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Adapted from J. D. Soule and J. K. Piper, Farming in Nature's Image, Island Press, Washington, D.C., 1992. With permission.

Adapted from J. D. Soule and J. K. Piper, Farming in Nature's Image, Island Press, Washington, D.C., 1992. With permission.

with time, (3) restoration of original soil structure and function following disturbance, (4) biodiversity of soil-dwelling organisms, (5) resistance to weed establishment, and (6) stable populations of beneficial insects.

Several studies have demonstrated that the reestablishment of perennial cover on retired cropland can reduce soil erosion while restoring soil quality. The greater root biomass associated with perennial grasses (Richter et al., 1990) gives carbon inputs into the soil that can be several times greater than those into cultivated soils (Anderson and Coleman, 1985; Buyanovsky et al., 1987; McConnell and Quinn, 1988) and reduces rates of nutrient leaching relative to annual crops (Paustian et al., 1990). Active soil organic matter, available nutrients, water-stable aggregates, and polysaccharide content may recover under perennial grasses fairly quickly (Jastrow, 1987; McConnell and Quinn, 1988; Gebhart et al., 1994; Burke et al., 1995).

The benefits of a perennial cover were recognized by the authors of the U.S. Conservation Reserve Program (CRP), authorized by Title XII of the 1985 Food Security Act. This program redirected monetary resources and human efforts toward soil conservation and indirectly toward control of agricultural non-point-source pollution. It was designed to protect the most vulnerable U.S. cropland, with a goal to shift 16 to 18 million ha of highly erodible land from annual crop production to perennial vegetation for 10 years. Overall, the program keeps about 595 million t of soil from eroding into U.S. streams and rivers annually, equivalent to a 21% reduction in erosion on cropland (Bjerke, 1991).

The second characteristic of prairies that contributes to sustainability is plant biodiversity. Benefits of plant biodiversity include (1) nitrogen supplied by legumes, (2) management of herbivorous insects and some plant diseases, (3) soil biodiversity, and (4) ecosystem stability.

Legumes play a critical role in supplying nitrogen to most natural ecosystems. Over periods of several years, perennial legumes can increase the concentrations of both carbon and nitrogen in the soil, as well as influence the size and activity of the microbial community (Berg, 1990; Halvorson et al., 1991). Similarly, legumes are important in providing nitrogen within many pastures as well as multiple cropping systems (Davis et al., 1986; Mallarino et al., 1990). Studies have consistently shown higher dry matter yields in grass/legume mixtures than in grass monocultures (e.g., Barnett and Posler, 1983; Posler et al., 1993).

Biodiversity also plays an important role in pest regulation. The presence of nonhost plant species can reduce insect density by interfering chemically or visually with host-finding behavior and thus colonization, feeding efficiency, movement among host individuals, and mate finding (Bach, 1980; Risch, 1981; Andow, 1990; Bottenberg and Irwin, 1992a; Coll and Bottrell, 1994). Moreover, reduced suitability of the microhabitat can reduce insect tenure time, oviposition, and larval survival, and can increase emigration rate (Tukahira and Coaker, 1982; Kareiva, 1985; Elmstrom et al., 1988). In some cases, diverse stands provide a more favorable habitat for parasitoids and predators, leading to reduced levels of insect herbivores (Letour-neau and Altieri, 1983; Letourneau, 1987). The weight of published evidence suggests that reduced resource concentration, rather than increased numbers of natural enemies, accounts for most of the observed herbivore reductions within polyculture (Andow, 1991).

Similarly, numerous studies have shown benefits of plant species diversity in minimizing certain plant diseases (Burdon, 1987), particularly those diseases vectored by insects (Zitter and Simons, 1980). Establishing host plants within diversified stands can reduce insect landing rate, and thus initial colonization of the plot, by interfering chemically or visually with host-finding behavior (Irwin and Kampmeier, 1989; Bottenberg and Irwin, 1992a,b). This can, in turn, lead to reduced levels of disease in mixtures relative to monoculture (Power, 1987; Allen, 1989; Bottenberg and Irwin, 1992c). Plant species diversity can thus lower the rate of pathogen transmission among individual host plants.

Besides these benefits, biodiversity can beget biodiversity. For example, Miller and Jastrow (1993) noted a significant relationship between underground fungal and floristic species richness in prairie restorations. Diversity of soil organisms can have profound effects on plant mycorrhizal associations, nutrient-uptake ability, and nutrient retention and cycling.

