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Response of blackwood (Acacia melanoxylon) regeneration to silvicultural removal of competition in regrowth eucalypt forests of north-west Tasmania, Australia
Forest Ecology and Management 177 (2003) 75±83
Response of blackwood (Acacia melanoxylon) regeneration to silvicultural removal of competition in regrowth eucalypt forests of north-west Tasmania, Australia S.M. Jenningsa,*, G.R. Wilkinsonb, G.L. Unwinc a
Forestry Tasmania, G.P.O. Box 207, Hobart, Tasmania 7001, Australia Forest Practices Board, 30 Patrick Street, Hobart, Tasmania 7000, Australia c University of Tasmania, Locked Bag 1-376, Launceston, Tasmania 7250, Australia b
Received 5 November 2001
Abstract Saplings of blackwood (Acacia melanoxylon) responded positively to release from competing young Eucalyptus obliqua in native forests of north-west Tasmania. In the treated area, 6 years after establishment of the regrowth forest on a cleared and burned seedbed, the young emergent eucalypts were culled by stem injection of herbicide. The dense sub-canopy (principally Pomaderris apetala) was also removed in two gap treatments (3.6 and 7.2 m diameter); each treatment applied to 27 single tree plots. During the 6 years following silvicultural treatment, periodic annual increment (PAI) of blackwood stem diameter increased from 0.8 cm per year in the untreated area to 1.4 cm per year where eucalypts (only) had been removed. The additional removal of the sub-canopy increased blackwood PAI (diameter) to 1.6 cm per year in the small gaps and 2.1 cm per year in the larger gaps. These are statistically signi®cant increases of 75, 100 and 160%, respectively. However, the larger sub-canopy gaps produced heavier, broader blackwood crowns, increased branch size and retention and reduced the length of branch-free bole. Maximum blackwood diameter and volume gains were therefore achieved at the expense of tree form and future log quality. In contrast, the removal of competing eucalypts (only) produced a smaller stem diameter response, but the remaining dense sub-canopy maintained excellent stem form and clear bole. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Blackwood; Acacia melanoxylon; Regeneration; Native forest silviculture; Gap treatment; Tree growth and form; Release from competition
1. Introduction Blackwood (Acacia melanoxylon R. Br.) is a valuable timber species which is commonly found as a * Corresponding author. Present address: Forestry Tasmania, P.O. Box 63, Smithton, Tasmania 7330, Australia. Tel.: 61-3-6452-4909; fax: 61-3-6452-4925. E-mail address: [email protected] (S.M. Jennings).
canopy or sub-canopy tree in a broad range of forests on tablelands and coastal escarpments in eastern Australia from north Queensland to southern Tasmania (Boland et al., 1984). The best timber for high value cabinet woods and veneers has traditionally been sourced from the swamps and wet eucalypt forests of north-west Tasmania (Jennings, 1998). Prospects for increasing the blackwood timber resource include the establishment of blackwood plantations
0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 3 2 6 - 2
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S.M. Jennings et al. / Forest Ecology and Management 177 (2003) 75±83
and fencing of native forest regeneration on suitable sites. The biggest asset for the regeneration of blackwood in native forests is the ground-stored seed resource which germinates after disturbances, including logging (Wilkinson and Jennings, 1994). Other important silvicultural considerations include tree form and browsing by native mammals. Blackwood has a tendency to grow as a suppressed or poorly formed tree unless suitable conditions are created through appropriate gaps in the forest canopy. Large gaps, where both top and side light are available, appear to optimise growth but at the expense of good form. Smaller gaps, where side shading suppresses lower branch growth, tend to be associated with good form but slower stem diameter growth (Jennings, 1998). Blackwood seedlings are highly sensitive to browsing damage by native mammals. The density of blackwood seedlings is signi®cantly reduced unless seedlings are afforded protection for at least the ®rst 2 or 3 years after germination. Measures such as fencing can substantially increase blackwood density and stocking within regeneration areas (Jennings and Dawson, 1998). Tasmania has in excess of 750 ha of young regeneration in State forests, in areas that previously carried blackwood as a sub-canopy species, which are intended to be managed primarily for future blackwood production. The standard regeneration treatment on these sites is to clearfell the mature forest, burn the logging slash, sow eucalypt (Eucalyptus obliqua) seed and erect fencing to provide browsing protection for the blackwood seedlings that germinate from ground-stored seed (Jennings and Dawson, 1998). These coupes span about 15 years in age since burning, and the early coupes have started to reach the target age for pre-commercial thinning. They contain variable densities of both blackwood and eucalypt seedlings although most coupes are adequately stocked with both of these species. In 1995, a trial was established to assess the effect of competition between the blackwoods and eucalypts, the effect of competition from the understorey species and the effect of removal of competition on the growth and form of blackwood saplings. This information will assist with the development of a pre-commercial thinning prescription for these regrowth areas.
