Mean Fire Interval Research Articles

Better lucky than good: How savanna trees escape the fire trap in a variable world.

Fire controls tree cover in many savannas by suppressing saplings through repeated topkill and resprouting, causing a demographic bottleneck. Tree cover can increase dramatically if even a small fraction of saplings escape this fire trap, so modeling and management of savanna vegetation should account for occasional individuals that escape the fire trap because they are "better" (i.e., they grow faster than average) or because they are "lucky" (they experience an occasional longer-than-average interval without fire or a below-average fire severity). We quantified variation in growth rates and topkill probability in Quercus laevis (turkey oak) in longleaf pine savanna to estimate the percentage of stems expected to escape the fire trap due to variability in (1) growth rate, (2) fire severity, and (3) fire interval. For trees growing at the mean rate and exposed to the mean fire severity and the mean fire interval, no saplings are expected to become adults under typical fire frequencies. Introducing variability in any of these factors, however, allows some individuals to escape the fire trap. A variable fire interval had the greatest influence, allowing 8% of stems to become adults within a century. In contrast, introducing variation in fire severity and growth rate should allow 2.8% and 0.3% of stems to become adults, respectively. Thus, most trees that escape the fire trap do so because of luck. By chance, they experience long fire-free intervals and/or a low-severity fire when they are not yet large enough to resist an average fire. Fewer stems escape the fire trap by being unusually fast-growing individuals. It is important to quantify these sources of variation and their consequences to improve understanding, prediction, and management of vegetation dynamics of fire-maintained savannas. Here we also present a new approach to quantifying variation in fire severity utilizing a latent-variable model of logistic regression.

Historical fire in the Appalachian Plateau of Ohio and Kentucky, USA, from remnant yellow pines

BackgroundKnowledge of historical fire regimes informs the restoration of woodland communities. In the Appalachian Plateau of Ohio and Kentucky, USA, little is known about the long-term history of fire in oak–pine communities, which are declining in the region. To address this knowledge gap, two sites with remnant fire-scarred yellow pines, Hatton Ridge in Kentucky and McAtee Run in Ohio, were studied to document aspects of the historical fire regime. Cross-sections from fire-scarred yellow pines were collected. Fire chronologies were constructed and fire intervals were calculated using standard dendrochronological methods.ResultsFires, the great majority of which occurred in the dormant season, were frequent at both sites from circa 1750 at Hatton and 1800 at McAtee, until the suppression period (1930 to present); only one fire was recorded after 1930. Mean fire intervals (MFI) for the entire period were nearly identical, 4.7 and 4.4 years at Hatton and McAtee, respectively. At both sites, MFIs were lowest in the industrial period (1850 to 1930). At Hatton, the MFI was 6.6 years before 1850 and 3.5 years from 1850 to 1930, while at McAtee, the MFI was 8.4 years before 1850 and 2.7 years from 1850 to 1930. At both sites, the occurrence of fire was not more frequent than expected in years associated with a drought. At McAtee, the majority of pine establishment occurred in pulses during two periods, 1770 to 1781 and 1853 to 1867, suggesting stand-scale canopy disturbances; the second pulse was associated with frequent burning. In contrast, large pulses of pine establishment were not found at Hatton.ConclusionsYellow pines were a component of these communities, which experienced frequent fire for at least 130 to 160 years. After more than 70 years with little or no fire, yellow pines are now a minor component of the overstory and pine regeneration is essentially absent. Although intensive management with partial harvesting and frequent fire would be required to restore oak–pine woodlands on appropriate sites, it would serve to sustain these increasingly uncommon communities.

