Bioenergy from trees
This article is corrected by:
- Errata: Corrigendum Volume 193, Issue 1, 288, Article first published online: 19 October 2011
26th New Phytologist Symposium: Bioenergy trees, INRA Nancy, France, 17–19 May 2011
Any cursory look at current newspaper headlines reveals escalating food prices, increased demands for energy consumption, and – often ignoring the unrealized economic costs associated with elevated atmospheric carbon – the rising costs of fossil fuels. Acknowledging that current energy strategies are unsustainable and detrimental to global health, it is not surprising that the UK Parliament, European Union, US Department of Energy, and Chinese Government have all set lofty benchmarks for greater reliance on plant-based bioenergy by the year 2020 (Buyx & Tait, 2011).
‘… lignocellulosic feedstocks have the potential to make a considerable contribution toward future energy benchmarks.’
The 26th New Phytologist Symposium – Bioenergy trees – was therefore a timely opportunity to address the role trees may play in future energy strategies. This intimate and energetic meeting was held at the Institut National de la Recherché Agronomique (INRA) in Nancy, centered in the heart of France’s beautiful Lorraine region. The topic of providing fuel from trees brought together people from disparate backgrounds – plant breeders, chemists, economists, engineers, geneticists, sociologists, and ecologists – and every continent. In conjunction with New Phytologist’s objective of having a mix of both young and more practiced scientists, the meeting allowed ample time for discourse on the development of bioenergy derived from woody biomass.
The desire to maximize bioethanol production from tree-based cellulosic materials was the central theme of the meeting. For the purposes of liquid fuel production, lignocellulosic feedstocks from fast-growing short-rotation tree species arguably show the most promise. Trees provide high productivity on a per hectare basis, just behind the C4 grasses switchgrass (Panicum) and Miscanthus (Jørgensen, 2011). Carefully managed, trees offer an opportunity for sustainable production, and – unlike other bioenergy crops – nonseasonally dependent harvesting and storage. First-generation bioethanol production from either sugar- (sugarcane (Saccharum officinarum) and sugar beet (Beta vulgaris)) or starch- (corn (Zea mays) and sorghum (Sorghum bicolor)) based materials elevates the price of agricultural commodities by subsequently driving up the cost of food (Karp & Richter, 2011). Algal biodiesel shows promise, but photosynthetic bioreactors are costly and still years away from general production (Wijffels & Barbosa, 2010). Despite hindrances to their utilization for bioethanol, lignocellulosic feedstocks have the potential to make a considerable contribution toward future energy benchmarks (Somerville et al., 2010).
Using genomics to optimize tree biomass
The greatest emphasis on tree biomass has been placed on poplar (Populus sp. hybrids), willow (Salix sp.), Eucalyptus, and temperate Pinus species. The promise of a model tree for plant genomics, population genetics, and cell biology came to fruition with the release of the poplar (Populus trichocarpa) genome (Tuskan et al., 2006). More recently, with the ongoing characterization of the poplar pan-genome, efforts have focused on understanding genetic diversity across natural populations, recording detailed phenotypic databases, and mapping genotypic data associated with desirable bioenergy traits (Neale & Kremer, 2011). By extending our basic knowledge of woody plant gene expression – from methylation to transcription factors – we hope to understand the genomic basis influencing plant growth and adaptation. Beyond elucidating plant growth at the genomic level, meeting participants stressed that the two main goals should be to understand the basic mechanisms behind cell wall formation and to reduce the level of chemical recalcitrance of lignocellulosic material in cell walls.
Advanced tree breeding approaches, such as genomic selection, have focused on maximizing both biomass yield and plant health by selecting for traits such as increased carbon partitioning to woody tissues, optimized growth forms, improved hydraulic conductivity, increased pulp yield, and stress adaptability and disease resistance (Wegrzyn et al., 2010). Target genes for maximized bioethanol yield are being identified using various methods, such as quantitative trait locus mapping and genetical genomics (Grattapaglia et al., 2009). Genetic modification techniques are used to promote or alter the expression of already existing genes linked to desirable traits or add genes via transgenic modification.
Understanding cell walls to take them apart
A large portion of the meeting discussion was devoted to addressing how genomics and molecular biology can provide tools for the elucidation and modification of plant cell walls. Lignocellulosic materials, the components of secondary cell walls, are inherently variable in trees but typically consist of cellulose (30–60%), hemicellulose (20–40%), and lignin (15–35%). With an end goal of modifying cell wall composition, researchers have been characterizing pathways (Joshi & Mansfield, 2007) and identifying transcription factors (Legay et al., 2010) underpinning the phenotypic and physicochemical attributes of cell walls, including wood chemistry and ultrastructure, optimum growth parameters, and tree physiology.
