Prescribed Burning Treatments Research Articles

Forest structural complexity and ignition pattern influence simulated prescribed fire effects

BackgroundForest structural characteristics, the burning environment, and the choice of ignition pattern each influence prescribed fire behaviors and resulting fire effects; however, few studies examine the influences and interactions of these factors. Understanding how interactions among these drivers can influence prescribed fire behavior and effects is crucial for executing prescribed fires that can safely and effectively meet management objectives. To analyze the interactions between the fuels complex and ignition patterns, we used FIRETEC, a three-dimensional computational fluid dynamics fire behavior model, to simulate fire behavior and effects across a range of horizontal and vertical forest structural complexities. For each forest structure, we then simulated three different prescribed fires each with a unique ignition pattern: strip-head, dot, and alternating dot.ResultsForest structural complexity and ignition pattern affected the proportions of simulated crown scorch, consumption, and damage for prescribed fires in a dry, fire-prone ecosystem. Prescribed fires in forests with complex canopy structures resulted in increased crown consumption, scorch, and damage compared to less spatially complex forests. The choice of using a strip-head ignition pattern over either a dot or alternating-dot pattern increased the degree of crown foliage scorched and damaged, though did not affect the proportion of crown consumed. We found no evidence of an interaction between forest structural complexity and ignition pattern on canopy fuel consumption, scorch, or damage.ConclusionsWe found that forest structure and ignition pattern, two powerful drivers of fire behavior that forest managers can readily account for or even manipulate, can be leveraged to influence fire behavior and the resultant fire effects of prescribed fire. These simulation findings have critical implications for how managers can plan and perform forest thinning and prescribed burn treatments to meet risk management or ecological objectives.

Enhanced post-wildfire vegetation recovery in prescribed-burnt Mediterranean shrubland: A regional assessment

The primary objective of prescribed burning (PB) treatment is to promote increased ecosystem resistance to high fire severity and ecosystem adaptive resilience after unplanned wildfires under worsening climates. Yet, empirical evidence involving regional-scale assessments with enough spatial representativeness on how vegetation will respond to PB-wildfire interactions in Mediterranean shrublands is absent. This is particularly relevant to provide comprehensive insights on the effectiveness of PB policies and programs in southern European countries, where shrublands are the main PB target. Here, we investigated the PB effect on vegetation responses in the short-term (1–3 years) after subsequent wildfire encounters in Mediterranean shrublands at the regional scale across mainland Portugal. We leveraged geospatial cloud-computing platforms and remote sensing data (optical and synthetic aperture radar -SAR-) and techniques with physical basis to procure reliable proxies for vegetation fractional cover (FCOVER), vigor (normalized difference vegetation index; NDVI) and structural complexity (cross-polarization ratio of C-band SAR backscatter data; SAR-CR) at the landscape scale throughout the time series. The role of site productivity and topographic configuration variables as putative drivers of vegetation responses was also analyzed. We found that the mitigating effect of PB on wildfire severity translated into stronger vegetation responses in treated areas than in untreated counterparts after the first PB-wildfire encounter. Although FCOVER, NDVI and SAR-CR recovered gradually in both areas, the recovery rate was markedly higher in PB-treated areas 3-years after the PB-wildfire encounter than at earlier times in the time series. The effect of PB treatment was stronger for the FCOVER recovery response than for NDVI and SAR-CR. The effect of the PB treatment on FCOVER and NDVI responses was modulated by site environmental characteristics, such that their differences between PB-treated and untreated counterparts were maximal for both higher altitudes and warmer, drier locations in the landscape.

