Loss of apical dominance, resulting in seedlings with more than one leader, generally referred to as multiple leaders (ML), has been a major problem in forest tree plantation in the tropics. A study to examine the effects of site preparation by burning on growth performance and multiple leaders (ML) formation, and its variation in eight-genotypes of <i>Acacia mangium </i> and <i>Acacia auriculiformis </i> was conducted. The design used for this field trial is a randomized complete block design with four replications at two sites (burnt and unburned) and eight genotypes (four from each species), and the effects were monitored for 24 months. There were significant differences at P < 0.05 between sites and genotypes for the number of ML trees/plot, basal diameter and survival. Differences among the genotypes, in terms of height, were also significant, but not between sites. The site × genotype interaction was significant only for ML trees per plot. Generally, the number and the size of ML trees per plot in the burnt site were higher and bigger than those in the unburned site. The number of ML trees per plot formed increased exponentially with time, reaching a peak in about four months after their first occurrence and then leveled off to an almost steady state thereafter at both sites. The average number of multiples (leaders) per tree ranged from 2 to 5 at the burnt site whilst at the unburned site, all the ML trees were of two leaders. <i>A. mangium </i> provenances were more responsive to burning than <i>A. auriculiformis </i>, whereas at the unburned site, the variation between them was not significant. Based on the above variation, it can be concluded that burning causes ML formation and can affect the growth of these selected <i>Acacia </i> genotypes.
Forest tree plantations in the tropics consist of exotic species mainly of tropical pines and acacias. Even though species trial results provide useful information on performance, adaptability and species-site interaction, extensive research is required to understand the physiology and the genetics of the introduced materials. Recently there has been upsurge of research on acacias throughout the tropics. Acacia mangium and Acacia auriculiformis are two of the fast growing tropical acacias, which have received the highest priority for genetic assessment and improvement other than Acacia aulococarpa and Acacia crassicarpa. Review of Turnbull [
Multiple leaders (ML) seedlings referred to seedlings with terminal shoots greater in length than 20% of the dominant or main leader [
The actual factors that cause the formation of multiple leaders in these species are limited. Although A. mangium shows strong apical dominance, on some sites trees still show a strong tendency to develop multiple leaders from the base. Srivastava [
In forestry practice, the multiple leader development may be denied or favored depending on management objectives and end-use products. Trees, which invariably have a shrub-like growth habit, would not be preferred when clear bole of higher quality is required for sawn timber production. Currently, multiple leader formation is posing a serious problem to A. mangium plantation industry in Peninsular Malaysia in terms of wood quality and added cost by the introduction of singling. Branch-related defects can significantly decrease the quality of plantation grown for solid-wood products. Studies on ML were initiated because the occurrence of ML trees in acacia plantation at some trial plots was becoming unacceptably high. Thus, field trial to examine the effects of site preparation by burning on growth performance and multiple leaders (ML) formation and its variation in eight genotypes of A. mangium and A. auriculiformis was conducted.
Seeds of eight genotypes, four each of A. mangium and A. auriculiformis from a seed consignment obtained from the Australian Tree Seed Center, CSIRO Forestry and Forest Products were used. Three of the A. mangium provenances were from Queensland (QLD) and one was from Papua New Guinea (PNG) while two of the A. auriculiformis were from Northern Territory (NT) one was from Papua New Guinea (PNG) and one was from Queensland (QLD). The seeds were treated prior germination, following the guidelines provided by CSIRO. They were then transplanted to polyethylene bags (12 cm × 18 cm), filled with a potting medium of 5:3:2 top soil, sand and tropical peat, respectively, two weeks after sowing. Seedlings were then raised in nursery and were given necessary care.
