Agroforestry: An Integrated Land-Use Management System for Production and Farmland Conservation
A COMPREHENSIVE ASSESSMENT OF U.S. AGROFORESTRY (LONG VERSION)
The Agroforestry Component of the Resource Conservation Act Appraisal for the SCS
Prepared for USDA SCS (Account 68-3A75-3-134)
February, 1994
Submitted by Committee Chair: H.E. 'Gene' Garrett , University of Missouri
Committee Members: W.B. Kurtz, University of Missouri; L.E. Buck, Cornell University; J.P. Lassoie, Cornell University; M.A. Gold, Michigan State University; H.A. Pearson, USDA ARS; L.H. Hardesty, Washington State University; J.P. Slusher, University of Missouri
TABLE OF CONTENTS
AGROFORESTRY DEFINED
Examples of practices
POTENTIAL ACREAGE FOR AGROFORESTRY
ECOLOGICAL BENEFITS
Protection Benefits
Crop protection
Snow capture
Wind damage
Microclimate modification
Livestock protection
Tree/groundcover interactions
Faunal diversity/crop yields
Plant/insect interactions
Environmental Benefits
Water quality
Nutrient reduction
Biomass removal/saturation in woody VBS
Sediment reduction and stream bank protection
Impacts on fish populations
Herbicide and pesticide reduction
Air quality
ECONOMIC BENEFITS
Production Economic Benefits
Trees and crops
Grazing/haying natural pine
Protection Economic Benefits
Soil erosion control
Shelterbelts
Federal Agroforestry Programs
Conservation Reserve Program (CRP)
Center for Semi-arid Agroforestry (CSA)
South Central Family Farm Research Center
Related Federal Technical Assistance and Cost-Share Programs
Stewardship Incentive Program (SIP)
Forestry Incentives Program (FIP)
Agricultural Conservation Program (ACP)
Environmental Easement Program (EEP)
Tree Planting Initiative
Sustainable Agriculture Research
Government Price Supports and Taxes
Agroforestry Research Extension and Education at Land Grant Universities
State Agroforestry and Related Programs
Association for Temperate Agroforestry (AFTA)
Non-profit membership groups
Summary of Policy Relating to Landowner Incentives
ACHIEVING AGROFORESTRY'S POTENTIAL
SUMMARY OF AGROFORESTRY DEVELOPMENT NEEDS
LIST OF TABLES (Note: All tables are in a separate 221 KB PDF file. Click here to view).
Table 1. Cropland Acres1 (1,000's) With an Erodibility Index (EI)2 <_ or > 8 on Nonfederal Rural Land
Table 2. Total Nonfederal Pasture and Rangeland Acres' (1,000's) and Acres With High and Medium Potential for Conversion to Cropland or Agroforestry (Based upon the 1982 NRI)
Table 3. Total Nonfederal Forestland Acres' (1,000's) and Acres with High and Medium
Potential for Production of Specialty Products (Based upon 1982 NRI)
Table 4. Orchard (fruit and nut) and other Horticultural (vineyard, bush fruit
and berry) Acres' (1,000's) on Nonfederal Rural Land
Table 5. Lineal' Miles2 Of Riparian Area' Streambank Vegetated With Shrubs
Table 6. Lineal' Miles of Riparian Area' Streambank Without Trees or Shrubs
Table 7. Percent of Land in the 18 Western States in Federal Ownership
Table 8. Summary of Minimum Woody Corridor Planting Needs in the United States1 (in 1,000's)
AGROFORESTRY: AN INTEGRATED LAND USE MANAGEMENT SYSTEM
FOR PRODUCTION AND FARMLAND CONSERVATION
"A farm can be regarded as a food factory and the criterion for its success is saleable products. Or, it can be regarded as a place to live, and the criterion for its success is harmonious balance between plants, animals and people; between the domestic and the wild; and between utility and beauty." - Aldo Leopold
Never in history has global concern for the consequences of human land use been more widely shared. Many regions of the world cannot meet their growing populations most basic needs for food, water and energy. Soil erosion adds to food, energy and transportation costs and threatens future food production capacity. Nonpoint source pollution from forest and agricultural lands restricts access to safe water. Loss of vegetation from land development and site degradation affects our aesthetic environment, global climate patterns and the quality of the air we breathe. Such problems are often the legacy of our success in maximizing production of one or more agricultural products in a financially optimal fashion without sufficient knowledge of, or regard for, impacts on future productivity and the environment. This was a rational economic choice when long-term consequences were unknown, when resources appeared to be relatively unlimited, and when technology promised means for further intensifying production. Now that we are discovering undesirable longer term consequences of current land-use systems, alternatives must be sought. one alternative is to model managed ecosystems after the structure and functions of naturally-occurring ones by reestablishing complexity in time, space and biodiversity. This would lead to a shift away from separating land uses on discrete parcels to integrating them on a landscape level. Agroforestry, which exploits the interactions between trees and crops (including livestock) when they are grown together, bridges the gap between production agriculture and natural resource management. This provides opportunities to integrate land uses on a landscape level. Furthermore, properly designed agroforestry systems provide environmentally and economically sound alternatives to unsustainable production systems.
In the past, progressive land management in North America meant increasingly intensive use of a site for production of a single product -- corn, rice, wood, or others, -- in which other values such as watershed and wildlife habitat were competitive, or at best, secondary. In contrast, agroforestry seeks to optimize production of multiple products and benefits by manipulating the interactions between components. For example, sheep grazing a forest plantation provide short-term revenue and reduce competition. Trees planted on a floodplain yield wood products, trap sediments during peak flows, reduce bank cutting and protect adjacent croplands and downstream water quality. Resource conservation and production goals are integrated rather than assumed to be tradeoffs. Several decades of development of agroforestry systems and their application in temperate nations such as New Zealand and China demonstrate the range of conditions under which this approach has been successful. Agroforestry requires shifting our thinking in both spatial and temporal domains, and demands skills in managing, rather than reducing complexity. Traditional disciplinary approaches to problem-solving such as the forester dealing with the trees, the soil scientist with the soil, and the hydrologist with the water, are no longer sufficient. This may be one reason why less developed nations that have never strictly segregated land uses are today's world leaders in development and application of agroforestry technologies. Agroforestry challenges land managers to transcend disciplinary boundaries and explore the potential synergism between production agriculture and natural resource management. Essential to this is an understanding of hierarchical scalar relationships within ecosystems and recognition that defined ecosystem "boundaries" exist primarily for managerial convenience.
Scientific exploration of complex, long-lived systems is difficult when only technique constrains the enterprise. Such exploration is nearly impossible when institutional and fiscal constraints also apply. Hence, the scientific basis for agroforestry lags behind the emerging need for its application as an alternative land use strategy in the United States. This gap is further exaggerated by the large clientele served. The farmer needs an immediate economic return; the consumer safe and affordable food; the parent, assurance of a safe future environment; and the general citizen, hope for global and interspecies equity. All are measured in different currencies and on different time scales. A forester may not be prepared to consider a tree as an individual landscape component. The farmer may see windbreaks as a loss of productive acres. The economist may not be able to value reduced soil erosion and so forth. The requirements of implementing complex systems over a range of environmental and social conditions are daunting.
Both the development and application of agroforestry has proceeded rapidly in many developing nations in the tropics. The problems confronting individuals and institutions were sufficiently urgent to justify bold approaches beyond the safe confines of scientifically defined management systems. While adoption has been slower in the temperate regions of the world, two decades of observational data and applied research suggest that agroforestry should be vigorously explored as a possible component of improved land-use strategies in the United States. Current interest in ecosystem management strongly suggests that we should embrace the complexity inherent to agroforestry and apply agroforestry principles, where appropriate, to better meet our current and future needs for the products and services of the land.
Agroforestry is an intensive land-management system that optimizes the benefits from the biological interactions created when trees and/or shrubs are deliberately combined with crops sad/or livestock.
Four key components of this definition are: (1) land use is intensive; (2) benefits are optimized; (3) biological interactions are increased; and (4) trees and/or shrubs are deliberately combined with crops and/or livestock.
Intensive land use--Agroforestry utilizes management practices for producing trees with crops and/or livestock. Management schemes incorporate intensive practices such as tree planting, annual cultivation, fertilization, irrigation, weed control, liming, grazing or combinations of these and other practices. Therefore, agroforestry manipulates the agroecosystem to achieve optimal benefits. Grazed forests or woodlands extensively managed as natural ecosystems also produce wood products and forage for livestock or wildlife but without using intensive approaches such as annual inputs of nutrients, herbicides, or mechanical treatments. These latter systems are often excluded from the agroforestry classification but, because of similar interactions between woody plants, forage supplies and livestock, they are considered a subset of the agroforestry land-use classification for management purposes.
Optimizes benefits--As a land-use system, agroforestry provides many benefits for humankind. These benefits may be economic, environmental, biological, or social. Optimization implies that the various factors can be combined, through compromise, to best serve the interests of the user and society. Since each user may have different objectives -- soil conservation, minimization of inputs, integrated pest management, greater profit, aesthetics -- optimization of system components will vary.
Biological interactions--Protective and productive benefits are realized from agroforestry practices. They are the products of biological interactions resulting from the proper mix of woody perennials, herbaceous species and livestock. These interactions affect soil, water and air quality, biological diversity, wildlife habitat, aesthetics, economics and, ultimately, rural community development. As an example, riparian filter strips may help reduce soil erosion from cropland and improve habitat for some particularly desired fish or wildlife species while enhancing visual landscapes.
Trees and/or shrubs deliberately combined with crops and/or livestock-An intentional combining of trees and/or shrubs with crops and/or livestock is an essential element of agroforestry systems. This aspect mimics the multispecies, multi-storied characteristic found in natural ecosystems while providing a variety of marketable commercial products. Crops may include conventional agronomic commodities such as corn, cotton and soybeans; specialty commodities such as ginseng, golden seal, shiitake mushrooms and honey; both warm- and cool-season tame pasture; and managed native forages -- forestland, woodland, pastureland and rangeland. Trees and shrubs may consist of high-value timber/veneer, nut/fruit producing woody perennials and others. Livestock are animals kept for use on the farm or raised for sale and profit.
