A review of the potential of growth regulators to stimulate seed crops in native eucalypt forest.  B. Roberts.  July 1999.  10pp. (unpubl.)

SUMMARY

The intensity of flowering of commercially important eucalypt species such as Eucalyptus regnans, E. obliqua and E. nitens varies naturally from year to year. Occasionally this has lead to inadequate seed supplies which have constrained regeneration operations following timber harvesting of those species. Accordingly, techniques that artificially induce or exaggerate a flowering response may prove beneficial in ensuring a more reliable seed supply and assist to improve provenance matching for native forest regeneration.

This review has found that the use of growth retardants/regulators like paclobutrazol have been successful in the promotion of flowering in genetically-improved E. nitens and E. globulus stock for plantation establishment. Products containing paclobutrazol such as Clipper^® can be administered through a tube (trunk injection) for passive uptake by the xylem. The technique relies on transpiration by the tree for uptake, which varies with tree growth rates and climatic conditions.

The response of eucalypts to treatment of paclobutrazol varies with species. More reliable inducement has been found with E. nitens compared with E. globulus. No references to the treatment of Monocalyptus species, such as E. regnans or E. obliqua, were found during the review. Therefore trials are needed to establish the response of these species which pose most difficulties for forest re-establishment in terms of seed supply. Appropriate application times and dose rates would also need to be explored.

Paclobutrazol produces an exaggerated flowering effect rather than inducing flowering per se. Therefore, any trial treatment should be considered at an individual tree level rather than a forest stand. Thus, dominant or co-dominant trees, that are more likely to exhibit better flowering, would be selected in order to maximise the benefits of the treatment.