The effects of mechanical disturbance and burn intensity on the floristic composition of two-year old aggregated retention coupes in Tasmanian wet eucalypt forests

Due to concerns about the ecological impacts of clearfelling in Tasmanian wet eucalypt forests, aggregated retention (ARN) was developed as an alternative harvesting method. It is predicted that, compared to clearfelling, ARN will have ecological benefits such as preservation of old-growth structures and improved regeneration of late successional species in harvested areas. However, early studies have indicated that ARN requires lower intensity regeneration burns and results in greater levels of mechanical soil disturbance than clearfelling. This study therefore aimed to assess the impacts of both soil disturbance and burn intensity on floristic composition, species dominance and species richness following harvesting in ARN coupes. Floristic surveys were conducted on six seedbed classes that reflected the combined effects of both burn intensity and soil disturbance in six two-year-old ARN coupes in southern Tasmania. The results showed that both factors had a significant influence on floristic composition. Higher burn intensities generally favoured colonising species such as eucalypts and Senecio minimus and resulted in lower species richness, while lower fire intensities favoured species regenerating from the seedbank and resulted in higher species richness. In contrast, mechanically disturbed seedbed generally had a high cover of the sedge Gahnia grandis but a low overall species richness due to reduced regeneration of eucalypts and understorey species. The prevalence of the six seedbed classes, and corresponding plant species composition, varied among four general harvesting-related disturbance categories; firebreaks, snigtracks, windrows of piled logging debris and the general harvested area. Maintaining a variety of burn intensities is likely to maximise plant biodiversity within ARN coupes, and soil disturbance should be minimised, with an emphasis on reducing the total area of firebreaks.

► Harvesting disturbance and burn intensity affect vascular plant regeneration.
► Compacted and disturbed firebreaks and snigtracks have low plant richness.
► Greatest plant diversity occurs on areas burnt to low or intermediate intensity.
► Very hot burns result in good eucalypt regeneration, but low overall richness.
► Forest managers are encouraged to minimise the area of disturbed firebreaks.