Toward a more ecologically informed view of severe forest fires

We use the historical presence of high-severity fire patches in mixed-conifer forests of the western United States to make several points that we hope will encourage development of a more ecologically informed view of severe wildland fire effects. First, many plant and animal species use, and have sometimes evolved to depend on, severely burned forest conditions for their persistence. Second, evidence from fire history studies also suggests that a complex mosaic of severely burned conifer patches was common historically in the West. Third, to maintain ecological integrity in forests born of mixed-severity fire, land managers will have to accept some severe fire and maintain the integrity of its aftermath. Lastly, public education messages surrounding fire could be modified so that people better understand and support management designed to maintain ecologically appropriate sizes and distributions of severe fire and the complex early-seral forest conditions it creates

Advancing dendrochronological studies of fire in the United States

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. Dendroecology is the science that dates tree rings to their exact calendar year of formation to study processes that influence forest ecology (e.g., Speer 2010 [1], Amoroso et al., 2017 [2]). Reconstruction of past fire regimes is a core application of dendroecology, linking fire history to population dynamics and climate effects on tree growth and survivorship. Since the early 20th century when dendrochronologists recognized that tree rings retained fire scars (e.g., Figure 1), and hence a record of past fires, they have conducted studies worldwide to reconstruct [2] the historical range and variability of fire regimes (e.g., frequency, severity, seasonality, spatial extent), [3] the influence of fire regimes on forest structure and ecosystem dynamics, and [4] the top-down (e.g., climate) and bottom-up (e.g., fuels, topography) drivers of fire that operate at a range of temporal and spatial scales. As in other scientific fields, continued application of dendrochronological techniques to study fires has shaped new trajectories for the science. Here we highlight some important current directions in the United States (US) and call on our international colleagues to continue the conversation with perspectives from other countries

The North American tree-ring fire-scar network

Fire regimes in North American forests are diverse and modern fire records are often too short to capture important patterns, trends, feedbacks, and drivers of variability. Tree-ring fire scars provide valuable perspectives on fire regimes, including centuries-long records of fire year, season, frequency, severity, and size. Here, we introduce the newly compiled North American tree-ring fire-scar network (NAFSN), which contains 2562 sites, >37,000 fire-scarred trees, and covers large parts of North America. We investigate the NAFSN in terms of geography, sample depth, vegetation, topography, climate, and human land use. Fire scars are found in most ecoregions, from boreal forests in northern Alaska and Canada to subtropical forests in southern Florida and Mexico. The network includes 91 tree species, but is dominated by gymnosperms in the genus Pinus. Fire scars are found from sea level to >4000-m elevation and across a range of topographic settings that vary by ecoregion. Multiple regions are densely sampled (e.g., >1000 fire-scarred trees), enabling new spatial analyses such as reconstructions of area burned. To demonstrate the potential of the network, we compared the climate space of the NAFSN to those of modern fires and forests; the NAFSN spans a climate space largely representative of the forested areas in North America, with notable gaps in warmer tropical climates. Modern fires are burning in similar climate spaces as historical fires, but disproportionately in warmer regions compared to the historical record, possibly related to under-sampling of warm subtropical forests or supporting observations of changing fire regimes. The historical influence of Indigenous and non-Indigenous human land use on fire regimes varies in space and time. A 20th century fire deficit associated with human activities is evident in many regions, yet fire regimes characterized by frequent surface fires are still active in some areas (e.g., Mexico and the southeastern United States). These analyses provide a foundation and framework for future studies using the hundreds of thousands of annually- to sub-annually-resolved tree-ring records of fire spanning centuries, which will further advance our understanding of the interactions among fire, climate, topography, vegetation, and humans across North America

