A synthesis of outcomes from the Warra Silvicultural Systems Trial, Tasmania: safety, timber production, economics, biodiversity, silviculture and social acceptability

The Warra Silvicultural Systems Trial was established in southern Tasmania from 1998 to 2007 in tall wet Eucalyptus obliqua forest to compare seven alternatives to the traditional clear-fell, burn and sow (CBS) harvesting method. The alternatives included CBS with understorey islands, patch-fell, strip-fell, dispersed retention, aggregated retention, single-tree/small-group selection and group selection. The effects of the treatments were compared at age three years against six criteria: safety for harvesting crews, rate of timber recovery, economic returns to the forest owner, old-growth biodiversity, regeneration and growth of eucalypts (silviculture), and social acceptability.

Combining all criteria, aggregated retention performed best, and is suggested to be the most suitable alternative for routine use in wet eucalypt forests if a management objective is to maintain old-growth structures and biodiversity at the stand (coupe) level. Aggregated retention presented no novel safety issues even though coupes contained a relatively greater amount of forest edge; harvesting was relatively productive, although total forest management costs increased by about $5 per tonne or cubic metre compared to clear-felling. Biodiversity outcomes were much superior to clear-felling, both at three years of age and (predicted) at rotation age, and the system had greater visual appeal and social acceptability than clear-felling. Regeneration burns in the aggregated retention treatments were operationally more difficult than with clear-felling and were somewhat less effective in creating a suitable seedbed, resulting in lower densities of eucalypt stems in the regenerating forest although stocking standards were still attained.

Old-growth biodiversity in commercial forests can be managed at the landscape level, through retention between coupes and through wildlife habitat strips that connect larger ecological reserves. However, internationally there is increasing recognition that maintaining old-growth elements at the coupe level is a worthwhile complementary practice for maintaining biodiversity in commercially-managed native forests. Managers of tall wet eucalypt forests need to balance the difficulties of regeneration burning and costs of aggregated retention against its ecological and social acceptability benefits.