If you are looking for a comprehensive article on poplar for biofuel production, this is for you.
Poplars (Populus spp.) are popular trees for landscape and agriculture use worldwide. They are known as “the trees of the people” (Gordon 2001) and are considered one of the most important families of woody plants for human use. Poplars’ incredibly fast growth has captured people’s interest for many years. Around the world, people have used these trees for thousands of years to build homes, make tools and medicines, and protect river banks. They were also planted for windbreaks and shelterbelts. Poplars were first planted commercially in the Pacific Northwest in the late 1800s, and commercial tree farms expanded during the last 45 years for the pulp and paper industry (Berguson et al. 2010).
Today, poplar uses are expanding to provide environmental benefits such as phytoremediation, soil carbon sequestration, reduction in sediment run-off, improvement in soil quality, and habitat for wildlife (Stanton et al. 2002). Poplars are also widely used for wood, veneer, and bioenergy. Researchers are working to improve poplars for bioenergy, carbon sequestration, phytoremediation, and watershed protection, and some argue that poplars can be an important component of solving twenty-first century economic and environmental problems as both human populations and greenhouse gases rise (Gordon 2001).
In the late 1970s, hybrid poplar was part of the U.S. Department of Energy's (DOE) Bioenergy Feedstock Development Program. The primary target was fuel for cogeneration of heat and electricity (Hansen et al. 1983). The DOE and others are now interested in poplar for liquid fuels. Poplars can be farmed as short rotation woody crops (SRWC) and harvested every two to five years (Balatinecz and Kretschmann 2001). Other short rotation woody crops being considered for biofuel include willows and eucalyptus. Poplar trees, when intensively cultured, can produce substantial amounts of energy ranging from 7,735 to 8,634 Btu/lb (depending on moisture content), which is equivalent to approximately 11 barrels of oil per acre per year (Isebrands et al. 1979).
Poplars are more desirable for biofuels than many other woody crops because of their fast growth, their ability to produce a significant amount of biomass in a short period of time, and their high cellulose and low lignin contents (Fig 1). For liquid fuels, the cellulose provides the carbohydrates to produce bioenergy and the low lignin content makes it easier to extract carbohydrates from the biomass. In addition, the development of poplar genotypes with improved yield, higher pest resistance, increased site adaptability, and easy vegetative propagation has made poplar a commercially valuable energy crop. The DOE also considers poplars to be one of the short rotation woody biomass crops that can be nationally developed (DOE 2011). Poplars have some advantages over other bioenergy crops such as grasses because the wood does not need to be stored, which allows harvest to occur throughout the year.
The search for alternatives to fossil fuels has led to a worldwide increase in poplar research for SRWC systems. In Germany, researchers found that SRWCs reduce environmental impacts when used for biofuels compared to fossil fuels (Roedl 2010). Trials of purpose-grown poplars for biofuels are occurring in Virginia (Brunner et al. 2009), the Southeast, the Lake States, and the Pacific Northwest (Advanced Hardwood Biofuels Northwest 2013a). These trials are funded by private industry, state universities, and federal programs such as the USDA’s Agriculture Food and Research Initiative and the Regional Biomass Feedstock Partnership developed by The Sun Grant Initiative and the US Department of Energy.
In North America, eastern cottonwood (Populus deltoides) and western black cottonwood (P. trichocarpa) are two common poplar species (Fig 2). Eastern cottonwood (P. deltoides) has a wide distribution in the eastern U.S. and Canada, and western black cottonwood (P. trichocarpa) is found in the western U.S., Canada, and Alaska (Fig. 3).
In areas where eastern and black cottonwood ranges overlap and naturally hybridize, researchers observed that these hybrids grew faster than either parent species. As a result, hybrid poplar tree improvement programs have been producing new hybrids for almost 100 years (Dickman and Stuart 1983). Work has also been done to cross native poplars with Asian black poplar (P. maximowiczii) and European black poplar (P. nigra).
