Volume 5 biomass and biofuel production 5 14 – woody biomass

Volume 5 biomass and biofuel production 5 14 – woody biomass Volume 5 biomass and biofuel production 5 14 – woody biomass Volume 5 biomass and biofuel production 5 14 – woody biomass Volume 5 biomass and biofuel production 5 14 – woody biomass Volume 5 biomass and biofuel production 5 14 – woody biomass

5.14.3 Forestland-Derived Resources

5.14.3.1.2 Environmental sustainability and the collection of primary forest residues

5.14.3.1.3 Economics of recovering primary forest residues

5.14.3.3.1 Primary mill residues

CAImax (maximum current annual increment) It is the slabs, peeler log cores, and sawdust that are generated in incremental growth of a tree or even-aged tree stand the processing of roundwood for lumber, plywood, and during the year when annual growth is maximized pulp

Coppice Creation of a multistemmed (bush-like) woody Rotation age The number of years between planting or crop by cutting the stems and allowing resprouting to resprouting of a tree crop and harvesting of the tree crop

Fuel treatment thinning This material is classified as rotation age may differ slightly

standing and downed trees in overstocked stands that, if Short rotation intensive culture (SRIC) is a silvicultural removed, would leave the forestlands healthier, more system based on short clear-felling cycles (rotations) productive, and much less susceptible to fire hazard generally between 1 and 15 years, employing intensive Fuelwood Wood that is harvested from forestlands cultural techniques such as fertilization, irrigation, and and combusted directly for useable heat in the residential weed control utilizing superior planting material This and commercial sectors and power in the electric utility term was coined early in the development of woody crops

MAImax (maximum mean annual increment) MAI is the terms Short Rotation Woody Crops (SRWC) in the average annual increase in volume or weight of individual United States and Short Rotation Forestry (SRF) in many trees or stands up to a specified point in time MAImax other countries The definitions are basically the same for identifies the year of the growth cycle in which the MAI is SRIC, SRWC, and SRF but the emphasis in the United maximized, which is also the optimum biological States is on the production of wood on agricultural land,

Primary forest residues, also called logging residues This of forestry approaches on forest land Short Rotation woody residue material largely consists of tops, branches Coppice (SRC) is a variant of the above approaches (and and limbs, salvable dead trees, rough and rotten trees, often included within the scope of the above terms) noncommercial species, and small trees This material is whereby the single stem trees are cut after the first year of often left in the forest growth to force a bush form that fully occupies the site

Comprehensive Renewable Energy, Volume 5 doi:10.1016/B978-0-08-087872-0.00520-5 263

within 3 or 4 years due to being planted at very high Single-stem woody crops A term used in this chapter to

densities Unfortunately, the acronym SRC has also been differentiate woody crops grown as single stem trees from

those grown in coppice associated with (Short Rotation Crops) starting in 2008 systems.

when an International Energy Agency/Bioenergy Group Urban wood residues The woody components of municipal

solid waste included both woody and perennial herbaceous crops as (MSW) and construction and demolition (C&D) part of the task study waste wood constitute urban wood residues.

5.14.1 Introduction

There are multiple sources of wood for bioenergy applications that include production of heat, electricity, and biofuels This overview will focus on recent analysis in the United States with brief mention of technology status in other countries The gathering and use of wood fuels for primary space heating and cooking applications will not be discussed

The major new or novel emerging sources of wood for bioenergy and also the potentially largest wood energy feedstock sources worldwide are purpose-grown woody crops produced both in coppice and single-stem production systems both of which are encompassed under the terms short-rotation woody crops (SRWCs) and short-rotation forestry (SRF) Willow species are particularly adaptable to high-density coppice management, but other hardwoods can be utilized In contrast, single-stem woody crop systems are normally planted at densities of 5000 stems per ha−1 or less Most hardwoods managed as single-stem crops in the first rotation will regrow as coppice crops in the following rotations if not replanted Hybrid poplars, cottonwoods, and eucalypts are all examples of hardwood trees being evaluated for bioenergy applications that also exhibit the ability to coppice However, coppicing is not a requirement for bioenergy applications as pines also have considerable potential for use as bioenergy feedstocks

Hardwoods (i.e., poplars and eucalypts) and pines (loblolly pine) will each receive specific attention as primary examples of single-stem woody crops because of the different history of development A 2006 review of the status of worldwide commercial development of bioenergy using energy crops showed that plantings of SRWC or any type of planted wood for bioenergy were still relatively small in most areas of the world [1] Exceptions were the countries like Brazil with 30 000 km2 of eucalyptus plantations largely used to produce charcoal and China with estimates of 70 000–100 000 km2 of woody crops used primarily for ‘fuelwood’ By contrast, Northern Europe, the part of the world with the largest use of willow for bioenergy (primarily district heating), was estimated to have only 180 km2 planted

Two potentially large bioenergy wood resources that already exist worldwide are logging residues from commercial harvesting operations and ‘thinnings’ generated by treatment of forests to reduce fuel loads (also referred to as fuel treatment thinnings) Although these are not ‘novel’ wood resources per se, they are included due to the significant resource potential currently existing and current efforts to reduce extraction and processing costs Issues surrounding the sustainability of these forest production systems are also addressed comprehensively

