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Key Message Hybrid saplings were more reactive to soil water deficit than Japanese and European larch. European larch had hydraulically safer wood and anisohydric behavior, Japanese and hybrid larch showed isohydric strategy. Abstract Deciduous larch species could be an alternative to evergreen conifers in reforestation, but little is known about drought sensitivity of their saplings. The effect of an experimental drought on hydraulics and quantitative wood anatomy was tested on saplings of European larch (EL, Larix decidua), Japanese larch (JL, Larix kaempferi) and their hybrid (HL). Across species, biomass, transpiration rate and relative water content were higher in controls than in drought stressed trees, but transpiration efficiency was lower. JL had the highest transpiration efficiency under drought, and EL the lowest, coinciding with slower growth of EL. Wood of EL formed before drought was hydraulically safer as shown by higher wall/lumen ratio and lower pit cavity area. EL neither had a significant increase in transpiration efficiency nor a reduction in transpiration rate under drought, suggesting that the stomata remained open under soil water deficit. HL saplings were the most reactive to water shortage, indicated by intra-annual density fluctuations and a decrease in relative water content of the sapwood. Significant reduction in transpiration by HL suggested a higher stomatal sensitivity, while the same leaf surface area was maintained and radial growth was still similar to its best parent, the JL. The latter showed a significantly lower leaf surface area under drought than controls. EL, with its hydraulically safer wood, followed an anisohydric behavior, while JL and HL revealed an isohydric strategy. Altogether, our results suggest species dependent acclimations to drought stress, whereby HL followed the strategy of JL rather than that of EL.

The ongoing climate warming affects most tree species across their biogeographic distribution range. The bottom and the top of an elevational gradient are appropriate environments to observe the effect of climate warming in contrasted temperature conditions.AimsTo retrospectively study the effect of the ongoing climate warming on Larix decidua trees located in warm and cold conditions, i.e., at the bottom (1200 m) and the top (2300 m) of an elevational gradient, respectively.MethodsDendroecological analysis of two groups of Larix decidua trees located at very low (1200 m) and very high (2300 m) elevations. Construction of climatic response curves for annual ring variables. Decomposition of the variance of the response models into their high and low frequency components.ResultsAt Briançon, in the French Alps, the increase of the daily maximum temperature during the last 50 years is already almost 3 °C for the March?October period. The results reveal contrasted behaviors at both ends of the investigated elevational gradient. Basal area increment, ring width, and their earlywood and latewood components increase or level off at the top of the gradient, while they all strongly decrease at the bottom. At the bottom, the low frequency warming effect explains the recent ring width decrease, with no influence of the high frequency temperature variation. At the top, both the low frequency warming effect and the high frequency temperature effect explain each about half of the ring width variation encompassed by the model. Latewood density displays the same trend as ring width, while earlywood density decreases at both ends of the gradient.ConclusionSuch opposed trends of the radial growth time trends between the top (2300 m) and the bottom (1200 m) of the gradient reflects the respectively favorable and unfavorable effects of climate warming at such ends of this Larix decidua elevational gradient. We propose that the strong ring width decrease observed at the warmer bottom announces a coming dieback. The corresponding wood density decrease will affect wood hydraulic properties in a way that is unclear. Fil: Rozenberg, Philippe. Institut National de la Recherche Agronomique; Francia Fil: Chauvin, Thibaud. Institut National de la Recherche Agronomique; Francia Fil: Escobar Sandoval, Margarita. Institut National de la Recherche Agronomique; Francia Fil: Huard, Frédéric. Institut National de la Recherche Agronomique; Francia Fil: Shishov, Vladimir. Siberian Federal University; Rusia Fil: Charpentier, Jean Paul. Institut National de la Recherche Agronomique; Francia Fil: Sergent, Anne Sophie. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche. Instituto de Investigaciones Forestales y Agropecuarias Bariloche. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones Forestales y Agropecuarias Bariloche; Argentina Fil: Vargas Hernandez, J. Jesus. No especifíca; Fil: Martinez Meier, Alejandro. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche. Instituto de Investigaciones Forestales y Agropecuarias Bariloche. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones Forestales y Agropecuarias Bariloche; Argentina Fil: Pâques, Luc. Institut National de la Recherche Agronomique; Francia

International audience; X-ray microdensitometry was performed on small wood samples after fungal exposure to assess wood density loss and its variability within a hybrid larch population. Microdensitometry was applied to thin wood slices obtained from increment cores collected from standing trees before and after 4 weeks of exposure to Coniophora puteana fungus. Density loss rates were measured and compared to mass loss percentages calculated on wood blocks following a 16-week decay test according to CEN TS 15083-1 (2005). The relationship between the standard mass loss and the density loss percentages was weak. Several explanations for the results are presented in this work, and methodological improvements are suggested to achieve a more accurate comparison. The proposed screening test has several advantages over the current standard method, as it is less invasive for the tree, less time consuming, and is therefore better suited for genetic studies and breeding. As predicted, due to extractive content variation and fungus behaviour, density loss after 4 w was more important in earlywood than in latewood and in inner heartwood than in outer heartwood. The new method also better determined the differences in decay among trees within the larch population than the standard method at 16 w.

