The cultivated sorghums are all varieties or hybrids from S. bicolor L. Moench. The sorghum species is quite diverse but very consistent both genetically and botanically. All S. bicolor is a genetic diploid with base chromosome number of n = 10 and 2n = 2x = 20. While there is evidence that the crop is an ancient tetraploid, it is a functional diploid [2]. The genome size is larger than rice but substantially smaller than corn (Zea mays) or other grasses species and the complete genome has been sequenced [3]. In addition to S. bicolor, there are several additional Sorghum species, but none are systematically cultivated and many are perennial. Those species have variable growth habits and chromosome numbers but they do represent an additional source of genetic variability if needed. Of these, Sorghum halepense is likely the most infamous because it is the persistent and aggressive weed known as johnsongrass.

Botanically, sorghum is a member of the Panicodeae family. It is a typical grass with a deep and fibrous root system, primary culm and the capacity for both basal and axillary tillering. Reproductively, sorghum has a complete flower that is exerted through the top leaf sheath prior to anthesis. Sorghum species are predominantly self-pollinated but outcrossing does occur at rates between 2 and 30%; the precise amount of cross-pollination is a function of both genotype and environment [4]. While it is self-pollinated, sorghum is highly amenable to commercial hybrid seed production through the use of cytoplasmic male sterility systems. The crop is typically grown and managed as an annual but it is technically a perennial; once harvested, it will regrow unless environmental conditions (e. g., freezing temperatures) preclude further growth. Among commonly grown cereal crops, sorghum has a high water use efficiency and very good heat and drought tolerance. Plant maturity and height are highly variable and influenced by both the genetics of the species and the environment.

As an energy crop, sorghum is unique in that there are several types of sorghum that are and can be used for biofuel or bioproduct production, with the defining factor in these crops being the primary source of fermentable carbohydrates. Grain sorghum, long used as feed grain, is now a significant contributor to biofuel production in the United States [5]. Sweet sorghum, long used as a sweetener, is now being deployed in conjunction with sugarcane in sugar to ethanol conversion facilities and biomass sorghums produce significant quantities of structural carbohydrates. Different types of forage sorghums, commonly grown for animal grazing, hay or silage, are integrated into the biomass sorghums and occasionally, and sometimes incorrectly, into the sweet sorghum group.

Most of the pine biomass used for energy in Europe and North America is first transported to pulp mills and lumber mills in the form of logs. After processing, the bark and sawdust compose a high proportion of the bioenergy produced in developed countries. In 2012, this “hitch a ride” method was the primary method of transporting wood fuel to power plants operated at pulp mills and OSB mills. The bark and wood “waste” is often burned for energy and, at some mills, the “waste” is sufficient to power the entire mill. Some mills that run on 100% wood fuel do not have to obtain any extra wood during the summer months. However, extra wood fuel (as bark or sawdust) may be required during the winter months when the “hitch a ride” system provides less energy than needed (due to colder temperatures outside). In contrast, some pines are harvested and sold to mills that produce wood pellets. Mill owners can increase the amount of this energy received by reducing specifications for top diameter and branchiness.

4.1.1.1 Main Features

To date, most of the information gained on the growth and physiology of Miscanthus has come from studies of Miscanthus x giganteus, which has been shown to reach a maximum aboveground yield of 46 t ha-1 under non-irrigated conditions [20] and 49 t ha-1 under irrigated conditions [21]. However, as detailed in Section 4.2.3 (Genetic Diversity for Biomass Production), the aboveground yield of Miscanthus can vary greatly depending on climatic and seasonal conditions, soil type, crop management and crop age.

The root system of Miscanthus x giganteus is composed of a coarse cluster of rhizomes from which a mass of fine roots grow [22,23]. These fine roots represent just under a third of the total belowground biomass and can reach a depth of 250 cm in sandy loams [22,23]. The rhizome biomass of M. x giganteus has been shown to reach a maximum of 23.8 tDM ha-1, by the end of summer, with this amount maintained until early winter [20].

