Research to develop trees as energy crops in the United States via short-rotation intensive culture made significant progress in the 1980s and 1990s. Projections indicate that yields of organic matter can be substantially increased by coppic­ing techniques and genetic improvements. Advanced designs of whole-tree harvesters, logging residue collection and chipping units, and automated plant­ers for rapid planting have been developed to the point where prototype units have been evaluated in the field and some are being manufactured for commercial use. It is expected that several additional devices will be offered for commercial use. The on going research is also leading to significant changes in forestry harvesting techniques. Clear-cutting is being phased out and partial harvesting or thinning operations are being phased in. New thinning technolo­gies have been proposed for testing in the forests of the Northwest after successful tests in California. The California research data show that the thin­ning of overgrown stands reduces tree mortality, provides healthier stands, and may offer biomass fuels at a cost that make it possible to operate wood — fueled power plants on a stand-alone basis at a profit in competition with market prices for electric power.

Some of the tree species that have been targeted for continuing research are red alder, black cottonwood, Douglas fir, and ponderosa pine in the Northwest; Eucalyptus, mesquite, Chinese tallow, and the leucaena in the West and South­west; sycamore, Eastern cottonwood, black locust, catalpa, sugar maple, poplar, and conifers in the Midwest; sycamore, sweetgum, European black alder, and loblolly pine in the Southwest; and sycamore, poplar, willow, and sugar maple in the East. Generally, tree growth in research plots is studied in terms of soil type and the requirements for site preparation, planting density, irrigation, fertilization, weed control, disease control, and nutrients. Harvesting methods are equally important, especially in the case of coppice growth for SRWC hardwoods. Although three species native to the region are usually included in the experimental designs, nonnative and hybrid species have often been tested in research plots as well. Advanced biochemical methods and techniques such as tissue culture propagation, genetic transformation, and somaclonal variation are being used in this research to clonally propagate individual geno­types and to regenerate genetically modified species.

After an intensive research effort over about 10 years, SRWC yields in the United States, based on accumulated data are projected to be 9, 9, 11, 17, and 17 dry t/ha-year in the Northeast, South/Southeast, Midwest/Lake, Northwest, and Subtropics, respectively (Wright, 1992). The corresponding research goals are 15, 18, 20, 30, and 30 dry t/ha-year. Hybrid poplar, which grows in many parts of the United States, and Eucalyptus, which is limited to Hawaii, Florida, southern Texas, and part of California, have shown the greatest potential thus far for attaining exceptionally fast growth rates. Both have achieved yields in the range of 20 to 43 t/ha-year in experimental trials with selected clones. Continuing research indicates that other promising species are black locust, sycamore, sweetgum, and silver maple.

Research on hybridizing techniques seem to be leading to super trees that have short growth cycles and that yield larger quantities of biomass. Fast­growing clones are being developed for energy farms in which the trees are ready for harvest in as little as 10 years and yield up to 30 m3/ha-year. Genetic and environmental manipulation has also led to valuable techniques for the fast growth of saplings in artificial light and with controlled atmospheres, humidity, and nutrition. The growth of infant trees in a few months is equiva­lent to what can be obtained in several years by conventional techniques.

Chemical injections into pine trees have been found to have stimulatory effects on the natural production of resins and terpenes and may result in high yields of these valuable chemicals. Combined oleoresin-timber production in mixed stands of pine and timber trees is under development, and it appears that when short-rotation forestry is used, the yields of energy products and timber can be substantially higher than the yields from separate operations.

One of the largest research projects on SRWC in the Western World, LEBEN or the Large European Bioenergy Project, was reported to be scheduled for initiation in the Abruzzo region of Italy in the mid-1980s and to be established near the end of that decade (Grassi, 1987; Klass, 1987). This project integrates SRWC production, the production of herbaceous energy crops and residues, and biomass conversion to biofuels and energy. About 400,000 t/year of biomass, consisting of 260,000 t/year of woody biomass from 700 ha and 120,000 t/year of agricultural residues from 700 ha of vineyards and olive and fruit orchards, will be used. Later, 110,000 t/year of energy crops from 1050 ha will be utilized. The energy products include liquid fuels (biomass —

derived oil), charcoal, 200 million kWh/year of electric power, and waste heat for injection into the regional agroforestry and industrial sectors. This project is still in the start-up stages in the mid-1990s.

One of the largest demonstration programs in the United States was started in 1993 in Minnesota where hybrid poplar is grown under short-rotation conditions on a few sites that total 2000 ha. As the results of this program are reported, a much more rigorous analysis of the potential of SWRC for energy will be possible. The ultimate approach to perfecting this technology, however, is to integrate large-scale biomass production with conversion. Little research of this type has been done. The assumptions and projections that have been made to evaluate the technology are based primarily on small-scale laboratory results, what others have reported as research results, or predictions about individual steps that make up the overall system. But this situation is starting to change as government-industry support of integrated biomass production and conversion research make it possible to examine the sustain­ability of these systems in detail. In the United States, several research projects in which virgin biomass production is integrated with conversion have been selected for field demonstration in plots that are expected to be a minimum of 405 ha (1000 ac) in size (Klass, 1996). This research will provide first-hand experience in operating integrated systems on a sustained basis in which a dedicated biomass feedstock is supplied to a conversion plant. The first group of biomass energy technologies to be scaled up consist of alfalfa production integrated with a gasifier-combined-cycle power plant in Minnesota, switch — grass production integrated with a power plant in Iowa in which biomass and coal are со-fired, hybrid willow production integrated with a power plant in New York in which biomass and coal are со-fired, and an innovative whole tree production system integrated with a power plant in Minnesota (Spaeth and Pierce, 1996). As these projects are implemented, others are expected to be added to the program.