvopastoral systems and riparian schemes using poplar and willowsReference Wilkinson72. Willow and poplar are widely used on unstable hillsides in silvopastoral systems for many reasons: they are easy to propagate and establish; have fast early growth rates; are tolerant of flooding and partially saturated soils; have extensive root systems with fine fibrous root mats that stabilize large soil masses; have high evapotranspiration rates that can help dry out swampy soils; provide shade, shelter and fodder for livestock; and may produce useful timber if managed appropriately (poplar only)Reference Wilkinson72.
Pest and disease regulation
Reduced pest and disease problems in agroforestry systems are predicted due to greater niche diversity and complexity than in monocropping systemsReference Stamps and Linit74. This can be attributed to a number of mechanismsReference Vandermeer75:
• Intermittent distribution of host plants makes it more difficult for pests to find the plants.
• One plant species acting as a trap-crop, which protects the other crop from herbivore attack.
• One plant species acting as a repellent to the herbivore.
• Higher predator and parasitoid densities due to higher plant diversity.
• Increased interspecific competition between pest and non-pest species.
Trees provide greater structural and microclimate diversity, greater temporal stability, greater biomass and surface area, alternate sources of pollen, nectar and prey, alternate hosts and stable refugesReference Stamps and Linit74, Reference Schmidt and Tscharntke76, Reference Dix, Johnson, Harrell, Case, Wright, Hodges, Brandle, Schoeneberger, Sunderman, Fitzmaurice, Young and Hubbard77. Agroforestry systems can be managed to enhance pest regulation, for example by providing sources of adult parasitoid food (e.g., flowers), and sites for mating, oviposition and resting sitesReference Young7, Reference Stamps and Linit74.
Higher insect diversities and natural enemy abundance, and lower abundances of pest species have been recorded in several agroforestry systemsReference Thevathasan and Gordon59, Reference Best, Whitmore and Booth78–Reference Murphy, Rosenheim and Granett86. For example, in an alley cropping system with peas (Pisum sativa) and four tree species (Juglans, Platanus, Fraxinus and Prunus) in Leeds, UK, higher insect diversities and natural enemy abundance and lower abundances of pea and bean weevils (Sitona spp.) and pea midge (Contarinia pisi) were recorded compared to a monoculture of peasReference Peng, Incoll, Sutton, Wright and Chadwick82. In this same silvoarable system, grain aphids (Sitobion avenae) populations in the winter barley crop were approximately half that of the arable controlReference Naeem, Compton, Phillips and Incoll81. This was attributed to an increase in cereal aphid predators, primarily hoverflies (Diptera: Syrphidae), which used the tree-strips as a refugeReference Phillips, Griffiths, Naeem, Compton and Incoll83.
While most focus has been on beneficial effects of trees on associated crops and livestock, the tree component may also benefit from pest suppression by livestock. In a study of farmed wild boar in Norway Spruce plantations in Austria, it was observed that mortality rates of the sawfly Cephalcia abietis, a defoliant whose larvae pupate in the soil for 21–33 months, increased by an additional 35.2% due to predation and soil disturbance by the boarReference Fürher and Fischer87. Similarly, local domestic breed pigs in South African plantations of Pinus patula killed 98% of the soil pupae population of the pest Nudaurelia cytherea (pine emperor moth) and 90% of a Pseudobunaea irius (poplar emperor moth) infestation (Van Den Berg, 1973, inReference Brownlow24).
However, some pest groups have been observed in higher numbers in agroforestry systems, and shifts in relative importance of pest groups may present novel management problems and influence crop choice. For example, Griffiths et al.Reference Griffiths, Phillips, Compton, Wright and Incoll88 observed increased slug populations and slug damage in agroforestry plots compared to arable controls in a silvoarable experiment in West Yorkshire, UK. This was attributed to the provision of refugia for slugs within the permanent, unploughed vegetated areas under the tree rows and a modified microclimate, which favor slug populations.
Biodiversity regulation
As ecosystem engineers and keystone species, trees provide a range of resources to other flora and fauna, and when integrated into agro-ecosystems, they can enhance structural heterogeneity and habitat stability of the landscapeReference Manning, Gibbons and Lindenmayer89. JoseReference Jose4 identifies five main ways that agroforestry contributes to the preservation of biodiversity:
1. By providing habitat for species that can tolerate a certain level of disturbance.
2. By helping to preserve germplasm of sensitive species.
3. By helping to reduce the rates of conversion of natural habitat and alleviate resource use pressure.
4. By providing connectivity by creating corridors between habitat remnants and the conservation of area-sensitive floral and faunal species.