Aboveground growth is triggered once temperatures reach 6 or 10°C with the emer­gence of stems from buds located just below the soil surface [24]. After stem emergence, tillering increases rapidly throughout May-July with up to 40 stems per plant recorded for M. x giganteus in the United Kingdom [25]. The number of productive shoots decreases over the course of the growing period, with the youngest tillers dying off. The oldest tillers continue to grow through to August-September and even October, depending on the climate and the time between emergence and flowering. The harvestable number of stems per plant varies between species, with M. sinensis silberspinne producing about 200 stems per plant or about five times as many stems as M. x giganteus (40 stems per plant) at three years of age [26].

Leaf area increases throughout the growing season to reach a peak at flowering, after which the canopy starts to senesce [27]. In Miscanthus x giganteus, the maximum leaf area index (LAI) obtained each season increases with crop age reaching about 7-8 m2 m-2 in summer for a three-year-old stand [28]. At the onset of plant senescence, all stems left standing gradually dry out during winter until February-March, when the crop is ready for harvest. The light extinction coefficient (k) through the leaf cover of the crop provides a measurement of the capacity of leaves to intercept light. The k-value for M. x giganteus has been recorded at 0.56 [27] and 0.68 [29] and for M. sinensis Goliath at 0.66 [30]. However, most of these values need to be validated, especially at the interspecific level.

Equally important to development of yield and quality potential of energy sorghum is the inherent protection of that yield potential. Thus, breeding for tolerance to both abiotic and biotic stress is critical for adaptation and productivity.

Abiotic stress is defined as a yield limiting factor caused by a non-biological source. Examples of abiotic stress include but are not limited to temperature, moisture and soil fertility. Of these factors, Boyer [92] estimated that drought was the largest single factor in reducing grain sorghum yield. The same situation will be true in bioenergy sorghum cultivars, especially since production will be reliant on rainfall and not irrigation. Drought stress is a complicated condition that initiates the expression numerous genes in a myriad of signal transduction pathways [93]. Drought tolerance has been studied for many years and several different mechanisms of tolerance have been described. Stay-green is a post­flowering drought tolerance trait that can be described as delayed senescence. Therefore, it can mitigate effects of terminal drought stress on the final yield processes including grain fill and stalk development. This trait is of special interest to breeders of dual purpose sorghum varieties [94]. The stay-green trait includes modifications in stalk properties that affect biotic stresses such as charcoal stalk rot [56], and lodging [62]. Stay-green plants tend to be superior in digestibility by ruminant animals due to higher basal stem sugars [95]. Stay- green may be of particular importance in sweet sorghum as sugar accumulation is optimized during grain fill. A second and less well understood drought tolerance mechanism is pre­flowering drought tolerance. This is tolerance prior to flowering and is physiologically different than stay-green. In fact, only a few sorghum genotypes possess both types of tolerance. Additional studies are necessary, but this tolerance may be of particular value in biomass sorghum hybrids since they do not enter the reproductive growth phase.

Aluminum toxicity can limit sorghum production in acidic soils, which are common in the humid tropics [96]. Genetic variability for aluminum tolerance has been observed [96,97] and is commonly used in sorghum breeding programs in Brazil. In fact, Magalhaes et al. [98] identified and cloned the aluminum tolerance gene AltSB5. Nitrogen use effi­ciency (NUE) in cereal crop production has been estimated to be at about 33% worldwide [99]. However, for lignocellulosic bioenergy production new experimental directions that emphasize improving biomass yield rather than grain production will need to be taken. Currently, experiments are being conducted using sweet sorghum to indentify genotypes and QTL that can improve NUE.

Biotic stresses are defined as yield limiting factors of a biological source, typically either an insect pest or a disease pathogen. There are numerous pathogens and pests that can reduce yield and quality of all types of sorghum. However, the relative importance of each biotic stress differs based on the type of sorghum and location of production. For example, biomass sorghum will generally be managed to eliminate or minimize reproductive growth. Consequently, pests and pathogens of reproductive growth are much less important. If grain is important, then breeding for resistance to midge and shoot fly are important because these pests reduce grain yield [100-103].

