North American Agroforestry. Группа авторов
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forage production while promoting high‐quality timber. Prescribed burns can prevent invasion by undesirable species while maintaining an open and productive understory. In semiarid areas, avoiding overgrazing is the most effective means of preventing the replacement of grasses by shrubs.
Ribbon Forests and Windbreaks
When wind encounters the edge of a forest, some of the air is deflected over the canopy for a distance of up to 20 tree heights (Cionco, 1985; Fritschen, 1985). If the forest occurs as a narrow strip, this deflection of air creates a protected zone to the leeward in which wind speed is reduced, wind‐related stresses such as desiccation are decreased, and snow deposition may increase.
This modification of microclimate is essential to the maintenance of ribbon forests (Billings, 1969; Peet, 1988) and is a fascinating feature of subalpine regions in the Rocky Mountains. Ribbon forests are arranged as alternating parallel strips of forest and moist alpine meadow oriented perpendicular to the prevailing winds. Snow accumulation to the lee of each forest strip inhibits seedling establishment, while tree growth rates at the far edge of each drift are increased by water from snowmelt and protection from desiccation by winter winds. Thus, the pattern and spacing of forest strips is determined by the effect of tree canopy structure on windspeed and snow deposition.
Ribbon forests are a classic model for one of the most common temperate agroforestry practices, windbreaks. Farm windbreaks are linear groups of trees that provide a sheltered microclimate for leeward fields. The extent and degree of shelter depends on the structural characteristics of the windbreak such as height, density, and orientation, and these can be manipulated to meet particular management goals (e.g., odor control). Dense windbreaks result in deposition of snow in drifts close to the leeward edge and act as living snow fences. More porous windbreaks cause snow to be distributed more evenly across the leeward field, a preferable situation if soil moisture conservation or protection of winter wheat from desiccation is the goal (Brandle & Finch, 1991; Mize, Brandle, Schoenberger, & Bentrup, 2008).
Riparian Forests
Particularly in arid and semiarid regions, riparian forests are often the only mesic vegetation type and serve a critical role as wildlife habitat. In Arizona and New Mexico, an estimated 80% of all vertebrates are dependent upon riparian habitat for at least part of their life cycle (Johnson, 1989). As linear features in the landscape, riparian forests may serve as corridors for the movement of many species between otherwise isolated patches of habitat (Forman & Godron, 1986). Woody vegetation also plays an integral role in the stabilization of streambanks (Smith, 1976), shading of streams reduces water temperature, and detritus inputs to the stream from the forest provide an energy source as well as habitat structure for aquatic organisms.
Because they occupy low spots in the landscape, riparian forests receive water and water‐borne nutrients and sediment from upland areas, filtering and trapping many of these inputs before they reach the streambed (Lowrance et al., 1984). These forests interact not just with adjacent fields but with systems throughout the landscape, linked through the hydrologic pathways of the watershed. In agricultural regions, this landscape‐level water quality function is particularly important. For example, despite large applications of N fertilizer to corn, peanut (Arachis glabrata Benth.), and other cropland in a Georgia Piedmont watershed, very little N left the watershed in streamflow due in part to accretion of N in the riparian forest biomass and denitrification in the saturated riparian soils (Lowrance, Leonard, Asmussen, & Todd, 1985). Maintenance of a young‐age forest through selective logging can improve the water quality function of the stand by maintaining plant nutrient uptake at a high rate (Welsch, 1991). An excellent overview on the ecological impact of developing riparian forests can be found in Oelbermann, Gordon, and Kaushik (2008).
Isolated Grasslands
In large areas in the Great Plains, particularly in the more xeric short‐ and mixed‐grass prairies, grasses and forbs exist largely independently of any woody species. The same situation exists in some of the larger high‐elevation meadows in the Rockies and, pre‐settlement, the grasslands of the Central Valley of California and the Palouse Prairie of eastern Washington.
How small a grassland can be before adjacent woodlands have a significant effect is, of course, a key question in terms of agroforestry design. Long‐term—i.e., hundreds or thousands of years—these grasslands did not remain isolated, as evidenced for example by the presence of conifers throughout the Great Plains during the various glacial periods (Axelrod, 1985).
General Ecological Principles
We can highlight four ecological principles that will be of particular use in designing and evaluating agroforestry practices:
1 Ecosystems are distinguished by spatial and temporal heterogeneity. An ecosystem or landscape consists of a mosaic of patches and linear components. The boundaries between patches are often the site of increased rates of processes such as nutrient and energy exchange, competition, water flow, and movement of organisms (Ranney, Bruner, & Levenson, 1981; Holland, Risser, & Naiman, 1991). For example, most of the removal of NO3 from water entering a Georgia riparian forest occurred in the first 10 m of a 55‐m‐wide forest (Lowrance, 1992). Designers of agroforestry practices should pay particular attention to the interfaces of woody and non‐woody components within their systems (Dix et al., 1995).Temporal variability is also important. Some variability, such as diurnal and seasonal environmental change or longer term successional change, is predictable and can easily be considered in designing practices; an example would be a winter wheat alley cropping practice in which the wheat completes most of its growth before the tree crop leafs out each spring (Chirko, Gold, Nguyen, & Jiang, 1996; Thevathasan & Gordon, 2004). Other sources of variability (e.g., drought) are less predictable but no less important to practice design and function.
2 Disturbance is a primary determinant of ecosystem structure and function. Ecosystems have adapted to various degrees and combinations of fire, drought, wind, flood, pest outbreaks, and other disturbances. Much of the heterogeneity in landscape structure is due to patterns of disturbance. Removal of a critical disturbance, for instance fire from an oak savanna, is a major disruption of system function and may trigger a structural shift to a closed forest (Bragg, Knapp, & Briggs, 1993).Management of agroforestry practices requires the appropriate application of disturbance to maintain the state that best meets management goals. In conventional row‐crop agriculture, tillage, a type of disturbance rarely seen in natural ecosystems, is required to inhibit normal successional processes and trajectories. Agroforestry managers need to consider the use of disturbance agents that mimic the natural disturbance patterns that maintain the agroecosystem at later stages of succession. Similarly, agroforestry practitioners should adopt management strategies such as pruning and root disking to reduce competitive interactions between trees and crops.Agroforestry practices must also be designed to handle sporadic, though inevitable, environmental stresses such as drought, high wind, intense rain, and extreme cold. Windbreaks, riparian forests, silvopasture, and other agroforestry practices that add perennial crops and groundcover to the farm will generally increase the system’s resilience and resistance to these stresses.
3 Perennialism is the most common condition in natural ecosystems. Annual plants dominate only after certain disturbances and are quickly replaced in the successional process by perennials. Disturbances severe enough to provide an opening for annuals also provide a window for accelerated loss of soil and nutrients from the system. These windows are generally short in natural systems, but if the disturbance is repeated regularly, as in row‐crop agroecosystems, the cumulative losses of soil and nutrients can greatly reduce the productive capacity of the system.Agroforestry practices provide one means of adding perennials to a conventional row‐crop farming system. There are many non‐woody perennials such as grasses, alfalfa, and clover (Trifolium spp.) that can also provide benefits. An optimal agroecosystem design will consider all potential perennial crops as well as appropriate annual crops.
4 Structural and functional diversity are important to ecosystem performance but are difficult to quantify. If an ecosystem includes