Immediately available (and lower cost) wood resources are already being obtained from primary and secondary processing wood residues from traditional wood products and urban wood wastes These sources include the bark residuals and black liquors generated by timber processing and paper pulp making and are largely utilized to produce heat and electricity Efficiency of these resources can be improved, and we provide recommendations on the potential expansion of these feedstocks

5.14.2 Novel Short-Rotation Woody Crops/Short-Rotation Forestry for Bioenergy Applications

5.14.2.1 Woody Coppice Production and Harvesting

The production of wood in very short rotations for fiber and energy originated in the United States in the 1960s with the testing of sycamore plantations planted at very high density, harvested at an early age, and allowed to sprout multiple stems (or coppice) for several rotations [2] Most hardwood species have the physiological capability for producing coppice sprouts though differing numbers of sprouts per stump and locations of sprouting buds create differences in form [3] High-density coppice production techniques have frequently been applied to poplars and eucalypts, but willow has undergone the most genetic selection for clones for high-productivity coppice culture [3] Willows have been grown as coppice crops since ancient times for basket making, wine trellises, and other uses Intensive efforts to develop high-yielding willow coppice crops were first initiated by researchers at the Swedish University of Agricultural Sciences, in Uppsala, Sweden, in the 1970s [4] Willow coppice research quickly spread to several other northern European countries, as well as the University of Toronto in Canada, and the State University of New York (SUNY) by the mid-1980s Yield trials of willow coppice are now ongoing in 15 states in the United States and six provinces in Canada (Figure 1)

Commercial implementation of willow coppice technology for energy occurred first in Sweden, with over 16 000 ha planted by the early 2000s The majority of the Swedish plantings occurred between 1991 and 1996 as a result of agricultural subsidies that included willow coppice production on surplus arable land, higher fossil fuel taxes, and an established biofuels market already

Figure 1 Resprouting of willow in western New York, USA, following a dormant season coppice Courtesy of State University of New York – Environmental Sciences & Forestry, SUNY-ESF, Woody Biomass Programs’ online images

using forest fuels [5] Since the late 1990s, new plantings of willow coppice for bioenergy has slowed; however, plantings for phytoremediation application have improved the economic viability of more recent plantings [6] By 2005, Poland had approxi­mately 6000 ha of commercial willow coppice plantations [7] In the United States, the Salix Consortium joined electric utility companies, universities, state and federal agencies, and private companies in the mid-1990s to commercialize willow biomass production With no subsidies available, only about 280 ha of willow biomass crops had been established in New York by 2000 [8] Commercial plantings are slowly expanding in the United States with the development of a commercial nursery to provide planting material Numerous field trials of new willow clones are being tested throughout the northeastern United States and southeastern Canada (Figure 2)

Handbooks available on the web provide excellent guidance on the latest advances in production techniques for willow coppice

[9, 10] although research on production techniques continues [7] Willow coppice grown on good agricultural soils will produce greater yields at an earlier age; however, willow coppice can be grown on soils that are marginal for traditional crops The soils should be imperfectly to moderately well drained, but excessively well-drained (coarse sands) and very poorly drained (heavy clay) soils are considered unsuitable A soil pH between 5.5 and 8.0 is required Current site preparation methods usually involve mowing to remove vegetation, application of a total kill herbicide (e.g., glyphosate), and tillage (no-till methods are being investigated) Effective weed control is critical to successful establishment One advantage to coppice production techniques is that full site occupation is rapidly achieved by the multiple-stem or ‘bush-like’ tree form, thus minimizing the amount of herbicide applications needed during a rotation

Willows are mechanically planted as unrooted dormant cuttings in early spring when the site is accessible Typical machines (e.g., the Salix Maskiner Step Planter® and the Egedal® Willow Planter) cut dormant 1.5–2 m whips of 1-year-old willow into 20 cm sections and insert them vertically into the ground Future planters may take a ‘lay-flat’ approach to establishing willow [11] Commercial willow biomass plantations in Sweden today contain about 12 000 cuttings ha−1 arranged in a ‘double-row’ system where between-row spacing is alternately 1.5 and 0.75 m and within-row spacing is about 0.75 m [12] The Willow Producers Handbook [9] suggests a similar double-row system with tighter within-row spacing, resulting in a somewhat higher density of about 14 760 plants ha−1(Figure 3) Recent research has tested production in stands containing up to 40 000 plants ha−1 [7] In all cases, the plants are cut back after the first growing season in order to promote sprouting (coppicing) Productivity is generally higher in the second and later coppice cycles Harvest of coppiced willows or poplars is conducted every 3–5 years during the period

of dormancy with the norm being 3 years The economic life span of a willow coppice plantation is generally believed to be less than 25 years [12]

Productivity of willow coppice varies greatly depending on soil, climate, management, and all the factors that normally affect yields of agricultural crops, including species, genotype, and rotation Experimental trials of fertilized and irrigated willows, grown in 3 or 4 years coppice rotations, have occasionally yielded more than 27 oven dry Megagrams (odMg) ha−1 yr−1 in the northeastern United States [8, 13], 30 odMg ha−1 yr−1 in southern Sweden [12], and 33 odMg ha−1 yr−1 in Poland [7] Numerous experimental trials in North American and Europe have produced willow coppice yields in the range of 7 20 odMg ha– −1 yr−1 [7, 8,

