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 decrease | chemical and energy inputs required to reduce specific pest populations on specific hosts |
| 18. Functional properties (such as community metabolism) are more robust (homeostatic‐resistant to stressors) than are species composition and other structural properties | despite drastic reduction in biodiversity and simplification of structure, system continues to be productive |
Table 3–4. Selected indicators of sustainability for agroecosystems, and the indicator values for the conventional and agroforestry farms described in Table 3–5.
| Indicator | Definition | Value indicating high sustainability | Value indicating low sustainability | Conventional farm | Agroforestry farm |
|---|---|---|---|---|---|
| Harvest a | weight of harvested crops and livestock, kg ha−1 (lb acre−1) dry weight | 7,952 (7,100) | 0 | 3,805 (3,397) | 3,923 (3,503) |
| Cultural energy input b | total non‐solar energy inputs, MJ ha−1 (MJ acre−1) | 0 |
59,259 (24,000)
|