Fire as a fundamental ecological process: Research advances and frontiers

© 2020 The Authors.Fire is a powerful ecological and evolutionary force that regulates organismal traits, population sizes, species interactions, community composition, carbon and nutrient cycling and ecosystem function. It also presents a rapidly growing societal challenge, due to both increasingly destructive wildfires and fire exclusion in fire‐dependent ecosystems. As an ecological process, fire integrates complex feedbacks among biological, social and geophysical processes, requiring coordination across several fields and scales of study. Here, we describe the diversity of ways in which fire operates as a fundamental ecological and evolutionary process on Earth. We explore research priorities in six categories of fire ecology: (a) characteristics of fire regimes, (b) changing fire regimes, (c) fire effects on above‐ground ecology, (d) fire effects on below‐ground ecology, (e) fire behaviour and (f) fire ecology modelling. We identify three emergent themes: the need to study fire across temporal scales, to assess the mechanisms underlying a variety of ecological feedbacks involving fire and to improve representation of fire in a range of modelling contexts. Synthesis: As fire regimes and our relationships with fire continue to change, prioritizing these research areas will facilitate understanding of the ecological causes and consequences of future fires and rethinking fire management alternatives.Support was provided by NSF‐DEB‐1743681 to K.K.M. and A.J.T. We thank Shalin Hai‐Jew for helpful discussion of the survey and qualitative methods.Peer reviewe

Fire history and tree recruitment in the Colorado Front Range upper montane zone: implications for forest restoration

Abstract. Forests experiencing moderate-or mixed-severity fire regimes are presumed to be widespread across the western United States, but few studies have characterized these complex disturbance regimes and their effects on contemporary forest structure. Restoration of pre-fire-suppression open-forest structure to reduce the risk of uncharacteristic standreplacing fires is a guiding principle in forest management policy, but identifying which forests are clear candidates for restoration remains a challenge. We conducted dendroecological reconstructions of fire history and stand structure at 40 sites in the upper montane zone of the Colorado Front Range (2400-2800 m), sampled in proportion to the distribution of forest types in that zone (50% dominated by ponderosa pine, 28% by lodgepole pine, 12% by aspen, 10% by Douglas-fir). We characterized past fire severity based on remnant criteria at each site in order to assess the effect of fire history on tree establishment patterns, and we also evaluated the influence of fire suppression and climate. We found that 62% of the sites experienced predominantly moderate-severity fire, 38% burned at high severity, and no sites burned exclusively at low severity. The proportion of total tree and sapling establishment was significantly different among equal time periods based on a chi-square test, with highest tree and sapling establishment during the pre-fire-suppression period . Superposed epoch analysis revealed that fires burned during years of extreme drought (95% CI). The major pulse of tree establishment in the upper montane zone occurred during a multidecadal period of extreme drought conditions in the Colorado Front Range (1850-1889), during which 53% of the fires from the 1750-1989 period burned. In the upper montane zone of the Colorado Front Range, historical evidence suggests that these forests are resilient to prolonged periods of severe drought and associated severe fires

Fire history and tree recruitment in the Colorado Front Range upper montane zone: implications for forest restoration

Abstract. Forests experiencing moderate-or mixed-severity fire regimes are presumed to be widespread across the western United States, but few studies have characterized these complex disturbance regimes and their effects on contemporary forest structure. Restoration of pre-fire-suppression open-forest structure to reduce the risk of uncharacteristic standreplacing fires is a guiding principle in forest management policy, but identifying which forests are clear candidates for restoration remains a challenge. We conducted dendroecological reconstructions of fire history and stand structure at 40 sites in the upper montane zone of the Colorado Front Range (2400-2800 m), sampled in proportion to the distribution of forest types in that zone (50% dominated by ponderosa pine, 28% by lodgepole pine, 12% by aspen, 10% by Douglas-fir). We characterized past fire severity based on remnant criteria at each site in order to assess the effect of fire history on tree establishment patterns, and we also evaluated the influence of fire suppression and climate. We found that 62% of the sites experienced predominantly moderate-severity fire, 38% burned at high severity, and no sites burned exclusively at low severity. The proportion of total tree and sapling establishment was significantly different among equal time periods based on a chi-square test, with highest tree and sapling establishment during the pre-fire-suppression period . Superposed epoch analysis revealed that fires burned during years of extreme drought (95% CI). The major pulse of tree establishment in the upper montane zone occurred during a multidecadal period of extreme drought conditions in the Colorado Front Range (1850-1889), during which 53% of the fires from the 1750-1989 period burned. In the upper montane zone of the Colorado Front Range, historical evidence suggests that these forests are resilient to prolonged periods of severe drought and associated severe fires

