Читать книгу North American Agroforestry - Группа авторов - Страница 70

Brent R. W. Coleman, Naresh V. Thevathasan, Andrew M. Gordon, and P. K. Ramachandran Nair

The primary natural ecosystems of North America are dominated by either perennial grasses or woody vegetation. Prior to European settlement, grasslands occupied 39% of the current United States, while forests and shrub‐dominated systems covered most of the remainder (Sims, 1988). These highly diverse assemblages of species evolved during millions of years in response to major changes in climate and physiography. Powered almost exclusively by solar energy, they have sustained production and provided enormous ecosystem services, generating a legacy of rich soils and other biological wealth.

Because of this legacy, most of these ecosystems have been converted to agroecosystems (agricultural systems) through the substitution of annual food plants (e.g., corn [Zea mays L.], wheat [Triticum aestivum L.] and soybean [Glycine max (L.) Merr.]) for the original perennial vegetation. Where climate or other conditions preclude the planting of row crops, native grassland is exploited through the substitution of domestic livestock for native herbivores, and forests are intensively managed for timber production. The conversion has been extensive and thorough; at the extreme are states like Illinois that have seen a decrease of 99.9% of prairie acreage, with approximately 930 ha (2,300 acres) remaining statewide (Steinauer & Collins, 1996). Of the 930,000 ha (2.3 billion acres) of total land area in the United States, 17% is classified as cropland, 29% is classified as grassland pasture and rangeland, 28% is classified as forest‐use land, 23% is classified as special use (parks and wildlife areas) or miscellaneous (wetlands, tundra, unproductive woodlands), and 3% as urban land (USDA, 2017). Approximately 43 million ha (106 million acres) are designated wilderness areas under the National Wilderness Preservation System, accounting for roughly 4.6% of the total land area of the United States (Watson, Matt, Knotek, Williams, & Yung, 2011). The landscape is now a “semi‐natural matrix” (Roberts, 1988) within which humans and all other species must survive.

A similar situation exists in southern Canada, especially in southern Ontario, Quebec, and the Maritime Provinces. Herbaceous and woody biomass crops are being cultivated on increasing areas across North America as a means of producing bioenergy or for use as animal bedding. Warm‐season grasses such as switchgrass (Panicum virgatum L.) and miscanthus (Miscanthus spp.) and fast‐growing woody species such as hybrid willow (Salix spp.) and poplar (Populus spp.) are growing in popularity among producers as a result of their high yields, low nutrient requirements, broad environmental tolerances, and environmental benefits such as enhanced C sequestration potentials compared with conventional agricultural crops (Coleman et al., 2018; Graham et al., 2019). The ability of these crops to grow on marginal lands is likely to contribute to increasing popularity going forward. The ecological principles explored in this chapter would apply equally to temperate agroforestry and biomass crop production systems developed in these regions.

The goal of this land‐use conversion has been to maximize the amount of net primary or secondary production from these systems that can be used by humans. In the short term, this goal has been met and a massive increase in food and wood supplies has been generated. However, the long‐term consequences of these conversions bring into question the sustainability of this level of production. For example, in addition to solar energy, U.S. agroecosystems use large amounts of fossil fuels to power machinery or produce other inputs such as fertilizer and pesticides. Irrigated corn production in Nebraska, for example, requires an estimated average of nearly 86 million kJ ha⁻¹ of fossil energy input (Pimentel, 2009), or 1 kJ input for each 1.65 kJ harvested. Conventional beef production requires 13 kg (29 lb) of grain and 30 kg (66 lb) of forage to produce 1 kg (2.2 lb) of beef, meaning fossil fuel energy inputs of 40 kJ kJ⁻¹ (or 40 kcal energy input per 1 kcal) of beef protein, without considering the energy costs of processing or transportation (Pimentel et al., 2008). This energy profligacy occurs in a country that imports 10.14 million barrels per day of petroleum (U.S. Energy Information Administration, 2018a). Additionally, the United States also heavily relies on shale oil for its own domestic production, accounting for nearly 60% of the total U.S. crude oil production (U.S. Energy Information Administration, 2018b), with fracking posing devastating environmental impacts and requiring significantly greater amounts of energy to extract compared with conventional drilling.

Soil degradation caused by erosion, salinization, waterlogging, and such other processes are major environmental issues that seriously impact land use. For example, approximately 30% of U.S. cropland has been severely damaged because of erosion, salinization, or waterlogging (Pimentel et al., 1995). Soil loss by erosion continues at a rate of 1.54 billion Mg of soil per year, with water erosion causing annual soil losses of approximately 900 Mg annually, and wind erosion causing soil losses of nearly 640 Mg annually (Natural Resources Conservation Service, 2015).

The continuing decline of genetic diversity in agriculture is yet another issue of major concern. Instead of the 250–300 plant species found in an equivalent area of tall‐grass prairie (Steiger, 1930), or the 100 species in a similar area of oak–hickory (Quercus spp.–Carya spp.) forest, a typical midwestern corn–soybean farm maintains only two species on a majority of the land area. In addition, genetic diversity within the major U.S. crops is quite low. Farmers who plant several hybrids or cultivars to increase their diversity are often planting essentially the same thing, under different names (National Academy of Science, 1972; Raeburn, 1995). These highly simplified single‐species systems are at increased risk from pest outbreak or climate extremes.

