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Bioenergy will be one of the most important renewable energy sources in the conversion from fossil fuels to bio-based products. Short rotation coppice Salix could be a key player in this conversion since Salix has rapid growth, positive energy balance, easy to manage cultivation system with vegetative propagation of plant material and multiple harvests from the same plantation. The aim of the present paper is to provide an overview of the main challenges and key issues in willow genetic improvement toward sustainable biofuel value chains. Primarily based on results from the research project "Optimized Utilization of Salix" (OPTUS), the influence of Salix wood quality on the potential for biofuel use is discussed, followed by issues related to the conversion of Salix biomass into liquid and gaseous transportation fuels. Thereafter, the studies address genotypic influence on soil carbon sequestration in Salix plantations, as well as on soil carbon dynamics and climate change impacts. Finally, the opportunities for plant breeding are discussed using willow as a resource for sustainable biofuel production. Substantial phenotypic and genotypic variation was reported for different wood quality traits important in biological (i.e., enzymatic and anaerobic) and thermochemical conversion processes, which is a prerequisite for plant breeding. Furthermore, different Salix genotypes can affect soil carbon sequestration variably, and life cycle assessment illustrates that these differences can result in different climate mitigation potential depending on genotype. Thus, the potential of Salix plantations for sustainable biomass production and its conversion into biofuels is shown. Large genetic variation in various wood and biomass traits, important for different conversion processes and carbon sequestration, provides opportunities to enhance the sustainability of the production system via plant breeding. This includes new breeding targets in addition to traditional targets for high yield to improve biomass quality and carbon sequestration potential.

Animal manure represents a major source of renewable energy that can be converted into biogas using anaerobic digestion. In order to most efficiently utilize this resource, it can be co-digested with energy dense, high biomethanation potential feedstocks such as energy crops. However, such feedstocks typically require pretreatments which are not feasible for small-scale facilities. We investigated the use of single-stage and the sequential co-digestion of comminuted but otherwise non-pretreated Salix with animal manure, and further investigated the effects of coppicing frequency and clone choice on biomethanation potential and the area requirements for a typical Swedish farm-scale anaerobic digester using Salix and manure as feedstock. In comparison with conventional single-stage digestion, sequential digestion increased the volumetric and specific methane production by 57% to 577 NmL L-1 d-1 and 192 NmL (g volatile solids (VS))−1, respectively. Biomethanation potential was the highest for the two-year-old shoots, although gains in biomass productivity suggest that every-third-year coppicing maybe a better strategy for supplying Salix feedstock for anaerobic digestion. The biomethane production performance of the sequential digestion of minimally pretreated Salix mirrors that of hydrothermally pretreated hardwoods and may provide an option where such pretreatments are not feasible.

Short rotation willow coppice (SRWC) is a recently commercialised agricultural crop in Sweden, producing biomass for energy. The expected lifespan of a SRWC plantation is more than 20 years, or at least 5 harvests. Consequently, understanding of long term stand development in relation to management, plant material and site characteristics is of great importance. The focus of this thesis is on SRWC biomass production during later cutting cycles. Long term development and dynamics have been studied during three cutting cycles by means of annual census of individual stools, scaled up to stand level. Methods for non-destructive biomass assessment, by means of allometric relations, have been developed and validated for different clones. A method for predicting willow shoot growth in field trials, under different nutrient and water conditions, based on characteristics found in pot-grown plants under corresponding conditions, has also been investigated. The best non-destructive method for assessing shoot weight of clones with bow-shaped and branching stems was found to be shoot dry weight related to the sum of the cross sectional areas of all shoots. In straight, un-branched clones, the measuring height could be elevated to 105 cm above stem base without loosing precision. A comparison of a destructive and a non-destructive method, applied on 12 different clones, showed a mean deviation of 2.5%. Stool mortality during the 1st cutting cycle was non-density dependent. Biomass production increased in the 2nd cutting cycle but high density dependent stool mortality at the end of the 2nd cutting cycle negatively influenced the production in the 3rd cutting cycle, which was lower than both the 1st and 2nd cutting cycle. A stool size hierarchy was established early and prevailed through all cutting cycles. Stool mortality occurred mainly among small stools. In the beginning of the 4th cutting cycle, biomass production stabilised due to compensatory growth of the remaining stools. Total leaf area and total nitrogen pool of pot-grown plants were good clone-specific characters for predicting shoot biomass growth in the field during the first cutting cycle and may be used for shortening the time needed to characterise new clones. The results suggest that sustainability in SRWC systems is enhanced by matching clone and site and adapting fertilisation and harvest timing to actual stand development.