International audience; AbstractKey messageAdvanced hybridization between first-generation interspecific hybrids is an attractive alternative to breed and mass-produce hybrid seed lots. Nevertheless it faces two major drawbacks, namely hybrid depression and inbreeding depression. Their impact may be highly detrimental on some traits (seed yield) especially for the highest level of consanguinity, but much less on some others (seed quality and early growth) as shown in this study for hybrid larch (Larix x eurolepis).ContextMass-production of 1st-generation larch hybrids (Larix x eurolepis) in interspecific hybridization orchards is problematic. Alternatives are searched such as advanced-generation orchards combining 1st-generation hybrid clones.AimsOur aim was to investigate how important hybrid and inbreeding depressions can be in 2nd-generation hybrids with a special focus on reproductive success (and early growth) and their consequences on the success of advanced-generation hybridization seed orchards.MethodsSecond-generation hybrids were created by control crosses among 18 1st-generation (F1) hybrid clones with 3 different levels of relatedness (OC (outcrossed), HS (half-sib) and FS (full-sib)). Seed yield and quality and growth in nursery were assessed for the 3 groups and compared with related F1 crosses. Hybrid breakdown and inbreeding depression were then estimated.ResultsF1 hybrids are fertile and mating among them is feasible. Nevertheless hybrid depression is observed when passing to 2nd-generation hybrids for crossing success rate (− 20%) and for seed yield (> 50% less filled seed/cone). Hybrid depression is reinforced by inbreeding and more severely with increasing consanguinity levels: negligible for HS, a supplementary drop of over 50% of seed yield is observed for FS. In contrast, seed quality traits as well as first-year growth are little impacted by both hybrid and inbreeding depressions. Nevertheless a great variation in inbreeding depression level exists among mother clones. Overall, total depression overpasses 70% for the FS parent group but is limited to less than 34% for OC parents for a synthetic trait (number of germinated seed/cone) and remains negligible for growth.ConclusionAdvanced hybridization is possible with larch but with a more or less severe impact on reproductive success depending on level of relatedness among the parents. Growth seems much less impacted than reproduction and this is a promising result. The proper choice of 1st-generation parents is unavoidable for success of 2nd-generation seed orchards.

Key message[br/] Despite variable dynamics of genetic diversification at the different altitudinal levels, strong gene flow tends to standardize larch genetic diversity: the larch forest distributed along the altitudinal gradient can be regarded as a single population.[br/] [br/] Context[br/] While in forest tree species many studies focus on the structure of the genetic diversity at the natural range and at the forest stand levels, few studies have worked at intermediate levels like the landscape level.[br/] [br/] Aims[br/] We tried to determine to what degree altitude variation can affect the genetic diversity and the local structure of the genetic diversity of European larch (Larix decidua Miller) at the landscape level.[br/] [br/] Methods[br/] Using microsatellite markers, we determined the between- and within-plot genetic structure and the spatial genetic structure (SGS) of four altitudinal plots distributed between 1,350 and 2,300 m a.s.l. in a European larch forest located in the French Alps.[br/] [br/] Results[br/] A homogenous neutral genetic structure was detected along this gradient. The intensity of the SGS was found to be stronger at 2,300 m and decreased at the 2,000-m plot. It was low or non-existent at the 1,700- and 1,350-m altitudinal levels.[br/] [br/] Conclusion[br/] Our results suggest that the genetic structure observed at the landscape level in this European larch forest was only slightly affected by climatic variation, human activities, or historical events. However, the variation of intensity of the SGS within the altitudinal plots indicates the existence of variable genetic dynamics, despite the globally uniform genetic structure along the altitudinal gradient.

International audience; Forest trees are long-lived organisms subject to repeated environmental constraints throughout their long lifetimes. They have developed various mechanisms enabling them to cope with fluctuating environmental conditions during their life span, and to survive to current climate change. Epigenetics has recently emerged as a powerful set of mechanisms regulating various developmental processes, plant growth and responses to environmental variations. Such epigenetic mechanisms, which may remain stable along tree life or across generations, constitute a source of rapid phenotypic variations potentially improving adaptation of the plants in situations in which naturally occurring mutations are very rare.In this review, we summarize recent advances in forest tree epigenomics. We first draw the particularities of trees and the available (epi) genomics resources and strategies. Then, we discuss the potential contributions of epigenetics to cope with global climate change and regulate various developmental processes, such as developmental transitions during the annual cycle, phenotypic plasticity in response to environmental variations and stress memory, as well as local adaptation. Finally, we propose some challenges for forest management and highlighted the need to take epigenetics into account in forest tree breeding strategies.

Despite the interest foresters have for inter-specific hybrid trees, still little is known about their quantitative genetics. This is especially true for the hybrid (HL) between Larix decidua (EL) and L. kaempferi (JL). Long-term, well-designed, multi-site experiments are necessary to estimate the parameters required for HL breeding programs. This paper presents the results from a diallel mating trial between nine EL and nine JL, set up in three contrasted sites. Growth traits (height, circumference), quality traits (wood density, stem form, heartwood proportion), and bud flush were measured from plantation to up to 18 years after plantation. Wood density and heartwood proportion were assessed using increment cores. We did a spatial analysis to take into account environmental heterogeneity at the tree level, and we fitted a multi-trait, Bayesian MCMC (Markov chain Monte Carlo) genetic model. Our study confirmed, in most situations, that HL expressed heterosis over its best parent for growth traits taking advantage of an early faster growth, with no loss in wood density. However, growth traits showed low levels of heritability. On the other hand, bud flush and stem flexuosity had high heritabilities, and wood density was clearly under JL control. Site-dependent heritabilities were expressed by EL. Additive genetic correlations were presented. The traits with high heritabilities showed high correlation between their performances in pure species and in hybridization, as well as high across-site correlations. The discussion focused on the interest of these genetic parameters for the hybrid larch breeding programs.

