Eva Marino

Fuel bulk density and fuel moisture content effects on fire rate of spread: a comparison between FIRETEC model predictions and experimental results in shrub fuels

Fuel bulk density and fuel moisture content effects on fire rate of spread were assessed in shrub fuels, comparing experimental data observed in outdoor wind tunnel burns and predictions from the physically-based model FIRETEC. Statistical models for the combined effects of bulk density and fuel moisture content were fitted to both the experimental and the simulated rate of spread values using non-linear regression techniques. Results confirmed a significant decreasing effect of bulk density on rate of spread in a power law in both laboratory burns and simulations. However, experimental data showed a lesser effect than simulations, suggesting a difference in the effective drag. Fuel moisture content effect was highly consistent, showing a similar exponential relationship with rate of spread in laboratory and in simulations. FIRETEC simulations showed similar orders of magnitude with predictions of two field-based empirical models, finding a significant correlation between rate of spread values. The study confirms the efficacy of the combined approach through experimental data and simulations to study fire behaviour.

Fire hazard after prescribed burning in a gorse shrubland: Implications for fuel management

Prescribed burning is commonly used to prevent accumulation of biomass in fire-prone shrubland in NW Spain. However, there is a lack of knowledge about the efficacy of the technique in reducing fire hazard in these ecosystems. Fire hazard in burned shrubland areas will depend on the initial capacity of woody vegetation to recover and on the fine ground fuels existing after fire. To explore the effect that time since burning has on fire hazard, experimental tests were performed with two fuel complexes (fine ground fuels and regenerated shrubs) resulting from previous prescribed burnings conducted in a gorse shrubland (Ulex europaeus L.) one, three and five years earlier. A point-ignition source was used in burning experiments to assess ignition and initial propagation success separately for each fuel complex. The effect of wind speed was also studied for shrub fuels, and several flammability parameters were measured. Results showed that both ignition and initial propagation success of fine ground fuels mainly depended on fuel depth and were independent of time since burning, although flammability parameters indicated higher fire hazard three years after burning. In contrast, time since burning increased ignition and initial propagation success of regenerated shrub fuels, as well as the flammability parameters assessed, but wind speed had no significant effect. The combination of results of fire hazard for fine ground fuels and regenerated shrubs according to the variation in relative coverage of each fuel type after prescribed burning enabled an assessment of integrated fire hazard in treated areas. The present results suggest that prescribed burning is a very effective technique to reduce fire hazard in the study area, but that fire hazard will be significantly increased by the third year after burning. These results are valuable for fire prevention and fuel management planning in gorse shrubland areas.

Flammability descriptors of fine dead fuels resulting from two mechanical treatments in shrubland: a comparative laboratory study

Mechanical treatments are traditionally used to modify the fuel complex in shrubland, but information about their actual effectiveness in reducing the risk of wildfire initiation is scarce. The effects of two mechanical fuel treatments (shrub clearing with crushing and manual removal) on flammability in a shrubland community in north-western Spain were compared. Three months after treatment, laboratory tests using a point-ignition source were conducted on the fine dead fuels to analyse the effect of type of treatment and fuel moisture content (FMC) under two conditions: (1) flaming; or (2) glowing+wind ignition source. Fuel load effect within each treatment was also studied. Time-to-ignition, flaming duration, number of burnt sides of the sample and fuel consumption ratio were assessed. Logistic models were developed to assess ignition and sustained combustion probabilities. Type of treatment and FMC significantly affected flammability under both experimental conditions tested. Slow smouldering was observed in fuels subjected to shrub clearing and removal, whereas crushing fuels were rapidly burnt with flaming phase combustion. In general, shrub clearing and removal appeared to be more effective in reducing wildfire hazard in these shrubland communities.

Estimation of Peak Heat Release Rate of a Forest Fuel Bed in Outdoor Laboratory Conditions

Experimental tests were carried out with an adapted bench-scale mass loss calorimeter (MLC) and also in an outdoor wind tunnel to estimate the heat release rate (HRR) of a forest fuel bed. The MLC apparatus uses a calibrated thermopile to quantify the HRR, as an alternative to the classical measurement of oxygen consumption due to combustion. Additional calibration of thermocouples to measure gas temperatures enabled estimation of HRR in experimental burnings conducted in the wind tunnel. The results showed a reasonable agreement between peak HRR (PHRR) values obtained in the MLC and in the wind tunnel, and also demonstrated that PHRR was significantly affected by the wind speed and the rate of spread. The proposed procedure is potentially useful as an initial step in monitoring HRR in outdoor forest fire experiments.

