The Role of Short-Rotation Woody Crops in Sustainable Development

Jim Shepard, National Council of the Paper Industry for Air and Stream Improvement, Gainesville, FL

Virginia Tolbert, Oak Ridge National Laboratory Oak Ridge, TN

Abstract

Trends in wood demand are closely correlated with population growth. While forest acreage in the United States has been essentially constant since 1930, the fraction of forest available for timber harvesting has decreased, particularly on public timberlands. National policies regarding the role of publicly-owned timberland have been changing toward ecosystem management, in which timber harvesting is an incidental consequence of management rather than an objective. Litigation, primarily concerning threatened and endangered species, has dramatically reduced planned harvests of public timber, particularly in the Pacific Northwest. The result is that total U.S. National Forest harvest volumes over the next 50 years are forecast to be half the levels of the previous several decades. National consumption of pulpwood is forecast to increase by 47% and lumber by 31% over the next 50 years. In addition, use of wood for bioenergy may increase substantially during this time period. How will these wood demands be met?

One answer is to increase wood production by increasing management intensity on existing timberland, especially in plantation forests. Another is to convert land currently in agriculture to timberland. Short- rotation woody crops can be used in both cases. But, what are the environmental consequences? Short- rotation woody crops can provide a net improvement in environmental quality at both local and global scales. Conversion of agricultural land to short-rotation woody crops can provide the most environmental quality enhancement by reducing erosion, improving soil quality, decreasing runoff, improving groundwater quality, and providing better wildlife habitat. Forest products companies can use increased production from intensively managed short-rotation woody crop systems to offset decreased yield from the portion of their timberland that is managed less intensively, e.g. streamside management zones and other ecologically sensitive or unique areas. At the global scale, use of short-rotation woody crops for bioenergy is part of the solution to reduce greenhouse gases produced by burning fossil fuels. Incorporating short-rotation woody crops into the agricultural landscape also increases storage of carbon in the soil, thus reducing atmospheric concentrations. In addition, the use of wood instead of alternatives such as steel, concrete, and plastics generally consumes less energy and produces less greenhouse gases.

Cooperative research can be used to achieve energy, fiber, and environmental goals. This paper will highlight several examples of ongoing cooperative research projects that seek to enhance the environmental aspects of short-rotation woody crop systems. Partnerships between government, industry, and academia are conducting research to study soil quality, use of mill residuals, nutrients in runoff and groundwater, and wildlife use of short-rotation woody crop systems. Such research is vital to assure the role of short-rotation woody crops as a sustainable way of meeting society's needs.

Keywords: environment, energy crops, bioenergy, biomass crops, wildlife, breeding birds, small mammals, soil, water quality, erosion, soil quality, hydrology, carbon sequestration

Introduction

Trends in wood demand are closely correlated with population growth. Between 1950 and 1991 world population increased from 2.5 billion to 5.2 billion; meanwhile, wood consumption increased from 1.5 to 3.5 billion cubic meters (Sutton 1994). Forest area in the United States has been relatively constant since about 1920 (Powell et al. 1993). However, the fraction of forest area available for timber harvesting has decreased, particularly on public forests in recent years (Haynes et al. 1995). National policies regarding the role of publicly-owned timberland have been changing toward ecosystem management, in which timber harvesting is an incidental consequence of management rather than an objective. Litigation, primarily concerning threatened and endangered species, has dramatically reduced planned harvests of public timber, particularly in the Pacific Northwest. The result is that total U.S. National Forest harvest volumes over the next 50 years are forecast to be half the levels of the previous several decades, while national consumption of pulpwood is forecast to increase by 47% and lumber by 31% over the same period (Haynes et al. 1995). In addition, the use of wood for energy may increase substantially during this time period (Moore 1996). How will these wood demands be met?

One answer is to increase wood production by increasing management intensity on existing timberland, especially in plantation forests. Another is to convert idle or marginally productive agricultural land to timberland. Short-rotation woody crop (SRWC) systems can be used in both cases. But, what are the environmental consequences? Production of SRWCs can provide a net improvement in environmental quality at both local and global scales. Preliminary results are showing that shifting from production of row crops on marginal or erosion-prone agricultural land to SRWCs can reduce erosion, improve surface and ground water quality, provide better wildlife habitat, and reduce carbon dioxide emissions.

