Effects of prescribed burning on surface runoff and erosion

© 2012 Dr. Jane Greenslade CawsonPrescribed burning – the deliberate use of fire to achieve management objectives – is used extensively in fire-prone vegetation for reducing fuel hazards and enhancing ecological values. As governments set ambitious targets for more prescribed burning, it is important to understand and manage the potential negative impacts, such as increased erosion. While globally there are many studies that consider the effects of prescribed burning on surface runoff and erosion, there are critical knowledge gaps for particular forest types (e.g. dry eucalypt forests) and in relation to understanding the factors controlling particular post-fire hydrologic and erosion responses, the likelihood of large impacts, the effects of spatial scale on the magnitude of an impact and the long-term risks of repeated burning. Therefore, the aim of thesis was to quantify the effects of prescribed burning on soil hydrologic properties, surface runoff and erosion in dry eucalypt forests in Victoria, Australia. This aim was addressed by examining the effects of two potentially important aspects of fire regimes – fire severity and burn patchiness – on soil hydrologic properties, surface runoff and erosion. Measurements were conducted in unburnt, low fire severity (scorched understorey and intact canopy) and high fire severity (burnt understorey and scorched canopy) areas at three dry eucalypt forest sites. Soil water repellency (using the critical surface tension test) and infiltration capacity (using ponded and tension infiltrometers) were measured at the point-scale for all sites immediately post-burn and then at six-month intervals. Rainfall simulations were used to measure runoff and erosion at the plot-scale (3 m2) six-weeks and 11-months post-burn at one site. Additionally, at one site runoff samplers (116 unbounded plots, 10 cm wide and approximately 100 m from the catchment divide) were used to measure runoff and erosion downslope of six burn categories: (1) high severity, (2) low severity, (3) unburnt, and low severity above (4) 1 m, (5) 5 m, and (6) 10 m wide unburnt patches. Prescribed burning resulted in higher runoff and erosion rates. Cumulative hillslope runoff volumes (over16-months) were approximately two orders of magnitude higher on burnt hillslopes and cumulative sediment loads were approximately three orders of magnitude higher. Water repellency increased following burning at two sites, but loss of vegetation cover appeared to be the primary driver for increased runoff and erosion in burnt areas, as fire-induced water repellency did not affect point-scale infiltration capacities. Fire severity differences had relatively little effect on runoff and erosion, presumably because surface vegetation cover was similar in the high and low fire severities. Unburnt patches were highly effective at reducing the connectivity of runoff and erosion from upslope burnt areas, with reductions in overall sediment loads of 96.6% and 99.8% for the 5 m and 10 m wide patches, respectively. The effectiveness of the unburnt patches at reducing runoff and erosion connectivity varied with patch width and rainfall intensity. For example, the 1 m wide unburnt patch reduced the overall sediment load by 92% for rainfall events with average recurrence intervals of < 10 years but was ineffective during a 10-year storm. Overall, the results suggested that despite higher plot-scale runoff and erosion rates post-burn, prescribed burns are unlikely to substantially affect runoff and erosion at the catchment-scale for most rainfall events given their inherent patchiness. Only during particularly intense storms, when unburnt patches become less effective at intersecting runoff and erosion, might severe erosion occur. From a management perceptive, the results suggest that to minimise runoff and erosion connectivity and potential water quality impacts following prescribed burning, there should be a fine-grained mosaic of burnt and unburnt patches throughout a burn (e.g. > 50% unburnt and patches 5-10 m wide) and unburnt streamside buffers. Such burn patterns may be achieved by the ignition pattern, and burning under mild conditions when there are moisture differentials throughout the burn area. While fire severity was found to be a less significant factor in relation to post-burn runoff and erosion rates, it is likely that lower fire severities are associated with more patchy burns and therefore it would be reasonable to aim for low severity burn outcomes

Shifting States, Altered Fates: Divergent Fuel Moisture Responses after High Frequency Wildfire in an Obligate Seeder Eucalypt Forest