Heterosis for growth traits was investigated on 2-yrold seedlings from related intra- and inter-specific progenies collected in hybridisation seed orchards combining Larix decidua (EL) and L. kaempferi (JL). Following taxa identification, seedlings phenology and growth were observed over a complete growing season in two experiments. Start, cessation, duration and rates of growth for the different taxa were explored in relation to hybrid vigour. Variable levels of heterosis were observed, depending on the genetic background of the parental species, the species over which superiority was evaluated, environmental conditions and the traits considered. While significant and consistent differences have been observed between hybrids and their related EL progenies for phenological growth traits, a better timing of the growth rhythm over the growing season together with a greater growth efficiency allowed for a consistently positive heterosis over EL. The situation appeared more complex with JL as heterosis could be either positive or negative. Besides the high variation in heterosis levels, the way heterosis for height built up over the growing season seemed also to be variable according to genetic backgrounds: cumulative constant superiority of the hybrid for growth rhythms along the growing season or greater growth rates during discrete periods of time.

International audience; Genetic variability of modulus of elasticity (MOE) was investigated in three genetic trials, including two progeny (16 years old) and one clonal (19 years old) trials of hybrid larch (Larix x eurolepis Henry). MOE was directly assessed on standing trees using the Rigidimeter, a bending device, and related to other traits including height, BH diameter and wood density. Mean MOE ranged from 5,183 to 9,228 MPa among families in the progeny trials and from 4,591 to 11,486 MPa in the clonal trial. Among traits studied, MOE was one of the most variable traits. It was strongly and positively related to wood density at both the individual and genotype mean levels. Interestingly too, wood stiffness did not seem, or only weakly, unfavourably linked to stem diameter at the phenotypical level, but it was negatively or not correlated to diameter at the genetic level. As well, MOE showed a high GxE stability over the two progeny trial sites. Narrow-sense heritabilities for MOE were moderate (around 0.36). In all three trials, they were lower than those for wood density or total height, and of the same level as for diameter. Improvement of wood stiffness of hybrid larch using the Rigidimeter seems possible and promising genetic gains are expected. Impacts of selection for growth traits on MOE are also discussed.

International audience; The performance of hybrid larch (Larix x eurolepis) stands may be altered by the fact that the seed lots produced in hybridization seed orchards always contain a proportion of non-hybrid seeds. We studied the effect of such hybrid/non-hybrid mixtures on the productivity and quality of young plantations, through a three-step process: determination of tree species identity with cytoplasmic DNA markers, comparison of hybrid and non-hybrid tree performance, and finally, assessment of thinning impact on hybrid percentage. Overall, we analyzed progenies from eight commercial seed orchards at three sites. Huge variations in hybrid percentage were found among orchard progenies. Globally, the results concerning survival, susceptibility to drought and to Meria laricis, growth, stem form and wood stiffness were clearly in favor of hybrids. In particular, they grew faster than the non-hybrid trees and exhibited a more homogeneous growth than the mixed progenies. Hybrid superiority was highest for orchards composed of a single maternal clone of a species which was poorly adapted to the planting site. Hybrid percentage was enhanced by successive thinnings. However, a serious loss of income is expected when planting progenies characterized by low initial hybrid percentage and high hybrid superiority. Moreover, variations in hybrid percentage proved to be problematic in seed orchard testing as seed orchard ranking may change depending on whether it is based on the performance of all trees or hybrids only. These variations reduce the reliability of the recommendations typically made to forest owners. For commercial and silvicultural reasons, we suggest setting a minimal threshold level of 60-70% for hybrid percentage in marketed Forest Reproductive Materials.; Les graines et plants produits en vergers à graines d'hybridation de mélèze ne sont jamais des purs hybrides Larix x eurolepis. La proportion d'hybrides étant susceptible d'influer sur les performances de ces matériels forestiers de reproduction (MFR), nous avons étudié l'impact de la pureté spécifique sur la productivité et la qualité de jeunes plantations. L'étude s'est déroulée en trois étapes : i) détermination taxonomique par marqueurs cytoplasmiques, ii) comparaison des performances des individus hybrides et non-hybrides et iii) évolution du pourcentage d'hybrides après éclaircies sylvicoles. Au total, nous avons étudié le comportement de MFR issus de 8 vergers commerciaux dans trois sites. Les MFR analysés contenaient des proportions d'hybrides éminemment variables, de 12% à 96%. D'une manière générale, nos résultats, qui concernent l'adaptation au site (survie, vulnérabilité à la sécheresse et à Meria laricis), la croissance (hauteur et circonférence), la forme et le module d'élasticité du bois, plaident clairement en faveur des hybrides. Ils se distinguent notamment des individus d'espèces pures (mélèze d'Europe ou du Japon) par une plus forte vigueur. A cet égard, la supériorité des hybrides est particulièrement grande quand les vergers sont composés d'un clone maternel unique et appartenant à une espèce moins bien adaptée au site de plantation. D'autre part, bien que les éclaircies s'accompagnent d'une augmentation du pourcentage d'hybrides, une perte de revenus significative est attendue si l'on utilise des MFR caractérisés par un faible taux d'hybrides et une forte supériorité des hybrides pour la croissance. En outre, les variations de composition des MFR sont sources de problèmes dans le cadre de l'évaluation des variétés, destinée à fournir des recommandations aux propriétaires forestiers, car leur classement est sujet à variations selon qu'on considère l'ensemble des arbres ou les seuls hybrides. En conclusion, pour des raisons commerciales et sylvicoles, nous suggérons de restreindre la commercialisation à des MFR dotés d'un taux d'hybrides minimal de 60-70%.