Erosion and Water Quality

A study to assess the environmental effects of converting conventional agricultural lands to SRWCs is ongoing at sites in Alabama, Mississippi, and Tennessee (Joslin and Schoenholtz in press, Thornton et al. in review). During the first few months of the first growing season, few differences in runoff water quality were observed between row crops and SRWCs because both still had substantial amounts of bare soil (Joslin and Schoenholtz in press). By the end of the first growing season, and during the following winter and spring of the second year, substantial differences in sediment lost via runoff were observed. In Mississippi, 16.2 Mg ha-1 of sediment was measured in runoff from conventionally-tilled cotton (Gossypium hirsutum) compared with 2.3 Mg ha-1 observed in runoff from cottonwood (Populus deltoides) over a 14-month period (Thornton et al. in review). Sediment loss from no-till corn (Zea maize) was three times that from sycamore (Platinus occidentalis) at the Tennessee site, although rates were much lower than at the Mississippi site (Thornton et al. in review). At the Alabama site, the sweetgum (Liquidambar styraciflua) SRWC treatment had greater sediment in runoff than no-till corn when a cover crop was not used in the SRWC treatment. With a fescue (Fescue elitor) cover crop, there were no differences between the row crop and SRWC (Thornton et al. 1996, Green et al. 1996, Tolbert and Wright in review). Nutrient concentrations in runoff were related to fertilizer applications and were generally higher from row crops than from SRWCs. Ground water nitrate concentrations exceeded EPA's maximum contaminant level of 10 mg l-1 nitrogen in several instances in the row crops but not in the SRWCs (Thornton et al. in review).

Soil Quality

Studies of small-scale planting of hybrid poplar in the north-central states have shown that over time significantly more organic matter built up under the SRWCs than under row crops or grasslands (Hansen 1993). Investigators assessing the environmental effects of converting land from row crops to SRWCs hypothesize that soil quality in different regions will be improved (Grigal and Berguson in review, Joslin and Schoenholtz in press). Improvements in soil porosity, bulk density, aggregate stability, soil organic matter, and infiltration are expected. These improvements may take several years to be detectable, however. Ongoing studies of SRWCs will identify the extent of differences in soil quality improvements over time for different soil type and regions.

An ongoing study in South Carolina is addressing the use of mill waste and residues as amendments to improve soil quality. Results to date are showing that paper mill residues provide more rapid and stable pH adjustment than agricultural residues alone (Camberato 1996, Tolbert and Schiller 1996). Field studies beginning in 1997 will verify these preliminary greenhouse results and will determine application rates that consider existing soil quality and SRWC nutrient requirements to enhance growth while minimizing the potential for soil and water quality impacts.

In the Tennessee study mentioned above, soil physical properties associated with soil quality were investigated in no-till corn, 1-year-old SRWCs, 12-year old sycamore and loblolly pine (Pinus taeda) plantations, and a 50-year-old forest (Bandaranayake et al. 1996). Findings generally confirmed the hypothesis of improvement in soil quality following replacement of row crops with SRWCs. Soil quality, as assessed by measurements of steady state infiltration, bulk density, and soil organic carbon, was highest in the 50-year-old forest and least in the corn crop. The 12-year-old sycamore and loblolly pine plantations had intermediate values for these soil parameters.

Carbon Storage

Use of SRWCs for bioenergy either for production of transportation fuels or for direct combustion has the obvious effect of reducing the amount of fossil fuel burned and thus reduces atmospheric CO2. Additional benefits can be gained when marginally productive or erosive agricultural cropland is replaced by SRWCs through carbon stored both in the soil (Hansen 1993) and in long-lived wood products (Marland and Marland 1992). Soil carbon may be lost in the early years of SRWC establishment due to mineralization of organic matter in the upper soil profile, but SRWCs should quickly become a net sink for carbon. Hansen (1993) found that soil carbon increased in 12- to 18-year- old hybrid poplar plantations at a rate of 1.6 Mg ha-1 y-1 more than in adjacent agricultural crops. Of course, such increases in soil carbon storage following agricultural conversions to SRWCs will not continue indefinitely. It is likely that a new equilibrium soil carbon level will be reached, with little long-term change under continued SRWC growth and harvest cycles. Grigal and Berguson (in review) concluded that changes in carbon storage and soil quality can be slowly changed over a one- to ten-year period by soil management. Johnson (1992) reviewed studies of soil carbon in chronosequences from abandoned agricultural land to aggrading forests. Most of the studies reviewed reported substantial net increases in soil C across a 40- to 50-year period relative to initial soil C under agricultural production.

Hydrology

Growing SRWCs on agricultural lands can change the hydrology compared with typical row crops. Sites are captured by SRWCs after one to three years and produce less runoff than row crops due to higher levels of evapotranspiration and soil cover. SRWCs quickly develop a forest floor after canopy closure that promotes rainfall interception and retention compared to row crops that are tilled at least once annually and thus have extended periods of bare soil each year. SRWCs generally have bare soil only during the first year or two following establishment and so have forest floor cover throughout most of each rotation. Transpiration rates on an equivalent leaf area level may not differ much between row crops and SRWCs, but SRWCs maintain higher leaf area throughout the year due to their perennial nature and so would be expected to transpire more on an annual basis.