Fire in central Piedmont as recorded by fire scars at Pilot Mountain State Park, NC

ABSTRACTFire has been well studied in the Appalachians but has not been well characterized within Piedmont xeric forests. Herein we describe a dendrochronological study at Pilot Mountain State Park, NC addressing the following: How frequently did this forest experience fire? Is fire occurrence correlated with drought conditions? How did fire activity change with the onset of suppression? How does the fire regime compare with xeric Appalachian forests and prior models of Piedmont fire frequency? Seventy samples documented 34 fires between 1849–2014. The first fire occurred in 1891 and the first two-scar fire occurred in 1904. The composite Mean Fire Interval and Weinbull Median Interval pre-suppression (pre-1930; 2.36 and 2.25 years, respectively) were almost half that of the suppression period (post-1930; 4.56 and 4.09 years, respectively). With suppression, the Decadal Fire Index and mean number of scars per fire declined significantly (p ≤ 0.05 and ≤ 0.001). The Pre-suppression Filtered Composite MFI (4.2 years) was lower than any reported for the Appalachians and agrees with previous model predictions. Fire activity was strongly correlated with drought 2 years prior, before suppression but not afterward. As documented here, fire scars on xeric forest pines may contain important records of the Piedmont’s fire history.

Multi-scale synthesis of historical fire regimes along the south-central US prairie\u2013forest border

BackgroundAlong the prairie–forest border in the south-central USA exists one of the most extensive areas of uncut forest in the nation (>323 750 hectares), providing unique potential for developing multi-century records of environmental changes through dendrochronological analyses. Twentieth century changes in vegetation, increased prescribed fire management, and recent years of elevated wildfire activity have increased interest in understanding the region’s long-term fire regime characteristics. To address this need, we analyzed and compared fire intervals, seasonality, severity, and extent based on fire-scar history datasets from three new and ten existing study sites.ResultsAt the study sites, mean fire intervals ranged from 3 to 10 years prior to Euro-American settlement and generally became more frequent after. The majority of fires occurred in the dormant season and resulted in low percentages of trees scarred. Coinciding with Euro-American settlement (EAS), fire frequencies appear to have varied by geography. At the regional scale, fire regimes have trended towards decreased fire severity and slightly decreased fire intervals over the past 300 years. Further, fires appear to have increased in extent from circa 1770 to the mid to late 1800s, after which it began to decline, circa 1920.ConclusionsAlthough frequent, descriptions of fire regimes since the eighteenth century should be characterized as time-dependent and spatially variable, likely depending on local socio-ecological influences. Similar influences may explain fire frequency increases following EAS, while fire severity trended lower.

Fire as a key driver of Earth's biodiversity.

Many terrestrial ecosystems are fire prone, such that their composition and structure are largely due to their fire regime. Regions subject to regular fire have exceptionally high levels of species richness and endemism, and fire has been proposed as a major driver of their diversity, within the context of climate, resource availability and environmental heterogeneity. However, current fire-management practices rarely take into account the ecological and evolutionary roles of fire in maintaining biodiversity. Here, we focus on the mechanisms that enable fire to act as a major ecological and evolutionary force that promotes and maintains biodiversity over numerous spatiotemporal scales. From an ecological perspective, the vegetation, topography and local weather conditions during a fire generate a landscape with spatial and temporal variation in fire-related patches (pyrodiversity), and these produce the biotic and environmental heterogeneity that drives biodiversity across local and regional scales. There have been few empirical tests of the proposition that 'pyrodiversity begets biodiversity' but we show that biodiversity should peak at moderately high levels of pyrodiversity. Overall species richness is greatest immediately after fire and declines monotonically over time, with postfire successional pathways dictated by animal habitat preferences and varying lifespans among resident plants. Theory and data support the 'intermediate disturbance hypothesis' when mean patch species diversity is correlated with mean fire intervals. Postfire persistence, recruitment and immigration allow species with different life histories to coexist. From an evolutionary perspective, fire drives population turnover and diversification by promoting a wide range of adaptive responses to particular fire regimes. Among 39 comparisons, the number of species in 26 fire-prone lineages is much higher than that in their non-fire-prone sister lineages. Fire and its byproducts may have direct mutagenic effects, producing novel genotypes that can lead to trait innovation and even speciation. A paradigm shift aimed at restoring biodiversity-maintaining fire regimes across broad landscapes is required among the fire research and management communities. This will require ecologists and other professionals to spread the burgeoning fire-science knowledge beyond scientific publications to the broader public, politicians and media.