2. Methods 2.1. Study site The study site is 1.5 ha in area, located within a 37 ha forest coupe in the north-west of Tasmania (latitude 408550 S, longitude 1448570 E). It is 1.5 ha and situated adjacent to the road. The site is one of the oldest fenced coupes and contains a high stocking of evenly distributed eucalypt and blackwood regeneration. The site receives about 1200 mm of rainfall annually and is approximately 60 m in elevation. The topography is gently undulating with brown clay soils derived from cambrian mudstone. Before harvesting, the site carried a sparse E. obliqua forest of 30±40 m in height with a blackwood sub-canopy at 20±25 m in height and a wet sclerophyll understorey (Pomaderris apetala, Phebalium squameum, Acacia spp.) from 10±15 m in height. 2.2. Site management prior to trial establishment The site was clearfelled in early 1988 and the harvesting residue burnt in February 1989 producing an excellent mineral soil seedbed. The coupe was aerially sown with E. obliqua seed and fenced with wire netting in April 1989. A regeneration survey in August 1990, in accordance with standards described in Forestry Tasmania (1996), showed a high stocking, with 85% of the total area mapped as fully stocked with eucalypt seedlings. In addition, blackwood seedlings were well distributed throughout the coupe. In January 1995, 6 years after establishment, 30 50 m2 circular plots were established systematically through the coupe. In each plot, all eucalypts and blackwoods were counted. The tallest two eucalypts and the tallest two blackwoods on each plot were measured for height and diameter. The average eucalypt stem density was 3500 stems per ha (spha) with an additional 1100 blackwood seedlings per ha. The tallest eucalypts averaged 10.5 m in height (1.75 m per year) while the tallest blackwoods averaged 5.4 m (0.9 m per year). Mean diameter (diameter breast height over bark) growth was 1.7 cm per year for eucalypts and 0.5 cm per year for blackwood. The site carried dense P. apetala averaging about 5 m in height, which formed a continuous sub-canopy with the blackwoods but which will eventually form the understorey.
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2.3. Silvicultural treatment
Table 1 Summary of silvicultural treatments
In April 1995, the eucalypt regeneration within a 1 ha area of the coupe was killed, to remove all eucalypt competition from the blackwood saplings. This was carried out by stem-injecting Glyphosate herbicide into each tree. Within this area blackwood trees were chosen for release from competition from the co-dominant understorey vegetation. The understorey species were manually removed to form circular gaps of two different sizes. The controls were selected from an adjacent untreated area of the coupe. Each treatment was applied to single tree plots. Blackwood trees of suitable diameter were selected for spacing from throughout the treatment area. Trees with good stem form were selected where possible. Plots were established using the blackwood tree as the
Treatment Treatment Eucalypts Gap Area of No. no. name present diameter competition of (m) removed (m2) trees 1 2 3 4
Control No gap Small gap Large gap
Yes No No No
NA NA 3.6 7.2
Nil Nil 10 40
27 27 27 27
centre of the plot. Plots were measured as blocks of four trees, one for each of the following treatments (Table 1). A pictorial representation of the treatments is shown in Fig. 1. The blocks were replicated 27 times. The diameters of the blackwood trees within the coupe varied widely
Fig. 1. Pictorial representation of the four treatments at age 6 years, showing the different levels of competition provided by the eucalypts and the P. apetala understorey, and the silvicultural gap treatments applied.
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at age 6 years, but in each case the four trees within a block were chosen so that there was less than 0.5 cm difference between the largest and smallest diameter at initial measurement. The actual average diameter difference between largest and smallest trees within each of the 27 blocks was 0.4 cm. The variation among blocks re¯ected the range of tree sizes within the coupe. The average diameter of trees in the smallest block was 3.2 cm while those in the largest block averaged 7.7 cm. Sample trees were selected in August 1995. Thinning of the gaps took place between August and November 1995 using small chainsaws and a large brush-cutter. The smaller gaps gave each tree a clear area equivalent to a spacing of 1000 spha, the larger gaps provided an equivalent spacing to 250 blackwood spha. 2.4. Measurement and analysis Initial diameter measurement took place in August 1995 at age 6 years. In the winter of 1996, the stem form of each tree was rated from 1 (for very good form) to 4 (for very poor form). The height of the lowest green branch and the height of crown break (an estimate of bole height) were also measured. From 1998 onwards, breast height diameter of sample blackwood trees was measured annually, with
height measurements repeated in 1998 and 2000. The form rating system was not used again as it was considered too subjective. In 1999 crown width was measured in two cardinal axes, and at the 2000 measurement, the basal diameter and number of green branches within the lowest 6 m of each tree stem was determined. Potential sawlog volume, bole volume and total stem volumes were calculated using Forestry Tasmania's LOGVOLS 97 (Goodwin, 1997). This model is designed to calculate volumes of eucalypts, but was used in the absence of a speci®c blackwood model. The equation for a thin barked species of eucalypt was considered to be suitable for blackwood. Heights, diameters and volumes were analysed using a one-way analysis of variance (ANOVA). Starting diameters were used as a covariant for further analysis of stem diameter. Differences among treatments were considered signi®cant at probability levels of P < 0:01. 3. Results 3.1. Diameter growth The blackwood saplings responded consistently to increased spacing and light availability at age 6 years.
Fig. 2. Mean diameter growth of blackwood stems for each treatment. Diameter at breast height over bark (DBHOB) (1.3 m). Error bars show least signi®cant differences (LSDs) at P 0:05, n 27.
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Fig. 3. Mean crown width of blackwood 4 years after treatment (bars show S.E., n 27).
