Variation in lateral shoot elongation patterns and hybrid vigor in full-sib families and interspecific hybrids of larch

Time of shoot initiation and cessation, and rate and duration of lateral sh oot elongation of full-sib families of Larix decidua Mill., L. laricina (Du Roi) C, Koch, and interspecific hybrids between L. decidua and L. kaempfer i (Lamb.) Carriere, L. kaempferi x decidua, L. laricina x decidua, and L, l aricina x kaempferi were explored in relation to hybrid vigor determined by height growth after 5 years. Height growth of L. laricina and L. decidua f amilies was significantly less than height growth of the interspecific hybr ids. Differences were observed for all lateral shoot elongation traits amon g L. laricina, L. decidua, and the interspecific hybrids. Lateral shoots of L.. laricina families were the first to both initiate and cease elongation , and grew for the shortest period of time. Overall, the length of the shoo t elongation period and date of cessation of shoot growth were significantl y correlated with total height growth after 5 years, but the time of shoot initiation was not. Rate of elongation of lateral shoots was strongly corre lated with growth increment in 1996, but was weakly correlated with total h eight growth after 5 years. Hybrid vigor in Larix seems to be partly a func tion of late cessation or increased duration of shoot elongation, or both.