Hybrid larch (Larix x eurolepis Henry), superiority over the parental species (Larix decidua Mill. and Larix kaempferi (Lamb.) Carr.) has been described by many tree breeders. This superiority concerns not only growth characteristics but also several economically important traits. In the literature, there is some ambiguity regarding the 3 following concepts: hybridization, hybrid superiority and heterosis. In particular, the superiority of hybrid larch has been claimed in many studies as due to heterosis. A detailed review of published results does not permit a decisive opinion on the subject as most of the results are based on punctual and limited observations. The interest in hybridization is not restricted to hybrid vigor but also includes combination and transfer of favorable characteristics. Several larch improvement strategies on inter- and intraspecific hybridization are discussed. Reciprocal recurrent selection seems particularly attractive. Nevertheless, none of the present strategies, including the F1 generation as a breeding population, can be excluded. Precise knowledge on genetic properties of traits selected for are required before any firm recommendation can be made. There is an urgent need for well designed, long-term experiments set up on several sites to obtain more insight into these delicate questions. Use of a two-level factorial mating design is recommended, in particular to avoid some of the approximations made in past experiments. Moreover, valuable information on genetic parameters, e..g. combining ability, heterosis, will be gained at both intra- and inter-species and intra- and intra-population levels and may help tree breeders in their choice of a more efficient hybridization strategy for the improvement of larch.
