Seed cone production enhanced by injecting 38-year-old Larix occidentalis Nutt. with GA4/7

The influence of injecting 38-year-old Larix occidentalis with gibberellin A_4/7 (GA_4/7) on seed and pollen cone production was studied in western Montana, U.S.A. Two natural stands thinned to several spacings in 1961 were chosen. The two widest spacings were used. In 1991, 60 trees selected for study at each site were randomly divided into two groups of similar DBH, half as controls and half injected in June 1991 with a solution of GA_4/7 in ethyl alcohol. The volume injected was adjusted to the DBH of each tree (60 mg per 5 cm diameter). In June 1994, half the trees treated with GA_4/7 in 1991 and half the untreated trees were injected with GA_4/7 in the same manner as before. In 1992 and 1995, seed cone production increased on GA_4/7-treated trees compared to the controls. Pollen cone production was significantly increased by GA_4/7 treatment but only at one site. Delayed effects of GA_4/7 on seed cone production was not evident in subsequent years after treatment. Foliar and shoot damage on treated trees was attributed to GA_4/7 treatment but most trees recovered completely the following year. Finally, GA_4/7 treatment did not affect cone length, potential seed per cone or filled seed per cone.