Mixed-severity Fires Research Articles

Historical pyrodiversity in Douglas-fir forests of the southern Cascades of Oregon, USA

Our understanding of forest dynamics and successional pathways in coastal Douglas-fir (Pseudotsuga menziesii var menziesii) forests with relatively frequent mixed-severity fires is limited by a lack of annually precise dendroecological reconstructions that combine records of historical fires and tree establishment. The processes by which old-forest heterogeneity developed under historical fire regimes with recurrent low- and moderate-severity fires has not been well studied at fine temporal scales and across spatial scales. We developed crossdated multi-century records of fire and tree establishment histories in old forests (170 – 550 years) at 34 plots distributed across six sites. Study sites include warm-dry to cool-moist Douglas-fir forest types found in the southern west Cascades of Oregon, USA. Spatial variability in historical fire frequency and fire effects resulted in tremendous diversity in forest developmental histories, age structure, and forest conditions. Most historical fire intervals were very frequent (<10 years) to frequent (<25 years) in dry Douglas-fir forests. Exceptionally high fire frequency and an abrupt decrease in fire frequency after European colonization in dry Douglas-fir forests adds to growing evidence and recognition of Indigenous fire stewardship in montane Douglas-fir forests. In moist forests where Douglas-fir is seral to western hemlock, fire intervals were frequent to moderately frequent (<50 years), but intervals varied substantially over time. Relatively young moist forests burned frequently while mature moist forests had long fire intervals (50–160 years). Nearly all tree establishment cohorts were preceded by either stand-replacing (28 %) or non-stand-replacing fires (64 %). However, tree cohorts only provided evidence of 16 % of historical fire events that we reconstructed from cambial fire scars. This study demonstrates that frequent fire can be an important driver of forest development and in some contexts shapes the structure of coastal old-growth Douglas-fir forests, which are often characterized as developing from endogenous disturbances during long fire-free periods. The high level of pyrodiversity we observed was associated with variation in and interactions of micro-climate, topography, fuels, and Indigenous fire stewardship. We recommend rigorous dendroecological reconstructions across the coastal Douglas-fir region to refine our understanding of the geography of fire-mediated forest developmental dynamics in this important forest type, to inform forest management, conservation, and ecocultural restoration.

Modeling Juvenile Stand Development and Fire Risk of Post-Fire Planted Forests under Variations in Thinning and Fuel Treatments Using FVS–FFE

In the past, the dry mixed conifer forests of California’s Sierra Nevada mountains experienced frequent low to mixed severity fires. However, due to fire suppression and past management, forest structure has changed, and the new fire regimes are characterized by large, high severity fires which kill a majority of the overstory trees. These new disturbance patterns require novel approaches to regenerate the forest as they are not adapted to large, high severity fires. We forecasted growth and fire behavior of young plantations for 100 years into the future using the Forest Vegetation Simulator (FVS) and its Fire and Fuels Extension (FFE). In these simulations, we tested combinations of different fuel treatments (mastication only, mastication with prescribed burning, and no fuels treatments) with different overstory thinning intensities (residual densities of 370 SDI (stand density index), 495 SDI, 618 SDI (TPH), and no overstory thinning) on stand growth and potential fire behavior using analysis of variance. We compared growth and crowning index at the end of the simulation and the simulation age when the flame length, basal area mortality, and fire type reached low severity between fuel treatment, thinning intensity, and original management of stands (plantation with PCT [precommercial thinning], plantation without PCT, and natural regenerating stands). These comparisons are essential to identify which fuel treatment categories reduce fire risk. We found an overall pattern of decreasing crown fire occurrence and fire induced mortality across all simulations due to increasing canopy base height and decreasing canopy bulk density. In particular, stands with mastication and prescribed burning transitioned from crown fire types to surface fires 10 years earlier compared to mastication only or no fuel treatment. Furthermore, pre-commercially thinned stands transitioned from crown fire states to surface fires 10 years earlier in the simulations compared to un-thinned and naturally regenerating stands. Stands with mastication and burning went below 25% reference threshold of basal area mortality 11 and 17 years earlier before the mastication only and no fuel treatment, respectively. In addition, pre-commercially thinned stands went below 25% basal area mortality 9 and 5 years earlier in the simulation compared to un-thinned or naturally regenerated stands, respectively. Mastication with prescribed burning (MB) was the most effective treatment for quickly reducing fire behavior by consuming surface fuels, thus drastically lowing flame length (e.g., surface flame length of MB was 0.6 m compared to mastication only [1.3 m] and no treatment [1.4 m]). Furthermore, intensive thinning reduced risk of active crown fires spreading through the stand. Prioritizing prescribed burning, when possible, and thinning (both pre-commercially and from below) are the most effective ways to quickly improve fire resistance in mixed conifer plantations. Our results highlight the different stressors that post-fire planted forests experience and how different silvicultural treatments interact over time to reduce fire risk, which demonstrates the importance of treating stands early and the effectiveness of surface fuel treatments.

