Environmental Impacts and Benefits of Agroforestry
Summary and Keywords
Agroforestry systems, the planting of perennial trees and/or shrubs with annual agronomic crops or pasture, have been proposed as more environmentally benign, alternative systems for agricultural production in both temperate and tropical regions of the world. Agroforestry provides a number of environmental benefits as confirmed by scientific literature. The four major environmental benefits of agroforestry are (1) climate change mitigation through carbon sequestration, (2) biodiversity conservation, (3) soil health enrichment, and (4) air and water quality improvement. In addition to environmental benefits, the economic benefits of multiple crops within agroforestry systems have also generated interest in their adoption by farmers the world over. The major negative impacts come from conversion or degradation of forests following certain traditional practices, which may not fit in the definition of modern agroforestry. Challenges remain for widespread adoption of agroforestry, particularly in the temperate world; however, a new resurgence of interest in this land-use practice among small-scale farmers has shed light on a path toward its possible success. Past evidence clearly indicates that agroforestry, as part of a multifunctional working landscape, can offer not only economic return, but also a number of ecosystem services and environmental benefits for a sustainable society.
Agroforestry is defined as a land-use management that optimizes the benefits (physical, biological, ecological, economic, and social) from biophysical interactions created when trees and/or shrubs are deliberately combined with crops and/or livestock. Four key criteria characterize agroforestry practices and distinguish them from other practices. To be called agroforestry, a land-use practice must satisfy all of the following four criteria:
Intentional: Combinations of trees, crops, and/or livestock are intentionally designed, established and/or managed to work together and yield multiple products and benefits, rather than as individual elements, which may occur together but are managed separately.
Intensive: Agroforestry practices are intensively managed to maintain their productive and protective functions, and often involve cultural operations such as cultivation, fertilization, irrigation, pruning and thinning.
Integrated: Components are structurally and functionally combined into a single, integrated management unit tailored to meet the objectives of the landowner. Integration may be horizontal or vertical, and above ground or below ground. Integration of multiple crops utilizes more of the productive capacity of the land and helps balance economic production with resource conservation.
Interactive: Agroforestry actively manipulates and utilizes the biophysical interactions among components to yield multiple harvestable products, while concurrently providing numerous conservation and ecological benefits.
During the 1970s, serious concern of the international scientific community over the problems of deforestation, environmental degradation and hunger gave large impetus to the formation of International Council for Research in Agroforestry, which later was renamed International Center for Research in Agroforestry and eventually the World Agroforestry Center. Cultivation of trees and crops in intimate combination has been in existence for millennia in many parts of the world. However, this ancient practice was not institutionalized until the formation of ICRAF in 1977 following a report commissioned by the International Development Research Center (IDRC) of Canada. The report, prepared by a team led by John Bene, identified the need to establish an international council that would support, plan, and coordinate research combining trees with agricultural crops in the tropics.
Tropical agroforestry has been practiced in many parts of the world for thousands of years. Many traditional farming systems in the tropics include some form of agroforestry. Even in temperate regions of the world, agroforestry has been practiced in one form or another for millennia. For example, Native Americans practiced several forms of agroforestry in the Americas. In Europe, the dehesas (Spain) and montados (Portugal) are some of the oldest examples of agroforestry practices that include scattered oak trees with an herbaceous understory. Modern agroforestry in the US context began with the establishment of windbreaks following the dustbowl of the 1930s.
Common Agroforestry Practices with Proven Environmental and Economic Benefits
Agroforestry practices are classified and cataloged in many different ways depending on species composition, arrangement of different components, functional aspects, and the socioeconomic context in which they were developed. Application of these practices responds to financial (e.g., rural unemployment, profitability), environmental (e.g., soil erosion, water quality), and social (e.g., quality of life) issues that are common to most regions of the world. There are differences between temperate and tropical agroforestry practices due to differences in ecosystems, their condition, and economic, social, cultural, and political realities. The most common agroforestry systems practiced in the world are given below:
Riparian and Upland Buffers: These are strips of permanent vegetation often consisting of trees, shrubs, and grasses. These strips are planted as buffers between agricultural land (farmland or pastureland) and water bodies (rivers, streams, creeks, lakes, wetlands) in order to reduce runoff and non-point source pollution, stabilize streambanks, improve aquatic and terrestrial habitats, and provide harvestable products. Upland buffers are placed along the contour within agricultural fields to reduce runoff and non-point source pollution, improve internal drainage, enhance infiltration, create wildlife habitat and provide travel corridors and provide harvestable products.
