Canopy Fuels Research Articles

Pre-outbreak forest conditions mediate the effects of spruce beetle outbreaks on fuels in subalpine forests of Colorado.

Over the past 30years, forest disturbances have increased in size, intensity, and frequency globally, and are predicted to continue increasing due to climate change, potentially relaxing the constraints of vegetation properties on disturbance regimes. However, the consequences of the potentially declining importance of vegetation in determining future disturbance regimes are not well understood. Historically, bark beetles preferentially attack older trees and stands in later stages of development. However, as climate warming intensifies outbreaks by promoting growth of beetle populations and compromising tree defenses, smaller diameter trees and stands in early stages of development now are being affected by outbreaks. To date, no study has considered how stand age and other pre-outbreak forest conditions mediate the effects of outbreaks on surface and aerial fuel arrangements. We collected fuels data across a chronosequence of post-outbreak sites affected by spruce beetle (SB) between the 1940s and the 2010s, stratified by young (<130yr) and old (>130yr) post-fire stands. Canopy and surface fuel loads were calculated for each tree and stand, and available crown fuel load, crown bulk density, and canopy bulk densities were estimated. Canopy bulk density and density of live canopy individuals were reduced in all stands affected by SB, though foliage loss was proportionally greater in old stands as compared to young stands. Fine surface fuel loads in young stands were three times greater shortly (<30yr) following outbreak as compared to young stands not affected by outbreak, after which the abundance of fine surface fuels decreased to below endemic (i.e., non-outbreak) levels. In both young and old stands, the net effect of SB outbreaks during the 20th and 21st centuries reduced total canopy fuels and increased stand-scale spatial heterogeneity of canopy fuels following outbreak. Importantly, the decrease in canopy fuels following outbreaks was greater in young post-fire stands than in older stands, suggesting that SB outbreaks may more substantially reduce risk of active crown fire when they affect stands in earlier stages of development. The current study shows that the effects of SB outbreaks on forest structure and on fuel profiles are strongly contingent on pre-outbreak conditions as determined by pre-outbreak disturbance history.

Fires Following Bark Beetles: Factors Controlling Severity and Disturbance Interactions in Ponderosa Pine

Previous studies have suggested that bark beetles and fires can be interacting disturbances, whereby bark beetle-caused tree mortality can alter the risk and severity of subsequent wildland fires. However, there remains considerable uncertainty around the type and magnitude of the interaction between fires following bark beetle attacks, especially in drier forest types such as those dominated by ponderosa pine (Pinus ponderosa Lawson & C. Lawson). We used a full factorial design across a range of factors thought to control bark beetle-fire interactions, including the temporal phase of the outbreak, level of mortality, and wind speed. We used a three-dimensional physics-based model, HIGRAD/FIRETEC, to simulate fire behavior in fuel beds representative of 60 field plots across five national forests in northern Arizona, USA. The plots were dominated by ponderosa pine, and encompassed a gradient of bark beetle-caused mortality due to a mixture of both Ips and Dendroctonus species. Non-host species included two sprouting species, Gambel oak (Quercus gambelii Nutt.) and alligator juniper (Juniperus deppeana Steud.), as well as other junipers and pinyon pine (Pinus edulis Engelm.). The simulations explicitly accounted for the modifications of fuel mass and moisture distribution caused by bark beetle-caused mortality. We first analyzed the influence of the outbreak phase, level of mortality, and wind speed on the severity of a subsequent fire, expressed as a function of live and dead canopy fuel consumption. We then computed a metric based on canopy fuel loss to characterize whether bark beetles and fire are linked disturbances and, if they are, if the linkage is antagonistic (net bark beetle and fire severity being less than if the two disturbances occurred independently) or synergistic (greater combined effects than independent disturbances). Both the severity of a subsequent fire and whether bark beetles and fire are linked disturbances depended on the outbreak phase of the bark beetle mortality and attack severity, as well as the fire weather (here, wind). Greater fire severity and synergistic interactions were generally associated with the “red phase” (when dead needles remain on trees). In contrast, during the “gray phase” (when dead needles had fallen to the ground), fire severity was either similar to, or less than, green-phase fires and interactions were generally antagonistic, but included both synergistic and neutral interactions. The simulations also revealed that the magnitude of the linkage between these two disturbances was smaller for fires occurring during high wind conditions, especially in the red phase. This complexity might be a reason for the contrasted or controversial perception of bark beetle-fire interactions reported in the literature, since both fire severity and the type and magnitude of the linkage can vary strongly among studies. These results suggest that, for fires burning in the gray phase following moderate levels of mortality, bark beetle-caused mortality may buffer rather than exacerbate fire severity. However, for fires burning under high wind speeds, regardless of the outbreak phase or level of mortality, the near complete loss of canopy fuels may push this ecosystem into an alternative state dominated by sprouting species.

