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As a renewable energy resource, woody biomass has a role to play in reducing worldwide fossil fuel consumption. Species in the genera Salix and Populus are widely used for this purpose through short rotation tree plantations under temperate latitudes. In warmer climates, common short rotation species include genera such as eucalyptus or black locust (Robinia) although this latter is also cultivated in temperate climates. The objective of our study was to compare the potential of poplar (Populus deltoides × P. nigra, clone 'Dorskamp') and black locust (Robinia pseudoacacia, provenances 'Nagybudmerii' and 'Nyirseg') for biomass production in the Northwest of France. We estimated biomass production and efficiency of water use of poplar compared to those of the two provenances of black locust, (1) at two planting densities (short rotation forestry: SRF, 1428 trees per ha vs. short rotation coppice: SRC, 7272 trees per ha), and (2) 2 and 4 years after planting. Only few effects of the initial planting density on plant growth and physiology were visible two years after planting. Stem height and circumference became higher in SRF than in SRC for both species after four years. Yield at plantation level of the first SRC rotation was almost twice as high for black locust than for poplar, while both species exhibited similar stem height or stem increase rate likely due to the development of more sylleptic branches in black locust. At the end of the second rotation, plantation yield was significantly higher for poplar due to black locust mortality. The combination of higher leaf N and greater WUE probably enhanced black locust performance under the dry conditions of our study site in 2010. During the fourth year (2012), the gap between the two species in terms of Δ 13 C, used as a surrogate of water-use efficiency, was less marked than during the first years due to the fact that poplar Δ 13 C decreased (increase in WUE), while black locust Δ 13 C slightly increased (decrease in WUE). To conclude, black locust appears as an interesting option for biomass production at sites prone to water shortage. However, at high planting density, poplar exhibited a lower mortality after harvest leading to an improved biomass production at the end of the second SRC rotation compared to black locust. The choice of species/cultivar must be adapted to the production system (SRC vs. SRF) and to the specific site conditions.

International audience; To make intensive bioenergy poplar plantations attractive on an economic and environmental point of view, their sustainable resource management is mandatory. These plantations are indeed often installed on marginal lands to avoid competition with food agriculture and large amounts of nutrients are repeatedly exported in the young harvested biomass. Besides the application of organic residues to reclaim impoverished soils, the use of species or genotypes efficiently using nutrients, and especially nitrogen, is also a way to slow down fertility losses in short-rotation poplar plantations. Our goal in the present study was to document the genotypic variability available in terms of yield, nitrogen-use efficiency (NUE), and nitrogen export rate, the links between these traits, and their plasticity in response to contrasting site conditions and during two rotations in a wide set of Populus deltoides × Populus nigra genotypes. A wide range of variation has been highlighted for biomass production, tree dimensions, and nitrogen use and export at both sites, and particularly at the most productive one. Environmental and genotype by environment effects were very strong and the ranking of genotypes was not maintained between the two sites for most studied traits. Because of climatic conditions less contrasted between sites during the second rotation, the differences were less marked than during the first rotation. The most productive and efficient genotypes to use nitrogen were also those responsible for the highest nitrogen exports during harvest. Indeed, although the most productive genotypes had high NUE, their high biomass production resulted in a more important nitrogen export at harvest than the less productive genotypes.

International audience; Because of their impressive growth rates, interspecific hybrid poplars are among the most extensively cultivated trees in temperate latitudes. In line with their general riparian origin, poplar trees are generally highly water-demanding such that the productivity of poplar plantations is tightly dependent on adequate water availability. However, in the current context of rational management of water resources, optimizing the carbon-water trade-off of such plantations is particularly important. Water-use efficiency (WUE, i.e. the amount of plant biomass produced per unit water used) has therefore emerged as a new relevant criterion to be accounted for in poplar selection, in conjunction with other common criteria already used such as disease resistance, productivity and wood quality. We propose here an overview of nearly 15 years of research on WUE in poplar hybrids (Populus deltoides × P. nigra and P. deltoides × P. trichocarpa, the two most commonly interspecific combinations used in Europe). These results were obtained under the framework of seven Ph.D. theses conducted along the 2000-2013 period. Four main questions will be addressed: (1) Is bulk leaf carbon isotope discrimination a good estimator of WUE assessed either at the leaf level (intrinsic WUE, i.e. net CO2 assimilation rate / stomatal conductance to water vapour) or at the whole-plant level (transpiration efficiency, i.e. biomass produced / water loss)? (2) To what extent does WUE vary among and within hybrid crosses and are hybrid rankings for WUE stable with ageing? (3) How do environmental conditions (drought, planting density, soil fertility) affect WUE? and (4) Are WUE and biomass production related and is this relationship modulated by environmental factors? The practical implications opened up by the results and the future research opportunities will then be presented.