Sustainability is “the concept about meeting today’s needs without compromising the ability of future generations to satisfy their needs, and it strives to achieve a balance between ecological preservation, economic vitality, and social justice” (World Commission on Environment and Development, 1987). Our current farming system faces declining domestic energy reserves, soil loss in excess of regeneration, and a rapidly increasing human population with a concomitant increase in demand for agricultural products. Although farmers have adopted practices such as contour planting, no‐till, and precision application of chemicals to reduce some of the negative effects of agriculture, farming systems based on monocultures or simple rotations of annuals are not sustainable without massive external inputs.

Further diversification of crops offers many advantages. The addition of herbaceous perennials such as alfalfa (Medicago sativa L.) and grasses increases the perennialism that is such a dominant feature of native ecosystems (Figure 3–1) (Van Andel, Bakker, & Grootjians, 1993). Reduced erosion, fixation of atmospheric N₂ by legumes, and reduced energy inputs (Heichel, 1978) are benefits of adding certain perennials to agroecosystems. Livestock offer further diversification and a mechanism for converting forages into higher value products (Bender, 1994).

Although forests are a major vegetation type in the United States, few farmers consciously integrate trees and other woody perennials into their farms as a way of increasing diversity and sustainability. This approach to farming is known as agroforestry and is defined generically as the integration of trees into agriculturally productive landscapes (Nair, 1993). Within a North American context, agroforestry can be more explicitly defined as: intensive land management that optimizes the benefits (physical, biological, ecological, economic, and social) from the biophysical interactions created when trees and/or shrubs are deliberately combined with crops and/or livestock (Garrett et al., 1994). A practice is deemed an agroforestry practice if it embraces four principal criteria as indicated by Gold and Garrett (Chapter 2) and shown in Table 3–1.

The key words in this definition are interactions, benefits, and optimizes. Biophysical interactions require a certain spatial and temporal proximity of the components. A woodlot on one corner of the farm, isolated from and not beneficially interacting with crops or livestock, does not constitute agroforestry by this definition (see discussion below). When trees, crops, and livestock are in close enough proximity to interact in a way that is significant to the farmer, agroforestry is created. The types of interactions depend on the species involved and their particular spatial and temporal relationships. Not all interactions are beneficial. For example, competition between trees and row crops for water, nutrients, and light can reduce row crop yields. Gray (2000) found that soybean yield and tree root quantity were negatively correlated, with 80% of tree roots being found in the A soil horizon. That study indicates, however, that soil cultivation suppresses tree root growth in the top portion of the soil, further establishing that appropriate tree management interventions can minimize competition with crops. Peng, Thevathasan, Gordon, Mohammed, & Gao (2015) also found a reduction in soybean yield when it was grown in a 26‐year‐old tree‐based intercropping site with silver maple (Acer saccharinum Marsh.), hybrid poplar (Populus deltoides × nigra), and black walnut (Juglans nigra L.). In addition to belowground competition, the trees, now roughly 15–20 m tall, also reduced incident photosynthetically active radiation, thereby reducing net assimilation, growth, and soybean yield. Obtaining optimal benefits from agroforestry requires knowledgeable selection, placement, and management of the woody and non‐woody components. Thinning and/or outright removal of some trees, as well as planting shade‐tolerant crops, are management options to consider as the system ages. A random mixture is unlikely to perform well.

Unfortunately, there is no single optimal agroforestry design that interested farmers and ranchers can be encouraged to adopt. Differences in climate, topography, soils, crops, and livestock exist at scales that range from the local to the continental. Agroforestry practices must be designed to fit the particular ecological, social, and economic context of the farm in question. Component interactions in agroforestry practices have been investigated to a small extent (Ong & Huxley, 1996), and while the emphasis has been on tropical systems (e.g., Rao, Nair, & Ong, 1997), some information is also available for temperate agroforestry systems (Thevathasan & Gordon, 2004). Whether we are considering temperate or tropical agroforestry, Muschler, in An Introduction to Agroforestry (Nair, 1993), pointed out “that the complexity and lifespan of agroforestry makes investigations of mechanisms and processes extremely difficult.” Leaving consideration of socioeconomic issues for later, how can we obtain the ecological knowledge necessary for the optimal design of a wide variety of temperate agroforestry practices?

Fig. 3–1. Hypothetical relationship between perennialism and sustainability in selected natural ecosystems and agroecosystems (based on Van Andel et al., 1993).

Table 3–1. The four key criteria that characterize agroforestry practices (modified from University of Missouri Center for Agroforestry, 2018, pp. 9–10).

Criteria	Description
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 that may occur together but are managed separately. Agroforestry is neither monoculture farming nor is it a mixture of monocultures.
Intensive	Agroforestry practices are created and 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, above‐ or belowground, simultaneous or sequential. Integration of multiple crops utilizes more of the productive capacity of the land and helps to balance economic production with resource conservation.
Interactive	Agroforestry actively manipulates and utilizes the interactions among components to yield multiple harvestable products while concurrently providing numerous conservation and ecological benefits.

The answer lies, at least in part, in the native ecosystems upon which U.S. agriculture is built. Highly sustainable, these systems were locally adapted to the environmental conditions under which they evolved. Natural ecosystems can provide models for the design of sustainable agroecosystems (Davies, 1994; Soule & Piper, 1992; Woodmansee, 1984). We believe that it is possible to identify structural and functional characteristics of natural ecosystems that contribute to their sustainability and then retain or incorporate these into agroecosystems while maintaining production. Regional and local differences in natural ecosystems can serve as guides for tailoring agroforestry practices that best fit a particular farm’s environmental conditions. Our goal in the remainder of this chapter is to illustrate some of the structural and functional relationships among woody and herbaceous vegetation in natural ecosystems of the United States and to show how these relationships apply to agroforestry practices.