5. By helping to conserve biological diversity by providing other ecosystem services such as erosion control and water recharge, thus preventing habitat degradation and loss.
There have been a number of studies investigating the role of agroforestry in supporting biodiversity, especially in the tropicsReference Bhagwat, Willis, Birks and Whittaker90, Reference McNeely and Schroth91. These studies demonstrate that agroforestry systems support floral and faunal assemblages that can be as species-rich, abundant and diverse as forests, but often with modified species compositions that include non-forest speciesReference Harvey and Gonzalez-Villalobos92. While agroforestry systems are unlikely to provide habitat for specialist forest species that require large tracts of undisturbed forest or woodland, they can support biodiversity in otherwise open landscapes and allow movement of species between habitat remnants, as well as buffer protected areas from the impacts of more intensive systemsReference Bhagwat, Willis, Birks and Whittaker90.
Several studies have shown the value of temperate woodland edge habitats such as windbreaks, hedgerows and other agroforestry systems for invertebrate and vertebrate biodiversity, particularly in landscapes otherwise devoid of wooded habitatsReference Bernier-Leduc, Vanasse, Olivier, Bussières and Maisonneuve93–Reference Gelling, Macdonald and Mathews96. The impact of agroforestry on biodiversity may extend beyond the landscape-scale; Perfecto et al.Reference Perfecto, Vandermeer and Wright97 consider the correlation between decreasing populations of songbirds in eastern USA and the elimination of shade trees from coffee agroforests in Latin American countries. Those species in decline were migratory species that overwintered in the southern countries, and were found in the forest-like habitats of traditional coffee farms with a diversity of shade tree species.
Natural hazards and extreme events
With the prediction of an increase in extreme events such as flooding and droughts, increasing the number of trees within the landscape can act as buffers to reduce the impact on the farming system and wider environment. By reducing surface runoff and increasing infiltration and soil water holding capacity, the risk of flash flooding following periods of heavy rainfall is reduced in agroforestry systems, with the tree roots and trunks acting as permeable barriers to reduce sediment and debris loading into rivers following floods. In New Zealand, widely spaced poplars reduced pasture production losses due to landslides during a cyclonic storm by 13.8% with, on average, each tree saving 8.4 m2 from failure (Hawley and Dymond, 1988, inReference Benavides, Douglas and Osoro28). Mature willow and poplar trees at 12 m spacing can reduce mass movement by 10–20% (Hicks, 1995, inReference Benavides, Douglas and Osoro28).
Cultural services
Cultural heritage values
Cultural aspects of agroforestry systems, particularly in temperate regions, are often overlooked, despite the long tradition of systems such as woodland and orchard grazing, alpine wooded pastures, pannage, the dehesa and parklandsReference McAdam, Burgess, Graves, Rigueiro-Rodríguez, Mosquera-Losada and Rigueiro-Rodríguez98. Lifestyles such as nomadism, transhumance and traditional techniques such as pollarding and hedge-laying, are integrated within such systems and the symbolic and cultural perception of these landscapes are shaped by local practices, laws and customsReference Ispikoudis, Sioliou, Mosquera-Losada, McAdam and Rigueiro-Rodríguez99.
Societal benefits
A key objective of implementing agroforestry systems in the tropics is improving livelihoods of poor rural smallholdersReference Garrity100. However, the societal benefits of temperate agroforestry have received less attention, with the focus limited primarily to economicsReference Mercer and Miller101. Integrating trees into the agricultural landscape has the potential to impact the local economy through diversification of local products, diversification of rural skills, improvements to the environment, landscape diversification and economic stability. Wood products such as wood fuel (either as logs or wood chips) need to be produced in close proximity to end-users to reduce transportation costs and make the business economically viable. This creates important links and business relationships between the end-user and local community businesses, so that the money that is paid to obtain these products is spent locally, thus stimulating the local economyReference Volk, Abrahamson, Nowak, Smart, Tharakan and White54.