For energy sorghum, insects of greatest interest to breeders are those that affect harvested plant organs. For biomass sorghum, two of the most important insect pests include green — bugs, which stunt growth in young plants, and the borers because they affect stalk integrity. Tunneling in the stalk can disrupt vascular tissues causing nutrient or water deficiencies in addition to general weakening of stalks and subsequent lodging [64]. Additionally, stalk borer damage causes wounds that provide entry points for stalk rot pathogens [104]. Grain sorghum crops in the United States are not generally seriously infested with stalk boring insects, but in energy cultivars these may become very serious crop pests. Genetic variation for resistance to selected stem borer has been reported but it is not complete and breeding may be difficult [101, 105, 106]. Effective insecticide and management practices, as well as transgenic sources of resistance, will likely be critical for controlling these pests.

Among the most important sorghum diseases are anthracnose Colletotrichum subline — olum, and downy mildew Peronosclerospora sorghi (Weston and Uppal) C. G. Shaw. Con­siderable effort has been put into developing cultivars that are resistant to these pathogens. Anthracnose is one of the most economically important sorghum diseases [107,108] and it is likely that this disease will become more important as production of energy sorghum increases. Anthracnose can affect all above ground plant organs and is among the most seri­ous pathogens of sorghum [109]. At any one time, pathogen populations can be made up of many pathotypes making breeding for resistance particularly challenging [63]. Estimated yield losses due to anthracnose alone have been reported in excess of 50% [110]. Gene-for — gene relationships have not been conclusively demonstrated and, therefore, assignment of races or pathotypes has been difficult [108]. However, genetic variation for resistance has been reported [109, 111].

Another pathogen that affects sorghum production worldwide is downy mildew. Genetic variation for resistance to this pathogen has also been reported [112-114]. Multiple races of downy mildew that infect sorghum have been identified and, consequently, resistance genes have come from multiple sources [113]. Resistance to single races of this pathogen is reported as simply inherited and, therefore, resistance to the pathogen in general is described as oligogenic in most cases [115]. For example, resistance to the ICRISAT Centre race has been described as fitting the expected ratio of a two locus model with complementary and inhibitory interactions [114]. Thus, it is feasible to pyramid resistance genes for multiple races as new ones are identified. Oh etal. [116], linked RFLP (restriction fragment length polymorphism) markers to resistance for pathotypes 1 and 3. However, the fragment patterns could not be reconciled with original mapping cross making it impossible to locate the QTL. Therefore, more research is required to develop useful marker assisted selection schemes for downy mildew.

7.2 Summary and Conclusions

Sorghum is among the most versatile of crop species with wide environmental adaptation and a diversity of end uses. The successful breeding history of grain and forage sorghum demonstrates that biomass and sweet sorghum hybrids can be developed using the same approaches. The genetic resources available to sorghum breeders, contained in both the cur­rently utilized breeding germplasm, as well as the extensive public germplasm collections, represent a rich repository of genetic variation, which is necessary for breeding progress. There remain significant needs and opportunities to develop the production logistics and management schemes to fully integrate sorghum and other grass genera into biomass pro­duction schemes that fit the needs of processing facilities.

Willow biomass crops are being developed as sustainable systems that simultaneously pro­duce a suite of ecological and environmental benefits in addition to a renewable feedstock for bioproducts and bioenergy [5,6,10]. The perennial nature of the willow production sys­tem provides a range of beneficial attributes, such an improved energy return in investment, reduced greenhouse gases, and changes to soil conditions and biodiversity.

A recent life cycle analysis of willow biomass crops in North America covered all the inputs and processes from the nursery through seven three-year harvest cycles. Establish­ment, harvest and delivery of the willow to an end user and removal of the willow stools after seven rotations were included. The study explored eight different scenarios based on differences in transportation distances to an end user (71 or 195 km), high or low-yield scenarios (11.8 or 9.2 odt ha-1 yr-1) and, the use of 0 or 100 kgN ha-1 once every three years following harvest. In addition, uncertainty analysis was conducted using data on vari­ations in leaf litter, yield, and belowground biomass. Across the eight scenarios, cumulative energy demand ranged from 446.5 ± 12.3 to 1055.2 ± 42.9 MJ odt-1 [36], which equates to an energy ratio of between 1:19 and 1:45 for willow biomass delivered to an end user. The largest fraction of energy demand across all scenarios was use of diesel fuel, of which 48-77% was used for transportation of willow chips from the field gate to the end user. Harvesting operations had a greater energy demand than other field processes due to the frequency of occurrence over the life of the crop and the size of the equipment that was used. A recent review of 26 studies of short-rotation willow and poplar systems found that net energy ratio across a range of scenarios ranged from 1:13 to 1:79 at the farm gate and 1 : 3 to 1 : 16 after delivery to an end user [37]. Results from the current study and those from a previous study in North America [38] are at the high end of this range because inputs to the willow system, such are fertilizer or fencing, are generally lower or non-existent, when compared to European recommendations.