Shoot-Level Flammability of Species Mixtures is Driven by the Most Flammable Species: Implications for Vegetation-Fire Feedbacks Favouring Invasive Species

Invasive species can cause shifts in vegetation composition and fire regimes by initiating positive vegetation-fire feedbacks. To understand the mechanisms underpinning these shifts, we need to determine how invasive species interact with other species when burned in combination and thus how they may influence net flammability in the communities they invade. Previous studies using litter and ground fuels suggest that flammability of a species mixture is nonadditive and is driven largely by the more-flammable species. However, this nonadditivity has not been investigated in the context of plant invasions nor for canopy fuels. Using whole shoots, we measured the flammability of indigenous-invasive species pairs for six New Zealand indigenous and four globally invasive plant species, along with single-species control burns. Our integrated measure of flammability was clearly nonadditive, and the more-flammable species per pairing had the stronger influence on flammability in 83% of combinations. The degree of nonadditivity was significantly positively correlated with the flammability difference between the species in a pairing. The strength of nonadditivity differed among individual flammability components. Ignitability and combustibility were strongly determined by the more-flammable species per pair, yet both species contributed more equally to consumability and sustainability. Our results suggest mechanisms by which invasive species entrain positive vegetation-fire feedbacks that alter ecosystem flammability, enhancing their invasion. Of the species tested, Hakea sericea and Ulex europaeus are those most likely to increase the flammability of New Zealand ecosystems and should be priorities for management.

Restoration benefits of re-entry with resource objective wildfire on a ponderosa pine landscape in northern Arizona, USA

Abstract Prescriptions for ecological restoration of ponderosa pine (Pinus ponderosa) forests of the American Southwest are generally aimed at approximating pre-fire-exclusion conditions by reducing tree densities and hazardous fuels, and reintroducing surface fire. Increasingly, land managers are utilizing natural fire ignitions to achieve restoration and hazardous fuels reduction objectives, but to minimize risk of escape and reduce potential for undesirable outcomes fires are commonly managed under weather and fuel moisture conditions that produce lower burn severities. Few studies have examined effectiveness of incremental approaches that attempt to restore these ecosystems by allowing repeated, low-severity wildfires to reburn sites. We analyzed a 12,035-ha landscape on the Kaibab National Forest in northern Arizona and compared forest structure, hazardous fuel loading, and potential fire behavior in areas that had burned once (single fire) to others that had burned twice (re-entry) over the preceding 13 years. Forest structure and hazardous fuels characteristics in re-entry areas tended to be closer to historical ranges of variability (HRV) than in single fire areas; however, we found few statistically significant differences and tree densities in both single and re-entry fires remained higher than HRV by up to 3.5-fold. Similarly, potential fire behavior variables were statistically similar between single and re-entry fire areas. Analysis of burn severity effects suggested that trends toward reduced tree density and canopy fuels in re-entry areas were consistently driven by “cleanup effects”, whereby re-entry fires produced low-severity effects in areas showing unburned/low severity after first-entry fires. Evidence of “additive effects”, whereby restoration and hazardous fuels reduction objectives were being met incrementally by repeated low-severity fire, was difficult to detect on the re-entry fire landscape. Thus, with similar management, several additional fire entries will be required to restore tree densities to within the historical range. Further, our findings suggest that managing resource objective wildfires to allow for more moderate-severity burning, even with a single entry, may be more effective for restoring these forests than repeated low-severity entries.