The response was evident after the ®rst year of measurement, but was not signi®cant among all treatments. By age 9 years, there was a signi®cant difference (F-test, P < 0:001) in mean diameter growth among all treatments, and this growth trend continued. At age 12 years, the controls (treatment 1) achieved a mean annual increment (MAI) (diameter) of 0.8 cm per year. The periodic annual increment (PAI) for the same trees over the last 6 years was still 0.8 cm per year showing steady diameter growth throughout the life of the trees. In the 6 years since treatment, PAI (diameter) increased to 1.4 cm per year for the no gap treatment, 1.6 cm per year for the small gap treatment and 2.1 cm per year for the large gap treatment. Patterns of blackwood diameter growth in response to treatment are shown in Fig. 2. 3.2. Crown width
age 6 years. Patterns of height growth are shown in Fig. 4. There is no signi®cant difference in height of the treatments 4 years after release. All treatments continued to grow steadily in height with MAI ranging from 0.95 m per year for the blackwoods still under a eucalypt canopy (control), to 1.0 m per year for the large gap treatment. 3.4. Blackwood stem form and branch habit Preliminary assessment of stem form of the blackwood trees before treatment showed consistently good form throughout the area. The mean form rating 1 year after treatment (on a scale of 1±4) ranged from 1.0 for the no gap treatment to 1.3 for the large gap treatment with the control and small gap treatment in being 1.1. The trees selected for the
The crowns of the blackwood saplings also responded to increased spacing and light availability (Fig. 3). Crown width for treatments without understorey clearing (treatments 1 and 2) were similar, while crowns in the gap treatments (3 and 4) responded by expanding into the release gaps in proportion to gap widths of 3.6 m for the small gap treatment and 7.2 m for the larger gaps. 3.3. Height growth Total tree height was unaffected by either eucalypt removal or by the establishment of gap treatments at
Fig. 4. Mean height of blackwood trees for each treatment (n 27).
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Fig. 5. Mean green height (stem height of lowest living branch) for each treatment (error bars show LSD at P 0:05, n 27).
trial showed good form with slight variation between treatments. The stem height to the lowest living branch on each blackwood tree (green height) was used to estimate current potential sawlog length (clear of persistent branches). Although total blackwood height did not vary signi®cantly among treatments (Fig. 4), mean green height was very different 5 years after treatment. The mean green heights for each treatment at ages 7 and 11 years are shown in Fig. 5. There were slight differences apparent between the treatments at age 7 years (1 year after treatment) but these were not signi®cant at the 0.01 level. At age 11 years, the no gap treatments (1 and 2) both averaged over 5.6 m to green height, which is close to our target of at least 6 m of clear bole. In the gap treatments (3 and 4) this was signi®cantly lower (F-
test, P < 0:001) with the large gap averaging just over 4 m to green height. Blackwood bole height was also in¯uenced by light availability. The number and size of living branches within the lowest 6 m of blackwood tree stems varied among treatments. Fig. 6 shows the mean number of live branches in 2 cm diameter classes, for each treatment. The no gap treatment (2) had the fewest branches in the lowest 6 m of blackwood stem. Due to removal of the eucalypt competition, some of these branches were larger in diameter than those of the controls (treatment 1). As the gap treatments resulted in progressively increased light availability, both the size and number of branches increased in response to treatment. The large gap treatment (4) had signi®cantly more branches of all sizes than all other treatments (F-test, P < 0:001).
Fig. 6. Mean number of branches in lowest 6 m of blackwood stems. Legend shows branch diameter classes (n 27 trees for each treatment).
S.M. Jennings et al. / Forest Ecology and Management 177 (2003) 75±83
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Fig. 7. Mean volume per tree of regrowth blackwood, by treatment and stem component (n 27).
3.5. Volume Using the lowest green branch as an indication of current potential sawlog height, and height of crown break for bole length, individual blackwood tree volumes were calculated using LOGVOLS 97 (see Section 2.4). The mean volume per tree by treatment and stem component is shown in Fig. 7. The most important factor contributing to the mean volume of each treatment is diameter growth. Despite the greatest potential sawlog length being produced by the no gap treatment (2), a signi®cantly greater sawlog volume was produced by the large gap treatment due to the higher diameter growth rate (F-test, P < 0:001). 4. Discussion Young blackwoods appear to have a great capacity to respond to changes in light availability within regrowth eucalypt forest, following canopy and subcanopy treatments. Under the conditions of this trial, the diameter response was rapid, and very consistent. PAI of stem diameter of up to 2.1 cm ha per year, achieved by releasing blackwood saplings from competition is similar to growth rates reported for plantation trials in New Zealand (New Zealand Forest Research Institute, 1982) and for trees planted into gaps in South Africa (Geldenhuys, 1996). These diameter growth rates are greater than for blackwood in plantations in Tasmania reported by Neilsen and Brown (1997).