Biodiversity can have ecosystem-level benefits, too. Several experimental studies have demonstrated that species-rich communities are more resilient and more efficient at using resources than species-poor communities (Naeem et al., 1994). McNaughton (1977; 1985) conducted experiments involving the grasslands of Serengeti National Park, Tanzania. Areas of roughly 16 m2 with different diversities were marked; then exclosures were fenced to prevent grazing by migrant herds of zebra, gazelles, and wildebeest. Grazers reduced the biomass of diverse areas by only about 25%, whereas the less diverse areas lost about 75% of their biomass. Nitrogen limits the number of plant species that coexist in Minnesota grasslands; thus Tilman and Downing (1994) manipulated the number present by varying the amount of nitrogen applied on 207 plots each of 16 m2. They started measuring biomass in 1987. The year 1988 featured a drought which reduced biomass differently on the different plots. After the drought, the species-diverse plots produced half their predrought biomass whereas the species-poor plots produced only about an eighth or less. In these studies, the more diverse communities were more resistant to change. Such studies indicate that plant biodiversity contributes directly to the resilience of grassland communities. Tilman et al. (1996) found that, in experimental plots of perennial grassland plant species, community productivity increased with plant biodiversity. Moreover, soil available nitrogen was used more completely in the more diverse plots, leading to less leaching potential.

Finally, resistance to invasion is another collective attribute of complex communities (Case, 1990; Drake, 1991). This property is important for the ability of a community to resist weeds and other exotic organisms.

AN AGRICULTURE MODELED ON THE PRAIRIE ECOSYSTEM Elements of the Prairie Model

Remnant plant communities of the North American prairie are persistent biotic assemblages in which complex webs of interdependent plants, animals, and microbes garner, retain, and recycle critical nutrients, and protect the soil. As such they provide our best models of the types of communities needed to restore sustainable diversity to compromised ecosystems. Such diverse species assemblages, whose composition varies across soil types, tend to retain species, resist invasion by exotics, and are resilient during short-term climatic variation. Agricultural systems modeled on natural grassland ecosystems would comprise diverse plantings of perennial species that would prevent soil loss, provide much of their own nitrogen requirement via symbiotic nitrogen fixation, and resist invasion by weeds as well as outbreaks of insect pests and plant pathogens. They would be structural and functional analogs of prairie plant communities, composed predominantly of representatives from four major plant guilds: perennial C3 and C4 grasses, nitrogen-fixing species (primarily legumes), and composites (Asteraceae). These functional groups include the majority of prairie vegetation (Kindscher and Wells, 1995; Piper, 1995).

A major objective of research toward a sustainable agriculture is to develop innovative methods of production that minimize negative environmental impacts. The CRP, although very successful in terms of soil preservation, is expensive ($1.8 billion/year) (Osborn, 1993) and provides no edible product from the idled land. Hence, the goal of the Land Institute is to develop polycultures of perennial grains that protect the soil and provide the restorative properties of a perennial cover while yielding significant amounts of edible grain. Grain-producing mixtures of perennial grasses, legumes, and composites (e.g., sunflower species) would mimic the vegetation structure and sustainable function of native grasslands in some fundamental ways. Species composition of such perennial mixtures would vary geographically and with soil type. Several promising candidates for a perennial grain agriculture have been identified and evaluated (Wagoner, 1990; Soule and Piper, 1992). Potentially, the sustainable features of such an agriculture include improved soil retention and health, more efficient use of land area, lower fossil fuel dependence, diversity within and between crops to reduce vulnerability to pest and pathogen outbreaks, and greater on-farm predictability and flexibility. Because approximately 20% of U.S. on-farm energy usage is associated with traction (Lovins et al., 1995), perennial grain agriculture, by reducing seedbed preparation and cultivation, application of synthetic chemicals, and irrigation will also translate into savings in energy and materials costs for farmers.

Hence, elements of the prairie model to mimic a natural systems agriculture are (1) herbaceous perennials as grains and (2) species grown in diverse fields. The working model, then, comprises several perennial grain species representing four functional groups (i.e., C4 grasses, C3 grasses, nitrogen-fixing legumes, and composites) planted together.

To design persistent and diverse prairie-like grain fields, one must be cognizant of the broad similarity of grassland communities as well as the details of their differences. Surveys of locally and regionally occurring species and their local distributions (e.g., Piper, 1995) suggest the types of species likely to participate. First, there are some broad rules that hold across locations (e.g., representation by each of the four major guilds). Second, although one cannot predict perfectly the composition of the final successful community, the general structure of natural communities supported on different soil types gives clues to the types of species to emphasize in the mix (e.g., nitrogen fixers on poor soils). Third, because occasional extreme years can limit diversity considerably (Tilman and El Haddi, 1992), high biodiversity should enable a community to weather better the wide precipitation swings that characterize continental climates.

Perennials as Grains

The development of perennial seed crops for agronomic mixtures consists of two interrelated efforts. The first is the breeding of new perennial grains. This can involve the domestication of currently wild species as well as the improvement of wide crosses between annual grain crops and their perennial relatives. The second area comprises long-term studies of intercrop compatibility within diversified plantings. This work involves studies to discern beneficial and inhibitory crop interactions, growth and seed yield patterns, and effects on soil quality.

