Fire-scarred Samples Research Articles

Fire History (1896–2013) in an Abies religiosa Forest in the Sierra Norte of Puebla, Mexico

The oyamel forests, as Abies dominated forests are commonly known as, register their largest distribution (95% of their population) along the Trans-Mexican Volcanic Belt (TMVB). Although efforts have been made to study these forests with various approaches, dendrochronology-based studies have been limited, particularly in pure Abies forests in this region. The objective of this study was to reconstruct fire regimes in an Abies religiosa forest in the Sierra Norte in the state of Puebla, Mexico. Within an area of 50-ha, we collected 40 fire-scar samples, which were processed and analyzed using dendrochronological techniques to identify 153 fire scars. The fire history was reconstructed for a period of 118 years (1896–2013), with low severity surface fires occurring mainly during in the spring (92.8%) and summer (7.2%). Over the past century, fires were frequent, with an mean fire interval (MFI) and Weibull median probability of (WMPI) of five years when considering all fire scars and less than 10 years for fires covering larger areas (fires recorded by ≥25% of samples). Extensive fires were synchronized with drought conditions based on Ring Width Indexes, Palmer Drought Severity Index (PDSI) and El Niño Southern Oscillation (ENSO). After 1983, we observed a change in fire frequencies attributed to regulated management. Longer fire intervals within the last several decades are likely leading to increased fuel accumulations and could potentially result in more severe fires in the future, threatening the sustainability of these forests. Based on our finding, we recommend management actions (silvicultural or prescribed fire) to reduce fuels and the risk of severe fires, particularly in the face of climatic changes.

Effects of climate on historical fire regimes (1451–2013) in Pinus hartwegii forests of Cofre de Perote National Park, Veracruz, Mexico

Cofre de Perote National Park (CPNP) in Veracruz, Mexico is part of the Transmexican Volcanic Belt, and its Pinus hartwegii forests reflect a balance between the various natural factors that represent the region's climatology and hydrology. Like many other areas in this region, the historical fire regimes of these forests and their relationship with climate are unknown, but are needed for sustainable management plans. The main objectives of this study were to reconstruct the historical fire regime in a Pinus hartwegii forest and decipher the influenced of climate. Our investigation focused in two study areas, Valle la Teta (VT) and Barranca Honda (BH). The VT study area was divided into three sites based on humidity and elevation: 1) Humid (VTH), 2) Dry Low (VTDL) and 3) Dry High (VTDH). The approximated area for each site was ​​30, 30, 35 and 50 ha, for VTH, VTDL, VTDH and BH, respectively. We collected 162 fire scarred samples to reconstruct the fire history for the last 550 years (1461−2013). The fire scarred samples contained 1240 fire scars, with most fires occurring in spring (95 %) or summer (5%). Prior to 1973, these sites were characterized by a frequent surface fire regime. In all four sites, the mean fire intervals ranged from 5 to 6 years (for fires that scarred ≥ 10 % of the samples) and 13–23 years (for fires that scarred ≥ 25 % of the samples). Extensive fires (≥ 10 %) coincided with significantly dry conditions based on the Standardized Precipitation Index (SPI), influenced by El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation Index (PDO). We also found a significant relationship between fire occurrence and ENSO, both in its warm phase, El Niño (21 fires ≥ 10 %) and in its cold phase, La Niña (32 fires ≥ 10 %). Synchronization of the cold phase of ENSO (La Niña) with the cold phase of the PDO (negative), facilitated severe drought conditions, resulting in fires with the greatest spatial extent. Since 1973, extensive fires have been absent from the study area most likely due to anthropogenic activities including active fire suppression. These results show a strong climate-fire relationship in these high elevation forests. The lack of fire in the last four decades is concerning and could potentially lead to unnatural stand-replacing fires, unless the historical fire regime is restored to maintain natural processes and increase forest resilience.

Using Tree-Rings to Reconstruct Fire History Information from Forested Areas.