High frequency wildfires can shift the structure and composition of obligate seeder forests and initiate replacement with alternative vegetation states. In some forests, the alternative stable state is drier and more easily burned by subsequent fires, driving a positive feedback that promotes further wildfire and perpetuates alternative stable states. Mountain Ash (Eucalyptus regnans (F.Muell.)) forests are highly valued for their biodiversity, water, timber and carbon. Fires are a natural part of the lifecycle of these forests, but too frequent fires can eliminate Mountain Ash and trigger a transition to lower stature, non-eucalypt forests which are dominated by understorey species. This study sought to better understand the fuel moisture dynamics of alternative stable states resulting from high frequency wildfires. A vegetation mosaic in the Central Highlands, Victoria created a unique opportunity to measure fuel moisture in adjacent forest stands that differed in overstorey species composition and time since fire. Specifically, we measured fuel moisture and microclimate at two eucalypt sites (9 and 79 years old) and three non-eucalypt sites (two 9 year old and one 79 year old). Fuel availability, defined here as the number of days surface fuels were below 16% and dry enough to ignite and sustain fire, was calculated to estimate flammability. Fuel availability differed between sites, particularly as a function of time since fire, with recently burnt sites available to burn more often (4&#8722;17 versus 0&#8722;3 days). There were differences in fuel availability between non-eucalypt sites of the same age, suggesting that high frequency fire does not always lead to the same vegetation condition or outcome for fuel availability. This indicates there is potential for both positive and negative flammability feedbacks following state transition depending on the composition of the non-eucalypt state. This is the first study to provide empirical insight into the fuel moisture dynamics of alternative stable states in Mountain Ash forests

Quantifying Litter Bed Ignitability: Comparison of a Laboratory and Field Method

Understanding the conditions when litter beds will ignite from firebrands is critical for predicting spot fire occurrence. Such research is either field- or laboratory-based, with limited analysis to compare the approaches. We examined the ability of a laboratory method to represent field-scale ignitability. The laboratory method involved collecting litter-bed samples concurrently with the field experiments and then reconstructing and burning the litter-bed samples in the laboratory. We measured the number of successful and sustained ignitions in the laboratory (n = 5) and field (n = 30 attempts). The laboratory and field results were more similar for successful (bias = 0.05) than sustained ignitions (bias = 0.08). Wind, fuel structure (in the field) and near-surface fuel moisture influenced the differences between the methods. Our study highlights the value in conducting simultaneous laboratory and field experiments to understand the scalability of laboratory studies. For our ignitability method, our results suggest that small-scale laboratory experiments could be an effective substitute for field experiments in forests where litter beds are the dominant fuel layer and where the cover of the near-surface fuel is low

Long-Term Response of Fuel to Mechanical Mastication in South-Eastern Australia

Mechanical mastication is a fuel management strategy that modifies vegetation structure to reduce the impact of wildfire. Although past research has quantified immediate changes to fuel post-mastication, few studies consider longer-term fuel trajectories and climatic drivers of this change. Our study sought to quantify changes to fuel loads and structure over time following mastication and as a function of landscape aridity. Measurements were made at 63 sites in Victoria, Australia. All sites had been masticated within the previous 9 years to remove over-abundant shrubs and small trees. We used generalised additive models to explore trends over time and along an aridity gradient. Surface fuel loads were highest immediately post-mastication and in the most arid sites. The surface fine fuel load declined over time, whereas the surface coarse fuel load remained high; these trends occurred irrespective of landscape aridity. Standing fuel (understorey and midstorey vegetation) regenerated consistently, but shrub cover was still substantially low at 9 years post-mastication. Fire managers need to consider the trade-off between a persistently higher surface coarse fuel load and reduced shrub cover to evaluate the efficacy of mastication for fuel management. Coarse fuel may increase soil heating and smoke emissions, but less shrub cover will likely moderate fire behaviour

Conditional Performance Evaluation: Using Wildfire Observations for Systematic Fire Simulator Development

Faster than real-time wildland fire simulators are being increasingly adopted by land managers to provide decision support for tactical wildfire management and assist with strategic risk planning. These simulators are typically based on simple forward rate-of-spread algorithms that were predominantly developed using observations of experimental fires. Given their operational use, it is important that fire simulators be assessed in terms of their performance for their intended use; predicting the spatial progression of wildfires. However, the conditions under which wildfires occur cannot be easily replicated experimentally. We describe and demonstrate a method for use in model development, whereby a dataset comprised of wildfire case-studies is used for evaluating the predictive performance of fire simulators. Two different versions of the model PHOENIX RapidFire were assessed, one incorporating a novel algorithm that accounts fine-scale spatial variation in landscape dryness. Evaluation was done by comparing simulator predictions against contemporaneous observations of 9 different wildfires that occurred in Australia. Performance was quantified using the sum of the Area Difference Indices—a measure of prediction overlap calculated for each prediction/observation pair. The two versions of the model performed similarly, with the newer version being marginally (but not statistically significantly) better when outcomes were summarised across all fires. Despite this, it did not perform better in all cases, with three of the 9 fires better predicted using the original model. Wildfire evaluation datasets were demonstrated to provide valuable feedback for model development, however the limited availability of data means power is lacking for detailed comparisons. With increasingly extreme weather conditions resulting from climate change, conditions under which wildfires occur are likely to continue to extend well beyond those under which fire models algorithms were developed. Consequently, the adoption of improved methods for collecting and utilising wildfire data is critical to ensure fire simulators retain relevance