International audience; Short rotation coppice (SRC) willow plantations may play an important role in the future for biomass production purposes. However, the high planting density schemes combined with the frequent harvests occurring in such plantations could rapidly deplete soil resources. The use of genotypes able to produce greater amounts of biomass by using the least water and nutrients may help mitigating this risk. This study aimed at assessing among six willow genotypes (1) the variability of traits related to productivity (e.g. aboveground dry biomass or stem height), leaf and wood nitrogen (N) contents, N exportation rate and water-use efficiency (WUE) as estimated through bulk leaf carbon isotope discrimination (Δ13C), (2) the relationships among traits, and (3) the plasticity of these traits and of the relationships among them across different sites. The six genotypes were grown under SRC at three sites in northern France differing primarily in pedoclimatic characteristics for two years. A significant genotypic variability was found for all traits, except for the N exportation rate. The pedoclimatic context impacted the values of all traits, and the genotypic ranking for traits related to productivity and for Δ13C. Δ13C was negatively correlated with total shoot dry biomass and/or height irrespective of the site, meaning that the most productive genotypes were also the most efficient to use water. In conclusion, no antagonism was detected between biomass production and WUE. The most productive and most water-use efficient genotypes were the ones responsible for the highest nitrogen removal from the plantation during harvest.

International audience; Poplar coppice plantations for biomass production can be conducted under either short rotation coppice (SRC) or short rotation forestry (SRF) systems, depending on planting density and rotation length. It is likely that differences in planting density affect tree physiology through competition for resource acquisition, including light, water and nutrients. In this paper, we hypothesized that the effects of planting density on growth and water-use efficiency (assessed through bulk leaf carbon isotope discrimination, Delta C-13) in poplar depend on site characteristics in terms of soil fertility and water availability. To test this hypothesis, 56 Populus deltoides x P. nigra genotypes were planted under both SRC and SRF and replicated at two sites differing for pedoclimatic conditions. At the most favorable site for growth, trees grown at the higher density (SRC) displayed higher stem height, lower stem circumference, higher specific leaf area, higher mass-based leaf nitrogen contents and higher Delta C-13, indicating that increased tree density mainly accentuated competition for light. Under less favorable conditions, trees grown under SRC still displayed lower stem circumference, higher specific leaf area and higher mass-based leaf nitrogen contents. However, stem height remained unaffected by increasing planting density while Delta C-13 was lower, likely because of increased competition for water availability. Genotypic rankings across planting densities were overall conserved while they were significantly modified across sites, suggesting that rankings for genotypic performances were much less affected by planting density than by site. Realized growth measured after 2 years (height and circumference) was weakly correlated with Delta C-13, but a negative relationship between Delta C-13 and growing season leaf increment rate was observed in most cases. The absence of trade-off between growth and water-use efficiency combined with the large genotypic variations observed for these traits confirms the potential for selecting genotypes with high water-use efficiency without counter-selecting on biomass production in P. deltoides x P. nigra.

This work aimed (1) at characterizing the impact of planting density (PD) and pedoclimatic conditions (sites) on the physiology of poplar and willow dedicated to biomass production, (2) at evaluating the genetic variability of productivity-related traits, water-use efficiency (WUE) and nitrogen-use efficiency (NUE) in a perspective of plant breeding, and (3) at assessing the relationships among these traits. Four experimental plantations were established in northern France. Our results show that PD affects biomass production and WUE depending on sites. Under most favorable growth conditions, trees grown at higher density displayed taller stems, lower stem circumference, and lower WUE, than trees grown at lower density indicating that increased tree density mainly accentuated competition for light. Under less favorable conditions, an increase of PD involved higher WUE and lower stem circumference, likely because of an increased competition for water. Positive links between biomass production and WUE were detected for poplar and willow under environmental conditions where competition for resources was reduced. A large canopy is associated to potentially more important transpiration water losses at plant level. This could be compensated at leaf level through a better stomatal regulation. The absence of antagonism between biomass production, WUE, and NUE suggests the possibility to select genotypes for these three traits independently. Moreover, the Genotype x Site interactions indicated the difficulty to identify good genotypes for a large set of site conditions suggesting that the selection has to be realized depending on the site conditions, Cette thèse visait à (1) caractériser l'impact de la densité de plantation (DP) et des conditions pédoclimatiques sur la physiologie des peupliers et des saules dédiés à la production de biomasse, (2) évaluer la variabilité génétique de caractères liés à la productivité, l'efficience d'utilisation de l'eau (WUE) et l'efficience d'utilisation de l'azote (NUE) dans une perspective de sélection variétale, et (3) à juger des relations entre ces caractères. Quatre dispositifs expérimentaux ont été installés dans le nord de la France. Nos résultats montrent que la DP affecte la production de biomasse et WUE de façon site-dépendante. Dans des conditions de croissance très favorables, les arbres cultivés à forte DP étaient plus haut, moins épais et avaient de plus faibles WUE, reflétant une compétition pour la lumière. Dans des conditions de croissance peu favorables, les arbres cultivés forte DP avaient de plus fortes WUE et de plus faibles circonférences, indiquant une compétition pour l'eau. Un lien positif entre biomasse aérienne et WUE a été détecté pour le saule et le peuplier dans les sites où la compétition était réduite. Une canopée plus développée s'associe à des pertes en eau par transpiration plus importante à l'échelle de l'arbre. Celles-ci sont compensées à l'échelle foliaire par une meilleure régulation stomatique. L'absence d'antagonisme entre WUE, NUE et la production de biomasse suggère la possibilité de sélectionner indépendamment sur les trois caractères. Cependant, les interactions Génotype x Site montrent la difficulté d'identifier des génotypes performants pour des stations d'accueil différentes. La sélection doit être réalisée de façon site-dépendante