Forest thinning and prescribed burning treatments reduce wildfire severity and buffer the impacts of severe fire weather

BackgroundThe capacity of forest fuel treatments to moderate the behavior and severity of subsequent wildfires depends on weather and fuel conditions at the time of burning. However, in-depth evaluations of how treatments perform are limited because encounters between wildfires and areas with extensive pre-fire data are rare. Here, we took advantage of a 1200-ha randomized and replicated experiment that burned almost entirely in a subsequent wildfire under a wide range of weather conditions. We compared the impacts of four fuel treatments on fire severity, including two thin-only, a thin-burn, a burn-only, and an untreated control. We evaluated four fire severity metrics—tree mortality, average bole char height, percent crown volume consumed (PCVC), and percent crown volume affected (PCVA)—and leveraged data from pre-fire surface and canopy fuels to better understand the mechanisms driving differences in wildfire severity among treatments and how they changed with fire weather.ResultsWe found strong mitigating effects of treatments on fire behavior and tree mortality, despite 20 years having elapsed since mechanical thinning and 10 years since the second entry of prescribed fire. The thin-burn treatment resulted in the lowest fire severity across all four metrics and the untreated control the highest. All four fire severity metrics were positively associated with pre-fire canopy and surface fuel loads, with the exception that PCVC (a fire severity metric related to crown fire behavior) was not associated with surface fuel load. The fire weather conditions under which fuel treatment was most effective varied among fire severity metrics. Fuel treatment benefit was maximized at intermediate burning index values for tree mortality, intermediate to high burning index values for PCVA, and high burning index for bole char height and PCVC.ConclusionsWe conclude that reducing canopy bulk density via mechanical thinning treatments can help to limit crown fire behavior for 20 years or more. However, reducing surface fuels is necessary to limit scorching and the total crown impacts associated with tree mortality. Further, while fuel treatment effectiveness may decline under the most severe fire weather conditions for fire severity metrics associated with tree mortality, it is maximized under severe fire weather conditions for fire severity metrics associated with crown fire behavior (bole charring and torching). Our results provide strong evidence for the use of fuel treatments to mitigate fire behavior and resulting fire severity even under extreme fire weather conditions.

Long-term efficacy of fuel reduction and restoration treatments in Northern Rockies dry forests.

Fuel and restoration treatments seeking to mitigate the likelihood of uncharacteristic high-severity wildfires in forests with historically frequent, low-severity fire regimes are increasingly common, but long-term treatment effects on fuels, aboveground carbon, plant community structure, ecosystem resilience, and other ecosystem attributes are understudied. We present 20-year responses to thinning and prescribed burning treatments commonly used in dry, low-elevation forests of the western United States from a long-term study site in the Northern Rockies that is part of the National Fire and Fire Surrogate Study. We provide a comprehensive synthesis of short-term (<4 years) and mid-term (<14 years) results from previous findings. We then place these results in the context of a mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak that impacted the site 5-10 years post-treatment and describe 20-year responses to assess the longevity of restoration and fuel reduction treatments in light of the MPB outbreak. Thinning treatments had persistently lower forest density and higher tree growth, but effects were more pronounced when thinning was combined with prescribed fire. The thinning+prescribed fire treatment had the additional benefit of maintaining the highest proportion of ponderosa pine (Pinus ponderosa) for overstory and regeneration. No differences in understory native plant cover and richness or exotic species cover remained after 20 years, but exotic species richness, while low relative to native species, was still higher in the thinning+prescribed fire treatment than the control. Aboveground live carbon stocks in thinning treatments recovered to near control and prescribed fire treatment levels by 20 years. The prescribed fire treatment and control had higher fuel loads than thinning treatments due to interactions with the MPB outbreak. The MPB-induced changes to forest structure and fuels increased the fire hazard 20 years post-treatment in the control and prescribed fire treatment. Should a wildfire occur now, the thinning+prescribed fire treatment would likely have the lowest intensity fire and highest tree survival and stable carbon stocks. Our findings show broad support that thinning and prescribed fire increase ponderosa pine forest resilience to both wildfire and bark beetles for up to 20 years, but efficacy is waning and additional fuel treatments are needed to maintain resilience.

