Variable retention silviculture in Tasmania's wet forests:
ecological rationale, adaptive management and synthesis of biodiversity
benefits.(Report)
Baker, Susan C; Read, Steve M
The recognition that biodiversity conservation requires more than a
system of reserves has led to the need to consider the outcomes of land
management actions, such as timber harvesting, in the matrix land
outside reserves. The design of harvesting systems can be guided by the
natural disturbance regime, which in Tasmania's lowland wet
eucalypt forests is infrequent, intense wildfire. Clearfell, burn and
sow silviculture has been used since the 1960s for harvesting these
forests but, while this system is practical and effectively regenerates
eucalypts in harvested coupes, it is predicted to lead to losses at the
coupe level of late-successional species and structures that would
survive into stands regenerating following natural wildfire. Variable
retention silviculture is thus currently being implemented as an
alternative to clearfelling in wet old-growth forest on public land
(state forest) in Tasmania. In contrast to clearfelling, variable
retention has the explicit ecological goal of maintaining some species,
habitats and structural legacies from the pre-harvest forest into the
harvested and regenerating stand. This paper synthesises biodiversity
findings from the Warra Silvicultural Systems Trial (SST), established
in 1997, and demonstrates that aggregated retention is the optimal form
of variable retention for ensuring coupe-scale persistence
('lifeboating') of mature-forest biodiversity. In addition to
providing retained forest, aggregates are also designed to facilitate
recolonisation of harvested areas by mature-forest species ('forest
influence'), and to provide connectivity across the forest stand.
In the last few years, more than 50 aggregated-retention coupes have
been harvested in mature forest across Tasmania. Development and
implementation of variable retention in Tasmania is an example of active
adaptive management, which we describe in relation to five steps for a
formalised adaptive management program, indicating how ecological
criteria are incorporated in operational guidelines for implementation
of aggregated retention.
Keywords: silvicultural systems; harvesting; regeneration;
management; adaptation; old-growth forests; biodiversity; forest
influences; eucalypts; Tasmania
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