Recreation and ecotourism
Agroforestry systems can provide recreational opportunities that can benefit the general public, as well as the landowner. Activities such as hunting, fishing, mountain biking, equestrianism and rural tourism can diversify income for farmers, while the public can benefit from improved health and enjoyment from agroforestry through sports and wildlife watchingReference McAdam, Burgess, Graves, Rigueiro-Rodríguez, Mosquera-Losada and Rigueiro-Rodríguez98. Cultural landscapes such as the dehesas of Spain and Portugal, and the wood pastures of the Alps, can create financial opportunities through ecotourism.
Aesthetic values
Public attitudes to agroforestry reflect society's view of non-market benefits connected with amenity, habitat, landscape and animal welfare. The visual impact of monocultures of crops or trees is unappealing for many people; integrating trees into agricultural landscapes can increase the diversity and attractiveness of the landscapeReference McAdam, Burgess, Graves, Rigueiro-Rodríguez, Mosquera-Losada and Rigueiro-Rodríguez98.
Supporting services
Soil formation and nutrient cycling
By promoting a closed system with internal recycling of nutrients, whereby nutrients are accessed from lower soil horizons by tree roots and returned to the soil through leaf fall, agroforestry systems enhance soil nutrient pools and turnover and reduce reliance on external inputs. For example, leaf fall from 6-year-old poplars resulted in mean soil nitrate production rates in the adjacent crop–alley up to double that compared to soils 8.0–15.0 m from the tree row, and nitrogen release from poplar leaf litter was equivalent to 7 kg N ha−1 yr−1Reference Thevathasan and Gordon59. Trees can also significantly influence nutrient additions to adjacent alley crops through intercepting rainfall, via throughfall (rainwater falling through tree canopies) and stemflow (rainwater falling down branches and stems). Zhang (1999, inReference Thevathasan and Gordon59) showed that these pathways contributed 10.99 and 15.22 kg N ha−1 yr−1 in hybrid poplar and silver maple systems, respectively.
There have been many studies assessing the value of green mulch from leguminous trees to enhance soil fertility for adjacent crops in tropical agroforestry systems (e.g.,Reference Yobterik, Timmer and Gordon102). However, relatively few of the 650 woody species that are able to fix atmospheric nitrogen occur in temperate regions; of these black locust (Robinia), mesquites (Prosopis), alder (Alnus) and oleaster (Eleagnus) have been investigated for their nitrogen-fixing potentialReference Jose, Gillespie and Pallardy12. Significant transfer of fixed nitrogen to crops has been observed in a study, which showed that 32–58% of the total nitrogen in alley-cropped maize came from nitrogen fixed by the adjacent red alder (Alnus rubra)Reference Jose, Gillespie and Pallardy12.
As many soil biological processes are performed by soil microorganisms, the presence of an abundant and diverse soil microbial community is essential to sustain productivity of an agroecosystem. In agroforestry systems, differences in litter quality between the tree and crop components promote spatial diversity in enzyme activities and microbial functioning and this spatial variation is enhanced by tree effects on microclimateReference Mungai, Motavalli, Kremer and Nelson103. Several studies have recorded higher microbial diversity, increased enzyme activity and greater stability in agroforestry alley cropping systems, attributable to differences in litter quality and quantity and root exudatesReference Mungai, Motavalli, Kremer and Nelson103–Reference Lee and Jose107.
Arbuscular mycorrhizal (AM) fungi enhance plant nutrient uptake and growth, soil stability and soil aggregation, litter decomposition rates, and could potentially enhance crop yields while reducing the need for chemical fertilizer inputReference Hijri, Sykorova, Oehl, Ineichen, Mader, Wiemken and Redecker108–Reference Schädler, Brandl and Kempel110. However, while AM fungal diversity tends to be low in conventionally managed agricultural soils, which has been attributed to negative effects of fertilization, fungicides, soil cultivations and low host diversity, it has been shown that agroforestry systems may enhance AM fungal richness compared to monocropped systemsReference Chifflot, Rivest, Olivier, Cogliastro and Khasa111. The role of AM symbioses in temperate regions have so far only been studied in intensive, high-input agroforestry; the potential of AM fungi to enhance plant growth in low-input and organic systems still needs quantifyingReference Chifflot, Rivest, Olivier, Cogliastro and Khasa111.
