Biomass 9 (1986) 125-133

Short-Rotation Forestry Research in the United States

W. A. Geyer and M. W. Melichar

Department of Forestry, Kansas State University, Manhattan, Kansas 66506, USA

(Received: 23 August, 1985)

ABSTRACT

In 1978 the United States Department of Energy initiated the Short Rotation Woody Crops Program* to create a technical information base and improve wood energy supplies. This national program focused on tree selection, management systems, equipment needs, economics, and nutrient requirements.

Close spacing and successive coppice rotations are critical to the short-rotation concept. Twenty-five species have been selected for use in various parts of the country. Woody biomass productivity rates from 382 nationwide, short rotation, intensive-culture research tests averaged 5"6 Mg/ha/yr. Increases of at least 125 % are expected through additional research.

BIOCUT, a microcomputer based cost-accounting model, has been developed to analyze short-rotation systems. Harvest and transportation costs represent 50 % of total production costs. Forage harvesting equip- ment is needed. Chunk cutting of wood bolts requires two-thirds less energy than chipping.

Future efforts of this national program should focus on coppice regeneration, harvesting techniques, and economic evaluation of multi- hectare monoculture viability tests.

Key words: Short-rotation forestry, woody biofuels, energy, biomass.

I N T R O D U C T I O N

Forests cover nearly one-third of the earth's land surfce and account for two-thirds of all dry matter production. 1 Much of the wood from

*Research sponsored by the Biomass Energy Technology Division, US Department of Energy under contract with Oak Ridge National Laboratory Martin Marietta Energy Systems, Inc.

125 Biomass 0144-4565/86/S03.50- © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain

126 W.A. Geyer, M. W. Melichar

these forests is used as fuel and although the global importance of fuel- wood has diminished during the last century, it has always been the primary fuel source in developing countries. Presently, it provides nearly 8% of global energy needs. 2 It has been estimated that over half of the timber cut in the world serves as fuel for cooking and heating. 3

In the US, the use of wood as a residential and industrial energy resource has increased by a factor of three since the 1960s, so that wood supplied about 6% of the industrial energy and nearly 10% of the nation's residential heating requirements in 1981. 4 About 25% of the wood cut in 1981 was for fuelwood. 5

It is expected that this increasing interest in renewable resources will encourage the shift from extensive forest management to more inten- sive cultural systems. 6 While wood products and forest residues will provide large amounts of fiber, forest plantations managed intensively for biomass production could contribute significantly to energy supplies.

Short-rotation intensive culture (SRIC) forestry is a silvicultural system that incorporates close spacing, intensive cultural techniques, and short cutting cycles. Significant to this concept are numerous har- vests resulting from coppice stands. This system was first studied in the US in the mid-1960s for fiber production. 7 Beginning in 1978, the con- cept was tested and improved for energy purposes with research focus- ing on dry matter production at competitive fuel costs.

The US Department of Energy, in its mandate to develop renewable national energy resources, considered wood the major potential source of renewable biomass energy. In 1978 it initiated the Short Rotation

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Short-rotation forestry research in the United States 127

Woody Crops Program (SRWCP) to create a technical information base and improve wood energy supplies. It is the first and only com- prehensive national program on short-rotation intensive culture in the country.

The overall objective of the SRWCP is to develop systems that produce woody biomass for energy at competitive prices. Research in the program focuses on five areas: (1) species screening and genetic selection, (2) stand management alternatives, (3) equipment needs, (4) economic evaluations, and (5) nutrient utilization and site productivity. There are 21 research institutions throughout the US conducting studies coordinated by the Oak Ridge National Laboratory in Oak Ridge, Tennessee. This report summarizes the activity and progress of the program. Much of the information is from annual reports prepared by the Oak Ridge National Laboratory. s-l°

PERFORMANCE GOALS/PRODUCTIVITY

Current productivity rate and cost performance, as compared to program goals, are shown in Fig. 1. Since 1977 nearly 50% of these goals have been attained. High productivity and low unit cost must be achieved for SRIC biomass production to compete as an alternative to other fuels. The short investment period (3-10 year harvest) could give SRIC an advantage over other forestry practices (20 + year harvest). Productivity varies between regions (Fig. 2), species, sites, and management techniques. The highest productivity rates are in the Lake States, Pacific Northwest, and Subtropics regions. The average woody biomass productivity rate from 382 nationwide SRIC research tests is 5"6 dry Mg/ha/yr (metric tons per hectare per year). Productivity can be expected to increase at least 125% by utilizing the best management techniques and selected species, clones, or hybrids.

SPECIES SCREENING AND GENETIC SELECTION

An increase in production yields to 20 + dry Mg/ha/yr will depend heavily on tree species selection and genetic breeding. Progress has been made in selecting species, clones, and hybrids that perform well on a variety of sites. Selection criteria include: high energy content, high dry-weight production, broad site-adaptability, coppicing ability, pest tolerance, cold hardiness, and drought tolerance. Table 1 lists 25 tree species that appear promising for use as of 1983. Their potential

128 W. A. Geyer, M. W. Melichar

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Fig. 2. Mean productivity by geographic region calculated from all programmatic data and from a subset of that data that represents successful management techniques and site

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use varies by geographic regions in the United States. Genetic research is being conducted on several species. As of 1982, 11 species had been identified with an average 52% gain in productivity through selection.

