Data and code for manuscript: Long-unburnt eucalypt forests are no less susceptible to crown fire in SE Australia.

Fire behaviour is dependent on the weather and the structure of vegetation through which it is burning. The structure of eucalypt forests can be altered by fire, hence the use of fire as a management tool to reduce fire risk. Forest structure is also changed by wildfires, leading to changes in the risk of high severity fire in the landscape, but how does the structure change with different times since fire? There is some evidence that high severity fire promotes dense shrub growth and altered canopy structure in the dry eucalypt forests of south-eastern Australia, but the forest structure may return to a pre-fire state over 30-50 years due to shrub senescence and canopy recovery. Two hypotheses were tested: 1) areas unburnt for at least forty years would have lower understory fuel cover and a greater vertical separation between understory and canopy than those burnt five or sixteen years previously and, 2) areas of high severity fire would have greater understory fuel cover and less vertical separation between understory and canopy than areas of low severity fire for at least sixteen years post fire. To test these hypotheses, the cover of different fuel strata (near surface, elevated, canopy) and other structural variables (fuel load, vertical separation) were measured at sites with differing times since fire: five years, sixteen years, and long-unburnt (more than forty years since fire). The five and sixteen-year time since fire sites were stratified by fire severity (low or high). We found that long-unburnt sites did not have lower understory cover than more recently burnt sites. Long-unburnt sites did have slightly shorter vertical separation between understory and canopy, but this was strongly tied to the specific vegetation type at a subset of those sites. There was an independent effect of time since fire on vertical separation, but it was difficult to disentangle the relationships. Understory cover was greater, and vertical separation less, after high severity fire, but only at sites with a time since fire of five years. At sixteen years, fire severity showed the opposite relationship with understory cover and vertical separation. These results suggest the probability of subsequent high severity fire is more likely five years after high severity fire in dry sclerophyll forests, but sixteen years post-fire the probability of high severity fire may be greater after low severity fire. The greater vertical separation between understory and canopy in sites unburnt for at least forty years suggests that the risk of high severity fire may be less in these forests. However, the lack of difference in the understory fuels and the small difference in separation means that any change in risk is likely to be minimal.