Wildland fires in the western United States are projected to increase in frequency, duration, and size. Characterized by widespread and diverse conifer forests, burning within this region may lead to significant terpenoid emissions. Terpenoids constitute a major class of highly reactive secondary organic aerosol (SOA) precursors, with significant structure-dependent variability in reactivity and SOA-formation potential. In this study, highly speciated measurements of terpenoids emitted from laboratory and prescribed fires were achieved using two-dimensional gas chromatography. Nearly 100 terpenoids were measured in smoke samples from 71 fires, with high variability in the dominant compounds. Terpenoid emissions were dependent on plant species and tissues. Canopy/needle-derived emissions dominated in the laboratory fires, whereas woody-tissue-derived emissions dominated in the prescribed fires. Such differences likely have implications for terpenoid emissions from high vs low intensity fires and suggest that canopy-dominant laboratory fires may not accurately represent terpenoid emissions from prescribed fires or wildland fires that burn with low intensity. Predicted SOA formation was sensitive to the diversity of emitted terpenoids when compared to assuming a single terpene surrogate. Given the demonstrated linkages between fuel type, fire terpenoid emissions, and the subsequent implications for plume chemistry, speciated measurements of terpenoids in smoke derived from diverse ecosystems and fire regimes may improve air quality predictions downwind of wildland fires.
Forests are the largest terrestrial carbon stock, and disturbance regimes can have large effects on the structure and function of forests. Many dry temperate forests in the western United States are adapted to a regime of frequent, low‐to‐moderate severity fire. The disruption of this disturbance regime over the last century has shifted forest conditions, making them more susceptible to high‐severity fire. Fuel treatments have been shown to effectively reduce wildfire hazard, often with co‐benefits to ecological values. However, the effects of fuel treatments on forest carbon are complex, often characterized by direct costs (e.g., carbon emissions from prescribed fire) and wildfire‐contingent benefits (increased resistance of live tree carbon to wildfire). In this study, we employ risk‐sensitive carbon accounting and empirical data from a replicated field experiment to evaluate the stand‐scale carbon effects of four management regimes over a 14‐yr period in a historically frequent‐fire adapted forest. All three active treatment regimes immediately increased stable live tree carbon stocks over no‐treatment controls. In most contexts examined, mechanical‐only or no‐treatment controls will maximize expected total carbon stocks when incorporating wildfire risk and the carbon stability of live biomass, dead biomass, and offsite forest products, although we acknowledge our wildfire modeling may underestimate C losses, particularly in the control stands. Undoubtedly, many other ecosystem and social values besides carbon will be important factors that influence fuel and restoration treatments.
Prescribed fire is a vital tool for mitigating wildfire hazard and restoring ecosystems in many western North American forest types. However, there can be considerable variability in fuel consumption from prescribed burns, which affects both hazard mitigation and emissions. In the present study, data from replicated, repeat-entry burns following a period of 100+ years of fire exclusion were used to provide a detailed quantification of fuel consumption as it varies by fuel type, size class, stand and prescribed burn number (first, second or third). Using model selection on a series of linear mixed-effects models, it was determined that total fuel load, proportion of overstorey pine, slope, canopy cover, basal area of live trees, burn number and stand influenced fuel consumption at a 0.04-ha scale. Specifically, overstorey pine composition had a positive effect on fuel consumption. Overall fuel consumption across the three burns averaged 45% of pre-burn fuel loads. Overall consumption was highest for the first burn at 65%, decreasing by 15–20% with each successive burn number. Fuel consumption was highly variable by fuel type, stand and tree species composition. This variability may be advantageous for managers seeking to foster structural diversity and resilience in forest stands.
1. Wildfire can both promote and erode resilience to future disturbances in fireadapted ecosystems. Through a combination of past fire exclusion and climate change, fire patterns and successional trajectories are shifting with potentially negative consequences for forest resilience. In particular, high-severity shortinterval reburns can lead to permanent transitions from forested to persistent non-forested ecosystems.2. To test conditions under which wildfires promote resilience or initiate vegetation transitions we leveraged high-resolution LiDAR data, field data and a natural experiment where two uncharacteristically severe wildfires burned the same area in California's Sierra Nevada mountains. Specifically, we evaluate what factors influence resistance to high-severity reburn and whether early forest recovery is evident following vegetation transition.3. Our findings indicate that topography and vegetative structure influenced resistance to high-severity effects of a second wildfire and that environmental heterogeneity played an important role. Forests that survived the initial burn were most resistant to subsequent high-severity fire when they were characterized by relatively dense but heterogeneous upper strata and a sparse understorey, located in variable and mesic terrain and burned under milder fire weather conditions.Early seral vegetation was most likely to resist repeat high-severity fire and potentially continue post-fire forest recovery when it was located in variable and mesic terrain and was characterized by relatively sparse understorey vegetation and a heterogeneous subcanopy. Some early seral areas that reburned at lower severity showed signs of conifer forest recovery. Vegetation structure and composition of areas that repeatedly burned at high severity are consistent with a transition to persistent shrubland or hardwood forests.
The increase of wildfire incidence in highly populated areas significantly enhances the risk for ecosystems and human lives, activities and infrastructures. In central and southern Italy, recent decades’ fire records indicate that 2007 and 2017 were extreme years in terms of the number of fires and total burned area. Among them, we selected large fire events and explored their features and drivers of burn severity. We used a standardized extraction procedure to identify large wildfires (>100 ha) from the MODIS burned areas database and Landsat multi-spectral images. We mapped burn severity with the Relative Difference Normalized Burn Ratio index and explored the main drivers of severity using topographic, land-cover and anthropogenic predictors. We selected 113 wildfires for a collective total burned area of over 100,000 ha. Large fires were more frequent in the southern than in the central and northern regions, especially in July and August. The average fire size was about 900 ha and occurred mainly in shrublands (30.4%) and broadleaf forests (19.5%). With a random forest model, we observed that the highest severity occurred in conifer plantations and shrublands, in highly populated areas and at lower elevations. Burn severity models, at the landscape or regional scales, can be very useful tools for pre- and post-fire forest management planning.
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