The extractive content of lignin and the brown-rot decay-resistance against Coniophora puteana and Poria placenta were studied in larch heartwood from different species and origin (Larix decidua var. decidua, L. decidua var. sudetica, L. kaempferi , L. × eurolepis). The study material consisted of 106 trees from a 39-year old provenance trial in France. The hot-water-soluble extractives were very variable (from5.66% to 20.50% of dry weight), but there was no significant variation between the investigated species and origins. In contrast, acetone extractives, the total amount of phenolics and lignin showed significant differences. The concentration of phenolics and lignin was significantly higher in L. kaempferi and in L. × eurolepis than in L. decidua. The total phenolics content was strongly correlated with decay-resistance in all investigated larch origins. A higher concentration of phenolics goes hand in hand with higher decay resistance and phenolics might therefore be a promising parameter to rapidly evaluate the level of decay-resistance in larch.

Le nombre optimal de ramets par clone dans des dispositifs de tests clonaux a ete etudie a l'aide de tirages aleatoires dans des donnees reelles de vigueur, de forme et de qualite du bois. Ces donnees provenaient d'un essai de 32 clones de merisier (Prunus avium) replique sur trois sites et âge de 7 ans et d'un essai monostationnel de 96 clones de meleze hybride (Larix × eurolepis) âge de 8 ans. La precision d'estimation des valeurs genetiques et des parametres genetiques de variance-covariance approche son maximum vers six a huit ramets par clone. Pour estimer les gains genetiques correspondant aux differentes situations pour le nombre de ramets, on a considere que le nombre d'arbres en test etait constant et que le nombre de genotypes etudies augmentait quand le nombre de ramets par clone diminuait. Dans ce cadre, avec six ramets par clone, l'esperance de gain pour des caracteres d'heritabilites comprises entre 0,20 et 0,70 est de 20 % plus importante que celle obtenue avec 18 ramets par clone, nombre de ramets par clone utilise frequemment dans les tests clonaux. Les resultats obtenus pour un index multilocal sont similaires a ceux obtenus pour des caracteres etudies dans un seul site.

Forest tree breeding has been ongoing for more than 70 years across Europe. It has successfully generated improved varieties for the major economical forest tree species. They are part of the present European forestry landscape and largely contribute to intensive wood production and other forest activities. In this book, we describe the state-of-art of breeding for the main forest tree species. We provide a comprehensive, unique and up-to-date overview of the major scientific results and breeding achievements gathered from the many programmes scattered across Europe. The book is divided into 10 chapters, each as a monograph corresponding to a species or group of species Abies spp., (Larix spp., Picea abies, Picea sitchensis, Pinus sylvestris, Pseudotsuga menziesii, and Mediterranean pines; Acer pseudoplatanus, Fraxinus excelsior, and Prunus avium). Each of them is written by a group of experts and focuses on the distribution and economical importance of the species; motivation for breeding and breeding objectives; intraspecific genetic variability, breeding populations and breeding strategy; forest reproductive material deployment including mass-propagation and, prospects and perspectives for joint research and breeding. The book is a unique and up-dated source of information for students, researchers and professionals interested in the genetics and domestication of forest tree species.

This work has been conducted under Research Infrastructure Concerted Action “Treebreedex” (http://treebreedex.eu, contract RICA-CT-2006-026076).; Vegetative propagation of forest trees offers advantages to both tree breeders and the forest industry. This review will describe benefits, type of vegetative propagation, and its integration into breeding programmes. Of all of the different methods for vegetative propagation, only rooted cuttings and somatic embryogenesis (and the combined use of both) offer any practical methods for large-scale commercial use. However, it is very difficult to fully appreciate the overall level of activity of the research and application of somatic embryogenesis of forest trees. Publications and reports only highlight a small fraction of the ongoing work. To this end, a survey was conducted across Europe (under EU Research Infrastructure Concerted Action “Treebreedex”) to document the species involved, the state-of-the-art of somatic embryogenesis, its stage of development and its application in tree improvement programmes and to commercial forestry. The results of this survey are presented and discussed. In addition, this review presents the challenges (biological, economic, public acceptance and regulatory) and their relationships to European forestry. Finally, a strategy to promote the use of this technology is proposed.

Plateforme Génobois; Heartwood is the most valuable part of larch timber when either natural durability or aesthetic aspects of the wood are required. Both properties are directly linked to chemical extractives and particularly to phenols. Based on a broad sample of trees (583) from European and Japanese larch and their interspecific hybrid, we investigated the variability in phenolic content, and particularly of two major compounds, Taxifolin (Tax) and Dihydrokaempferol (DHK), and their link with wood colour. At the individual wood sample level, phenolic contents ranged from 6.0 to 55.9 mg eq. Tax/g DW. Taxifolin was the most abundant constituent (range: 1.3–41.7 mg/g DW) compared with DHK (0.5–13.7 mg eq. Tax/g DW). A high variability among taxa, genotypes and individual trees within taxa and within trees was observed. Japanese larch had the highest amount of total phenols and of Taxifolin and European larch the lowest. For DHK, Japanese larch was the poorest compared with European larch. Hybrid larch had both a high content of Taxifolin and of DHK. Variability for colour parameters was on average weaker than for phenolic content but still large enough to show significant differences between taxa. Correlations between colour parameters and extractives were moderate to weak. At the mean genotype level, a good link (r > 0.51, p < 0.001) was found between a* (red–green axis in CIELAB) and total phenols and Taxifolin contents and between L* (Lightness) and DHK content (r > 0.54). The broad variability observed in this study at different levels for phenolics offers opportunities for breeders to genetically improve the quality of larch heartwood, in particular in relation to decay resistance.

