Carl N. Skinner

Fire treatment effects on vegetation structure, fuels, and potential fire severity in western U.S. forests

Forest structure and species composition in many western U.S. coniferous forests have been altered through fire exclusion, past and ongoing harvesting practices, and livestock grazing over the 20th century. The effects of these activities have been most pronounced in seasonally dry, low and mid-elevation coniferous forests that once experienced frequent, low to moderate intensity, fire regimes. In this paper, we report the effects of Fire and Fire Surrogate (FFS) forest stand treatments on fuel load profiles, potential fire behavior, and fire severity under three weather scenarios from six western U.S. FFS sites. This replicated, multisite experiment provides a framework for drawing broad generalizations about the effectiveness of prescribed fire and mechanical treatments on surface fuel loads, forest structure, and potential fire severity. Mechanical treatments without fire resulted in combined 1-, 10-, and 100-hour surface fuel loads that were significantly greater than controls at three of five FFS sites. Canopy cover was significantly lower than controls at three of five FFS sites with mechanical-only treatments and at all five FFS sites with the mechanical plus burning treatment; fire-only treatments reduced canopy cover at only one site. For the combined treatment of mechanical plus fire, all five FFS sites with this treatment had a substantially lower likelihood of passive crown fire as indicated by the very high torching indices. FFS sites that experienced significant increases in 1-, 10-, and 100-hour combined surface fuel loads utilized harvest systems that left all activity fuels within experimental units. When mechanical treatments were followed by prescribed burning or pile burning, they were the most effective treatment for reducing crown fire potential and predicted tree mortality because of low surface fuel loads and increased vertical and horizontal canopy separation. Results indicate that mechanical plus fire, fire-only, and mechanical-only treatments using whole-tree harvest systems were all effective at reducing potential fire severity under severe fire weather conditions. Retaining the largest trees within stands also increased fire resistance.

Fire‐climate interactions in the American West since 1400 CE

[1]xa0Despite a strong anthropogenic fingerprint on 20th Century wildland fire activity in the American West, climate remains a main driver. A better understanding of the spatio-temporal variability in fire-climate interactions is therefore crucial for fire management. Here, we present annually resolved, tree-ring based fire records for four regions in the American West that extend back to 1400 CE. In all regions, years with high fire activity were characterized by widespread yet regionally distinct summer droughts. Overall fire activity was high in late Medieval times, when much of the American West was affected by mega-droughts. A distinct decline in fire activity in the late 16th Century corresponds with anomalously low temperatures during the Little Ice Age and a decline in Native American fire use. The high spatiotemporal resolution of our fire record discloses a time-frequency dependent climatic influence on wildfire regimes in the American West that needs to be accounted for in fire models.

Fire-climate interactions in forests of the American Pacific Coast

We investigate relationships between climate and wildfire activity between 1929 and 2004 in Pacific coast forests of the United States. Self-Organizing Mapping (SOM) of annual area burned in National Forests (NF) in California, Oregon, and Washington identifies three contiguous NF groups and a fourth group of NF traversed by major highways. Large fire years in all groups are dry compared to small fire years. A sub-hemispheric circulation pattern of a strong trough over the North Pacific and a ridge over the West Coast is characteristic of large fire years in all groups. This pattern resembles the Pacific North American (PNA) teleconnection and positive phase of the Pacific Decadal Oscillation (PDO). A reverse PNA and negative PDO phase characterizes small fire years. Despite the effect of fire suppression management between 1929 and 2004, forest area burned is linked to climatic variations related to large-scale atmospheric circulation patterns.

Climatic influences on fire regimes in montane forests of the southern Cascades, California, USA

The relationship between climate variability and fire extent was examined in montane and upper montane forests in the southern Cascades. Fire occurrence and extent were reconstructed for seven sites and related to measures of reconstructed climate for the period 1700 to 1900. The climate variables included the Palmer Drought Severity Index (PDSI), summer temperature (TEMP), NINO3, a measure of the El Nino-Southern Oscillation (ENSO), and the Pacific Decadal Oscillation (PDO). Fire extent at the site and regional scale was associated with dry and warm conditions in the year of the fire and regional fire extent was not associated with ENSO or PDO for the full period of analysis. The relationship between regional fire extent and climate was not stable over time. The associations of fire extent with PDSI and TEMP were only significant from ∼1775 onward and the associations were strongest between 1805 and 1855. PDO and fire extent were also associated during the 1805-1855 period, and ENSO was associated with fire extent before 1800, but not after. The interannual and interdecadal variability of the fire response to temperature and drought suggests that increased periods of regional fire activity may occur when high interannual PDSI variation coincides with warm decades. Additional keywords: American Pacific coast, climatic variation, El Nino-Southern Oscillation, fire ecology, Pacific Decadal Oscillation.

