Photochemical Evolution of the 2013 California Rim Fire: Synergistic Impacts of Reactive Hydrocarbons and Enhanced Oxidants

Large wildfires markedly alter regional atmospheric composition, but chemical complexity challenges model predictions of downwind impacts. Here, we elucidate key facets of gas-phase photochemistry and assess novel chemical processes via a case study of the 2013 California Rim Fire plume. Airborne in situ observations, acquired during the NASA Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) mission, illustrate the evolution of volatile organic compounds (VOC), oxidants, and reactive nitrogen over 12 hours of atmospheric aging. Measurements show rapid formation of ozone and peroxyacyl nitrates (PNs), sustained peroxide production, and prolonged enhancements in oxygenated VOC and nitrogen oxides (NOx). Measurements and Lagrangian trajectories constrain a 0-D puff model that approximates plume photochemical history and provides a framework for evaluating key processes. Simulations examine the effects of 1) previously-unmeasured reactive VOC identified in recent laboratory studies, and 2) emissions and secondary production of nitrous acid (HONO). Inclusion of estimated unmeasured VOC leads to a 250 % increase in OH reactivity and a 70 % increase in radical production via oxygenated VOC photolysis. HONO amplifies radical cycling and serves as a downwind NOx source, although two different HONO production mechanisms (particulate nitrate photolysis and heterogeneous NO2 conversion) exhibit markedly different effects on ozone, NOx, and PNs. Analysis of radical initiation rates suggests that oxygenated VOC photolysis is a major radical source, exceeding HONO photolysis when averaged over the first 2 hours of aging. Ozone production chemistry transitions from VOC-sensitive to NOx-sensitive within the first hour of plume aging, with both peroxide and organic nitrate formation contributing significantly to radical termination. To simulate smoke plume chemistry accurately, models should simultaneously account for the full reactive VOC pool and all relevant oxidant sources.

Pyrolyzed Substrates Induce Aromatic Compound Metabolism in the Post-fire Fungus, Pyronema domesticum

Wildfires represent a fundamental and profound disturbance in many ecosystems, and their frequency and severity are increasing in many regions of the world. Fire affects soil by removing carbon in the form of CO2 and transforming remaining surface carbon into pyrolyzed organic matter (PyOM). Fires also generate substantial necromass at depths where the heat kills soil organisms but does not catalyze the formation of PyOM. Pyronema species strongly dominate soil fungal communities within weeks to months after fire. However, the carbon pool (i.e., necromass or PyOM) that fuels their rise in abundance is unknown. We used a Pyronema domesticum isolate from the catastrophic 2013 Rim Fire (CA, United States) to ask whether P. domesticum is capable of metabolizing PyOM. Pyronema domesticum grew readily on agar media where the sole carbon source was PyOM (specifically, pine wood PyOM produced at 750°C). Using RNAseq, we investigated the response of P. domesticum to PyOM and observed a comprehensive induction of genes involved in the metabolism and mineralization of aromatic compounds, typical of those found in PyOM. Lastly, we used 13C-labeled 750°C PyOM to demonstrate that P. domesticum is capable of mineralizing PyOM to CO2. Collectively, our results indicate a robust potential for P. domesticum to liberate carbon from PyOM in post-fire ecosystems and return it to the bioavailable carbon pool.

Pyrolyzed substrates induce aromatic compound metabolism in the post-fire fungus,Pyronema domesticum

ABSTRACTWildfires represent a fundamental and profound disturbance in many ecosystems, and their frequency and severity are increasing in many regions of the world. Fire affects soil by removing carbon in the form of CO2and transforming remaining surface carbon into pyrolyzed organic material (PyOM). Fires also generate substantial necromass at depths where the heat kills soil organisms but does not catalyze the formation of PyOM.Pyronemaspecies strongly dominate soil fungal communities within weeks to months after fire. However, the carbon pool (i.e. necromass or PyOM) that fuels their rise in abundance is unknown. We used aPyronema domesticumisolate from the catastrophic 2013 Rim Fire (CA, USA) to ask ifP. domesticumis capable of metabolizing PyOM.P. domesticumgrew readily on agar media where the sole carbon source was PyOM (specifically, pine wood PyOM produced at 750 °C). Using RNAseq, we investigated the response ofP. domesticumto PyOM and observed a comprehensive induction of genes involved in the metabolism and mineralization of aromatic compounds, typical of those found in PyOM. Lastly, we used13C-labeled 750 °C PyOM to demonstrate thatP. domesticumis capable of mineralizing PyOM to CO2. Collectively, our results indicate a robust potential forP. domesticumto liberate carbon from PyOM in post-fire ecosystems and return it to the bioavailable carbon pool.IMPORTANCEFires are increasing in frequency and severity in many regions across the world. Thus, it’s critically important to understand how our ecosystems respond to inform restoration and recovery efforts. Fire transforms the soil, removing many nutrients while leaving behind both nutritious necromass and complex pyrolyzed organic matter, which is often recalcitrant. Filamentous fungi of the genusPyronemastrongly dominate soil fungal communities soon after fire. While Pyronema are key pioneer species in post-fire environments, the nutrient source that fuels their rise in abundance is unknown. In this manuscript, we used a P. domesticum isolate from the catastrophic 2013 Rim Fire (CA, USA) to demonstrate thatP. domesticummetabolizes pyrolyzed organic material, effectively liberating this complex pyrolyzed carbon and returning it to the bioavailable carbon pool. The success of Pyronema in post-fire ecosystems has the potential to kick-start growth of other organisms and influence the entire trajectory of post-fire recovery.