The trial was established at the Dunbar Estate of the How Swee Sdn. Bhd. Aur Gading near Kuala Lipis in Pahang, Peninsular Malaysia in August 2010. It is located 4˚20.5'N latitude and 101˚55.5'E longitude, at an altitude of 91.5 m above sea level. The soil of the trial site were described as deep, brownish yellow to yellowish brown fine sandy loam, weak to moderate medium to fine sub angular blocky structure, friable, well drained with sporadic water logging in some part. The terrain was undulating and the parent material was either Shale or Riverine. The soil types are Bungor and Gong Yong series (Malaysian soil classification system). Two sites, burnt and unburned, were selected within the Estate, based on the recent history of land preparation for planting. Site preparation was carried out in some selected compartments, which were either mechanically cleared or mechanically cleaned and followed by broadcast burning of the old rubber trees (Hevea brasilensis). The sturdiest of the seedlings from the nursery production run were selected, conditioned and hardened-off at the nursery. They were then dispatched and transported to the field trial site a week prior to out planting for the purpose of further hardening-off at the planting site. Seedlings of similar size and vigor were selected for the purpose of field establishment. The experimental design used was a randomized complete block (RCBD) with four replications per site. Within each replication, eight genotypes were assigned randomly to eight single line plots of sixteen trees, planted at a spacing of 3 m × 3 m.
Six periodic measurements of the ML formation, survival percentage, total tree height and basal diameter were measured for 24 months. Because of the ML formation an adjusted basal diameter was calculated using the geometric mean procedure for all ML trees. The data of the two sites combined and of each individual site were analyzed separately using SAS, version 6.2, for variance to detect the significance of variation of the sources and their interaction.
The result showed significant differences between site preparation methods for survival and basal diameter, ML, p < 0.05, but not for height. There were also significant differences between genotypes for ML, survival, height and basal diameter, and site × genotype interaction for ML, p < 0.05, but not for survival, height and basal diameter (
There was also significant difference between the burnt and unburned sites in terms of ML trees/plot. The number of ML trees/plot at the burnt site was higher than the unburned site (
Source of variation | df | ML | Survival (%) | Height | Basal diameter |
---|---|---|---|---|---|
Site | 1 | 805.14* | 11956.05* | 0.04 ns | 453.10* |
Genotype | 7 | 67.27* | 443.47* | 3.42* | 2848.36* |
Blocks | 3 | 2.02 ns | 422.72 ns | 1.61* | 524.24* |
Site × Genotype | 7 | 71.39* | 87.81 ns | 0.38 ns | 113.24 ns |
*Significant different at p < 0.05; ns: not significant different at p < 0.05.
Site | ML trees/plot (No) | Survival (%) | Height (m) | Diameter (cm) |
---|---|---|---|---|
Burnt | 7.56a | 95.11a | 5.63a | 93.78a |
Unburned | 0.47b | 67.77b | 5.58a | 88.46b |
Means followed by the same letter in a column are not significantly different at p < 0.05 by Duncan's New Multiple Range Test.