Annual tree-ring patterns are a source of ecological and environmental information including the history of fires in forested areas. Tree-ring based fire histories include three fundamental phases: field collection, laboratory methods (preparation and dating), and data analysis. Here we provide step-by-step instructions and issues to consider, including the process for selecting the study area, sampling sites, plus how and which fire-scarred trees to sample. In addition, we describe fire-scar sample preparation and dating which are done in the laboratory. Finally, we describe basic analysis and relevant results, including examples from studies that have reconstructed fire history patterns. These studies allow us to understand the historical fire frequency, changes in those frequencies related to anthropogenic factors, and analyzes of how climate influences fire occurrence over time. The description of these methods and techniques should provide a greater understanding of fire history studies that will benefit researchers, educators, technicians, and students interested in this field. These detailed methods will allow new researchers to this field, a resource to start their own work and achieve greater success. This resource will provide a greater integration of tree-ring aspects within other studies and lead to a better understanding of natural processes with forested ecosystems.

Fire regimes and structural changes in oak-pine forests of the Mogollon Highlands ecoregion: Implications for ecological restoration

Ponderosa pine (Pinus ponderosa) forests occur at their warmer, drier environmental limits in the Mogollon Highlands ecoregion (MHE) of the Southwestern United States, and are commonly found in stringers or discrete stands that form ecotones with interior chaparral. These “rear edge” forests are likely to be highly vulnerable to rapid changes in structure and composition with climate warming, drought, and wildfire. There is increasing interest in understanding historical conditions, ecosystem changes, and restoration needs for MHE forests. However, comprehensive reconstruction analysis of fire regimes and stand structure has not been done for these systems, which differ from many montane ponderosa pine forests by having an abundance of understory shrubs. In this study we used demographic data from field plots, fire scar samples, and dendroecology to reconstruct historical fire regimes and landscape structure at ponderosa pine-dominated sites that spanned a range of environmental conditions on the Prescott and Tonto National Forests. We found strong evidence of historical surface fire regimes with mean fire intervals ranging 1.3–15.6 years across the five MHE sites during the period 1700–1879. We found very little evidence of historical high-severity fire at any study site. Historical forest structure was open with tree densities ranging 84.7–136.4 trees ha−1 and stand basal area (BA) ranging 4.5–8.4 m2 ha−1. Historical composition showed codominance of ponderosa pine, Arizona white oak (Quercus arizonica), Emory oak (Q. emoryi), and Gambel oak (Q. gambelii). Thus, oak species and likely other hardwoods were important historical components of these ecosystems. Contemporary forests are greater in stand density and BA by 359–703% and 285–502%, respectively, compared to historical estimates. In addition, we observed contemporary shifts in species composition. Changes related to disruption of historical fire regimes have increased susceptibility of ponderosa pine forests in the MHE to rapid shifts in structure and composition that may come about with climate change and high-intensity wildfire. Meeting fuels reduction and ecological restoration goals will be challenging for land managers due to vigorous regeneration responses of shrubs to tree thinning, prescribed burning, or other management activities. Managers will be required to balance attention to historical reference conditions, conservation of biological diversity, and needs for fuels management.

Fire human-climate interaction in Atlas cedar forests of Aurès, Northern Algeria

Forty-one fire-scarred samples from two new Cedrus atlantica sites in the Ouled-Yaâgoub Forest were collected and combined with the two previous sites from the Chelia Forest reported in Slimani et al. (2014) to develop a regional analysis of fire history in the Aurès Mountains, northern Algeria. Prior to 1850, cedar forests were characterized by high frequency, low-intensity surface fire regimes. Fire frequency declined after 1850 after the elimination of the Barbary lion and the enforcement of the first laws governing land and forest use in Algeria. Most of the fire events in both forests occurred in the summer season, the latter part of the growing season, or in the dormant season. We compared past fires occurrences and an October-June precipitation reconstruction. On a regional scale, precipitation was significantly reduced in the months prior to the fire occurrence. In addition, precipitation during the first year preceding the fire years was significantly above average. This study provides a baseline for planning and justifying improved ecosystem management programs.