The “cottonwood” species, as opposed to other poplars like the aspens, have received the most attention for energy tree farms because of their propensity to interbreed and their ability to reproduce from dormant hardwood stem cuttings. These characteristics allow for rapid, economical propagation and deployment of new, fast-growing hybrids. The “cottonwood” poplars are bred as interspecies hybrids and managed as clones. These clones can be used to establish hybrid poplar tree farms in most of the U.S. where there is sufficient water. Research is also being done to find hybrid varieties that grow well in the alkaline soils of the arid and semi-arid regions of the intermountain West (O’ Neill et al. 2010).
Poplars are genetically diverse and their ability to hybridize creates even more variation among the clones. To optimize biomass production levels, it is important to consider how different clones respond to different climatic factors (Zalesny et al. 2009) and select the appropriate varieties for each region and sites within regions.
Poplar energy plantations will probably occupy idle, retired, or low-productivity cropland in order to avoid competition with food production. Careful attention to proper clonal selection, site preparation and weed management, fertility, and moisture will be critical to ensure success on these “marginal” sites. In drier regions, irrigation may be required.
Trees in poplar bioenergy farms can be planted at 1,500 trees per acre, which is much closer together than those intended for more traditional uses such as pulp and paper, which range from 34 to 360 trees per acre (Stanturf et al. 2001). This is done to speed the accumulation of biomass per acre rather than produce large individual trees. Studies are underway now to determine the economics of various planting systems. Depending on the management system being used, poplar energy farms may contain as few as 700 to as many as 5,700 trees per acre (Miller and Bender 2012). Low-density tree farms are much less expensive to plant and produce fewer but larger stems, which may not be ready to harvest for eight to ten years. High-density plantings are expensive to plant but produce many small stems and more biomass that are ready to harvest within two or three years. Also of importance is the spacing between the stems. Optimal spacing studies are currently underway at poplar tree farms in the Pacific Northwest. Researchers are investigating optimal spacing for bioenergy, such as twin-row spacing with trees at three feet intervals within each row and ten feet between the twin rows.
Planting can be done with unrooted hardwood cuttings 0.5 to 3 feet long with viable buds or with rooted cuttings that are bare root or in containers (Stanturf et al. 2001; Fig. 4). Many nurseries have hybrid poplar available to the public. The preferred cuttings or planting stock will be available from organizations researching the best clones for biofuels, including GreenWood Resources, Inc. in the Pacific Northwest and ArborGen in the southeastern U.S.
In early spring after the ground has thawed, unrooted cuttings are generally planted manually. Efficient mechanical planting methods are also being developed (van Oosten 2006). In general, three quarters of the cutting should be placed in the soil with at least two inches above ground. It is important to place cuttings in the soil with the buds pointing upright. For longer cuttings, more of the cutting may be above the surface to promote the growth of multiple shoots. The unrooted cuttings develop adventitious roots and the first leaves appear within a few weeks of planting.
Since poplar grows well in poor soils, fertilizer inputs are low compared to other crops. Nutrient inputs need to be based on local soil analysis. Nitrogen fertilizer application has not produced significant yield increases in preliminary Lake States’ trials. Weed control should only be needed for one or two years until the poplars form a closed canopy. Pesticides may be needed to inhibit cottonwood leaf beetle and other pests. There are several common damaging or fatal diseases, including leaf rusts and stem cankers such as Septoria musiva (Newcombe et al. 2001). Planting pest-resistant clones is the most effective way to avoid losses from pests. Fortunately, pest resistance can be bred into poplar clones, but there are strong regional differences in response. In the Mississippi River Valley, for example, native eastern cottonwoods are less susceptible to disease than the hybrid poplars. In the Pacific Northwest, poplar hybrids with a P. trichocarpa parent do well but they tend to be highly sensitive to the diseases that are present in the Lake States region.
As seen in the video by Advanced Hardwood Biofuels Northwest (2013c), poplar utilized for biofuels will be grown as SRWC. The rotation cycles will be much shorter than traditional poplar plantations. The length of the cycle will vary in different regions, with longer ones in more northern latitudes and shorter ones in more southern latitudes. In New York and Michigan, studies have been done on three-year rotations (Tharakan et al. 2003). In the Pacific Northwest, seven-year rotations have been studied (DeBell et al. 1996) and currently there are research trials on two- or three-year rotations (Advanced Hardwood Biofuels Northwest, 2013b). The poplar biomass can be harvested again in another two to three years as sprouts emerge from cut root stalks (coppice), forming new stands of poplar. This pattern can be repeated several times before replanting is required.