Climatic influence on fire regime (1700 to 2008) in the Nazas watershed, Durango, Mexico

BackgroundFire scars are the primary source of physical evidence used to date past fires around the world, and to estimate parameters of historical fire regimes and fire-climate relationships. Despite an increase in studies about historical fire regimes and the relationship between fire and climate, these studies are still limited for Mexican forests. Our goal was to characterize the historical fire regime in two sites with different elevations, to assess changes in fire frequency, and to determine the relationship between fire regimes and climate patterns in a conifer-dominated forest in the Upper Nazas watershed of the Sierra Madre Occidental, Mexico. We were able to cross-date 50 fire-scarred samples of Pinus arizonica Engelm., P. strobiformis Engelm., P. teocote Schlecht. & Cham., P. durangensis Martínez, and Pseudotsuga menziesii (Mirb.) Franco and identify a total of 596 fire scars.ResultsEvidences are compelling evidences on forest fires occurred frequently in the study area, starting in the mid eighteenth century and through the mid twentieth century. The season of fire occurrence was determined for 560 (94%) fire scars. Most fire scars at both of our sampling locations, found at low and high elevation sites, occurred in the spring (525, 93.8%) and only 6.2% occurred in the summer. The mean fire interval and Weibull median fire interval values were < 10 years for all fire-scar filter (all samples, ≥10% and ≥ 25%), and the average per-sample fire interval was < 25 years.ConclusionsOur results suggest fire frequency has changed over time at our study site, becoming less common and less extensive since the 1950s. In addition, extensive fires occurred in dry years as indicated by low precipitation, negative NIÑO 3 Sea Surface Temperature (SST) values, and negative Palmer Drought Severity Index (PDSI) values. Fire frequencies must be maintained within the amplitude of their historical regime, with the objective of reducing the risk of severe fires and preserving ecological and hydrological benefits in the watershed.

Long-term succession of closed boreal forests at their range limit in eastern North America shows resilience to fire and climate disturbances

The distribution of vegetation zones in northeastern North America forms a gradual transition from closed to open forests, and to tundra. Closed forests near the open forest/tundra boundary generally correspond to residual patches of a once larger forest community. Whether these forest patches have survived as untouched, fire-free communities or as resilient communities during the Holocene remains unknown. To answer to this question, we reconstructed the successional history of two black spruce closed-crown forests and one jack pine forest near the northern limit of closed forest ecosystems. Stand-scale postfire succession was analysed based on charcoal macrofossils. Extensive 14C dating and botanical identification of charcoal fragments located at the organic soil surface and buried in the mineral podzolic solum were used to reconstruct the successional pathways of the sites. The forest sites developed in a context of continuous fire disturbance, except for a short interval of several centuries. The fire history of the three sites is similar with a fire frequency of 34–37 fires over the last 5000–5800 years, which corresponds to a mean fire interval of 150–180 years. Based on parametric and non-parametric estimators, the regional fire regime of the Laforge area (combined fire data of two sites located about 7 km from each other) was most likely characterized by 38 fires over the last 5000–5800 years, i.e., at an interval of about 150–160 years. Birch (Betula papyrifera) was present in the spruce sites between 5800 and 3000 cal. years BP. Jack pine arrived at the pine site about 3000 cal. years BP, at the time when birch was extirpated from the spruce forests. The closed-crown forests near the open forest/tundra boundary are resilient postfire ecosystems which regenerated successfully since mid-Holocene. Compositional change of these forests, which led to the decline of birch but maintained the dominance of conifers, appears related to cooler and wetter conditions of late Holocene. Climatic conditions also slowed the eastward migration of jack pine, which explains the late arrival of this species at its northeastern range. Our data suggest that fire disturbance in these sites of the boreal biome has been a continuous and positive regenerative process since mid-Holocene despite the influence of climatic change on the tree flora.

Fire regime on a cultural landscape: Navajo Nation.

