Eucalypts, especially blue gum (Eucalyptus globulus), have been extensively planted in Portugal and nowadays dominate most of its forest landscapes. Large-scale forestation programs can intensify ...fire activity, and blue gum plantations are often viewed as highly flammable due to the nature and structure of the fuel complex. The role of eucalypt plantations in the fire regime of Mediterranean climate regions is increasingly debated following the recent catastrophic wildfires in Portugal and elsewhere. In this study we examined the effects of eucalypt forestation on burned area (BA), fire size, and fire severity in Portugal. This was based on fire and vegetation mapping and statistics, fire weather data, satellite imagery, and national forest inventory data. Eucalypt BA comprised an average of 12.5% of total BA (1980–2017) and did not increase over time and with eucalypt expansion. Eucalypt metrics did not explain interannual BA variability after accounting for the effects of other variables. Forest fires started within eucalypt stands were the least likely to become large, and large fire size was irresponsive to forest composition. Likewise, forest type was a generally minor influence in mega-fire severity and accounted for just 1.4–8.6% of surface fuel-hazard metrics variation. In general, large-scale conversion of maritime pine to eucalypt stands (1970–2015) implied lower fuel accumulation. Fire activity results are consistent with fuel hazard results and express trade-offs between short-rotation forestry and fire behaviour in blue gum stands, with high spotting potential versus modest crown fire likelihood. We found no support for the contention of a modified fire regime as a result of eucalypt forestation in Portugal, but the rising undermanaged and abandoned blue gum estate, especially after large-fire seasons, is a concern for the future. However, it remains to be determined whether post-fire eucalypt regrowth is a higher fire threat than native vegetation in the same context.
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•Eucalypt expansion did not increase burned area in Portugal.•Fire size independent of forest composition•Slight decrease of mega-fire severity in eucalypt stands•Forest type is a minor influence on fuel hazard.•Fire activity reflects trade-offs between short-rotation forestry and fire behaviour.
A laboratory experimental program addressing fire spread in fuel beds composed of dead foliage litter and vertically placed quasi-live branches, representative of many natural fuel complexes, was ...carried out for either still-air or wind conditions. Fuel-bed characteristics, fire spread rate, flame geometry, and fuel consumption were assessed and empirical models for estimating several parameters were developed. Weighted fuel moisture content (18%–163%) provided good estimates of fire-behaviour characteristics and accounted for most of the variation in still-air and wind-driven spread rate (0.1–1.3 m·min
−1
). When predicting still-air fire spread rate, fuel height was the most relevant fuel-bed structural parameter and fuel type had significant influence, whereas for wind-driven spread, the effect of foliar fuel-bed density was dominant and fuel type became irrelevant. Flame length (0.4–2.2 m) increased from still-air to wind-assisted (8 km·h
−1
) fire spread, but its height remained constant. The fraction of total fuel load and mean woody fuel diameter consumed by fire were reasonably predicted from weighted fuel moisture content alone, but predictions for the latter variable improved substantially by adding foliar fuel load.
Background
The study of wildfire interactions (i.e., spread limitation and reburns) is gaining traction as a means of describing the self-limiting process of fire spread in the landscape and has ...important management implications but has scarcely been attempted in Europe. We examined to what extent previously burned areas restricted the development of individual large wildfires (> 500 ha) in mainland Portugal.
Results
For the 1984–2021 period, we (1) modeled the proportion of large wildfire perimeters coinciding with transitions to shorter time since fire (TSF), i.e., locations where fire spread ceased upon encountering assumedly less flammable fuels, and (2) characterized the prevalence of different TSF in the composition of the area burned by large wildfires in relation to available TSF. Only 4% of the large wildfires did not comprise edges intersecting past wildfires. Low TSF (especially up to 8 years) resulted in large-wildfire perimeter limitation at TSF transitions. This effect was further enhanced by high historical burn probability and proximity to roadways and watercourses. Perimeter limitation did also increase under high (but not very high or extreme) fire danger, benefiting from maximum seasonal firefighting preparedness. TSF prevalence in the composition of large-wildfire area was extremely variable and thus an overall weak pattern emerged, with minimum and maximum prevalence respectively at TSF < 2 years and TSF ≥ 6 years.