Patterns similar to energy demand were also found for greenhouse gas emissions for willow biomass crops across all scenarios because fossil fuel use is the largest source of emissions in the system [36]. Among the eight scenarios, greenhouse gas emissions ranged from-138.3 ± 22.5 to -52.7 ± 14.7 kg CO2-eq odt-1 (~-6.9 to -2.7gCO2 eqMJ-1). Carbon sequestration in the belowground portion of the willow system provided a large sink [39] that more than compensated for carbon emissions associated with crop production and management across all eight scenarios. As a result, the willow biomass crop system ended up being a carbon sequestration system, in addition to producing woody biomass that can be used to generate bioenergy, biofuels or bioproducts.

The perennial nature and extensive fine-root system of willow crops reduces soil erosion and non-point source pollution relative to annual crops, promotes stable nutrient cycling and enhances soil carbon storage in roots and the soil [39-42]. In addition, the crop is constantly in its rapid juvenile growth stage, so demand for nutrients is high, resulting in very low leaching rates of nitrogen even when rates of applications exceed what is needed for plant growth [43-45]. The period with greatest potential for soil erosion and non-point source pollution is during the first 1V2 years of establishment, when cover is often limited because weeds need to be controlled and the willow canopy has not closed. The use of a winter rye cover crop has proven to be effective at providing soil cover without impeding establishment of the willow crop [46] and trials with a spring planting of low growing white clover have also been effective [22]. Since herbicides are only used to control weed competition during the establishment phase, the amount of herbicides applied per hectare is about 10% of that used in a typical corn (Zea mays)-alfalfa (Medicago sativa) rotation in upstate New York [47].

Birds are one indicator of the biodiversity supported by willow biomass crops that have been studied in the United States. A study of bird diversity in willow biomass crops over several years found that these systems provide good foraging and nesting habitat for a diverse array of bird species [48]. Thirty-nine different species made regular use of the willow crops and 21 of these species nested in them (Figure 12.5). The study found that diversity increased as the age of willows and size of plantings increased, and also that birds have preferences for some willow varieties over others [49]. The number of bird species supported in willow biomass crops was similar to natural ecosystems, such as early succession habitats and natural, intact eastern deciduous forest ecosystems. The positive impact of willow on bird diversity was also supported in a recent assessment as part of a multidisciplinary study in Europe [10]. Instead of creating monocultures with a limited diversity across the landscape, willow biomass crops will increase diversity relative to open agricultural land or arable crop fields.

Tools are under development to assess biomass yield and composition and their related traits in Miscanthus.

© Inra — E. Rosiau

For biomass production, Zub et al. [26] showed that biomass yield during the second year could be used to predict the biomass yield of the third year, whatever the harvest date. This correlation requires further investigation over a longer period and on a wider sample of genotypes to determine whether the yield difference between harvest dates is the same for juvenile and mature phases of the crop.

In addition, Zub et al. [150] found that the aboveground volume including the stem number, the stem diameter and the plant height was a good predictor of plant biomass yield. Within genotypes, strong positive relationships were observed between biomass yield and the aboveground volume regardless of crop year (equal to 0.70 and 0.82 for autumn and winter harvests during the second year, respectively).