STAND MANAGEMENT

Stand management is the largest research area in the SRWCP and involves tree propagation, plantation establishment, weed control, fertilization, irrigation, spacing, rotation length, pest and disease con- trol, and coppicing. Stand management practices are dependent on the species, site, and desired end product.

Plant propagation and tissue-culture research complement the gains in productivity from short growing cycles. Cultivars possessing desir- able characteristics can be reproduced asexually, thereby producing

Short-rotation forestry research in the United States

TABLE 1 The 25 Species That Appear Most Promising for SRIC as of 1983

129

Species Regions of good performance ~

NW W/SW GP LS MW S/SE NE SB

Acer saccharinum (silver maple) x × Ailanthus altissima (ailanthus) x Alnus glutinosa (European alder) x Alnus rubra (red alder) x Atriplex canescens (fourwing saltbush) x Elaeagnus umbellata (autumn olive) x Eucalyptus grandis (eucalyptus) LiquMambarstyraciflua (sweetgum) x Pinus clausa (sand pine) x Pinus elliottii (slash pine) × P. nigra x P. densiflora (hybrid pine) x Pinus sylvestris (Scotch pine) x Pinus taeda (loblolly pine) x P latanus occidentalis (American x

sycamore) Populus deltoides (eastern cottonwood) × Populus hybrid (hybrid cottonwood) x Populus spp. (hybrid poplar) x × × Populus tremuloides (quaking aspen) × Populus trichocarpa (black cottonwood) × Prosopis alba (mesquite) x Robinia pseudoacacia (black locust) × × × Salix spp. (willow) x Sapium sebiferum (Chinese tallow tree) x Tamarix (tamarisk) x Ulmuspumila (Siberian elm) x

×

"NW= Northwest, W/SW-West/Southwest, GP = Great Plains, LS = Lake States, MW = Midwest, S/SE = South/Southeast, NE = Northeast, SB = Subtropics.

large n u m b e r s of genetical ly identical plants. H y b r i d popla rs have rece ived the most at tention. Af te r a 2 -year es tab l i shment per iod , 20 cm cutt ings f rom s too l -bed o rcha rds should p rov ide enough cutt ings annual ly to plant an a rea 120 t imes larger than the s too l -bed area. T i s sue cul ture research has also been c o n d u c t e d on several addi- t ional h a r d w o o d t ree species, especia l ly sweetgum, mesqui te , and alder.

Con t ro l of compe t ing vegeta t ion in h a r d w o o d plantings is essential. P lanta t ion es tab l i shment t echniques and w e e d cont ro l m e t h o d s are

130 W.A. Geyer, M. W. Melichar

site and species-specific. Frequently, site preparation is necessary in the season prior to planting. In such cases, application of a contact herbicide is followed by plowing and disking. Site preparation in the season of planting involves disking, followed by application of a pre- emergent herbicide. Post planting weed control is best achieved by cultivation and strip application of herbicides.

Fertilization is required on most sites for maximum biomass produc- tivity. While fertilization is usually cost effective, mixed planting with nitrogen-fixing species is an alternative to fertilizer application. How- ever, trial plantings of sycamore and sweetgum mixed with European alder or black locust have resulted in lower total biomass production than when grown in monoculture. Similar reductions were found in mixed stands of black cottonwood and red alder.

Spacing and rotation-length studies have determined that growth at close spacing (0.2 m) is very rapid initially but declines after two to four growing seasons. Closer spacing requires frequent harvests to prevent stand deterioration. Plantings at wider spacing (2.5 m) are characterized by slower stand volume growth, thus requiring longer rotation periods (7-10 years) to achieve productivity similar to that of more closely spaced plantings.

Pests and diseases may present problems in short-rotation mono- culture plantations. Conditions that favor rapid tree growth also can favor pathogen and insect infestations. Periodic monitoring of pests and diseases is necessary to assure that no problems and/or pathogens are present.

Successive coppice rotations are critical to the short-rotation con- cept. Time of harvest, stump height, and tree age affect stump survival and coppice response. Dormant season harvest is recommended for most SRIC species. Second rotation productivity rates are expected to be at least 20% greater than first rotation rates. Although data is limited, some studies have shown increases as high as 100% in 3-year- old stands.

ECONOMIC EVALUATION

Increasing the economic efficiency of producing woody biomass from SRIC involves assessment of the market potential and determination of the most efficient species, management, and harvesting strategies.

BIOCUT, a microcomputer based cost-accounting model, has been developed to analyze the economic viability of SRIC systems. Two operational systems, mesquite production in the semiarid Southwest and hybrid poplar growth in the temperate zone of central

Short- rotation forestry research in the United States 131

Pennsylvania, were evaluated by this model. These evaluations showed the optimum rotation age to be 5 or 6 years. Selling prices used in the model ranged from $30 to $40 per dry Mg. Optimum rotation age is shortened by higher selling prices or discount rates and is also sensi- tive to biomass productivity variations and the number of coppice harvests. Moderate variation in establishment, cultural management, and harvesting costs had little effect on rotation age. These evaluations are based on research-sized plots. The SRWCP plans to establish full- scale operational plantings larger than 75 ha to quantify productivity rates and evaluate costs.