Prediction of Forest Canopy and Surface Fuels from Lidar and Satellite Time Series Data in a Bark Beetle-Affected Forest

Wildfire behavior depends on the type, quantity, and condition of fuels, and the effect that bark beetle outbreaks have on fuels is a topic of current research and debate. Remote sensing can provide estimates of fuels across landscapes, although few studies have estimated surface fuels from remote sensing data. Here we predicted and mapped field-measured canopy and surface fuels from light detection and ranging (lidar) and Landsat time series explanatory variables via random forest (RF) modeling across a coniferous montane forest in Colorado, USA, which was affected by mountain pine beetles (Dendroctonus ponderosae Hopkins) approximately six years prior. We examined relationships between mapped fuels and the severity of tree mortality with correlation tests. RF models explained 59%, 48%, 35%, and 70% of the variation in available canopy fuel, canopy bulk density, canopy base height, and canopy height, respectively (percent root-mean-square error (%RMSE) = 12–54%). Surface fuels were predicted less accurately, with models explaining 24%, 28%, 32%, and 30% of the variation in litter and duff, 1 to 100-h, 1000-h, and total surface fuels, respectively (%RMSE = 37–98%). Fuel metrics were negatively correlated with the severity of tree mortality, except canopy base height, which increased with greater tree mortality. Our results showed how bark beetle-caused tree mortality significantly reduced canopy fuels in our study area. We demonstrated that lidar and Landsat time series data contain substantial information about canopy and surface fuels and can be used for large-scale efforts to monitor and map fuel loads for fire behavior modeling at a landscape scale.

Fuel accumulation in a high-frequency boreal wildfire regime: from wetland to upland

Although it is increasingly accepted that young (e.g., ≤30 years) stands originating from wildfire are considerably less flammable than older stands in the boreal forest of North America, the role of fuel availability and structure in this phenomenon has not been thoroughly investigated. As a regional study in a high-frequency fire regime, detailed wildfire fuel loading and structure were measured in 66 sites including both wetlands and uplands in the Boreal Plains landscape of Wood Buffalo National Park in northwestern Canada. Overall, a significant increase in total flammable biomass occurred in upland sites over 97 years, but this increase was not consistently observed in wetlands, except where there was dense tree cover. Fuel accumulation was highly moderated by canopy fuels, as surface fuels were relatively constant across differing site types and time since fire, averaging 0.4 kg·m−2. Significant but gradual canopy fuel accumulation was observed in moist conifer upland forests dominated by mature black spruce (Picea mariana (Mill.) B.S.P.) or white spruce (Picea glauca (Moench) Voss) over 100 years since fire. Outside of these mature moist conifer uplands, there was no difference in total fuel loading between other upland forests and across the gradient of treed to open wetlands.

The effect of silvicultural treatments on fire behaviour potential in radiata pine plantations of South Australia

Large plantations of radiata pine (Pinus radiata Don) are susceptible to widespread damage from wildfires that compromise the sustainability of forest production and downstream industries. Fuel management zones, strategically located plantation areas where an intensive silvicultural prescription comprising multiple prunings and thinnings aim to constrain the onset and spread of crown fires, have been implemented for decades in Australia, although their effectiveness has yet to be quantified. We aimed to quantify the effect of these silvicultural-based fuel management prescriptions on fuel complex structure and fire behaviour potential. We characterised fuel complex structure associated with the various silviculture treatments and contrasted it with stands representative of the standard silvicultural regime. This information and local weather were used to estimate fire potential in fuel management zones and control stands.We found the pruning and thinning fuel treatments significantly changed the plantation fuel complex, relocating ladder and canopy fuels to the surface layer, while breaking up fuel vertical continuity. Key beneficial effects such as the decrease in the vertical continuity of the fuel complex were found to be long lasting, whereas disadvantageous effects such as the increase in surface fuel load were short lived. The fuel management zone prescriptions resulted in significantly lower fireline intensity (e.g., first thinning reduced mean intensity during Very High Fire Danger days from 8613 to 760kWm−1) and likelihood of crowning (e.g., second pruning reduced the incidence of crowning during Very High Fire Danger days from 25% to nil) under a broad range of burning conditions. Analysis of the effect of silvicultural operations conducted in fuel management zones should not view each operation independently, but as a process where a succession of treatments transforms a highly “flammable” fuel complex into a low one early in the rotation.

