About one-third of the world’s land area is forestland. Broad-leaved evergreen trees are a dominant species in tropical rain forests near the equator (Spurr, 1979). In the northern hemisphere, stands of coniferous softwood trees such as spruce, fir, and larch dominate in the boreal forests at the higher latitudes, while both the broad-leaved deciduous hardwoods such as oak, beach, and maple and the conifers such as pine and fir are found in the middle latitudes. Silviculture, or the growth of trees, is practiced by five basic methods: exploi­tative, conventional extensive, conventional intensive, naturalistic, and short- rotation (Spurr, 1979). The exploitative method is simply the harvesting of trees without regard to regeneration. The conventional extensive method is the harvesting of mature trees so that natural regeneration is encouraged. Conventional intensive silviculture is the growing and harvesting of commer­cial tree species in essentially pure stands such as Douglas fir and pine on tree farms. The naturalistic method has been defined as the growth of selected mixed tree species, including hardwoods, in which the species are selected to match the ecology of the site. The last method, short-rotation silviculture (short-rotation woody crop or SRWC, short-rotation intensive culture or SRIC), has been suggested as the most suitable method for energy applications. In this technique, trees that grow quickly are harvested every few years, in contrast to once every 20 or more years. Fast-growing trees such as cottonwood, red alder, and aspen are intensively cultivated and mechanically harvested every 3 to 6 years when they are 3 to 6 m high and only a few centimeters in diameter. The young trees are converted into chips for further processing or direct fuel use and the small remaining stems or stumps form new sprouts by vegetative growth (coppicing) and are intensively cultivated again. SRWC production affords dry yields of several tons of biomass per hectare annually, often without large energy inputs for fertilization, irrigation, cultivation, and harvesting, so that the energy balance is positive.

It should be noted that although the prime purpose is to produce wood fiber for the manufacture of paper products, the pulp and paper companies have operated large tree plantations that yield energy as a by-product for decades. Heat, steam, and electricity are produced from wood wastes and also black liquor, which is generated in the paper manufacturing process (Chapter 5). Almost two-thirds of all renewable fuels consumed by the U. S. industrial sector is accounted for by the industry’s use of black liquor (U. S. Dept, of Energy, 1995). The pulp and paper industry produces well over half of its own energy needs and clearly has a great interest in sustained-yield forestry. In the United States, several pulp and paper companies are developing SRWC technology to provide improved methods for supplying fiber to pulp mills and by-product energy (Stokes and Hartsough, 1994). In 1994, approximately 20,000 ha of SRWC systems were operated in the United States by the pulp and paper industry; 40,000 to 80,000 ha were projected to be operated by the year 2000.

Historically, trees are important resources and still serve as major energy resources in many developing countries. No fewer than 1.5 billion people in developing countries derive at least 90% of their energy requirements from wood and charcoal, and at least another billion people meet at least 50% of their energy needs this way (National Academy of Sciences, 1980). Hundreds of species in the seven genera Acacia, Casuanna, Eucalyptus, Pinus, Prosopis, and Trema are used as fuelwood in developing countries (Little, 1980). Several studies of temperate forests indicate productivities from about 9 to 28 t/ha — year, while the corresponding yields of tropical forests are higher, ranging from about 20 to 50 t/ha-year (Nichiporovich, 1967). These yields are obtained using conventional forestry methods over long periods of time, 20 to 50 years or more. Productivity is initially low in a new forest, slowly increases for about the first 20 years, and then begins to decline. Coniferous forests will grow

even in the winter months if the temperatures are not too low; they do not exhibit the yield fluctuations characteristic of deciduous forests.

One of the tree species that has been studied in great detail as a renewable energy resource is the eucalyptus (Mariani, 1978), evergreen hardwood trees that belong to the myrtle family, Myrtaceae, and the genus Eucalyptus. There are approximately 450 to over 700 identifiable species in the genus. The eucalyptus is a rapidly growing tree native to Australia and New Guinea, and is widely grown in the United States, especially in Southern California and Hawaii for a variety of construction purposes. High-density plantings (17,790 trees/ha) in Southern California of E. grandis harvested twice annually have been reported to yield in excess of 22 dry t/ha-year (Sachs, Gilpin, and Mock, 1980). It appears to be a prime candidate for energy use because it reaches a size suitable for harvesting in about 7 years. Several species have the ability to coppice after harvesting, and as many as four harvests can be obtained from a single stump before replanting is necessary. In several South American countries, eucalyptus trees are converted to charcoal and used as fuel. Eucalyp­tus wood has also been used to power integrated sawmill, wood distillation, and charcoal-iron plants in western Australia. Several large areas of marginal land in the United States may be suitable for establishing eucalyptus energy farms. These areas are in the western and central regions of California and the southeastern United States.

Various species and hybrids of the genus Populus are some of the more promising candidates for SRWC growth and harvesting as an energy resource (Sajdak et al, 1981). The group has long been cultivated in Europe and more recently in the eastern United States and Canada. Populus hybrids are easily developed and the resulting progeny are propagated vegetatively using stem cuttings. Consequently, there are hundreds of numbered or named clones established throughout the eastern United States. Summaries of record SRWC small-plot yields for Populus hybrids have shown production levels of 15-20 dry t/ha-year (Hansen, 1988) and yields of 30-40 dry t/ha-year have been projected as attainable goals through genetic engineering (Ranney, Wright, and Layton, 1987). SRWC growth of hybrid poplar clone D-01 has been reported to afford yields of biomass that range as high as 112-202 green t/ha — year (56-101 dry t/ha-year at 50 wt % moisture) (Dula, 1984). These results were reported with very high-density plantings that have been termed wood — grass in which the crop is grown like grass and is harvested several times each growing season. However, there is some dispute regarding the benefits of woodgrass growth vs SRWC growth (Wright et al, 1989). Several investigators have not been able to reproduce these results (с/. DeBell and Clendenen, 1991), although the high-density planting technique seems to have some potential benefits.

It was concluded from early studies that deciduous trees are preferred over conifers for the production of woody biomass for conversion to biofuels (InterTechnology Corp., 1975). Conifers are used as fuel in many parts of the world, including the United States, but the long-term research effort to develop woody species as dedicated energy crops emphasizes mostly deciduous species (с/. Ferrell et al, 1993; Wright, 1994; Ferrell, Wright, and Tuskan, 1995). Several deciduous species can be started readily from clones, resprout copiously and vigorously from their stumps at least five or six times without loss of vigor, and exhibit rapid initial growth. They can also be grown on sites with slopes as steep as 25%, where precipitation is 50 cm or more per year. It has been estimated that yields between about 18 and 22 dry t/ha-year are possible on a sustained basis almost anywhere in the Eastern and Central time zones in the United States from deciduous trees grown in dense plantings. Table 4.10 lists deciduous trees that were judged in early work to have desirable growth characteristics for plantation culture and that have been shown to grow satisfactorily at high planting densities for short and repeated harvest cycles.