Behaviour and effects of prescribed fire in masticated fuelbeds

Mechanical mastication converts shrub and small tree fuels into surface fuels, and this method is being widely used as a treatment to reduce fire hazard. The compactness of these fuelbeds is thought to moderate fire behaviour, but whether standard fuel models can accurately predict fire behaviour and effects is poorly understood. Prescribed burns were conducted in young ponderosa pine (Pinus ponderosa Laws.) forests at two sites in northern California where the midstorey layer dominated by shrubs had been masticated. Surface fuels were raked from the base of a subset of trees before burning. Rate of spread and flame length were estimated for both backing and heading fires, soil heating measured with thermocouples and tree fire injury recorded. Standard fuel models often over-predicted rate of spread or under-predicted flame length. Custom models generally provided a better balance between the slow rates of spread and moderate flame lengths observed in prescribed burns. Post-fire tree mortality was most strongly associated with crown scorch and tree size; raking fuels from the base of trees did not improve survival. Under severe fire weather conditions, fire behaviour and effect models as well as observations from wildfires suggest that mastication may be more effective for moderating fire behaviour than reducing residual tree mortality. Treating masticated fuels with prescribed burns could potentially improve the resilience of stands to wildfire.

Socioecological transitions trigger fire regime shifts and modulate fire–climate interactions in the Sierra Nevada, USA, 1600–2015 CE

Significance Twenty-first–century climate change is projected to increase fire activity in California, but predictions are uncertain because humans can amplify or buffer fire–climate relationships. We combined a tree-ring–based fire history with 20th-century area burned data to show that large fire regime shifts during the past 415 y corresponded with socioecological change, and not climate variability. Climate amplified large-scale fire activity after Native American depopulation reduced the buffering effect of Native American burns on fire spread. Later Euro-American settlement and fire suppression buffered fire activity from long-term temperature increases. Our findings highlight a need to enhance our understanding of human–fire interactions to improve the skill of future projections of fire driven by climate change. Large wildfires in California cause significant socioecological impacts, and half of the federal funds for fire suppression are spent each year in California. Future fire activity is projected to increase with climate change, but predictions are uncertain because humans can modulate or even override climatic effects on fire activity. Here we test the hypothesis that changes in socioecological systems from the Native American to the current period drove shifts in fire activity and modulated fire–climate relationships in the Sierra Nevada. We developed a 415-y record (1600–2015 CE) of fire activity by merging a tree-ring–based record of Sierra Nevada fire history with a 20th-century record based on annual area burned. Large shifts in the fire record corresponded with socioecological change, and not climate change, and socioecological conditions amplified and buffered fire response to climate. Fire activity was highest and fire–climate relationships were strongest after Native American depopulation—following mission establishment (ca. 1775 CE)—reduced the self-limiting effect of Native American burns on fire spread. With the Gold Rush and Euro-American settlement (ca. 1865 CE), fire activity declined, and the strong multidecadal relationship between temperature and fire decayed and then disappeared after implementation of fire suppression (ca. 1904 CE). The amplification and buffering of fire–climate relationships by humans underscores the need for parameterizing thresholds of human- vs. climate-driven fire activity to improve the skill and value of fire–climate models for addressing the increasing fire risk in California.

Science synthesis to support socioecological resilience in the Sierra Nevada and southern Cascade Range.

A team of scientists integrated recent research to inform forest managers, stakeholders, and interested parties concerned with promoting socioecological resilience in the Sierra Nevada, southern Cascade Range, and Modoc Plateau. Among the focal topics were forest and fire ecology; soils; aquatic ecosystems; forest carnivores including Pacific fisher, marten, and California spotted owl; air quality; and the social, economic, and cultural components of socioecological systems. The synthesis adopted a holistic perspective by focusing on issues that cross scientific disciplines and considering the integrated nature of terrestrial and aquatic systems and the interconnections between restoration of ecological processes and the social and economic concerns of communities. A central theme is the importance of restoring key ecological processes to mitigate impacts of widespread stressors to socioecological resilience, including changes in climate, fire deficit and fuel accumulations, air pollution, and pathogens and invasive species. Key findings from the synthesis were that (1) efforts to promote resilience of socioecological systems increasingly consider the interaction of social values and ecological processes in pursuit of long-term mutual benefits and social learning for local communities and larger social networks; (2) strategic placement of treatments to reduce hazardous fuel accumulations and to restore fire as an ecosystem process within large landscapes can lower the risk of uncharacteristically large, severe, and dangerous fires, and their associated impacts to sensitive wildlife species; and (3) science suggests a need for active treatment in some riparian and core wildlife habitat to restore fire and its ecological benefits. Forest landscape management will need to be adaptive as the impacts of stressors and treatments on a range of socioecological values are determined by further research and monitoring.