Modelling the vertical distribution of canopy fuel load using national forest inventory and low-density airbone laser scanning data.

The fuel complex variables canopy bulk density and canopy base height are often used to predict crown fire initiation and spread. Direct measurement of these variables is impractical, and they are usually estimated indirectly by modelling. Recent advances in predicting crown fire behaviour require accurate estimates of the complete vertical distribution of canopy fuels. The objectives of the present study were to model the vertical profile of available canopy fuel in pine stands by using data from the Spanish national forest inventory plus low-density airborne laser scanning (ALS) metrics. In a first step, the vertical distribution of the canopy fuel load was modelled using the Weibull probability density function. In a second step, two different systems of models were fitted to estimate the canopy variables defining the vertical distributions; the first system related these variables to stand variables obtained in a field inventory, and the second system related the canopy variables to airborne laser scanning metrics. The models of each system were fitted simultaneously to compensate the effects of the inherent cross-model correlation between the canopy variables. Heteroscedasticity was also analyzed, but no correction in the fitting process was necessary. The estimated canopy fuel load profiles from field variables explained 84% and 86% of the variation in canopy fuel load for maritime pine and radiata pine respectively; whereas the estimated canopy fuel load profiles from ALS metrics explained 52% and 49% of the variation for the same species. The proposed models can be used to assess the effectiveness of different forest management alternatives for reducing crown fire hazard.

Extrapolating Forest Canopy Fuel Properties in the California Rim Fire by Combining Airborne LiDAR and Landsat OLI Data

Accurate, spatially explicit information about forest canopy fuel properties is essential for ecosystem management strategies for reducing the severity of forest fires. Airborne LiDAR technology has demonstrated its ability to accurately map canopy fuels. However, its geographical and temporal coverage is limited, thus making it difficult to characterize fuel properties over large regions before catastrophic events occur. This study presents a two-step methodology for integrating post-fire airborne LiDAR and pre-fire Landsat OLI (Operational Land Imager) data to estimate important pre-fire canopy fuel properties for crown fire spread, namely canopy fuel load (CFL), canopy cover (CC), and canopy bulk density (CBD). This study focused on a fire prone area affected by the large 2013 Rim fire in the Sierra Nevada Mountains, California, USA. First, LiDAR data was used to estimate CFL, CC, and CBD across an unburned 2 km buffer with similar structural characteristics to the burned area. Second, the LiDAR-based canopy fuel properties were extrapolated over the whole area using Landsat OLI data, which yielded an R2 of 0.8, 0.79, and 0.64 and RMSE of 3.76 Mg·ha−1, 0.09, and 0.02 kg·m−3 for CFL, CC, and CBD, respectively. The uncertainty of the estimates was estimated for each pixel using a bootstrapping approach, and the 95% confidence intervals are reported. The proposed methodology provides a detailed spatial estimation of forest canopy fuel properties along with their uncertainty that can be readily integrated into fire behavior and fire effects models. The methodology could be also integrated into the LANDFIRE program to improve the information on canopy fuels.

Mastication treatment effects on vegetation and fuels in piñon-juniper woodlands of central Colorado, USA

