Coppiced
Wood density is an important property when assessing the raw material quality for pulping since it affects pulp yield and other aspects of the pulp and papermaking processes, as well as the economics of forest production and mill processing. Most breeding programmes have therefore taken wood density as one of the determining selection criteria. This has been the case of eucalypts used for the pulp industry, for which the breeding traits have included wood density in addition to those related to tree growth (
The plantations are managed as a coppice system, with a first cycle of single-stem trees (first rotation) followed by two or three coppice cycles until the final harvest, stump removal and replanting. The species regenerates easily through stump coppice after harvesting. Since the coppice regrowth benefits from the established root system, the coppice rotations without replanting are advantageous, provided that the first rotation was well established and the stump regrowth was sufficient (
The wood quality of
However, there is very little information on the quality of coppiced wood and the differences in relation to the previous single-stem rotation. Information on the influence of silvicultural practices on the technological quality of coppiced trees is also particularly sparse.
The objective of the present study is to provide insight on the stem quality of coppiced
The pulping company CELBI (now ALTRI) established the Alto do Vilão trial with commercial seeds in March 1975. The trial includes two blocks, each with five plots with different tree spacings (m×m): 3×2 (1667 trees ha-1), 3×3 (1111 trees ha-1), 4×3 (833 trees ha-1), 4×4 (625 trees ha-1) and 5×4 (500 trees ha-1). The blocks are located side by side and the spacings are distributed according to decreasing density in one of the blocks and the opposite in the other block.
The trial was harvested in March 1993, at 18 years of age, as the first rotation, and the stumps were left to sprout for a second rotation cycle. The thinning of the shoots took place in October 1995, 2.6 years after harvest, following the usual practice of leaving shoots with dimensions comparable to those of the first rotation of the same age (
Sampling was conducted in a total of 25 stumps by randomly selecting five stumps in each spacing plot. As described, the stumps had a variable number of stems; seven stumps with three stems/stump, six stumps with two stems/stump, and 12 stumps with one stem/stump. From each sampled tree, 10 cm thick discs were cut at different stem heights: base, 1.30 m and every 2 m along the stem until the top, corresponding to a 7 cm diameter.
The wood and bark were separated. Estimates of basic density were obtained using the water immersion method by determining the green saturated volume and the oven-dry weight (TAPPI Test Method - T258 om 98, TAPPI Press, Atlanta, GA, USA). The influence of the initial planting density and stump density on wood density at breast height was studied using a one-way ANOVA analysis. Statistical analysis was conducted using the software package SJHNASUAU© for Windows Version 2.0 (Jandel Corporation, San Jose, CA, USA), with α = 0.05.
The trees harvested in the second cutting had been also measured at the end of the previous first cutting, therefore allowing for direct comparison between the two rotation cycles, since the trees were 18 year old in both cutting cycles. At the harvest of the first rotation of this trial, the overbark diameter at 1.3 m above the ground and the tree height were determined and the wood basic density was measured at dbh (
For comparison of tree and stand volume production between the two cutting cycles, the individual tree total volume was calculated for the first and second rotations using the volume equation model developed for
Bark volume was calculated as the difference between the tree and stemwood volumes.
The average wood basic density at breast height for all the eucalypt coppiced trees was 567 kg m-3, ranging from 553 to 597 kg m-3 among spacings. For the same trees at 18 years of age in the first rotation, the range of wood density was between 563 and 594 kg m-3, with an average of 582 kg m-3 (
The differences in wood density between the coppiced and the single stem trees were of a small magnitude. On average, the wood from coppiced trees was 2.5% less dense than that of the single stem trees. The difference was statistically significant (P=0.039) only for the 3×3 m spacing (
A few studies on
An effect of stump density on the wood basic density was found (P = 0.036), though the differences were statistically significant only between the widest (4×4) and the closest (3×3 and 3×2) spacings. The between-tree variability in each spacing was small, with coefficients of variance of the means below 5% in all cases. However, the stand density did not significantly influence wood density in the first rotation (
Wood density did not show a correlation with growth rate, as measured by tree diameter (R² = 0.0154, P = 0.433), confirming previous reports that growth rate is not correlated with wood density in
The axial variation of the wood basic density showed in all cases a similar profile in the coppiced trees, increasing along the stem from 581 to 636 kg m-3, respectively at the base and top (
The bark content in 18-year-old
The bark density at breast height was, on average, 473 kg m-3, ranging from 455 to 487 kg m-3 among different spacings (
Very few data exist on
The average tree volume in the coppice was lower than in the first rotation because the individual trees were smaller by about 40 to 66% (
Stand density was an important factor in stand productivity during the first rotation. In the closest spacing of 3×2 m, the trees were smaller and increased regularly with the widening of the spacing, until the 4×5 m spacing, respectively 0.32 m3 and 0.67 m3. However, the tree dimensions did not compensate for the difference in the number of trees in the different spacings, and the volume production per ha was highest in the 3×2 spacing (666.8 m3 ha-1) and lowest in the 4×4 (300 m3 ha-1) and 4×5 (320 m3 ha-1) spacings.