The regrowth blackwoods at Togari are growing under almost ideal conditions, on a fertile site with adequate moisture throughout the year, and so it is likely that the limiting factor for their growth is light availability. This was also suggested by the rapid increase in crown width following the gap treatments, where additional space and light was created in the subcanopy. Within the range of silvicultural treatments applied the more light that trees received, the faster they grew. Competition from both eucalypt and understorey species limited blackwood growth, with diameter growth rates increasing with the progressive removal of eucalypt and sub-canopy competition (Figs. 2 and 3). However, as observed elsewhere in the district, the Pomaderris will reach an expected height limit at about 10±15 m, and the most competitive blackwood saplings will then overtop the understorey. At this stage, all of the treatments without eucalypt competition could be expected to grow at the same diameter growth rate with their crowns above the understorey level, receiving light from both side and top. To some extent this trend has occurred already for the no gap and small gap treatments. After an initial advantage to treatment 3, given by the clearing of a small gap, treatments 2 and 3 are now both advantaged predominantly by removal of the eucalypts and their diameter growth curves are parallel. The large gap treatment still has the advantage of some sidelight and diameter growth continues at a faster rate (Fig. 2). The effect of clearing sub-canopy gaps may therefore be a short-term gain, giving a growth advantage to selected regrowth blackwood trees, which lasts about 4 years for small gaps and at least 6 years in the case of
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S.M. Jennings et al. / Forest Ecology and Management 177 (2003) 75±83
the large gaps. Any long-term gain in growth will be from the removal of the eucalypts. The current diameter growth trends indicate that the control trees will continue to grow more slowly under a eucalypt canopy than the trees in all release treatments. The degree of release is also correlated with the number of spha. Large gap release equates to 250 spha while small gap release equates to 1000 spha. It is not possible to retain more blackwood stems for each of these treatments, respectively. Higher densities can be retained for no gap treatments. It is not yet known at what age the untreated regrowth stands may reach a ®nal stem density for blackwood, or at what age it is most advantageous to thin these stands to ®nal crop stocking. As with thinning operations for other species (Smith et al., 1997) (Medhurst et al., 2001), blackwood height growth did not vary signi®cantly among treatments. Height growth of 0.95±1 m per year is similar to that reported by Neilsen and Brown (1997) for blackwood plantations in Tasmania. The form of the blackwood saplings was predominantly in¯uenced by the sub-canopy vegetation rather than the eucalypt overstorey. This is shown in both Figs. 5 and 6. The branch habits of treatments with no understorey gaps, but with and without eucalypts, (treatments 1 and 2) were similar, while the removal of the competing sub-canopy vegetation (treatments 3 and 4) produced a signi®cantly different crown size and branch pattern. Where a densely stocked sub-canopy was retained, the lower branches of young blackwood were suppressed, and the dead branches were shed rapidly from the heavily shaded lower bole, with prompt and effective occlusion. The no gap treatment (2) showed the best blackwood stem form as light measurements showed that the Pomaderris also bene®ted from the absence of eucalypt competition and provided heavier shading where the eucalypts were removed. Removal of the sub-canopy vegetation increased the number and size of retained branches. In the small gaps, where sub-canopy removal was minimal, the branches in the lower 6 m of the stem were still being shed. In the large gaps, branches in the 4±6 cm diameter class were common, and with continued light availability it is likely that these will be permanent and the sawlog length is now set. New Zealand Forest Research Institute (1983) also concluded that control of tree form and branch size is directly proportional to the height of the close surrounding vegetation.
In terms of stem volume, short fast growing boles produce greater volumes than slightly longer boles growing at a slower rate. Maximum stem volume was produced in response to maximum removal of competition, and stem volume increment was proportional to light availability to the blackwood crowns. However, blackwood is not being grown for ®bre volume but as a high quality product suitable for clearwood and veneer. Short branchy logs are not the product sought by the furniture industry. It would therefore seem that some compromise should be sought between volume and quality. 5. Conclusion Removal of eucalypts does not impair blackwood form and does produce a long-term growth advantage to the blackwoods in young regrowth forests in northwest Tasmania. Removal of the competing sub-canopy species gives blackwoods at least a short-term growth advantage and allows selection of crop trees. However, removal of large amounts of the sub-canopy compromises effective sawlog length. To retain the naturally good form of the regrowth blackwoods, large subcanopy gaps should not be created until a clear branchfree sawlog length of at least 6 m has developed. However, by this time it is likely that the growth advantage from gaps will be minimal, as the blackwood crowns will then be emerging from understorey competition. It is likely that a prescription based on the selective removal of eucalypts and other competing species (blackwoods of poor form, other Acacia spp., very close or large Pomaderris) near selected blackwood stems would provide the basis for an effective method of increasing the growth-rates of the blackwoods. Retention of the great majority of the Pomaderris sub-canopy should ensure effective lower branch suppression and high quality sawlog production in these stands. Acknowledgements We would like to thank Ann LaSala for the volume calculations and Joanne Dingle for assisting with the statistical calculations. Grant Denholm provided the illustration of the treatments. We also thank John
S.M. Jennings et al. / Forest Ecology and Management 177 (2003) 75±83
Hickey for support and comments on an earlier version of the manuscript. References Boland, D.J., Brooker, M.I.H., Chippendale, G.M., Hall, N., Hyland, B.P.M., Johnston, R.D., Kleinig, D.A., Turner, J.D., 1984. Forest Trees of Australia. Nelson, CSIRO. Forestry Tasmania, 1996. Native Forest Silviculture Technical Bulletin No. 6, Regeneration Surveys and Stocking Standards. Hobart, Tasmania. Geldenhuys, C.J., 1996. The Blackwood Group System: it's relevance for sustainable forest management in the southern Cape. S. Afr. For. J. 177, 7±21. Goodwin, A., 1997. LOGVOLS 97. Forestry Tasmania, Hobart, unpublished. Jennings, S.M., 1998. Managing native forests for blackwood (Acacia melanoxylon) production in north-western Tasmania. Aust. For. 61, 141±146.
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Jennings, S.M., Dawson, J.K., 1998. Fencing eucalypt coupes for blackwood regeneration. Tasforests 10, 103±113. Medhurst, J.L., Beadle, C.L., Neilsen, W.A., 2001. Early-age and later-age thinning affects growth, dominance and intraspeci®c competition in Eucalyptus nitens plantations, dominance and intraspeci®c competition in Eucalyptus nitens plantations. Can. J. For. Res. 31, 187±197. Neilsen, W.A., Brown, D.R., 1997. Growth and silviculture of Acacia melanoxylon plantations in Tasmania. Tasforests 9, 51± 70. Smith, D.M., Larson, B.C., Kelty, M.J., Ashton, P.M.S., 1997. The Practice of Silviculture: Applied Forest Ecology. Wiley, New York, pp. 51±52. New Zealand Forest Research Institute, 1982. Australian Blackwood (Acacia melanoxylon). What's New in Forest Research, No. 105. New Zealand Forest Research Institute, 1983. Interplanting. What's New in Forest Research, No. 121. Wilkinson, G.R., Jennings, S.M., 1994. Regeneration of blackwood from ground-stored seed in the north arthur forests, northwestern Tasmania. Tasforests 6, 69±78.