The contribution of Indigenous stewardship to an historical mixed-severity fire regime in British Columbia, Canada.

Indigenous land stewardship and mixed-severity fire regimes both promote landscape heterogeneity, and the relationship between them is an emerging area of research. In our study, we reconstructed the historical fire regime of Ne Sextsine, a 5900-ha dry, Douglas fir-dominated forest in the traditional territory of the T'exelc (Williams Lake First Nation) in British Columbia, Canada. Between 1550 and 1982CE, we found median fire intervals of 18years at the plot level and 4years at the study-site level. Ne Sextsine was characterized by an historical mixed-severity fire regime, dominated by frequent, low-severity fires as indicated by fire scars, with infrequent, mixed-severity fires indicated by cohorts. Differentiating low- from mixed-severity plots over time was key to understanding the drivers of the fire regime at Ne Sextsine. Low-severity plots were coincident with areas of highest use by the T'exelc, including winter village sites, summer fishing camps, and travel corridors. The high fire frequency in low-severity plots ceased in the 1870s, following the smallpox epidemic, the forced relocation of Indigenous peoples into small reserves, and the prohibition of Indigenous burning. In contrast, the mixed-severity plots were coincident with areas where forest resources, such as deer or certain berry species, were important. The high fire frequency in the mixed-severity plots continued until the 1920s when industrial-scale grazing and logging began, facilitated by the establishment of a nearby railway. T'exelc oral histories and archeological evidence at Ne Sextsine speak to varied land stewardship, reflected in the spatiotemporal complexity of low- and mixed-severity fire plots. Across Ne Sextsine, 63% of cohorts established and persisted in the absence of fire after colonial impacts beginning in the 1860s, resulting in a dense, homogeneous landscape that no longer supports T'exelc values and is more likely to burn at uncharacteristic high severities. This nuanced understanding of the Indigenous contribution to a mixed-severity fire regime is critical for advancing proactive fire mitigation that is ecoculturally relevant and guided by Indigenous expertise.

Fire history and vegetation data reveal ecological benefits of recent mixed-severity fires in the Cumberland Mountains, West Virginia, USA

BackgroundWithout periodic fire, fire-adapted plant communities across the Central Hardwood Forest Region (CHF) in the USA have undergone significant changes in forest structure and species composition, most notably a decrease in oak regeneration and herbaceous diversity and an increase in shade-tolerant, fire-sensitive tree species. In this study, we conducted a comparative analysis of two mixed pine-oak (Pinus-Quercus) forests with different land management histories in the Cumberland Mountains of southern West Virginia where fire ecology and fire effects are understudied. We reconstructed the fire history of both sites from fire-scarred shortleaf pine (Pinus echinata Mill.) and pitch pine (Pinus rigida Mill.) trees to describe variation in the fire regimes over time. We also made plant community measurements that spatially coincided with fire-scarred pines to assess present-day plant community structure in relation to recent fire history.ResultsBefore 1970, fires at Hite Fork and Wall Fork occurred frequently and almost exclusively in the dormant season, every 7–8 years on average. The fire regimes diverged in the Post-Industrial era (1970–2020), during which there was a single fire at Wall Fork, while six major fires, scarring more than 40% of sampled trees, occurred between 1985 and 2017 at Hite Fork. Four of these dormant-season fires correspond to late fall incendiary fires in the observational record. These differences in recent fire history had large effects on plant community structure. Recent mixed-severity fires at Hite Fork likely caused mortality of pole-sized trees and opened the canopy, creating conditions favorable for pine recruitment and resulted in significantly higher species richness in the herbaceous layer compared to Wall Fork, which exhibited the effects of mesophication.ConclusionsOur results suggest that frequent mixed-severity fire in pine-oak forests of the Cumberland Mountains can meet management objectives by reducing mesophytic tree abundance, increasing herbaceous diversity and pine recruitment, and generally promoting forest heterogeneity.