The genotypes showed significant differences in terms of ML tree/plot formation at the burned site. It ranged from 1 (A. auriculiformis, Wenlock River) to 14.75 (A. mangium, Tully Mission Beach). Here, A. mangium showed significant differences between its provenances but A. auriculiformis did not. The genotypes also showed significant differences in terms of ML tree/plot formation at the unburned site. It ranged between nil (A. auriculiformis, Elizabeth River) to 1 (A. auriculiformis, May River). At this site A. auriculiformis showed significant differences between its provenances but not for A. mangium. The mean ML trees/plots of both sites combined also showed significant difference between the genotypes. It ranged from 0.75 (A. auriculiformis, Wenlock River) to 7.5 (A. mangium, Tully Mission Beach). A. mangium showed significant difference between its provenances. It ranged from 5.5 (SE Daintree) to 7.5 (Tully Mission Beach). But A. auriculiformis did not show any significant difference (
Generally, A. mangium produced remarkably higher percentage of ML trees/ plot in the burnt site compared to A. auriculiformis. The value ranged from 66.67% (SE of Daintree) to 93.66% (Tully Mission Beach) for A. mangium provenances while it ranged from 6.25% (Wenlock River) to 20.66% (Mary River) for A. auriculiformis. However, the variation between the species was not pronounced in the unburned site. It ranged from 1.82% (Tully Mission Beach) to 5.13% (SSO Kuranda) for A. mangium while it ranged from nil (Elizabeth River) to 8.88% (Mary River) for A. auriculiformis (
Burnt site | Unburned site | ||||||||
---|---|---|---|---|---|---|---|---|---|
Genotype | ML (no) | Ht (m) | Diameter (mm) | Survival (%) | ML (tree/plot) | Ht (m) | Diameter (mm) | Survival (%) | ML (tree/plot) |
A. mangium | |||||||||
Kuraida, FN-QLD | 13.25ab | 6.5a | 110.05a | 89.06ab | 0.50ab | 6.8a | 114.93a | 60.94b | 6.88a |
Oriomo Province WP | 13.00ab | 6.4a | 114.88a | 93.69ab | 0.50ab | 5.1ab | 1O3.l3b | 64.06ab | 6.75ab |
Tully-Mission Beach | 14.75a | 6.4a | 120.06a | 98.44a | 0.25ab | 5.7b | 1O5.12b | 85.94a | 7.50a |
SE of Daintree | 10.75b | 5.83ab | 96.97b | 84.38b | 0.25ab | 5.5b | 92.76c | 51.56b | 5.50b |
A. auriculiformis | |||||||||
SSO Fiji | 2.00c | 5.4b | 87.24bc | 100.00a | 0.75ab | 5.6b | 85.05c | 65.63ab | 1.38c |
Elizabeth River | 2.50c | 5.2bc | 80.34cd | 96.88a | 0.00b | 4.lb | 67.45ed | 70.31ab | 1.25c |
Wenlock River | 1.00c | 4.lbe | 70.91d | 100.00a | 0.50ab | 5.3b | 75.19d | 73.44ab | 0.75c |
Mary River | 3.25c | 4.4c | 69.80d | 98.44a | 1.00a | 4.9b | 63.88e | 70.31ab | 2.13c |
Overall Site Mean | 7.56 | 5.6 | 93.78 | 95.11 | 0.47 | 5.6 | 88.46 | 67.77 | 4.02 |
SD | 5.88 | 0.77 | 19.9 | 5.7 | 0.31 | 0.54 | 18.71 | 10.02 | 1.29 |
Means followed by the same letter in a column are not significantly different at p < 0.05 by Duncan' New Multiple Range Test.
Genotype | ML (trees/plot) | ML (%) | ||
---|---|---|---|---|
Burnt | Unburned | Burnt | Unburned | |
A. mangium Tully Mission Beach | 12 | 0.5 | 93.66 | 1.82 |
A. mangium SSO Kuranda | 9.5 | 0.75 | 92.98 | 5.13 |
A. mangium Oriomo Province WP | 11.25 | 0.75 | 83.87 | 4.88 |
A. mangium SE Daintree | 9.5 | 0.25 | 66.67 | 4 |
A. auriculiformis Mary River | 3.25 | 1.25 | 20.63 | 8.88 |
A. auriculiformis Elizabeth River | 4.5 | 0.25 | 16.13 | 0 |
A. auriculiformis SSO Fiji | 3.5 | 0.75 | 12.5 | 7.14 |
A. auriculiformis Wenlock River | 1.5 | 0.5 | 6.25 | 4.26 |
It took about 7 to 8 months for the planted trees to fully develop and obviously display the ML growth habit.