Response of blackwood (Acacia melanoxylon) regeneration to silvicultural removal of competition in regrowth eucalypt forests of north-west Tasmania, Australia S.M. Jenningsa,*, G.R. Wilkinsonb, G.L. Unwinc a
Forestry Tasmania, G.P.O. Box 207, Hobart, Tasmania 7001, Australia Forest Practices Board, 30 Patrick Street, Hobart, Tasmania 7000, Australia c University of Tasmania, Locked Bag 1-376, Launceston, Tasmania 7250, Australia b
Received 5 November 2001
Abstract Saplings of blackwood (Acacia melanoxylon) responded positively to release from competing young Eucalyptus obliqua in native forests of north-west Tasmania. In the treated area, 6 years after establishment of the regrowth forest on a cleared and burned seedbed, the young emergent eucalypts were culled by stem injection of herbicide. The dense sub-canopy (principally Pomaderris apetala) was also removed in two gap treatments (3.6 and 7.2 m diameter); each treatment applied to 27 single tree plots. During the 6 years following silvicultural treatment, periodic annual increment (PAI) of blackwood stem diameter increased from 0.8 cm per year in the untreated area to 1.4 cm per year where eucalypts (only) had been removed. The additional removal of the sub-canopy increased blackwood PAI (diameter) to 1.6 cm per year in the small gaps and 2.1 cm per year in the larger gaps. These are statistically signi®cant increases of 75, 100 and 160%, respectively. However, the larger sub-canopy gaps produced heavier, broader blackwood crowns, increased branch size and retention and reduced the length of branch-free bole. Maximum blackwood diameter and volume gains were therefore achieved at the expense of tree form and future log quality. In contrast, the removal of competing eucalypts (only) produced a smaller stem diameter response, but the remaining dense sub-canopy maintained excellent stem form and clear bole. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Blackwood; Acacia melanoxylon; Regeneration; Native forest silviculture; Gap treatment; Tree growth and form; Release from competition
1. Introduction Blackwood (Acacia melanoxylon R. Br.) is a valuable timber species which is commonly found as a * Corresponding author. Present address: Forestry Tasmania, P.O. Box 63, Smithton, Tasmania 7330, Australia. Tel.: 61-3-6452-4909; fax: 61-3-6452-4925. E-mail address: [email protected] (S.M. Jennings).
canopy or sub-canopy tree in a broad range of forests on tablelands and coastal escarpments in eastern Australia from north Queensland to southern Tasmania (Boland et al., 1984). The best timber for high value cabinet woods and veneers has traditionally been sourced from the swamps and wet eucalypt forests of north-west Tasmania (Jennings, 1998). Prospects for increasing the blackwood timber resource include the establishment of blackwood plantations
0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 3 2 6 - 2
76
S.M. Jennings et al. / Forest Ecology and Management 177 (2003) 75±83
and fencing of native forest regeneration on suitable sites. The biggest asset for the regeneration of blackwood in native forests is the ground-stored seed resource which germinates after disturbances, including logging (Wilkinson and Jennings, 1994). Other important silvicultural considerations include tree form and browsing by native mammals. Blackwood has a tendency to grow as a suppressed or poorly formed tree unless suitable conditions are created through appropriate gaps in the forest canopy. Large gaps, where both top and side light are available, appear to optimise growth but at the expense of good form. Smaller gaps, where side shading suppresses lower branch growth, tend to be associated with good form but slower stem diameter growth (Jennings, 1998). Blackwood seedlings are highly sensitive to browsing damage by native mammals. The density of blackwood seedlings is signi®cantly reduced unless seedlings are afforded protection for at least the ®rst 2 or 3 years after germination. Measures such as fencing can substantially increase blackwood density and stocking within regeneration areas (Jennings and Dawson, 1998). Tasmania has in excess of 750 ha of young regeneration in State forests, in areas that previously carried blackwood as a sub-canopy species, which are intended to be managed primarily for future blackwood production. The standard regeneration treatment on these sites is to clearfell the mature forest, burn the logging slash, sow eucalypt (Eucalyptus obliqua) seed and erect fencing to provide browsing protection for the blackwood seedlings that germinate from ground-stored seed (Jennings and Dawson, 1998). These coupes span about 15 years in age since burning, and the early coupes have started to reach the target age for pre-commercial thinning. They contain variable densities of both blackwood and eucalypt seedlings although most coupes are adequately stocked with both of these species. In 1995, a trial was established to assess the effect of competition between the blackwoods and eucalypts, the effect of competition from the understorey species and the effect of removal of competition on the growth and form of blackwood saplings. This information will assist with the development of a pre-commercial thinning prescription for these regrowth areas.