European larch (Larix decidua) is a native species from the Alps and from several mountainous ranges in Central Europe. Its early introduction outside its native range, especially in lowlands western and northern Europe has been problematic for ecological (phenology) and sanitary (canker) reasons due to the lack of knowledge of its genetic variability. As a substitute, Japanese larch (Larix kaempferi) and then hybrids between European and Japanese larches have been preferred in these regions together with Siberian larches in northern Scandinavia. Results from intensive provenances testing (as early as in the 1940s) have been the corner stone on which breeding programmes have developed both at the intra- (mostly in Central and Eastern Europe) and inter-specific (mostly in North-Western Europe) levels. Inter-specific hybridization is probably the most emblematic and promising breeding strategy for larch, taking benefit of both species trait complementarity, hybrid vigour (heterosis) and hybrid homeostasis. While seed orchards are now the main source for dissemination of genetic progress for European larch, mass-production of hybrid larch remains problematic either through sexual (hybridization seed orchard) or asexual (vegetative propagation by cutting) ways and still impedes large deployment of hybrids in Europe. While progress is expected from current research on this topic (coupling of somatic embryogenesis with bulk vegetative propagation by cutting, 2nd generation hybridization orchard), other hot issues include decoding of heterosis, evaluation of genetic variability for the most original larch wood properties (heartwood formation and durability), adaptation to climate, etc. Joined efforts at European level are expected for genetic resource conservation, breeding and deployment, and tackling of several scientific and technical bottlenecks.

• Direct assessment of modulus of elasticity (MOE) on standing trees is attractive for breeders to evaluate genotypes prior to selection: this can be done using the Rigidimeter, a bending-based measurement device. • In this study, we tested its reliability to properly rank genotypes by relating trunk MOE with MOEs estimated with a vibrating analysis system (Bing) on different types of conditioned wood specimens from the same trees (boards and standardised 2$\times$2$\times$30 cm-clear-wood specimens). One hundred and ten trees from different genotypes of hybrid larch (Larix x eurolepis) were tested. • Mean trunk MOE was 7 300 MPa with a similar value obtained for sawn boards. Clear-wood specimens MOE increased from pith to bark from less than 6 000 MPa to nearly 9 000 MPa. Moderate correlations (r = 0.48-0.61) were found at the individual tree level between trunk MOE and MOE of wood samples. • Single specimen MOE was shown to be strongly related to a linear combination of trunk MOE and sample position. • At the genotype mean level, trunk MOE was highly correlated with wood samples MOE (r = 0.80-0.91). Ranking of genotypes based on trunk MOE was mostly consistent with that based on standardised specimens. • It was concluded that besides other operational advantages which are discussed, the Rigidimeter is a valuable tool for breeders to routinely evaluate and rank genotypes for stiffness prior to further selection., • L'évaluation directe du module d'élasticité (MOE) sur arbre debout intéresse les améliorateurs pour l'évaluation de la valeur des génotypes avant sélection : le Rigidimètre permet cette mesure sur arbre debout grâce à une mesure de la déviation du tronc sous l'effet d'une contrainte connue. • Dans cette étude, nous avons testé sa fiabilité en comparant ce module avec celui obtenu sur divers échantillons de bois séchés, grâce à un système d'analyse vibratoire (Bing). Cent dix arbres, issus d'un test de descendances de mélèze hybride (Larix x eurolepis), ont été analysés. Les pièces de bois comprenaient pour chaque arbre : (i) une planche centrale brute (4 cm d'épaisseur et 80 cm de longueur) tirée du billon de pied, (ii) la même planche délignée, et (iii) des éprouvettes standardisées (2$\times$2$\times$30 cm). • Le module de tronc sur pied atteignait en moyenne 7 300 MPa (2 180–12 174 MPa) avec une amplitude au niveau familial de 5 052 MPa à 8 948 MPa. Le MOE des planches était légèrement plus faible (7 256–7 182 MPa). Celui des éprouvettes normalisées variait de moins de 6 000 MPa au niveau de la moëlle à 9 000 MPa vers l'écorce. Des corrélations modérées (0,48–0,61) ont été trouvées au niveau individuel entre MOE sur arbre debout et MOE des échantillons de bois. • Cependant, il semble possible d'estimer le module des éprouvettes normalisées à partir entre autre du module des arbres sur pied et de la position de l'échantillon dans l'arbre. • Au niveau génotypique, le module de tronc était très fortement corrélé aux MOE des échantillons (0,80–0,91) et le classement des génotypes est apparu fiable. • Outre ses autres avantages (e.g. mise en place rapide, évaluation non destructive), le Rigidimètre est donc un outil bien adapté aux besoins des améliorateurs pour évaluer et classer leurs génotypes pour leur rigidité.

