Environmental Enhancement Using Short-Rotation Woody Crops and Perennial Grasses as Alternatives to Traditional Agricultural CropsVirginia R. Tolbert1 and Andrew Schiller 1Biofuels Feedstock Development Program, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6422 (corresponding author). 2Oak Ridge Institute for Science and Education, Oak Ridge, TN 37831. ESD Pub. No. 4513. Sponsored by the Biofuels Systems Division, U.S. Dept. of Energy, under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corporation. From Environmental Enhancement Through Agriculture: Proceedings of a Conference, Boston, Massachusetts, November 15-17, 1995, Center for Agriculture, Food and Environment, Tufts University, Medford, MA. |
Introduction: Alternative Energy Sources Short-rotation woody crops (SRWC) and herbaceous crops (perennial grasses)
are receiving increasing interest as potential alternative energy sources that
also can provide extensive environmental benefits. These benefits can be viewed
as occurring on a scale ranging from global (reductions in greenhouse gases) to
a specific site (decreases in erosion and the need for chemicals compared with
traditional row crops). When grown as biomass feedstocks, these woody and
herbaceous crops can be significant sources of energy and fiber in some regions
of the country. Besides offering an alternative energy resource, they can
provide crop diversity and both economic and environmental benefits to local
agricultural communities. The potential environmental benefits of
short-rotation woody crops and herbaceous energy crops compared with
traditional row crops include improved soil quality and stability (reduced
erosion), cover for wildlife, and lower inputs of energy, water, and
agrochemicals (McLaughlin, 1992; The need to reduce national dependence on imported oil and the opportunity
to develop the nation's potential to produce high-yielding biomass energy crops
have prompted significant research on both the agricultural production and
energy conversion technologies necessary to achieve this potential ( Woody and herbaceous biomass crops are not seen as competing for land with traditional row crops such as corn and soybeans. Rather, they are perennial crops that can be grown on more marginal lands where erosion is a problem, where soil stabilization is needed, or where the economic returns to the farmer's labor and capital are low. Once established, maintaining perennial biomass crops requires little input of labor or resources compared with annual cycles of planting and harvesting of traditional row crops. Traditional agricultural crops need good quality soils that are easy to work and that have adequate water and nutrient availability to produce high yields and a satisfactory profit for the farmer. Because market forces and farmers' economic needs drive how much of each crop is planted, agricultural production may extend onto marginal sites to meet these needs. Agricultural production on these more marginal, erosive sites harms the environment. In meeting the need for food, fiber and fuel, agriculture inevitably will affect the very resources that it relies on for productivity and profitability. With the need for alternative fuel feedstocks, biomass crops may become an economically competitive option for many farmers in the future. Whether this will happen is not yet known (Walsh, 1995); it will depend on economics, public acceptance, and other factors that are not included in this address of the environmental effects of crop production. State and federal support and commitments by utilities to purchase biomass feedstocks may be required initially to encourage production of these crops and to offer growers a reasonable economic return. This paper focuses on three areas in which biomass crops may provide environmental benefits while providing income to farmers and agricultural communities: soil loss, water quality, and native wildlife habitat. We highlight physical and management differences between traditional row crops and biomass crops and the ways that the latter may be environmentally preferable to row crops. We also present some results from research supported by the Biofuels Feedstock Development Program (BFPD) at Oak Ridge National Laboratory to quantify the potential environmental benefits of biomass crop production. |
Potential Energy CropsHerbaceous energy crops Perennial grass crops potentially can provide farm income as both energy feedstocks and forages (McLaughlin et al., 1994). An example is switchgrass (Panicum virgatum), a sod-forming, warm-season grass that was an important component of the native North American tallgrass prairie. After evaluating yield and agronomic data on 34 herbaceous species, BFDP selected switchgrass for further research and development as a primary bioenergy candidate. This choice was based on its high yields and excellent versatility in early field trials and its many positive environmental attributes. Switchgrass has the economic advantage that it can be harvested using traditional agricultural equipment to produce large round bales that can either be stored on-site until needed or transferred directly to conversion facilities following harvest. Depending on the region and the time of harvest, farmers may be able to get a second growth for use as forage for livestock. Switchgrass and other native prairie grasses have become increasingly important as forage grasses, particularly in the Midwest, because of their ability to grow during hot summer months when water availability can limit growth of most other species (Moser and Vogel, 1995). Because of its extensive rooting system, switchgrass provides environmental advantages such as increased soil organic matter, soil stability, and lower requirements for energy, water, and agrochemicals compared with traditional row crops (McLaughlin, 1992). Its extensive rooting system