Some prominent examples of agroforestry practices applicable to the U.S. temperate zone include: (1) riparian vegetative buffer strip (VBS) systems -- a combination of vegetative types established on stream and river banks -- to regulate microenvironments and protect fish habitats or for regulating nonpoint source waterway pollution; (2) tree-agronomic crop systems such as alley cropping or intercropping -- planting rows of trees at wide spacings and cropping the alleyways -- for increasing and/or diversifying farm incomes, abating soil erosion and nutrient loading, and protecting watersheds; (3) tree-animal systems -- silvopastoral or forest livestock grazing, the intensive management of forages grown with trees for pasture purposes -- which are especially prominent in the west and south leading to economic, wildlife habitat, fire protection, and forest management benefits; (4) windbreak systems (shelterbelts) of noted importance in the plains and western states for protecting and enhancing production of crops and animals and stabilizing microenvironments; and (5) natural forest/specialty crop systems -- forest farming, the development of suitable microenvironments in natural forest stands for growing specialty crops such as mushrooms and ginseng.
POTENTIAL ACREAGE FOR AGROFORESTRY
While few data exist which would allow a reliable estimation of the degree of current use of various agroforestry practices, vast acreages of forestland, pastureland, rangeland, cropland and stream corridor suitable for agroforestry occur throughout the United States (Tables 1 through 6). The adoption and application of practices varies by region and is driven by local tradition, economic factors and land-ownership patterns (government vs private).
Approximately 32% of the U.S. land area is in federal ownership but the percentage varies significantly by regions and even within regions by states. In the western states, where opportunities exist for agroforestry practices such as riparian buffer strips that affect water quality and wildlife habitat, and silvopastoral management of forest plantations in lieu of herbicide use, federal ownership ranges from 89.5% in Alaska and 81.7% in Nevada to a low of 6% in Colorado (Table 7). In contrast, in the northern plains -- Kansas, Nebraska, North Dakota and South Dakota -- federal ownership ranges from a low of 1.4% in Kansas and Nebraska to a high of only 6.4% in South Dakota. The prospects for application of agroforestry practices to federal lands are probably limited in the short-run by the long time frames committed to existing land-use plans. However, mounting pressure to resolve land-use issues such as anadromous fish habitat, forest health, threatened and endangered species and rural community stabilization is leading to a new openness toward nontraditional land-use decisions on federal land. In spite of this, in the short-term, the application of agroforestry principles primarily will be limited to the private sector.
The growing intensity of land-use practices, the increasing public interest in biological conservation and legislated requirements for resource impact assessments, all serve to encourage private landowners to evaluate the potential of agroforestry to meet their needs. Agroforestry systems could provide landowners with land-management options which have environmental benefits and provide sustainable economic gains. When land-management advisors were surveyed recently in Washington state, all of the respondents indicated that agroforestry could be useful in USDA-mandated conservation plans.1
The potential for agroforestry on highly erodible cropland is enormous. More than 112 million acres of non-federal cropland have an erodibility index (EI) of greater than 8 (Table 1). These lands could benefit greatly from the application of agroforestry practices, such as windbreak systems or alley cropping, designed for both production and protection.
In the Midwest, where considerable agroforestry research has been conducted, the five states of Missouri, Illinois, Indiana, Iowa and Ohio have more than 19 million acres of cropland with an EI greater than 10.2 Approximately 9 million of these acres are recommended for forestry plantings and would be ideally suited for agroforestry. In the plains states of Nebraska and Kansas, an additional 16 million acres of cropland are highly erodible (EI > 8) and would lend themselves well to windbreak or alley cropping systems (Table 1). Furthermore, within the United States there exists over 131 million acres of pastureland and another 405 million acres of rangeland (Table 2). Approximately 18 million of the pastureland acres are rated as having high potential for conversion to cropland and should, therefore, lend themselves to agroforestry. Another 40 million acres have medium potential for conversion. More than 8 and 40 million acres, respectively, of the rangeland also are rated as having high or medium potential for conversion to cropland and, we may assume, agroforestry.
Silvopastoral agroforestry has great potential in the United States but perhaps nowhere is it greater than in the pine belt of the south and in the west (Table 3). In the south, the pine and oak-pine forestland totals about 99 million acres. This land either produces or has the potential to produce significant forage. resources. Much of it could be successfully converted to silvopastoral management. In the Pacific region -- California, Oregon, Washington -- nearly 40 million acres of forest exist. Of this, approximately two million acres (Table 3) are rated as having good potential for conversion to cropland and could produce high-quality wood/forage under sound management practices. Millions of additional acres could be seeded to forages and, through intensive silvopastoral management, result in significant economic gains for the region without harming trees or environment. Forestland grazing, including plantation grazing, is the most common agroforestry-related practice in Washington state, and perhaps in the region.3 Moreover, recent reviews conclude that properly managed forest and plantation grazing can benefit tree growth. The impact of forest grazing on ecosystem processes is not fully understood at this time. However, it is generally believed that under conditions of intensive silvopastoral management, impacts are directly related to the intensity of grazing.
Forested areas also provide opportunities for the production of specialty crops which are sold for ornamental, culinary or medicinal uses. While silvopastoral management and the production of specialty crops may be incompatible, forest landowners not interested in livestock grazing may choose to develop enterprises such as the production of food items (mushrooms, sugar maple), medicinal plants (ginseng, golden seal) or even shade-tolerant ornamental plants which require the microenvironment of forested acreage. Specialty crops are already a multi-million dollar business in many parts of the United States -- northeast, lake states, Appalachia and the Pacific northwest. In Kentucky alone, ginseng production is a $5 million per year businees.4 Much of the forested acreage available in the United States would readily lend itself to a variety of enterprises relating to the production of specialty crops. Much of the acreage under orchard management (Table 4) could also be adapted to accommodate specialty crops under agroforestry management. Seven states -- Michigan, New York, Florida, Georgia, Texas, Washington and California -- each have more than 180,000 acres of orchards which might benefit from conversion to agroforestry.
An agroforestry practice of enormous potential and in need of immediate broadscale adoption is the riparian vegetative buffer strip (VBS). Properly designed VBS help control water quality, flow regime, physical habitat and energy inputs in streams while providing wood products. They reduce the effects of traditional farming on stream quality while creating suitable habitat for a variety of wildlife species including aquatic inhabitants. Of special concern are streambanks without adequate vegetative protection where only shrubs and/or grasses or forbs are found (Tables 5 and 6). Many streams found throughout the United States could be greatly improved through the establishment of proper vegetation along their edges. In the corn belt alone -- Illinois, Indiana, Iowa, Missouri and Ohio -- where agricultural chemicals are used extensively, more than 85,000 miles of stream and river banks are unprotected by trees or shrubs. An additional 80,000 miles are unprotected or have only minimal protection in the heavily farmed states of Kansas, Nebraska, North Dakota and South Dakota. California has more than 40,000 miles of unprotected stream and river banks.
The potential for agroforestry varies by regions, reflecting the diverse landscapes, values and regional/local economies. Agroforestry adoption emerges and changes as a function of complex socio-economic factors, combined with the innovative capacity of landowners to respond to respective market forces and social values. Notwithstanding these qualifications, it appears that the potential for agroforestry is vast, as is the acreage where it may be applied.
Properly designed agroforestry systems patterned after natural ecosystems yield many benefits. Such systems require active manipulation of vegetation diversity. Landscape diversification of crop-fields, through the establishment of windbreaks, shelterbelts, woody hedges, alley cropping systems, and others leads to:
- increased productivity during successional changes;
- decreased weed competition;
- increased self-maintenance and internal regulation;
- enhanced biological regulation of major insect problems;
- increased efficiency in use of solar, radiation;
- increased soil organic matter;
- increased biodiversity in agricultural landscapes;
- decreased agriculturally derived contaminants in riparian zones;
- decreased wind and water erosion;
- increased uptake and fixation of atmospheric carbon dioxide;
- increased nutrient retention via greater exploitation of soil profiles; and
- improved economic efficiency.
ECOLOGICAL BENEFITS
Protection Benefits
When carefully designed, woody and herbaceous corridors such as hedgerows, windbreaks, shelterbelts, wooded alleys and forest edges, can be need to protect soil, conserve moisture and improve dryland crops. Corridors provide habitat that benefit wildlife within agricultural landscapes and provide diversity that supports natural enemies of crop pests. Benefits to wildlife include protection from wind and adverse weather, escape or refuge cover, food and foraging sites, reproductive habitat and travel areas.5 6
Negative effects of woody corridors upon crops include a reduction in available land for planting, the potential for increased wind turbulence, competition for soil moisture and nutrients, allelopathy and shading. All of these potentially negative effects occur in immediate proximity to the woody corridor. Most can be mitigated through proper species selection, corridor design and management (e.g., corridor root pruning).
Beneficial effects leading to increased crop yields are afforded by woody corridors. Increases are attributed to reduced wind erosion, improved microclimate, snow retention and reduced crop damage by high winds.7 Twenty year-old woody corridors in which trees and shrubs occupy only 6% of the land unit have been shown to provide adequate crop protection in Nebraska.8 Land protected from wind is, in general, more productive per unit area than unprotected land and total crop output increases. Reducing the number of acres required to achieve crop production goals reduces requirements for crop inputs including fossil fuels and fertilizer.