Prescribed fire promotes regeneration in a mature eastern white pine forest

Although evidence indicates that fire exclusion may result in substantial short- and long-term changes to forest stand structure and composition, the long-term effects on eastern white pine (Pinus strobus L.) stands remain largely unknown. We investigated the response of trees and understory vegetation after prescribed burning in a mature white pine stand in northern Ontario, Canada. Results indicate that a single prescribed burning treatment improved regeneration of white pine and other mid-shade tolerant species compared to controls whereas repeated burning favoured shade intolerant species such as trembling aspen (Populus tremuloides Michx.). Unburned control plots had a substantial understory component of shade tolerant balsam fir (Abies balsamea [L.] Mill.). We simulated stand compositions after 70 years post-treatment and found that prescribed burn plots had much higher volume of white pine compared to unburned plots. Overall, the results highlight fire severity as a driving influence on the successional trajectory of white pine stands. The findings support low to moderate prescribed burning of mature white pine-dominated forests as an effective means to retain a white pine component within areas of the Great Lakes-St. Lawrence forest region.

Signatures of prescribed fire in the microbial communities of Cornus florida are largely undetectable five months post-fire.

Prescribed burn is a management tool that influences the physical structure and composition of forest plant communities and their associated microorganisms. Plant-associated microorganisms aid in host plant disease tolerance and increase nutrient availability. The effects of prescribed burn on microorganisms associated with native ecologically and economically important tree species, such as Cornus florida L. (flowering dogwood), are not well understood, particularly in aboveground plant tissues (e.g., leaf, stem, and bark tissues). The objective of this study was to use 16S rRNA gene and ITS2 region sequencing to evaluate changes in bacterial and fungal communities of five different flowering dogwood-associated niches (soil, roots, bark, stem, and leaves) five months following a prescribed burn treatment. The alpha- and beta-diversity of root bacterial/archaeal communities differed significantly between prescribed burn and unburned control-treated trees. In these bacterial/archaeal root communities, we also detected a significantly higher relative abundance of sequences identified as Acidothermaceae, a family of thermophilic bacteria. No significant differences were detected between prescribed burn-treated and unburned control trees in bulk soils or bark, stem, or leaf tissues. The findings of our study suggest that prescribed burn does not significantly alter the aboveground plant-associated microbial communities of flowering dogwood trees five months following the prescribed burn application. Further studies are required to better understand the short- and long-term effects of prescribed burns on the microbial communities of forest trees.

Effects of Prescribed-Fire Timing on Stocker Cattle Performance, Forage Biomass Accumulation, and Native Plant Species Composition

Objective: The objective of this experiment was to document the effects of prescribed-fire timing on stocker cattle performance, forage biomass accumulation, soil cover, and plant species composition in the Kansas Flint Hills. Study Description: A total of 1,416 yearling stocker cattle were assigned to one of three prescribed-burn treatments: spring (April 9 ± 5.1 days), summer (August 23 ± 4.9 days), or fall (September 29 ± 8.7 days) over a 4-year period. Calves were grazed from May to August for 90 days. Individual body weights were recorded at the beginning and end of the grazing season. Native plant composition and soil cover were evaluated annually using a modified step-point method, and forage biomass was measured biannually. The Bottom Line: Flint Hills ranchers can employ late-summer prescribed fires to manage sericea lespedeza (Lespedeza cuneata) infestations without negatively impacting stocker cattle growth performance, forage biomass accumulation, or native rangeland plant species composition.

Short-term recovery of soil and pine tree canopy after late prescribed burning in a semi-arid landscape

Climate change worsening due to global warming and progressive abandonment in rural areas mean that wildfires are increasing in extent and severity terms, and are one of the major disturbances in the Mediterranean Basin. To mitigate these disturbances, preventive management tools need to be used. Fire employment is being implemented, known as prescribed burnings, as forestry actions to change vegetation lines both vertically and horizontally to eliminate forest fuel load continuity. This study aimed to know the ecological effects of late prescribed burning treatments under mixed trees. Prescribed burns were carried out in October 2019 in the municipality of Ayna, Albacete (SE Spain). To assess the short-term (12-month) fire impact on soil, we measured soil respiration with a CO2 flow chamber and analyzed soil physico-chemical properties. We also used minidisc infiltrometers to analyze soil repellency and soil hydraulic conductivity over a 1-year monitoring period. In addition, we evaluated the effects on tree strata by performing chemical nature analyses of fallen needles in Pinus pinaster after prescribed burning. According to our initial hypotheses, the results did not show significant changes in any parameter evaluated during the study period. However, some variables were affected in the first 3 months, needlefall and in microbiological activity, such as variations in soil physico-chemical properties, which can be due to long dry seasons combined with prescribed burning. This study attempts to observe and make known the effects that low-intensity prescribed burning has on soil and needlefall, which are relevant for updating forest management tools.