12, 14, 15] (Table 1) Unfortunately, average commercial yields of willow coppice have generally been lower First-rotation yields

of the first commercial harvests in the United States (winter of 2001/2002) averaged only 7.5 odMg ha−1 yr−1 [8], though second-rotation harvests and new clone harvests are reported to average about 11.4 odMg ha−1 yr−1 [25] Early commercial production in Sweden averaged as low as 2.6, 4.2, and 4.5 odMg ha−1 yr−1 for first-, second-, and third-cutting cycles, respectively, though some farmers achieved yields double or triple the average [12] Proper establishment and tending (including fertilization)

N

Yields of Willow Biomass Crops in Regional

Minne sota

Wl 4.6 odt/acre/year (1999)

St Lawrece Co., Massena, NY

Maine 4.1 odt/acre/year (1993)

Chittenden Co., Burlington , Wisconsin

Vermont 5.5 odt/acre/year (1997) Jefferson Co , Belleville,

NY 6.0 odt/acre/year (2005)

Michigan

New Hampshire 5.2 odt/acre/year (1998)

Madison Co., Canastota, NY 5.0 odt/acre/year (1998) Columbia Co., Arlingtion, Wl Chautauqua Co., Sheridan, New York

7.2 odt/acre/year (1999) NY 3.6 odt/acre/year (1998)

Massa chusetts Onondage Co., Tully, NY

Onondage Co., Tully, NY 4.0 odt/acre/year (1998) Phode lsland 4.9 odt/acre/year (2005)

Connecticut

New Castle Co., Smyrna,

DE 5.2 odt/acre/year (1998) Maryland

Delaware Yields represent the best clone at Queen Annes Co., Queenstown ,

each site at the end of the first

State Boundaries

(1999) Year in the lable indicates

' 2009 Soutl 0 25 50 Figure 2 Map of willow test locations in the United States Courtesy of Tim Volk of SUNY-ESF

Figure 3 Double-row spacing for coppice willow plantings in the United States Courtesy of SUNY-ESF Woody Biomass Programs’ online images

and better clones were linked to higher performing farmers US research has determined that annual fertilization with about

100 kg ha−1 annually of commercial fertilizer or addition of manures or biosolids is needed to obtain commercially viable yields

[25] Modeling of yield potential based on oat crops in Sweden suggests that commercial willow coppice yields could easily be doubled in Sweden with appropriate silviculture [12] Across Europe, yields are estimated to range from 3.5 to 15.1 odMg ha−1 yr−1 [26]

Poplars are frequently included in high-density coppice production trials [14, 20, 21, 27] Considering all else equal, the best coppiced willow clones generally outperform the best poplar clones under coppice management [14, 20] (see comparisons in

Table 1) However, poplars can perform well in high density For example, a high-density (18 000 trees ha−1) species comparison

S viminalis � S purpurea, 14.5 1 (1–4 average) Fertile site 40 000 one clone

T, W, F in Viterbo, Italy P alba clone 24.5–29.8 3 (2) 792, 22, 45 10 000 1999 [19]

(all)

P nigra clone 22.9–29.0 Coppice after first Fertile site 10 000 1999

rotation

Medium-intensity culture – small plot yields

T, W in Central Scotland Populus hybrid 9.0 3.2 (1) 0, 0, 0 10 000 1989 [20]

‘Balsam spire’

Willow ‘Bowles Hybrid’ 14.0 3.2 (1) 0, 0, 0 10 000 1989 [20]

T, W, F in Tully, NY, USA Average willow clones, first 8.4–11.6 4 (1) 100, 0, 0 14 326 Early [8]

Modeled commercial coppice yields

NE in Sweden Better willow clones 8–9 ∼ 4 (1) Lower water (12 to 20) NA [22]

Average Swedish grower Average willow clones 4.4 4.2 (3) 0, 0, 0 to very (12 to 20) NA [12]

d Stem age represents the growth year in which the stand reached maximum mean annual increment (MAImax

e Age of MAImax not verifiable but stand had reached expected harvest age for the planting density and was believed to be close to MAImax

f

g Age of MAImax

h Age selected is year of peak average annual yield of annually coppice harvests between years 3

trial in Canada found two poplar clones that equaled or outperformed the yields of nine willow clones over a 4-year first rotation

[21] Furthermore, recent high-density (10 000 trees ha−1) trials of three poplar species in Italy produced yields as high as 20.9–25.8 odMg ha−1 yr−1 during a second (coppice) rotation with optimal culture conditions and current climate conditions and

up to 28–31 odMg ha−1 yr−1 in elevated CO2 conditions [19] It is likely to be significant that in both high-yield scenarios, poplar trees were not coppiced during the establishment year Most available poplar and eucalypt clones, while adaptable to coppice techniques, appear to perform better if allowed to grow in the single-stem form for at least 2–3 years after planting even if planted at high density [18, 28] Some poplar clones only perform well when planted at much wider spacing or when thinned as soon as crown closure occurs