Snow capture -- where a significant amount of annual precipitation falls in the form of snow, entrapment and retention by woody corridors has been found to increase crop yields and provide a layer of insulation preventing winter kill of sensitive crops such as winter wheat.9
Wind damage -- Small-seeded, shallow-sown crops, and newly emerged crops are very susceptible to direct wind damage. Severe postemergent crop damage can also occur due to sandblast -- the effect of soil particles abrading plant tissue. The effects of sandblast injury varies from reduced yields and delayed maturity to mortality. Woody corridors trap wind-borne soil and reduce wind velocity, thereby reducing wind erosion and sandblast injury. Crop damage can also occur due to lodging. Shelterbelts have been found to reduce wind-induced lodging. In one case, lodging of oats was prevented to a distance of 10 h (height of trees) behind a single-row Douglas fir shelterbelt.8
The overall benefit of shelterbelts on crop yields, via reduced soil erosion, depends on the relationship between erosion and crop productivity. Commonly, only small annual reductions in productivity are found to be due to the loss of topsoil. The magnitude of these losses depends upon soil texture, root zone depth and soil regeneration rates as well as the amount of soil lost. For example, reports suggest that shelterbelts increase yields of small-grain cereals in low-yielding fields more than in high-yielding fields due to differences in soil texture and consequent moisture availability. 10
Microclimate modification -- This can influence all production components of an agroforestry system. Agronomic crops may differ significantly in their responsiveness to shelter and associated microclimate modifications. Sheltering, however, generally improves growing conditions due to higher soil moisture, humidity and night-time carbon dioxide levels as well as lower evaporation and night-time air temperatures. Of greatest importance is the moderation of extreme temperatures, resulting in lower respiration rates. Collectively, these modifications tend to improve crop yield and quality and accelerate maturity.11
Windbreaks are especially important for livestock in northern climates. Properly positioned trees and shrubs can provide much needed protection for pastures, feedlots and calving areas. Reducing wind speed lowers animal stress, improves animal health and increases feeding efficiency of livestock.12 Canadian researchers have demonstrated that cattle on winter range require an additional 20% increase in feed energy, above maintenance, to offset the direct effects of exposure to a combination of cold temperatures and wind. Adequate wind protection has been found to reduce the direct effects of cold by more than half.13 Similar findings have been reported for swine and dairy animals.14
Underplanting black walnut with legume or grass covers can effect positive changes in the phonology of walnut. Compared with clean cultivation, the maintenance of legume or grass covers in walnut plantations delays bud break 6 to 12 days, thereby decreasing possible frost damage. Underplanting walnut with winter annual legumes also has been found to accelerate the onset of dormancy.15 Furthermore, seeding of plantations with cool season legumes, even without chemical weed control, can accelerate tree establishment and growth.
Alnus spp., non-leguminous nitrogen fixers, are recognized as having the potential for use in agroforestry management systems similar to that of legumes in agriculture. The concept of "intercropping vigor", in which dry matter production of mixed cultures exceeds that of pure plantings has been reported.16 The major benefits derived from the use of woody nitrogen fixers include the realization of optimal biomass yields per unit of land area, a reduction or elimination of the need for applied nitrogen fertilizer, improvement in soil fertility and soil physical properties, suppression of soil pathogens, and improved growth of associated species in mixed cropping systems.
Mixtures of annuals and perennials result in better nutrient capture and tighter nutrient cycling. Extensive, deep root systems and associated mycorrhizal fungi enable woody plants to enrich soil by recycling leached or unavailable nutrients at rhizosphere depths beyond the reach of annual plants. In addition, the trapping, retaining and recycling of nutrients contained in rainfall is enhanced.
Long-rotation, mixed-cropping systems using nitrogen-fixing trees as nurse plants usually require that the nurse crop be harvested, poisoned, or removed before the final harvest of the timber crop. However, reported benefits may outweigh the inconvenience. Mixed plantings of nitrogen fixing autumn olive (Eleangus umbellata) with walnut stimulates walnut growth. After 10 years, walnut grown with autumn olive were 80% taller and 104% larger in diameter than those grown alone. In addition, the mixed plots were higher in soil nitrogen, lower in soil moisture, and had lower soil and air temperatures.17 Similar responses have been found using other species, suggesting an improvement in economic viability when intermediate quality sites planted with high-value tree species are interplanted with nitrogen fixers.
Diversity in habitat and fauna through the application of agroforestry principles appears to bring stability to agricultural landscapes. A recent review of shelterbelt literature indicates that at least 108 bird species and 28 mammalian species use shelterbelt habitats. Of the 108 bird species found within agricultural landscapes, 29 have been reported to benefit substantially from the microenvironments created. Similarly, research in the Midwest indicates the importance of grassed waterways, woody and herbaceous edges around crop fields, fencerow corridors and riparian areas to birds in agricultural landscapes.18 19
Of practical importance, birds as predators of insects may affect crop paste within the crop field, the edge habitat or the wooded corridor. Birds living in a wooded corridor forage in the crop field and consume insect pests blown or attracted to the edge. Calm winds on the leeward side of wooded corridors appear well suited for bird and bat insect predation. Downy woodpeckers have been found to be important predators of European corn borers in North Dakota, Louisiana and Arkansas. Woodpeckers consume the larvae by pecking into the corn stalks after harvest. Other studies have found bird predation on insect pests in tobacco and fruit orchards. Although it is unlikely that birds or other vertebrates could eliminate a pest population, they appear capable of keeping pest numbers below thresholds at which damage becomes an economic problem
Vegetation change is an important tool in controlling agricultural pests. Crop species receive protection from insects when grown in association with other species through the following mechanisms: 1) reduced visibility of pest target; 2) diluted number of potential hosts with increased diversity; 3) physical interference with pest movement; 4) creation of environments which are less favorable to population buildups; and 5) creation of environments which are more favorable to parasites and predators.
On a broad scale, agricultural landscapes are composed of a large number of individual units that vary in size, shape and composition. Agroforestry must be integrated at the landscape level where concern is for the entire crop field, woodlot, fencerow, hedgerow, wetland, and orchard as a dynamic system. Each of the individual units, however, can be considered as separate ecosystems interacting with each other where plants, animals, nutrients and insects flow easily between them. In agricultural landscapes many of these flows are influenced by humans as is the case with insect pests. Insect populations in agricultural landscapes are particularly dynamic. Their spatial distribution is seldom static, with populations flowing through multiple habitats with multiple spatial patterns over the course of a season. The spatial patterns of vegetation strongly influence the biology of arthropods both directly and indirectly. In particular, corridors, that are a common feature in many agroforestry systems and distinct as narrow strips of land different from the matrix on either side, can play significant roles.
Due to their extensive edge to volume ratios, corridors harbor organisms that tend to have extensive interactions with adjacent elements. Tree and hedge rows, irrespective of their application, may increase or decrease pest density in an associated crop depending upon a number of factors. These uncultivated habitats can, and sometimes do, serve as a reservoir of pest species acting as a source of initial infestation for adjoining crop fields or orchards. These habitats may be important as over wintering sites for predators or as a source of food. Alternatively, association with wooded edge habitats has been reported to reduce the infestation of certain pests. An increased abundance of natural enemies and more effective biological control of pests is often found where uncultivated habitats, frequently corridors, occur in association with crops. Many feel that through management, the positive aspects of corridors can be accentuated while the potential negative aspects are eliminated. For example, some forest-dwelling carabids -- predators of insect pests -- are never found more than 150 feet from a forest edge. Hedgerows, however, have been shown to serve as corridors for forest carabids to penetrate as much as 12.5 miles into nonforested agricultural landscapes greatly enhancing their ability to help control pest species.20
Studies in Michigan have shown significantly higher ichneumonid wasp parasitism of European corn borer larvae near wooded edges than near non-wooded edges or field interiors. Microclimatic effects involving temperature, wind flow and shading, coupled with the spatial arrangement and proximity of adult food -- flowering plants in noncrop areas -- are believed to contribute to the higher predation.21 These and subsequent data strongly suggest that local landscape structure and its management can play an important role in pest regulation. However, careful design of species rich crop systems with field-margin vegetation or within field-plant diversity is required.
Great concern exists over the quality of our air, water and soil resources. Water and air quality and control of soil erosion have been significantly improved as a result of the application of agroforestry practices.
Agriculture-derived contaminants such as sediment, nutrients, and pesticides, constitute the largest diffuse source of water quality degradation in the United States. Surface runoff and subsurface flow from sites of farm animal waste application and crop and pasture fertilization can cause significant nutrient loading to down slope water sources unless appropriate management techniques are applied. Excessive pollutants have deleterious ecological impacts on the receiving waters of streams and lakes. Catchment strategies are advocated that use best-management practices and techniques that incorporate natural physical and biological processes to reduce, convert or store pollutants on the land before they enter aquatic systems. Bioassimilative strategies have been adopted for mitigation of the negative impacts of agricultural practices. These include the use of riparian VBS which are accepted management practices under the Conservation Reserve Program (CRP).
Riparian VBS have been adopted as a viable tool for stream and river restoration and management. Research has demonstrated that inclusion of trees and shrubs in VBS within agricultural systems can improve water quality.22 Nutrient uptake and removal by the soil and vegetation in a wooded riparian ecosystem has been shown to prevent agricultural upland outputs from reaching stream channels. The mitigating benefits of VBS are maximized if they are established in smaller headwater streams whose lengths dominate drainage networks.23
In small to mid-size streams, narrow bands of woody vegetation parallel to the stream can stabilize stream banks, moderate temperatures, reduce sediment input and provide essential organic matter sources to the lotic community. Partially as s response to the heavy flooding along the upper Mississippi River in 1993, the use of riparian VBS for stream bioremediation is expected to increase.
Woody VBS function as bioassimilative transformers, changing the chemical composition of compounds. Under oxygenated soil conditions, resident bacteria and fungi mineralize runoff derived nitrogen which is then available for uptake by soil bacteria and plants. Field-applied nutrients moving to streams and ground water are reduced due to absorption by roots within the tree/shrub/grass strip. Greater infiltration of nutrient transporting water occurs within the strip than in cultivated soil. Planting or retaining riparian VBS are effective and economically feasible procedures for reducing nitrogen and phosphorus inputs. Processes involved include retention of sediment-bound nutrients in surface runoff, uptake of soluble nutrients by vegetation and microbes, and absorption of soluble nutrients by organic and inorganic soil particles. Forested VBS 90 to 150 feet in width have been shown to reduce nitrogen in groundwater by 68 to 100% and in surface runoff by 78 to 98%. Forested VBS 50 to 150 feet wide reduce phosphorus concentrations in surface waters by 50 to 85%.23
Mature riparian forests have been shown to be excellent nutrient sinks and buffers, yet little research has been conducted on the short-term effectiveness of newly established riparian forests. However, densely planted poplar were recently established in Iowa to provide a riparian buffer for conventional row-crop agricultural land. In certain instances, the re-established riparian buffer decreased nitrate concentrations in the shallow soil profile from 25 to 5 parts per million during the first year of eatablishment.24
Biomass removal/saturation in woody VBS
Both woody and grass VBS require periodic maintenance to sustain optimal performance. Evidence points to leakage of phosphorus in woody and grass VBS due to nutrient saturation. Saturation problems with VBS can be moderated by continual harvest and careful management of the wood and grass component. Harvest of vegetation causes reversion to a state of active growth and biomass accumulation recreating a condition of high site demands for water and nutrients. Maintenance of an intermediate successional state of active growth/biomass accumulation helps maintain the VBS and minimizes loss of nutrients.25
Sediment reduction and stress bank protection
Sediment loading and deposition also contribute to water-quality problems. Both experimental and field studies indicate that narrow VBS -- 30 to 90 feet -- that augment shallow sheet flows can protect streams from excessive sediment loading and reduce sediment input to surface water. Reductions of sediment yields by woody VBS are achieved via the following mechanisms: 1) deep root systems which resist scouring under swift current conditions; 2) dense, well-ramified top growth for storage of nutrients and pesticides; 3) resistance to flooding and drought; and 4) ability for rapid recovery following heavy sediment loading. Wooded VBS have been shown to be more effective in stabilizing stream banks than grass VBS due to the deep, perennial root systems.26
Wooded VBS between 30 and 90 feet in width have been shown to reduce solar radiation inputs and effectively moderate stream water temperatures.27 The principal advantage of shading on water quality is found in the improved habitat for many aquatic insects via reduced stream water temperature, higher dissolved oxygen and reduced denitrification rates. Partial shade is preferred when attempting to maximize both the overall ecological health of the riparian environment and control specific submerged and emergent aquatic plants.