Fire has played an important role in the evolutionary environment of global ecosystems, and Indigenous peoples have long managed natural resources in these fire‐prone environments. We worked with the Navajo Nation Forestry Department to evaluate the historical role of fire on a 50 km2 landscape bisected by a natural mountain pass. We used fifty 5‐ha circular plots to collect proxy fire history data on fire‐scarred trees, stumps, logs, and snags in a coniferous forest centered on a key mountain pass. The fire history data were categorized into three groups: All (all 50 plots), Corridor (25 plots closest to Buffalo Pass drainage), and Outer (remaining 25 plots, farther from pass). We assessed spatial and temporal patterns of fire recurrence and fire‐climate relationships. The landscape experienced frequent fires from 1644, the earliest fire date with sufficient sample depth, to 1920, after which fire occurrence was interrupted. The mean fire interval (MFI) for fire dates scarring 10% or more of the samples was 6.25 years; there were 13 large‐scale fires identified with the 25% filter with an MFI of 22.6 years. Fire regimes varied over the landscape, with an early reduction in fire occurrence after 1829, likely associated with pastoralism, in the outer uplands away from the pass. In contrast, the pass corridor had continuing fire occurrence until the early 20th century.Synthesis. Fires were synchronized with large‐scale top‐down climatic oscillations (drought and La Niña), but the spatially explicit landscape sampling design allowed us to detect bottom‐up factors of topography, livestock grazing, and human movement patterns that interacted in complex ways to influence the fire regime at fine scales. Since the early 20th century, however, fires have been completely excluded. Fuel accumulation in the absence of fire and warming climate present challenges for future management.

Ecological legacies of anthropogenic burning in a British Columbia coastal temperate rain forest

AbstractAimFew long‐term fire histories have been reconstructed in coastal temperate rain forests, and little is known regarding the spatial and temporal characteristics of lightning and human ignitions. We use a multidisciplinary approach to assess the impact, scale and ecological legacies of historic fires.LocationWe focus on perhumid temperate rain forests located on the central coast of British Columbia, Canada.MethodsWe reconstructed 700 years of temporal and spatial aspects of fire activity with 30 plots on Hecate Island using fire scars and forest‐stand establishment. We then conducted a paired study of 20 former indigenous habitation and control sites on 15 islands to relate fire activity to patterns of human settlement. We mapped 15 years of lightning strike densities and use mixed‐effects modelling to assess whether fire activity predicted the distribution and abundance of traditional plants.ResultsSixteen low‐ and mixed‐severity fires were recorded from 1376 to 1893. The abundance of traditional plants and the density of western redcedar trees were best predicted by the location of former habitation sites and shorter mean fire intervals. Lightning is too rare to explain the pattern of fire activity in the study area. No fire activity was detected after 1893, coinciding with the relocation of indigenous groups from the study area.Main conclusionsFire was strongly associated with former indigenous habitation sites during the periods of occupation. People likely utilized fire as a tool for resource management to influence the densities of specific plants by creating mosaics of vegetation in different stages of succession. By assessing the ecological impacts of historic fire events, we gain a better understanding of the abrupt changes that occurred in the 20th century. Our ability to understand present‐day temperate rain forest ecosystems may be compromised if we underestimate the role of humans in driving historic fire activity.

The theory, direction, and magnitude of ecosystem fire probability as constrained by precipitation and temperature.