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.

Fire History at the Confluence of the Driftless Area and Central Sand Plains of Wisconsin: A Case Study from Castle Mound Pine Forest State Natural Area

Castle Mound Pine Forest State Natural Area (CMPF) is a 48-ha reserve at the confluence of the Driftless Area and Central Sand Plains of Wisconsin. Here, we report the first tree-ring—based fire history study for central Wisconsin and examine the relationships among fire, forest structure and composition, and historical land use at the site. We crossdated 12 fire-scar samples from Pinus resinosa stumps and inventoried and cored 83 trees along four transects to quantify the fire history, forest composition, and forest age structure at the site. The fire history data span the years 1788–2006 and include 15 years when trees were scarred by a fire on the site. Most fire scars were recorded in the earlywood of the recording growth ring, suggesting spring or early summer fires. The mean fire return interval for the site was six years when calculated between the first fire in 1841 and the last fire in 1923. The time-since-fire at the time of our study was 91 years. The canopy of the site was dominated by P. resinosa, individuals of which represent the oldest trees on the site. The subcanopy and recent age structure was dominated by mesic species, indicating that the forest at CMPF is transitioning away from the historical dominance by P. resinosa to a more closed forest following the cessation of fire at the site. Our data highlight an opportunity to establish ecological baseline data for this area to inform fire management and restoration activities.

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.

Fire and climate in Mongolia (1532–2010 Common Era)

AbstractRecent increases in wildland fire, warming temperatures, and land use change have coincided in many forested regions, making it difficult to parse causes of elevated fire activity. Here we use 20 multicentury fire scar chronologies (464 fire scar samples) from Mongolia to evaluate the role of climate forcing of fire in the context of livestock grazing and minimal fire suppression. We observe no change in fire return intervals post‐1900; however, since the 1500s, periods of drought are coincident with more fire and shorter fire return intervals. We observe same year and some antecedent year effects of drought on fire, a pattern typical of semiarid forests elsewhere. During the instrumental period, drought remains an important driver of fire; however, limited fire activity in recent decades may be due to the coincidence of drought and intensive grazing that have synergized to reduce fuel continuity and fire spread.

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.

Fire Synchrony and the Influence of Pacific Climate Variability on Wildfires in the Florida Keys, United States

We investigated relationships between climate variability and wildfires in endangered pine rockland communities in the Florida Keys, United States, using fire-scarred samples from the canopy dominant Pinus elliottii var. densa. To test broad-scale, spatiotemporal relationships between wildfires and climate, we compared cross-dated fire-scar chronologies from two islands in the lower Florida Keys, Big Pine Key (BPK) and No Name Key (NNK), to measured values of the Atlantic Multidecadal Oscillation (AMO), North Atlantic Oscillation (NAO), El Niño-Southern Oscillation (ENSO; NIÑO3.4), Pacific Decadal Oscillation (PDO), Interdecadal Pacific Oscillation (IPO), and divisional temperature and precipitation over the period from 1856 to 1956. Large-scale climate anomalies captured by ENSO (NIÑO3.4) and IPO indexes had combined effects on widespread fires. Superposed epoch analysis revealed that widespread fires on BPK occurred during years that were drier than average and when constructive phases (years of combined warm [positive] or cool [negative] phases) of ENSO and the IPO occurred three years and one year prior to fires. Positive phases of the PDO were also significantly associated with widespread fires three years prior to events, but the PDO was not influential one year prior to fires. Although fire years were temporally synchronous between the two islands during the period between 1818 and 1924 (n = 10), we did not find significant relationships between climate and fire on the smaller NNK, which suggests that island size influences the ability to detect broad-scale climate forcing of wildfires in the Florida Keys.

