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The effects of integrating trees into production agriculture systems are far‐reaching, and address not only on‐farm needs, but also numerous agriculturally‐related problems causing increasing concern around the world. Growing trees in combination with crops and livestock has been shown to enhance crop yields (Kort, 1988; Dupraz et al., 2018b), improve animal health (Brunetti, 2006; Pent et al., 2022) and reduce losses, conserve soil and recycle nutrients, and reduce environmental impacts of agriculture (Udawatta et al., 2002; Blanco‐Canqui et al., 2004; Dosskey et al., 2007; Lerch et al., 2017; Schulte et al., 2017), while producing various tree and specialty products (Gold et al., 2004; Mori et al., 2018). The postulated effects of agroforestry in the United States and Canada are presented in the form of verifiable agroforestry concepts (Table 2–3). Increasing amounts of data exist to support and prove these concepts. Current research and on‐the‐ground practices will continue to confirm and modify these concepts in the coming years.

Fundamentally, as the definition of agroforestry implies, the benefits from agroforestry are derived from the biophysical interactions created when trees and/or shrubs are combined with crops and/or livestock. Interactions refer to the influence of one component on the performance of the other components, and on the system as a whole. We seek to optimize these interactions to favor mutualism and commensalism and minimize competition and predation. Interactions include both above‐ground and below‐ground effects. Although the dynamics of component interactions is a complex research challenge (Jose and Holzmueller, 2022), the net effect of interactions is of practical significance, and creates the biophysical success or failure of an agroforestry practice. The observable net effects of component interactions are expressed by the terms complementary, supplementary, and competitive (Anderson and Sinclair, 1993; Ong et al., 2015; Jose and Holzmueller, 2022). Component interactions represent processes at the tree–crop interface and tree–crop–animal interface. These interactions can be positive (e.g., stress reduction, yield enhancement, soil retention, water capture) or negative (e.g., competition, allelopathy, pest enhancement) (Jose et al., 2004; Jose and Holzmueller, 2022). Consequently, it is imperative that agroforestry practices be properly designed and managed to optimize desired positive interactions and minimize the negative ones (Dupraz et al., 2019).

Our perception of agroforestry, its benefits, and its relative importance also depend on the scale of interest (e.g., field, farm, watershed, landscape). At the farm scale, benefits that accrue to the landowner are of primary importance while societal benefits are secondary. At larger landscape and watershed scales, societal benefits of conservation (e.g., water quality) are often valued equally with community viability (e.g., economic production) (Bentrup and Kellerman, 2004; Garrity, 2005). Thus, at the individual farm ownership scale, agroforestry focuses on utilizing the productive niches within the farm to meet the owner’s conservation and income needs. At the landscape scale (Hillbrand et al., 2017; Kremen and Merenlender, 2018), agroforestry practices help create buffer zones (National Research Council, 1993; Schultz et al., 2022) within agricultural systems that enhance vital ecological services required for sustainability. At the watershed scale, agroforestry practices directly support community‐based land stewardship by addressing both conservation and economic goals (Curtis et al., 1995; Jordan et al., 2007; USDA, 2017).

Agroforestry shares fundamental concepts and principles with sustainable agriculture, agroecology, and permaculture. These include judicious use of inputs, maintaining soil quality and productivity, minimizing the environmental impacts of agriculture, utilizing natural processes where possible and practical, and providing for human health, safety, and quality of life (Lovell et al., 2010; Ferguson and Lovell, 2014; Liebman and Schulte, 2015; Krebs and Bach, 2018). Agroforestry can contribute to the integrity of agroecosystems through the creation of buffer zones (National Research Council, 1993; Schoeneberger et al., 2006; USDA, 2017). These buffer zones expand the structural and spatial diversity of the system, and enhance certain ecological services generated from biodiversity and nutrient cycling (Edwards et al., 1993; Udawatta et al., 2017). Ultimately, management strategies must utilize agroforestry and other practices that generate biodiversity and nutrient cycling processes to provide ecological services like soil and nutrient retention, water capture and cycling, microclimate moderation, waste assimilation, and pest management (Costanza et al., 1997; Matson et al., 1997; Geertsema et al., 2016; LaCanne and Lundgren, 2018).