The effects of climate on wildland fire confronts society across a range of different ecosystems. Water and temperature affect the combustion dynamics, irrespective of whether those are associated with carbon fueled motors or ecosystems, but through different chemical, physical, and biological processes. We use an ecosystem combustion equation developed with the physical chemistry of atmospheric variables to estimate and simulate fire probability and mean fire interval (MFI). The calibration of ecosystem fire probability with basic combustion chemistry and physics offers a quantitative method to address wildland fire in addition to the well-studied forcing factors such as topography, ignition, and vegetation. We develop a graphic analysis tool for estimating climate forced fire probability with temperature and precipitation based on an empirical assessment of combustion theory and fire prediction in ecosystems. Climate-affected fire probability for any period, past or future, is estimated with given temperature and precipitation. A graphic analyses of wildland fire dynamics driven by climate supports a dialectic in hydrologic processes that affect ecosystem combustion: 1) the water needed by plants to produce carbon bonds (fuel) and 2) the inhibition of successful reactant collisions by water molecules (humidity and fuel moisture). These two postulates enable a classification scheme for ecosystems into three or more climate categories using their position relative to change points defined by precipitation in combustion dynamics equations. Three classifications of combustion dynamics in ecosystems fire probability include: 1) precipitation insensitive, 2) precipitation unstable, and 3) precipitation sensitive. All three classifications interact in different ways with variable levels of temperature.

Restoring and managing low-severity fire in dry-forest landscapes of the western USA.

Low-severity fires that killed few canopy trees played a significant historical role in dry forests of the western USA and warrant restoration and management, but historical rates of burning remain uncertain. Past reconstructions focused on on dating fire years, not measuring historical rates of burning. Past statistics, including mean composite fire interval (mean CFI) and individual-tree fire interval (mean ITFI) have biases and inaccuracies if used as estimators of rates. In this study, I used regression, with a calibration dataset of 96 cases, to test whether these statistics could accurately predict two equivalent historical rates, population mean fire interval (PMFI) and fire rotation (FR). The best model, using Weibull mean ITFI, had low prediction error and R2adj = 0.972. I used this model to predict historical PMFI/FR at 252 sites spanning dry forests. Historical PMFI/FR for a pool of 342 calibration and predicted sites had a mean of 39 years and median of 30 years. Short (< 25 years) mean PMFI/FRs were in Arizona and New Mexico and scattered in other states. Long (> 55 years) mean PMFI/FRs were mainly from northern New Mexico to South Dakota. Mountain sites often had a large range in PMFI/FR. Nearly all 342 estimates are for old forests with a history of primarily low-severity fire, found across only about 34% of historical dry-forest area. Frequent fire (PMFI/FR < 25 years) was found across only about 14% of historical dry-forest area, with 86% having multidecadal rates of low-severity fire. Historical fuels (e.g., understory shrubs and small trees) could fully recover between multidecadal fires, allowing some denser forests and some ecosystem processes and wildlife habitat to be less limited by fire. Lower historical rates mean less restoration treatment is needed before beginning managed fire for resource benefits, where feasible. Mimicking patterns of variability in historical low-severity fire regimes would likely benefit biological diversity and ecosystem functioning.

Spatial interpolation and mean fire interval analyses quantify historical mixed-severity fire regimes

Tree-age data in combination with fire scars improved inverse-distance-weighted spatial modelling of historical fire boundaries and intervals for the Darkwoods, British Columbia, Canada. Fire-scarred trees provided direct evidence of fire. The presence of fire-sensitive trees at sites with no fire scars indicated fire-free periods over their lifespan. Sensitivity analyses showed: (1) tree ages used in combination with fire-scar dates refined fire boundaries without biasing mean fire return intervals; and (2) compared with derived conservative, moderate and liberal thresholds (i.e. minimum burn likelihood cut-off values), fixed thresholds generated area burned estimates that were most consistent with estimates based on the proportion of plots that recorded historical fires. Unweighted and weighted spatial mean fire intervals (50–56 and 58–68 years respectively) exceeded dendrochronological plot-level (38-year) estimates based on fire scars only. Including tree-age data from fire-sensitive trees to calculate landscape-level fire interval metrics lengthened the mean return intervals, better representing historical high-severity fires. Supplementing fire scars with tree ages better reflects the spatiotemporal diversity of fire frequencies and severities inherent to mixed-severity fire regimes.

