Harris, F 2007, 'The effect of competition on stand, tree, and wood growth and structure in subtropical Eucalyptus grandis plantations', PhD thesis, Southern Cross University, Lismore, NSW.
Copyright F Harris 2007
In this study, the effect of competition on stand, tree, and wood structure was examined in the context of the viability of high-density eucalyptus plantations for the production of quality timber. The study was based on a 4-year-old Eucalyptus grandis trial planted in south-eastern Queensland in March 1999 by Greenfield Resource Options P/L. Stocking densities of 250, 1,000, 5,000 and 10,000 stems/ha allowed growth traits in extreme stockings to be compared. High stocking densities led to greater stand growth and also greater inequality in tree size. The largest trees in high stocking densities were smaller than those in low stocking densities, however high stocking densities had more trees in the largest cohort and they were more uniform in size. Consequently the total stem volume of the largest 1,000 st/ha was similar for all stocking densities, and the total stem volume of the remaining trees increased with stocking density. The allocation of tree biomass skewed away from crown production and towards stem production as stocking density (general competition) increased and as stem diameter (competitive status) decreased. Dominant trees in high stocking densities had similar aboveground biomass accumulation per unit leaf area compared to dominant trees in low stocking densities, but had a larger proportion of biomass allocated to the stem when compared to the crown. Dominant trees in high stocking densities therefore had similar tree growth efficiency but better stem growth efficiency than dominant trees in low stocking densities. Increased competition appeared to restrict the growth of dominant trees by restricting resource capture rather than by reducing the efficiency of growth, since the tree growth efficiency of dominant trees was not affected by competition. Examination of the stem wood structure revealed that the largest trees in high stocking densities exhibited more desirable wood properties including more uniform wood density, less variability in wood anatomy, and better branch-shed (hence low knot content) than the largest trees in the low stocking densities. This suggests that densely stocked plantations could be of better value for timber production than lightly stocked plantations. The results illustrate the importance of including stand structure in forest research since a failure to do so will underestimate the productivity of the largest trees in densely stocked stands and does not adequately account for the structural benefits of high stocking density. The findings are based on a young plantation, however they indicate that densely stocked plantations could be used to provide an early cash return from a harvested biomass crop with no detrimental effect on a retained solid hardwood crop of the largest 1,000 st/ha. The results of this study indicate that the perception that densely stocked plantations cannot produce an equivalent volume of sawlogs of similar quality wood to that produced from lightly stocked plantations is incorrect.