WOOD-HANDLING EQUIPMENT AND FUEL CHARACTERIZATION

Harvest and transportation costs are estimated to represent over 50% of the total production costs in SRIC forestry. Existing and newly designed harvesting equipment is being evaluated for use in SRIC plantations. Previous studies demonstrated that single-stem cutting is not compatible with SRIC harvest needs and multiple stem cutting is required. Equipment must be capable of harvesting at least 20 Mg/hr and utilizing continuous harvest concepts similar to, but sturdier than, forage harvesters.

Conventional testing of feller/bunchers and wood bailing have been evaluated. Bundling (tying groups of trees together) and billeting (cutting trees into short lengths) have potential for drying and storage. Chunking (cutting wood into larger than normal chips) studies have shown energy consumption to be one-third that of conventional chipping. Pressing (roller squeezing) green roundwood tree bolts is a promising method for removing moisture.

Most research suggests that the heating value of wood varies signifi- cantly between, but not within, species. Observations on a limited number of species indicate low heritability of the heating value of wood. The estimated heating value across species, genera, geographic regions, and wood age varies from 18 750 to 22 570 kJ/drykg. Average values for conifers and hardwoods are 21 370 and 19 680 kJ/ dry kg, respectively.

PLANT PRODUCTIVITY AND SITE NUTRIENTS

Physiological processes of plant growth are being studied to determine the causal relationships of biomass productivity and nutrient avail-

132 W.A. Geyer, M. W. Melichar

ability, movement, and uptake by the plant. These processes are assimilation, seasonal energy allocation, photosynthate storage and mobilization, water and nutrient uptake, and soil-plant nutrient cycling.

A variety of silvicultural practices and species are being studied to determine their effects on these processes. Preliminary tests comparing single and multiple nitrogen fertilizer applications at 200 kg/ha (during a year) showed no significant difference in biomass production. There is no growth advantage to multiple fertilizer applications and there is a potential for groundwater nitrate pollution from these treat- ments.

FUTURE EFFORTS OF SRWCP

The best species for SRIC and successful stand establishment proce- dures have been identified. Future research in the SRWCP should focus on maximizing biomass production and minimizing costs.

Successful coppice regeneration and harvesting techniques are criti- cal to the viability of this concept. Several projects are testing factors that affect coppice growth. Since harvesting represents nearly 50% of short-rotation wood costs, equipment now being designed will be tested under field conditions.

Genetic improvement research will determine potential genetic gain in productivity for additional important biomass species already identified.

Operational plantation tests are planned in cooperative efforts with industry. These will implement research results and facilitate technical transfer of the information into an operational system to make the SRIC system a viable, renewable, energy resource alternative.

REFERENCES

1. Spurr, S. H. & Vaux, H. J. (1976). Timber: Biological and economic potential. Science, 919,752-6.

2. Smith, N. (1981). Wood: an ancient fuel with a new future. Worldwatch paper 42. Worldwatch Institute, Washington, DC, 48 pp.

3. Eckholm, E. (1975). The other energy crisis: Firewood. Worldwatch paper 1, Worldwatch Institute, Washington, DC, 22 pp.

4. Hewett, C. E. & Glidden, Jr., W. T. (1982). Marke t pressures to use wood as an energy resource - - current trends and a f inanc ia l assessment, Resources

Short-rotation forestry research in the United States 133

Policy Center, Thayer Sch. of Eng., Dartmouth College, Hanover, VT, 24 PP.

5. Skog, K. E. & Watterson, I. A. (1984). Residential fuelwood use in the United States. J. Forestry, 82,742-7.

6. Hanover, J. (1980). Horizons in research on primary productivity of forests. In: Proc. 6th North Amer. For. Biol. Workshop, Edmonton, Alberta, pp. 93-104.

7. McAlpine, R. G., Brown, C. L., Herrick, A. M. & Ruark, H. E. (1966). 'Silage' sycamore. Forest Farmer, 26 (6), 6-7, 16.

8. Ranney, J. W. & Cushman, J. H. (1982). Short rotation woody crops pro- gram: Annual progress report for 1981, Oak Ridge National Laboratory, ORNL/TM-8120, 65 pp.

9. Cushman, J. H., Wright, L. L., Trimble, J. L. & Ranney, J. W. (1983). Short rotation woody crops program: Annual progress report 1982, Oak Ridge National Laboratory, ORNL-5973, 128 pp.

10. Ranney, J. W., Trimble, J. L., Wright, L. L., Perlack, R. D., Wenzel, C. R. & Cushman, J. H. (1984). Short rotation woody crops program: Annual pro- gress report for 1983, Oak Ridge National Laboratory, ORNL-6085, 91 pp.