For biomass production, Hodgson et al. [59] developed Near-Infrared Reflectance Spec­troscopy (NIRS) calibration models for biomass quality to determine acid detergent lignin (ADL), acid detergent fiber (ADF), and neutral detergent fiber (NDF) from sample spectra of M. x giganteus, M. sacchariflorus and M. sinensis. The corresponding concentrations were predicted with a good degree of accuracy based on the coefficient of determination (values of R2 being higher than 0.80), standard error of calibration, and standard error of cross-validation values.

Regarding the statistical analysis, the residual model error in the analysis of variance model needs to be small to enable the comparisons between genotypes for quantitative traits. In Miscanthus, the residual term was high during the second and third years of the crop (Zub et al. [150]) and could hamper inter-genotypic comparisons for traits such as aboveground biomass yield or related traits. Without more plots or samples (it is indeed important for the
breeder and the producer to save place and cost), one way for reducing the residual term is to take into account intra-genotypic competition effect in the statistical model [150]. As it implies observations at the plant level, intragenotypic effect assessment requires easy — to-measure variables, such as the stand volume as a predictor of the aboveground biomass [150,151].

Modeling of emergence and plant growth using three and four-parameter logistic func­tions and the Gompertz function were tested to best describe the dynamics of crop emer­gence and of plant growth. The Gompertz function was found to be the best to estimate emergence dynamics while four-parameter logistic to estimate growth dynamics [151].

In contrast to the genetic tools, where intensive research has been conducted during the last decade, the phenotypic evaluation represents a bottleneck for Miscanthus, since high throughput tools are still required today.

4.4 Conclusion

The development of renewable energy sources is being investigated across the world and there is a growing demand for bioenergy feedstock that does not compete with food production and which has a low environmental impact. Miscanthus is developing as a serious player in the renewable energy sector. To realize its potential, new varieties are needed with the productivity and processing traits required for bioenergy production. This will require a full exploration of the genetic resource base of Miscanthus and its related species and the development of appropriate genetic tools. Many such projects are in progress throughout the world and the next decade is likely to deliver exciting new developments for Miscanthus as a renewable energy source.

The amount of energy captured by pines over a one-year period will vary with stand age, species and cultural practices. Pine trees that are less than three years old typically have a limited amount of foliage and, therefore, have not captured much energy (when compared to maize or perennial grass species). However, once pines have “captured the site” and are producing a high percentage of shade, the ability to capture energy increases. For species like Pinus tadea and Pinus elliottii, the amount of energy captured (i. e. stored as aboveground wood) in a given year typically peaks around ages 10-19 years. This peak may be around age 10 years when growth rates are high while lower growth rates may result in a peak near age 19 years. After this peak, the net energy captured in a year declines as the stand matures.

The amount of energy contained in one cubic meter of pine depends on the unextracted specific gravity, which changes with stand age. The specific gravity of a five-year-old Pinus taeda sapling may be 330 kg/m3 (from stump to a height of 3 m) while that of a bone-dry, 3 m log from a 50-year-old stand may be 500 kg/m3. In this example, the older log contains 50% more energy than an equal volume of the younger log. Therefore, it should not be assumed that the energy in a green tonne of 14-year-old pine logs will be the same as that contained in 28-year-old logs.

Sometimes the estimated energy per volume of wood varies by as much as 16% because wood shrinks when it dries. Therefore, one might overestimate the energy captured by pines if one overestimates dry mass productivity. Estimates of the dry mass per cubic meter could vary from 500 to 562 kg (Table 10.2) depending on if the volume is measured soon after the tree is harvested (i. e. green) or soon after the wood is removed from a drying oven. Therefore, overestimates may occur when specific gravity is determined using the oven-dry volume as the denominator.

The amount of energy contained in a pine stand (aboveground biomass = 500 m3/ha) might be 1150 MWh/ha (LHV) but this might end up producing only 250 MWh of electricity.

The greater value (1150 MWh/ha) assumes wood (25% od moisture content) will produce

2.3 MWh/m3 when burned in a wood boiler (97% efficiency). The lower value (250 MWh) assumes the wood is used in a power plant to produce electricity (21% efficiency). Some wood-fired power plants may convert one Mg of waste wood into 0.8 MWh of electricity.