For small trees grown on two- to three-year rotations, a fully mechanized New Holland harvester may be a more economically attractive option. A modified forage harvester (University of Minnesota 2010) cuts and chips the trees as it moves along the row (FAO 2008; Fig. 5).
After harvest, the chips are ideally shipped directly to the biorefinery. The moisture content at harvest ranges from 40-58%, and the biorefinery is not expected to need dry chips. If the biorefinery is not able to take the chips, they may be stored for the short term where they were harvested.
Poplar yield depends on climate, site quality, clone, age, spacing, and silvicultural conditions and treatments (Isebrands 2007). In general, yield is lower in unirrigated and unfertilized tree farms in the upper Midwest and higher in the Southeast where growing seasons are long and the Pacific Northwest where irrigation and soil moisture may be more abundant. Yield estimates range from 1.25 to 8.61 dry tons per acre per year (Table 1). Geneticists are working on increasing the biomass yield, adaptability, and pest resistance of poplar. To increase yield over time, it will be important to select the best clones for particular sites within each region (Davis 2008).
For poplars to become a successful biofuel crop, there must be a suitable and available land base in close proximity to potential biorefineries. Because poplar may be targeted for marginal and idle lands, there may be difficulty in obtaining water for irrigation. For small landowners to be successful at growing poplar, they may need to form co-ops and obtain specialized technical support. Local communities will also need to accept poplar plantations and biorefineries. The larger political, social, and economic forces, especially the price of a barrel of oil compared to liquid biofuels, will be among the greatest challenges to overcome (Dickman 2006).
If the biorefinery can utilize poplar chips that also contain leaves and bark, harvest can occur during the growing season as well as the dormant season. Research is underway to investigate the conversion technology of producing biofuels from clean poplar chips compared to chips containing leaves and bark (Townsend 2013). Some conversion processes may only be able to utilize clean poplar chips, which will require that the trees be grown on longer rotations and harvested with more traditional timber methods to remove the bark before being sent to the biorefinery.
Landowners will naturally decide which crops they will grow on their land. Understanding the costs and returns from poplar energy crops will allow growers to consider this alternative crop and decide where it would best be grown on their fields.
Similar to other agricultural row crops, poplar production costs may include land rental, site preparation, planting material (cuttings), planting, weed and pest control, fertilizer, irrigation, root stock removal, labor and management, crop insurance, harvesting, and transportation. However, many of these cost factors will differ widely across regions and scenarios regarding the ease of establishing and harvesting poplars. For the best estimate, potential growers will need to gather key information on current farming expenditures of other traditional crops. Land cost is a significant part of the cost of producing poplar as an energy crop, and utilizing marginal or less productive cropland may significantly reduce the cost compared to other productive lands. When planted on marginal land, the break-even prices of biomass feedstock are most sensitive to changes in biomass yield and harvest costs (Khanna and Huang 2010).
Current research on growing hybrid poplar as an SRWC for biofuel production shows great promise to increase profitability. Cash flow models of production costs and expected yields from the DOE (2011) billion ton update show that poplar biomass ranges from $25 to $60 per dry ton. This estimate is comparable to other dedicated biomass production systems and does not include the cost of transportation from the field to the biorefinery. A preliminary study of short-rotation poplars in Michigan showed 4 to 5 dry tons/acre yield where the break-even price of poplar biomass was estimated to be $108 (delivered to the mill) based on $60/ton (James et al. 2010). The preliminary study suggests 60% yield gains in poplar biomass per acre to compete with production cost of corn-based biofuels.