Windbreaks: Windbreak practices, linear planting of trees and/or shrubs, include shelterbelts, timberbelts and hedgerows and are often managed as part of a crop or livestock operation. Field windbreaks are used to protect a variety of wind-sensitive crops, control wind erosion, and to provide other benefits such as improved pollinator and wildlife habitat. Following the Dust Bowl of the 1930s the United States planted 220 million tree seedlings to establish 30,000 km of windbreaks in the Great Plains of the United States. The windbreaks returned the Great Plains to agricultural production again by stabilizing soil. Livestock windbreaks help with animal welfare by reducing stress and mortality, feed and water consumption, and odor. Timberbelts are managed windbreaks designed to increase the value of the forestry component. Windbreaks used for odor control are called Vegetative Environmental Buffers (VEB). Properly designed VEBs can cut down odor from concentrated animal feeding operations by 20 to 30 %.
Alley Cropping: This practice combines trees planted in single or multiple rows with agricultural or horticultural crops cultivated in the alleyways between the tree rows. High-value hardwoods such as oak (Quercus sp.), walnut (Juglans sp.), ash (Fraxinus sp.), chestnut (Castanea sp.) and pecan (Carya illinoensis (Wangenh.) K. Koch) are favored species in alley cropping practices in temperate regions, and many can provide high-value lumber or veneer logs. Crops grown in the alleys, and nuts from trees, provide annual income while the longer-term wood crop matures. Maize, wheat, cotton, and forage species are commonly grown in the alleys. When specialty crops such as herbs, fruits, vegetables, nursery stock, or flowers are grown in the alleys, the microclimate created by the trees enables greater yield of these sensitive high-value crops. Tree species planted in the tropics often include nitrogen-fixing trees that can enhance soil quality. Planting of fast growing timber species such as Populus sp. also has become common in the tropical and subtropical regions of the world.
Silvopasture: This practice combines trees with forage (pasture) and livestock production. Silvopasture can be established by adding trees to existing pasture, or by thinning an existing forest stand and adding (or improving) a forage component. The trees are managed for high-value sawlogs, and at the same time they provide shelter for livestock, protecting them from temperature stresses and reducing food and water consumption. Forage and livestock provide short-term income at the same time a crop of high-value sawlogs is being grown, providing a greater overall economic return from the land. A key principle in silvopasture is the sustainable management of all three components of the system—trees, forage, and livestock. Silvopasture is one of the ancient agroforestry practices. For example, traditional silvopasture known as “dehesas” of the Iberian Peninsula has a long history. A dehesa is a wooded pasture with regularly spaced trees (~ 40 per ha) and is managed to promote high levels of tree growth and acorn production as well as healthy grasslands. Trees in dehesas are pruned throughout life to increase their acorn production. During fall and winter months, these acorns provide forage to livestock. Wood from the thinning and pruning processes is also sold to make extra money. Dehesas cover over three million ha on the Iberian Peninsula in Spain and Portugal and have been delineated as preserve lands by the European Union.
Forest Farming: This is a practice in which crops are cultivated beneath the canopy of an existing forest or woodlot. The understory crops in a forest farming system are called non-timber forest products (NTFPs). These crops include certain bee products, maple syrup, edible and medicinal plants, craft material, fruits, nuts, and mushrooms. Examples of high-value specialty crops include ginseng (Panax quinquefolium L.), log-grown shiitake mushrooms (Lentinula edodes (Berkeley) Pegler), decorative ferns and spring ephemerals sold for medicinal, food, and decorative uses. Overstory trees are managed for nuts or fruits or timber. There are opportunities to incorporate sustainable forest management practices for the benefit of NTFPs. For example, prescribed fire can be used as a tool to promote the growth of several medicinal plants. Silvicultural treatments such as regeneration harvests can promote the growth of many economically important understory plants.
Homegardens: This is a multistory tree–crop combination, sometimes in association with domestic animals, with definite boundaries and a house (Kumar & Nair, 2004). This agroforestry system supplements socioeconomic needs, generates employment for family labor, provides animal feeds and environmental services, and contributes toward food security of indigenous households. Thus, it serves a variety of biophysical, economic, and sociocultural functions for the owner. Homegarden agroforests are known for their role in conserving and enhancing biodiversity in agriculture-dominated landscapes. Although thought of primarily as a tropical agroforestry practice, homegardens have a long tradition in temperate regions of the world as well. For example, homegardens are a common agroforestry practice in the southwestern United States. The satoyama system of Japan is another example of a temperate traditional homegarden.