If the pine stand mentioned above was 25 years old, the yield would be 20 m3/ha/yr or 46 MWh/ha/yr. Likewise, if it took 50 years to produce this volume, the yield would be cut in half (i. e. 10 m3/ha/yr or 23 MWh/ha/yr). As a comparison, 23 MWh/ha is equivalent to

13.5 barrels of crude oil (in theory). A hectare of solar panels might yield about 990 MWh of electricity per year. As a comparison, it might take a year and 21 hectares to capture the same amount of wood energy with a pine plantation.

Grain sorghum is grown primarily for starch, a primary component of the grain and like corn, a substantial portion of the U. S. grain sorghum crop (approximately 33%) is used for ethanol production [5]. The primary goal of most grain sorghum breeding programs is to enhance and protect productivity of the grain crop. While variation for seed composition (i. e., starch) exists in sorghum [6,7], simply increasing grain yield increases ethanol yield, because total starch per unit area is increased. In addition to maximizing productivity, protection of yield potential is equally important, especially drought tolerance, since production of grain sorghum occurs mostly drier areas of the world.

Sweet sorghum genotypes produce high quantities of simple sugars by having a very juicy stalk and high concentrations of soluble sugars. Traditionally, sweet sorghums have been used as a sweetener; extracted juice is cooked and sugars are concentrated in syrup. Sweet sorghums are known in various parts of Africa and they became popular in Asia and North America in the seventeenth and eighteenth centuries [8]. In the early twentieth century, the United States was producing 20 million gallons of sorghum syrup annually. Production dropped after World War II as crystal sugar became more available; today, sorghum syrup production is essentially artisanal and it is concentrated within the southeastern United States. Sweet sorghums are usually tall, with thick stalks and low grain yields compared to grain sorghum varieties. For energy production, sweet sorghum is processed in a similar manner to sugarcane; in fact, sugarcane processes and equipment provide a logical starting point for utilizing sweet sorghum [9]. Sweet sorghums have relatively high biomass yield potential; Hunter and Anderson [10] estimated that sorghum has the potential to produce up to 8000 liters of ethanol per hectare, or about twice as much as that of maize and 30% more than sugarcane ethanol in Brazil. Much of the carbohydrate content of the stalk juice of sweet sorghum is sucrose and/or glucose and it is fermentable without starch hydrolysis. This has advantages and disadvantages because the fermentation process can proceed without pretreatment (advantage) but fermentation must be initiated quickly because of the instability of the sugars in the stalks and/or juice (disadvantage). Preliminary results indicate that there can be a reduction of 16.8% sugar yield if juice extraction is delayed by 48 hours [11]. The residue (bagasse) is also a rich source of structural carbohydrates that can be used for energy production or as an animal feed [12].

For these reasons, the duration of the harvest season is important for sweet sorghum production and crop complementation production systems or storage processes must be developed [13]. Subtropical and tropical regions, which have longer harvest seasons, have an inherent production advantage for sweet sorghum. In addition, there is a logical and economically beneficial complementation between sweet sorghum and sugarcane and this can extend the harvest and milling season [14]. Sweet sorghum complements sugarcane in this scheme because it can be harvested twice in one year in tropical environments and because of its enhanced drought tolerance and water use efficiency.

Energy sorghum is a specific type of photoperiod sensitive sorghum that, when grown in long day environments, accumulates large quantities of biomass [1, 15]. Because they are photoperiod sensitive, they will not flower until day length drops below a specific length of time under temperate climates and the plants will never reach anthesis due to cold temperatures [16]. This characteristic causes an extended vegetative growing season, allowing the plant to capture and convert solar energy into biomass, provided adequate moisture is available for growth. In addition, a plant that is in a vegetative growth stage is inherently more drought tolerant than when in reproductive growth stages. Biomass sorghum cultivars will reduce growth during periods of drought but then resume growth when moisture is available. Therefore, this reduces the crop’s sensitivity to drought stress and its timing. Biomass sorghum is produced primarily for structural carbohydrates but it does produce some non-structural carbohydrates, albeit at lower levels than either sweet sorghum or grain sorghum.