The recalcitrance of lignin is perhaps the key scientific challenge for establishing highly efficient biofuels from lignocellulosic biomass (Rubin, 2008) and a number of participants focused on this discussion topic. Goals for plant biologists and wood chemists include producing plants with less lignin and making the remaining lignin more accessible to degrading enzymes. During the meeting, John Ralph (University of Wisconsin, USA) perhaps said it best when he stated ‘What we want to do is make lignin chemically inactive while not making it biologically inactive’.
From field to fuel
A significant benefit of producing woody biomass from trees is the ability to utilize marginal lands not suitable for food or other agricultural crops; however, sustainable cultivation presents a number of potential problems (Taylor, 2008). Details of tree biomass accumulation for bioethanol yields in field settings are minimal and edaphic conditions found frequently on marginal sites, such as poor soil quality, can affect both abiotic and biotic plant stress, amplify susceptibility to insect herbivory and disease, and increase lignin content, all of which may translate into reduced biomass (Karp & Shield, 2008). Trees may be the least nutrient-intensive bioenergy crop, and their symbiotic associations, such as mycorrhizas, may increase water use efficiency and improve phosphorus and nitrogen uptake (Luo & Polle, 2009). Willow is conceivably leading the way in field studies (Karp et al., 2011), evidenced here by many oral and poster presentations underscoring its value in bioenergy experiments. Field trials for tree species of interest are needed, particularly in relation to overall bioenergy yield.
The production of ethanol fuel from woody biomass is accomplished by the process of saccharification, which involves the physical breaking of biomass material, pretreatment, and separation to liquid and solid components (Richard, 2010). To improve saccharification, pretreatments are utilized and these handling steps most often involve high-pressure steam with the addition of lactic or sulfuric acid or sulphur dioxide (Monavari et al., 2011). While lignocellulose pretreatment technologies are still being developed and optimized at the fuel production phase, chemical treatments to improve the yield of biomass feedstocks before harvest were suggested during the meeting discussion. Although the fermentation phase has been well optimized, significant improvement is possible when considering the pretreatment phase of the process.
The use of life cycle analysis (LCA) could provide important insights, particularly as it has been used to scrutinize the efficiency of biofuel production. The meeting consensus suggested that LCA should critically address the feasibility of energetic, economic, and environmental sustainability of cellulosic ethanol. Potential benefits and tradeoffs of cellulosic bioethanol can be investigated comprehensively using process modeling, techno-economic analysis and attributional LCA. To make lignocellulosic bioethanol economically feasible, at least in the immediate future, all components of the input material should be utilized. For example, ethanol can first be produced from carbohydrates, and the remaining soluble organic compounds and solids can be used to produce electricity, biogas, secondary products and agricultural fertilizers (Murphy et al., 2011).
How do we get there from here?
Following discussion of the use of trees in future energy schemes, many of the symposium participants left with more questions than answers, a sign of what had been a very productive meeting. Participants called for more collaborative science across multiple disciplines and agreed that all stakeholders could benefit from the development of more specific research questions.
The meeting consensus recognized gaps in our current knowledge and identified technology bottlenecks existing in three research areas. First, we need to improve our knowledge in planta in an attempt to advance genomic selection, rapid trait identification of bona fide growth characters, and the characterization of pathways for the biosynthesis of cellulose, hemicelluloses, and lignins. Secondly, we must advance our knowledge of how plants perform in the environment and how this translates into increased growth. Numerous researchers stressed our need for field trials or common garden studies to determine natural population variability, develop surveys for novel trait breeding, and make in situ measurements of plant performance. Finally, our current ability to move from field to fuel needs to be improved. To produce even a portion of the bioenergy earmarked in the next decade, a large increase in the scale of cellulosic material processing and number and size of bioreactors is needed.
The greatest obstacles toward the utilization of woody tree biomass, and bioenergy in general, are policy hinderances, at both local and global scales. Noting the difficulty in initiating, changing or enhancing policy, some suggested that we, as scientists, should develop synergistic interactions with government policy makers, at many different levels, from the start. Additional open public discourse on the science of tree transgenics is needed (Strauss et al., 2009) and field to fuel demonstration sites – many of which are underway – will show current technology and collaborative science in effect.
The production of renewable and environmentally sustainable energy is one of the principal goals, if not the most pressing goal, currently facing scientists. Despite numerous and considerable hurdles to accomplishing this goal, the research on lignocellulosic biofuels presented at this meeting suggests that tree-derived woody biomass, in conjunction with other forms of renewable energy, can satisfy a portion of our energy needs. The New Phytologist Symposium: Bioenergy trees was a great success because it opened up scientific discussion and dialogue, communicated cutting edge knowledge from advancing and veteran researchers in the realm of plant-based bioenergy, and outlined research directions and future goals so that we can move from tree pulp to gas pump.
I wish to thank the organizing committee of Francis Martin, Michele Morgante, Andrea Polle, Steve Strauss, Gail Taylor and Jerry Tuskan for their efforts toward an interesting and productive meeting and Brian Ellis for his thoughtful post meeting discussion on the state of lignocellulosic bioenergy.