Urban Food Forests: Urban food forests are emerging as a form of agroforestry in temperate regions of the world. These urban agroforests share similarities to homegardens of the tropics, but often lack livestock or a homestead. They are a mixture of multiple tree and shrub species and may or may not contain an herbaceous crop component. These multistrata systems are integrating elements of urban agriculture, urban forestry and the principles of agroforestry to improve sustainability and resiliency of urban ecosystems. A growing number of organizations and city governments are adopting this concept in urban environments, primarily because of their importance in ensuring food security of a rapidly growing urban population, particularly the poor. Many such urban food forests built on formally vacant degraded sites are already providing local fresh food along with multiple benefits of improved air and water quality, carbon sequestration, wildlife habitat and recreational value in cities in the developed world.
Taungya: This is a system of forest management in which land is cleared and planted in high-value timber species with food crops planted in between tree rows by local people during the early establishment phase of the plantation. Taungya was developed by the British in Myanmar during the 19th century to establish teak (Tectona grandis) plantations. Following canopy closure, the land would be managed as a timber plantation until harvesting of trees when the cycle is repeated. It is practiced in other parts of the world as well. Studies have shown that taungya is beneficial in terms of tree seedling survival and food crop production, benefiting both the farmers and the plantation department (usually a government agency). While the farmers produce food, they also tend to the young tree seedling, keeping them weed free which, in turn, reduces the overall plantation establishment costs.
Improved Fallows: This practice is a modified and improved form of slash-and-burn agriculture. Farmers use improved fallow to rapidly replenish soil fertility so that the length of the fallow period can be shortened. Farmers scatter seeds or plant seedlings of fast-growing plants—usually legumes- after harvest of the crops from the site. The trees and shrubs are left to occupy the site for several months or years. During the fallow period, the plants accumulate nitrogen, enrich the soil and conserve soil water. When trees are removed at the end of the fallow, improvements in soil quality in terms of soil chemical and physical properties (e.g., organic matter, nutrients, soil porosity, water-holding capacity etc.) help increase crop yield.
Environmental Benefits of Agroforestry
Agroforestry provides a number of environmental benefits or positive impacts. These benefits, often referred to as ecosystem services, are classified into four categories: (1) climate change mitigation through carbon sequestration; (2) biodiversity conservation; (3) soil health enrichment; and (4) air and water quality improvement. Overall, the discussion in this paper cuts across the four major categories of ecosystem services (provisioning, regulating, cultural and supporting) identified by the Millennium Ecosystem Assessment (2005) and follows the description provided by Jose (2009) and Jose et al. (2018).
Climate Change Mitigation through Carbon Sequestration
At the historic Paris climate conference (COP21) in December 2015, 195 countries adopted the first-ever universal, legally binding global climate deal that set out a global action plan to put the world on track to avoid dangerous climate change by limiting global warming to well below 2 °C and pursuing efforts to limit it to 1.5 °C. Biological carbon sequestration can play a significant role in mitigating carbon emissions in the future to reach the below 2 °C goal. Carbon sequestration involves the removal and storage of carbon from the atmosphere in carbon sinks (such as oceans, vegetation, or soils) through physical or biological processes. Agroforestry, as a system that combines a perennial component (trees and/or shrubs) with agronomic crops (annual usually), offers great promise to sequester C both above and belowground. Agroforestry practices have been approved as a strategy for soil C sequestration under afforestation and reforestation programs and also under the Clean Development Mechanisms of the Kyoto Protocol (IPCC, 2007). Adoption of agroforestry practices has greater potential to increase C sequestration of predominantly agriculture-dominated landscapes than monocrop agriculture (Nair, Nair, Kumar, & Showalter, 2010). Within agroforestry systems, C can be stored in above and belowground biomass, soil, and living and dead organisms. The quantity and quality of residue supplied by trees/shrubs/grass in agroforestry systems enhance soil C concentration. In addition, C stored by trees could stay in soils or as wood-products for extended periods of time. If agroforestry systems are managed sustainably, C can be retained in these systems for centuries. The amount of C stored on a site is a balance between long-term fluxes. However, the net C gain depends on the C content of the previous system that the agroforestry practice replaces.