Harvesting of willow and poplar coppice should only be performed during the dormant season if resprouting is desired Eucalyptus will resprout during most of the year, but most species tested have shown less vigor when cut in late summer [3] Harvesting technology for short-rotation coppice is generally the most expensive portion of its production, and the area is developing rapidly Case New Holland (CNH) has been particularly active in testing and modifying existing harvesting heads for traditional crops Initial field trials of willow harvesting with a new CNH fb130 header were performed in the United States and United Kingdom in 2008 and 2009 Based on the UK harvest trial, the header was able to harvest and chip willow stems up to

200 mm thick and 12.5 m tall at speeds of 12.5 kph This rate would allow harvest of as much as 8 ha day−1 The chips were blown directly into a truck following behind or beside the tractor with the harvest header Other harvesters for woody coppice include sugar cane harvesters made by Austroft, forage harvesters made by Class, various versions of the Bender made by Salix Maskiner, and other harvesters that are adaptations of existing farm equipment Tests have recently been conducted in Italy on poplar coppice with stems between 2 and 7 cm using several types of Class foragers Results showed that harvest costs are a function of field stocking and machine power in fields with annual yields ranging from 9 to 15 odMg ha−1 yr−1 and harvested yields up to 70 green Mg [29] The current trend in wood coppice harvesting is toward powerful units fitted with a very strong harvest header (Figure 4) The economics of willow biomass crop production in the United States has been analyzed using a publicly available cash flow model, EcoWillow v.1.4 (Beta) [30] The EcoWillow model incorporates all stages of willow field production: site preparation, planting, maintenance, and harvesting over multiple rotations The model also includes transportation to an end user The base case scenario in EcoWillow shows an internal rate of return of 5.5% over seven 3-year cycles (22 years) and payback is reached in the thirteenth year Harvesting, establishment, and land rent/insurance are the main expenses making up 29%, 25%, and 18%, respectively, of the total undiscounted costs The remaining costs (undiscounted) including crop removal, transport, administrative costs, and fertilizer applications account for about 28% of the total costs of willow production

Cost reduction can occur both through genetic selection for high yield and more efficient harvesting technology Reducing the frequency of harvesting operations can also reduce costs Additionally, methods to reduce the cost of the planting stock (currently 63% of establishment costs in the EcoWillow baseline) can decrease the overall upfront capital for planting Another strategy for reducing costs is to combine coppice production for bioenergy with provision of phytoremediation or other environmental services that result in additional income This is seen as one of the best opportunities for creating win–win scenarios of providing a profit to farmers as well as keeping the feedstock costs to bioenergy facilities low

5.14.2.2 Single-Stem Hardwoods

Research and commercialization of single-stem hardwood crops (such as hybrid poplars, cottonwoods, eucalypts, and sycamore)

on short rotations for fiber and energy began in late 1960s and early 1970s at several locations in the United States with substantial involvement of the US Forest Service [31–33] However, technology and cultivation practices were developed to a much fuller extent

Figure 4 Picture of Case New Holland coppice harvester and chipper blowing chips into a tractor-pulled transfer bin Courtesy of Tim Volk, SUNY-ESF

Figure 5 Very short-rotation eucalyptus in Brazil Courtesy of Laercio Couto, RENEBIO, www.renebio.org.br

in the United States as a result of the Short-Rotation Woody Crops Program (SRWCP) initiated in 1981 by the US Department of Energy and managed by scientists at the Oak Ridge National Laboratory (ORNL) in Tennessee [34] In the United States and Canada, the concept of growing trees as row-crops on short rotations, was originally (and often still is) referred to as short-rotation intensive culture and defined as

a silvicultural system based upon short clear-felling cycles, generally between one and 15 years, employing intensive cultural techniques such as fertilization, irrigation, and weed control, and utilizing genetically superior planting material [35]

The term SRWC was adopted in the United States around 1989 [36] to focus on the agricultural approach to wood production In Europe, the term ‘short-rotation forestry’ is more frequently used to convey the same concept Although many countries have grown eucalyptus, poplars, cottonwoods, and other hardwoods as row-crops for pulpwood for decades and have, since the 1970s, also considered these hardwoods for energy, there seems to be a recent renewed interest in single-stem short-rotation technology (Figure 5) Many other hardwood species have been evaluated for and found to be suitable for both single-stem and coppice production systems such as sweetgum, sycamores, black locust, birches, beeches, silver maple, and others The short-rotation concept was originally linked to only hardwood culture since most definitions included the concept of relying on coppice regeneration after the first harvest Coppice regeneration is no longer included as an inherent component of single-stem production

