Variable retention in Tasmania, Australia: trends over 16 years of monitoring and adaptive management

Background: Variable retention harvesting using aggregated retention (ARN) has been applied in some Tasmanian wet eucalypt forests for > 15 years. Implementation of ARN in Tasmania differs from most other regions, as forest influence (or the proximity of harvested areas to long-term retention) is primarily used to distinguish ARN from clearfelling rather than retention level, and broadcast burning is used as a regeneration treatment, resulting in a preference for edge aggregates instead of isolated retained islands. Clear emphasis on site-level retention of biological legacies and forest influence to facilitate re-establishment of mature forest species ensures that ARN coupes achieve the ecological objectives associated with retention forestry.

Results: Spatial and survey data collected from operational ARN coupes illustrates the development of this silvicultural practice over time and allows comparisons with conventional clearfelling. Over 90% of ARN coupes have met the forest influence target of > 50%. The number of ARN coupes harvested per year has varied, but mean retention levels (29%) and the mean area harvested per coupe (24 ha) have remained the same. Forest influence in ARN coupes has decreased over time, as have perimeter-to-area ratios, largely due to a decrease in the number of island aggregates. Although these measures of complexity have decreased, ARN coupes still have much greater forest influence, retention, and perimeter-to-area ratios when compared to conventional clearfell, burn and sow (CBS) coupes. The observed decrease in boundary complexity should facilitate regeneration burning in ARN coupes, although no increase in the proportion of burnt seedbed was observed. The proportion of burnt seedbed is strongly correlated with eucalypt seedling stocking and density in ARN coupes, and these attributes were all significantly lower in recent ARN coupes compared to clearfells. These differences appear to be due to achieving better burns in recent CBS coupes, rather than poorer ones in ARN coupes.

Conclusions: Although the area that can feasibly be harvested by ARN is limited by the requirement for regeneration burning, ARN provides clear biodiversity benefits and satisfactory silvicultural outcomes and is now firmly embedded as a viable alternative to clearfelling in Tasmanian wet eucalypt forests.