Impacts of Postfire Management Are Unjustified in Spotted Owl Habitat

In mixed-conifer forests inhabited by California spotted owls, land managers hypothesize that without human intervention natural conifer regeneration will take many decades or longer to begin within interior areas of large high-severity fire patches, due to long distances from live tree seed sources. As a result, widespread post-fire logging, followed by sprayed application of herbicides and planting of conifer seedlings, are used to create tree plantations. These are activities routinely conducted in spotted owl territories following fires, despite current data that indicate this approach has adverse impacts on spotted owl occupancy. Land managers acknowledge such impacts, but continue these forest management practices, assuming they are a necessary harm, one that is warranted to ensure the later return of mature conifer forests used by spotted owls for nesting and roosting. However, few data have been gathered to test this hypothesis. At 5 years post-fire, we surveyed field plots on a grid within large high-severity fire patches in spotted owl habitat within the Rim fire of 2013 in the Sierra Nevada, California. In our analysis the percentage of plots lacking conifer regeneration decreased significantly with larger plot sizes, a finding contrary to previous studies which assumed vast “deforested” areas in wildland fires, a bias created by small plot size. We found higher conifer regeneration closer to live-tree edges, but we consistently found natural post-fire conifer regeneration at all distances into interior spaces of large high-severity fire patches, including >300 m from the nearest live trees. Distance from live-tree edges did not affect pine dominance in post-fire regeneration. The post-fire natural conifer regeneration reported in our results suggests that the adverse effects of current post-fire management in spotted owl habitat are not necessary practices that can be justified.

Influence of wildfire severity on geomorphic features and riparian vegetation of forested streams of the Sierra Nevada, California, USA

Fires are a common feature of many landscapes, with numerous and complex ecological consequences. In stream ecosystems, fire can strongly influence fluvial geomorphic characteristics and riparian vegetation, which are structural components of stream–riparian ecosystems that contribute to biodiversity and ecosystem function. However, the effects of fire severity on stream–riparian ecosystems in California’s Sierra Nevada region (USA) are not well described, yet critical for effectively informing fire management and policy. At 12 stream reaches paired by fire severity (one high-severity burned, one low-severity burned), no significant differences were found in riparian plant community cover and composition or stream geomorphic characteristics 2–15 years following wildfire. In addition, minimal changes in riparian vegetation and stream geomorphic properties were observed in the first summer following the extensive and severe Rim Fire. However, an upstream-to-downstream influence of multiple fire occurrences was observed over the previous 81 years within each catchment on stream geomorphic metrics, including sediment size, embeddedness and channel geometry, at our study reaches. The inconsistent effects of wildfire on stream–riparian vegetation and geomorphic characteristics over space and time may be related to time since fire and precipitation.

Short-term resilience of Great Gray Owls to a megafire in California, USA

ABSTRACT Throughout western North America, longer, hotter fire seasons and dense fuels are yielding more frequent, larger, and higher-severity wildfires, including uncharacteristically large “megafires.” Wildlife species associated with late-seral forest characteristics may be particularly vulnerable to habitat loss stemming from changing fire regimes. The Great Gray Owl (Strix nebulosa) is a state-listed endangered species in California that typically nests in large snags in well-shaded forests adjacent to montane meadows. The 2013 Rim Fire burned 104,000 ha in Yosemite National Park and Stanislaus National Forest, making it the largest recorded fire in California’s Sierra Nevada. The fire perimeter contained 23 meadows known to be occupied by Great Gray Owls during the decade prior to the fire, representing nearly a quarter of all known or suspected territories in California at the time. We analyzed 13 yr (2004–2016) of Great Gray Owl detection/non-detection data from 144 meadows in the central Sierra Nevada, including meadows inside and outside the Rim Fire perimeter in Yosemite National Park and on Stanislaus National Forest. During 3 yr of surveys after the fire, Great Gray Owls were detected at 21 of 22 meadows surveyed within the fire perimeter that were occupied during the decade prior to the fire. Bayesian hierarchical modeling revealed that, rather than decreasing after the fire, persistence of owls at meadows actually increased on both National Park Service (NPS) and non-NPS lands, while colonization rates exhibited no significant change. Within the burned area, these dynamics were unrelated to forest structure variables describing post-fire stands around individual meadows. Notably, post-fire increases in owl persistence occurred both inside and outside the fire perimeter, suggesting factors other than the fire were likely favorable to Great Gray Owls during the post-fire years. Great Gray Owls appear to have been largely resilient to effects of the Rim Fire during the 3 yr after it burned.

Multi-scale assessment of post-fire tree mortality models

Post-fire tree mortality models are vital tools used by forest land managers to predict fire effects, estimate delayed mortality and develop management prescriptions. We evaluated the performance of mortality models within the First Order Fire Effects Model (FOFEM) software, and compared their performance to locally-parameterised models based on five different forms. We evaluated all models at the individual tree and stand levels with a dataset comprising 34174 trees from a mixed-conifer forest in the Sierra Nevada, California that burned in the 2013 Rim Fire. We compared stand-level accuracy across a range of spatial scales, and we used point pattern analysis to test the accuracy with which mortality models predict post-fire tree spatial pattern. FOFEM under-predicted mortality for the three conifers, possibly because of the timing of the Rim Fire during a severe multi-year drought. Locally-parameterised models based on crown scorch were most accurate in predicting individual tree mortality, but tree diameter-based models were more accurate at the stand level for Abies concolor and large-diameter Pinus lambertiana, the most abundant trees in this forest. Stand-level accuracy was reduced by spatially correlated error at small spatial scales, but stabilised at scales ≥1ha. The predictive error of FOFEM generated inaccurate predictions of post-fire spatial pattern at small scales, and this error could be reduced by improving FOFEM model accuracy for small trees.