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Research at Warra supersite improves forestry policy

Research at the Warra supersite in Tasmania has led to improvements in harvesting methods for use in tall, wet eucalypt forests that also protect native fauna and improve the look of forest that is growing back after harvesting.

The aggregated retention method has been developed as an alternative to the clear-fell, burn and sow (CBS) method that has been used in Tasmania’s tall, wet eucalypt forests since the 1960s. In CBS, the whole patch is clear-felled and the slash left behind is burned intensely to provide optimal conditions for new eucalypt seedlings to grow. With aggregated retention, patches of trees are retained intact in the area being harvested.

Dr Tim Wardlaw is one of the scientists who has been studying ways of improving forestry and its social acceptability while protecting biodiversity. He runs the Warra site, which is part of TERN’s Australian Supersite Network.

‘CBS is safe, efficient and economical, and the eucalypt species grow back quickly – but there are some problems with it, too,’ Tim says.

‘One is that you start with beautiful forest that people love but it’s ugly in the early years after it’s been harvested, burned and sown for the new forest that develops down the track. People don’t like it and that’s bad PR for forestry, including sustainable forestry.

‘The other is that although eucalypts grow back quickly, late-successional species don’t; they’re plants that are slow to grow such as leatherwood (Eucryphia lucida) and animals such as the green rosella (Platycercus caledonicus) that depend on mature trees. They’re not adapted to the large openings and intense fires that happen as part of a CBS operation, so they are slow to move back into the regenerating forest from adjoining forest that hasn’t been harvested.

‘We’re increasingly understanding that wildfires, the natural disturbance in these forests, are much more variable than we once thought. Some areas do burn intensely, as in CBS, but other areas burn less intensely or not at all. This variability helps late-successional species to persist in the forests. Using aggregated retention for some of our harvesting should better mimic what happens naturally. Wood from managed native forests is one of the most sustainable products society uses. Adopting variable retention will strengthen its sustainability.’

The 15,900 ha Warra supersite was established as a long-term ecological research site in 1997. The original site lies between the Huon and Weld rivers, and the supersite extends it to include the landscape stretching from the Tasmanian Wilderness World Heritage Area to the Huon estuary. It is dominated by tall, wet forests of Eucalyptus obliqua, the most extensive forest type in Tasmania. The structure of the forest has been influenced by periodic wildfires. The Warra site contains a complex mosaic of mature, multi-age and single-age forests that originate from past wildfires and from forest that is regenerating after being harvested.

The project that led to forestry practices becoming more sophisticated and sensitive to wildlife and human social needs, the Silvicultural Systems Trial (SST), began in 1998. There was a major review of the SST in 2008 when most ‘treatments’ (that is, CBS, aggregated retention and so on) had reached their third anniversary after harvest. Researchers found that communities of birds, ground beetles, and plants had changed markedly in the harvested areas of all treatments, but remained similar to unharvested forest in the retained forest patches of aggregated retention and strip-fell treatments. While eucalypts regenerated best in CBS, most other treatments reached acceptable stocking levels by age three. The next review is planned for 2015, when most treatments reach their 10th anniversary.

Convinced by the research results, Forestry Tasmania has begun to use aggregated (or variable) retention, even though it costs a little more to harvest timber in this way.

‘Translating the scientific findings from the SST into policy and practice meant bringing together scientists, planners and operations staff to develop feasible practices that achieved the ecological, economic and social objectives,’ Tim says.

‘Among other things, we had to evaluate the harvesting operations. At the end of four years of using aggregated retention we found that we could successfully harvest trees, protect the patches of mature forest retained in the aggregates, and provide good conditions for regeneration in the harvested areas. We will keep monitoring forest in the Silvicultural Systems Trial to see if late-successional species recolonise the harvested areas in variable retention areas faster than in CBS. This monitoring acts as a sentinel to tell us when to monitor more widely in areas harvested.’

In other research at the Warra supersite, scientists found that, 30 to 50 years after harvesting, forests can support similar populations of species sensitive to disturbance as mature forests, if they are within 400 metres of mature forest.

‘Knowing this helps us determine how to balance harvesting and retention in politically and ecologically sensitive areas of tall, wet eucalypt forest,’ Tim says. ‘We tested this along a gradient of forest environments, contrasting the biodiversity of largely pristine landscapes with that in increasingly altered landscapes, that is towards the farming end of the scale. The results are crystal clear: if there is mature, undisturbed forest nearby, the harvested forest can return to something akin to its original state.’

Tim said the research has vindicated the comprehensive, adequate and representative (CAR) reserve system developed for the regional forest agreements that Commonwealth and state governments signed in the 1990s. The CAR system is used to sustain forest-dependent species throughout their range using a network of reserves and complementary management outside reserves.

‘The CAR reserves aren’t distributed evenly throughout the landscape – for example, many of the large, formal reserves abut national parks, where there is already a high level of protection – but we found that uneven distribution didn’t really matter. Mature forests remaining in the more intensively managed production forest areas have similar populations of species as those in and around the large reserves. It shows we can manage biodiversity and human aesthetic needs in forestry areas,’ Tim says.

This knowledge will be used to fine-tune forest practices regulations and forestry policy, to ensure that biodiversity is protected. The Warra site is a case study in the Multi-Scale Plot Network book on monitoring environmental change that CSIRO will publish next year.

French intern students Elsa Libis and Gregoire Thauvin measuring coarse woody debris in one of 
the plots used in the CAR reserves study
Alison Phillips emptying a pitfall trap used to sample 
beetles in the Silvicultural Systems Trial at Warra

Published in TERN e-Newsletter August 2012

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