Abstract
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 Acacia mangium and Acacia auriculiformis 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. A. mangium provenances were more responsive to burning than A. auriculiformis, 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 Acacia genotypes.
Highlights
Forest tree plantations in the tropics consist of exotic species mainly of tropical pines and acacias
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 result of the present study revealed that the problem of multiple leaders (ML) growth habit in relation to site preparation method might be a potential problem for A. mangium, in particular
Summary
Forest tree plantations in the tropics consist of exotic species mainly of tropical pines and acacias. Many national and international provenance trials of A. mangium and A. auriculiformis have been reported from different regions and localities such as from China [2], Indonesia [3] [4], Malaysia [5] [6], Thailand [7] [8], Zaire [9] and Vietnam [10] [11]. Reports from Malaysia and Thailand, in particular, indicated that despite their successful introduction and adaptability, both A. mangium and A. auriculiformis provenances grown in plantation and trial sites have shown problems in tree form, growth habit and develop multiple leaders (ML) right from the base of the trunk and in some cases heavy epicormic branching with excessive low forking branch
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