Fire and legacy effects of logging on understorey assemblages in wet-sclerophyll forests

Abstract

Wet sclerophyll forests of south-eastern Australia typically experience wildfire once or twice a century. However, disturbance regimes have changed drastically in recent decades due to clear-fell logging and altered fire regimes. To date, botanical research on disturbances in wet-forests has focussed on individual elements of disturbance regimes, such as intensity, at single points in time, largely neglecting past disturbance history. Studies of the impact of previous disturbance history on plant responses to successive disturbance events are important to our understanding of vegetation dynamics. Here we investigate the response of wet-forest understorey species to two important elements of disturbance regimes – timing and type – and trajectories of change in these vegetation communities. In surveys separated by 15 years over 128 sites, we recorded the frequency of occurrence of 21 understorey species from stands with disturbance histories ranging from 4 years post clear-fell logging to 150 years post wildfire. Approximately half our sites were burnt in the 2009 Black Saturday wildfires. This provided an opportunity to examine the effects of inter-fire interval and the legacy effects of clear-fell logging. Generalised linear mixed modelling showed that many of the species studied appear to be at risk of population decline as a result of clear-fell logging. Unlike wildfire, clear-fell logging led to changes in the understorey, having a long-lasting impact on the presence of key wet-forest taxa that rely on vegetative regeneration. These include large shade-bearing shrubs like Hedycarya angustifolia R.Cunn., Bedfordia arborescens Hochr. and Olearia argophylla (Labill.) Benth., which were resilient to recurrent wildfire but responded negatively to recent wildfire in sites with a history of clear-fell logging. Negative effects of a short inter-fire interval were limited to a few species.