| Summary: | International audience Since its discovery in Dunkeld (Scotland) at the beginning of the 20th century, hybrid larch (HL) (Larix x eurolepis) raised a strong interest among foresters and, later on, breeders. Dozens of hybridization orchards have been established in several European countries. All of them include selected genotypes of European Larch (EL) (Larix decidua Mill.) and Japanese larch (JL) (Larix kaempferi (Lamb.) Carr.) but they differ in the mother species (EL, JL, both), the number of clones used on both maternal and paternal sides (from one to several dozens) and in the planting design. However, the products of these seed orchards are composed of a mixture of hybrid and pure species seeds, the latter resulting from intraspecific crossings and/or self-pollination. Molecular markers show highly variable hybrid percentages among orchards but also, for a given orchard, among years. In addition to the number of mother clones, the proportion of hybrid seeds depends on several factors, e.g. clonal contributions to pollen and seed-cone production and flowering overlapping that are related to genetic factors, climatic conditions and seed orchard management (flower stimulation, supplemental mass pollination). Considering the extent of hybrid rate fluctuations (between 10% and 90%) it is important to study the effect of species purity on stand productivity and quality. The consequences are also discussed in the context of seed orchard testing. Our data have been mainly collected in two "twin" trials managed by Irstea that aimed at comparing seed lots from six European hybridization orchards (64 seedlings per plot x 4 blocks for each variety). They were planted in 1995 in two sites, at mid-elevation (800 and 1000 m). The first one is subjected to an oceanic climate favourable to JL whereas the second one, more continental, is more suitable to EL. Bedrock is granite in site 1 and limestone in site 2. Taxa were first identified (100-120 trees per variety) by INRA using markers based on cytoplasmic DNA. It ...
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