Effect of stand density on longitudinal variation of wood and bark growth in fast-growing eucalyptus plantations

Abstract

The influence of tree spacing on the wood/bark ratio is unknown in young fast-growing Eucalyptus trees. The objective of this study was to evaluate the effect of plant spacing on the wood and bark production along the Eucalyptus stem. Four genetic materials were planted in four spacings: 3×1 m, 3×2 m, 3×3 m and 3×4 m. Three 5-year-old trees from each clone and in each plant spacing were harvested. Cross-sectional discs (thickness: 30 mm) were cut from each tree along the stem (0%, 25%, 50%, 75% and 100% of the total tree height) and at 1.3 m above ground, totaling 288 disks (4 spacings × 4 clones × 3 replicates × 6 axial positions). The wood thickness was measured at six random and equidistant points around the perimeter using a gauge and means were calculated from each disc. Six cross diameters were measured for each debarked disc. After obtaining the averaged bark thickness and wood diameter, the bark content was calculated as the ratio between the surface area occupied by the bark and the total area of the stem in each level. In the narrowed plant spacing (3×1), the trees had a mean diameter of 7.4 cm, while at the spacing 3×4 the diameter of the trees was 91% higher (14.11 cm) at breast height. The increase in plant spacing from 3 to 12 m2 per tree resulted in an increase in bark thickness (56.7%) from 1.94 mm to 3.04 mm, but caused a reduction of bark content (16%) from 9.66% to 8.11%. Our findings show that trees grown under wider spacing tend to produce thicker bark. The bark thickness and the effect of plant spacing on the bark thickness decreased in the base-top direction. The correlation between bark thickness and wood diameter increases from 0.682 to 0.825 with the increase of spacing between trees. In contrast, the bark thickness to bark content correlation decrease from 0.735 to 0.15 with increased plant spacing. The stand density significantly affected the variation of the stem diameter, bark thickness and bark content of Eucalyptus plantations.