of wood, but it remains an option for hardwood species

The Populus genera (including cottonwoods, aspens, balsam poplars, white poplars, and hybrids) contain many species native to Europe, North America, Asia, and North Africa including some of the most studied of all forest tree species A black cottonwood clone was the first tree species to have its genome sequenced, involving the participation of scientists worldwide [37, 38] A review of the silviculture and biology of SRWC in 2006 [39] traces the recognition of the value of poplars back to the Roman Empire as well as ancient Asian cultures The poplars were used in single-stem form for timber, windbreaks, and roadway lining, and as coppiced forms for fuelwood and forage Early explorers carried poplar trees from the Americas and Asia back to Europe and natural hybrids were first recognized in the mid-1700s The first controlled crosses of selected hybrid poplar parents was performed in 1912 in London’s Kew Garden, but by 1924, wide-scale breeding had been initiated in the United States, and by the 1930s, many countries had poplar breeding programs in place [39] Many international and national organizations are dedicated to the study and distribution of knowledge about Populus species Poplars have attained such recognition and study due to their rapid growth characteristics, ease of experimental manipulation and clonal propagation, large phenotypic diversity, ease of hybridization, and more recently availability of a nearly complete genomic map [37] Much research is presently being directed toward using the knowledge gained to develop new clones with special properties that will increase the already high value of poplars for producing fuels and chemicals

Eucalyptus has been characterized as “an ideal energy crop with certain species and hybrids having excellent biomass produc­tivity, relatively low lignin content, and a short rotation time” [40] Though more than 700 species of Eucalyptus exist, most are native only to Australia and nearby islands and less than 15 species are commercially significant Eucalypts are claimed to be the

‘most valuable and widely planted hardwood in the world’, occupying 18 million ha in 90 countries [41] India has large areas of low-intensity/low-productivity plantings, while Brazil has the largest amount of land dedicated to intensive cultivation of eucalypts China has the largest commitment to establishing new eucalypt plantations at a rate of 3500–43 000 ha yr−1 [41]

Brazil leads the world with experience in selecting improved genotypes and developing short-rotation production techniques for eucalyptus [42] Four species of eucalyptus and their hybrids account for 80% of plantations worldwide, and of those, Eucalyptus grandis is the most widely planted species, showing the fastest growth and widest adaptability of all eucalypt species (Figure 6) However, the Brazilian bioenergy eucalyptus plantings are using hybrids of E grandis with combinations of Eucalyptus urophylla, Eucalyptus tereticornis, and Eucalyptus camaldulensis (Figure 5) Several eucalyptus species are being planted in Hawaii and the subtropical regions of the US mainland Eucalyptus genome sequencing is ongoing [43] along with efforts to modify lignin contents

Figure 6 Eucalyptus grandis in pastoral forestry systems in Brazil Courtesy of Laercio Couto, RENEBIO, www.renebio.org.br

and other wood quality traits [41] of importance to bioenergy/biofuel utilization Eucalyptus lignin levels are slightly higher than most other fast-growing hardwoods; therefore, the best, immediate bioenergy use may be the production of electricity (thermo­chemical processes), syngas (which can be transformed to many products), or charcoal for industrial processes, as has been done in Brazil for many years

Single-stem short-rotation research in the United States in the late 1970s and early 1980s focused on outcomes affecting tree density management and mean annual growth In particular, when comparing poplar densities in the range of 500–100 000 trees

ha−1, an abrupt change in the rotation age–density relationship was observed between 2000 and 4000 trees ha−1 such that the age at which maximum mean annual increment was achieved could be reduced by nearly half [34] This led to recommending planting densities in the range of 2500–4000 trees ha−1 (1000–1600 trees per acre) to optimize for rotations of 5–8 years This density range was initially used in many commercial plantings for short-rotation hardwood pulpwood production in the United States However, the desire for product flexibility (for both energy and pulp) led to using lower densities (∼ 700 to 800 stems ha−1) and longer rotations (7–12 years) with a resulting increase in individual stem size with lower bark to wood ratios (Figure 7) Although the planting densities differ, the same interest in product flexibility was recently given as a rationale for renewed interest in research on hybrid poplars and aspens in Sweden [44] A negative consequence of lower densities is the increased risk of weed competition for nutrients and water, requiring higher levels of mechanical and chemical weed control in the early years A possible solution is to plant at higher densities, and then to remove some wood for energy when the stand closes canopy

Planting strategy depends to a great extent on the planned density Multiple-row mechanical planters are almost always used for very high-density plantings (such as the willow planters described earlier) The current approach in the United States to planting most poplar or cottonwood cuttings and rooted hardwoods at commercial densities is to use an experienced planting crew that plants the trees by hand using a dibble stick to create the planting hole and a well-placed stomp of the foot to close the dirt around the hole To facilitate cultivation in two directions for weed control, the field is ‘cross-checked’ with a tractor prior to planting to establish the desired planting pattern The alternative for the planting of many hardwood seedlings and cuttings is to use a single- or multiple-row ‘planter’ pulled by a tractor This involves individuals sitting on the planter and feeding the cuttings or seedlings into a slot Time and labor requirements are reduced, but this approach fails to produce evenly spaced plantings suitable for cross-cultivation The approach is satisfactory for plantings with relatively tight in-row spacing and wide between-row spacing (e.g., 0.5 � 3.0 m spacing), which only require tillage and fertilizer applications in one direction More efficient planter designs are under development The author has observed a prototype multiple-row mechanical planter in operation that can simultaneously plant multiple rows (row number is spacing dependent) of hardwood cuttings with greatly improved speed and accuracy [45] As the demand for novel wood energy crops increases, it is anticipated that multiple new planting equipment designs will become commercially available

