Regeneration in tropical eucalypt forest on Melville Island

The eucalypt species of the evergreen savanna forest in monsoonal Australia have a population structure characterized by mature trees, an abundance of short stems in the ground layer and low densities of saplings. Other common trees of these forests, such as the broadleaved species, are almost exclusively represented by these short woody sprouts. The distinctive structure of the eucalypt forests begs the question as to how these forests are perpetuated. This study seeks to examine tree regeneration in the eucalypt forests of Melville Island in the wet-dry tropics of Australia. The first part of the thesis adresses the influence of the physical environment on regeneration by examining edpahic influences on vegetation structure, growth rates and fire response. The second part of the thesis concentrates on the evergreen eucalypts forests and examines possible causes for the supression of woody sprouts. An attempt was made to define the soil moisture conditions across the range of savanna types. However, elucidating the relationship between edaphic conditions and vegetation patterns is difficult because of the effect of rockiness and soil texture on root penetration. It is reasonably certain that evergreen eucalypts are excluded from one intensively studied site because a clay layer impedes root growth. This low forest is dominated by semi-deciduous eucalypts and the growth of all woody sprouts is slower than in the tall forest. Despite less vigorous growth, broadleaved tree species, such as Terminalia ferdinandiana and Planchonia careya, occupy the mid-canopy at this site. It is suggested that the tall evergreen eucalypts, Eucalyptus miniata, Eucalyptus nesophila and Eucalyptus tetrodonta, suppress the growth of other tree species. Overwood suppression is evident within the tall forest. Tree seedlings grew more rapidly under killed trees than live overwood. Furthermore, clearfelled sites develop saplings although the replacement of the tree canopy occurs sporadically. Few eucalypt saplings developed in an extensive experiment designed to examine the influence of eucalypt and wattle trees and herbage on tree regeneration. The saplings of two tree species were weakly associated with natural canopy gaps. The saplings of three other species had no such association. It is suggested that underground organs of middleage and with well developed root systems are the most likely to become trees. Underground organs are extensively damaged by termites, and relief from this form of herbivory may contribute to the accession of saplings. It is demonstrated that moisture conditions, below 50 cm, are generally adequate for tree growth throughout the year. This helps to explain why the new growth of woody sprouts after fire, occurs during the driest months. These ground layer stems stop growing when water is abundant during the wet season. The hypothesis that competition for moisture provides the mechanism of overwood suppression is difficult to equate with these facts. The hypothesis that competition for nutrients controls the suppression of young trees is not supported by an experiment where woody sprouts showed no response to fertilizing. Patterns of insect abundance and damage levels indicate that the growth of woody sprouts is not tuned to the activities of phytophagous insects. There was no relationship between stem height and levels of insect damage. Furthermore, experimental protection of plants using pesticide did not increase growth. Thus, it seems that suppression of tree stems cannot be attributed to insects. Shredded Eucalyptus tetrodonta leaves suppressed seedlings of the same species when applied to pots in a nursery experiment. It is suggested that allelopathic interactions may be important for suppressing regeneration. Relief from fire is another factor that contributes to tree regeneration. However, the effect of this agent on forest structure seems dependent on certain site conditions. The development of tree saplings in a rocky eucalypt forest after longterm fire protection contrasts with limited sapling development in another fire protected forest on deep soil. Many features of the regeneration process remain unclear but this thesis provides sufficient evidence to postulate a comprehensive model describing a common regeneration strategy in tropical eucalypt forest.