Woody crops can be divided into hardwoods and softwoods. Hardwoods include willow, eucalyptus, birch and oak as shown in Figure 1.6. Softwoods include pine and spruce (see Figure 1.7).
a) Willow
Willow, a short rotation coppice, forms the genus Salix. It grows very rapidly during the juvenile stage (Murphy et al., 1996). This type of tree species is established by plant cuttings from one-year old wood, which are then inserted into the ground in the spring. At the end of the first growing season, they are cut to ground level to promote the growth of multi-stemmed stool (Dawson, 2007). The growth reaches up to 4 m in the first year and continues rapidly to heighten to 6-7 m at harvest in the third year (Dawson, 2007). Moreover, the willow coppice may be harvested six to eight times on a three-year cycle through the crop’s lifespan of 15 to 20 years (McCracken, 2006; Dawson, 2007). Willow grows best in mildly acidic soils (pH 5- 7) and one important advantage that it has compared to other crops is that less insecticides, fungicides and herbicides are needed in willow coppice plantations (Murphy et al., 1996). They are only necessary during the first and second year of the plantation. Dawson (2007) suggested that the yield can be expected to be in the range 7-12 tonnes dry matter (tDM) per hectare per year or 21-36 tDM on a three year harvest cycle.
b) Eucalyptus
Eucalyptus comes from the family, Myrtaceae. Many species of Eucalyptus are well grown in numerous countries such as Florida, South Africa, Brazil, Uruguay, Portugal and Venezuela (Gonzalez et al., 2011). However, in the UK, Leslie et al (2012) stated that there are only a few Eucalyptus species that are able to survive and grow well in the cold climate, namely
Eucalyptus gunnii and Eucalyptus nitens. These two species have very rapid early growth,
where their growth rates are 1.5-2.0 m per year and > 2.0 m per year respectively (FCS, 2010). Therefore, they have been used for short rotation coppice (Leslie et al., 2012). More interestingly, E.gunnii and E.nitens are also eligible as candidates for short rotation forestry (McKay, 2011). In central Florida, Eucalyptus is very promising for co-firing in coal-based plants (Rockwood et al, 2008).
c) Oak
Quercus is the old Latin name for oak and there are about 450 species known to be found in
Europe, Asia, North Africa, North America and South America (Simpfendorfer, 1992). Its family is said to be one of the largest and hardest in the world (Simpfendorfer, 1992). Oak trees can survive up to hundreds of years, of at least 200 years and a maximum of 600 years. The growth rate of oak trees depends on the species, where it varies from slow to rapid. An example of a slow growth oak tree is the white oak, Quercus alba, where it grows 10-15 feet within a span of 10-12 years (Simpfendorfer, 1992). An example of a moderate growth oak tree is the Southern red oak tree, Quercus falcate, where it grows 25 feet every 20 years. An example of a fast growth oak tree is water oak tree, Quercus nigra, where it grows 25 feet every 10 years. Oak woods are widely used for furniture and housing industries (Tumuluru et
al., 2012). Its sawdust is a by-product available from the timber industry and is said to be
valuable for bioenergy purposes (Tumuluru et al., 2012).
d) Birch
Birch is a relatively short-lived and broad-leaved deciduous tree of the genus Betula, where there are about 60 species found in northern Europe, Asia and North America (Simpfendorfer, 1992). Birch trees are medium-sized trees growing to between 40 and 50 feet. They are considered to be moderate to fast growing trees, where their growth rate is 1.25 feet per year for the first 10-20 years (Simpfendorfer, 1992). Simpfendorfer (1992) suggested that since birch is very short-lived, it is required to be sown within a few days of falling. Silver Birch is the species that is always planted. Interestingly, recent Energy Crops Scheme
is now supporting more hardwood species, where one of them is silver birch, even though it gives lower yield than willow and poplar (BEC, 2007b).
Willow Eucalyptus
Birch Oak
Figure 1.6. Photo images of hardwood trees (Source: www.2020site.org/trees/).
e) Pine
Pines are among the most well-known coniferous trees in the genus Pinus, with about 70 species. They are mostly found in most of Northern hemisphere, throughout temperate and subtropical regions of the world (Simpfendorfer, 1992). They can also be found past the Equator in parts of Southeast Asia (Simpfendorfer, 1992). These pines are characterised by their needle-shaped leaves and their heights range from 45 to 135 feet. Pine trees are evergreens, in which their leaves do not change in colour in the fall and do not shed in the winter months (2020site, 2012). Viana et al (2010) stated that one pine species that is capable
of providing a regular supply to meet fuel demand is Pinus pinaster (maritime pine), which is originally from France and Portugal. It has a fast growth rate and is reported to be able to grow well over a wide range of soil and rainfall conditions (FPC, 2006). Another fast growth pine species is Pinus taeda (loblolly pine), which is an important source for saw timber and pulp wood. It is abundant in Southern U.S and has been studied for ethanol production and can be economically competitive compared to production of ethanol from corn stover and other lignocellulosic materials (Frederick et al., 2008). The other species is Loblolly pine and it is considered a dedicated energy crop, at which there has been a considerable interest in using this pine as a feedstock for the production of transportation fuels (Frederick et al., 2008). It was reported in Georgia that short rotation plantation for loblolly pine was carried out intensively (10 to 12 years) and as a result, 26.6 m3 per hectare per year was produced (Borders and Bailey, 2001 in Gonzalez et al., 2011).
f) Spruce
Spruce trees belong to the genus Picea and can grow up to 100 feet tall. They have attractive pyramid shape and stiff needles. These coniferous trees can live up to 800 years and are able to tolerate extreme weather conditions (2020site, 2012). They can be found in Europe while most grow in North America. In Sweden, trees are used as raw materials for industrial needs and source of energy, for example Picea abies (Norway spruce) (Johansson, 1999). Johansson (1999) reported that spruce plantations grow very fast especially if the soil is very fertile, where the spruce can grow 2 to 3 feet per year on their first 25 years. On a poor soil, it can grow on an average of a foot per year.
g) Larch
Larch is a deciduous in the genus, Larix with about ten species identified around Europe, Asia and North America (Simpfendorfer, 1992). Its height can go up to 120 feet. It can survive in cooler temperature Northern hemisphere so they grow mostly in the mountainous regions (2020site, 2012). The leaves are soft, flat-looking needles and shed during autumn. Larch is very hard that it is able to withstand most forest fires. One example of a larch species is the European larch, Larix decidua, which has a slow to moderate growth rate (12 to 18 inches per year) and long been used in the timber industry for building constructions and provide heat in homes.
Pine Spruce Larch
Figure 1.7. Photo images of softwoods and their respective needle-like leaves (Source:
www.2020site.org/trees/).