Variable retention silviculture in Tasmania's wet forests: ecological rationale, adaptive management and synthesis of biodiversity benefits

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.