The first forest fire history of the Burabai Region (Kazakhstan)from tree rings of Pinus sylvestris

Forest fires are one of the most important events causing an abrupt decrease in tree-ring width. Although humans are the main cause of forest fires, extreme weather or climate change may promote the frequency and severity of fires. The purpose of this study is to reconstruct historical fires in the Burabai Region of Kazakhstan including their frequency and seasons. Five tree cross-sections with fire scars were collected from two different sites, Akylbai and Burabai. After sanding the transversal surfaces of the cross-sections, the year and season of fire scars were determined. We identified 15 fires during the last 300 years in the region. The mean fire interval is 27 years, with minimum and maximum fire intervals of 9 and 53 years, respectively. The seasonally determined fire scars generally occurred during the middle earlywood formation (77%), and less in early earlywood (22%). Three site chronologies of Pinus sylvestris L. were also used to find the effect and extent of the fires. The negative effect of fires on tree growth lasted 1 to 8 years after the fire. Four of the fire scars had long-term negative effects (up to 10 years) on tree growth in the years of 1759, 1779, 1871, and 1952. Fires of 1759, 1797, 1824, 1833, 1852, and 1871 are seen in both sites, suggesting that these fires spread over large areas.

Spatial, temporal and latitudinal components of historical fire regimes in mixed conifer forests, California

AbstractAimThis study seeks to document and compare historical temporal and spatial components of fire regimes in two watersheds in mixed conifer forests of the western slope of the Sierra Nevada, California, USA.LocationWatersheds in the southern Sierra Nevada (Sugar Pine, 2358 ha) and north‐central Sierra Nevada (Last Chance, 3021 ha), California, USA are compared.MethodsTemporal (frequency, return interval, season) and spatial (extent, fire rotation, spatial mean fire interval) fire regime metrics were reconstructed from fire scar samples. Superposed epoch analysis (SEA) was used to examine relationships between fire occurrence and the Palmer drought severity index (PDSI) at each site. Thin plate splines were introduced as a tool for interpolating historical fire extent from dendroecological data. Point fire return intervals were compared between sites to better understand possible influences of historical Native American burning practices.ResultsDifferences emerged between sites in temporal and spatial fire regime metrics. The northern site had longer fire return intervals, more synchronized fire years, fewer point intervals &lt; 4 years, longer fire rotation period and longer spatial mean fire interval. The northern site showed a significant reduction in PDSI values during fire years, whereas this climate–fire relationship in the southern site was likely decoupled by frequent Native American burning. Thin plate spline interpolation effectively reduced discontinuities at sample points compared to inverse distance weighting methods.Main conclusionsDifferences in both temporal and spatial fire regime metrics between sites were likely due to interplay in latitudinal influence on climate as well as differential Native American burning practices. Reconstruction of historical fire areas via geographical interpolation of fire scar data holds great promise for spatially explicit fire frequency reconstruction. The use of thin plate spline interpolation methods has the potential to reduce the impact of ‘false negatives’ in dendroecological data from frequent‐fire forests.

Frecuencia histórica de incendios (1779-2013) en bosques de pino-encino de la comunidad de Charcos, Mezquital, Durango

Historical fire regimes were reconstructed using dendrochronological methods to provide basic information for understanding this phenomenon. In order to reconstruct the fire history in a pine-oak forest in Mezquital, Durango, we collected 78 samples from different fire-scarred tree species (Pinus durangensis, P. arizonica, P. ayacahuite, P. teocote and Pseudotsuga menziesii); 73 and 27 % of the samples were taken from live and dead trees, respectively. By applying dendrochronological methods a total of 75 samples (96 %) and 535 fire scars were dated. Fire regimes were reconstructed from 1746 to 2013, with 1779 being the year of the first recorded fire. A mean fire interval (MFI) of 2.0 years and a Weibull median probability interval (WMPI) of 1.8 years (all scars) were reconstructed. Considering 25 % of scars or more (large fires), a MFI and a WMPI of 7.0 and 5.9 years, respectively, were determined. In total, 92.2 % of fires were categorized as having occurred in the spring and only 7.8 % in the summer. There was no significant (P < 0.05) influence of rainfall and the El Nino Southern Oscillation (ENSO) on fire frequency; however, extensive fires were positively associated with decreased rainfall and ENSO events (La Nina phase).