Resprouting in European beech confers resilience to high-frequency fire

AbstractEuropean beech (Fagus sylvatica L.) can regenerate successfully from seeds after mixed-severity fires with mid-to-long fire return intervals (&amp;gt;60 years). However, if fire return interval is lower than the age of sexual maturity, post-fire seeding will be limited, leaving vegetative resprouting as the only viable option for recovery. This means that the forecasted increase in fire frequency driven by climate change may erode beech forest resilience to fire. Here, we surveyed tree regeneration in a European beech forest affected by two consecutive fires, in 2003 and 2017, and applied experimental clipping of tree saplings to address the following questions: (1) What is the fire resistance and post-fire recovery via resprouting of tree saplings? (2) Which factors drive post-fire resprouting of beech saplings? (3) Does post-fire clipping of tree saplings increase the probability of survival and resprouting vigor? We monitored 2195 beech saplings and 953 saplings of other tree species during three consecutive years, from 2018 to 2020. Almost all beech saplings were top-killed by fire, and two-thirds of them died completely. However, 3 years after the second fire, 30 per cent of beech saplings survived by resprouting from the base. Post-fire resprouting was less likely in small-diameter saplings and in those more injured by fire. Overall, the second fire did not cause a major decline of beech regeneration and consequently did not alter the dominant species composition of post-fire recovery. Given the low specific resistance to fire, post-fire resprouting of saplings is therefore a key component of beech resilience to short-interval fires. The effects of clipping on post-fire survival and resprouting vigor were very limited, suggesting the unsuitability of actively clearing burned beech regeneration as a post-fire management prescription. In conclusion, basal resprouting from beech saplings after fire-induced top-kill led to a higher-than-expected resilience of beech to short-interval fires (i.e. circa 15 years).

Fire regime and management in Canada's protected areas

Wildland fires are a vital ecological process in Canada's protected areas (PAs), resulting in both ecological benefits and detrimental disturbances. In the face of rapid global changes, maximizing fire's ecological benefits while minimizing its negative impacts is particularly difficult but essential for PA fire management. To facilitate fire management in Canada's PAs, we conducted a comprehensive review of fire regimes and management strategies in PAs using several scholarly sources and national databases. We found that a high proportion of protected areas are in the remote boreal forest region or the Montane Cordillera. Fire regimes varied from the high-intensity fires in the boreal forest region to the more complex mixed-severity fires in the Montane Cordillera ecozone. Numerous fire tools and science-based management plans have been developed to address the varied and complex fire regimes. However, many challenges are still presented under longer fire seasons with increasingly severe burning across the country. To cope with these challenges, we proposed a protection-conservation integrated fire management framework. It is expected to improve the capability of the existing tools in guiding both fire protection and ecological fire use in Canada's PAs.

Have western USA fire suppression and megafire active management approaches become a contemporary Sisyphus?

Fire suppression policies and “active management” in response to wildfires are being carried out by land managers globally, including millions of hectares of mixed conifer and dry ponderosa pine (Pinus ponderosa) forests of the western USA that periodically burn in mixed severity fires. Federal managers pour billions of dollars into command-and-control fire suppression and the MegaFire (landscape scale) Active Management Approach (MFAMA) in an attempt to contain wildfires increasingly influenced by top down climate forcings. Wildfire suppression activities aimed at stopping or slowing fires include expansive dozerlines, chemical retardants and igniters, backburns, and cutting trees (live and dead), including within roadless and wilderness areas. MFAMA involves logging of large, fire-resistant live trees and snags; mastication of beneficial shrubs; degradation of wildlife habitat, including endangered species habitat; aquatic impacts from an expansive road system; and logging-related carbon emissions. Such impacts are routinely dismissed with minimal environmental review and defiance of the precautionary principle in environmental planning. Placing restrictive bounds on these activities, deemed increasingly ineffective in a change climate, is urgently needed to overcome their contributions to the global biodiversity and climate crises. We urge land managers and decision makers to address the root cause of recent fire increases by reducing greenhouse gas emissions across all sectors, reforming industrial forestry and fire suppression practices, protecting carbon stores in large trees and recently burned forests, working with wildfire for ecosystem benefits using minimum suppression tactics when fire is not threatening towns, and surgical application of thinning and prescribed fire nearest homes.