The potential of agroforestry systems to sequester carbon varies depending upon the type of the system, species composition, age of component species, geographic location, environmental factors, and management practices. A large number of studies have estimated the carbon sequestration potential of agroforestry systems during the last two decades. For example, Nair et al. (2009) showed that the carbon sequestration potential of the vegetation component (above and belowground) varied from 0.29 Mg ha-1 yr-1 in a fodder bank agroforestry system of West African Sahel to 15.21 Mg ha-1 yr-1 in mixed-species stands of Puerto Rico. Soil carbon estimates ranged from 1.25 Mg ha-1 in a Canadian alley cropping system to 173 Mg ha-1 in an Atlantic Coast silvopastoral system in Costa Rica.
Attempts have also been made to quantify carbon sequestration potential in the United States. Udawatta and Jose (2011) concluded that silvopastoral systems, the most common form of agroforestry in North America, had the greatest potential to sequester C in the U.S. Using a sequestration potential of 6.1 Mg C ha–1 yr–1 on 10% marginal pasture land (23.7 million ha) and 54 million ha of forests, they estimated total C sequestration potential for silvopastoral lands in the Unites States as 474 Tg C yr–1. Similarly, they estimated that alley cropping could be practiced on 10% of the 179 million ha cropland in the U.S., which could sequester 60.9 Tg C yr–1. Based on several published data they estimated that the average above and belowground C sequestration potential was 2.6 Mg C ha–1 yr–1 for riparian buffers. The total river and stream length in the U.S. is approximately 5.65 million km (3.533 million miles. Lakes and estuaries occupy 16.8 million ha and 22.7 million ha, respectively. If a 30 m wide riparian buffer is established along both sides of 5% of total river length (a total land area of 1.69 million ha), the potential C sequestration by riparian buffers along rivers in the U.S. could be as high as 4.7 Tg C yr–1. Like other agroforestry practices, windbreaks also offer promise for C sequestration. In addition to C sequestered by trees, windbreaks provide additional C sequestration due to improved crop and livestock production and energy savings. Udawatta and Jose (2011) estimated that the total C sequestration potential for windbreaks was 8.79 Tg C yr–1. Overall, the C sequestered by agroforestry could help offset the U.S. emission rate of 1600 Tg C yr–1 from burning fossil fuel (coal, oil, and gas) by 34 %. These estimates indicate the important role of agroforestry as a promising CO2 mitigation strategy in the United States, and possibly in other countries of North America.
Estimates are also available for global carbon sequestration potential of agroforestry systems. According to Dixon (1995) a total of 585 to 1,215 million ha of land in Africa, Asia and the Americas is under agroforestry, which translates to a global potential to sequester 1.1–2.2 Pg of carbon (vegetation and soil) over 50 years. Improving current management (e.g., better management of trees on croplands) in existing agroforestry and converting unproductive croplands and grasslands to agroforestry (630 million ha) could sequester an additional 17,000 and 586,000 Mg C y–1, respectively, by 2040 (IPCC, 2000). Kumar and Nair (2011) provides the latest synthesis on this topic.
Despite our attempts to reverse biodiversity loss it continues at an alarming rate with one endangered species lost every 20 minutes. The factors contributing to this trend are many, including over-exploitation of species, alien species invasions, environmental pollution and contamination, global climate change, and habitat loss. Sustainable land-use approaches that can combine production and conservation functions are important in conserving biodiversity in human-dominated landscapes, and agroforestry has been promoted as such a land-use system. As part of a multifunctional working landscape, agroforestry has been demonstrated to play a major role in conserving and even enhancing biodiversity from farms to the landscape level in both tropical and temperate regions of the world (Jose, 2012). The mechanisms by which agroforestry systems contribute to biodiversity have been examined by various authors. In general, agroforestry plays five major roles in conserving biodiversity (Jose, 2009; 2012): (1) agroforestry provides habitat for species that can tolerate a certain level of disturbance; (2) agroforestry helps preserve germplasm of sensitive species; (3) agroforestry helps reduce the rates of conversion of natural habitat by providing a more productive, sustainable alternative to traditional agricultural systems that may involve clearing natural habitats; (4) agroforestry provides connectivity by creating corridors between habitat remnants which may support the integrity of these remnants and the conservation of area-sensitive floral and faunal species; and (5) agroforestry helps conserve biological diversity by providing other ecosystem services such as erosion control and water recharge, thereby preventing the degradation and loss of surrounding habitat.