increases the efficiency of nutrient and water uptake and provides strong energy storage reserves that contribute to more stable yields during stress years. The extensive rooting system of switchgrass reduces soil erosion and enhances nutrient storage and availability. This increases the plant's ability to persist for a number of years without the annual replanting cycle and soil and nutrient losses associated with annual row crops. These attributes make perennial grasses valuable for reclaiming marginally productive sites that have been degraded by erosion and soil depletion (Aguilar et al., 1988). Short-rotation woody crops One hundred twenty-five tree species have been examined at various levels of detail by the BFDP since 1978 to identify those with the greatest potential for rapid growth, wide adaptability, and resistance to insect pests and diseases. To date, the species with the greatest potential for extensive development are poplar (Populus spp.), willow (Salix spp.), sweetgum (Liquidambar styraciflua), sycamore (Platanus occidentalis), and maple (Acer spp.) (Wright, 1994). Experimental yields of SRWC have been 2 to 5 times those currently obtained in natural forest stands and conifer pulpwood plantations in the United States. The increased yields of these tree species are the result of good matching of species to site, careful establishment techniques, use of improved clones or carefully selected seedlings, and recognition of the importance of weed control until the canopy closes (Wright, 1994). Methods for establishing SRWC are the same as for most other agricultural crops, although no-till establishment has not been effective because weeds must be removed and any underlying hardpan must be broken for maximum root penetration. Wright (1994) provides more detailed information on site establishment, maintenance, and harvesting needs for SRWCs. A possible obstacle to production of woody crops is the need for traditional forest harvesting equipment that may not be readily available to agricultural growers. Studies of small-scale plantings of hybrid poplar in the North Central states have shown that over time, trees grown on tilled agricultural lands that previously were in prairie grasses sequestered significant quantities of soil carbon as organic matter (Hansen, 1993). Unlike perennial grasses, which can cover the soil surface in one to two years, tree crops continue to leave most of the soil surface exposed (assuming weeds are controlled or removed) until the canopy closes in three to five years after establishment. Until then, soil erosion and loss of organic matter continue, although losses are less than with annual crops. Once the tree canopy has closed, weed control no longer is required because there is little light penetration and weed growth, consequently, is low. Fertilization requirements for SRWC depend on soil type and fertility. Studies are currently being conducted by the BFDP to determine fertilization and irrigation requirements of hybrid poplars in the Pacific Northwest and of sweetgum, sycamore, and cottonwood (Populus deltoides) clones in the Southeast to maximize yield while limiting production costs. |
Environmental Enhancement Using Biomass CropsThe BFDP's environmental research task currently supports several studies to help determine the environmental effects of converting agricultural lands to biomass crop production. These studies, which are conducted by several universities under subcontracts with the BFDP, investigate how soil characteristics change with conversion of agricultural land to biomass crops, the fate of chemicals applied to energy crops compared with annual agricultural crops, and the effects on habitat availability for native wildlife species. Information from these studies will help identify how environmental enhancement can be achieved through biomass crop production as part of the existing agricultural setting. Soils and nutrients Studies by the University of Minnesota, Duluth, are characterizing agricultural soils planted to biomass crops. These studies will provide information on how organic matter changes over time and whether biomass crops can be used to reclaim eroded sites and eventually restore their productivity. The study addresses the potential for biomass crops in a farm rotation system to increase crop diversity, minimize erosion, and increase soil fertility, while diversifying the economic base of individual farms. Other studies are examining the fate of chemicals applied to short-rotation woody crops both with and without cover crops, to a herbaceous crop (switchgrass), and to traditional row crops. This work will quantify the movement of nutrients and herbicides across the soil surface as well as into the soil and groundwater beneath research-scale sites. The study in Alabama, part of a three-site southeastern study funded jointly with the Tennessee Valley Authority, also is addressing the amount and chemical composition of surface water runoff from switchgrass, corn, and tree crops to quantify erosion and nutrient movement. These studies will provide information on uptake, release, and off-site movement of nutrients and pesticides on various soil types, and will help define hydrologic and environmental pathways of chemicals applied to biomass crops and their long-term effects on the agricultural environment. A study conducted by the BFDP in conjunction with an industrial partner in the coastal plain of South Carolina is determining the growth response of cottonwood clones, sycamore, and sweetgum to different irrigation and fertilization regimes. The nutrient content of water percolating through each experimental plot is being monitored to develop guides for the application of nutrients and water to maximize growth and minimize costs to growers. A related study by Clemson University, supported by the BFDP and the National Council for the