Historical Fire Regimes and Stand Dynamics of Xerophytic Pine–Oak Stands in the Southern Appalachian Mountains, Virginia, USA

Fire-dependent yellow pine (Pinus) forests are included within the temperate deciduous forest of eastern North America. These forests, which occupy dry slopes and typically contain xerophytic oaks (Quercus), have receded under fire suppression. Understanding historical fire regimes is essential for interpreting and managing these stands. To characterize fire history and vegetation dynamics, we conducted a dendroecological study of fire-scarred trees and age structure in pine stands at four sites in the Appalachian Mountains. Fire interval estimates suggest that before fire suppression began in the early to middle 1900s, fires occurred at approximately three- to eleven-year intervals. Short intervals were probably maintained in part by large-extent fires that spread from sparse ignition points. Fire frequency showed no long-term temporal trend (e.g., no wave of fire) from the middle 1700s through early 1900s despite land–use intensification, including industrial logging and associated wildfires during the late nineteenth and early twentieth centuries. Fire occurrence was associated with drought at two sites. Age–structure analyses evoke pyrogenic pine–oak communities that predated industrial disturbances and persisted under a regime of frequent, mixed-severity fires that was likely maintained through a positive feedback with the flammable vegetation. Competing species were established under more recent fire suppression, however, and are poised to replace the pines.

Historical and future global burned area with changing climate and human demography

Summary Wildfires influence terrestrial carbon cycling and represent a safety risk, and yet a process-based understanding of their frequency and spatial distributions remains elusive. We combine satellite-based observations with an enhanced dynamic global vegetation model to make regionally resolved global assessments of burned area (BA) responses to changing climate, derived from 34 Earth system models and human demographics for 1860–2100. Limited by climate and socioeconomics, recent BA has decreased, especially in central South America and mesic African savannas. However, future simulations predict increasing BA due to changing climate, rapid population density growth, and urbanization. BA increases are especially notable at high latitudes, due to accelerated warming, and over the tropics and subtropics, due to drying and human ignitions. Conversely, rapid urbanization also limits BA via enhanced fire suppression in the immediate vicinity of settlements, offsetting the potential for dramatic future increases, depending on warming extent. Our analysis provides further insight into regional and global BA trends, highlighting the importance of including human demographic change in models for wildfire under changing climate.

Fire legacies, heterogeneity, and the importance of mixed-severity fire in ponderosa pine savannas