Figure 7 Hybrid poplars near harvest age (∼ age 7) in the US Pacific Northwest Courtesy of Lynn Wright, WrightLink Consulting

Poplar and eucalyptus growth rates and yields at harvest are influenced by water availability, fertility, soil, sunlight levels, genetics, and whether the stand has been allowed to reach its maximum mean annual increment (MAImax) The length of the rotation required to achieve MAImax is heavily influenced by the planting density and the response of the trees to competition and growing-degree days Table 2 contains selected representative published data on single-stem hardwood row-crop yields Selected Populus hybrids have achieved highest yields in the United States in the Pacific Northwest where they have access to groundwater or drip irrigation including nutrients (fertigation), long days with plenty of sunshine, and relatively cool nights The best yields achieved are represented by the Populus trichocarpa � Populus deltoides (t�d) hybrid (11–11) grown in very small plots on 4 year rotations where the first rotation was estimated to produce 27.5 odMg ha−1 yr−1 and the second (coppice) rotation produced

43 odMg ha−1 yr−1 (assuming 100% survival) [46, 47] Similar first-rotation yields were replicated by similar t�d hybrids in later small plot studies [48] The production of t�d hybrid 11–11 in larger experimental plots produced a maximum of about

18 odMg ha−1 yr−1 [49], which is more likely to represent the upper yield potential of selected clones grown under optimal conditions on a commercial scale in the US Pacific Northwest For the North Central, Midwestern, and northeastern portions of the United States, yields of selected single-stem Populus hybrid clonal plantings at or near MAImax have ranged from about 9 to

15 odMg ha−1 yr−1 [50–55] in small experimental plantings and less in first-generation larger-scale plantings [51] The best Populus clones differ considerably with each location Pure P deltoides (eastern cottonwood) clones are a better choice for most areas of the southern United States that experience frost and heavy infestation by the fungal disease, Septoria Total aboveground yields of P deltoides grown in operational plantations primarily for pulp in the Mississippi Delta region have been estimated to range from 6.7

to 12.5 [56] But many published results show lower yields for poplars and other hardwoods outside the Mississippi Delta region

[57, 58] Modeling assessments have suggested that fertilized P deltoides stands could yield 20 odMg ha−1 yr−1 or more on bottom­land sites in latitudes above about 35 degrees North, dropping to as low as 5 odMg ha−1 yr−1 on sandy soils in southern Georgia (∼ 31 degrees latitude north) [61] (Figures 8 and 9) The few recently published yield reports [20, 44, 62, 63] on single-stem poplar and other hardwoods produced in Europe (Table 3) appear to fall within the same range as the coppice crop yields summarized in

Table 1

Recent US studies are showing very high potential for Eucalyptus species at US latitudes below about 31 degrees North In central Florida, Eucalyptus species have been observed to yield 17–32 odMg ha−1 yr−1 after 3–5 years of growth on a clay settling area [59]

Table 2 Selected hardwood single-stem yields in North America with culture intensity and N levels included

Stem Total rotation Planting Yield (dry) c age d N, P, K density trees Plant Culture intensity a location Genotype b (Mg ha−1 yr −1) (rotation) (kg ha−1) (ha−1) year References

Experimental yields

T, W, F in US Pacific Northwest P trichocarpa � deltoides 27.5 4 (1)e 225, 0, 0 6 944 1979 [46, 47] (WA) (15 tree plots) clones 11-11 43.0 4 (2)e Fertile site

T, W, I in US Pacific Northwest P trichocarpa � deltoides 18.4 4 (1) 0, 0, 0 Fertile 10 000 1986 [49]

T, W (small plots) in US North Populus hybrids top five 13.5–15.0 6 (1)f 0, 0, 0; 1 076 1995 [50] Central; (WI, MN, IA) clones

T, W, I (small plots) in US Populus hybrids 11.4 7(1) 0, 0, 0 10 000 1981 [51]

T, W (small plots) in US North Populus hybrids 8.7 6(1) 0, 0, 0 10 000 1981 [51]

T, W (large plantings) in US Populus hybrids 4.8–9.5 6 (1) to 0, 0, 0 1 682 1988 [55] North Central, six sites (WI, Average of DN17, DN34, 9 (1)

T with pest control in US North P deltoides 11.5 8 (1) 0, 0, 0 200 1998 [52] Central (IA) 91 � 04–03

W, I (first year) in US Midwest Populus hybrids 1112, 10.6 5 (1)e 0, 0, 0 10 000 2000 [53]

T, W, I in US Northeast (PA) Populus hybrid NE388 12.9 3 (2) 0, 0, 0 21 570 1981 [54]