Open-pollinated seed orchards of European and Japanese larch produce mixed progenies combining a highly variable proportion of hybrids along with pure parental species. For several reasons, it is desirable to identify and to sort out hybrids from the pure species at the seedling stage. Taxa identification of 1- to 2-year-old seedlings was attempted using non-destructive assessment of several traits, including morphology, phenology, growth and architecture parameters. Two sets of progenies originating from 10 hybridisation seed orchards were used, relying in a first step on taxa identification of individual seedlings with diagnostic molecular markers. Based on 21 traits assessed, some clear trends in pure species and hybrid features were apparent but due to the large and overlapping ranges of taxa characteristics, no single parameter allowed unambiguous identification of taxa. Combination of traits through linear discriminant analysis made possible correct classification of 90.2% up to 98.6 % of individuals depending on the orchard although there were a few problematic orchards. Two traits appeared particularly pertinent for discriminating young plant taxa, namely 1st-year leaf retention and the bark colour of 2nd-year shoot increments. Results were corroborated using progenies from several orchards and over two experimental periods., Les vergers d'hybridation de mélèzes produisent des proportions très variables de graines hybrides mélangées à des graines de mélèze d'Europe et/ou du Japon. Afin de commercialiser des plants réellement hybrides, il serait souhaitable de pouvoir trier les hybrides des espèces pures en pépinière. Nous avons donc tenté d'identifier des plants de 1-2 ans à l'aide de critères morphologiques, phénologiques, de croissance et d'architecture et en utilisant des méthodes non destructives. L'étude a été réalisée à partir de deux jeux de descendances issues de dix vergers d'hybridation européens, chaque plant ayant été au préalable identifié grâce à des marqueurs moléculaires. Des différences sont apparues entre hybrides et espèces pures pour plusieurs des 21 caractères évalués mais, en raison de la forte variabilité des caractères étudiés, aucun d'entre eux n'a permis une identification suffisamment précise des taxons. En revanche, la combinaison de ces caractères dans des analyses linéaires discriminantes a abouti à une classification correcte de 90.2% à 98.6% des plants. Deux caractères sont apparus particulièrement pertinents pour différencier les taxons : la marcescence en fin de première année et la couleur de l'écorce des pousses de deuxième année. Ces résultats ont été validés sur le second jeu de descendances élevé dans des conditions différentes du premier.

Genetic markers from the nuclear, chloroplast, and mitochondrial genomes were developed to distinguish unambiguously among four larch species [Larix laricina (Du Roi) K. Koch, Larix decidua (Mill.), Larix kaempferi (Lamb.) Sarg., and Larix sibirica (Ledeb.)] used in intensive forestry in eastern North America. Nine random amplified polymorphic DNA (RAPD) fragments had good diagnostic value, and 3 out of 12 nuclear genes were found to harbor fixed interspecific polymorphisms implicating a total of 17 single nucleotide polymorphisms (SNPs) and 2 indels. The sequencing of five mtDNA introns (cox1-intron1, matR-intron1, nad1-intron b/c, nad3-intron1, and nad5-intron1) and four cpDNA regions (matK, trnL-intron, trnT–trnL and trnL–trnF intergenic spacers) resulted in the identification of 14 sites with fixed interspecific differences among the four species. Including the ten Larix species, one polymorphic site per 47 nucleotide sites sampled was observed for nuclear genes, one per 283 sites for cpDNA, and one per 374 sites for mtDNA. The phylogeny of the genus could be estimated from variation among the ten species detected in two cpDNA intergenic regions and four mtDNA introns. There was congruence between cpDNA and mtDNA phylogenies with three large groups delineated: the North American, North Eurasian, and South Asian taxa. The position of L. sibirica differed between organelle genomes. It was regrouped with South Asian species on the cpDNA tree, but with its North Eurasian congenerics on the mtDNA tree. To simplify the detection of diagnostic DNA sequence polymorphisms among the four main Larix species, cleaved amplified polymorphic sequence (CAPS) assays were developed from the polymorphisms identified in the various genomes. Seventeen primer–enzyme combinations were tested, and six were selected for their high level of informativeness. These new species-specific diagnostic markers should be useful for the certification of larch breeding materials and hybrid stocks used in intensive forestry in the northern hemisphere.

International audience; Hybrids between European and Japanese larches combine the properties of both parental species (drought resistance, canker resistance, stem straightness) and exhibit a fast growth rate. They are produced in seed orchards, generally by natural pollination. Seeds are collected and used for afforestation as interspecific hybrids. However, there are no convenient tests to assess the interspecific hybrid proportion. In the present study, we developed diagnostic molecular markers suitable for the individual identification of hybrids, whatever their developmental stage. Our strategy involved testing a combination of maternally inherited markers from the mitochondrial genome (mtDNA) and paternally inherited markers from the chloroplast genome (cpDNA). Hybrids were then identified by the presence of a mitochondrial sequence inherited from one parental species and a chloroplast sequence inherited from the other parental species. To achieve this aim, markers discriminating both parental species were first sought. Amplifications of mitochondrial and chloroplast sequences were performed using specific PCR primers. After testing 33 primer pairs in combination with nine restriction enzymes, we detected one mitochondrial marker, f13 which was amplified in Japanese larch and absent in European larch, and one chloroplast marker, ll-TaqI which showed different restriction patterns depending on the species. A restriction fragment of 601 bp was obtained in Japanese larch while two fragments of 120 bp and 481 bp were observed in European larch. These patterns were found in all 197 individuals tested from the two pure species. These markers were then used for the evaluation of the hybrid proportion in a seed lot produced from seed orchards; this was assessed as between 43% and 53% depending on the parental species. The male and female parental species could be determined for each progeny.