Globally, savanna ecosystems are shifting outside of “safe operating spaces” due to removal of their primary self-reinforcing feedback—fire—and subsequent erosion of disturbance legacies. Restoring savannas will require reinstating fire feedbacks. But knowledge gaps in the nature of historic fire regimes and how mechanisms such as time-since-fire and fire severity interact to produce disturbance legacies hinders development of ecologically relevant restoration targets. A theory-based approach for determining restoration targets is to compare structures produced by time-since-fire/fire severity interactions to structures that fostered animal communities that historically inhabited savannas. Here, we use a space-for-time substitution to quantify interactive effects of time-since-fire and fire severity on vegetation structures related to known animal community habitat preferences by surveying sites in 10-year-old and 27-year-old mixed-severity fires that occurred in an eastern ponderosa pine (Pinus ponderosa) savanna where fire was excluded since European settlement. Our specific objectives are to 1), quantify the relative strengths and interactive effects of time-since-fire and a full fire severity gradient on multivariate vegetation structure across landscape patches and 2), assess relationships between multivariate vegetation structures and time-since-fire/fire severity classes across landscape patches. We used a stratified random design to distribute 112 sampling plots by fire severity (unburned, low, moderate, and high) and time-since-fire (10 years and 27 years) and measured stand structure, tree cavity characteristics, and coarse woody debris. The interaction of time-since-fire and fire severity drove structure complexity for 27 years post-fire, but fire severity explained > 14 times the amount of variation in structure than time-since-fire alone. A full fire severity gradient (low-, moderate-, and high-severities) and structural changes within patches that experienced different fire severities generated sufficient patch-level heterogeneity to foster animal communities historically native to eastern ponderosa pine savannas. Structures generated by low fire severity alone, which is a common goal of fire management in easternmost ponderosa pine savannas, did not reflect sufficient structural complexity to support the diversity of endemic animal species. This indicates information legacies (i.e., represented by the distribution of species traits in a community) were shaped by mixed severity fire regimes, which provides further support for the scientific premise that management goals seeking to minimize variation in fire regimes (e.g., low intensity and low-severity fires only) is less able to support a full array of biodiversity. Rather, mixed-severity fire is an important driver of structural heterogeneity, fosters diverse information legacies, and enhances ponderosa pine savanna resilience to extreme fire.

Creating forest management units with Hot Spot Analysis (Getis-Ord Gi*) over a forest affected by mixed-severity fires

ABSTRACTThe rehabilitation of degraded subtropical natural forests is a global concern. A detailed assessment of their structure is a challenging and costly prerequisite because diverse structures exist depending on the cause and degree of degradation. Recent remote sensing concepts and technologies provide a detailed picture of actual forest structure, even in difficult terrain. When it comes to planning and implementing rehabilitation measures on the ground, however, meaningful forest management units (FMUs) must be created that are large enough to allow technical implementation, but which are also homogenous in structure. To date, the delineation of FMUs has, in most cases, been achieved qualitatively based on expert knowledge. The aim of this contribution is to develop and demonstrate a method for creating and delineating meaningful FMUs based on quantitative information acquired from remote sensing and spatial statistics. Therefore, a case study was conducted in a 3940-ha fire-degraded forest area in the Argentinean cloud forest of Yungas Pedemontana. A plot-based field inventory and an aerial survey with an unmanned aerial vehicle were conducted. The Adjusted Canopy Coverage Index (ACCI), as a metric for stand structure, was formulated to predict basal area from canopy height models. A SPOT6 image of the area was object-based segmented and classified into four fire-severity strata by training it with the ACCI values. The resulting classification presented a mosaic pattern in which the stands are homogenous but far too small (average 3129 m2) for planning adaptive management. Therefore, features in close proximity with similar structure (i.e. ACCI values) were aggregated using the Hot Spot Analysis (Getis-Ord Gi*) tool from the Arc geographic information system environment to create FMUs. Clusters were calculated at four scales: 10, 20, 30 and 40 ha (resulting in threshold radii of 178, 252, 309 and 357 m, respectively), using ACCI values as the variable of aggregation. As a result, average cluster areas were obtained of 33.9 ha for the shortest threshold distance of analysis and 138.5 ha for the greatest threshold distance. The tool significantly aggregated between 30.7% and 60.8% of the area into either coldspots or hotspots of ACCI, facilitating the delineation of FMUs for the planning of adaptive rehabilitation measures. There is a trade-off, however, between the gain in area of the FMUs and the loss of homogeneity: for a 357 m distance threshold, 12% more of the area was misclassified, compared with a 178 m threshold.

Burn severity and heterogeneity mediate avian response to wildfire in a hemiboreal forest

