Growth of red ironbark between 1972 and 1995 on research plots in the Heathcote forest.  J. D. Kellas, D. A. Oswin and A. K. Ashton.  November 1993.  11pp. (unpubl.)

SUMMARY

Results are presented from a series of research plots located in the vicinity of Costerfield, north of Heathcote, in typical box-ironbark forest, to better understand the development of red ironbark (Eucalyptus tricarpa) in these forests.

The first study was established in 1972, to determine the response (basal area increment) of individual trees to various degrees of release from competing regrowth and overwood. The second study was to determine the 'zone of influence' surrounding individual red ironbark trees, and the third study monitored the growth of a 'permanent plot' established in a 'fully stocked' stand which had been excluded from harvesting for approximately 30 years. Collectively, the data could provide a basis to simulate various thinning regimes and predict yields on an area basis in red ironbark forests.

The results showed that individual red ironbark trees retain a capacity to respond to reductions in competition in fully stocked stands. For regrowth (dbhob <20 cm), the response was rapid with the removal of competing overwood and could be further enhanced by also reducing the regrowth competition. For overwood (dbhob >20 cm) the response was slower. For both regrowth and overwood trees, total competition from all competitors appears more important than that from overwood or regrowth alone.

Maximal response of individual trees appeared to occur when the radius of the area free of competition was between 33 and 50 times tree diameter (as determined using a 2m² ha^-1 and 1m² ha^-1 optical prism, respectively). This would require an extensive opening around individual trees, which would compromise total site productivity and other environmental values, particularly where coppice is the predominant form of regeneration.

In terms of diameter increment over the 23 years of the first study, the average for regrowth varied between 0.21 and 0.46 cm y^-1 and 0.34 and 0.55 cm y^-1 for overwood; growth rates which are relatively slow compared to the more productive forest types. In fully stocked stands or in the control treatments, the diameter and basal area increments of red ironbark was very small (diameter averaging approximately 0.2 cm y^-1).

These studies show that while the box-ironbark forests have low productive capacity relative to forests in the higher rainfall zones, significant growth responses can be expected with appropriate thinning regimes but responses may be limited to less than 10 years, requiring periodic thinning for sustained responses.