The 2019–2020 Australian forest fires are a harbinger of decreased prescribed burning effectiveness under rising extreme conditions

There is an imperative for fire agencies to quantify the potential for prescribed burning to mitigate risk to life, property and environmental values while facing changing climates. The 2019–2020 Black Summer fires in eastern Australia raised questions about the effectiveness of prescribed burning in mitigating risk under unprecedented fire conditions. We performed a simulation experiment to test the effects of different rates of prescribed burning treatment on risks posed by wildfire to life, property and infrastructure. In four forested case study landscapes, we found that the risks posed by wildfire were substantially higher under the fire weather conditions of the 2019–2020 season, compared to the full range of long-term historic weather conditions. For area burnt and house loss, the 2019–2020 conditions resulted in more than a doubling of residual risk across the four landscapes, regardless of treatment rate (mean increase of 230%, range 164–360%). Fire managers must prepare for a higher level of residual risk as climate change increases the likelihood of similar or even more dangerous fire seasons.

Evaluating the effect of prescribed burning on the reduction of wildfire extent in Portugal

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.

Effects of burn season on fire-excluded plant communities in the southern Appalachian Mountains, USA

Following decades of fire exclusion, managers are increasingly implementing prescribed fire in southern Appalachian forests. To date, the use of prescribed fire in the region has often been focused on reducing hazardous fuel loads and has typically occurred in the dormant season. Understanding the effects of burning in different periods of plant growth may reveal how burn season influences patterns of vegetative succession. In this study, we compared the effects of prescribed burn treatments conducted in the dormant season (January-early April) vs. the early growing season (mid-late April) on changes in plant abundance by understory, midstory, and overstory forest strata. Plant groups were distinguished by growth habit, stem origin, functional characteristics, and species of management interest (red maple (Acer rubrum L.) and mountain laurel (Kalmia latifolia L.)). Burn season had minimal effect on understory cover, density, richness, or diversity. In the midstory, early growing season burns were more effective in reducing shrub density than dormant season burns (−1,585 ± 188 ha−1 vs. −813 ± 240 ha−1, respectively), with greater differences among smaller stems. Early growing season burns also reduced midstory red maple density to a greater degree than dormant season burns (−356 ± 57 ha−1 vs. −219 ± 69 ha−1), a response that was not observed among other mesophytic hardwood species. Burning slightly reduced canopy cover, but neither canopy cover nor overstory density response varied by burn season. Our results demonstrate that managers may find increased opportunities to promote forest restoration objectives in the southern Appalachians by extending the use of prescribed fire into the early growing season.

Tree growth responses to extreme drought after mechanical thinning and prescribed fire in a Sierra Nevada mixed-conifer forest, USA