Recent studies have shown the importance of accounting for burn severity when assessing the effects of forest fires on avian communities. We add to this growing literature with one of the first studies to assess these effects in boreal and hemiboreal regions of North America. We conducted point counts in control and treatment areas for 2–3 years before (2009–2011) and 4–5 years after (2012–2016) the Pagami Creek Fire in northern Minnesota, USA. Our primary objectives were to (1) assess the effects of burn status, burn severity and burn heterogeneity on avian communities and patterns of abundance in individual bird species and (2) compare these results when counts were either restricted to 100 m count radii or corrected for potential detection heterogeneity. Species richness and total abundance of birds were best described by a burn severity by year interaction, with lower-severity burns having similar patterns to control areas and higher-severity burns showing reduced abundance and richness following the fire. Nevertheless, values associated with higher-severity burns returned to near-control levels after five years. Of 43 species analyzed, 27 showed detectable responses to the fire through shifts in abundance. Nearly equal proportions of these species had generally positive versus negative effects of the fire. Sixteen (59%) of these fire responses were mediated by burn severity, while four (15%) and seven (26%) were best predicted by burn heterogeneity or burn status, respectively. Of the species whose response to the fire was mediated by burn severity, the majority (81%) had decreased abundance as burn severity increased. However, all species responding to burn heterogeneity or burn status showed positive relationships to these metrics. Results were similar between analytical methods that adjusted for species detectability vs those that did not, with 78% of species having the same interpretation of fire effects. Burn severity and heterogeneity, in addition to the sole distinction of whether an area was burned, dictate the response of birds to forest fire in hemiboreal forests. Mixed-severity fires, which include a range of burn severities and associated vegetation change, will likely benefit the most bird species in this region. When possible, management should focus on retaining this spectrum of post-fire conditions, though of primary importance is the inclusion of wildfire in forest planning and conservation efforts.

Combining Satellite and UAV Imagery to Delineate Forest Cover and Basal Area after Mixed-Severity Fires

In northern Argentina, the assessment of degraded forests is a big challenge for both science and practice, due to their heterogeneous structure. However, new technologies could contribute to mapping post-disturbance canopy cover and basal area in detail. Therefore, this research assesses whether or not the inclusion of partial cover unmanned aerial vehicle imagery could reduce the classification error of a SPOT6 image used in an area-based inventory. BA was calculated from 77 ground inventory plots over 3944 ha of a forest affected by mixed-severity fires in the Argentinian Yungas. In total, 74% of the area was covered with UAV flights, and canopy height models were calculated to estimate partial canopy cover at three tree height classes. Basal area and partial canopy cover were used to formulate the adjusted canopy cover index, and it was calculated for 70 ground plots and an additional 20 image plots. Four classes of fire severity were created based on basal area and adjusted canopy cover index, and were used to run two supervised classifications over a segmented (algorithm multiresolution) wall-to-wall SPOT6 image. The comparison of the Cohan’s Kappa coefficient of both classifications shows that they are not significantly different (p-value: 0.43). However, the approach based on the adjusted canopy cover index achieved more homogeneous strata (Welch t-test with 95% of confidence). Additionally, UAV-derived canopy height model estimates of tree height were compared with field measurements of 71 alive trees. The canopy height models underestimated tree height with an RMSE ranging from 2.8 to 8.3 m. The best accuracy of the canopy height model was achieved using a larger pixel size (10 m), and for lower stocked plots due to high fire severity.

Accommodating Mixed-Severity Fire to Restore and Maintain Ecosystem Integrity with a Focus on the Sierra Nevada of California, USA

Existing fire policy encourages the maintenance of ecosystem integrity in fire management, yet this is difficult to implement on lands managed for competing economic, human safety, and air quality concerns. We discuss a fire management approach in the mid-elevations of the Sierra Nevada, California, USA, that may exemplify similar challenges in other fire-adapted regions of the western USA. We also discuss how managing for pyrodiversity through mixed-severity fires can promote ecosystem integrity in Sierran mixed conifer and ponderosa pine (Pinus ponderosa Laws) forests. To illustrate, we show how coarse-filter (landscape-level) and complementary fine-filter (species-level) approaches can enhance forest management and conservation biology objectives as related to wildfire management. At the coarse-filter level, pyrodiverse mixed-severity fires provide landscape heterogeneity. Species and ecosystem characteristics associated with pyrodiversity can be maintained or enhanced by accommodating moderately severe fires, which hasten restoration by recreating a complex vegetation mosaic otherwise at risk from suppression. At the fine-filter level, managers can select focal species and species of conservation concern based on the degree to which those species depend on fire and accommodate their specific conservation needs. The black-backed woodpecker (Picoides arcticus [Swainson, 1832]) is an ideal focal species for monitoring the ecological integrity of forests restored through mixed-severity fire, and the California spotted owl (Strix occidentalis occidentalis [Xantus de Vesey, 1860]) is a species of conservation concern that uses post-fire habitat mosaics and is particularly vulnerable to logging. We suggest a comprehensive approach that integrates wildland fire for ecosystem integrity and species viability with strategic deployment of fire suppression and ecologically based restoration of pyrodiverse landscapes. Our approach would accomplish fire management goals while simultaneously maintaining biodiversity.