International audience

International audience

Black locust is an interesting option for biomass production at sites prone to water shortage because the species combines water-use efficiency and a biomass yield largely superior to that of poplars under the conditions of the study. Black locust (Robinia pseudoacacia L.) is an interesting tree species for woody biomass production. However, its potential for this purpose has been much less studied and characterized than that for species from the Salicaceae family (i.e., poplar and willow). The objective of our study was to evaluate the potential of black locust for biomass production as compared to that of poplar. We estimated biomass production, growth habit, and efficiency of water use of two provenances of black locust (1) compared to those of poplar (Populus deltoides Bartr. ex Marsh. × P. nigra L.), (2) at two planting densities, and (3) 2 and 4 years after planting. Black locust had a very different growth habit, much higher biomass production, and larger water-use efficiency than poplar. These differences were exacerbated during the driest year of the experiment. However, black locust responded very badly to harvesting. Black locust was more productive and more efficient in terms of water use than poplar, especially during the driest year of the experiment.

International audience; Because world resources of fossil fuels are rapidly decreasing, it is necessary and urgent to find alternative sources of energy. By 2020, the European Union aims at reaching 20% of the share of total energy produced by renewable energy. One way to achieve this is to use crops specifically dedicated to the production of woody biomass. Currently, woody biomass comes mainly from logging residues or from the waste wood industry. However, cropping systems specifically dedicated to the production of woody biomass as short rotation coppice are presently gaining in importance. In a context of large-scale production of biomass produced from fast growing woody species, it is important to optimize the production by selecting species and genotypes adapted to local conditions. Plant material can be selected either directly for biomass production or for potential determinants of wood production such as phenology, ramification, leaf growth and composition, or water- and nutrient-use efficiencies (WUE and NUE). Then, wood production can be maximized by playing either on the choice of the plant material (species and genotypes within species) for a given pedoclimatic context and/or on the adopted cultural system (notably through planting densities / rotation length). The prerequisite is to evaluate the strength of the relationships between wood production and its potential determinants, and their reliability (plasticity) for the different species, cultural systems, and pedoclimatic contexts. Studies have already been conducted to analyze the effect of the interaction between genotype and environment. The present study will provide new information about this interaction as it takes into account factors that were not studied concurrently in previous studies. Our experiment proposes to study the influence of genetic and environmental factors (and their interaction, GxE) on phenological, architectural, functional (WUE and NUE), and foliar determinants of wood production. The genetic factor is considered at inter-specific level (comparison of three fast-growing species, poplar, willow and black locust) and at intraspecific level (clonal tests for each species). The environmental factor consists of different pedoclimatic contexts in France and different cultural systems (more or less intensive). Two experimental sites at the West and East sides of France are used in this experiment. These sites are located (1) in French Brittany (47°60’N, 1°91’W) with an oceanic climate and (2) in Burgundy (47°18’N, 5°19’E) with a continental climate. Determinants of productivity such as NUE, WUE, bud and leaf phenology, ramification, growth in height and girth, leaf increment, and foliar structure and function will be analyzed throughout the growing season with repeated measurements and observations (from March to October 2010) at the two sites, for the different clones of the three species, and for different planting densities. This monitoring is done under the framework of a French project called SYLVABIOM (2008 to 2012). The SYLVABIOM project is supported by the Bioenergy thematic of the French research national agency (ANR).