or whose leaves capture sunlight at different heights in the canopy, this structural and functional diversity may increase the efficiency of the system in using resources and maintaining production. However, many species function similarly, so species diversity alone is a poor measure of functional diversity (Olson & Francis, 1995). Agroforestry provides an obvious way to increase the structural diversity of a row‐crop farm. While this change in diversity will undoubtedly have functional effects, the farm manager needs to carefully consider the relationship between structure and function as it applies to their management goals. Random additions of woody perennials will increase species diversity but are unlikely to produce optimal economic or ecological results.
Indicators of Agroecosystem Sustainability
In the remainder of this chapter, we examine the system‐level effects of adding agroforestry practices to a conventional farm, such as those related to microclimate, as illustrated in Figure 3–3. However, to fully judge the effects, indicators of economic and social sustainability as well as of ecological sustainability must be considered. To be sustainable, an agroecosystem has to be profitable and it has to meet societal demands for food and fiber. If changes to a farm are made solely to improve the ecological trends illustrated in Table 3–3, the effect on the overall system may be negative. We should note that long‐term gains may be justifiable reasons for introducing systems that in the short term may not be overly economically viable.
Fig. 3–3. Hypothetical changes in energy and nutrient fluxes, pools and conditions of existence, upon the introduction of trees via agroforestry systems into agricultural systems (from Brenner, 1996; reproduced with permission of CABI, Wallingford, UK).
The issue of sustainability and the choice of indicators of agroecosystem condition have been considered frequently (Harrington, 1992; Lefroy & Hobbs, 1992; Stockle, Papendick, Saxton, Campbell, & van Evert, 1994; Campbell, Heck, Neher, Munster, & Hoag, 1995; Thevathasan et al., 2014). Although the debate continues about which group of indicators is most appropriate, there has been considerable convergence among the choices. We have compiled a suite of indicators (Table 3–4) based on our examination of Appendix 3‐1, Odum (1985), (Table 3–3), and Francis, Aschmann, & Olson (1997), on indicators of functional sustainability of farms. This group of indicators reflects our summary view of agroecosystem sustainability, i.e., in an increasingly resource‐poor world, farms that maintain a high rate of conversion of solar energy into marketable crops, minimize ancillary energy and material inputs, and preserve their natural capital (e.g., soil) will be the most sustainable.
Although it is fairly easy to determine which trend in an indicator favors sustainability, it is more difficult to quantify the particular values of an indicator that represent high or low sustainability. As indicated in the footnotes to Table 3–4, we set upper and lower bounds for our indicators based on benchmark farming systems in the region, such as irrigated continuous corn (e.g., high energy inputs), the properties of a particular soil (e.g., 11 Mg soil erosion per hectare is the tolerance limit for a Sharpsburg silty clay loam with 4–6% slope), or on economic benchmarks (e.g., the poverty level for a family of four). The goal is to ground the evaluations in a realistic assessment of the range of conditions in the region of interest.
Table 3–3. Trends expected in stressed ecosystems (Odum, 1985) and the evidence for these trends in a corn–soybean farm relative to a prairie or oak–hickory ecosystem (drawn from Appendix 3‐1).
| Trend | Farm characteristics in support |
|---|---|
| Energetics | |
| 1. Community respiration increases | tillage increases decomposition of soil organic matter |
| 2. P/R (production/respiration) becomes unbalanced (< or >1) | system production exceeds respiration due to export of net primary productivity (NPP) from system |
| 3. P/B and R/B (maintenance/biomass structure) ratios increase | data not available |
| 4. Importance of auxiliary energy increases | 17.3 × 103 MJ ha−1 input (as fertilizer, fuel, labor, etc.) |
| 5. Exported or unused primary production increases | 450 g kg−1 (45%) of NPP exported as grain |
| Nutrient cycling | |
| 6. Nutrient turnover increases | see no. 7 |
| 7. Horizontal transport increases and vertical cycling of nutrients decreases | internal N cycling decreases from 960 to 560 g kg−1 (96 to 56%) of total N flows |
| 8. Nutrient loss increases (system becomes more “leaky”) | loss of N from farm is 7 to 50 times greater than from natural ecosystems |
| Community structure | |
| 9. Proportion of r‐strategists increases | annual crops replace perennials |
| 10. Size of organisms decreases | corn smaller than oak and soybean smaller than tall grasses |
| 11. Lifespans of organisms or parts (e.g., leaves) decrease | crops are annuals |
| 12. Food chains shorten | not shortened, but food web complexity likely reduced as one consumer (humans) co‐opts almost half of NPP |
| 13. Species diversity decreases and dominance increases | two species dominate |
| General system‐level trends | |
| 14. Ecosystem becomes more open (i.e., input and output environments become more important as internal cycling is reduced) | inputs of cultural energy and chemicals, and export of harvested crops are essential to system maintenance |
| 15. Autogenic successional trends reverse (succession reverts to earlier stages) | system maintained at first year of secondary succession by annual tillage |
| 16. Efficiency of resource use decreases | annual NPP reduced despite large inputs of external materials and energy |
|
17. Parasitism and other negative interactions increase, and mutualism and other positive interactions
|