Forest mastication treatments are increasingly utilized by land managers as a means of reducing tree cover for fire hazard mitigation and other habitat objectives in piñon-juniper (P-J) woodlands. Mastication converts trees into small pieces (e.g., wood chips), in the process creating canopy openings, redistributing fuel from the canopy to the surface, converting large diameter to small diameter fuels, and covering the ground with piles of woody debris. We measured vegetation and fuels at 192 sites in 24 pairs of 1–11-year-old mastication treatments and untreated adjacent controls in P-J woodlands of the Arkansas River valley, Colorado, and used paired t-tests, mixed-effects models, and gradient analysis (non-metric multidimensional scaling) to assess mastication effects.Treatments were associated with major, persistent ecological shifts relative to controls. Tree cover and canopy fuels were reduced in treatments; concomitantly, dead and down woody surface fuels, forb, and graminoid cover were elevated. Treatments exhibited much higher frequency, richness, and cover by a suite of non-native plant species including cheatgrass (Bromus tectorum). Non-native plant expansion appears linked to the disturbance associated with treatment activities, reductions in tree canopy, and alterations to ground cover, and effective mitigation of increases by these species may necessitate both pre- and post-treatment control measures. Shifts from native-dominated woodlands to open, weedy, herb- and shrub-dominated communities are likely to change patterns of abundance and habitat use by woodland- and forest-dependent wildlife. Decreased canopy fuels and increased herbaceous surface fuels including exotic annuals are expected to alter potential fire behavior. We encourage managers carrying out P-J mastication projects to explicitly address potential trade-offs between desired treatment outcomes and potentially unwelcome impacts, and how these might be mitigated. It may also be worth considering whether or not tree removal treatments will be warranted given anticipated climate change impacts to these woodlands.

Climate warming alters fuels across elevational gradients in Great Basin bristlecone pine-dominated sky island forests

Little is known about fuel characteristics and dynamics in GBBP communities, and current monitoring programs inadequately quantify the surface and canopy fuels of this system. Using the Forest Inventory Analysis (FIA) plot variables of tree species, height, diameter at breast height (DBH), canopy base height (CBH), coarse (CWD) and fine (FWD) woody debris counts, and canopy fuels measurements, this paper examines the effects of climate-induced changes to fuel loading, fire hazard and risk on predicted changes in fire behavior and severity. Field transects were installed using FIA protocols along environmental gradients. Plots were located every 22 chains or ∼440m along random transects on Mt. Washington in the Great Basin National Park (GBNP) and in the nearby Mt. Moriah Wilderness, NV. Additional plots were installed at Notch Peak (UT), Cave Mountain (NV), and Wheeler Peak (GBNP, NV). Linear regression showed that all classes of FWD decreased with elevation, and only 1000-h fuels remained constant across elevational transects. This, combined with lower CBH and foliar moisture and increasing temperatures due to climate change, increases fire potential at the Great Basin bristlecone pine treeline, threatening the oldest individuals of this iconic species. New information about discontinuous fuels will aid in management of high elevation alpine treeline forests.

Impacts of lodgepole pine dwarf mistletoe (Arceuthobium americanum) infestation on stand structure and fuel load in lodgepole pine dominated forests in central Colorado

Parasitic plants are capable of causing substantial alterations to plant communities through impacts on individual host plants. Lodgepole pine dwarf mistletoe is an important parasite in forests of the western USA that causes reductions to productivity and is thought to alter wildland fuel complexes. These impacts are hypothesized to vary with infestation severity. To test this, we used a linear mixed modeling approach to evaluate the relationship between dwarf mistletoe infestation severity and parameters representing stand structure and surface and canopy fuels in infested lodgepole pine stands in central Colorado. Infestation severity was negatively related to live basal area, average tree size, canopy base height, canopy fuel load, and canopy bulk density, and was positively related to the loading of woody surfaces fuels greater than 0.64 cm in diameter. No relationship was detected between infestation severity and live tree density, or live crown ratio. These results confirm the long-held assumption that dwarf mistletoe increases surface fuel loading in lodgepole pine communities, but also suggest that infested stands have reduced amounts of available canopy fuel. These findings have implications for potential fire behavior and highlight the importance of dwarf mistletoe in predicting the spatial and temporal dynamics of wildland fuels.

