Jane Cawson

Surface runoff and erosion after prescribed burning and the effect of different fire regimes in forests and shrublands: a review

This paper examines the state of knowledge about the effects of prescribed burning on surface runoff and erosion at point to catchment scales in forests and shrublands. Fires can increase surface runoff and erosion by removing vegetation, changing soil hydrologic properties and providing a readily erodible layer of sediment and ash. Catchment-scale studies in prescribed-burnt areas usually report minimal impacts from the burn. However, measurements at smaller spatial scales suggest that large changes to hydrologic properties and processes do occur, and a debris-flow example from Australia demonstrates that large catchment-scale impacts are possible. It appears that existing catchment-scale studies in prescribed burns do not capture these large events as the sample size (i.e. number of studies) is too small relative to the infrequency of such events. Furthermore, numerous knowledge gaps across all spatial scales limit understanding of the processes contributing to post-prescribed burn runoff and erosion. Understanding the influence of fire regime characteristics on post-fire runoff and erosion is particularly important in the context of prescribed burning, as fire regimes can be manipulated to reduce erosion and water-quality impacts. Therefore, two directions for future research are recommended: (1) process-based studies to understand the factors controlling surface runoff and erosion, particularly in relation to aspects of the fire regime; and (2) landscape-scale surveys to quantify large erosion events.

Is aridity a high-order control on the hydro–geomorphic response of burned landscapes?

Fire can result in hydro–geomorphic changes that are spatially variable and difficult to predict. In this research note we compile 294 infiltration measurements and 10 other soil, catchment runoff and erosion datasets from the eastern Victorian uplands in south-eastern Australia and argue that higher aridity (a function of the long-term mean precipitation and net radiation) is associated with lower post-fire infiltration capacities, increasing the chance of surface runoff and strongly increasing the chance of debris flows. Post-fire debris flows were only observed in the more arid locations within the Victorian uplands, and resulted in erosion rates more than two orders of magnitude greater than non-debris flow processes. We therefore argue that aridity is a high-order control on the magnitude of post-wildfire hydro–geomorphic processes. Aridity is a landscape-scale parameter that is mappable at a high resolution and therefore is a useful predictor of the spatial variability of the magnitude of post-fire hydro–geomorphic responses.

Dryness thresholds for fire occurrence vary by forest type along an aridity gradient: evidence from Southern Australia

ContextWildfires are common in localities where there is sufficient productivity to allow the accumulation of biomass combined with seasonality that allows this to dry and transition to a flammable state. An understanding of the conditions under which vegetated landscapes become flammable is valuable for assessing fire risk and determining how fire regimes may alter with climate change.ObjectivesWeather based metrics of dryness are a standard approach for estimating the potential for fires to occur in the near term. However, such approaches do not consider the contribution of vegetation communities. We aim to evaluate differences in weather-based dryness thresholds for fire occurrence between vegetation communities and test whether these are a function of landscape aridity.MethodsWe analysed dryness thresholds (using Drought Factor) for fire occurrence in six vegetation communities using historic fires events that occurred in South-eastern Australia using logistic regression. These thresholds were compared to the landscape aridity for where the communities persist.ResultsWe found that dryness thresholds differed between vegetation communities, and this effect could in part be explained by landscape aridity. Dryness thresholds for fire occurrence were lower in vegetation communities that occur in arid environments. These communities were also exposed to dry conditions for a greater proportion of the year.ConclusionsOur findings suggest that vegetation driven feedbacks may be an important driver of landscape flammability. Increased consideration of vegetation properties in fire danger indices may provide for better estimates of landscape fire risk and allow changes to fire regimes to be anticipated.

Messmate stringybark: bark ignitability and burning sustainability in relation to fragment dimensions, hazard score and time since fire

Messmate stringybark is common in forests across south-eastern Australia. The bark of these trees is persistent and produces firebrands that contribute to house loss and the difficulty of fire suppression during wildfires. The trees typically survive fire with the amount of bark depleted. We compared two common methods to assess messmate bark fuels: (1) field-based hazard assessment, and (2) desk-based assessment using mapped time since fire. Our measurements included space-for-time field surveys and laboratory flammability tests. Although several physical properties of bark could be approximated from both assessment methods, some bark properties important to flammability were not captured. Ignitability was found to be dependent on the amount of char on bark fragments and could be predicted by the site assessment methods, whereas sustainability was dependent on bark fragment dimensions and could not be predicted by current methods. Bark fragment properties were found to be partially a function of tree size. Overall, these findings indicate that current bark assessment methods do not capture all the key bark properties that contribute to messmate bark’s flammability. Further research is warranted to improve bark assessment methods so they better reflect bark’s contribution to fire behaviour.