Historical fire in longleaf pine (Pinus palustris) forests of south Mississippi and its relation to land use and climate

AbstractWe characterized historical fire regimes in Pinus palustris (longleaf pine) forests of southern Mississippi with regard to global and regional coupled climate systems (e.g., El Niño–Southern Oscillation) and past human activity. The composite fire chronology spanned 1756–2013 with 132 individual scars representing 89 separate fire events. The mean fire interval was 2.9 yr, and mean intervals were significantly different between identified time periods (e.g., settlement period vs. management period). Evidence of biannual fire activity (up to three fires occurring within a 12‐ to 15‐month period) was found coeval with a peak in livestock grazing and logging from the 1850s through the 1880s. Connections were also found between historical fire and Pacific climate variability (e.g., El Niño–Southern Oscillation and Pacific Decadal Oscillation; P &lt; 0.05), yet the fire–climate linkage was likely at least partially masked by substantial human land use activities over the past several centuries. Coupled climate and human land use activity controlled the historical fire regime over the past ca. 240 yr. Although the many fire adaptions of P. palustris yield limitations in tree‐ring‐based fire history studies (e.g., thick bark), we highlight the efficacy of considering the height at which fire scars are analyzed along the bole as a way to glean a more accurate depiction of historical fire occurrence, especially in ecosystems characterized by a frequent, low‐severity fire regime. This study suggests growing‐season fire prescribed at a 2‐ to 3‐yr interval would be the first step toward simulating historical landscape conditions and fire activity, should that be the goal by land managers.

Fire Regime in Marginal Jack Pine Populations at Their Southern Limit of Distribution, Riding Mountain National Park, Central Canada

In central Canada, long fire history reconstructions are rare. In a context where both anthropogenic and climate influences on fire regime have changed, Parks Canada has a mandate to maintain ecological integrity. Here we present a fire history derived from fire-scarred jack pine (Pinus banksiana Lamb.) trees growing at their southern distribution limit in Riding Mountain National Park (RMNP). In Lake Katherine Fire Management Unit (LKFMU), a subregion within the park, fire history was reconstructed from archival records, tree-ring records, and charcoal in lake sediment. From about 1450 to 1850 common era (CE) the fire return intervals varied from 37 to 125 years, according to models. During the period 1864–1930 the study area burned frequently (Weibull Mean Fire Intervals between 2.66 and 5.62 years); this period coincided with the end of First Nations occupation and the start of European settlement. Major recruitment pulses were associated with the stand-replacing 1864 and 1894 fires. This period nevertheless corresponded to a reduction in charcoal accumulation. The current fire-free period in LKFMU (1930–today) coincides with RMNP establishment, exclusion of First Nations land use and increased fire suppression. Charcoal accumulation further decreased during this period. In the absence of fire, jack pine exclusion in LKFMU is foreseeable and the use of prescribed burning is advocated to conserve this protected jack pine ecosystem, at the southern margins of its range, and in the face of potential climate change.

Scale Dependence of Oak woodland Historical Fire Intervals: Contrasting the barrens of Tennessee and Cross timbers of Oklahoma, USA

Characterization of scale dependence of fire intervals could inform interpretations of fire history and improve fire prescriptions that aim to mimic historical fire regime conditions. We quantified the temporal variability in fire regimes and described the spatial dependence of fire intervals through the analysis of multi-century fire scar records (8 study sites, 332 trees, 843 fire scars) derived from two historically post oak (Quercus stellata Wangenh.) woodland landscapes. Despite large differences in fire environment conditions, study sites (∼1 km2) burned frequently (mean fire interval [MFI] ≤10 yr) before Euro-American settlement (pre-EAS), with sites in Tennessee showing higher overall fire frequency than sites in Oklahoma, USA. Pre-EAS MFIs decreased exponentially with increasing spatial extent from individual trees (∼1 m2) to landscapes (∼100 km2). The relationship between MFI and spatial extent may help to explain how historical observations of annual burning could be recorded in woodlands, when experimental studies suggest that this is too frequent for tree recruitment. Further investigations of spatial dependence of fire intervals would improve our ability to relate historical and experimental fire data to present day fire prescriptions, and vice versa.