Fuel management effectiveness in a mixed heathland: a comparison of the effect of different treatment types on fire initiation risk

Fuel management is commonly used to reduce fire risk in fire-prone shrubland, but information about the real efficacy of the different techniques is scarce. In this study, we assessed in the laboratory the effects of different treatment types on fire initiation risk in a mixed heathland. The effects of two mechanical treatments and of prescribed burning were compared with untreated vegetation. Flammability tests were performed in samples of the regenerated shrubs and fine ground fuels present 2 years after treatments. Results indicate that all treatments were effective in reducing fire initiation risk in regenerated shrubs, but not in fine ground fuels. Recovery of vegetation differed between treatments, and treatment type had a significant effect on flammability, mainly affecting fire sustainability. Wind speed had a minor effect on shrub fuel flammability, whereas fuel moisture had a significant effect. The flammability of fine ground fuels differed significantly depending on fuel moisture content, even at the low levels tested. Logistic models were fitted to predict successful fire sustainability, and the probability of initial propagation was obtained as a function of treatment type, fuel moisture content and fuel structural characteristics. This study provides new insights into wildfire prevention in shrubland, and compares the effectiveness of different fuel treatment techniques.

Short-term effect of fuel treatments on fire behaviour in a mixed heathland: a comparative assessment in an outdoor wind tunnel

Fuel management is one of the main challenges for wildfire prevention in the Mediterranean region, where wildfires have important environmental and socioeconomic effects. Different treatments are usually applied in fire-prone shrubland to try to modify its flammability. However, a knowledge gap on the effectiveness of fuel management techniques still exists. We studied the effects of two mechanical treatments (shrub crushing and shrub clearing with removal) and of prescribed burning, on fire behaviour, and compared them with untreated vegetation. Experimental burns in 0.8 × 6 m samples of regenerated shrubs 2 years after treatments were performed in an outdoor wind tunnel. All fuel treatments effectively modified fire behaviour, but no significant difference between treatment types was observed. Shrub fuel structure was the main factor affecting fire behaviour. Reduction of fuel load and height, especially necromass fraction, decreased flame height and fire intensity but did not affect fire rate of spread. Moisture contents of live and dead fuel fractions were not significant as independent parameters, but the average moisture level of the shrub fuel complex showed a relevant effect in determining fire behaviour. Temperature regime within and above the shrubs was also related to shrub fuel structure. This study contributes to understanding fuel management in shrubland by providing information about different fuel treatments effects on fire behaviour.

Vertical forest structure analysis for wildfire prevention: Comparing airborne laser scanning data and stereoscopic hemispherical images

Abstract Vertical fuel structure is critical for fire hazard assessment in forest ecosystems. Forest stands with ladder fuels are more prone to crown fires because of canopy fuel continuity. However, characterization of ladder fuels is difficult in the field and few studies have developed explicit measurement procedures to account for these hazardous fuel situations. This study compares vertical profiles derived from airborne laser scanning (ALS) data and stereoscopic hemispherical images obtained in Pinus sylvestris stands in central Spain to test their ability to detect the presence or absence of vertical fuel continuity (VFC). Vertical fuel profiles based on canopy cover fraction estimations at different height strata were assessed at plot level and compared with field observations. The quadratic form distance (QFD) was the metric used to quantify the similarity between histogram distributions defined by the vertical profiles from different datasets. Logistic regression analysis was tested to discriminate areas with and without VFC from ALS data at two threshold levels (15% and 30%). The vertical fuel profiles of canopy cover showed a different level of correspondence depending on the relative amount of ladder fuels. Significant logistic models were found (p  090) for different combination of ALS metrics, with low percentiles (up to P30), canopy relief ratio (CRR) and the percentage of returns normalized by height strata (PRN) up to 8 m as the best predictors to identify the presence of VFC. Results indicated that both datasets were useful in retrieving variability of forest fuel distribution, but further methodological improvements (e.g. understory segmentation in stereoscopic images, new algorithms to better account for occlusions, or ground calibration for laser attenuation in ALS) are needed to increase accuracy in highly continuous areas.