Conclusions
Large wildfire limitation in Portugal is hampered by fast fuel build-up after fire, indicating a short-lived fire-hazard reduction effect under the prevailing Mediterranean humid climate of the study region. Nonetheless, such effect should be considered when planning fuel-reduction treatments and can be used opportunistically during large-wildfire suppression operations.
Near-instantaneous estimation of the moisture content of live fuels is complicated because of the large control exerted by physiological mechanisms. The commonly accepted reference method for ...measuring fuel moisture content is oven drying, which is time consuming. This study evaluates the use of a moisture analyzer (ML-50, A&D Company, Limited, Tokyo, Japan) for measuring the foliar moisture content of two common European species. The moisture of live leaves of Arbutus unedo L. (strawberry tree) and Quercus robur L. (pedunculate oak) was measured within a period of 15 min using two drying temperatures and compared with the oven-dried value. Correction factors were determined for estimating the oven-dried moisture content based on the measurement by the moisture analyzer. The power delivered during the drying process plays an important role in the moisture measured by the analyzer in relation to the oven-dried value. Increasing the drying time beyond the minimum period necessary for obtaining a reliable prediction of the oven-dried moisture does not significantly change the moisture measured at lower temperatures. The moisture analyzer is able to estimate the live foliage moisture content with high accuracy.
Behaviour of slope and wind backing fires Rossa, Carlos G; Davim, David A; Viegas, Domingos X
International journal of wildland fire,
01/2015, Letnik:
24, Številka:
8
Journal Article
Recenzirano
Laboratory experiments of backing fires with slope (–60 to 0°) and wind (–4.5 to 0ms-1) were carried out in fuel beds of dead Pinus pinaster Ait. needles and straw at a 0.6-kgm-2 fuel load, ...evaluating rates of spread and flame geometry. Wind velocity measurements inside and above the fuel beds were also carried out. Increase in fuel moisture content decreased the ratio between downslope and level-ground rates of spread (ROS). The ROS decrease with slope angle followed by an increase agreed well with flame geometry data that provided an estimation of the amount of radiation reaching the fuel bed. Features of slope backing fire behaviour could be reasonably estimated based on no-slope fire spread rate. Evidence was found that fuel moisture influenced the ROS of backing fires with wind, despite with an effect opposite to that of slope. Reduced penetration of air into the fuel beds explains the small ROS variation and results suggest that for an increasingly deep fuel bed, the mean ROS tends asymptotically to the no-wind ROS.
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•The spatial interaction between wildfires and prescribed fires is studied.•Wildfires find prescribed burnt areas often and in the first years after treatment.•The fraction of ...unburned treated area upon wildfire encounter is typically low.•Wildfire behaviour and treatment size dominate the intersection outcome.•Higher effectiveness requires scaling-up treatments.
Adoption of prescribed burning is increasing as the treatment chosen to decrease fuel hazard in southern Europe but little is known about how it affects wildfire activity. We assessed the effectiveness of prescribed burning treatments by analysing the survival of treatment units to wildfire in mainland Portugal (2005–2017). We examined the time-dependency of treatment-wildfire encounters through survival analysis, and evaluated treatment effectiveness as the intersection outcome in terms of the unburned fraction of the treatment. Generalized linear modelling supplemented by regression tree analysis was used to attain the second objective. Prescribed fire treatments were frequently (42% of the total number of units) intersected by wildfire, which occurs soon after treatment: the probability of an encounter peaked 2 years after treatment and its cumulative value grew at a diminishing rate with fuel age. Of all treated units, 58% burned entirely upon encounter and the median unburned fraction was 0.01 owing to the prevalence of intersections with large and presumably fast spreading and high intensity wildfires. Larger treatments burned less in area but the effect of wildfire characteristics was largely prevalent over the effect of treatment size. The unburned fraction of treated units seldom responded to fuel age, which we discuss based on biophysical influences, treatment effort, and fire suppression strategy. The high encounter rate but low effectiveness in decreasing burned area within treatments and, seemingly, nil effect for practical purposes on wildfire size indicates that prescribed burning is not impacting wildfire extent in Portugal. Our findings indicate the need to scale-up prescribed burning activity to effectively contribute to decrease wildfire size, but also improvements in fire management planning and operations in general.
