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Forest plantations play a crucial role in addressing the growing demand for wood-based products and biomass for renewable energy. However, their expansion is often limited by competition with food crops. Agroforestry, which integrates trees within agricultural plots, and mixed forest plantations have emerged as promising ways to reconcile these needs, especially when they include nitrogen-fixing species. This study evaluates the performance of mixed forest and agroforestry plantations including nitrogen-fixing species, such as alder and alfalfa, compared to monocultures. Based on an experimental trial in northeastern France, we compared the biomass production, and its time course, for poplar (Populus nigra × P. deltoides) and alder (Alnus glutinosa) trees in monocultures to their biomass production when associated together in the same stand, and when associated to crops (poplar with alfalfa and alder with grass) at three different scales (tree, plantation, and cropping system). After nine years of growth, at the tree level, both poplar and alder biomass was higher in the agroforestry systems than in the forest mixture or the monocultures. However, at the plantation level, the wood yield of the poplars and alders did not significantly differ between the mixtures and the monocultures at the end of the rotation. At the cropping system level, the two agroforestry systems exhibited a Land Equivalent Ratio significantly superior to 1, indicating higher productivity in these systems compared to crop and tree monocultures. The two agroforestry systems including nitrogen fixators showed very promising results, while the poplar and alder association was disappointing. • Poplar and alder biomass was higher in the agroforestry than in the monocultures. • Wood yields did not differ between the mixtures and the monocultures. • The 2 agroforestry systems exhibited a Land Equivalent Ratio largely superior to 1. • The growth of the non-fixing species was stimulated by the presence of the N 2 -fixer. • The poplar and alder association was disappointing. [ABSTRACT FROM AUTHOR]

• 148 case studies, from 34 plantations, were inventoried from the literature. • Mixed-tree plantations were 18% more productive than the non-N 2 fixing monocultures. • The effect was significantly different from 0 under temperate conditions only. • The effect was negatively correlated with biomass production in the monoculture. • The success of the mixture seems limited to low productivity sites. The inclusion of N 2 -fixing tree species in tree plantations has the potential to increase biomass production compared to monocultures. Both successes and failures have been described in the literature; however, it is still difficult to distinguish a general pattern and to disentangle the factors influencing the mixture effect. The first objective of this study was to provide an overview of the published data on the effect of the introduction of N 2 -fixing trees in tree plantations through a meta-analysis approach and to calculate a mean effect of mixed-tree plantations on biomass production compared to monocultures of the non N 2 -fixing species in stands 2–20 years of age. The second objective was to evaluate the effects of (1) climate zone (temperate vs. tropical), (2) the species used (eucalypts vs. other non N 2 -fixing species, and leguminous tree species vs. other N 2 -fixing species), (3) the proportion of N 2 -fixing species compared to the non-fixing species, and (4) plant developmental stage. A total of 148 case studies from 34 experimental plantations under tropical (68 case studies) and temperate (80 case studies) conditions were identified from the literature. The global mixture effect was significantly positive, mixed-tree plantations being 18% more productive than the non N 2 -fixing monocultures, and this effect was significantly different from zero under temperate conditions (24% more productive) but not under tropical conditions (12% more productive). Indeed, the sites where the positive mixture effect was significantly different from zero were mostly located in a temperate climate, where soil nitrogen is generally considered less available than in tropical latitudes. Intermediate and high proportions of N 2 -fixing species gave similar positive results (27% more productive), while low proportions had no significant impact. Neither plantation age nor type of N 2 -fixing species (legume trees vs. other N 2 -fixing species) had any significant effect. In conclusion, it appears that climate is the main factor influencing the success of the mixture; however, it also seems that the degree of mixture success is more marked on sites with low biomass production where the monoculture is the least productive. [ABSTRACT FROM AUTHOR]

Trees have a long lifespan and must continually adapt to environmental pressures, notably in the context of climate change. Epigenetic mechanisms are doubtless involved in phenotypic plasticity and in stress memory; however, little evidence of the role of epigenetic processes is available for trees growing in fields. Here, we analyzed the possible involvement of epigenetic mechanisms in the winter-dormant shoot apical meristem of Populus × euramericana clones in memory of the growing conditions faced during the vegetative period. We aimed to estimate the range of genetic and environmentally induced variations in global DNA methylation and to evaluate their correlation with changes in biomass production, identify differentially methylated regions (DMRs), and characterize common DMRs between experiments. We showed that the variations in global DNA methylation between conditions were genotype dependent and correlated with biomass production capacity. Microarray chip analysis allowed detection of DMRs 6 months after the stressful summer period. The 161 DMRs identified as common to three independent experiments most notably targeted abiotic stress and developmental response genes. Results are consistent with a winter-dormant shoot apical meristem epigenetic memory of stressful environmental conditions that occurred during the preceding summer period. This memory may facilitate tree acclimation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Highlights: [•] Planting density affects growth and WUE depending on pedoclimatic conditions. [•] Under favorable conditions, increasing planting density accentuates plant competition for light. [•] Under limiting water availability increasing planting density accentuates plant competition for water acquisition. [•] No trade-off between growth potential and WUE. [Copyright &y& Elsevier]

Abstract: Short rotation coppice culture systems are characterized by a high stool and shoot density, and by rotation durations between 2 and 3 years for species showing an extremely fast growth such as poplar. With the objective to study the long-term biomass production evolution and dynamic of such systems, a high-density plantation with 17 poplar (Populus) clones belonging to six parentages was established in April 1996 and coppiced in December 1996, January 2001, and February 2004. At the end of each growing season, stool survival, shoot diameter, and the number of shoots per stool were measured. Biomass production was estimated directly by harvesting a number of shoots or indirectly via allometric power equations. Stool survival rates differed among parentages and from year to year irrespective of number of rotations; the highest rates were for P. nigra and the lowest rates were for P. deltoides × P. nigra and P. deltoides × P. trichocarpa parentages. In the second and third rotations, the P. trichocarpa × P. deltoides hybrids showed very high mortality rates. The number of shoots per stool varied among rotations and parentages, and shoot elimination was parentage specific. The number of shoots per stool increased with increasing rotation number, while they decreased from year to year within each rotation. A single general allometric equation linking shoot diameter to shoot biomass could be used to estimate biomass production for most years and most clones. Exceptions were found for the year 2001 and for clone Hazendans. Biomass production differed among parentages and among rotations. The different parentages also differed in their strategy of biomass accumulation. Although P. nigra had the highest biomass production over 11 years, the parentage ranking of biomass production shifted over rotations. The P. trichocarpa × P. deltoides hybrids showed high biomass production in the first rotation, but low in the second and third rotations. Biomass production increased from year to year within one rotation, while it decreased with rotation number. Among the parentages examined, P. nigra and P. trichocarpa were the best performing ones and the best adapted to the multiple coppice rotations. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]