Responses of tree species to a severe fire indicate major structural change to Eucalyptus-Callitris forests

In many fire-prone habitats fires may be relatively frequent but of low severity or small areal extent. However, these same habitats may occasionally be subject to large, severe fires when extreme conditions and ignitions coincide. After > 50 years without significant fire, a mega-fire burnt > 50,000 ha of Eucalyptus-Callitris forest in southeastern Australia. We assessed the impact of this fire on vegetation structure at a landscape scale by quantifying post-fire responses of 11 tree species over 97 sites with varying fire severity. At low severity over 60 % of Callitris trees survived by escaping crown scorch, but they were almost all killed at higher severity. Fewer eucalypts escaped crown scorch (33 % at low fire severity) but there was no evidence of mortality due to the fire. Most eucalypts were topkilled (55 %) but less frequently at low (39 %) compared to moderate (55 %) or high (74 %) fire severity. Larger trees were less likely to suffer topkill. Taken together these results indicate that this wildfire has caused major changes to vegetation structure within the area burnt. Death of Callitris trees reduced canopy tree density by 25 % and a high proportion of eucalypt topkill has resulted in a shorter, more open forest. Recovery of the tallest structural components through eucalypt regrowth and maturation of Callitris may require fire-free intervals of several decades. Any fires within this period may extend the recovery time and lead to declines in populations of the obligate-seeding Callitris species.