International audience; Larch heartwood is appreciated for its good mechanical properties, its colour and its texture, and it is often used outdoors because of its natural durability (decay resistance). In this study the colour of larch heartwood was studied in relation to extractives and decay resistance, with the aim to estimate durability of larch heartwood from its colour. On a total of 293 trees colour in the CIE L*a*b* space (L* lightness, a* red/green axis, b* yellow/blue axis), extractives content (acetone and hot-water extractives, amount of phenolics) and the brown-rot decay resistance were determined. For calculating the relative decay resistance ( x), mass loss after inoculation for 16 weeks with two fungi [ Coniophora puteana (Schum.ex.Fr.) Karst., Poria placenta (Fr.) Cke, European standard EN 113] of larch heartwood samples was compared to Scots pine ( Pinus sylvestris L) sapwood reference samples (EN 350-1). Different species [Japanese larch ( Larix kaempferi Lamb.), Hybrid larch (Larix deciduax L. kaempferi) and European larch ( L. decidua Mill.)], provenances and age classes (38-year, >150-year) were included. Japanese larch heartwood turned out to be significantly more reddish (higher a*-values) compared to the European larch provenances. Reddishness of the hybrids was intermediate. The red hue (+a*) was strongly correlated with the amount of phenols ( r =0.84) and decay resistance ( r =0.63) and therefore suitable for prediction of both parameters. The results suggest that colour measurements of larch heartwood could be of benefit in tree breeding programs and for an optimised utilization of larch timber.

Predictive models for heartwood and bark proportions have been developed for Japanese larch (Larix kaempferi (Lambert) Carr.) planted in South of Belgium. These models have been tested to evaluate the way of applying them to European larch (Larix decidua Miller) and hybrid larch (Larix eurolepis Henry) growing in west-European lowlands. For the heartwood proportion, independent variables selected are stand age and dbh. The model set up for Japanese larch has a R-square of 0.859. In average, European larch has lower heartwood proportion. With reference to bark proportion, selected independent variable is dbh. Residual variability is far more important for bark models ( R-square of 0.456). European larch has, in average, higher bark proportion whereas hybrid larch has lower. An example of the application of these models in connection with a growth model is presented. It consists in an evaluation of the impact of dynamic silviculture on heartwood and bark proportion., Des modèles prédictifs de la proportion de duramen et d'écorce à 1,3 m de hauteur ont été développés pour les peuplements de mélèze du Japon (Larix kaempferi (Lambert) Carr.) plantés en Région wallonne (Belgique). Ces modèles sont testés pour évaluer dans quelle mesure ils peuvent être appliqués aux mélèzes d'Europe (Larix decidua Miller) et aux mélèzes hybrides (Larix eurolepis Henry) plantés dans les régions de basse altitude de l'Europe de l'Ouest. Pour la proportion de duramen, les variables explicatives retenues sont l'âge du peuplement et le diamètre à 1,3 m. Le modèle présente un coefficient de détermination de 0,859 pour le mélèze du Japon. En moyenne, le mélèze d'Europe présente des proportions de duramen plus faibles. En ce qui concerne la proportion d'écorce, la variable explicative retenue est le diamètre à 1,3 m. La variabilité résiduelle de ce modèle est beaucoup plus importante ( R2 de 0,456). Le mélèze d'Europe présente en moyenne des proportions d'écorce plus importantes tandis que celles du mélèze hybride sont plus faibles. Un exemple d'utilisation de ces modèles en combinaison avec un modèle de croissance est présenté. Il permet d'évaluer l'influence d'une sylviculture dynamique sur les proportions de duramen et, dans une moindre mesure, d'écorce.

International audience; The use of wood quality components in forest trees selection is hampered by long delays before phenotypic evaluation. Marker-assisted selection opens new possibilities since selection becomes feasible even before trait expression. In the present study, we have analysed 224 13-year-old hybrid larches derived from eight progenies from a single female parent in order to localise genomic regions controlling several wood traits. Traits were assessed on radial cores by microdensitometry, a method that provides estimates of wood density and radial growth and avoids tree destruction. Measurements were assessed individually on six consecutive rings. QTL detection was achieved by ANOVA using 68 AFLP markers distributed on average each 20 cM along larch genome. Significant QTLs for at least two out of six studied years are distributed on seven linkage groups. Applications towards marker-assisted selection are discussed.; Localisation des régions du génome contrôlant des composantes microdensitométriques de la qualité du bois chez des mélèzes hybrides. Les paramètres de qualité du bois sont peu pris en compte dans l'amélioration des arbres forestiers en raison des délais d'évaluation très longs. La sélection assistée par marqueurs ouvre de nouvelles perspectives en rendant possible une sélection précoce pour des caractères à expression tardive. Dans ce travail, nous avons analysé huit descendances de mélèze hybride (Larix decidua$\times$L. kaempferi) issues d'un même parent et comportant 224 arbres âgés de 13 ans, afin de localiser sur une carte génétique les régions du génome qui contrôlent divers paramètres de qualité du bois. Ces paramètres ont été mesurés sur des carottes de bois par microdensitométrie. Cette méthode permet d'évaluer, de façon non destructrice, de nombreuses composantes de la densité du bois et également de la croissance radiale. Les mesures sont effectuées sur six cernes consécutifs. La localisation des QTL a été menée à l'aide de 68 marqueurs AFLP distants en moyenne de 20 cM et par analyse de variance. Les QTL significatifs pour au moins deux des six années analysées se répartissent sur sept groupes de liaison. Les applications de ces résultats sont discutées essentiellement dans les perspectives de la sélection assistée par marqueurs.

International audience; Rigidimeter is a device developed in order to determine the modulus of elasticity (MOE) of standing trees. It was used on a hybrid Larch clonal test located at INRA, Orléans, France. Some of the examined trees were felled and remeasured. The goal of this study was to contribute to the validation of the Rigidimeter, as a fast non-destructive and reliable tool for rapid evaluation of MOE, an important mechanical property of wood. Two measurements in orthogonal directions improve the accuracy especially in the presence of reaction wood. We have showed that two measurements of diameter including bark with a precision of $\pm 1$ mm are sufficient to obtain a reliable second moment of area, which is an important parameter for the estimation of trunk MOE. Bark MOE is small compared to wood MOE, but bark thickness influences the measurement of tree diameter. A correction may be necessary to account for the presence of bark. We propose a method for estimating MOE of different trunk layers, such as bark and mature wood layer.

