Investigating optimal site conditions for flower bud production in Eucalyptus smithii orchards in South Africa

ABSTRACT

Eucalyptus smithii is an important commercial forestry species in South Africa. The species’ shy and erratic flowering tendencies hinder its genetic improvement and seed production. Breeders and seed producers are highly dependent on paclobutrazol (PBZ) applications for encouraging flower and seed production in E. smithii orchards. During the summer of 2003/04, an E. smithii flowering field trial series was established across a range of high elevation (1568–1828 m asl), cool temperate (13.8–15.5°C mean annual temperature (MAT)) commercial forestry sites in South Africa. The main objective was to investigate the interaction between PBZ and a range of climatic and landscape factors on E. smithii floral bud (umbel) production. The main aim was to define optimum environmental conditions for E. smithii umbel production and potential orchard establishment. In the fifth and sixth years after planting, 64% and 83% of the control (nil PBZ) grafted trees and 94% and 94% of PBZ-treated trees produced umbels. In the fifth year, PBZ application increased mean (across-site) umbel production per tree by 132%. Regardless of whether PBZ was applied or not, mean umbel production varied markedly across sites. In the absence of PBZ application, E. smithii test orchards located on south-west facing slopes in high elevation (>1550 m asl) areas at the lower end of the applied MAT range were generally the most productive on the basis of umbel crop. The study demonstrated that, through careful site selection, dependency on PBZ to achieve satisfactory umbel production in reproductively mature E. smithii trees may be substantially reduced. Within the elevation and MAT ranges applied in the trial series, E. smithii orchards situated in low positions in the landscape were at high risk of being severely damaged by frosts.

KEYWORDS:

• Cold tolerance
• flowering
• flower induction
• seed orchards
• site-species matching
• chilling requirement
• eucalypt
• Eucalyptus smithii

Acknowledgements

The establishment and ongoing maintenance and measurement of the trials comprised a team effort. The ICFR technical staff, and foresters within the funding companies, who contributed in this regard are gratefully acknowledged. Kabir Peerbhay (ICFR) is acknowledged for compiling the GIS map in Figure 1. Andrew Morris and Tammy Swain (ICFR) are thanked for reviewing the draft manuscript and for their useful comments. Our ICFR funders are thanked for making the resources available to undertake the research.

Disclosure statement

No potential conflict of interest was reported by the authors.