2. Methods 2.1. Study site The study site is 1.5 ha in area, located within a 37 ha forest coupe in the north-west of Tasmania (latitude 408550 S, longitude 1448570 E). It is 1.5 ha and situated adjacent to the road. The site is one of the oldest fenced coupes and contains a high stocking of evenly distributed eucalypt and blackwood regeneration. The site receives about 1200 mm of rainfall annually and is approximately 60 m in elevation. The topography is gently undulating with brown clay soils derived from cambrian mudstone. Before harvesting, the site carried a sparse E. obliqua forest of 30±40 m in height with a blackwood sub-canopy at 20±25 m in height and a wet sclerophyll understorey (Pomaderris apetala, Phebalium squameum, Acacia spp.) from 10±15 m in height. 2.2. Site management prior to trial establishment The site was clearfelled in early 1988 and the harvesting residue burnt in February 1989 producing an excellent mineral soil seedbed. The coupe was aerially sown with E. obliqua seed and fenced with wire netting in April 1989. A regeneration survey in August 1990, in accordance with standards described in Forestry Tasmania (1996), showed a high stocking, with 85% of the total area mapped as fully stocked with eucalypt seedlings. In addition, blackwood seedlings were well distributed throughout the coupe. In January 1995, 6 years after establishment, 30 50 m2 circular plots were established systematically through the coupe. In each plot, all eucalypts and blackwoods were counted. The tallest two eucalypts and the tallest two blackwoods on each plot were measured for height and diameter. The average eucalypt stem density was 3500 stems per ha (spha) with an additional 1100 blackwood seedlings per ha. The tallest eucalypts averaged 10.5 m in height (1.75 m per year) while the tallest blackwoods averaged 5.4 m (0.9 m per year). Mean diameter (diameter breast height over bark) growth was 1.7 cm per year for eucalypts and 0.5 cm per year for blackwood. The site carried dense P. apetala averaging about 5 m in height, which formed a continuous sub-canopy with the blackwoods but which will eventually form the understorey.
S.M. Jennings et al. / Forest Ecology and Management 177 (2003) 75±83
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2.3. Silvicultural treatment
Table 1 Summary of silvicultural treatments
In April 1995, the eucalypt regeneration within a 1 ha area of the coupe was killed, to remove all eucalypt competition from the blackwood saplings. This was carried out by stem-injecting Glyphosate herbicide into each tree. Within this area blackwood trees were chosen for release from competition from the co-dominant understorey vegetation. The understorey species were manually removed to form circular gaps of two different sizes. The controls were selected from an adjacent untreated area of the coupe. Each treatment was applied to single tree plots. Blackwood trees of suitable diameter were selected for spacing from throughout the treatment area. Trees with good stem form were selected where possible. Plots were established using the blackwood tree as the
Treatment Treatment Eucalypts Gap Area of No. no. name present diameter competition of (m) removed (m2) trees 1 2 3 4
Control No gap Small gap Large gap
Yes No No No
NA NA 3.6 7.2
Nil Nil 10 40
27 27 27 27
centre of the plot. Plots were measured as blocks of four trees, one for each of the following treatments (Table 1). A pictorial representation of the treatments is shown in Fig. 1. The blocks were replicated 27 times. The diameters of the blackwood trees within the coupe varied widely
Fig. 1. Pictorial representation of the four treatments at age 6 years, showing the different levels of competition provided by the eucalypts and the P. apetala understorey, and the silvicultural gap treatments applied.
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at age 6 years, but in each case the four trees within a block were chosen so that there was less than 0.5 cm difference between the largest and smallest diameter at initial measurement. The actual average diameter difference between largest and smallest trees within each of the 27 blocks was 0.4 cm. The variation among blocks re¯ected the range of tree sizes within the coupe. The average diameter of trees in the smallest block was 3.2 cm while those in the largest block averaged 7.7 cm. Sample trees were selected in August 1995. Thinning of the gaps took place between August and November 1995 using small chainsaws and a large brush-cutter. The smaller gaps gave each tree a clear area equivalent to a spacing of 1000 spha, the larger gaps provided an equivalent spacing to 250 blackwood spha. 2.4. Measurement and analysis Initial diameter measurement took place in August 1995 at age 6 years. In the winter of 1996, the stem form of each tree was rated from 1 (for very good form) to 4 (for very poor form). The height of the lowest green branch and the height of crown break (an estimate of bole height) were also measured. From 1998 onwards, breast height diameter of sample blackwood trees was measured annually, with
height measurements repeated in 1998 and 2000. The form rating system was not used again as it was considered too subjective. In 1999 crown width was measured in two cardinal axes, and at the 2000 measurement, the basal diameter and number of green branches within the lowest 6 m of each tree stem was determined. Potential sawlog volume, bole volume and total stem volumes were calculated using Forestry Tasmania's LOGVOLS 97 (Goodwin, 1997). This model is designed to calculate volumes of eucalypts, but was used in the absence of a speci®c blackwood model. The equation for a thin barked species of eucalypt was considered to be suitable for blackwood. Heights, diameters and volumes were analysed using a one-way analysis of variance (ANOVA). Starting diameters were used as a covariant for further analysis of stem diameter. Differences among treatments were considered signi®cant at probability levels of P < 0:01. 3. Results 3.1. Diameter growth The blackwood saplings responded consistently to increased spacing and light availability at age 6 years.
Fig. 2. Mean diameter growth of blackwood stems for each treatment. Diameter at breast height over bark (DBHOB) (1.3 m). Error bars show least signi®cant differences (LSDs) at P 0:05, n 27.
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Fig. 3. Mean crown width of blackwood 4 years after treatment (bars show S.E., n 27).