Resilience of European larch (Larix decidua Mill.) forests to wildfires in the western Alps

European larch is a dominant species in the subalpine belt of the western Alps. Despite recent increases in wildfire activity in this region, fire ecology of European larch is poorly understood compared to other larch species around the world. This study aims to assess whether European larch forests are resilient to fires, and to find out the factors that drive such resilience. We assessed the recovery of larch forests along a gradient of fire severity (low, moderate, high) based on the abundance and dominance of post-fire larch regeneration. We established 200 plots distributed among burned larch forests in nine wildfires that occurred between 1973 and 2007 in the western Alps. We included variables regarding topography, climate, fire severity, fire legacies, ground cover, grazing intensity, and time since fire. To evaluate potential drivers of larch recruitment, we applied generalized linear mixed models (GLMM) and random forests (RF). Larch regeneration was much more abundant and dominant in the moderate- and high-severity fire classes than in the low-severity class. More than half of the plots in the moderate- and high-severity classes were classified as resilient, i.e., post-fire larch regeneration was enough to recover a larch stand. GLMM and RF produced complementary results: fire severity and legacies, such as snags, canopy cover and distance to seed source, were crucial factors explaining post-fire larch recruitment. This study shows that fire has a positive effect on larch regeneration, and we conclude that European larch forests are highly resilient to mixed-severity fires in the western Alps.

Differential impacts of wildfire on the population dynamics of an old-forest species.

Ecological disturbances shape and maintain natural communities, but climate change and human land use can alter disturbance regimes and affect population persistence and vital rates in unpredictable ways. Species inhabiting landscapes shaped by wildfire have evolved mechanisms allowing them to persist under this dynamic disturbance type, which creates habitats of varying quality for these species. We utilized data from a 26-yr demographic study of northern spotted owls to analyze the influence of wildfire on apparent survival and recruitment rates. Wildfires occurred across different years and affected different spotted owl territories, which allowed us to implement a retrospective Before-After-Control-Impact (BACI) analysis and model the potential effect of wildfire extent and severity. Our results indicated that mixed-severity fires that burned at predominantly low-severity had little effect on survival and recruitment while fires characterized by more medium to high burn severities negatively affected spotted owl survival, with varying effects on recruitment. Reduced survival and increased recruitment rates on some territories affected by medium to high severity fires suggested that post-fire habitat quality was reduced resulting in territories that were marginally capable of supporting owls. We hypothesize these territories may have represented "sinks" that were supported by nearby "source" territories in a spatially heterogeneous landscape created by the mixed-severity fire regime of the region.

The Ecological Importance of Mixed-Severity Fires: Nature's Phoenix

The Ecological Importance of Mixed-Severity Fires: Nature's Phoenix

Tree mortality and structural change following mixed-severity fire in Pseudotsuga forests of Oregon’s western Cascades, USA