A study comparing row crops with SRWCs in Alabama, Mississippi, and Tennessee observed few significant differences in the amount of runoff during the first 14 months following establishment (Thornton et al. in review). This is not surprising since there is little difference in canopy cover, rooting depth, and litter layers between these crop systems during the first year. Hydrologic differences should become expressed during the second and subsequent years due to differences between annual and perennial cropping.

Richardson and McCarthy (1994) used a field-scale hydrologic simulation model (DRAINMOD) to compare hydrology among several alternate land uses in eastern North Carolina. In their simulation they separated pine plantation silviculture into an early period (1-3 years) and closed canopy period (4 years +). A 20-year simulation on a 404 ha area found that young pine plantations had 7% less runoff annually than agriculture, and older, closed canopy pine plantations had 26% less runoff than agriculture. Studies of SRWCs in the southeastern and north-central states are expected to demonstrate similar reductions in runoff as the research plots mature. The reduced runoff can also be tied to improved surface and groundwater quality as nutrients and chemicals applied for weed and pest control are retained on the SRWC sites.

Wildlife

Wildlife implications of conversion of agricultural fields to SRWCs and other energy crop systems have been discussed by Christian et al. (1994), Graham et al. (1995), Tolbert and Schiller (1996), and Tolbert and Downing (1995). Benefits include habitat for early successional species, the potential for improving habitat for interior forest species by connecting fragmented forests with SRWC plantings, and use as linear corridors for wildlife travel in predominantly agricultural landscapes (Schiller and Tolbert 1996). Additionally, forest products companies can use increased production from intensively managed SRWCs to meet their raw material needs while offsetting decreased yields from the portions of their timberland that are managed less intensively, e.g., streamside management zones, ecologically sensitive or unique areas, and other areas managed primarily for wildlife (Hughes 1992).

Christian (in review) used snow-tracking to study how medium-sized mammals and deer used small hybrid poplar plantations and adjacent lands in Minnesota, South Dakota, and Wisconsin during winter. Eight plantations, all 3-4 ha in size and 5-6 years old, were studied. Deer used hybrid poplar plantations for travel, but concentrated use was not observed. Medium-sized mammals such as squirrels, rabbits, and hares also rarely used the plantations. Winter use of these SRWCs was similar to adjacent open land. Christian et al. (1994) also found that small mammals using hybrid poplar plantings were more similar to grasslands and row crops than to forested areas.

Use of SRWCs in Minnesota by breeding birds was studied in 12 hybrid poplar plantings ranging in age from 1 to 8 years and in size from 4 to 30 ha. More individual birds and more species were found in these SRWCs than in croplands, but less than in nearby native forest and scrub habitats (Hanowski et al. in press). Bird use of these SRWCs was influenced by the structure of the plantation's vegetation, with increased use in more structurally complex habitats. Bird use in these plantations seemed also to be influenced by the plantation's landscape context. However, it is not clear how plantation size, shape, and landscape arrangements influence habitat quality for different species of breeding birds. The relatively new science of landscape ecology is only just beginning to provide land managers with information on alternatives regarding how to configure forest plantations in landscapes containing agricultural fields, roads, towns, and natural forests (Robinson et al. 1995).

The study of wildlife habitat quality in SRWCs planted for bioenergy is relatively recent. Most early bioenergy SRWCs were installed as research plots used to assess performance of different species and clones and are usually small in area (Schiller and Tolbert 1996). Knowledge about wildlife use of these plantings may not be applicable to operational-sized SRWC plantations. Wildlife use of operational- scale plantations grown for pulpwood and solid wood products has been extensively studied (NCASI 1993, Allen et al. 1996). Information on wildlife use of these plantations, at least in young ages, should be useful in assessing how wildlife will use operational-scale SRWCs.

Conclusions

Studies of how soil nutrients and physical properties change with incorporation of short-rotation woody crops into industrial and agricultural landscapes can help assess the environmental effects of these crops produced in different regions of the country. Information on environmental changes associated with conversion of erosive or marginally productive lands to intensive short-rotation tree crop management can help match tree crop species, site characteristics, and nutrient requirements to maximize productivity and both economic and environmental benefits. For forest products companies, SRWCs offer a way to offset the production losses associated with managing a portion of their timberland for non-timber objectives. Documenting how SRWCs managed for fiber and energy can simultaneously provide environmental benefits can increase the value and acceptance of these crop systems for industry, producers, environmental groups, and the general public.

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File posted on March 17, 1998; Date Modified: February 21, 1999