On the contrary, the spacing effect in the second rotation was not observed at tree level. This shows that a coppiced stand somewhat loses the effect of the initial planting density and has a more irregular space occupation by the tree crowns, probably in relation to the specific local situation,
In coppiced eucalypt plantations, it is usually assumed that peak production occurs in the second rotation, as a result of the favorable effect of a fully developed root system, and explaining the faster initial growth of shoots as compared to that of seedlings (
The average stem bark content, in percentage of stem volume, was 17.4%, providing 25 to 52 ton ha-1 of bark respectively for the widest (4×5 m) and closest spacing (3×2 m).
The wood of coppiced
Overall, the differences between the first and second cutting cycles refer to individual tree volume and stand productivity, which are largely dependent on the stand density resulting from the number of sprouts left per stump. The silvicultural operation of coppice thinning should take this into account.
The work was carried out with the base funding to
Average wood density of parent and coppiced trees for the different spacings. Bars represent the standard deviation.
Axial variation of wood basic density of 18-year-old
Axial variation of bark basic density of 18-year-old
Ratio of volume production (m3 ha-1) between the 2nd and 1st rotations of
Variation of wood and bark basic density at breast height and bark content in percentage of stem volume in the 18-year-old coppiced
Rotation | Variable | Spacing (m × m) | ||||
---|---|---|---|---|---|---|
4×5 | 4×4 | 4×3 | 3×3 | 3×2 | ||
2nd | Wood density (kgm-3) | 582 ± 26 | 597 ± 37 | 574 ± 32 | 557 ± 24 | 553 ± 38 |
Bark density (kgm-3) | 456 ± 88 | 487 ± 21 | 483 ± 44 | 482 ± 25 | 455 ± 34 | |
Bark content (% of stem volume) | 17.4 ± 0.1 | 17.4 ± 0.1 | 17.4 ± 0.1 | 17.4 ± 0.2 | 17.4 ± 0.1 | |
1st | Wood density (kgm-3) | 594 ± 43 | 583 ± 44 | 590 ± 40 | 580 ± 34 | 563 ± 42 |
Bark content (% of stem volume) | 14.4 ± 0.5 | 13.4 ± 2.5 | 14.0 ± 0.7 | 13.8 ± 0.7 | 13.4 ± 1.1 |
Biometric data for
Rotation | Variable | Spacing (m × m) | ||||
---|---|---|---|---|---|---|
4×5 | 4×4 | 4×3 | 3×3 | 3×2 | ||
2nd | Tree volume (m3) | 0.30 ± 0.07 | 0.20 ± 0.08 | 0.30 ± 0.15 | 0.25 ± 0.17 | 0.35 ± 0.13 |
Volume per stump (m3) | 0.64 ± 0.30 | 0.48 ± 0.39 | 0.55 ± 0.34 | 0.38 ± 0.30 | 0.40± 0.16 | |
Volume over bark (m3 ha-1) | 320.0 | 300.0 | 458.2 | 422.2 | 666.8 | |
Bark content (% of stem volume) | 17.4 ± 0.1 | 17.4 ± 0.1 | 17.4 ± 0.1 | 17.4 ± 0.2 | 17.4 ± 0.1 | |
Volume of bark (m3 ha-1) | 55.7 | 52.2 | 79.7 | 73.5 | 116.0 | |
Wood production (ton ha-1) | 155 | 149 | 218 | 195 | 306 | |
Bark production (ton ha-1) | 25 | 25 | 38 | 35 | 52 | |
Stem bark content (% weight) | 13.63 | 14.19 | 14.64 | 15.06 | 14.32 | |
1st | Tree volume (m3) | 0.67 ± 0.32 | 0.51 ± 0.26 | 0.45 ± 0.19 | 0.38 ± 0.20 | 0.32 ± 0.11 |
Volume over bark (m3 ha-1) | 292.2 | 284.6 | 321.7 | 341.6 | 378.9 | |
Bark content (% of stem volume) | 14.4 ± 0.5 | 13.4 ± 2.5 | 14.0 ± 0.7 | 13.8 ± 0.7 | 13.4 ± 1.1 | |
Volume of bark (m3 ha-1) | 48.2 | 45.9 | 54.0 | 60.8 | 76.8 |