Pulp and Paper Industry for Air and Stream Improvement, is addressing the use of paper and pulp sludges applied either alone or in combination with agricultural wastes as soil amendments for production of hardwood crops. This study offers the opportunity to: 1) determine the potential for using agricultural residues to enhance productivity and soil quality; 2) address the quantity and composition of the residue required to increase productivity; 3) provide environmental benefit; and 4) minimize economic costs for agricultural producers. All these studies will help address how tree crops' response to different nutrient regimes varies with species, soils and climate. Therefore, they will help us predict the effects of large-scale deployment of biomass crops in agricultural landscapes. Biodiversity SRWCs and switchgrass energy crops can enhance biological diversity by creating or increasing the habitat for native species. For example, fast-growing energy trees planted around existing forest remnants may buffer the forest habitat from predation by edge-using species such as the raccoon (Procyon lotor) and the brown-headed cowbird Molothrus ater). The additional wooded area provided by these buffers may increase the habitat value of the adjacent forest remnants for various disturbance-sensitive species, including an entire suite of migrant birds that need interior forest habitat to survive and breed. Many of these interior forest birds are Neotropical migrants that migrate between breeding areas in temperate North America and tropical wintering areas in Latin America and the Caribbean. These migrants are declining as the result of forest fragmentation and competition from edge-using species (Wilcove, 1985; Blake and Karr, 1987; Robinson et al., 1995). Some of these Neotropical migrants, such as the prairie warbler (Dendroica discolor) and the yellow-breasted chat (Icteria virens), require large expanses of regenerating forest, and may benefit directly from habitat created by SRWCs. Others, such as the sedge wren (Cistothorus platensis) rely on grassland habitats, and could benefit from switchgrass energy crops or from switchgrass planted to buffer existing native grasslands from other land uses, such as traditional row crops. In other areas, small plantings or windbreaks of native grasses or SRWCs can provide additional edge habitats in primarily agricultural landscapes for species that can access different habitat types near their preferred habitat. Placing native grasses and SRWCs to buffer existing forests, to form linear corridors for wildlife movement between otherwise isolated forests or grasslands, or to provide greater habitat diversity in agriculturally dominated areas all can enhance biodiversity, but the benefit will depend on how wildlife species use the habitat provided by energy crops. The role of these crops in enhancing biodiversity has been an area of
active research supported by the BFDP on several sites since 1992. Research on
small-scale hybrid poplar plantings in Minnesota has shown that these plantings
support increased bird diversity compared with row crops; however, the young
tree plantations were less valuable as habitat than were the adjacent forests
and shrubs (Hanowski et al., 1994). Few bird species
that require mature forest habitat have been found in the young woody
plantings, which are dominated by more common bird species ( Research in the Southeast by the BFDP and the National Audubon Society in conjunction with an industrial partner is addressing which energy crops, planting sizes, ages, management regimes, and placement in the landscape are of the greatest benefit for regional biodiversity. BFDP-sponsored research has shown that energy crops provide greater habitat structure and benefits for native biodiversity than do row crops, and could benefit biodiversity on a regional level with large-scale production of biomass energy crops. |
ConclusionsConversion of agricultural lands to energy crops is being closely monitored to determine its environmental effects and possible benefits. Research on short-rotation woody crops and herbaceous energy crops conducted or supported by the U.S. Department of Energy's BFDP has shown that these crops have significant environmental benefits and can contribute to the economic viability and diversity of individual farms and agricultural communities. Studies on a research scale have shown that biomass crops grown on more erodible agricultural lands increase soil stability, nutrient retention, and soil organic matter. These studies can provide initial guidance in determining the environmental effects and benefits of converting larger areas of land to biomass crop production in different regions of the U.S. If biomass crops grown under specific management techniques and located on more marginally productive or erosive land are found to be valuable for improving soil and water quality, decreasing chemical use and runoff, and improving native biodiversity, it will be important to determine cost/benefit ratios for growers who want to consider planting energy crops to provide both economic and environmental benefits. Determining what works best and developing guidelines on how to produce energy corps at a profit while benefiting the environment are important priorities. Information on how wildlife use biomass crops is important for growers who want to manage their biomass plantings to provide additional economic value from such activities as hunting and wildlife viewing leases. We need to develop an environmental decision structure to help determine the best locations and management methods for large-scale plantings to achieve environmental benefits for soil, water, and wildlife diversity while meeting the economic needs of individual growers and regional and local agricultural communities. |
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