The effect of salvage logging on surface fuel loads and fuel moisture in beetle-infested lodgepole pine forests

Widespread tree mortality from mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins) outbreaks has prompted forest management activities to reduce crown fire hazard in the Rocky Mountain region. However, little is known about how beetle-related salvage logging and biomass utilization options affect woody surface fuel loads and fuel moisture dynamics. We compared these attributes in salvage-logged lodgepole pine (Pinus contorta var. latifolia Engelm. Ex S. Wats.) stands harvested using either biomass removal (whole-tree harvest) or biomass retention (bole only harvest) prescriptions with untreated MPB-infested stands. Both prescriptions roughly doubled 1-h and 10-h fuel loads compared to untreated forest. Biomass retention left ten times more 1000-h fuels compared to biomass removal prescription (28 vs 3Mgha−1). Overall, the woody fuel load was more than twice as high with biomass retention compared to biomass removal (60 vs 25Mgha−1). Fuel moisture content was lower in salvage logged units compared to untreated forest plots, but it did not differ among the biomass prescriptions. Fine (10-h) and heavy (1000-h) fuels dried to a critical ignition threshold 3–8weeks earlier in the two prescriptions, respectively, compared to the untreated forests. Salvage logging removes canopy fuels and crown fire hazard, but we found that depending on the amount of biomass retained it can both increase surface fuel load and decrease fuel moisture compared to untreated stands. In the coming years, snag fall will transfer crown to surface fuels in untreated beetle-killed stands adding coarse surface fuel loads surpassing those in treated stands.

EFFECT OF FUEL TREATMENTS ON CROWN FIRE BEHAVIOR IN ALEPPO PINE FORESTS OF GREECE: A SIMULATION STUDY

This study models the potential effect of various fuel treatments on crown fire behavior by using two crown fire simulation models in Aleppo pine (Pinus halepensis Mill.) stands. Fuel complex characteristics that influence fire behavior were measured in a representative Aleppo pine stand in northern Greece, using standard surface and canopy fuel inventory methods. Nine different fuel treatments (control, prescribed burning, controlled grazing, thinning without slash removal, thinning with slash removal, thinning and prescribed burning with slash removal, thinning and controlled grazing with slash removal, thinning and prescribed burning without slash removal, thinning and controlled grazing without slash removal) were simulated by adjusting surface fuels (total load, load-by-size class, depth) and canopy fuels (available canopy fuel load, canopy base height, canopy bulk density) under dry and normal fuel moisture conditions. Under normal conditions, stand thinning followed by prescribed burning and/or controlled grazing with slash removal prevented crown fire initiation in most cases. Under dry fuel moisture conditions, the simulation showed that the proposed fuel treatments did not appear to be effective in averting crown fire initiation and spread. Combinations of prescribed fire and/or controlled grazing can be used to reduce wildfire hazard in Aleppo pine forests. Management of Aleppo pine forests should include an analysis of how the proposed actions (logging, thinning, etc.) may affect fuel complex properties. Fuel treatment plans should be an essential component of forest management in Aleppo pine forests in Greece, in order to reduce fire hazard.

Landscape variation in tree regeneration and snag fall drive fuel loads in 24-year old post-fire lodgepole pine forests.

Escalating wildfire in subalpine forests with stand-replacing fire regimes is increasing the extent of early-seral forests throughout the western USA. Post-fire succession generates the fuel for future fires, but little is known about fuel loads and their variability in young post-fire stands. We sampled fuel profiles in 24-year-old post-fire lodgepole pine (Pinus contorta var. latifolia) stands (n=82) that regenerated from the 1988 Yellowstone Fires to answer three questions. (1) How do canopy and surface fuel loads vary within and among young lodgepole pine stands? (2) How do canopy and surface fuels vary with pre- and post-fire lodgepole pine stand structure and environmental conditions? (3) How have surface fuels changed between eight and 24years post-fire? Fuel complexes varied tremendously across the landscape despite having regenerated from the same fires. Available canopy fuel loads and canopy bulk density averaged 8.5Mg/ha (range 0.0-46.6) and 0.24kg/m3 (range: 0.0-2.3), respectively, meeting or exceeding levels in mature lodgepole pine forests. Total surface-fuel loads averaged 123Mg/ha (range: 43-207), and 88% was in the 1,000-h fuel class. Litter, 1-h, and 10-h surface fuel loads were lower than reported for mature lodgepole pine forests, and 1,000-h fuel loads were similar or greater. Among-plot variation was greater in canopy fuels than surface fuels, and within-plot variation was greater than among-plot variation for nearly all fuels. Post-fire lodgepole pine density was the strongest positive predictor of canopy and fine surface fuel loads. Pre-fire successional stage was the best predictor of 100-h and 1,000-h fuel loads in the post-fire stands and strongly influenced the size and proportion of sound logs (greater when late successional stands had burned) and rotten logs (greater when early successional stands had burned). Our data suggest that 76% of the young post-fire lodgepole pine forests have 1,000-h fuel loads that exceed levels associated with high-severity surface fire potential, and 63% exceed levels associated with active crown fire potential. Fire rotations in Yellowstone National Park are predicted to shorten to a few decades and this prediction cannot be ruled out by a lack of fuels to carry repeated fires.