An estimated 128 M trees died during the 2012–2016 California drought, largely in the southern Sierra Nevada Range. Prescribed burning and mechanical thinning are widely used to reduce fuels and restore ecosystem properties, but it is unclear if these treatments improve tree growth and vigor during extreme drought. This study examined tree growth responses after thinning, prescribed burning, and extreme drought at the Teakettle Experimental Forest, a historically frequent fire mixed-conifer forest in the southern Sierra Nevada of California, USA. Mechanical thinning (no thin, understory thin, and overstory thin) and prescribed burning (unburned, fall burning) were implemented in 2000–2001. Using annual growth data from increment cores, over 10,000 mapped and measured trees, and lidar-derived metrics of solar radiation and topographic wetness, we had two primary questions. First, what were the growth responses to thinning and prescribed burning treatments, and did these responses persist during the 2012–2016 drought? Second, what tree-level attributes and environmental conditions influenced growth responses to treatments and drought?Thinning increased residual tree growth and that response persisted through extreme drought 10–15 years after treatments. Growth responses were higher in overstory versus understory thinning, with differences between thinning types more pronounced during drought. Species-specific growth responses were strongest with overstory thinning, with sugar pine (Pinus lambertiana) and incense-cedar (Calocedrus decurrens) having higher growth responses compared to white fir (Abies concolor) and Jeffery pine (Pinus jeffreyi). For individual trees, factors associated with higher growth responses were declining pretreatment growth trend, smaller tree size, and post-treatment low neighborhood basal area. Growth responses were initially not influenced by topography, but topographic wetness became important during extreme drought. Mechanical thinning resulted in durable increases in residual tree growth rates during extreme drought over a decade after thinning occurred, indicating treatment longevity in mitigating drought stress. In contrast, tree growth did not improve after prescribed burning, likely due to fire effects that reduced surface fuels, but had little effect on reducing tree density. Thinning treatments promoted durable growth responses, but focusing on stand-level metrics may ignore important tree-level attributes such as localized competition and topography associated with higher water availability. Mechanical thinning was effective at improving growth in trees that had been experiencing declining growth trends, but was less effective in improving growth responses in large old trees of higher ecological importance.

Previous wildfires and management treatments moderate subsequent fire severity

We investigated the relative importance of daily fire weather, landscape position, climate, recent forest and fuels management, and fire history to explaining patterns of remotely-sensed burn severity – as measured by the Relativized Burn Ratio – in 150 fires occurring from 2001 to 2019, which burned conifer forests of northeastern Washington State, USA. Daily fire weather, annual precipitation anomalies, and species’ fire resistance traits were important predictors of wildfire burn severity. In areas burned within the past two to three decades, prior fire decreased the severity of subsequent burns, particularly for the first 16 postfire years. In areas managed before a wildfire, thinning and prescribed burning treatments lowered burn severity relative to untreated controls. Prescribed burning was the most effective treatment at lowering subsequent burn severity, and prescribed burned areas were usually unburned or burned at low severity in subsequent wildfires. Patches that were harvested and planted <10 years before a wildfire burned with slightly higher severity. In areas managed within 5 years after an initial fire, postfire harvest and planting reduced prevalence of stand-replacing fire in reburns. However, overall, postfire management actions after a first wildfire only weakly influenced the severity of subsequent fires. The importance of fire-fire interactions to moderating burn severity establishes the importance of stabilizing feedbacks in active fire regimes, and our results demonstrate how silvicultural treatments can be combined with prescribed fire and wildfires to maintain resilient landscapes.

Survival of prescribed burning treatments to wildfire in Portugal

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.

The Kings River Experimental Watersheds: Infrastructure and data

AbstractThe Kings River Experimental Watersheds (KREW) were established in 2002 to expand our knowledge of catchment physical, chemical, and biological processes in Sierra Nevada headwater forests, and to better understand the impacts of prescribed burning and forest thinning on these processes. Two elevation strata (high and low) were selected for the KREW sites, with four independent catchments and one nested catchment within each stratum. Both high and low elevation study areas were instrumented for continuous measurements of meteorology, streamflow, and turbidity. Atmospheric and stream chemistry, suspended sediment concentration, and bedload sediment delivery were measured on a regular schedule. Soil chemical and physical properties and vegetation were systematically sampled before and after the initial thinning and prescribed burning treatments, which were implemented between 2012 and 2016. Post‐treatment data collection continues today as we explore opportunities for the second round of possible treatments. The critical research infrastructure and long‐term baseline data collection has been instrumental in building partnerships with downstream managers, end users, non‐governmental organizations, academic researchers, and national research programmes. Contributions to date include fundamental understanding of magnitude and variability of nutrient deposition; carbon, nutrient, and major ion dynamics in headwater streams; aquatic algae and macroinvertebrate populations; vegetation composition and structure; and streamflow responses to precipitation in the two elevation strata. Data from the experimental watersheds also support calibration and validation of diverse hydrologic models used for water resources planning.

Do forest fuel reduction treatments confer resistance to beetle infestation and drought mortality?

