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Regeneration in tropical eucalypt forest on Melville Island

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Fensham, R J(Roderick John) (1990) Regeneration in tropical eucalypt forest on Melville Island. PhD thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (PhD)
Keywords: Eucalyptus, Regeneration (Botany)
Copyright Holders: The Author
Copyright Information:

Copyright 1990 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Includes bibliographical references (leaves 218-247). Thesis (Ph.D.)--University of Tasmania, 1992

Date Deposited: 08 Dec 2014 23:59
Last Modified: 16 Aug 2016 01:20
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