Little information exists related to the time required or the type of woody VBS needed to restore farm streams to their former undegraded condition. However, the requirement may be less than previously thought. In 1985, a woody VBS was established in southern ontario. A 1.2-mile section of a small degraded stream was planted with 6,500 hybrid poplar, silver maple, alder, black walnut, green ash and red oak trees. After only 4 years, the tree plantings averaged 8 dried tons of biomass/acre and within the poplar plantings, radiation loadings to the middle of the stream were reduced by almost 40%.28
Woody VBS act as a source of nutrients and energy for streams in the form of dissolved carbon and organic detritus. These materials help restore and maintain the aquatic food chain providing nutrients to benthic detritivores. As they are consumed by larger benthic fauna and ultimately by fish they help transfer energy up the food chain. In the southern ontario study", fish sampling documented the presence of endangered red-sided dace in 1987 and brook trout by 1990. Although the headwaters of the creek are spring fed and have a resident population of brook trout, they had not been found at this location prior to streamside tree planting.
Herbicide and pesticide reduction
Herbicides and pesticides are reduced due to decomposition resulting from organic matter in woody and grass VBS. Furthermore, the extensive network of roots within the strip serves as sites for capture and bioremediation of chemicals before the pollutants move offsite. Within the soil and litter layers of woody VBS, many toxic chemicals, including pesticides, are transformed to nontoxic forms by microbial decomposition, oxidation, reduction, and solar radiation.
Global warming resulting from the buildup of trace gasses in the atmosphere is a potential problem of major proportions for the 21st century. Projections for increases in global surface temperatures range from 5° to 15°C. A major contributing factor is the dramatic increase in ambient carbon dioxide levels. Long-term studies clearly demonstrate increases which approach 1 part per million per year. Biomass-derived carbon dioxide emissions, especially from fossil-fuel burning, account for a significant percentage of the increases I observed. Trees and other green plants require large quantities of this carbon dioxide to satisfy their photosynthetic requirements and thus serve as a carbon sink.
Although oceans store a far greater quantity of carbon than terrestrial ecosystems, at present our ability to manage terrestrial ecosystems is greater. Tree planting in conventional forestry and agroforestry programs presents many opportunities to sequester atmospheric carbon dioxide and mitigate the impacts of global warming. Young, fast-growing trees take up carbon at high rates. If the end uses of forest products are durable goods such as furniture, carbon is stored in these materials for decades or centuries beyond the life of the trees. Alternatively, woody biomass burned for energy as a substitute for fossil fuels poses an effective mechanism to reduce the depletion of nonrenewable fossil fuel-derived carbon at no net increase in carbon dioxide emissions.
Trees, as long-lived perennials, are an effective storage system for sequestered carbon and carbon dioxide.29 Studies have demonstrated that an "average tree" sequesters about 13 pounds of carbon and carbon dioxide per year. If in agroforestry, trees were planted using a 10 by 40-foot tree spacing, the spacing often recommended for alley cropping, these 108 trees per acre could potentially tie up 1381 pounds of C02 per acre per year or, assuming 60 years as the average life of these trees, 82,866 pounds per acre of carbon and carbon dioxide during the rotation. If this were extrapolated just to the millions of acres in the Conservation Reserve Program (CRP) a substantial step could be made towards mitigating the carbon dioxide problem.
A program designed to meet minimum shelterbelt planting needs for crop protection and reduced wind erosion in the United States has been estimated to result in the establishment of 1.9 million miles of woody corridors containing 1.3 billion trees on 5 million acres (Table 8). Simultaneously, these woody corridors would sequester 89.8 million tons of carbon dioxide. A more aggressive program of woody corridor establishment, designed with the goal of protecting 40 percent of U.S. cropland would require planting an additional 1 billion trees or shrubs. These additional woody plants would occupy 3.4 million acres and would sequester an additional 86.8 million tons of carbon dioxide. The cumulative benefit of such an aggressive planting program to establish 2.3 billion trees would result in sequestration of 176.6 million tons of carbon dioxide.
ECONOMIC BENEFITS
Production Economic Benefits
Agroforestry adds a significant element of diversity to an existing or planned agricultural system. The added diversity associated with such an endeavor is, in general, financially advantageous. It provides a farm operator the opportunity to develop a portfolio of short- and long-term investments thus allowing for some risk spreading through diversification. In addition, for some farmers with land particularly unsuitable for crop production, agroforestry provides an avenue for that land to be removed from crop production over an extended period of time as the trees mature. Likewise, agroforestry produces economically valuable social benefits, some directly measurable, others not, by ensuring resource conservation.
Agroforestry, from a production economics standpoint, is representative of the multiple output model where several fixed and variable inputs are combined to produce at least two products. Implicit in this model is that quantities produced of all outputs can be varied by deliberate management decisions. Thus, one of the objectives in agroforestry is to achieve production efficiency -- optimization of the combination of inputs and outputs in attaining a stated production objective over a given time period. Simply expressed, the typical adoption decision is to add trees to the land as long as the increase in overall benefits from each additional tree is greater than the overall benefits foregone from the crop which the tree is replacing. The optimal point of output combination of trees and crops is where additional trees (or crop) when substituted for the other product leads to a decline in the overall benefit through time.
Various agroforestry systems have been evaluated on a financial basis for both production and protection. A number of these systems are described and their economic aspects discussed.
In the Midwest, the most common tree/crop system studied has been alley cropping. Due to growing conditions and markets, openly spaced systems containing high-value hardwoods typically have been examined. The open condition has led to concern regarding the quality of agroforestry-grown wood. However, early indications suggest no adverse effects on wood quality. To the contrary, parameters such as height, diameter and specific gravity benefit. Moreover, based upon early growth rates, a 50 to 60-year veneer log rotation is projected for trees grown under alley cropping instead of the customary 80 to 100 years.30
Numerous economic assessments have been conducted on agroforestry management regimes incorporating agricultural crops with trees. Alley crops shown to be viable from both economic and biological perspectives include soybeans, corn, milo, wheat, cool-season forages, sod species such as bluegrass, energy plantings such as sycamore, poplar and silver maple, and livestock. Typical internal rates of return range from 4 to 11%.31 In general, returns tend to increase with management complexity and site quality. However, obvious factors such as market value of the crops grown, cash-flow relationships and even risk-taking influence profitability. Tree species which yield especially high-value wood products, along with fruit or other marketable products, have obvious advantages over low-value species which produce only wood products. Enterprises such as livestock grazing have shown great potential but also carry a degree of risk due to the increased chance of tree damage. Further, an increased investment-must be incurred early in the rotation for fence construction and maintenance if grazing is to begin immediately following tree establishment.
Findings of special interest have been reported from Illinois where interplantings of black walnut with various common crops were accomplished using conventional tillage, minimum tillage and no-till. The primary purpose of the work was to ascertain the economic viability of agroforestry systems for marginal quality soils. Black walnut in an agroforestry management regime was found to produce land expectation values comparable to those of traditional agricultural systems, particularly at the lowest discount rate of 4%.32 Another option on marginal quality soils is to plant nitrogen-fixing nurse crops, such as black alder or autumn olive, with commercially valuable tree species to reduce managerial inputs and enhance growth. Reports suggest that valuable tree species interplanted with nitrogen-fixing trees offer an economically viable alternative to conventional row crops on marginal lands.33
Several agroforestry management systems involving integration of natural stands of timber with grazing have been evaluated in terms of their physical inputs and outputs. However, little information has been published on the, economic costs and benefits. In the southern coastal plain, research on grazing longleaf-slash pine forests over a 40-year rotation has shown average annual net returns of $7.64 per acre for the grazing operation and $87.56 per acre for the timber.34 In studies of domestic forages established under 20-year old loblolly pine and evaluated over a 5-year period, two out of five alternatives considered yielded greater internal rates of return than the open-grown forage alternative.35
To evaluate a system incorporating deferred grazing to enable tree establishment, a system of haying for the first three years followed by grazing for the next three was evaluated under slash pine.36 Wide spacing permitted harvesting hay for forage while waiting for the trees to become large enough to permit grazing. This system was found to provide early, regular returns for the landowner and eliminate the need for internal fencing. Yet another potentially valuable product coming from pine stands is pine straw needles) used as a mulch in landscaping. Recent developments in the pine straw industry permit landowners to supplement their income through agroforestry systems designed to include pine straw harvesting every few years. With prices in excess of $100 per acre being paid, combinations of wood, forage, livestock and pine straw can yield attractive financial gains. Furthermore, financial analyses of the incremental gains in pine straw production associated with investments in nitrogen and phosphorus fertilization have demonstrated that intensive management of stands is financially profitable.37
Protection, as discussed earlier, is one function of an agroforestry system. Economically, the loss of soil and chemicals from the land are problems that impact society as off-site "externalities." That is, their costs are not incorporated into the production decision framework by the individual on whose farm they originate.
Adoption of practices that will result in a measure of protection benefiting society are borne by the landowner, unless society is willing to share the cost and responsibility through some form of assistance. An example is the Conservation Reserve Program (CRP). An evaluation of benefits emanating from the CRP suggest that some $6 to $14 billion in present value could result if its full enrollment goal of 45 million acres were achieved.38 39 A scenario in which 10$ of the acreage enrolled by 1990 was in trees resulted in higher water quality benefits than a baseline comparison. Further, policies to reduce soil loss were found to improve wildlife habitat as well as increase water quality through reduction of sediment delivery to streams.
