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Effects of variable retention harvesting on productivity and growth in wet eucalypt forests

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Scott, RE (2013) Effects of variable retention harvesting on productivity and growth in wet eucalypt forests. PhD thesis, University of Tasmania.

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Abstract

Society’s changing expectations for native forest management and an improved
understanding of wet-forest ecology have led to the adoption of variable retention silviculture
in Tasmania’s old-growth wet eucalypt forests. Variable retention aims to maintain
biodiversity and ecosystem function in managed forests by retaining patches of forest or
individual trees. Retained areas are intended to provide continuity of structure and function,
enhance landscape connectivity, and influence the regenerating forest. However, these
ecological goals must be balanced against silvicultural considerations such as achieving
successful regeneration and avoiding damage to retained trees.
This study is the first to assess regeneration success and related silvicultural outcomes after
operational variable retention harvesting in wet eucalypt forests, and to compare these to
outcomes after conventional clearfell, burn and sow harvesting. A total of 38 aggregated
retention (ARN) coupes and 31 paired clearfell, burn and sow (CBS) coupes harvested from
2003 – 2009 and regenerated from 2007 – 2010 were monitored for up to three years to
address questions concerning forest influence and retention levels, the persistence of
aggregates, the effects of site preparation including new ‘slow burning’ methods, and early
regeneration results.
Early silvicultural outcomes after operational ARN harvesting in old-growth wet eucalypt
forests were generally satisfactory, and compared favourably with outcomes after
conventional CBS harvesting. There were no differences in eucalypt seedling stocking,
density or height between ARN and CBS coupes at one year of age. At three years of age,
seedling density and height did not vary with silvicultural system, and stocking was only 5%
lower in ARN coupes when two outliers were removed. This early regeneration success in the ARN coupes is attributed to the high proportion of burnt seedbed achieved in the regeneration
burns on these coupes, the adoption of aerial sowing as a standard operating procedure, and
the absence of any increase in browsing pressure or edge-related growth suppression.
Seedling height and density were strongly related to the state of the seedbed, and increased
with increasing burn intensity, confirming that the creation of burnt seedbed is essential for
good early regeneration in wet eucalypt forests. The higher perimeter-to-area ratio of ARN
coupes resulted in a higher proportion of the harvested area being affected by firebreaks,
although this decreased in more recently harvested openings due to changes in coupe design.
Soil disturbance and compaction associated with firebreaks were found to affect soil physical
and chemical properties and to reduce eucalypt seedling height growth by 40-60%. To reduce
soil disturbance and potential impacts on regeneration, it is recommended that firebreaks be
established only where absolutely necessary, and firebreak widths be minimised wherever
possible.
Windthrow and harvesting damage were not significantly increased by ARN harvesting, but
2.5 times as much unharvested forest was affected by the regeneration burn in ARN coupes
compared to CBS coupes, due largely to burning in the retained aggregates. It is
recommended that island aggregates be at least 1 ha in size to avoid excessive burn damage
and reduce windthrow risk. The longer-term effects of ARN harvesting on eucalypt
productivity remain unknown, and more detailed examination of edge effects is required, but
these early results indicate that initial silvicultural goals for regeneration can be met after
variable retention harvesting in wet eucalypt forests.

Item Type: Thesis (PhD)
Keywords: Eucalyptus, Australia, silvicultural systems, regeneration, firebreak, seedbed
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Date Deposited: 01 Oct 2013 23:58
Last Modified: 15 Sep 2017 01:06
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