• Energy Research

AbstractBioenergy 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.

Quantitative trait loci (QTL) for growth traits and water-use efficiency have been identified in two water regimes (normal and drought-treated) and for a treatment index. A tetraploid hybrid F2 population originating from a cross between a Salix dasyclados clone (SW901290) and a Salix viminalis clone ('Jorunn') was used in the study. The growth response of each individual including both above and below ground dry-matter production (i.e. shoot length, shoot diameter, aboveground and root dry weight, internode length, root dry weight/total dry weight, relative growth rate and leaf nitrogen content) was analysed in a replicated block experiment with two water treatments. A composite interval mapping approach was used to estimate number of QTL, the magnitude of the QTL and their position on genetic linkage maps. QTL specific for each treatment and for the treatment index were found, but QTL common across the treatments and the treatment index were also detected. Each QTL explained from 8% to 29% of the phenotypic variation, depending on trait and treatment. Clusters of QTL for different traits were mapped close to each other at several linkage groups, indicating either a common genetic base or tightly linked QTL. Common QTL identified between treatments and treatment index in the complex trait dry weight can be useful tools in the breeding and selection for drought stress tolerance in Salix.

Increasing loss of biodiversity in agricultural landscapes is often debated in the bioenergy context, especially with respect to non-traditional crops that can be grown for energy production in the future. As promising renewable energy source and additional landscape element, the potential role of short rotation coppice (SRC) plantations to biodiversity is of great interest. We studied plant species richness in eight landscapes (225 km2) containing willow and poplar SRC plantations (1,600 m2) in Sweden and Germany, and the related SRC α-diversity to species richness in the landscapes (γ-diversity). Using matrix variables, spatial analyses of SRC plantations and landscapes were performed to explain the contribution of SRC α-diversity to γ-diversity. In accordance with the mosaic concept, multiple regression analyses revealed number of habitat types as a significant predictor for species richness: the higher the habitat type number, the higher the γ-diversity and the lower the proportion of SRC plantation α-diversity to γ-diversity. SRC plantation α-diversity was 6.9 % (±1.7 % SD) of species richness on the landscape scale. The contribution of SRC plantations increased with decreasing γ-diversity. SRC plantations were dominated more by species adapted to frequent disturbances and anthropo-zoogenic impacts than surrounding landscapes. We conclude that by providing habitats for plants with different requirements, SRC α-diversity has a significant share on γ-diversity in rural areas and can promote diversity in landscapes with low habitat heterogeneity and low species pools. However, plant diversity enrichment is mainly due to additional species typically present in disturbed and anthropogenic environments.

The short rotation biomass crop willow (Salix genera) has been of interest for bioenergy but recently also for biofuel production. For a faster development of new varieties molecular markers could be used as selection tool in an early stage of the breeding cycle. To identify markers associated with growth traits, genome-wide association mapping was conducted using a population of 291 Salix viminalis accessions collected across Europe and Russia and a large set of genotyping-by-sequencing markers. The accessions were vegetatively propagated and planted in replicated field experiments, one in Southern Sweden and one in Central Sweden. Phenology data, including bud burst and leaf senescence, as well as different growth traits were collected and measured repeatedly between 2010 and 2017 at both field environments. A value of the plasticity for each accession was calculated for all traits that were measured the same year in both environments as the normalized accession value in one environment subtracted by the corresponding value in the other environment. Broad-sense accession heritabilities and narrow-sense chip heritabilities ranged from 0.68 to 0.95 and 0.45 to 0.99, respectively for phenology traits and from 0.56 to 0.85 and 0.24 to 0.97 for growth traits indicating a considerable genetic component for most traits. Population structure and kinship between accessions were taken into account in the association analyses. In total, 39 marker-trait associations were found where four were specifically connected to plasticity and interestingly one particular marker was associated to several different plasticity growth traits. Otherwise association consistency was poor, possibly due to accession by environment interactions which were demonstrated by the low structure adjusted accession correlations across environments (ranging from 0.40 to 0.58). However, one marker association with biomass fresh weight was repeatedly observed in the same environment over two harvest years. For some traits where several associations were found, the markers jointly explained over 20% of the accession variation. The result from this study using a population of unrelated accessions has given useful information about marker-trait associations especially highlighting marker-plasticity associations and genotype-by-environment interactions as important factors to take account of in future strategies of Salix breeding.

Abstract This study assessed the effects of stand structure and fertilisation with wood ash and/or sludge on wood fuel quality of Salix viminalis. The relative proportions of bark and wood in 1-, 2- and 3-year-old shoot populations were determined. The concentrations of essential elements (N, P, K) and heavy metals (Cu, Zn, Cd, Ni) in bark and wood were used to assess the wood fuel quality in harvestable shoot biomass. Controlled field experiments were conducted on two newly harvested commercial short-rotation willow coppice fields. Five treatments were applied: sewage sludge at the maximum legally permitted amount; ash; two sludge–ash mixtures supplying the maximum and twice the maximum permitted sludge–ash amount; and a control receiving mineral nutrients only. The proportion of bark in the willow stands was decreasing with the age of the shoot population. The shoot population with few large stems, compared to that with many small stems, had a lower proportion of element-rich bark in the harvestable shoot biomass, meaning better quality of the wood fuel. Overall, wood fuel quality in terms of mineral concentrations was influenced by the age of the shoot population at harvest, stand structure, management practices (e.g. planting density, fertilisation) and site conditions (soil type, element availability). Our results imply that harvestable shoot biomass of willows grown as few large stems have better wood fuel quality, compared to harvestable shoot biomass of many small stems. Increased length of cutting cycle improves the wood fuel quality.

Abstract Willow (Salix spp.) is an attractive biomass resource for many regions, but is today grown commercially mainly in cool-temperate areas. It is unclear whether modern willow hybrids bred for cool-temperate climate are capable of regulating strong water losses when exposed to warm and/or dry climate. The objective was to assess leaf scale water relations (evapotranspiration, E; and stomatal conductance, GS) and corresponding leaf traits in six wild and hybrid willows field-grown in Central Sweden (cool and well-watered), Northern Portugal (warm and dry), and Northern Italy (warm and well-watered). Diurnal courses of E, GS and leaf temperature were recorded, plant heights measured, and leaves sampled for assessment of specific leaf area (SLA) and area-based leaf N content (Na). Height growth, GS, SLA and Na varied between the genotypes, but genotype environment interaction was important only for plant height and GS. Thus, genotypic variation in leaf scale E was mostly caused by stomatal (GS) and not by non-stomatal (leaf temperature) genotypic variation. Leaf scale E was positively correlated with Na when assessed across the drought gradient. It is concluded that the willow hybrids bred for cool-temperate climate (in Scandinavia) are capable of regulating strong water losses when exposed to warm and/or dry climate (in Southern Europe), provided that water supply is good. The ability to regulate water losses under warm and dry conditions in the short term is a pre-condition to high water use efficiency and improved growth in warm and dry environments also in the long term.