Adoption of agricultural conservation practices for the benefit of society at large by a farmer who must absorb a large portion of the initial coat is not likely to occur often, even if over the long term the farm operator realizes a net benefit. Economically rational farm operators are more likely to adopt conservation practices which benefit them directly and in the more immediate future. A policy interpretation of the results of a cross-sectional, time series study of farmers' investments in land improvements found that farmers invest in land improvements in response to expected increases in income. Farmers have an added incentive to invest in conservation practices if farmland prices increase relative to the cost of improvements. Furthermore, government programs that idle land during the growing period provide an incentive for added investment.40
Some farm operators, however, for non-economic reasons, adopt conservation practices that do not yield immediate, short-term benefits. Investment in conservation measures other than conservation tillage has been found in certain instances to be positively related to perception of an erosion problem, farm size, income and existence of a conservation plan. Furthermore, some studies have found that a sense of stewardship relative to protection of natural resources has a positive relationship to farmers' use of conservation practices.
On-farm soil erosion which generally proceeds at an imperceptible rate can exact a gradual but heavy toll on a farm business. Perhaps the most noticeable effect of erosion, from the farmer's standpoint, might be in the cost of fertilizers. It has been reported that a loss of five tons of soil per acre per year, within the tolerance limits of many soils, requires an annual addition of $24 per acre of commercial fertilizers to maintain production levels.41
Moreover, as a result of erosion, the lives of reservoirs are shortened as they fill with sediment. Estimates of the cost of cropland sediment in reservoirs run as high as $200 million annually.42 The largest costs occur in the northern plains and corn belt regions where water storage capacity is least. In these two regions, nearly two-thirds of the sediment entering reservoirs comes from cropland. Communities that rely on reservoirs for residential water supplies must remove the sediment in the process of purifying their water. In Ohio alone, studies show that a 25% reduction in soil erosion would result in a savings of $2.7 million in water treatment costs each year.43 Such findings imply even greater savings for areas experiencing excessively high erosion rates.
Financial aspects of using agroforestry rather than mechanical means to control erosion have been evaluated in Missouri. Three "representative farms" of different sizes containing a composite mix of the most common soils in the region were developed and several agroforestry systems along with terracing and grass strip cropping were simulated for each farm size to reduce soil erosion from typical crop rotations on the highly erodible soils to tolerance (T) levels. Tree species planted included black walnut for timber and nuts, Scotch pine for Christmas trees, and American sycamore for industrial fuelwood. Agroforestry enterprises were found to be more profitable than using conventional erosion control measures.44
The economic benefits of shelterbelts are quite variable and location specific due to the indirect nature of their contribution. While numerous studies have been published describing the specific gains in increased crop yields, few financial evaluations of the costs associated with generating the gains are available. A detailed examination of the financial aspects of several field windbreak systems over a range of crops, yields, prices, discount rates, and windbreak establishment costs has been conducted for eastern Nebraska. Three systems utilizing windbreaks at spacings of 635, 420, and 218 feet were evaluated in fields planted to either soybeans, winter wheat or corn. All three designs were found to yield positive net benefits over an estimated 50 year life span. The lowest return was from the widest spacing, although a positive present net worth at a 11% discount rate was still realized. Positive were yielded by the other designs at discount rates as high as 17%.45
Three general benefits of an economic nature are typically ascribed to agroforestry. They are spreading (sharing) of fixed costs because of the joint-production relationship; reduction of the initial time period required for income from land devoted exclusively to tree production; and diversification of income sources, in effect spreading the risk generally associated with a monoculture. Likewise, from an operational standpoint, agroforestry to some extent increases flexibility in agricultural operations since many silvicultural practices may be delayed with little or no detrimental effect until free time is available. Of additional importance, however, is that agroforestry offers a means by which farmers can phase out some or all of their row crop production over time, at a rate approximating the rate of depreciation of their farm equipment.
Agriculture, forestry and rural development policies at federal, state and local levels have important effects on the development of agroforestry and realization of its benefits. Numerous policies within these broad arenas and limited provisions that are aimed directly at agroforestry will influence the allocation of resources to this activity by landowners, public institutions and non-profit membership groups. While it is beyond the scope of this effort to analyze or even identify the full range of policies that may affect agroforestry in its broadest sense, the following discussion highlights programs and traditions in key institutions that appear likely to influence agroforestry development most directly.
The Food, Agriculture, Conservation and Trade Act (FACTA) of 1990 contains two
explicit provisions for agroforestry.
Conservation Reserve Program (CRP)
The Conservation Reserve Program as established by the Food Security Act of 1985 offers agricultural producers long-term rental agreements, acreage payments and cost-share assistance to establish permanent vegetative cover on selected cropland. The Conservation Program Improvements Act (Title XIV of Food, Agriculture, Conservation, and Trade Act of 1990 - FACTA) amends the 1985 act to allow cost sharing for alley cropping of agricultural commodities when such land is planted to hardwood trees, provided that operators offer to reduce annual rental payments by 50% in exchange for permission to produce agricultural commodities. It also provides for windbreaks and shelterbelts without requiring enrollment of the entire field and allows the operator to specify, within limits, the terms of such contracts for these features.
Center for Semi-arid Agroforestry (CSA)
The Forest Stewardship Act of 1990 (FACTA - Title XII - State and Private Forestry, Ch. 2, Sec. 1243) provides for the establishment of a Semiarid Agroforestry Research, Development and Demonstration Center (Center for Semi arid Agroforestry). Through participation by federal or state governmental entities, land grant colleges and universities, state agricultural experiment stations, state and private foresters, the National Arbor Day Foundation and other non-profit foundations, the Center is charged with conducting research to develop sustainable agroforestry systems on semiarid lands in 17 western states. Such systems are intended to minimize topsoil loss and water contamination and stabilize or enhance crop productivity.
A broad set of specific objectives and activities is identified for this specialized research including biological, sociological, demographic and economic studies aimed at increasing the use of improved forestry conservation and agroforestry practices. In addition to in-house and cooperative research, the Center is charged with transferring agroforestry technology to state forestry agencies, the U.S. Department of Agriculture's Soil Conservation Service (SCS), soil conservation districts and other state and federal agencies and organizations. It also is to conduct cooperative demonstrations of agroforestry practices and integrated conservation systems under different climatic and soil conditions that will provide opportunities to introduce new plant materials from tree improvement programs and SCS plant materials centers. Finally, it is to conduct information and education programs in concert with the National Arbor Day Foundation.
Operationally, the CSA encourages the development of state agroforestry programs which would identify goals and priorities for that state and provides some resources for attaining them. The Center would complement state activities by conducting or funding some research, cost-sharing needed demonstrations and providing technology transfer and education to meet local needs. Likewise, CSA activities are intended to complement Soil Conservation Service tree-planting programs by enhancing the traditional U.S. Department of Agriculture's Forest Service role of conducting research on trees for agricultural lands. CSA aims also to enhance the Forest Stewardship Program by increasing understanding and acceptance of the benefits of tree planting on agricultural lands.
Funds for the Center, however, have not been appropriated. Therefore the Center's core operating expenses, which in 1992 amounted to $350,000, have been drawn from allocations to pre-existing Forest Service programs within the research, state and private, and international branches. Project funding is sought from outside the Center on a competitive basis as opportunities are identified.
Given the Center's extremely limited funding base, the Forest Service has directed the Center to limit its research and development focus for the present time to the 10 Great Plains states, but to remain an advocate for agroforestry in the westernmost semi-arid states as well. Cooperative projects in which the Center is presently engaged focus on: 1) developing more stress and pest-resistant multi-purpose trees for agroforestry systems; 2)modeling agroforestry systems in the context of global change; 3) developing riparian buffer systems; 4) identifying interactions between agricultural crops, insects and birds in agroforestry systems -- in the context of a neo tropical migratory bird study; and 5) developing cooperative agroforestry research and development with scientists in northern Mexico.
South Central Family Farm Research Center
The U.S. Department of Agriculture's, Agricultural Research Service (ARS) initiated an agroforestry systems research program at the South Central Family Farm Research Center in Booneville, Arkansas in 1992. Through the reallocation of in-house ARS funds, an initial funding level of $240,000 per year was established. The program's aim is to devise and assess agroforestry systems that are compatible with combined livestock, tree, pasture and wildlife production, and evaluate the biological, ecological, environmental, and socio-economic interrelationships among the combined multiple resources.
Related Federal Technical Assistance and Cost-Share Programs
Stewardship Incentive Program (SIP)
FACTA's Title XII (State and Private Forestry) also authorizes the cooperative Stewardship Incentive Program to stimulate enhanced management of nonindustrial private forest lands through cost-sharing of approved practices. The program is authorized to spend up to $100 million annually through 1995. For fiscal year 1991 the appropriation was $19.9 million. The 1992 fiscal year appropriation was $19.7 million.
SIP provides cost-share assistance if non-industrial private landowners have developed an approved forest stewardship plan. Approved activities and practices eligible for cost-share assistance in each state include establishment, management, maintenance and restoration of forests for shelterbelts, windbreaks, aesthetic quality and other conservation purposes as well as a range of productive purposes.
Forest Service and state foresters have leadership responsibilities for SIP at the national and state level, respectively. Each state forester in consultation with the State Forest Stewardship Committee will determine the cost-share levels, practice priorities and minimum acreage requirements. The U.S. Department of Agriculture's, Agricultural Stabilization and Conservation Service (ASCS) provides administrative assistance by accepting applications and arranging for disbursed payments. Technical responsibilities for SIP practices may be assigned to other agencies and resource professionals through memoranda and cooperative agreements with SCS and the U.S. Department of Agriculture's, Cooperative Extension Service serving as key partners in developing and implementing SIP.
Forestry Incentives Program (FIP)
The Forestry Incentives Program was authorized by Congress in 1973 under (Title X) the Agriculture and Consumer Protection Act and in 1974 was implemented through the U.S. Department of Agriculture's, Agricultural Conservation Program (ACP). In 1975, FIP became a separately funded program. A total of 3 million acres have been planted on nonindustrial private lands under FIP through 1992. FIP is distinguished in that its primary objective is to increase national timber supplies through authorization of cost-share payments for reforestation and timber stand improvement, firebreak construction and site preparation for natural regeneration.