Trees that develop multiple leader growth habit ultimately lose their timber value. The intensive investigation reports made by Hofstra et al. [
Age (month) | Height (m) | Diameter (mm) | ML (tree/plot) | Survival (%) | ||||
---|---|---|---|---|---|---|---|---|
Burnt | Unburned | Burnt | Unburned | Burnt | Unburned | Burnt | Unburned | |
4 | 0.927 ± 0.20 | 0.976 ± 0.23 | 10.462 ± 1.58 | 8.725 ± 1.76 | 0.00 ± 0.00 | 0.00 ± 0.00 | 96.68 ± 6.35 | 83.38 ± 14.7 |
7 | 1.573 ± 0.29 | 1.555 ± 0.37 | 22.108 ± 4.14 | 15.597 ± 3.56 | 0.00 ± 0.00 | 0.00 ± 0.00 | 96.48 ± 6.34 | 76.75 ± 17.1 |
9 | 2.276 ± 0.35 | 2.181 ± 0.40 | 36.54 ± 11.98 | 25.124 ± 6.56 | 5.187 ± 3.45 | 0.43 ± 0.67 | 96.26 ± 6.53 | 75.77 ± 17.8 |
13 | 3.594 ± 0.50 | 3.168 ± 0.54 | 52.476 ± 11.70 | 39.196 ± 9.70 | 6.438 ± 4.38 | 0.531 ± 0.72 | 96.09 ± 6.69 | 72.85 ± 17.8 |
16 | 4.583 ± 0.70 | 3.964 ± 0.62 | 67.499 ± 14.50 | 56.499 ± 13.58 | 6.531 ± 4.31 | 0.625 ± 0.79 | 95.89 ± 6.66 | 68.53 ± 19.3 |
23 | 5.632 ± 0.89 | 5.582 ± 0.96 | 93.78 ± 20.52 | 88.459 ± 19.66 | 6.781 ± 4.54 | 0.625 ± 0.79 | 95.10 ± 8.06 | 67.77 ± 18.5 |
Note: Mean ± Standard Deviation.
timber trees development. The result of the present study in agreement with the suggestion that environment has much influence on form and growth habit. In this context, about half (49.09%) of total number of planted trees at burnt site had multiple trunks as compared to only 4.25% at the unburned site. The result of the present study also revealed that the problem of ML growth habit in relation to site preparation method might be a potential problem for A. mangium, in particular. All four A. mangium provenances at the burnt site had significantly higher number of ML trees per plot compared to all four A. auriculiformis provenances (
Regarding the survival rate, the result of the trial indicated that all the genotypes survived well at the burned site with a site mean of 95.11%, ranging between 84% - 100%, while the survival at the unburned site was comparatively low, with an overall site mean of 66.67%, ranging from 54.56% to 73.44%. However the mean of both sites combined ranged from 67.90% to 92.13%. A. mangium from the SE of Daintree showed the lowest survival rate. Generally, the results of the survival rate obtained in this study were comparable with the trends experienced in many of the A. mangium and A. auriculiformis provenances trials in the humid tropics [
The result of the 24 months observation, however, showed that there was no significant difference in terms of height between the provenances of each of the two Acacia species but only between the genotypes (
The results of the study are in agreement with the suggestion that environment has much influence on form and growth habit of Acacia genotypes. The use of fire to clear land for A. mangium and A. auriculiformis genotypes induces ML formation. It was also observed that A. mangium provenances were more responsive to ML formation than A. auriculiformis provenances. The number ML/tree ranged from two to five at the burnt site compared with only two at the unburned site for both species. The problem of ML growth habit of Acacia provenances in relation to site preparation method might be a potential problem for establishment of commercial plantation area, if the high quality grade timber was the target of a plantation. If fuel wood or small construction poles or even pulp and paper production is the objective, then the ML formation would be accepted and not greatly affect the end product, and in fact, it may be desirable in some cases. However, this could be utilized for better planning, and management of plantation area with option of other suitable methods depends on the objective of the end product, site condition and environment factors.
This work was funded mainly by Intensification of Research in Priority Area (IRPA) grant provided by Ministry of Science, Technology and the Innovative (MOSTI). This work is also partly supported by the scholarship grant from the Acacia project of Trop Bio Research, Universiti Putra Malaysia. We are indepted to Mr Khongsak Pinyopusarerk of CSIRO Forestry and Forest Products, Australia for providing seeds lots and Mr How Swee the owner of Dunbar Estate, Aur Gading, Kuala Lipis for the establishmant of the field trial. Authors also thank all colleagues for providing support and facilities and those who contributed directly or indirectly to improve this manuscript.