North American Agroforestry. Группа авторов

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North American Agroforestry - Группа авторов


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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.

Schematic illustration of hypothetical changes in energy and nutrient fluxes, pools and conditions of existence, upon the introduction of trees via agroforestry systems into agricultural systems.
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
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