A fire history derived from Pinus resinosa Ait. for the Islands of Eastern Lac La Croix, Minnesota, USA.

We reconstructed fire occurrence near a fur-trade era canoe travel corridor (used ca. 1780-1802) in the Quetico-Superior region west of Lake Superior to explore the possibility of human influence on pre-fire suppression rates of fire occurrence. Our research objectives were to (1) examine the spatial and temporal patterns of fire in the study area, (2) test fires' strength of association with regional drought, and (3) assess whether reconstructed fire frequencies could be explained by observed rates of lightning fire ignition over the modern period of record. We developed a 420-year fire history for the eastern portion of Lac La Croix in the Boundary Waters Canoe Area Wilderness (BWCAW). Seventy-one fire-scarred samples were collected from remnant Pinus resinosa Ait. (red pine) stumps and logs from thirteen distinct island and three mainland forest stands. Collectively these samples contained records of 255 individual fire scars representing 79 fire events from 1636 to 1933 (study area mean fire intervals [MFI] 3.8 yr). Reconstructed fires were spatially and temporally asynchronous and not strongly associated with regional drought (P > 0.05). When compared to the conservative, tree-ring reconstructed estimate of historical fire occurrence and modern lightning-caused fires (1929-2012), a noticeable change in the distribution and frequency of fires within the study area was evident with only two lightning-ignited island fires since 1934 in the study area. Our results suggest a high likelihood that indigenous land use contributed to surface fire ignitions within our study area and highlights the importance of examining the potential effects of past indigenous land use when determining modern approaches to fire and wilderness management in fire-adapted ecosystems.

Historical dominance of low-severity fire in dry and wet mixed-conifer forest habitats of the endangered terrestrial Jemez Mountains salamander (Plethodon neomexicanus)

Anthropogenic alteration of ecosystem processes confounds forest management and conservation of rare, declining species. Restoration of forest structure and fire hazard reduction are central goals of forest management policy in the western United States, but restoration priorities and treatments have become increasingly contentious. Numerous studies have documented changes in fire regimes, forest stand structure and species composition following a century of fire exclusion in dry, frequent-fire forests of the western U.S. (e.g., ponderosa pine and dry mixed-conifer). In contrast, wet mixed-conifer forests are thought to have historically burned infrequently with mixed- or high-severity fire—resulting in reduced impacts from fire exclusion and low restoration need—but data are limited. In this study we quantified the current forest habitat of the federally endangered, terrestrial Jemez Mountains salamander (Plethodon neomexicanus) and compared it to dendroecological reconstructions of historical habitat (e.g., stand structure and composition), and fire regime parameters along a gradient from upper ponderosa pine to wet mixed-conifer forests. We found that current fire-free intervals in Jemez Mountains salamander habitat (116–165years) are significantly longer than historical intervals, even in wet mixed-conifer forests. Historical mean fire intervals ranged from 10 to 42years along the forest gradient. Low-severity fires were historically dominant across all forest types (92 of 102 fires). Although some mixed- or high-severity fire historically occurred at 67% of the plots over the last four centuries, complete mortality within 1.0ha plots was rare, and asynchronous within and among sites. Climate was an important driver of temporal variability in fire severity, such that mixed- and high-severity fires were associated with more extreme drought than low-severity fires. Tree density in dry conifer forests historically ranged from open (90treesha−1) to moderately dense (400treesha−1), but has doubled on average since fire exclusion. Infill of fire-sensitive tree species has contributed to the conversion of historically dry mixed-conifer to wet mixed-conifer forest. We conclude that low-severity fire, which has been absent for over a century, was a critical ecosystem process across the forest gradient in Jemez Mountains salamander habitat, and thus is an important element of ecosystem restoration, resilience, and rare species recovery.