Long-term overstory and understory change following logging and fire exclusion in a Sierra Nevada mixed-conifer forest

In many forests of the western US, increased potential for fires of uncharacteristic intensity and severity is frequently attributed to structural changes brought about by fire exclusion, past land management practices, and climate. Extent of forest change and effect on understory vegetation over time are not well understood, but such information is useful to forest management focused on restoring biodiversity and resilience to these ecosystems. We re-measured three large (4ha) historical “Methods of Cutting” (MC) plots in mixed-conifer forest of the central Sierra Nevada installed in 1929 to evaluate the effects of different logging methods. Trees ⩾10cm were surveyed across the entire plots and understory vegetation (tree seedlings, shrubs, and herbaceous species) was quantified within quadrats in the old-growth condition in 1929 prior to logging, later in 1929 after logging, and again in 2007 or 2008. We also compared forest structure in the MC plots with an adjacent unlogged “control” area and collected fire scar samples from nearby stumps to evaluate the historical fire regime. The contemporary tree density in the MC plots (739treesha−1) was 2.4times greater than the 1929 pre-logging density (314treesha−1). Trees in the small and intermediate size classes (10–75cmdbh) were significantly over-represented, and trees in the larger size classes (>90cmdbh) generally significantly under-represented, compared with historical conditions. The proportion of pine dropped from 37% of tree basal area in 1929 to 21% in 2007/08. Density of small to intermediate sized trees was similar in the contemporary logged and unlogged control plots, suggesting that over the long term, ingrowth may have been influenced more by lack of fire than historical logging. Change to non-tree vegetation was most pronounced for shrub cover, which averaged 28.6% in 1929 but only 2.5% in 2008. CART analysis indicated that the highest shrub cover in 1929 was in areas having four or fewer trees growing within 15m to the south of the quadrat, suggesting that reduced light was the most likely explanation for the decline over time. Herbaceous species richness in 2008 was significantly lower than in 1931, two years after logging, but did not differ significantly from 1929, prior to logging. Understory vegetation should benefit from thinning or prescribed fire treatments that lead to a greater abundance of higher light environments within stands.

Fire History and Age Structure of an Oakpine Forest on Price Mountain, Virginia, USA

Fire history is an important aspect of the natural disturbance pattern of many types of forested ecosystems. Nonetheless, many forests and corresponding management plans lack quantitative information on fire interval, frequency, and seasonality. This project examined the fire history at Price Mountain, Virginia, using fire scar samples and tree-ring analyses from live tree chronologies. Additionally, this project investigated the fire scarring potential of two little-studied species, black gum (Nyssa sylvatica) and sourwood (Oxydendrum arboreum), as well as described the age-structure of the current stand. We hypothesized that fire frequency would be high prior to the fire suppression era, given the proximity to an historical railroad track at the base of the mountain and susceptibility to lightning due to elevation. Six major fire years occurred between 1861 and 1925 at an average interval of 14 years, followed by a period of no fires. Two-thirds of the fires burned early in the season. There was an initial establishment of sourwood and chestnut oak (Quercus prinus) from 1930–1940 as well as another establishment peak between 1950 and 1960 after a major logging event. Pine (Pinus pungens and Pinus rigida) species established between 1870 and 1930. Reconstructed fire history and age structure informs land managers that repeated fires occurred in this Appalachian ridge top forest and that modern forest structure is in part the legacy of historic fires and fire suppression.

Fire history of mixed conifer ecosystems in the Great Basin/Mojave Deserts transition zone, Nevada, USA