The response was evident after the ®rst year of measurement, but was not signi®cant among all treatments. By age 9 years, there was a signi®cant difference (F-test, P < 0:001) in mean diameter growth among all treatments, and this growth trend continued. At age 12 years, the controls (treatment 1) achieved a mean annual increment (MAI) (diameter) of 0.8 cm per year. The periodic annual increment (PAI) for the same trees over the last 6 years was still 0.8 cm per year showing steady diameter growth throughout the life of the trees. In the 6 years since treatment, PAI (diameter) increased to 1.4 cm per year for the no gap treatment, 1.6 cm per year for the small gap treatment and 2.1 cm per year for the large gap treatment. Patterns of blackwood diameter growth in response to treatment are shown in Fig. 2. 3.2. Crown width
age 6 years. Patterns of height growth are shown in Fig. 4. There is no signi®cant difference in height of the treatments 4 years after release. All treatments continued to grow steadily in height with MAI ranging from 0.95 m per year for the blackwoods still under a eucalypt canopy (control), to 1.0 m per year for the large gap treatment. 3.4. Blackwood stem form and branch habit Preliminary assessment of stem form of the blackwood trees before treatment showed consistently good form throughout the area. The mean form rating 1 year after treatment (on a scale of 1±4) ranged from 1.0 for the no gap treatment to 1.3 for the large gap treatment with the control and small gap treatment in being 1.1. The trees selected for the
The crowns of the blackwood saplings also responded to increased spacing and light availability (Fig. 3). Crown width for treatments without understorey clearing (treatments 1 and 2) were similar, while crowns in the gap treatments (3 and 4) responded by expanding into the release gaps in proportion to gap widths of 3.6 m for the small gap treatment and 7.2 m for the larger gaps. 3.3. Height growth Total tree height was unaffected by either eucalypt removal or by the establishment of gap treatments at
Fig. 4. Mean height of blackwood trees for each treatment (n 27).
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Fig. 5. Mean green height (stem height of lowest living branch) for each treatment (error bars show LSD at P 0:05, n 27).
trial showed good form with slight variation between treatments. The stem height to the lowest living branch on each blackwood tree (green height) was used to estimate current potential sawlog length (clear of persistent branches). Although total blackwood height did not vary signi®cantly among treatments (Fig. 4), mean green height was very different 5 years after treatment. The mean green heights for each treatment at ages 7 and 11 years are shown in Fig. 5. There were slight differences apparent between the treatments at age 7 years (1 year after treatment) but these were not signi®cant at the 0.01 level. At age 11 years, the no gap treatments (1 and 2) both averaged over 5.6 m to green height, which is close to our target of at least 6 m of clear bole. In the gap treatments (3 and 4) this was signi®cantly lower (F-
test, P < 0:001) with the large gap averaging just over 4 m to green height. Blackwood bole height was also in¯uenced by light availability. The number and size of living branches within the lowest 6 m of blackwood tree stems varied among treatments. Fig. 6 shows the mean number of live branches in 2 cm diameter classes, for each treatment. The no gap treatment (2) had the fewest branches in the lowest 6 m of blackwood stem. Due to removal of the eucalypt competition, some of these branches were larger in diameter than those of the controls (treatment 1). As the gap treatments resulted in progressively increased light availability, both the size and number of branches increased in response to treatment. The large gap treatment (4) had signi®cantly more branches of all sizes than all other treatments (F-test, P < 0:001).
Fig. 6. Mean number of branches in lowest 6 m of blackwood stems. Legend shows branch diameter classes (n 27 trees for each treatment).
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Fig. 7. Mean volume per tree of regrowth blackwood, by treatment and stem component (n 27).
3.5. Volume Using the lowest green branch as an indication of current potential sawlog height, and height of crown break for bole length, individual blackwood tree volumes were calculated using LOGVOLS 97 (see Section 2.4). The mean volume per tree by treatment and stem component is shown in Fig. 7. The most important factor contributing to the mean volume of each treatment is diameter growth. Despite the greatest potential sawlog length being produced by the no gap treatment (2), a signi®cantly greater sawlog volume was produced by the large gap treatment due to the higher diameter growth rate (F-test, P < 0:001). 4. Discussion Young blackwoods appear to have a great capacity to respond to changes in light availability within regrowth eucalypt forest, following canopy and subcanopy treatments. Under the conditions of this trial, the diameter response was rapid, and very consistent. PAI of stem diameter of up to 2.1 cm ha per year, achieved by releasing blackwood saplings from competition is similar to growth rates reported for plantation trials in New Zealand (New Zealand Forest Research Institute, 1982) and for trees planted into gaps in South Africa (Geldenhuys, 1996). These diameter growth rates are greater than for blackwood in plantations in Tasmania reported by Neilsen and Brown (1997).