The ASCS has responsibility for program administration while the Forest Service takes the leadership role for technical assistance. State service foresters approve landowners forest management plans and their performance of practices before payments are made. The county ASCS committee has the power of final approval of applicants to receive funding. FIP could serve as an excellent model for a national agroforestry incentives program.
Agricultural Conservation Program (ACP)
The long-standing ACP, among other activities, provides cost-share assistance to agricultural producers for tree planting to meet conservation objectives. The ASCS subsidizes tree cost to farmers and makes them available through nurseries. The program is consistent with the goals of agroforestry and marks a comparatively long tradition of ASCS and SCS involvement in promoting tree based conservation practices. Compared with other conservation measures that the ASCS has encouraged under the ACP, however, trees have played a minor role.
Environmental Easement Program (EEP)
Chapter 3 of the Agricultural Resources Conservation Program (Subtitle C of FACTA) authorizes an Environmental Easement Program. This program provides for public acquisition of permanent or long-term easements from willing owners of eligible farms or ranches to ensure the continued conservation and improvement of soil and water resources. In return for the creation of an easement on qualifying lands, landowners are prohibited from harvesting or selling Christmas trees or nuts, though allowed to undertake customary forestry practices such as thinning, pruning or tree-stand improvement. The terms of such easements also limit the production of any agricultural commodities except as they will benefit wildlife, and prohibit harvesting, grazing or other commercial use of forage.
These EEP provisions not only fail to take advantage of the potential for agroforestry to help accomplish the program's conservation objectives, but uniformly limit the owner's range of options for implementing the requisite natural resources conservation management plan. By summarily excluding practices that incorporate the foregoing components, opportunities are lost for the landowner to generate income which in turn could help reduce federal expenditures on conservation. A re-examination of the justification and the practical basis for the exclusion of agroforestry-type practices appears warranted.
Tree Planting Initiative
The EEP provides also for a Tree Planting Initiative to encourage reforestation of marginal agricultural lands, promote tree planting to reduce soil erosion, improve water quality and provide for the sustained production of the commodity and non-commodity resources that these lands can provide to meet the nation's needs. Under the Initiative, the Secretary of Agriculture is encouraged to use the following programs to accomplish this policy: Conservation Reserve (1985), Agriculture Conservation (1970), and Cooperative Forestry Assistance (1978) and provisions of State and Private Forestry (Title XII, FACTA).
Indeed, a principal intent of the initiative is to foster collaboration among agencies and programs concerned with the protective and productive roles of trees in agricultural areas. It thus could have a direct and beneficial role in assisting the development of agroforestry practices provided that technical assistance providers from the respective agencies are adequately aware of the potential roles and benefits of agroforestry in addressing the initiative's objectives.
Sustainable Agriculture Research
Title XVI of the FACTA provides for sustainable agriculture research, where sustainable agriculture is defined as "... integrated systems of plant and animal production practices having a site-specific application that will, over the long-term -- satisfy human food and fiber needs, enhance environmental quality and the national resource base upon which the agricultural economy depends, make efficient use of non-renewable resources and on-farm resources, integrate natural biological cycles and controls, sustain the economic viability of farm operations and enhance the quality of life for farmers and society as a whole ..." This mission speaks directly to potential roles for agroforestry. Known presently as Sustainable Agricultural Research and Extension (SARE), the program to date has funded few research projects that focus on either agroforestry or on farming systems that include active tree components. Greater recognition of agroforestry by this program could have an important effect. The conventional one-to-three year time frame for the completion of studies, however, constrains the investigation of many key agroforestry topics and questions.
Government Price Supports and Taxes
U.S. agricultural policy rests on a complex system of price supports and taxes which, among other objectives, aim to influence production levels and consumer prices for respective commodities. Income tax provisions and price supports are highly influential in the use and management of agricultural and private forest lands. In addition to stabilizing markets, an increasingly important use of agricultural policy instruments during the past decade has been to limit the production of certain commodities in order to reduce "problems" of overall surplus. Since 1990, loans, payments and acreage reduction programs have placed additional emphasis on conserving soil sad water resources for lands withdrawn from production.
However, the surplus production of high-value timber in the United States has not occurred and all indications are that the value of wood and wood products will continue to grow appreciably. A policy focus on growing high quality timber under field or pasture conditions in agroforestry systems provides an opportunity for government to overcome problems of commodity surplus without having to subsidize growers to produce less. It will be critical to the promotion and development of agroforestry that markets for key tree crops produced in agroforestry systems, and nuts in particular, are encouraged to flourish despite projected increases in levels of production.
Federal tax policy with respect to forestland has aimed principally to limit the financial burden of management on the private landowner. And, the policy, through capital gains provisions of the income tax, helps to syncopate the incidence of taxation with the realization of profit from timber sales. Certain state tax laws provide further incentives to landowners to undertake forest management. This is achieved not only through the state income tax, but also by allowing the deferment of property taxes on lands under active forest management until a significant volume of timber is harvested or until the property is sold or transferred.
Policy instruments to encourage agricultural conservation include federal income tax provisions that allow farmers' soil and water conservation costs to be expensed rather than capitalized as long as the measures have been approved by the SCS or comparable state agency.46 This would include tree planting costs. In addition, payments farmers receive for certain conservation and environmental protection programs may be excluded from income. This includes payments for purchase and installation of capital improvements and the implementation of best-management practices, except for payments that are expenses on Schedule F (the federal income tax form for profits and loss from farming).
In addition to these various income tax exemptions, deferments are allowed for farmland owners who enter into permanent easement agreements for various conservation purposes. Such allowances are based on the decline in the value of such properties to the owner.
It is evident that there is adequate precedent in the use of policy instruments that could encourage landowners to invest in agroforestry. on the other hand, the effect of more widespread application of agroforestry as a conservation device on public subsidy and tax programs has not been evaluated. It could be hypothesized for example that, over time, a combination of public education about the benefits of agroforestry and minor adjustments in income and property tax policy would reduce the need for direct public subsidies for production reduction and natural resources conservation.
Agroforestry Research, Extension and Education at Land-Grant Universities
There are no provisions in the enabling legislation for land-grant universities or for Cooperative Extension Service that would prohibit or limit agroforestry research, extension or education. Indeed agroforestry programs are developing at a number of land-grant institutions and interest in agroforestry at universities and colleges throughout the nation is increasing. Generally, such programs have emerged from departments of forestry or natural resources. Successful programs, however, are highly interdisciplinary and involve faculty, staff and students from a range of departments and state agricultural experiment stations. Furthermore, they depend on successful integration of research and extension early in the research process due to the critical role of landowner input and feedback in the comparatively complex and long-term studies required.
Among the most longstanding and rigorous agroforestry research, extension and education programs are those at the Universities of Missouri and Florida, State Universities in Michigan, Washington, Colorado and Iowa, and Cornell University in New York. The emergence of agroforestry as a domain of knowledge generation at these institutions has depended however on the vision, energy and determination of a few key individuals. These individuals have directed scarce research and education resources to agroforestry despite the absence in most cases of funding dedicated specifically to these activities. Further growth of these programs is limited directly by lack of recognition by policy decision-makers and the associated lack of funds.
For faculty who are not tenured, significant involvement in agroforestry research and education can pose risks to their institutional security due to the still marginal statue of agroforestry as a scholarly or practical pursuit. Due to the complex and long-term nature of much agroforestry research, on-farm methodologies are often the most practical and sometimes the only feasible means of answering many of the relevant questions. Certain constituent disciplines, however, may question the robustness of such research designs. Thus, young faculty members who may be enthusiastic about agroforestry are also concerned about jeopardizing their credentials and may fail to pursue research or teaching in this field.
Agroforestry's continued marginal status with respect to funding and program development at leading land-grant and other academic institutions throughout the country stems partly from insufficient recognition of agroforestry as a domain of professional practice. Until public agency positions for "agroforesters" are identified, it may not appear justifiable to academic administrators to invest in the education and training which in turn could support extended research programs. In the meantime, there is a common perception among the comparatively few professionals who do identify with agroforestry that resource managers are not receiving the training needed to provide adequate technical assistance in integrated resource management.
State Agroforestry and Related Programs
Missouri, in 1990, was the first state to adopt specific agroforestry legislation. Designed to complement the extended CRP, Missouri's agroforestry legislation is limited to alley cropping defined as the use of trees planted or otherwise established on land with grass strips or row crops or both between the lanes. Based on the demonstrated effectiveness of this technology in controlling soil loss from highly erodible land in Missouri, state policy makers sought to encourage more landowners to enroll greater acreages in alley cropping than would have come about through federal incentives alone. The state initiative compensates farmers for the difference between payments they would have received by adopting a non-agroforestry CRP practice and the amount allowable -- 50% reduction in rental payments -- for alley cropping
The legislation makes no explicit provisions for agroforestry research or extension. The University of Missouri, however, has perhaps the best developed agroforestry research and extension program in the United States. Indeed the state's innovative agroforestry legislation and landowner incentive program can be attributed in large part to the strength of this program.
Since 1990, South Dakota and Iowa have followed Missouri's initiative in providing added incentives for CRP enrollees to adopt agroforestry practices. Illinois' Forestry Development Act authorizes cost sharing for tree planting which is not tied to CRP, and presently has legislation pending which would further subsidize CRP tree planting.
Association for Temperate Agroforestry (AFTA)
As a result of the diverse interest in agroforestry, academic, agency professionals and landowners joined together in 1991 to form the Association for Temperate Agroforestry. This growing society provides an umbrella for temperate agroforestry activities via networking and information-sharing services to its members. AFTA was formed as an outgrowth of biennial conferences on temperate zone agroforestry that have been held since 1989. AFTA is now their primary sponsor. The conferences have been instrumental in the intellectual and the practical development of agroforestry. In recognition of the multidisciplinary and interagency perspectives that effective agroforestry development requires, from the beginning these meetings have included landowners and agency professionals as well as academic representatives.
Non-profit membership groups
There is a vast though often unarticulated network of membership groups who are sympathetic to agroforestry and sometimes actively engaged in its development. Associations of forest owners, Christmas tree, nut or fruit producers, shiitake mushroom or ginseng growers, bee keepers, fallow deer and other alternative livestock farmers, organic growers, permaculture designers and various conservation and environmental organizations have members who consciously practice agroforestry and many more who manage land and resources by means that are consistent with agroforestry principles.47 Presently, there is no consistent public policy that unites these groups in purpose or provides incentives for them to promote or support agroforestry development.