Windbreaks, the most understood and widely‐used agroforestry practice, best illustrate the cascade of benefits from agroforestry practices at multiple scales (Fig. 2–2, also see Brandle et al., 2022). Windbreaks create farm scale buffer zones, which generate numerous benefits (physical, biological, ecological, and social). Physically, they slow the wind, create a microclimate more favorable to plant growth, decrease windborne soil erosion, reduce physical damage to emerging and sensitive crops, increase snow capture and distribution (Heavey and Volk, 2014), and protect crops and livestock from climatic extremes. Biologically, they provide habitat for natural enemies of crop pests (Altieri et al., 2017; Yang et al., 2019), enhancing biological controls and provide habitat for wildlife. Ecologically, windbreaks increase water capture and cycling, reduce runoff and flooding, and help maintain soil quality and productivity. Economically, windbreaks increase crop yields, reduce animal feed costs, increase the survival of newborns and can be designed to incorporate marketable products (Josiah et al., 2004; Baker et al., 2018). Social benefits consist of landscape diversity and protection of human environments from wind, dust, noise, and odors, as well as enhancement of wildlife and associated recreational opportunities. The outcome is multiple benefits that accrue to both the landowner and to society (Fig. 2–2).

Fig. 2–2. Principles and benefits derived from the windbreak agroforestry practice.

Through the introduction of trees and the interactions they generate, agroforestry can significantly contribute to desirable ecosystem level services. Overall this can result in more structurally diverse (both above and below ground) agroecosystems that are richer in plant and animal biodiversity and have improved system self‐maintenance and resistance to environmental stresses (Jose and Holzmueller, 2022; Udawatta et al., 2017; USDA, 2017). Based on these ecological principles, agroforestry practices can be an important tool to restore land use sustainability, overall ecosystem health, and be used to reclaim degraded lands. Thus, agroforestry is more than a set of practices; it is the incremental addition of trees to farming systems and farming landscapes, resulting in the generation and enhancement of desired ecological services considered vital for sustainable land use (Jordan et al., 2007; Geertsema et al., 2016; USDA, 2017).

To be effective, agroforestry must follow a grassroots approach tailored to: i) the individual landowner’s special interests, problems, and needs (Rule et al., 2000); ii) the available productive niches; and iii) the local soil and climate conditions and existing and potential markets (Gold et al., 2004, 2013, 2018). This same approach also applies to community‐based land stewardship (Curtis et al., 1995; Garrity 2005; Schoeneberger et al., 2006). Thus, although we now recognize six categories of agroforestry practices for nomenclature purposes, these practices are not rigid. Rather, the application of agroforestry should be seen as a common‐sense approach tailored to local needs and conditions (Sobels et al., 2001; Rule et al., 2000; Rios‐Diaz et al., 2018). The result is an approach that is similar to whole farm planning in that the emphasis is on the design of a system for a given field, farm, or watershed rather than the promotion of a particular land use option (Gold et al., 2013, 2018).

As found in the chapters within this volume, over the past decade, there has been rapid development of agroforestry science in support of agroforestry practices. Appropriate technologies, information, and tools have been developed to design agroforestry practices to achieve specific production and conservation objectives at the local level (Bentrup and Kellerman, 2004; Shelton et al., 2005; Gold et al., 2013, 2018; Wilson et al., 2018). Based on strong supporting data obtained from multiple studies accumulated at numerous sites over many years, agroforestry science is well on the way toward developing the principles (i.e., component interactions and ecosystem functions) that underlie these practices (Nair, 2007; Lovell et al., 2017; USDA, 2017; Munsell and Chamberlain, 2019).

As a science‐based on interacting components within practices (i.e., combinations of trees, crops and livestock), agroforestry draws upon knowledge from many different disciplines. Beginning in 1990 and evolving rapidly in the past decade, a critical process‐level, science‐based approach to agroforestry research has gradually emerged. An understanding of component interactions is being assembled which will enable applications to be designed in a predictable manner. The bottom line for agroforestry is to be able to locally apply technologies that generate predictable and positive interactions, and optimize them for the benefit of the farmer and associated land resources, and for society as a whole. To achieve the bottom line, a fusion of top‐down and bottom‐up approaches are needed that are market‐focused and result in the development of robust social networks (Rule et al., 2000; Valdivia et al., 2022) at multiple social and spatial scales (i.e., landowner, community, state, region, and nation).