There has been a tremendous growth in the number of publications since 2000 dealing with the biodiversity values of agroforestry. While comprehensive reviews and synthesis have been rare, Schroth and colleagues (2004) provide the most comprehensive review in a tropical context with examples from many different countries. Shade coffee is an agroforestry system that has been shown to enhance biodiversity compared to traditional agricultural practices. Multistrata cacao (Theobroma cacao) agroforestry with timber, fruit, and native forest species also contribute to biodiversity conservation by providing habitat for avian, mammalian, and other species, enhancing landscape connectivity, and reducing edge effects between forest and agricultural land. Similar results have been reported from several types of multistrata tropical agroforestry systems (e.g., homegardens, shade coffee, shade cacao, silvopasture) where bats, birds, and mammals were abundant, and diverse as forest ecosystems. However, in many situations, the species composition of these assemblages would be highly modified with fewer forest-dependent species, more non-forest species and different dominant species. Windbreaks and riparian buffers offer the only woody habitat for wildlife in many agriculture-dominated landscapes. Increased bird and animal diversity has been reported from riparian habitats compared to adjacent crop fields. In almost all cases, agroforestry systems have contributed to conservation efforts in human-dominated landscapes by serving as habitats to a large number faunal species, including species of known conservation concern.
Another major benefit of agroforestry that has received considerable attention is its role in promoting pollinators. According to FAO, the value of annual global food production that relies on the direct contribution of pollinators is between $235 and $577 billion. Pollinator populations have been on a steady decline for years due to a number of reasons, including pesticides, habitat destruction, and disease outbreaks. For example, Colony Collapse Disorder (CCD) has been decimating adult worker honeybees (Apris mellifera L.) in North America. The characteristics of properly designed agroforestry systems can help counteract many of these negative factors causing pollinator decline. On a predominantly agriculture-dominated landscape, presence of trees and shrubs in agroforestry configurations will serve as important habitats for a diverse native pollinators. Studies in shade-grown coffee plantations in Latin America have shown the beneficial effects of trees on pollinators in those systems compared to the sun-grown coffee plantations. Agroforestry can reduce the risk of pollinators’ exposure to pesticide in two different ways. Certain practices such as windbreaks or hedgerows can effectively reduce drift from pesticides. Pesticide use is generally low in agroforestry due to lower incident of pests and diseases in agroforestry compared to monoculture cropping systems. Higher diversity of plants in agroforestry generally helps in increasing the number of beneficial insects while reducing the harmful ones. For example, Brandle, Hodges, and Zhou (2004) observed greater density and diversity of insect populations in windbreaks due to the heterogeneity of the edges that provided varied microhabitats and a variety of hosts, prey, pollen, and nectar sources.
Agroforestry also helps conserve floral diversity. For example, homegardens, both from the neo tropics and old-world tropics, are known for their high floristic diversity. Many ecologists consider such systems structurally and functionally the closest mimics of natural forest ecosystems. In a review, Kumar and Nair (2004) reported species richness of tropical homegardens varying from 27 (Sri Lanka) to 602 (West Java). Negash, Yirdaw, and Luukkanen (2012) from the southeastern Rift Valley escarpment of Ethiopia reported a total of 58 woody species, belonging to 49 genera and 30 families from three different agroforestry systems. Twenty-two of these species were of special interest for conservation, according to IUCN Red lists and local criteria. Bardhan, Jose, Biswas, Kabir, and Rogers (2012) concluded that homegardens could function as “intermediary” for conserving tree species diversity in Bangladesh. There was a 30 % similarity between homegardens and natural forests and the species richness increased as the size of homegardens increased. In a country like Bangladesh where natural forests have declined drastically, agroforestry could serve as an important ecological tool in conserving tree species diversity.
Temperate homegardens have also played a major role in conserving biodiversity over time. Although they are not widely known, traditional homegardens of the southwestern United States harbor a diversity of trees, shrubs, vines, and herbaceous plants that yield vegetable crops, fruits and nuts, forage and fodder, poles and posts, firewood, and flowers for consumption in the home or sale. The satoyama systems of Japan is an indigenous landscape that evolved through long-term interaction between human beings and their local environment. It is characterized by integrated landscapes comprised of paddy fields, farmland, woodlots, grasslands, ponds and canals, and human settlements in close proximity to one another. The satoyama system is known for its rich biodiversity and its success in conserving it for centuries.