Given the ubiquity of such groups throughout the country, the often innovative character of their membership, the important educational role they play on behalf of their members and the public, and the spirit of volunteerism that infects moat such groups, public policy that would direct and support their activities in agroforestry is likely to have early and far-reaching payoffs. Well-designed, small-grant programs directed at such groups, perhaps incorporating some linkage with land-grant managed research and extension, are likely to generate the type of practical knowledge and information needed to significantly enhance agroforestry innovation and adoption nation-wide. Small grants may also be used beneficially to enhance the synergy among respective membership groups whose dominant agendas presently do not warrant their direct interaction. Few if any incentives are available for these groups to help develop and promote agroforestry. Providing such incentives on a competitive basis would likely have a significant impact at comparatively little public cost.
Summary of Policy Relating to Landowner Incentives
Programs that provide cost-sharing incentives have been highly effective with respect to stimulating landowner interest and enrollment. The CRP program has become competitive and is fully subscribed. Furthermore, many of the practices encouraged by the program, particularly those with tree-planting components, are now viewed to have cumulative environmental and economic benefits well beyond those anticipated by program planners. Thus, there is considerable speculation and planning at present concerning the future of CRP lands and the potential for using agroforestry as a mechanism that is attractive to land owners for continuing CRP-type practices after present contracts have expired.
The Conservation Reserve Program, however, excludes many joint crop/animal and tree production options and remains narrowly focused on alley cropping, windbreaks and shelterbelts. CRP could be more attractive to landowners if certain barriers to joint production were relaxed, especially those involving grazing, and if the minimum number of trees per acre was reduced to better accommodate agroforestry practices or fully stocked tree plantations. The Stewardship Incentive Program, unlike CRP, has given no explicit consideration to agroforestry practices. However, like the CRP, in many states SIP enrollment is fully subscribed. The program however is young and its direct benefits remain to be evaluated.
There is a lack of general knowledge of the implications of state and federal taxes and price support policies for land use in general, and agroforestry in particular. While such phenomena may be understood by agricultural economists I and related professionals who study them, there is a need to make these linkages more transparent so that technical assistance providers and landowners can factor them into long-term management strategies more effectively. Likewise, it is important that the role of subsidies and taxes in landowner decision-making about agroforestry be systematically evaluated, and that the role of agroforestry in reducing the need for public subsidies for conservation be examined.
Furthermore, it is extremely important that obvious disincentives to the practice of agroforestry be re-evaluated in light of what is known about its benefits to conservation. In particular, the Environmental Easement Program should be considered not only with respect to the prohibition of certain agroforestry-type land uses, but also in terms of public subsidy reductions that may be possible by incorporating agroforestry into this program.
The need and potential for agroforestry land use systems throughout the United States is becoming increasingly well documented through scholarly research as well as agency mandates and program evaluations. However, a host of real and perceived problems relative to education/information, research and policy must be addressed and resolved before the full potential benefits of agroforestry can be realized. These problems are:
Education/information
1) a high level of uncertainty and accompanying risk associated with agroforestry as an unproven land use system;
2) the perception of high establishment costs and level of management expertise required;
3) a perception that the economic gains from agroforestry are lower than from more traditional production agricultural practices;
4) an unwillingness to adopt systems with rotation lengths which may exceed the landowner's life expectancy;
5) a perception by many agencies that landowners would be unwilling to follow management prescriptions necessary for developing and maintaining economically and ecologically profitable agroforestry systems;
Research
6) a weak basic and applied research base to support agroforestry management decisions;
7) lack of a strong research, teaching and extension infrastructure committed to agroforestry;
8) minimal opportunities for graduate study which emphasizes domestic agroforestry and minimal employment opportunities for trained agroforesters;
9) an inability to recruit young scientists due to the dearth of funding and a perception that applied research is not sanctioned or rewarded in academia;
Policy
10) exclusion of agroforestry by current public cost-share programs due to minimum acceptable tree planting densities, and the lack of federal policy relating specifically to agroforestry; and
11) a resistance from organized commodity groups who feel subsidized producers entering the market through federally funded programs, such as CRP, represent unfair competition.
ACHIEVING AGROFORESTRY'S POTENTIAL
In view of the problems facing today's landowners and our nation, and the emphasis being placed on developing sustainable agricultural and natural resource systems, agroforestry can have lasting environmental and social impacts. To realize this potential will require a concerted effort on the part of many, including established agricultural and natural resource institutions ouch as land-grant universities and USDA agencies. Through a unified effort to enhance our knowledge of agroforestry and to extend knowledge already available, many of the current constraints to the adoption of agroforestry would disappear. The first priority should be to provide national leadership in the form of an agroforestry coordinator. The designee, a USDA employee, would coordinate agroforestry activities in research, education, extension and policy while networking agencies and institutions working within this field.
Agroforestry systems and techniques are beat developed in an interdisciplinary I context through regular and interactive participation by scientific researchers, technical specialists from respective fields and agencies, and landowners. Thus, programs are needed that explicitly combine conventional research and extension functions in a holistic manner and that actively involve landowners in establishing and implementing research agendas.
The second priority is to secure funding for agroforestry. A major limitation to agroforestry research, teaching and extension is the lack of funding available due to disciplinary boundaries. Agroforestry is a hybrid field and, as such, receives only a fraction of the funds available to the parent disciplines of forestry and production agriculture. Funding for agroforestry need not be through new programs, although new monies are an option. Many existing programs could accommodate agroforestry if sufficient resources, specific directives, and administrative will existed. The requirement is that dollars be earmarked specifically for agroforestry programs which adhere to the strict definition of this field of study.
Some agroforestry technologies are already well enough developed for broad implementation but are not being widely applied, often due to the lack of technical assistance available to the landowner. Opportunities exist for land-grant universities, Cooperative Extension Service, SCS and Forest Service, as well as state and local agencies and private entities, to provide consultation and informative materials to better guide landowners and land managers in implementing agroforestry practices. Technical advisors may recognize the need for such programs but often lack the expertise required to implement them. Hence, training for technical advisors would enhance implementation of agroforestry practices.
Furthermore, in the medium term there will be a need for more and better developed agroforestry curricula at the undergraduate and graduate levels, particularly if the land-grant system and related resource management agencies establish a stronger focus on agroforestry development. The enormous and growing interest in this field by students is an important indicator of the likelihood that agroforestry education will improve as research and practice develops. For this to occur, however, it is critical that each reasonably distinct "agroforestry region", or collection of states with similar agroforestry needs and potential, generate the resources necessary to conduct long-term, interdisciplinary studies in which students can meaningfully participate. This in turn requires a university-level commitment to allocate state resources to the establishment of secure faculty positions in this field.
A compelling argument can be made for public support of agroforestry practices. Many of the benefits -- atmospheric carbon dioxide reduction, cleaner water, erosion control, increased biodiversity -- affect all people. Currently, however, the landowner or manager bears the cost of implementing agroforestry although the landowner may not be the primary beneficiary. To optimize public benefit, some form of cost-sharing may be appropriate for those practices shown to provide real and necessary public benefits. It is critical to evaluate our system of subsidies and taxes and identify measures that encourage more and better agroforestry practices. During the process, it is important that we develop agroforestry land-management systems that are sufficiently attractive financially for landowners to adopt on their own merit. Such is and should continue to be a central goal in research aimed at developing sustainable agroforestry systems.
To further encourage agroforestry it will be important that federal income tax Schedule F -- as completed by agricultural producers with gross sales of more than $10,000 per annum -- provides that the incidence of taxation is syncopated with market receipts, rather than the overall value of the enterprise. In general, income tax provisions that presently apply to forest owners should be evaluated for their applicability to agricultural producers who incorporate a significant tree component into their enterprise for conservation and/or productive purposes. Depending also on the projected state or local level benefits to be realized, property tax codes may warrant redesign to compensate owners for creating positive "externalities".
SUMMARY OF AGROFORESTRY DEVELOPMENT NEEDS
Agroforestry provides the landowner the opportunity to develop a portfolio of short- and long-term investments which, with the proper combination of trees/shrubs and crops/livestock, provides both economic and environmental benefits. However, full implementation of agroforestry within the United States will require the development of an extensive agroforestry infrastructure (research, teaching, extension and policy). This infrastructure must be established with the ultimate user, the landowner, in mind and designed to provide a rapid turn-around of systems research findings.
Achieving the goal of full implementation will require:
a) collaborative and flexible yet adequately coordinated and empowered institutional mechanisms for planning and managing agroforestry development activities;
b) funds earmarked specifically for agroforestry land use to address the interdisciplinary and often long-term nature of agroforestry research and the multi-dimensional, inter-sectoral nature of effective technical assistance and technology transfer; and
c) modified perspectives among public agencies concerned with land and resource management which recognize the demonstrated and potential benefits of agroforestry throughout the country to address conservation and sustainable development objectives often at considerably less cost than the dominant alternative strategies.
A "Commission an Agroforestry" should be established to determine the role of agroforestry in the United States. This commission should be charged to:
a) identify broad national and more focused regional agroforestry needs;
b) determine research, teaching and technology transfer requirements to satisfy those needs; and c) recommend policies and programs required to ensure that the future needs are adequately and efficiently met.
In view of the current level of interest in agroforestry as a land-use system, and the degree of urgency associated with implementing certain agroforestry practices to improve environmental quality, an interim strategy is proposed and presented here.
a) The USDA Soil Conservation Service should assume a role in eliminating the barriers to more widespread and effective agroforestry practice. The SCS's long-term professional leadership position in farmland conservation technology development and implementation, its prominent collaborative activities with other resource management entities in many states and its physical presence at a local level throughout the country, places the agency in a strategic position to assist in agroforestry development.
b) Annual funding of $15 to 25 million should be established to support agroforestry research, teaching and outreach programs throughout the country. The funds should be allocated to the many forms of education, technology development and transfer needed to adequately enhance the agroforestry knowledge and information base among resource management agencies, land management professionals, policy makers and landowners.
c) A broad and flexible institutional mechanism should be established to routinely assess agroforestry development opportunities and priorities throughout the country and to allocate funds earmarked for agroforestry on a competitive basis. The following two-tiered strategy is proposed:
Federal level -- A National Agroforestry Advisory Board could be established within the USDA. An agroforestry coordinator would be designated as a full time director to facilitate the activities of the board. The board and coordinator would advise regional agroforestry consortia. Members also would advise their respective parent organizations on strategies for enhancing agroforestry development in the context of their on-going programs. The board would be comprised of representatives of USDA and other agencies, institutions and private organizations with key expert and facilitative roles to play in developing agroforestry. The board would monitor and help coordinate activities of the regional consortia including "vision-sharing", education, professional training, research, technology transfer and policy development. Initially, the board could sponsor a national-level workshop to identify key opportunities for linkage among regions as discussed below.