Many landowners do not rely on economic equations (e. g. net present value, internal rate of return or equal annual equivalent) to determine the “optimal” rotation age or planting spacing when growing pines. Often, they ignore the time value of money and, instead, adopt objectives that overshadow profit motives. As a result, some use shorter rotations (e. g. 8-year) or plant more seedlings (e. g. > 1700/ha) or spend more for intensive management than would be optimal for profit maximization (Figure 10.3). When economics is the primary objective, then the optimal rotation age will be a function of the desired interest rate. For example, when the interest rate charged by a financial company is 5%, the optimal rotation for pine biomass on some sites might be 17 years (Table 10.7). In contrast, a shorter rotation might be used when the landowner borrowed money at a 12% rate.

Landowners who are risk-adverse typically prefer short rotations over long ones. This might occur when the risk of losses due to fire, insects, disease or hurricanes is high. As a result, short biomass rotations might be attractive to some, especially when the expected return on investment is greater than 9%. Some landowners might be willing to accept a reduction of $9/ha/yr in equal annual equivalent (Table 10.7) if it resulted in reducing the risk of losing trees to beetles and disease.

In the cases where there is only one price per green tonne (regardless of tree size), the economic rotation is no greater than the age at maximum mean annual increment. Typically, the year when maximum mean annual increment occurs varies with site productivity. Sites high in productivity (e. g. >15 Mg/ha/yr) will achieve shorter “biological” rotations than

low productivity sites (e. g. <8 Mg/ha/yr). For example, some productive sites may result in a “biological” rotation of 19-20 years while the economically attractive rotation may be 13-17 years (Table 10.7).

Forest plantations are often on sites that generate lower rental rates for alternative land uses, such as crop agriculture or animal grazing. In addition, government policies that lower property tax rates for forest land or offer conservation payments offset potential rent from other land uses. As a result, the economic analyses of forest plantations seldom include annual rent as an input. The exclusion of market-based annual rent payment yields, on balance, longer economic rotations than when annual rent is included.

Although profits could be achieved for a landowner who sells biomass to a refinery that produces synthetic diesel fuel, the economic incentive is often greater when the grower is also the end user. For example, one green tonne of pulpwood may be worth $9 in the forest, $26 at the roadside, and $32 at the power plant. However, for a homeowner, the wood might be sufficient to offset $135 in fuel oil (2012 prices). Therefore, one green tonne of pine biomass is worth perhaps 12 times more to a landowner who burns wood as an energy source (to furnish heat to their business) when compared to a landowner who sells pine logs on the open market (to a pellet mill or a Biomass Fluid Catalytic Cracking (BFCC) plant).

Some landowners add value to their pine logs by splitting and drying the wood for use as firewood. In some regions of the United States, split, air-dried pine firewood is currently sold for $65/m3. A landowner could either sell a green cubic meter of pines for $9 to a wood dealer or, after drying and splitting, could deliver it to homeowners for seven times that amount. Homeowners who purchase firewood do so because the cost of heating their home with wood is less than heating with fuel oil.

The least economical use of pine is as a substitute for coal, but one of the most economical uses is as a substitute for gasoline. During the Second World War, supplies of gasoline were limited and, therefore, many vehicles in Europe were converted to run on wood gas (also known as producer gas). In Sweden, there were over 70 000 of these vehicles at that time.

Currently, only a few individuals own vehicles that can be powered with wood gas. The senior author of this chapter actually owns a modified truck that can run on either gasoline or wood gas (Figure 10.4). This vehicle gets about 8.5 km per liter of gasoline or 2.4 kg of dry pine blocks («12% od moisture content). What is surprising is that the engine is more efficient when running on wood-gas. If one liter of untaxed gasoline costs $2 and one kg of dry pine block cost $0.06, then annual fuel costs (assuming 34 000 km) would be $8000 when using gasoline compared with a cost of only $240 when using wood gas.

10.3 Government Regulations

Globally, differences in governmental regulations can affect a landowner’s desire to estab­lish pine biomass plantations. Therefore, even when the price of coal is essentially the same in two countries, government policies can greatly affect the incentive for establish­ing plantations of pine. For example, policies regarding carbon dioxide have resulted in a decline in planting pines in New Zealand. In South Africa, policies regarding water limit the areas where pines may be established for energy (but establishing grasses for energy is permitted). Some regional governments have regulations concerning wood smoke pollution and the associated health effects.

