This book is included in the Natural Disasters section.
Executive Summary The size and severity of recent fire episodes are widely attributed to altered fuel profiles as a result of fire exclusion and fire regime disruption in many ecosystems. Current national fuels management initiatives propose widespread application of prescribed fire and other treatments both to reduce the potential of catastrophic wildfire and to restore the structure and function of altered ecosystems. However, the chain of hypotheses that link historic fire regimes to appropriate fuel treatment application has not received a systematic assessment. This project seeks to provide such an assessment with a series of quantitative literature syntheses that focus on the following questions: 1) Are the effects of 20th Century fire exclusion on fire frequency related to historic fire regimes? 2) Are 20th century changes in fuel conditions and fire hazard most apparent in ecosystems where fire was historically most frequent? 3) Is there a relationship between historic fire regimes and fuel treatment efficacy? 4) Can geographic variables be used as predictors of historic fire regimes to facilitate their incorporation into fuel management planning? We standardized the information contained in 75 North American fire history studies to develop a general linear model that predicts historic fire frequency from geographic information (i.e., latitude, longitude, elevation, and aspect). Spatially and temporally standardized mean fire free periods (MFFP) were calculated each study. Development of the fire frequency model employed weighted least squares regression with each study weighted in proportion to statistical confidence about its MFFP estimate. The final model resulted from a 3-way cross-validation procedure and includes the following as predictors of log-transformed historic MFFP: latitude, longitudinal region (east of the Rockies, west of the Cascades/Sierra-Nevadas, and Intermountain West), elevation, and a two-way interaction between elevation and the eastern region. The model is highly significant and can provide reasonable estimates of historic MFFP at sites with unknown fire history in many regions of North America, particularly the western US. Fire exclusion effect sizes were calculated for each of the fire history studies used to develop the historic fire frequency model. We used the statistical techniques of meta-analysis to determine if there has been an overall change in fire frequencies in the 20th Century and whether this change varies by historic fire regime as predicted from our historic fire frequency model. We found a statistically significant overall mean effect size of fire exclusion when calculated from the combined evidence of all studies. However, the test statistic for heterogeneity among studies was highly insignificant, indicating that variability between studies in mean fire exclusion effect sizes is overwhelmed by uncertainty about the means. Nonetheless, we found that the effect of fire exclusion across ecosystems decreases with marginal significance as historic fire frequency decreases. Unlabeled photo pairs depicting historic versus recent vegetation conditions at seven diverse locations in the western U.S. were evaluated by 32 wildland fire professionals. Their ratings demonstrated a large and significant increase in perceived crown fire potential and a moderate and significant increase in fire severity potential, but no change in spread rate potential. Perceived changes in crown fire potential and potential fire severity were both significantly related to the historic fire regime of forested photo locations, with the greatest amount of change perceived where fire was historically most frequent. Finally, we synthesized the results of fire severity assessments in eight recent wildfires that were sampled with standardized methods in adjacent treated and untreated stands. Sampled sites occurred in a variety of conifer forests throughout the Western U.S. and treatments included reduction of surface fuels and crown fuels, both in isolation and in combination. Meta-analysis of these studies indicated that treatment effectiveness is most significantly related to differences in tree size (mean diameter) between treated and untreated stands, but we also found historic fire frequency to be a marginally significant predictor. Our results suggest that fuel treatments will be most effective when they complement ecosystem restoration objectives, such as the removal of small trees from ecosystems that historically experienced frequent fire. We conclude that the historic fire frequency model produced by this project can facilitate incorporation of fire regime considerations into fuels management planning. Table of Contents Executive Summary 1. Project Background 1.1 Rationale 1.2 Objectives and Hypotheses 2. Geographic Variations in Historic Fire Regimes 2.1 Methods 2.1.1 Information Sources 2.1.2 Statistical Analysis 2.2 Results 2.2.1 Study characteristics 2.2.2 Fire frequency model 2.3 Discussion 3. Meta-analysis of 20th Century Changes to Historic Fire Regimes 3.1 Methods 3.1.1 Information Sources 3.1.2 Statistical Analysis 3.2 Results 3.2.1 Study characteristics 3.2.2 Hypothesis tests 3.3 Discussion 4. Relating Historic Fire Regimes to 20th Century Fire Potential 4.1 Methods 4.1.1 Information Sources 4.1.2 Statistical Analysis 4.2 Results 4.3 Discussion 5. Assessment of Historic Fire Regime as a Factor in Fuel Treatment Effectiveness 5.1 Methods 5.2 Results and Discussion 6. Conclusions 7. Literature Cited Appendix A: Dissemination of Project Results Appendix B: Publications included in the fire history syntheses Appendix C: Published studies that quantify a fuel treatment effect on wildfire potential
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