Seed crop maturation in five eucalypt species from the high elevation mixed species forests of East Gippsland.  M. D. Murray and D. Terrell.  May 2004.  28pp. (unpubl.)

SUMMARY

The availability of mature eucalypt seed is a vital component of the regeneration strategy following commercial timber harvesting in High Elevation Mixed Species (HEMS) forests of East Gippsland. Understanding the maturation process of capsules and seed and being able to determine the maturity of seed crops are necessary for appropriate use of seed tree silvicultural systems and collection of seed for artificial sowing.

This project investigated the maturation of capsules and seed of the five main eucalypt species in HEMS forest in East Gippsland: Eucalyptus cypellocarpa, E. denticulata, E. obliqua, E. fastigata, and E. viminalis. The usefulness of visual indicators of maturity such as position of capsules in the canopy, and colour of capsules, seed and chaff was assessed. Viability (viable seeds per kilogram) and germinative energy (rate) and final percent of germination under controlled conditions were also investigated as possible indicators of maturity.

Contrary to expectations, capsule colour was not a reliable indicator of maturity for most species studied. Brownish capsules could generally be relied on to be mature, however capsules could also maintain a green appearance well after seed had matured, particularly in the gum species (E. cypellocarpa, E. denticulata and E. viminalis).

Germinative energy, percent germination, and viability were good indicators of seed maturity, increasing as capsules aged post-flowering, as was the colour of extracted seed and chaff. These four indicators were most reliable when considered in unison rather than individually.

Older capsules are found further down the branch away from the foliage than younger capsules. In some species, particularly E. obliqua and E. fastigata, and in some trees the differentiation between capsule crops (from successive years) is obvious due to significant growth of the branches in the intervening 12-month period. However, depending on the seasonal conditions and species, there may be very little space between capsule crops making them difficult if not impossible to differentiate on the basis of position in the canopy alone. This effect was often noticed in older trees.

For all five species, the time required from flowering to fully mature seed was determined to be 9 or 10 months. Given the expense involved in seed collection and the problems associated with extraction, storage and use of seed that is not fully mature, it is recommended that seed collection activities be concentrated on crops that are greater than 10 months old. In addition, seed crops on seed trees should be mature at the expected time of seedbed preparation and inducement of seed fall.

Further investigation of the maturation processes of capsules and seed of E. fastigata and E. denticulata particularly is warranted as some doubts remain regarding the maturation periods of these species. The main indicators of maturity (particularly germinative energy) were found to be less useful in E. fastigata than in the other species. Difficulties arose during collection of E. denticulata with regard to identifying different capsule crops and finding sufficient capsules of differing ages for comparison.

The monitoring and testing of capsule crops from individual trees from flowering to full maturity and seed shed would help determine the timing of maturation and the possible factors such as early or late flowering and seasonal conditions that influence it. The influence of seed maturity, particularly germinative energy, on regeneration success is an additional area of interest as it would assist in determining the importance of collecting and sowing with mature seed under various conditions. Further investigation into the effect of storage of maturing seed is also recommended.