10.4 Final Comments

In various countries, growing pines in plantations, for uses other than energy, is an econom­ically viable enterprise. In addition, pine firewood and pine residue will continue to provide energy to homes and mills throughout the world. For example, bioenergy (from pine and other sources) provides more energy to Sweden than oil or hydropower or nuclear power. Although the technology to produce electricity and liquid fuels from pine is available, so far few pine plantations have been established solely for the production of bioenergy. This is partly because pine biomass can also be obtained from thinnings, mill residues, harvest residues and from pine scraps transported to landfills. However, the number of pine biomass plantations might increase dramatically if the prices paid (per Mg) by biomass plants skyrocket.

Claims about short-rotation woody biomass crops have been made for more than four decades. During that period, many predictions about prices, yields, and seedling planting rates have not been achieved. Therefore, we are hesitant to make any claims regarding the future extent of short-rotation pine plantations. However, those considering harvesting pines on an eigh-year rotation for bioenergy may wish to consider the following points:

• Some bioenergy reports have likely overestimated the expected yield/ha of short-rotation pine plantations (on average sites) harvested before the year 2030.

• Few (if any) landowners will invest $43 to grow and harvest a dry Mg of pine biomass when the price paid at the farm gate is only $40 per dry Mg.

• Most planting densities recommended for pine biomass are not the economically opti­mum for a landowner who sells harvested wood at the roadside and who wishes to maximize the land expectation value.

• Several studies that evaluate the optimum tree planting density do not consider any additional costs associated with harvesting small diameter logs.

• Unrealistic goals for the establishment of biomass plantations (of any species), will prove difficult to achieve.

• Some researchers have an inherent bias against pine, since there are few researchable bottlenecks for large-scale establishment of pine plantations.

• Some individuals have ignored the law of diminishing returns when stating that increasing silviculture intensity will lower the unit cost of producing pine biomass.

• In order to achieve 5.3 million ha of short-rotation plantations by the year 2022, it is first necessary to be able to produce a sufficient quantity of planting stock (perhaps 1 billion plants per year for energy crops plus an additional 0.9 billion seedlings for longer-rotation pines).

4.3.1 Establishment

Successful establishment is critical for Miscanthus because it ensures biomass production in the year of planting [63-65], which in turn improves its frost tolerance in areas such as northern Europe [24]. In addition, a well established crop ensures rapid growth in the second year, with more nutrients translocated to the rhizome upon which winter survival and re-growth depend [24, 66, 67].

The sterile hybrid Miscanthus x giganteus, which must be vegetatively propagated, makes up the majority of Miscanthus currently cultivated in Europe [65]. Miscant — hus x giganteus is propagated using either macro or micropropagation methods. In macro­propagation, small rhizome sections are obtained through mechanical division and planted out. In micropropagation, plantlets are generated via tissue culture and then established in the field. Other genotypes, such as M. sinensis, can be propagated either vegetatively or by seed [68,69].

Successful establishment depends on many factors acting individually or in combination [63,64] (Table 4.2).

Rhizome or macropropagation appears to be the best propagation method because:

• Seed production is limited in northerly latitudes where the growing period is too short to ensure sufficient flowering and fertile seed production. Soil temperatures in spring in some areas such as northern Europe (Denmark, UK, etc.) are not high enough for seed germination [64,65, 75].

• Micropropagation results in a lower survival rate during winter of the first year [66, 75, 84-86] and is more expensive [64, 69, 75] than macropropagation. It is more suited to areas such as southern Europe with mild winters and low frost risk.

• Stem segment propagation requires high temperatures (about 25-30°C) to be successful [64,87]. Such conditions are not achieved in temperate climates. This method also results in lower emergence rates than rhizome propagation [87] and is impractical as the best time to cut stems is in late summer while planting occurs in spring, making it necessary to store the stem-propagated plants over winter [75].

Much of the research on establishment of has been done in Europe, where Miscanthus is being investigated as a bioenergy feedstock [88, 89]. However, the findings can be transferred to other environments, such as North America [39].