The regrowth blackwoods at Togari are growing under almost ideal conditions, on a fertile site with adequate moisture throughout the year, and so it is likely that the limiting factor for their growth is light availability. This was also suggested by the rapid increase in crown width following the gap treatments, where additional space and light was created in the subcanopy. Within the range of silvicultural treatments applied the more light that trees received, the faster they grew. Competition from both eucalypt and understorey species limited blackwood growth, with diameter growth rates increasing with the progressive removal of eucalypt and sub-canopy competition (Figs. 2 and 3). However, as observed elsewhere in the district, the Pomaderris will reach an expected height limit at about 10±15 m, and the most competitive blackwood saplings will then overtop the understorey. At this stage, all of the treatments without eucalypt competition could be expected to grow at the same diameter growth rate with their crowns above the understorey level, receiving light from both side and top. To some extent this trend has occurred already for the no gap and small gap treatments. After an initial advantage to treatment 3, given by the clearing of a small gap, treatments 2 and 3 are now both advantaged predominantly by removal of the eucalypts and their diameter growth curves are parallel. The large gap treatment still has the advantage of some sidelight and diameter growth continues at a faster rate (Fig. 2). The effect of clearing sub-canopy gaps may therefore be a short-term gain, giving a growth advantage to selected regrowth blackwood trees, which lasts about 4 years for small gaps and at least 6 years in the case of
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the large gaps. Any long-term gain in growth will be from the removal of the eucalypts. The current diameter growth trends indicate that the control trees will continue to grow more slowly under a eucalypt canopy than the trees in all release treatments. The degree of release is also correlated with the number of spha. Large gap release equates to 250 spha while small gap release equates to 1000 spha. It is not possible to retain more blackwood stems for each of these treatments, respectively. Higher densities can be retained for no gap treatments. It is not yet known at what age the untreated regrowth stands may reach a ®nal stem density for blackwood, or at what age it is most advantageous to thin these stands to ®nal crop stocking. As with thinning operations for other species (Smith et al., 1997) (Medhurst et al., 2001), blackwood height growth did not vary signi®cantly among treatments. Height growth of 0.95±1 m per year is similar to that reported by Neilsen and Brown (1997) for blackwood plantations in Tasmania. The form of the blackwood saplings was predominantly in¯uenced by the sub-canopy vegetation rather than the eucalypt overstorey. This is shown in both Figs. 5 and 6. The branch habits of treatments with no understorey gaps, but with and without eucalypts, (treatments 1 and 2) were similar, while the removal of the competing sub-canopy vegetation (treatments 3 and 4) produced a signi®cantly different crown size and branch pattern. Where a densely stocked sub-canopy was retained, the lower branches of young blackwood were suppressed, and the dead branches were shed rapidly from the heavily shaded lower bole, with prompt and effective occlusion. The no gap treatment (2) showed the best blackwood stem form as light measurements showed that the Pomaderris also bene®ted from the absence of eucalypt competition and provided heavier shading where the eucalypts were removed. Removal of the sub-canopy vegetation increased the number and size of retained branches. In the small gaps, where sub-canopy removal was minimal, the branches in the lower 6 m of the stem were still being shed. In the large gaps, branches in the 4±6 cm diameter class were common, and with continued light availability it is likely that these will be permanent and the sawlog length is now set. New Zealand Forest Research Institute (1983) also concluded that control of tree form and branch size is directly proportional to the height of the close surrounding vegetation.
In terms of stem volume, short fast growing boles produce greater volumes than slightly longer boles growing at a slower rate. Maximum stem volume was produced in response to maximum removal of competition, and stem volume increment was proportional to light availability to the blackwood crowns. However, blackwood is not being grown for ®bre volume but as a high quality product suitable for clearwood and veneer. Short branchy logs are not the product sought by the furniture industry. It would therefore seem that some compromise should be sought between volume and quality. 5. Conclusion Removal of eucalypts does not impair blackwood form and does produce a long-term growth advantage to the blackwoods in young regrowth forests in northwest Tasmania. Removal of the competing sub-canopy species gives blackwoods at least a short-term growth advantage and allows selection of crop trees. However, removal of large amounts of the sub-canopy compromises effective sawlog length. To retain the naturally good form of the regrowth blackwoods, large subcanopy gaps should not be created until a clear branchfree sawlog length of at least 6 m has developed. However, by this time it is likely that the growth advantage from gaps will be minimal, as the blackwood crowns will then be emerging from understorey competition. It is likely that a prescription based on the selective removal of eucalypts and other competing species (blackwoods of poor form, other Acacia spp., very close or large Pomaderris) near selected blackwood stems would provide the basis for an effective method of increasing the growth-rates of the blackwoods. Retention of the great majority of the Pomaderris sub-canopy should ensure effective lower branch suppression and high quality sawlog production in these stands. Acknowledgements We would like to thank Ann LaSala for the volume calculations and Joanne Dingle for assisting with the statistical calculations. Grant Denholm provided the illustration of the treatments. We also thank John
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Hickey for support and comments on an earlier version of the manuscript. References Boland, D.J., Brooker, M.I.H., Chippendale, G.M., Hall, N., Hyland, B.P.M., Johnston, R.D., Kleinig, D.A., Turner, J.D., 1984. Forest Trees of Australia. Nelson, CSIRO. Forestry Tasmania, 1996. Native Forest Silviculture Technical Bulletin No. 6, Regeneration Surveys and Stocking Standards. Hobart, Tasmania. Geldenhuys, C.J., 1996. The Blackwood Group System: it's relevance for sustainable forest management in the southern Cape. S. Afr. For. J. 177, 7±21. Goodwin, A., 1997. LOGVOLS 97. Forestry Tasmania, Hobart, unpublished. Jennings, S.M., 1998. Managing native forests for blackwood (Acacia melanoxylon) production in north-western Tasmania. Aust. For. 61, 141±146.
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Jennings, S.M., Dawson, J.K., 1998. Fencing eucalypt coupes for blackwood regeneration. Tasforests 10, 103±113. Medhurst, J.L., Beadle, C.L., Neilsen, W.A., 2001. Early-age and later-age thinning affects growth, dominance and intraspeci®c competition in Eucalyptus nitens plantations, dominance and intraspeci®c competition in Eucalyptus nitens plantations. Can. J. For. Res. 31, 187±197. Neilsen, W.A., Brown, D.R., 1997. Growth and silviculture of Acacia melanoxylon plantations in Tasmania. Tasforests 9, 51± 70. Smith, D.M., Larson, B.C., Kelty, M.J., Ashton, P.M.S., 1997. The Practice of Silviculture: Applied Forest Ecology. Wiley, New York, pp. 51±52. New Zealand Forest Research Institute, 1982. Australian Blackwood (Acacia melanoxylon). What's New in Forest Research, No. 105. New Zealand Forest Research Institute, 1983. Interplanting. What's New in Forest Research, No. 121. Wilkinson, G.R., Jennings, S.M., 1994. Regeneration of blackwood from ground-stored seed in the north arthur forests, northwestern Tasmania. Tasforests 6, 69±78.