North American Agroforestry. Группа авторов

      The size and machinery complement of each synthetic farm was determined from a survey and analysis of Nebraska farms (Bernhardt, 1994), and a schedule of operations was developed for each farm based on best management practices for east‐central Nebraska. The economic performance of the two systems was then quantified with a model developed by Olson (1998), and erosion and nutrient losses were evaluated with PLANETOR, a farm‐scale environmental and economic model (Center for Farm Financial Management, University of Minnesota). Energy and nutrient budgets for each farm were compiled from published values of the embodied energy of farm inputs (Pimentel, 1980) and crop nutrient and energy contents (Church, 1984; Holland, Welch, et al., 1991). The values of each indicator for the two farms are given in Table 3–4.

A rapid appraisal of Table 3–4 suggests that the agroforestry farm is more sustainable than the conventional corn–soybean farm. Although the systems perform similarly as measured by production indicators (e.g., harvest, energy capture efficiency, water use efficiency), the agroforestry farm does better economically (net income, capital borrowing) and in some measures of resource conservation (e.g., erosion, N loss). Neither system has a sustainable nutrient balance in that each exports considerably more N and P than it imports.

      Of course, there is no way to tell from system‐level indicators how much of the improvement in the performance of the agroforestry farm is due to its woody perennial components. The underlying performance data (not shown) indicate that the tree components had a major impact on economic returns. Christmas trees and hazelnuts (Corylus L.) were very profitable, and windbreaks increased crop yields more than enough to compensate for the land taken out of production. Tree crops (with grassed alleys) eliminated water erosion on the land they occupied, although for the whole farm, alfalfa was equally important in reducing water erosion. Windbreaks provided no benefit in reducing wind erosion because soil loss by wind is insignificant on these soils when adequate residue is left each fall.

      A final observation concerns the definition of agroforestry. The windbreaks on this model farm, by interacting with the field crops (biologically and physically), clearly meet the definition of agroforestry. The Christmas trees and hazelnut shrubs, although woody perennials, are planted in blocks and may have only minimal biophysical interaction with other components of the farming system. Does the inclusion of block plantings of trees on a farm necessarily constitute agroforestry? Not by the definition given earlier in this chapter (see Gold & Garrett, 2008), although other definitions of agroforestry would accept such a system on the landscape if it was developed in a temporal sense (Gordon, Newman, Coleman, & Thevathasan, 2018).

      Without question, when the distribution of labor is considered (data not shown) on these two farms, there are advantages to having incorporated woody perennials into the farm system. The conventional farmer is very busy in the spring and early fall, with much less to do in‐between. On the agroforestry farm, the hazelnuts require a great deal of labor for harvest in late July and early August, and Christmas tree sales provide work in late November and December. The inclusion of block plantings of tree crops represents both an economic and a social interaction with other components of the farm but not necessarily one of a biophysical nature. Agroforestry, in North America, is currently defined in terms of five individual practices, with a sixth one added recently (see Chapter 2); however, as it continues to evolve, a broader definition at farm and landscape scales may become appropriate.

Ecological Goods and Services

      Ecological goods and services are defined as logical benefits resulting from the “normal” functioning of an ecosystem. Maximum production of such goods and services is associated with unstressed agroecosystems, which within the context of agroforestry would constitute a variety of temporal and spatial configurations of trees on the farming landscape. Humans benefit from the maintenance of these goods (e.g., fresh water) within the ecosystem, and the “flow” of these services (e.g., greenhouse gas mitigation) to other systems.

      Agroforestry systems, regardless of type, are capable of providing numerous ecological goods and services, of a range of complexities, over long periods of time (Hunt, 2005; Jose, 2009; Nair, Gordon, & Mosquera‐Losada, 2008). Indeed, agroforestry systems can be designed and engineered to provide specific quantities of particular goods and services. Nonetheless, the universal application of ecological principles to agroforestry system design and management is nearly impossible as a result of the many varied types of systems in existence—from riparian management systems that link terrestrial and aquatic systems to more traditional systems that integrate perennial plants with annual crops, with or without animals. The broad geographical range across which agroforestry systems may be successfully implemented and the scale at which interactions occur—from landscape to individual plant—also complicates the development of a universal understanding of nutrient and energy flows and the relationship of these to system productivity.

      Although systems will differ in the nature and types of environmental services provided, some generalizations can be stated. Most agroforestry systems will tend to improve soils, including productivity, largely through the incorporation of organic matter and C into upper soil profiles from the production of annual litterfall from the tree component. As a result of the presence of perennial root systems, soil erosion relative to monocropped agroecosystems is often minimized.

With respect to the maintenance and proliferation of biodiversity, agroforestry systems often enhance the components of biodiversity, at scales ranging from the stand (farm level) to regional landscapes (Jose, 2009; Nair et al., 2008). Jose (2009) categorized the contributions of agroforestry systems toward biodiversity conservation into five major roles, which include: (a) species habitat provisioning; (b) germplasm preservation for sensitive species; (c) reduction of the rates of natural habitat loss through providing a more productive and sustainable alternative to traditional agricultural systems that may involve clearing natural habitats; (d) creation of corridors to connect habitat remnants for floral and faunal species; and (e) prevention of land degradation and habitat loss through provisioning additional ecosystem services such as erosion control and soil health enhancements. The potential for agroforestry systems to contribute to biodiversity conservation is especially high in the fragmented agricultural landscapes of North America, where widespread conventional agricultural practices are undoubtedly contributing to species decline across the continent. Gibbs et al. (2016) found that in a mature temperate tree‐based intercropping system, avian species richness was nearly 1.5 times greater than in a conventionally managed sole‐crop agricultural system (32 vs. 23 unique bird species). Additionally, avian diversity was more than twice as great in the tree‐based intercropping system than in the sole‐crop agricultural system (Shannon–Wiener diversity index values of 2.9 vs. 1.2). Tree‐based intercropping systems may therefore be one method for slowing or even reversing the decline of migratory bird populations in North America, as seen during the last several decades.

      Agroforestry systems are being promoted as a means of mitigating climate change as a result of their C sequestration potentials. In all systems, the storage of C is enhanced (Jose, 2009, 2019; Nair et al., 2008; Thevathasan & Gordon, 2004), not only through the perennial nature of the trees, but also through increased soil C storage. The C sequestration potential for agroforestry systems is dependent on the type of agroforestry system, in addition to species composition and age, geographic location, environmental factors, as well as system management practices (Jose, 2009). A 2006 study examining the C sequestration potentials in a 13‐yr‐old temperate tree‐based intercropping system found that the carbon sequestration potential of systems incorporating barley (Hordeum vulgare L. ‘OAC Kippen’) and hybrid poplar (Populus deltoids × Populus nigra clone DN‐177) were four times greater than in a barley and Norway spruce (Picea abies L.) system and five times greater than the examined sole‐cropped barley system, with net C fluxes of 13.2, 1.1, and −2.9 Mg C ha⁻¹ annually (Peichl, Thevathasan, Gordon, Huss, & Abohassan, 2006). Wotherspoon, Thevathasan, Gordon, and Voroney (2014), utilizing the same research site, also quantified the C sequestration