Although the SRWC approach is completely different than long-rotation (10 to 20 years) poplars from the pulpwood industry, there are lessons that could be learned from the previous studies on long-rotation poplar production costs and returns. For example, in the Southeast, Gallagher et al. (2006) estimated that delivered break-even costs of poplar biomass to a pulp mill (assumed transport distance of 31 miles) are between $75.58 to $89.28/dry ton. More research and improvements in poplar yield and production costs may make poplar biofuels more feasible.
Poplar trees provide numerous environmental benefits including conservation of soil, water, and biodiversity. Studies show that short rotation poplars reduce soil erosion and surface runoff (Langeveld et al. 2012). Poplar trees can also be used to remove contaminants from municipal biosolids (Felix et al. 2008) and for phytoremediation (Marmiroli et al. 2011). In agriculture areas, short rotation poplars can reduce stream flow rates and the amount of nitrogen and sediments entering streams (Updegraff et al. 2004). In terms of biodiversity, research in the northern plains and Midwest found an increase in bird diversity in poplar plantings compared with traditional row crops (Hanowski et al. 1997).
Theoretical life cycle assessments (LCAs) of biofuels made from hybrid poplars show their potential to substantially reduce greenhouse gas (GHG) emission compared with traditional fossil fuels (González-Garcìa et al. 2010, Adler et al. 2007). In fact, short rotation poplar may actually be carbon negative, sequestering more carbon than is released (Langeveld et al. 2012). Poplar may be better in this respect than alternative crops like switchgrass and corn-soybean rotations (Adler et. al. 2007). It is important to note that these LCAs depend largely on assumptions about many things, including the cropping system used, the previous land use, the location of the crop, the biofuel conversion system employed, and the fuel end use. Because commercial production of both hybrid poplar energy crops and bio-based fuels is in its infancy, real-world examples on which LCAs can be conducted are generally lacking. Even so, the preliminary LCAs that have been done suggest key areas where cropping systems and fuel production systems might be improved to further improve GHG balances. Key among these are reduction of fertilizer, water, and fossil fuel inputs per unit of biomass grown and transported.
There are environmental concerns with producing poplar-based biofuels, including impacts on water and air quality such as isoprene release during crop growth (Ashworth et al. 2013). Other concerns being examined include GHG debts incurred and nitrogen mobilized during land use change (Nikièma, et. al. 2012). Improvements in production systems should eventually be able to ameliorate these environmental concerns.
Fast growth and wide site adaptability have made poplar a suitable tree species to grow for multiple purposes including biofuels production. In some locations, they also are known for being useless when markets disappear. For poplars to be a viable commercial scale feedstock for biofuels, stable markets must be established and contracts must be made so the burden is not solely on the grower. Co-ops for growers may make this more feasible.
As a biofuel crop/feedstock, poplars may have an advantage over other feedstocks because poplars can potentially be harvested in different seasons and thus provide a continuous supply of feedstock to the biorefinery. Poplars also produce higher amounts of energy than other feedstocks and are predicted to displace more gasoline and diesel than corn, soybeans, reed canary grass, and switchgrass (Adler et al. 2007). The amount of energy produced does depend on poplar management and production input. Moreover, compared to other wood sources, it is much easier to convert hybrid poplar into liquid biofuels. The high genetic variation among poplars and many desirable traits provide an opportunity to develop them as ideal feedstock for biofuels.
Poplars show promise as a feedstock for biofuels. The trees are fast growing and able to produce large quantities of biomass in a short amount of time, easy to propagate and cultivate, and can grow in many regions. Poplar wood can be easily and sustainably converted into liquid transportation fuels. Before poplars can be grown on large scales for biofuels, some technical and management details need to be worked out, including the best clones for bioenergy, disease resistance, optimal spacing, harvesting technology, and available lands for growing poplar. Support will be needed to assist smaller landowners in forming poplar growing co-ops to share technical resources and equipment. For poplar to become an important component of the twenty-first century renewable energy portfolio, research must continue to develop the clones, technologies, and financial feasibility.
Dickmann DI, Isebrands JG, Eckenwalder JE, Richardson J. (eds.) 2001. Poplar Culture in North America. NRC Research Press, Ottawa, Ontario, Canada. 397 pp.