For this purpose, the importance of light availability and the effects of gap size and within gap position on the growth and morphology of seedlings were investigated in canopy gaps of 15, 30 and 45 m diameter, as well as in a clearfell and beneath closed canopy. Average total light availability and its variability along the north-south axis increased with increasing gap size. Relative light intensity within gaps at 2 m above ground was highest at the centre, increasing from 25%, through 52% to 67% across the increasing gap size continuum. Relative light intensity declined with increasing distance from the gap centre. In addition to the horizontal light variation, vertical light gradients were examined.

This revealed that, on average, 5% more diffuse light was received at 2 m than at ground level and 7 % more at 6 m than 2 m. Whereas the eucalypt overstorey surrounding the gaps had little influence on light transmission within gaps, the dense secondary rainforest tree stratum surrounding gaps strongly influenced light transmission. Seedlings of E.obliqua displayed high plasticity in carbon allocation. Light availability significantly influenced diameter growth, height to diameter ratio, crown architecture, leaf are, specific leaf area, leaf area ratio, light compensation point and above- and below-ground biomass of seedlings planted in the north, centre and south positions of gaps and in a clearfell treatment and beneath a closed canopy. Higher quantities of light increased seedling stem diameter, crown width, leaf area, light compensation point and leaf, branch, stem and root mass. Height to diameter ratio, specific leaf area and leaf area ratio decreased with increasing light availability.

Strong relationships were found between diffuse light availability and seedling crown width and shoot and total seedling biomass. Light compensation points of E.obliqua were lower than in many other shade-intolerant species. These changes in morphology and physiology with light climate suggest that E.obliqua is moderately shade tolerant in the seedling stage. To assess whether the light climate in gaps would also be suitable to later growth stages with less shade-tolerant foliage, the relative light intensity at the base of sapling crowns with intermediate foliage was determined at a nearby site. Minimum relative light intensity for intermediate E.obliqua foliage was 27%. The area of the 30 m and 45 m diameter gaps with relative light intensities above 27% at 2 m above the ground were 93 and 97% respectively.

However, the entire area of the 15 m diameter gap had relative light intensities below 27% which will not provide sufficient light levels to sustain intermediate sapling foliage. Thus a minimum gap size requirement of 30 m diameter in old-growth E.obliqua mixed forest was recommended for the initial regeneration phase. This study indicates that seedling biomass was reduced in gaps compared to the clearfell treatment. However, adequate seedling growth and survival in all gaps suggests gaps can provide suitable conditions for the early growth of planted E.obliqua seedlings. Care must be taken to extrapolate results from this study to natural regeneration or later growth stages of this species.