Higher levels of soil organic matter in agroforestry systems also positively influence soil invertebrate communitiesReference Park, Newman and Cousins112, Reference Price and Gordon113. In a poplar–arable silvoarable system, soil organic matter, soil arthropod abundance and cumulative body mass were higher in samples taken close to the trees, with lower levels in the crop alleys attributed to frequent cultivations, lower litter inputs and a reduction in tree root densitiesReference Park, Newman and Cousins112.
Valuing ecosystem services in agroforestry
Recently, there has been considerable interest in placing a monetary value on the delivery of ecosystem services or public goods, such as soil protection and carbon sequestration. Porter et al.Reference Porter, Costanza, Sandhu, Sigsgaard and Wratten114 calculated the values of market and non-market ecosystem services of a novel combined food and energy agroforestry system in Taastrup, Denmark. Belts of fast-growing trees (hazel, willow and alder) for bioenergy production are planted at right angles to fields of cereal and pasture crops, and the system is managed organically with no inputs of pesticides or inorganic N. Field-based estimates of ecosystem services, including pest control, nitrogen regulation, soil formation, food and forage production, biomass production, soil carbon accumulation, hydrological flow into ground water reserves, landscape aesthetics and pollination by wild pollinators produced a total value of US$1074 ha−1 of which 46% is from market ecosystem services (production of food, forage and biomass crops) and the rest from non-market ecosystem services. Porter et al.Reference Porter, Costanza, Sandhu, Sigsgaard and Wratten114 then extrapolated these values to the European scale and calculated that the value of non-market ecosystem services from this novel system exceeds current European farm subsidy payments. Obviously there are many challenges involved with using an ecosystem services or public goods approach to developing a support scheme for sustainable agricultural practices, but there has been much progress in the field of ecological economics recently and increased awareness at policy level of the potential of this approachReference Cooper, Hart and Baldock115.
Carbon credits
One particular area of environmental services where there has been more progress is the potential of an agroforestry approach to conserve and sequester C while maintaining land for food production and reducing deforestation and degradation of remaining natural forests. The 1997 Kyoto Protocol provides a mechanism by which countries that emit carbon in excess of agreed limits can purchase carbon credits from countries that manage carbon sinks. Leading the way with establishing tradable securities of carbon sinks to off-set emissions, Costa Rica invested US$14 million in 1997 for the Payment for Environmental Services (PES), with 80% of funding coming from a tax on fossil fuels and 20% from international trading of carbon from public protected areas. This scheme led to the reforestation of 6500 ha, the sustainable management of 10,000 ha of natural forests and the preservation of 79,000 ha of private natural forestsReference Montagnini and Nair49. In 2003, the scheme was expanded to include agroforestry systems, and the Costa Rican government budgeted US$400,000 for the integration of agroforestry management into the C trading schemes with payments depending on the number of trees present on the farmReference Oelbermann, Voroney and Gordon116. Introducing carbon payments to landowners and managers of agroforestry systems in temperate regions opens the way to obtaining additional income from these systems and may increase the attractiveness of establishing an agroforestry system, as well as adding value to established systems such as riparian buffers, shelterbelts, silvopastoral and silvoarable systems.
Conclusions
This review demonstrates that temperate agroforestry can provide an alternative approach to land management that, through the re-integration of trees and agriculture, can balance productivity with protection of the environment. With careful design and management, agroforestry systems can be more productive, and by producing a wide range of products, including food, fuel, fodder and timber, avoid the food or fuel conflict for land. Establishing trees on agricultural land can help to mitigate many of the negative impacts of agriculture, for example by regulating soil, water and air quality, supporting biodiversity, reducing inputs by natural regulation of pests and more efficient nutrient cycling, and by modifying local and global climates. The challenge now lies in promoting the adoption of agroforestry as a mainstream land use through research, dissemination of information and policy changes. In a second paper on temperate agroforestry, we discuss further how to develop modern systems that integrate ecological farming and agroecological advances to achieve sustainable intensification. 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 systemReference Smith, Pearce and Wolfe117.
Acknowledgements
We gratefully acknowledge the support of the Ashden Trust which allowed us to carry out this literature review. Thanks also to Helen Pearce and Louisa Winkler for their valuable comments on an earlier draft.