T, W, F in US Mississippi delta P deltoides multiple 6.7–12.5 10 (1) ∼ 100, 0, 0 1 537–1 685 1980s [56]

d Stem age represents the growth year in which the stand reached maximum mean annual increment (MAImax

e Age of MAImax not verifiable but stand had reached expected harvest age for the planting density and was believed to be close to MAImax

f Age of MAImax

g

with yields of 14–24 odMg ha−1 yr−1 on muck soils [60] Eucalyptus grandis is the highest yielding tree crop in South Florida, while Eucalyptus amplifolia has the advantage of being more frost-tolerant Two new species of interest for United States are Eucalyptus benthamii and Eucalyptus Macarthurii, which have demonstrated both fast growth potential and sufficient frost tolerance to be considered for most of the Gulf and Atlantic Coastal plains of the Southeastern United States [64] Intensively managed eucalyptus plantations in Brazil have recently achieved average, stem-only, productivities of about 22.6 odMg ha−1 yr−1 with current operational rates of fertilization and up to 30.6 odMg ha−1 yr−1 with irrigated Total biomass is likely about 30% higher The study documenting those yields showed that water supply is the limiting factor for plantation productivity in Brazil [65]

Attaining economically viable yields, wherever the location, requires use of clonal material selected for high yield potential, establishment on marginal to good agricultural land, intensive site preparation to minimize weed seeds and break-up hardpans, and weed control until crown closure Weed control is preferably accomplished with only herbicide applications, but cultivation is often also necessary to achieve adequate control Except in very high fertility areas, some fertilization with nitrogen (N), phosphorus (P),

and potassium (K) will likely be required, but should be applied no sooner than the second year of growth to avoid stimulating weed competition Additional applications may be needed every other year Efforts should be made to minimize wasteful fertilizer additions by basing application levels on soil and foliage analysis; small additions of micronutrients may be very helpful in some cases The best management approaches for single-stem bioenergy production (adjusted to match clones, soils and climate) should

be expected to result in reaching crown closure by the end of the second growing season, and produce operational harvest yields in the range of 11–16 odMg ha−1 yr−1 Early economic studies suggested that best returns would result from harvesting the stands in the dormant season and allowing coppice regrowth for the second and third rotations [66, 67] However, harvesting equipment, which can efficiently cut both single-stem trees and multistemmed trees grown on 4–10 year rotations, is not available, and the logistical advantages of year-round harvesting to provide a continuous supply are large Coppice management of 4–10 years rotation hardwood stands has been used in the southeastern United States but rarely elsewhere [56] Furthermore, as long as breeding research continues to show the potential for yield improvements of 20–100% within a single rotation, replanting after each rotation remains a viable option [68]

Table 3 Selected hardwood single-stem yields in Europe with culture intensity and N levels included

Total rotation Planting Yield (dry) c Stem aged N, P, K density trees Plant Culture intensity a location Genotype b (Mg ha−1 yr −1) (rotation) (kg ha−1) (ha−1) year Reference

Poplar single-stem experimental yields

T, W in Central Scotland, Populus hybrid 11.63 6.2 (1) 0, 0, 0 10 000 1989 [20]

Alnus rubra

T, W, F in Normandy, P trichocarpa � deltoides

T, F in Brandenburg, P trichocarpa � deltoides

d Stem age represents the growth year in which the stand reached maximum mean annual increment (MAImax

e Age of MAImax not verifiable but stand had reached expected harvest age for the planting density and was believed to be close to MAImax

5.14.2.3 Single-Stem Softwoods

Pines comprised 32% of the tree species planted for production purposes around the world in 2005 [69] and 83% of tree species planted in the southern United States [70] Softwoods in the southern United States already contribute 40% of the US total annual industrial wood supply of round wood [71] and 40% of southern softwoods is used for pulpwood and composites Since the fiber industry has long used both bark and black liquor to produce energy for running the pulp mills, southern pines are already a significant contributor to US biomass energy Improvements in pine silviculture have resulted in improving southern US pine productivity by a factor of about 6 times since the 1940s, and the number of planted acres of pines increased from zero in the 1940s

to 15.243 million ha by 2006 with loblolly pine (Pinus taeda) being the most commonly planted species on 12.2 million ha [70] The shift from natural pine stands to intensively managed pine plantations for fiber production is one of the major success stories in the world for plantation forestry [72] Loblolly pines are now considered one of the most productive species for bioenergy in the southern United States [73]

Many steps contributed to improving the productivity of loblolly pine in the south [74] Naturally regenerated low-productivity forests were common practice from the 1920s through the 1950s Improved nursery and field planting practices begun in the 1950s [72] resulted in whole tree aboveground yields tripling by the 1970s Seed orchards dedicated to seed improvement were first established in the late 1950s First-generation improved seeds increased the value of plantation wood by 20% and second-generation improved seeds being used now add another 14–23% yield increase Nursery production of superior (larger) bare-root seedlings involve planting improved seeds in specialized beds with controlled conditions for 8–12 months, top pruning, lifting, and careful grading (Figure 10) Planting into a well-prepared and maintained site is critical to rapid growth The importance of hardwood competition control was recognized by the early 1970s First methods of control were entirely mechanical, but by the late 1970s, herbicides were added By 1990, site preparation was predominately chemical, with limited mechanical site preparation involved Fertilization of pine plantations was initiated in the late 1960s but was implemented slowly during the 1970s and 1980s [75] Average productivity increased rapidly from the 1970s to 1990s primarily as a result of implementing the use of improved site preparation, hardwood competition control, and genetically improved seeds At a national level, average yields in managed plantations increased from 1.1 to 5.6 odMg ha−1 yr−1 from 1920 to 1990 (Conversation with John Stanturf verified that reported weights in his 2003 paper and in most other forestry papers are expressed

as total aboveground green weights To be consistent with other biomass literature, all reported green weights are converted to oven dry weights by assuming 50% moisture content.)