Soil Health Enrichment
While soil degradation threatens the ability of soil to sustain food production and provide necessary ecosystem services globally, there is a looming challenge of increasing food production by 70 % by 2050 to support a global population of approximately 9.1 billion people. Agroforestry’s potential to enhancing and maintaining soil quality has been widely recognized as a major benefit and is well-documented. Healthy soil is one of the most critical resources for the health of both natural and agro ecosystems so that they can continue to produce food and provide ecosystem services (Dollinger & Jose, 2018). The term “soil health” has often been used synonymously with soil quality in the literature. Doran and Safley (1997) define soil health as “the continued capacity of soil to function as a vital living system, within ecosystem and land-use boundaries, to sustain biological productivity, maintain the quality of air and water environments, and promote plant, animal, and human health.” According to the USDA, soil health is “the continued capacity of the soil to function as a vital living ecosystem that sustains plants, animals and humans” (USDA-NRCS, 2018). While soil quality is used in the context of both natural and managed ecosystems, soil health is primarily used in the context of managed ecosystems, primarily agroecosystems (Lal and Stewart, 2010). Many authors use soil quality and health interchangeably, but refer to the continued productive capacity of the soil resulting from its physical, chemical and biological properties. Agroforestry is proven to enrich the physical, chemical and biological properties of the soil and thus soil health.
The incorporation of leguminous trees, shrubs, and crops that are able to fix nitrogen is fairly common in tropical agroforestry systems. Other non N-fixing trees can also enrich soil by improving soil physical, chemical and biological properties by adding organic matter and recycling nutrients. A number of both original research and synthesis articles, has described the benefit of agroforestry in terms of soil enrichment, particularly in the tropical context (e.g., Nair & Latt, 1997; Young, 1997). For example, in a study by Yadav, Yadav, Chhipa, Dhyani, and Ram (2011) of various multipurpose tree-based agroforestry systems in India revealed that integration of multipurpose trees (MPTs) into croplands had increased microbial biomass and C : N ratios for all agroforestry treatments. This was likely due to additions of organic matter through litterfall over many years in agroforestry. These authors also reported increased levels of enzyme activity, specifically of dehydrogenase and alkaline phosphatase, as much as 1.60–1.87fold and 88.3–163 %, respectively, in comparison to sole cropping. The study also found that nutrient flux of C, N, and P was 1.38 to 1.85 times higher in the agroforestry systems compared to sole cropping. These results are consistent with findings from other parts of the tropics where significant increases in these soil health indicators as a result of agroforestry have been reported. Sloping Agricultural Land Technology (SALT) is a contour planting of nitrogen-fixing trees as soil binder, fertilizer generator, and livestock feed source on sloping hillsides with annual and perennial food crops grown in the spaces between the hedgerows. Developed and practiced primarily in the Philippines, the SALT model has been proven to protect and enrich soil. Farmer-managed natural regeneration (FMNR) on agricultural land is restoring soil fertility on millions of acres degraded farmlands throughout Africa. FMNR is the encouragement of regeneration of trees and shrubs that sprout from stumps, roots, and seeds found in degraded soils. These trees are proven to improve soil fertility and moisture so that crops planted in combination with them have greater yield. For example, results from Faidherbia albida based land-use systems in Africa have shown increased yield of associated crops. These effects are often attributed to nitrogen fixation by F. albida and nitrogen addition to the soil by dropping its foliage during the cropping season. Shedding of the leaves during the cropping season eliminates competition for water with the crops. Further, the soil water-holding capacity is increased around the trees, which reduces crop wilting during dry periods.
Although not as rich, evidence of similar complementary interactions benefitting soil health exists in temperate agroforestry literature as well. For example, N-fixing trees such as alder (Alnus rubra) have been shown to transfer fixed N to associated crops in alley cropping systems in temperate regions. Seiter, Ingham, Horwath, and William (1995) showed that 32 % to 58 % of the total N in alley-cropped maize was derived from N fixed by red alder, and that nitrogen transfer increased with decreasing distance between the trees and crops. Greater organic matter accumulation and higher microbial biomass have also been reported in alley-cropping systems compared to monoculture crops in temperate North America. Udawatta, Gantzer et al. (2008) and Udawatta, Kremer et al. (2008) demonstrated improvement in several chemical, physical and biological properties (e.g., soil aggregates stability, porosity, soil carbon, soil nitrogen, and soil enzyme activity) in soils under agroforestry buffers compared to row crops.