Are Wildfire Mitigation and Restoration of Historic Forest Structure Compatible? A Spatial Modeling Assessment

In response to catastrophic wildfires, wide-reaching forest management policies have been enacted in recent years, most notably the Healthy Forests Restoration Act of 2003. A key premise underlying these policies is that fire suppression has resulted in denser forests than were present historically in some western forest types. Therefore, although reducing the threat of wildfire is the primary goal, forest managers commonly view fuel treatments as a means to restore historic forest structure in those forest types that are outside of their historic range of variation. This study evaluates where both wildfire mitigation and restoration of historic forest structure are potentially needed in the ponderosa pine–dominated montane forest zone of Boulder County, Colorado. Two spatial models were overlain: a model of potential fireline intensity and a model of historic fire frequency. The overlay was then aggregated by land management classes. Contrary to current assumptions, results of this study indicate that both wildfire mitigation and restoration of historic forest structure are needed in only a small part of the study area, primarily at low elevations. Furthermore, little of this land is located on Forest Service land where most of the current thinning projects are taking place. We question the validity of thinning as a means both to reduce the threat of wildfire and to restore historic forest structure in the absence of site-specific data collection on past and present landscape conditions

A Spatial Model of Mechanical Thinning Location and Forest Management Outcomes in the Wildland-Urban Interface

In fire-prone areas of the western United States, mechanical thinning is often seen as a way to achieve two outcomes: Wildfire mitigation and restoration of historical forest structure. In this study, a spatial modeling approach is used to (1) find which forests are likely to be thinned under different criteria; (2) for these forests, evaluate whether wildfire mitigation and restoration of historical forest structure are potentially needed; and (3) determine whether these results change under alternative assumptions related to weather and fire history. Effectively, the spatial models in this study allow us to “test” thinning criteria to see if they lead to the selection of land where the stated management goals are needed in the study area of the montane zone of Boulder County, Colo. The spatial modeling results indicate that common management practices — such as thinning dense stands on Forest Service land near communities — may be inappropriate if the desired outcome is both wildfire mitigation and restoration of historical forest structure. Instead, modeling results suggest that lower elevation forests in the study area should receive priority. Though specific to the montane zone of Boulder County, the results of this study support wider criticisms of national fire policy

Modeling Wildfire Potential in Residential Parcels: A Case Study of the North-Central Colorado Front Range

This study evaluated if present-day wildfire potential (i.e. potential fireline intensity and percentage crown fire) differs for residential parcels developed at different time periods in the north–central Colorado Front Range. To answer this question, a model of wildfire potential was built based on 2001 fuels and vegetation and compared the output to actual fire severity of the 2002 Hayman and 2004 Picnic Rock fires (measured by satellite imagery). Except for low-load fuel types such as grass, the modeled wildfire potential corresponded well to observed fire severity. Wildfire potential was then evaluated within 7 classes: developed (1880–1944, 1945–1959, 1960–1974, 1975–1989, 1990–2005) and undeveloped (either zoned or not zoned for development). The results suggest that there is one class characterized by relatively low wildfire potential (developed 1880–1944) and three classes characterized by relatively high wildfire potential (developed 1960–1974 and the two undeveloped parcel classes). These results hold both for 99th percentile (extreme) and 50th percentile (average) fuel conditions. The results suggest that under current zoning regulations, future structures are likely to be built on parcels that, on average, have somewhat higher potential fireline intensity and higher percentage of crown fire compared to currently developed parcels. However, the location of future development may be influenced by forest changes, such as the visual degradation and perceived fire hazard of trees killed by the continuing mountain pine beetle outbreak. Overall, this study introduces an improved method for quantifying wildfire potential in the rapidly developing wildland–urban interface that could be applied to other areas