•We analyse leverage, the effect of past fire on reducing subsequent wildfire.•Wildfire extent is driven mainly by fire weather and ignition density.•Past fire, as a whole, reduces subsequent ...wildfire extent.•Return-for-effort of prescribed burning is 1:5 in the worst-case scenario.•Scaling-up of prescribed fire is expected to increase its leverage.
Prescribed burning (PB) is increasingly recognised as a viable, cost-effective technique for reducing wildfire risk. Yet, quantification of the effect of PB on the reduction of wildfire extent in southern Europe is non-existent. We used 35 years of fire mapping data in Portugal to analyse wildfire regime metrics in nine landscapes before (1985–2004) and during (2005–2020), the period when systematic use of PB to reduce fuel load began. We employed robust linear mixed models and generalized linear mixed models to examine the role of annual wildfire extent drivers at landscape scales and quantified the effect of PB on the reduction of subsequent wildfire extent, i.e., leverage. We found evidence of larger wildfires during the PB era, suggesting that other influences are counteracting its putative effect. The most important variables explaining wildfire extent were fire weather and ignition density, with positive effect, and past wildfire and PB extent, with negative effect, corresponding to leverage. We obtained a total past fire (PB and wildfire) leverage of 0.28, i.e., approximately 4 ha of past fire decrease subsequent wildfire by 1 ha, increasing to 0.84 if only landscapes with high wildfire incidence are considered. The analyses failed to unequivocally identify a direct leverage effect of PB but a 1:5 return-for-effort is reasonable as a conservative estimate. Increasing the size of PB treatments and treatment effort in the landscape in combination with fire-modelling assisted planning would enhance PB leverage, making it more effective at reducing wildfire extent, rather than functioning only as a supplement to the effect of previous wildfires.
Analizirana je uloga pojedinih svojstava ložišta u stopi širenja šumskog požara, na temelju ravnoteže toplinske energije, iz koje je razvijen poznati Rothermelov model širenja požara (1972), ...pokazujući da niti visina ložišta, jednako kao ni količina i gustoća ne utječu izravno na ravnotežu toplinske energije. Utjecaj tih parametara, često izvedenih iz empirijskih opisa stope širenja, nužno proizlazi iz neizravnih utjecaja na mehanizme prijenosa topline. Udio topline prenesen iz plamena na nesagorijelo gorivo, pretežno ovisi o sadržaju vlage u gorivu te je neovisan od stope širenja te oblika plamena. S obzirom na to da empirijski modeli uglavnom impliciraju mehanizme širenja požara u opisivanju njegovog ponašanja, rezultati ove studije mogu biti od pomoći u osmišljavanju i ocrtavanju budućih eksperimenata i pristupa.
We analysed the role of some fuel bed properties on forest fire-spread rate based on the thermal energy balance upon which the well-known fire-spread rate model of Rothermel (1972) was developed, showing that neither fuel bed height, load or density directly influence the thermal energy balance. The influence of such parameters, often inferred from empirical descriptions of spread rate, must result from indirect effects on heat transfer mechanisms. The fraction of heat transferred from the flame to the unburned fuel depends mostly on fuel moisture content and is independent of spread rate and flame geometry. Because empirical models usually implicitly assume the underlying mechanisms of fire spread for describing fire behaviour, this study results can assist at idealizing and delineating future experiments and approaches.
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