AbstractClimate change is amplifying the frequency and severity of droughts and wildfires in many forests. In the western United States, fuel reduction treatments, both mechanical and prescribed fire, are widely used to increase resilience to wildfire but their effect on resistance to drought and beetle mortality is not as well understood. We followed more than 10,000 mapped and tagged trees in a mixed‐conifer forest following mechanical thinning and/or prescribed burning treatments in 2001 through the extreme 2012–2016 drought in California. Mortality varied by tree species from 3% of incense‐cedar to 38% of red fir with proportionally higher mortality rates in the larger size classes for sugar pine, red fir, and white fir. Treatment reductions in stem density were associated with increased diameter growth and rapidly growing trees had lower rates of mortality. However, the ultimate effects of treatment on drought‐related mortality varied greatly by treatment type. All species had neutral to reduced mortality rates following mechanical thinning alone, but treatments that included prescribed burning increased beetle infestation rates and increased mortality of red fir and sugar pine. Fuel reduction treatments appear to benefit some species such as Jeffrey pine, but can reduce resistance to extreme drought and beetle outbreaks in other species when treatments include prescribed burning. In a non‐analog future, fuel reduction treatments may require modification to provide resistance to beetle infestation and severe droughts.

Variable thinning and prescribed fire influence tree mortality and growth during and after a severe drought

California’s high density, fire-excluded forests experienced an extreme drought accompanied by warmer than normal temperatures from 2012 to 2015, resulting in the deaths of millions of trees. We examined tree mortality and growth of mixed-conifer stands that had been experimentally treated between 2011 and 2013 with two different thinning treatments, one with more structural variability (HighV) and one with less structural variability (LowV), applied alone or in combination with prescribed burning. Tree mortality between 2014 and 2018 varied by species ranging from 42% of white fir (Abies concolor) to 18% of sugar pine (Pinus lambertiana), 12% of incense cedar (Calocedrus decurrens) and 10% of yellow pine (P. ponderosa and P. jeffreyi). Lower overall tree mortality rates at this location relative to drier locations in the southern Sierra Nevada suggested that drought effects may have been ameliorated by lower water deficits due to our site’s more northerly location and deep, productive soils in combination with reductions in tree competition following thinning and burning. Averaged across burn treatments, thinning reduced the overall mortality rate between 2014 and 2018 from 34% to 11%. A total of 23% of the basal area was lost in the unthinned control treatments during this time period, while basal area was unchanged in the thinned treatments, with growth offsetting mortality. There was no significant difference in mortality or basal area change between LowV and HighV, suggesting that leaving trees at variable spacing may not compromise growth or resilience of the stand during a drought. Overall tree mortality was greater in the prescribed burn treatments, most pronounced in the smaller tree size classes, and varied by species, with burning having a significant effect on incense cedar and all pines, but not white fir. Trees with greater competition (Hegyi index) were more likely to die, particularly when also burned. Burning, however, consumed surface fuels and lowered fire hazard. With predictions of warmer droughts and greater weather variability, reducing forest density (basal area) and keeping surface fuel loads low will be important for building greater resilience to future drought stress and wildfire.

Cost-Effective Prescribed Burning Solutions Vary Between Landscapes in Eastern Australia

Fire management agencies undertake a range of fire management strategies to reduce the risk posed by future wildfires. This can include fuel treatments (prescribed burning and mechanical removal), suppression and community engagement. However, no agency has an unlimited budget and numerically optimal solutions can rarely be implemented, if indeed they do exist. Agencies are trying to quantify the extent to which their management actions reduce risk across multiple values in the most cost-effective manner. In this paper, we examine the cost-effectiveness of a range of prescribed burning strategies across multiple landscapes in south-eastern Australia. Landscapes considered include vegetated areas surrounding the cities of Hobart, Melbourne, Adelaide, Canberra and Sydney. Using a simulation approach, we examine the potential range of fires that could occur in a region with varying levels of edge and landscape prescribed burning treatment regimes. Damages to assets are measured for houses, lives, transmission lines, carbon and ecological assets. Costs of treatments are estimated from published models and all data are analysed using multi-criteria decision analysis. Cost-effectiveness of prescribed burning varies widely between regions. Variations primarily relate to the spatial configuration of assets and natural vegetation. Regions with continuous urban interface adjacent to continuous vegetation had the most cost-effective fuel treatment strategies. In contrast, those regions with fragmented vegetation and discontinuous interfaces demonstrated the lowest cost-effectiveness of treatments. Quantifying the extent to which fuel treatments can reduce the risk to assets is vital for determining the location and extent of treatments across a landscape.