Spatiotemporal Variability of Wildland Fuels in US Northern Rocky Mountain Forests

Fire regimes are ultimately controlled by wildland fuel dynamics over space and time; spatial distributions of fuel influence the size, spread, and intensity of individual fires, while the temporal distribution of fuel deposition influences fire’s frequency and controls fire size. These “shifting fuel mosaics” are both a cause and a consequence of fire regimes. This paper synthesizes results from two major fuel dynamics studies that described the spatial and temporal variability of canopy and surface wildland fuel characteristics found in US northern Rocky Mountain forests. Eight major surface fuel components—four downed dead woody fuel size classes (1, 10, 100, 1000 h), duff, litter, shrub, and herb—and three canopy fuel characteristics—loading, bulk density and cover—were studied. Properties of these fuel types were sampled on nested plots located within sampling grids to describe their variability across spatiotemporal scales. Important findings were that fuel component loadings were highly variable (two to three times the mean), and this variability increased with the size of fuel particles. The spatial variability of loadings also varied by spatial scale with fine fuels (duff, litter, 1 h, 10 h) varying at scales of 1 to 5 m; coarse fuels at 10 to 150 m, and canopy fuels at 100 to 600 m. Fine fuels are more uniformly distributed over both time and space and decayed quickly, while large fuels are rare on the landscape but have a high residence time.

A fire modeling approach to assess wildfire exposure of valued resources in central Navarra, Spain

Wildfires are a growing threat to socioeconomic and natural resources in the wildland–rural–urban intermix in central Navarra (Spain), where recent fast-spreading and spotting short fire events have overwhelmed suppression capabilities. A fire simulation modeling approach based on the minimum travel time algorithm was used to analyze the wildfire exposure of highly valued resources and assets (HVRAs) in a 28,000 ha area. We replicated 30,000 fires at fine resolution (20 m), based on wildfire season and recent fire weather and moisture conditions, historical ignition patterns and spatially explicit canopy fuels derived from low-density airborne light detection and ranging (LiDAR). Detailed maps of simulated fire likelihood, fire intensity and fire size were used to assess spatial patterns of HVRA exposure to fire and to analyze large fire initiation and spread through source-sink ratio and fire potential index. Crown fire activity was estimated and used to identify potential spotting-emission hazardous stands. The results revealed considerable variation in fire risk causative factors among and within HVRAs. Exposure levels across HVRAs were mainly related to the combined effects of anthropic ignition locations, fuels, topography and weather conditions. We discuss the potential of fire management strategies such as prioritizing mitigation treatment and fire ignition prevention monitoring, informed by fine-scale geospatial quantitative risk assessment outcomes.

Effect of stand, topographic, and climatic factors on the fuel complex characteristics of Aleppo (Pinus halepensis Mill.) and Calabrian (Pinus brutia Ten.) pine forests of Greece

In this study we examined the effect of stand, topographic, and climatic factors on the fuel complex characteristics of Aleppo (Pinus halepensis Mill.) and Calabrian (Pinus brutia Ten.) pine forests of Greece, based on measurements in fifty six (56) plots dominated by Aleppo pine (28 plots) and Calabrian pine (28 plots) in four different forest sites. The data were analyzed through Random Forest regression, linear regression, and quantile regression. The results showed that for all the examined factors, basal area, stand density index and stand height were the most important factors explaining fuel complex characteristics in the region for both species. Topography and climate factors were not found to be significant factors for canopy fuels. However, elevation and annual precipitation showed a minor effect mainly on the shrub fuel layer of Calabrian pine. The analysis and the conclusions of the study can be important inputs for forest management decisions (thinning, fuel treatments, stand regeneration planning) quantifying the effect that certain fuel management actions can have on the fuel hazard that the stands of these two important Mediterranean pines represent.