Implementation of silviculture and genetic improvements very much accelerated in the 1990s as a result of the nonproprietary research conducted by university–industry cooperatives During the 1980s and 1990s, cooperative research clearly confirmed the benefits to pine productivity of fertilizing with both N and P, especially in mid-rotation Further research published since 2000 has shown the need for micronutrients on certain soil types [72, 76, 77] Several research studies published since 2000 have demon­strated the yield improvement effects of management intensity levels [58, 78–84] Selected examples are shown in Table 4 Third-generation seeds from selected parents were deployed around year 2000 [85]

Figure 10 Southern pine seedlings being lifted Courtesy of Thomas D ‘Tom’ Landis, USDA Forest Service, Bugwood.org

Table 4 Loblolly pine yields from silvicultural trials in southeastern United States

Culture intensity a

location Genotypeb

Yield (dry) c (Mg ha−1 yr −1)

Stem age d

Total rotation N, P, K (kg ha−1)

Plant density trees(ha−1)

Plant year Reference Very high intensity

W, I, F in Bainbridge, GA Improved second-generation

Improved second-generation family 7–56

Average commercial

16.0 (7.1) 15.2 (6.8) 8.3 (3.4)g

[78] [79] [61]

Improved second-generation family 7–56

Seven full-sib first-generation family + mix

11.6 (5.2) 11.2 (5.0) 8.1 (3.6)d

Low-intensity or experimental controls and modeled result

W in Bainbridge, GA Four improved

second-generation families

T (dry site) in Waycross,

GA

Improved second-generation family 7–56

8.1 (3.6)b 8.4 (3.8)

a

b

c

aboveground biomass, except for Borders et al [79]

d Stem age represents the growth year in which the stand reached maximum mean annual increment (MAImax

e Age of MAImax not verifiable but stand had reached expected harvest age for the planting density and was believed to be close to MAImax

f Age of MAImax

g

h

At present, most loblolly pines stands in the South are currently managed for a combination of pulp and timber so that thinning

is incorporated into the management (Figure 11) The stands are planted on average at about 1480 trees ha−1 for a 25-year rotation, with a thinning at age 15 [73] An analysis published in 2010 supported this approach, suggesting that production of loblolly pine exclusively for biofuels using intensive site preparation was unprofitable at yields between 5.3 and 7.8 odMg ha−1 yr−1 [86]

Figure 11 Age 14 loblolly pine planted for pulpwood in the southern United States Courtesy of David Stephens, Bugwood.org

However, the study also showed that the most intensive management approaches were optimal for maximizing yields With many new studies showing the benefits of weed control and fertilization, those practices have become considerably more common [75] Average operational yields in the southeastern United States were reported in 2003 to be about 9 odMg ha−1 yr−1 [74] Companies are predicting future operational yields of 13–18 odMg ha−1 yr−1 as a result of greater intensity of management [74] and deployment

of third-generation seed sources Field studies comparing intensive silviculture to current less intensive practices have demonstrated that total aboveground yields can be increased 2–4 times with complete control of competing vegetation and yearly fertilization [80, 87] Analysis of some scenarios has indicated that although the cost of intensive management is higher, yields are also higher and thus returns are also higher [88] New growth and yield models, not tied to original site index assessments, are needed to more accurately model intensively managed pine plantations and to predict total aboveground biomass yields available for bioenergy (i.e., inclusion of branches and foliage as well as stem and bark)

A likely future scenario for pine management will include markets for both bioenergy, pulp and timber For this reason, a recent proposal suggests alternating planting densities in each row, a tightly spaced row for bioenergy that would be harvested in 7–8 years and a widely spaced row for lumber production to be harvested at 18–22 years [73, 89] In addition, future management techniques are predicted to include ‘clonal plantations, whole rotation resource management regimes, use of spatially explicit spectral reflectance data as a major information source for management decisions, active management to minimize insect and disease losses, and more attention to growing wood for specific products’ [88] Thus, the economic analysis (discussed below) assumes a future scenario when optimal production of loblolly pine produced for energy is clear-cut at around age 8 and replanted with improved genotypes

5.14.2.4 Single-Stem Harvest and Handling

The harvest and handling equipment and systems used to collect and transport single-stem woody crops grown for biomass in the United States are essentially the same as those used for logging of pulpwood-sized trees with a feller-buncher (Figure 12) [90] Current pulpwood logging techniques use several different pieces of equipment for felling, extracting, and transporting pulpwood from forests [91] At a minimum, a feller-buncher fells and stacks whole trees and a second piece of equipment (usually a skidder)

Figure 12 John Deere feller-buncher shown harvesting hybrid poplars is also used for pine harvesting Courtesy of Lynn Wright, WrightLink Consulting