Predicting potential cambium damage and fire resistance in Pinus nigra Arn. ssp. salzmannii

Fire management can play a key role in ensuring stand maintenance in future scenarios of global change, particularly in Pinus nigra stands, which are known to be adapted to low-intensity surface fires through characteristics such as thick bark. In this study, laboratory tests were carried out to quantify cambium damage and fire resistance in P. nigra, by using a mass loss colorimeter device in a vertical configuration for the first time. In addition, low-intensity prescribed burning treatments were conducted in the field, and the field and laboratory data were compared. The following variables were used as proxy measures to assess cambium damage: time that temperature remained above 60 °C, heating rate and maximum absolute temperature in the inner bark area. The data were analysed using a Bayesian hierarchical approach (generalized linear mixed model). A threshold heat flux (25 kW m−2) for the time to ignition of bark was identified. A critical temperature of 60 °C was reached in the cambium during the combustion phase, after the flame was extinguished. The laboratory experiments showed, for the first time, the influence of flame residence time on the potential cambium damage. A bark thickness of 17 mm can be considered the threshold level for preventing critical temperatures being reached in Pinus nigra stands. The influence of bark thickness on protection against fire was confirmed, as was the importance of the coefficient of variation of bark thickness. The field results showed that flame characteristics (maximum temperature and residence time) were the most significant predictors of cambium damage. The combination of fire intensity and exposure time at low heat fluxes is more important than bark in determining cambium damage and may have important implications in the field of forest fuel management and in the ecology of pine forests.

Restoration Treatments Improve Overstory Tree Resistance Attributes and Growth in a Ponderosa Pine/Douglas-Fir Forest

Research Highlights: This study provides much needed insight into the development of resistance to disturbance and growth dynamics of overstory trees in response to restoration-based fuel reduction, and will be useful to scientists and managers attempting to better grasp the relative merits of restoration treatment types. Background and Objectives: Restoration-based fuel reduction treatments are common in dry, fire-prone forests of the western United States. The primary objective of such treatments is to immediately reduce a stand’s crown fire hazard. However, the impact of these treatments on residual trees is relevant to assess their longevity and resistance to future disturbances. In this study, we evaluate the effects of restoration on retained overstory ponderosa pine (Pinus ponderosa Lawson &amp; C. Lawson) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees in western Montana, where treatments were experimentally implemented 13 years prior as part of the national Fire and Fire Surrogate study. Materials and Methods: We examined tree attributes in response to the following replicated treatments: thin-only, burn-only, thin + burn, and a no-action control. We analyzed three different tree attributes that confer resistance to common disturbances: height-to-diameter ratio (resistance to wind), bark thickness (resistance to surface fire), and growth efficiency (resistance to bark beetles). Results: Our models suggest that thinning (with or without burning) alters tree attributes relative to the control in a manner that may increase tree resistance to wind and snow breakage, surface fire, and biotic agents such as bark beetles. Further analysis of annual growth of ponderosa pine and Douglas-fir varied by treatment type: thinning-based restoration (thin-only and thin + burn) increased diameter growth for both species, crown length and width in ponderosa pine, and crown length in Douglas-fir relative to unthinned treatments. Burning (burn-only and thin + burn) did not significantly affect tree growth relative to unburned treatments. Conclusions: While low-severity prescribed burning treatments are often used for restoration and have various ecosystem benefits, this study demonstrates that thinning (alone or in addition to burning) produces more measureable, beneficial results to overstory tree disturbance resistance metrics and growth.