Modeling spatial and temporal dynamics of wind flow and potential fire behavior following a mountain pine beetle outbreak in a lodgepole pine forest

Patches of live, dead, and dying trees resulting from bark beetle-caused mortality alter spatial and temporal variability in the canopy and surface fuel complex through changes in the foliar moisture content of attacked trees and through the redistribution of canopy fuels. The resulting heterogeneous fuels complexes alter within-canopy wind flow, wind fluctuations, and rate of fire spread. However, there is currently little information about the potential influence of different rates and patterns of mortality on wind flow and fire behavior following bark beetle outbreaks. In this study, we contrasted within-canopy wind flow and fire rate-of-spread (ROS) at two different ambient wind speeds using FIRETEC for two differing bark beetle attack trajectories for a lodgepole pine (Pinus contorta) forest. These two attack trajectories represent different realizations of a bark beetle outbreak and result in different amounts and patterns of mortality through time. Our simulations suggested that the mean within-canopy wind velocities increased through time following the progression of mortality. In addition, we found that for a given level of mortality, a bark beetle outbreak that resulted in a higher degree of aggregation of canopy fuels had greater mean within-canopy wind velocities due to the channeling of wind flow. These findings suggest that bark beetle mortality can influence the mean within-canopy wind flow in two ways: first, by reducing the amount of vegetation present in the canopy acting as a source of drag; and second, by altering spatial patterns of vegetation that can lead to channeling of wind flow. Changes in the fire rate-of-spread were positively related to the level and continuity of bark beetle mortality. Peak rates of spread were between 1.2 and 2.7 times greater than the pre-outbreak scenario and coincided with a high level of mortality and minimal loss of canopy fuels. Following the loss of canopy fuels the rate of fire spread declined to levels below the initial phases of the outbreak in low wind speed cases but remained above pre-outbreak levels in high wind speed cases. These findings suggest that the rate and pattern of mortality arising from a bark beetle outbreak exerts significant influence on the magnitude and timing of alterations to the within-canopy wind flow and rate of fire spread. Our findings help clarify existing knowledge gaps related to the effect of bark beetle outbreaks on fire behavior and could explain potential differences in the reported effects of bark beetle outbreaks on fire behavior through time.

Utah juniper and two-needle piñon reduction alters fuel loads

Juniper (Juniperus spp.) and piñon (Pinus spp.) trees have encroached millions of hectares of sagebrush (Artemisia spp.)–bunchgrass communities. Juniper–piñon trees are treated to reduce canopy fuel loads and crown fire potential. We measured the effects of juniper–piñon infilling and fuel-reduction treatments on fuel load characteristics at four locations in Utah. At each location, treatment areas were burned, left untreated, or trees were cut or masticated in a randomised complete-block design. We measured standing and downed fuels by size and type along 30-m transects on 15 subplots (30 × 33 m) per location before and 1–3 years after treatment. Increased tree cover was associated with decreased shrub and herbaceous fuel loads (P &lt; 0.01). By 2 years post-treatment, herbaceous fuel loads were greater than pretreatment in all treated areas (P &lt; 0.01). Cut and mastication treatments increased surface woody 10- and 100-h fuel loads and wood/bark cover (P &lt; 0.01). Masticated-tree depth was a good estimator of fuel loads (R2 = 92). The conversion of canopy fuels to surface fuels reduced fuels that enable crown fire and extreme fire intensity. Cool-season prescribed fire may need to follow mechanical treatments to reduce surface fuel and the potential for wildfire damage to perennial understorey vegetation.