Regional level -- Consortia could be developed within "agroforestry regions" which link USDA professionals with land-grant universities and other sources of research, technology transfer and technical assistance capability within the region. Such consortia would recommend funding for agroforestry development by regions. Each consortium would develop an agroforestry plan that reflects research, technology transfer and technical assistance needs and development priorities. The priorities would be incorporated into guidelines for education, training, research, technology transfer and policy development project proposals. Consortium members could draw upon expert review panels convened to assess the merit and feasibility of implementing the respective proposals in that region.
1 Lawrence, J.H. and L.H. Hardesty. 1992. Mapping the territory: agroforestry awareness among Washington State land managers. Agroforestry Systems 19(1):27-36.
2 Noweg, T.A. and Kurtz. 1987. Eastern black walnut plantations: an economically viable option for Conservation Reserve lands within the Corn Belt. No. J. Appl. For. 4:158-160.
3 Lawrence, J.H., L.H. Hardesty, R.C. Chapman and S.J. Gill. 1992. Agroforestry practices of non-industrial private forest landowners in Washington state. Agroforestry Systems 19:37-55.
4 Hill, D.B. 1991. Agroforestry specialty crops. In: Garrett, H.E. (ed.) Proc. 2nd Conference on Agroforestry in North America, Aug. 18-21, 1991, Springfield MO, Univ. Missouri, Columbia, pp. 210-220.
5 Johnson, R.J., J.R. Brandle, R.L. Fitzmaurice and K.L. Poague. 1992. Vertebrates for biological control of insects in agroforestry systems. Symp. on Biol. Control of Forest Pests in the Great Plains: Status and Needs. July 13-16, 1992, Bismarck, N.D. Great Plains Ag. Coun. Pub. No. 145. pp. 77-84.
6 Landis, D.A. 1993. Arthropod sampling in agricultural landscapes: ecological considerations. fin: L.P. Pedigo and G.D. Buntin (eds.). Handbook of Sampling Methods for Arthropod Pests in Agriculture. (In press).
7 Kort, J. 1988. Benefits of windbreaks to field and forage crops. Agriculture, Ecosystems and Environment 22/23:165-190.
8 Brandle, J.R., D.L. Hintz and J.W. Sturrock (eds.). 1988. Windbreak Technology. Elsevier Science Publishers. 598 p.
9 Scholten, H. 1988. Snow distribution on crop fields. Agric. Ecosys. Environ. 22/23:363380.
10 Stoeckler, J.H. 1962. Shelterbelt influence on Great Plains field environment and corps. Prod. Res. Pap No. 62, 26 p., USDA For. Serv., Lake States For. Exp. Stn., St. Paul, MN.
11 Rosenberg, N.J.. 1974. Microclimate: The Biological Environment. John Wiley, New York. 315 p.
12 Quam, V., L. Johnson, B. Wight and J.R. Brandle. n.d. Windbreaks for livestock operations. Technical Note (in preparation). North Dakota State University.
13 Webster, A.J.F. 1970. Direct effects of cold weather on the energetic efficiency of beef production in different regions of Canada. Canadian Journal of Animal Science. 50:563-573.
14 Hintz, D.L. 1983. Benefits associated with feedlot and livestock windbreaks. USDA Soil Conservation Service, Midwest National Technical Center, Technical Note 190-LI-1, 15p. Lincoln, NE.
15 Van Sambeek, J.W. and G. Rink. 1982. Physiology and silviculture of black walnut for combined timber and nut production. In: Black Walnut for the Future. USDA. Forest Service N.C. Forest Expt. Sta. Gen. Tech. Rep. NC-74.
16 Dawson, J.O. 1983. Dinitrogen fixing plant symbioses for combined timber and livestock production. In: D.B. Hannaway (ed.) Proc. International Hill Lands Symp. Foothills for Food and Forests, Corvallis, OR. pp. 95-112.
17 Funk, D.T., R.C. Schlesinger and F. Ponder, Jr. 1979. Autumn olive as a nurse plant for black walnut. Bot. Gaz 140(suppl):S110-S114.
18 Johnson, R.J., J.R. Brandle, R.L. Fitzmaurice and K.L. Poague. 1992. Vertebrates for biological control of insects in agroforestry systems. Symp. on Biol. Control of Forest Pests in the Great Plains: Status and Needs. July 13-16, 1992, Bismarck, N.D. Great Plains Ag. Coun. Pub. No. 145. pp. 77-84.
19 Best, L.B., R.C. Whitmore, and G.M. Booth. 1990. Use of cornfields by birds during the breeding season: the importance of edge habitat. American Midland Naturalist 123:84-99.
20 Burel, F. 1989. Landscape structure effects on carabid beetles spatial patterns in western France. Landscape Ecology 2:215.
21 Landis, D.A. and M.J. Haas. 1992. Influence of landscape structure on abundance and within-field distribution of European corn borer larvae parasitoids in Michigan. Environ. Entomol. 21:409-416.
22 Lowrance, R.R. 1992. Groundwater nitrate and denitrification in a coastal plain riparian forest. J. Environ. Qual. 21:401-405.
23 Osborne, L.L. and M.J. Wiley. 1988. Empirical relationships between land use/cover and stream water quality in an agricultural watershed. J. Envir. Manage. 26:9-27.
24 Licht L.A. and J.L. Schnoor. 1990. Poplar tree roots for water quality improvement. ASAE Paper No. 40-2057, ASAE, St. Joseph, MI.
25 Osborne, L.L. and D.A. Kovacic. 1993. Riparian vegetated buffer strips in water quality restoration and stream management. Freshwater Biology 29:243-258.
26 Erman, D.C., J.D. Newbold and K.B. Roby. 1977. Evaluation of streamside buffer strips for protecting aquatic organisms. California Water Resources Center Technical Report, University of California, Davis.
27 Feller, M. C. 1981. Effects of clear-cutting and slash-burning on stream temperature in southwestern British Columbia. Water Resources Bulletin 17:863-867.
28 Gordon, A., P. Williams and N.K. Kaushik. 1992. Advances in Agroforestry: Crops, livestock and fish have it made in the shade. Highlights of Agricultural and Food Research in ontario 15(3):2-7.
29 Henderson, S., and R.K. Dixon (eds.). 1993. Management of the terrestrial biosphere to sequester atmospheric CO2. Inter-Research. 140 p.
30 Cutter, B.E. and H.E. Garrett. 1993. Wood quality in alley cropped eastern black walnut. Agrofor. Sys. 22:25-32.
31 Garrett, H.E., J.E. Jones, W.B. Kurtz and J.P. Slusher. 1991. Black walnut (Juglans L.) agroforestry -- its design and potential as a land use alternative. The Forestry Chronicle 67:213-218.
32 Lottes, G.J. 1985. Economics of integrated forestry systems in Illinois. Unpub. M.S. thesis, 185 p. Univ. of Illinois, Champaign-Urbana.
33 Gordon, J.C. and J.O. Dawson. 1982. Potential uses of nitrogen-fixing leguminous trees and shrubs in commercial forestry. Bot. Gaz. 140 (suppl):88-90.
34 Pearson, H.A. 1990. Silvopasture: forest grazing and agroforestry in the southern coastal plain. In: Proc. Mid-South Conf. on Agroforestry Practices and Policies. p. 25-42.
35 Clason, T.R. 1993. Economic implications of silvopastures on southern pine plantations. 1n: Proc. 3rd North American Agroforestry Conference. 11 p. (in press).
36 Lewis, C.E., G.W. Burton, W.G. Monson and W.C. McCormick. 1984. Integration of pines and pastures for hay and grazing. Agrofor. Sys. 2:31-41.
37 Morris, L.A., E.J. Jokela and J.B. O'Connor, Jr. 1992. Silvicultural guidelines for pinestraw management in the southeastern United States. Georgia For. Res. Pap. 88, Research Division, Georgia For. Comm. 11 p.
38 Ribaudo, M.O., D. Colacicco, L.L. Lagner, S. Piper and G.D. Schaible. 1990. Natural resources and users benefit from the CRP. USDA Econ. Res. Serv. Agr. Eton. Rep. No. 627, t Washington, D.C. 51 p.
39 Miranowski, J.A. and R.L. Bender. 1982. Impact of erosion control policies on wildlife habitat on private lands. J. Soil Water Cons. 37:288-291.
40 Nielsen, E.G., J.A. Miranowski, and M.J. Morehart. 1989. Investments in soil conservation and land improvements: factors explaining farmers' decisions. USDA Econ. Res. Serv. AER-601, Wash., DC.
41 Willis, W.O. and C.E. Evans. 1977. Our soil is valuable. J. Soil Water Cons. 32:258-259.
42 Crowder, B.M. 1987. Economic costs of reservoir sedimentation: a regional approach to estimating cropland erosion damages. J. Soil Water Cons. 42:194-197.
43 Forster, D.L., C.P. Bardos, and D.D. Southgate. 1987. Soil erosion and water treatment costs. J.Soil Water Cons. 42(5):349-352.
44 Kurtz, W.B., S.E. Thurman, M.J. Monson, and H.E. Garrett. 1991. The use of agroforestry to control erosion - financial aspects. The For. Chron. 67:254-257.
45 Brandle, J.R., B.B. Johnson and T. Akeson. 1992. Field Windbreaks: are they economical? J. Prod. Agric. 5:393-398.
46 Casler, G.L. and S. F. Smith, 1993. Farm income tax management and reporting reference manual, A.E. Extension 93-16. Department of Agricultural, Resource and Managerial Economics, Cornell University, Ithaca, New York.
47 Buck, L.E. and S. Matthews. Preliminary findings from a comparative study on agroforestry practices and knowledge systems in New York State and ontario, Canada. Poster at 3rd Conference on Agroforestry in North America. August, 1993, Ames, IA. (in press).