Mixed-severity fires are increasingly recognized as common in Pseudotsuga forests of the Pacific Northwest and may be an important mechanism for developing or maintaining their structural diversity and complexity. Questions remain about how tree mortality varies and forest structure is altered across the disturbance gradient imposed by these fires. Therefore, we sampled live and dead trees at 45–1.0ha plots, each with four 0.10ha nested subplots, stratified across an unburned, low, moderate and high-severity fire gradient. Burned plots were primarily sampled 10 and 22years post-fire, except three plots sampled four years post-fire. We estimated probability of mortality for fire-tolerant (Douglas-fir, incense-cedar, sugar pine) and fire-intolerant (western hemlock, western redcedar, true fir) trees from 5079 samples. The probability of mortality varied across all species and fire-severity classes, but the greatest difference was observed between fire-tolerant and fire-intolerant functional groups. Probability of mortality decreased with increasing DBH for all species except western hemlock that did not increase its fire resistance with increasing size. Some large, fire-tolerant trees survived high-severity fire but only in 31% of our plots. Snag fall and fragmentation was estimated for trees killed by the fire from 2746 sampled snags and logs 10 and 22years post-fire. The proportion of snags fallen decreased with increasing DBH for all species, and was positively correlated with fire severity, except for Douglas-fir that had a higher proportion felled following low-severity fire. Snag fragmentation rates were positively correlated with DBH and fire severity for all species. Individual tree and snag estimates were scaled to plot-level structural attributes and contrasted by their coefficient of variation within- and among-plots from unburned to moderate-severity, as well as across all sampled conditions. Structural attributes varied more within- than among-plots, likely a result of patchy mortality that increased with fire severity. Although vertical and horizontal structural diversity increased at sub-hectare scales, the highest variability occurred for all structural attributes when compared across all sampled conditions. We conclude that the range of fire effects imposed by mixed-severity fire is important in creating structural diversity in Pseudotsuga forests of the Pacific Northwest and should be promoted through progressive fire management programs aimed at maintaining this forest’s fire regime now and in the future.

Topography, Fuels, and Fire Exclusion Drive Fire Severity of the Rim Fire in an Old-Growth Mixed-Conifer Forest, Yosemite National Park, USA

The number of large, high-severity fires has increased in the western United States over the past 30 years due to climate change and increasing tree density from fire suppression. Fuel quantity, topography, and weather during a burn control fire severity, and the relative contributions of these controls in mixed-severity fires in mountainous terrain are poorly understood. In 2013, the Rim Fire burned a previously studied 2125 ha area of mixed-conifer forest in Yosemite National Park. Data from 84 plots sampled in 2002 revealed increases in tree density, basal area, and fuel buildup since 1899 due to fire exclusion. A dendroecological fire history and reconstruction of forest structure in 1899 showed that this area historically experienced frequent, low-severity fire. In contrast with this region’s historical fire regime, burn severity from Landsat imagery showed that this area burned at mixed-severity in the Rim Fire, with 13% of plots classified as unchanged, 31% low severity, 32% moderate severity, and 24% high severity. A random forest model was used to identify the controls of fire severity in this portion of the Rim Fire, using daily area burned, daily fire weather, and fuels and vegetation data for the surface and canopy. Topography, tree species composition, and cover of forbs and shrubs best explained the fire severity. As an example of a re-entry burn, this study demonstrates how fire exclusion alters fire–vegetation interactions, leading to uncharacteristically severe burns and potentially new fire-vegetation dynamics.

Historical Northern spotted owl habitat and old-growth dry forests maintained by mixed-severity wildfires

Reconstructing historical habitat could help reverse declining animal populations, but detailed, spatially comprehensive data are rare. For example, habitat for the federally threatened Northern spotted owl (NSO; Strix occidentalis caurina) was thought historically rare because low-severity fires kept forests open and habitat restricted to fire refugia, but spatial historical data are lacking. Here I use public land-surveys to spatially reconstruct NSO habitat and old-growth forests in dry forests in Oregon’s Eastern Cascades in the late-1800s. I used reconstructions of forest structure across about 280,000 ha, including 9,605 tree records and 2,180 section-line descriptions. I was able to reconstruct likely NSO nest trees, nest stands, and foraging and roosting habitat, based on modern NSO habitat studies. Historical nest stands, including sufficient nest trees, were predicted across 22–39 % and foraging and roosting habitat across 11–68 % of the study area, thus neither were rare. More habitat than expected occurred in forests with preceding mixed-severity fires. Early post-fire succession produced foraging and roosting habitat. Mid- to late-succession produced nesting habitat. Late-succession after high-severity fires can also provide NSO habitat. Old-growth forests, covering 76 % of study-area forests, also likely link to preceding mixed-severity fires. Mixed- and high-severity fires strongly shaped historical dry forests and produced important components of historical NSO habitat. Focus on short-term loss of nest sites and territories to these fires is mis-directed. Fuel treatments to reduce these natural fires, if successful, would reduce future habitat of the NSO in dry forests.