Trade-off de l'hétérosis pour la croissance chez le mélèze : de la vigueur, mais à quel prix ?

Climate change challenges tree breeders to select tree genotypes that not only grow well but also maintain adaptive capacity under increasing environmental stresses. Ecologists with different schools of thought emphasize the use of slow-growing species with a conservative strategy. Interspecific hybridization, often favored for breeding, promotes high yield through fast-growing, short-rotation genotypes, further enhanced in hybrids by heterosis and trait complementation. This thesis sits at the center of these two visions, combining the promising tool of interspecific hybridization with the broader need to integrate adaptive traits for future climate challenges. We took advantage of a decade-long clonal experimental trial in Orléans combining genetic material of the three species of Larch (European (Larix decidua Mill.) and Japanese Larch (Larix kaempferi (Lamb.) Car.), and their hybrids (Larix eurolepis A. Henry), sampled from an intra-/inter-specific diallel mating design. The same genetic material (clones) was planted in two plots contrasted for their soil water availability (irrigated and non-irrigated). Our objective was to study the differential mortality and growth responses of the three taxa to drought and the possible trade-offs of growth with some adaptive traits. The first chapter explored mortality dynamics and found that Japanese Larch was the most affected, European Larch the least, and Hybrid Larch remained intermediate. Maximum temperature and VPD during late spring-early summer were the most critical parameters and time period, besides summer soil water shortages. Mortality was closely associated with growth at individual ring and clonal levels, but in different ways. Annual radial growth in the year preceding death was consistently lower in trees that eventually died. Importantly, we showed that clonal vigor was, whatever the taxa, positively associated with survival. Building on these findings, addressing environmental challenges requires assessing a broader range of traits, particularly those related to tree hydraulics. However, such evaluations are often time-consuming, costly, and labor-intensive. In Chapter II, we developed good to moderately reliable prediction models for key traits using a high-throughput phenotyping approach through NIRS. These include N, C and lignin contents, H/G ratio and and ∆13C (water-use efficiency, WUE). Building on the environmental insights from Chapter I and the methodological advances from Chapter II, the last Chapter focused on the variability of expression of growth among the three Larix taxa, and possibly how adaptive traits could drive performance. Hybrid larch often outperformed growth of both parental species, with stronger relative heterosis observed under drought conditions. It was very sensitive to soil water availability in contrast to European larch, the least plastic species. Japanese larch was the most vulnerable to cavitation (P50), hybrid larch was on average intermediate but with values close to Japanese larch for some combinations, and European larch was the most resistant. WUE was higher in Japanese and hybrid larches compared to European larch. Fast growth did not necessarily come at a cost, as it was generally positively associated with WUE and growth recovery after drought; however, some unfavorable trade-offs particularly for P50, were observed in hybrids. P50 and resilience traits exhibited sufficient variability and moderate correlations with growth, supporting their value as predictive traits for selecting genotypes with growth and adaptation, whereas WUE showed limited variability and thus offered less predictive potential. Hybrid larch offers promising growth advantages by combining high productivity with intermediate to good adaptive capacities. This study should be continued through integration of more structural, phenological and physiological traits and the study of their interrelationships to get a more insight in heterosis development.