Interacting effects of fire severity, time since fire and topography on vegetation structure after wildfire

Fire is an important disturbance in forest ecosystems globally. Many of the effects of fire on forest processes are mediated through effects on vegetation structure. Understanding how fire properties, fire regimes and environmental variation interact to affect structure is required in the face of predictions of increasing size and severity of fires - “megafires”. Here, we investigated the influence of topographic position, fire severity, and time since last fire on vegetation structure in foothill eucalypt forests, two years after a large wildfire in south-eastern Australia. We found that forest gullies had significantly greater structural complexity than forest slopes; but that fire severity and time since last fire influenced the structure of vegetation in gullies and slopes in similar ways. Two years after wildfire, severely burnt gullies and slopes (tree crowns scorched or consumed) had a reduced canopy cover, but a denser cover of eucalypt saplings up to 4 m tall, than gullies and slopes that had not been burnt. Compared with severe fire, understorey fire had much less influence on the structure of vegetation, with a significant effect only on slopes. There was little effect of the time since last fire prior to 2009 (≤3 years vs. ≥20 years) on subsequent vegetation structure after wildfire in either gullies or slopes. Finally, we found that fire did not homogenise the structure of gullies and slopes: vegetation structure in paired gullies and slopes did not become more similar following understorey or severe fire. Two years after a large wildfire, heterogeneity in the structural complexity of forest vegetation was evident at both the site and landscape scale. At the landscape scale, fire-induced heterogeneity in vegetation structure, arising from spatial variation in fire severity, provides habitat structures of differing quality for plants and animals. At a finer scale, the rapid return of distinct vegetation structure between adjacent gullies and slopes is important for the persistence of species that depend on a complex vegetation structure or require fine-scale heterogeneity in vegetation.