Examples of the first approach, the domestication of wild perennials, are experiments with the cool-season grasses mammoth wildrye and intermediate wheatgrass, the warm-season grass eastern gamagrass, the legume Illinois bundleflower, and Maximilian sunflower. Promising examples of the second approach, the development of perennial grains via wide crosses between annual grains and wild congeners, include studies with hybrid grain sorghum (Sorghum bicolor x S. halepense) and "Permontra" hybrid perennial rye (Secale cereale x S. montanum).

An obvious consideration before any new crop is adopted is yield. Seed yields of any new perennial grains need to be sufficiently high and stable to make their adoption by farmers compelling. A second consideration is the possible loss of long-term viability as a species is selected for higher seed yield. Studies of several perennial species (Reekie and Bazzaz, 1987; Horvitz and Schemske, 1988; Piper, 1992; Piper and Kulakow, 1994), however, have indicated that, within the ranges investigated, there are no strict "trade-offs" between increased seed yield, vegetative growth, likelihood of future reproduction, or survivorship. Jackson and Dewald (1994) demonstrated conclusively that a severalfold seed yield increase in eastern gamagrass was not associated with a decline in plant growth or survivorship. Similarly, there was no apparent trade-off between higher seed yield and rhizome production in crosses between annual and perennial sorghum species (Piper and Kulakow, 1994). Such results hold promise for research to increase seed yield without losing the perennial nature of a species.

Species Diversity

Intercropping, the simultaneous raising of different crops in the same place, makes use of species that complement one another spatially or seasonally. Relative to monocultures, intercropped systems can display more efficient use of land, labor, or resources, increased yield, and reduced loss to insects, diseases, and weeds (Francis, 1986; Vandermeer, 1989). Overyielding, a yield advantage in mixture relative to monoculture, can occur when interspecific competition in a mixture is less intense than intraspecific competition or where plant species enhance the growth of one another. Many factors can lead to overyielding. Crops may be released from competition for light by having different light requirements or differences in architecture that minimize shading. Roots of different species may explore different soil layers, or crop species may have complementary nutrient requirements or uptake abilities. Differences in seasonal period of nutrient uptake among crops can also promote overyielding. Intercrops may be more productive than monocultures crops for improving soil fertility, controlling soil erosion, lowering risk of total crop failure, and decreasing crop losses to insects and diseases. Thus, intercropping may satisfy several crop production goals simultaneously.

One difficulty facing the plant breeder is that it may be difficult to predict from its performance in monoculture how a crop will behave in polyculture. For example, some plants change their root architecture or patterns of nutrient uptake when grown in association with different species (Goodman and Collison, 1982; Jastrow and Miller, 1993). Shorter plants that are vigorous in monoculture may be shaded out by taller neighbors in polyculture. Thus, selection of varieties for use in perennial polyculture is inherently more complex than selection of varieties for monocultures due to interactions between variety and cropping system that may be unpredictable. So, in addition to the traits needed by any viable crop (e.g., adaptation to the growing environment, tolerance of or resistance to prevailing insect or disease organisms, and reasonably high and stable yield potential), scientists breeding crops for polyculture must also select for or against competitive ability, shade tolerance, and modifications to plant architecture that allow coexistence (Smith and Francis, 1986). Hence, evaluation of species interactions is critical in designing a breeding program for polycultures. Moreover, in perennial systems the outcomes of species interactions may differ in different years (e.g., Barker and Piper, 1995).

Mechanisms that reduce overlap in resource demand among coexisting species usually involve differences in location and timing of resource use. Roots of neighboring species may explore different soil layers or a requirement might be met by different resources. Mixtures of legumes with nonlegumes frequently demonstrate yield advantages over monocultures because the two species are tapping different N sources which minimizes competition for this nutrient. Similarly, intercrops may be released from competition for light, and show greater overall productivity, if canopies of component crops occupy different vertical layers (Davis et al., 1984; Clark and Francis, 1985). Differences in length of the growing period or in the seasonal periods of nutrient uptake among crops (e.g., Piper, 1993a) can also reduce direct competition and thus promote overyielding (Francis et al., 1982; Smith and Francis, 1986).

Alternatively, a plant may benefit its neighbors by providing cover (Vandermeer, 1980), nitrogen (Wagmare and Singh, 1984), pest or disease protection (Risch et al., 1983; Burdon, 1987), protection from desiccating winds (Radke and Hagstrom, 1976), physical support against lodging (references in Trenbath, 1976), enhancement of mycorrhizal associations (Jastrow and Miller, 1993), or by attracting pollinators (Rathcke, 1984). Indirect facilitation can occur where one species "traps" nutrients that would otherwise leach or be lost from the system, and which later become available to other species (e.g., Agamathu and Broughton, 1985).

The Land Institute's ongoing research agenda in Natural Systems Agriculture revolves around four basic agronomic questions:

1. Can a perennial grain yield as well as an annual grain crop?

2. Can a perennial grain polyculture overyield?

3. Can a perennial grain polyculture provide its own nitrogen fertility?

4. Can a perennial grain polyculture manage weeds, insect pests, and plant pathogens?

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