Not all studies show improvements in soil quality through agroforestry practices. A study by Tornquist, Hons, Feagley, and Haggar (1999) in the Sarapiqui region of Costa Rica showed no significant difference in soil properties between agroforestry under tropical hardwoods and pasture. A range of physical, chemical and biological soil properties were analyzed for both systems. Total nitrogen, soil organic carbon, and C : N ratios did not appear to have been influenced by five years of agroforestry management. Another study investigating the effect of agroforestry on the structure and function of microbial community under a gradient of land-use intensification (from planted forest and woodlots, agroforestry plots, agricultural fields, and eroded, uncultivated areas) showed no difference after 20–30 years of utilizing agroforestry practices compared to traditional fields (Lagerlöf et al., 2014). While there are isolated mixed results regarding agroforestry’s role in improving soil, the overwhelming trend is of positive benefits and hence supports the claims of agroforestry proponents.
Air and Water Quality and Quantity Improvement
Agroforestry practices such as windbreaks and shelterbelts can improve air quality in agriculture-dominated landscapes. These practices reduce wind chills, protect crops, provide wildlife habitat, remove atmospheric carbon dioxide and produce oxygen, reduce wind velocity. By modifying wind patterns and the microenvironment, these practices can reduce wind erosion and particulate matter in the air. They can also reduce noise pollution, and mitigate odor from concentrated livestock operations or Confined Animal Feeding Operations (CAFO). Odor-causing chemicals and compounds are carried on aerosols. Vegetative Environmental Buffers (VEB) can filter airstreams of particulates by removing dust, gas, and microbial constituents. Tyndall and Colletti (2007) suggested that when planted in strategic designs, VEBs could effectively mitigate odor in a socioeconomically responsible way.
Agroforestry’s role in improving water quality is well documented. In conventional monoculture cropping systems, less than half of the applied N and P fertilizer is taken up by crops. As a result, excess fertilizer is either washed away along with surface runoff or leached into the ground water supply, which has the potential to contaminate water sources and thereby decreasing water quality. Agricultural surface runoff can result in excess sediment, nutrient, and pesticide delivery to receiving water bodies and is a major contributor to eutrophication in lakes and oceans all over the world. Riparian buffers have been proposed as a means to reduce non-point source pollution from farms. Riparian buffers will reduce velocity of runoff, enhance water infiltration, reduce sediment movement, and increase nutrient retention thereby improving water quality. Buffers also reduce nutrient movement into ground water by taking up the excess nutrients. Trees with deep rooting systems often serve as the “safety net” whereby excess nutrients that have been leached below the rooting zone of agronomic crops are taken up by tree roots. These nutrients are then recycled back into the system through root turnover and litterfall, increasing the nutrient use efficiency of the system. Several studies have shown that agroforestry vegetative buffers reduce non-point source pollution from row crop agriculture (e.g., Anderson, Udawatta, Seobi, & Garrett, 2009). For example, Udawatta, Garrett, and Kallenbach (2012) examined non-point source pollution reduction as influenced by agroforestry buffers in watersheds under grazing and row crop management in Missouri, USA. Runoff water samples were collected from 2004 through 2008 and analyzed for sediment, total nitrogen (TN), and total phosphorus (TP). Results showed that agroforestry and grass buffers significantly reduced runoff, sediment, TN, and TP losses to streams for both watersheds. Buffers in association with grazing and row crop management reduced runoff by 49 % and 19 %, respectively compared to control treatments. Average sediment loss for grazing and row crop management watersheds was 13.8 and 17.9 kg ha-1 yr-1, respectively. In general, grass and agroforestry buffers reduced sediment, TN, and TP losses by 32 %, 42 %, and 46 % compared to the control treatments.