Genetic linkage maps have been increasingly developed for a wide variety of plants, using segregating populations such as F2s or backcrosses between inbred lines. These pedigrees are rarely available in outbred species like forest trees which have long generation times. Thus genetic mapping studies have to use peculiar pedigrees and markers in appropriate configurations. We constructed single-tree genetic linkage maps of European larch (Larix decidua Mill.) and Japanese larch [Larix kaempferi (Lamb.) Carr.] using segregation data from 112 progeny individuals of an hybrid family. A total of 266 markers (114 AFLP, 149 RAPD and 3 ISSR loci) showing a testcross configuration, i.e.heterozygous in one parent and null in the other parent, were grouped at LOD 4.0, θ=0.3. The maternal parent map (L. decidua)consisted of 117 markers partitioned within 17 linkage groups (1152 cM) and the paternal parent map (L. kaempferi) had 125 markers assembled into 21 linkage groups (1206 cM). The map distance covered by markers was determined by adding a 34.7-cM independence distance at the end of each group and unlinked marker. It reached 2537 cM and 2997 cM respectively for European larch and Japanese larch, and represented respectively a 79.6% and 80.8% coverage of the overall genome. A few 3:1 segregating markers were used to identify homologous linkage groups between the European larch and the Japanese larch genetic maps. The PCR-based molecular markers allowed the construction of genetic maps, thus ensuring a good coverage of the larch genome for further QTL detection and mapping studies.

283 clones de meleze hybride (Larix × eurolepis et L. × leptoeuropaea) issus de 14 familles de plein-freres ont ete inocules avec un isolat agressif de L. willkommii au printemps 1990. Chaque clone etait represente par trois ramets greffes âges de 3-4 ans et a recu 15 inoculations. L'inoculum etait constitue de pailles de sorgho colonisees, introduites dans un trou de 1 mm de diametre inflige a des rameaux de 2 ans. Une premiere phase d'installation du parasite dans les tissus a conduit a un taux d'infection de 27,9 % au bout de 18 mois. La grande majorite des clones (80-90 %) etaient receptifs et une forte variabilite clonale etait observee pour tous les symptomes. Aucune influence, ni du sens du croisement (L. × eurolepis vs L. × leptoeuropaea), ni de l'origine des parents de meleze d'Europe (Alpin vs Sudetes) n'a pu etre mise en evidence. Au bout de 39 mois, presque toutes les lesions etaient cicatrisees, le taux d'infection residuel etant de 2,2 %. La tres faible sensibilite du meleze hybride au L. willkommii est ainsi confirmee par voie experimentale. La vigueur de l'hote contribue probablement a cette resistance. La transmission de cette resistance par le parent japonais est suggeree mais elle meriterait d'etre verifiee.

Quarante clones de Larix decidua originaires du centre de la Pologne ont ete inocules dans trois sites francais (Orleans, Peyrat-le-Chateau, Cendrieux) avec un isolat de Lachnellula willkommii au printemps 1991. Chaque clone represente par 3 a 15 ramets âges de 11 ans, a recu 20 a 40 inoculations. Les symptomes ont ete suivis pendant trois saisons de vegetation. Le taux general d'infection obtenu a varie de 10 a 30 % selon les sites, confirmant une sensibilite faible a moderee de la race « polonica » etudiee et revelant un effet marque du site sur la reussite et le deroulement de l'infection. Une forte variabilite clonale (a noter l'absence de clone immun) est mise en evidence ainsi qu'un controle genetique non negligeable de la resistance au niveau des moyennes clonales, mais le classement des clones varie selon les sites. La variabilite residuelle (intraclonale) forte, due aux effets du milieu reste un obstacle a la realisation de tests a portee generale. Des progres sont attendus d'une meilleure connaissance de l'interaction hote-parasite, en particulier par la definition des conditions optimales d'expression de la maladie (en particulier pour le choix du site experimental) et d'autre part par une reduction des bruits de fond lies a l'experimentation (par exemple homogeneisation de la taille des organes inocules).

Random Amplified Polymorphic DNAs (RAPD) were used for estimating genetic distances between 12 European larches (Larix decidua) and 12 Japanese larches (L. kaempferi) that were the parents in a factorial mating design. One hundred and eleven fragments were used for establishing genetic distances based on Jaccard's coefficient between parents. Thirteen fragments differentiated the larch species. The genetic distance between individuals of the same species (D J =0.39 in the Japanese larch and 0.45 in the European larch) was lower than the genetic distance between species (D J =0.72). A UPGMA dendrogram based on genetic distances clearly clustered each larch species, confirming the speciation at a molecular level. Correlations between genetic distances of the parents and performances of the hybrid families were established for various quantitative traits. Significant values were found for growth characters and branch insertion angle, which suggested an effect of general heterozygosity level on hybrid traits. These correlations also evolved with tree age: the maximal correlation was noticed on 6-year-old trees for height. The lack of correlation between parental genetic distances and hybrid performances for the other quantitative traits suggested that these characters were controlled by fewer genes. The results of this study show that crosses between genetically distant parents produce hybrids with excellent growth performances; this represents a potential selection criterion of the genitors.