Veterinary antibiotics are a threat to soil due to their potential to alter the quality and quantity of native microbial communities and enhance the development and spread of antibiotic-resistant bacteria. A large quantity of veterinary antibiotics administered in livestock can be released into the soil through deposition of manure. In addition to their ability to reduce erosion, and nutrient run off from farms, recent evidence suggests that agroforestry buffers can also reduce veterinary antibiotics in the soil (Chu, Goyne, Anderson, Lin, & Udawatta, 2010; Lin, Goyne, Kremer, Lerch, & Garrett, 2010, Lin, Lerch, Goyne, & Garrett, 2011; Unger et al., 2013). Microbial communities associated with the root systems of certain tree species, planted as buffers or in silvopastoral configurations, can disintegrate antibiotics that are deposited on the soil surface through livestock manure. Similarly, agroforestry buffers have been recommended as a cost-effective approach to mitigate herbicide and pesticide transport in surface runoff derived from agronomic operations (Lin, Lerch, Goyne et al., 2011; Lin, Lerch, Kremer et al., 2011).
Although not a lot of evidence is available, agroforestry has been shown to increase water infiltration, thereby improving water recharge and availability in agricultural fields and watersheds. For example, on steep hillsides, agroforestry terraces help reduce runoff and enhance infiltration, hence increasing water availability for adjacent cropping areas.
Environmental Impacts of Agroforestry
There are certain traditional practices that are often classified as agroforestry, but may result in negative impacts on the environment. Two common examples of such potentially detrimental practices are slash-and-burn agriculture and grazing in the forests. Slash-and-burn agriculture (also known as swidden agriculture or shifting cultivation) involves clearing natural forests followed by burning to prepare the land for agriculture. Crops are planted in the nutrient-rich soil until native soil fertility is exhausted and then the land is abandoned. A new forest land will be cleared and the process will be repeated. This practice contributes to 70 % of deforestation in Africa, 50 % in Asia, and 30 % in Latin America, and accounts for 14 million hectares of tropical forest loss globally every year. Slash-and-burn not only results in loss of habitat for flora and fauna, but increases the risks for soil erosion, landslides, water quality issues, and air pollution. Alley cropping, forest farming, and improved fallows are modern agroforestry solutions or alternatives to slash-and-burn agriculture.
Grazing in forests and woodlands has a long history in many parts of the world. If the key principle of silvopasture—managing all three components—is followed, this practice will qualify as a sustainable agroforestry practice. If not, it can be a threat to the long-term sustainability of forest ecosystems. Long-term intensive grazing without regards to the sound principles of carrying capacity and sustainable forest management can lead to regeneration failure in forests. It can also result in soil compaction, low water-infiltration capacity, soil erosion, and eventual vegetation changes—perhaps even desertification in arid environments.
Agroforestry, as an integrated land-use practice, has been proven to generate a number of environmental benefits while yielding economically viable products, including food. While there are some negative impacts associated with certain traditional practices, this article has attempted to highlight the environmental benefits, commonly referred to as ecosystem services, in four categories. Over three decades of strong scientific data are available to prove these services; yet, acceptance of agroforestry by landowners and farmers is still not satisfactory in many parts of the world, particularly in the temperate region. The four categories of services discussed are: (1) “Climate Change Mitigation through Carbon Sequestration”; (2) “Biodiversity Conservation,” (3) “Soil Health Enrichment,” and (4) “Air and Water Quality Improvement.” While the potential of agroforestry systems to sequester carbon varies depending upon the type of the system, species composition, age of component species, geographic location, environmental factors, and management practices, it is clear from the available evidence that agroforestry is a promising CO2 mitigation strategy in both tropical and temperate regions of the world. Scientists and practitioners agree that sustainable land-use approaches that can combine production and conservation can conserve biodiversity in human-dominated landscapes. Agroforestry is such a land-use approach and when incorporated as part of a multifunctional working landscape, cannot only conserve, but also enhance biodiversity from farms to the landscape scale. The role of agroforestry in enhancing and maintaining soil quality and health has been widely recognized. Healthy soil is one of the most critical resources for the health of both natural and agro ecosystems so that they can continue to produce food and provide ecosystem services. Agroforestry has been proven to improve nearly every major soil property (chemical, physical and biological) that is used in measuring soil health. Agricultural surface runoff containing sediments, nutrients, pesticides, herbicides and veterinary antibiotics can cause major water quality issues. Agroforestry offers a solution for this problem as well. It is well documented that certain agroforestry practices like riparian and upland buffers can reduce non-point source pollution from farms to a considerable extent. The negative impacts are limited primarily to two traditional practices, slash-and-burn agriculture and grazing in the forests. However, there are sustainable alternatives to both practices in modern agroforestry. The available evidence of environmental benefits combined with the commodity benefits of agroforestry should help promote its adoption worldwide.
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