Vegetation processes in terrestrial ecosystems are closely linked with wildfire regime, but fire histories at the boundary between the Great Basin and Mojave Deserts of North America are relatively sparse. We investigated wildfire regime and its driving factors before and after Euro-American settlement in high-elevation mixed-conifer ecosystems that are found as “mountain islands” in south-eastern Nevada, USA. Field-based results obtained at the Clover Mountains were compared with those already published for Mt. Irish, less than 100 km away, and also to remotely sensed information provided by the LANDFIRE project, which is commonly used for natural resource management. Annually resolved wildfire history at the Clover Mountains was derived back to year 1500 from fire scar samples taken from 139 ponderosa pines (Pinus ponderosa) located in six stands. During the 1785–2007 period, when at least 20 recorder trees (and a total of 241 fire scars) were available, the Clover Mountains were characterized by frequent (mean fire interval <10 years) low-severity fires, half of which scarred more than 10 % of recorder trees. The 1877 and 1946 fires scarred 50 % or more of recorder trees and spread to four out of six sampled stands. After the 1946 event, the site has experienced a 61-year fire-free period tied to fire suppression activity starting in the mid-1900s. In comparison with Mt. Irish, the Clover Mountains showed a longer mean fire return interval, larger fires, and some patchy high-severity events, even before Euro-American settlement. Variations in ecosystem composition and associated fire regime in these high-elevation mixed-conifer woodlands were not adequately captured by remotely sensed data used for vegetation management, revealing a need for additional field-based assessments of fire regime characteristics in this region.

Human influences on fire regimes and forest structure in the Chihuahuan Desert Borderlands

Recent research in isolated sites in southwestern North America suggests that spatial variation in land use history can result in differences in the dates of fire exclusion onset and the extent to which forests have changed in response to fire cessation. We investigated the relationships among fire, land use, and forest structure in three mountain ranges that straddle the Texas–Mexico border including The Davis Mountains Preserve of The Nature Conservancy and Big Bend National Park in west Texas, and The Maderas del Carmen Protected Area in northern Coahuila, Mexico. We quantified fire regime characteristics (frequency, size, severity, and seasonality) and forest stand structure using dendroecology. Frequent low severity fire was a dominant force shaping forest structure and species composition historically. Mean fire return intervals prior to fire exclusion (1700–1900) ranged from 1 to 29years for fires scarring >25% of the fire-scar samples. The per-sample fire return intervals ranged from 15 to 25years signifying historically frequent fires at any one location on the landscape. The contemporary period was characterized by a doubling of fire return intervals across all three sites, probably as a result of fire exclusion through livestock (sheep) grazing. The presence of older trees in the tree age data suggested that fires were predominantly low in severity and that tree regeneration occurred during fire free intervals. Our results show that the disruption of frequent, low severity fire regimes in our sites resulted in widespread tree regeneration. This study documents the importance of sheep grazing as a causal factor of fire exclusion in west Texas and northern Mexico, which occurred in the early to mid-1900s, well after the 1880s cattle boom.

Fire history in western Patagonia from paired tree-ring fire-scar and charcoal records

Abstract. Fire history reconstructions are typically based on tree ages and tree-ring fire scars or on charcoal in sedimentary records from lakes or bogs, but rarely on both. In this study of fire history in western Patagonia (47–48° S) in southern South America (SSA) we compared three sedimentary charcoal records collected in bogs with tree-ring fire-scar data collected at 13 nearby sample sites. We examined the temporal and spatial correspondence between the two fire proxies and also compared them to published charcoal records from distant sites in SSA, and with published proxy reconstructions of regional climate variability and large-scale climate modes. Two of our three charcoal records record fire activity for the last 4 ka yr and one for the last 11 ka yr. For the last ca. 400 yr, charcoal accumulation peaks tend to coincide with high fire activity in the tree-ring fire scar records, but the charcoal records failed to detect some of the fire activity recorded by tree rings. Potentially, this discrepancy reflects low-severity fires that burn in herbaceous and other fine fuels without depositing charcoal in the sedimentary record. Periods of high fire activity tended to be synchronous across sample areas, across proxy types, and with proxy records of regional climatic variability as well as major climate drivers. Fire activity throughout the Holocene in western Patagonia has responded to regional climate variation affecting a broad region of southern South America that is teleconnected to both tropical- and high-latitude climate drivers-El Niño-Southern Oscillation and the Southern Annular Mode. An early Holocene peak in fire activity pre-dates any known human presence in our study area, and consequently implicates lightning as the ignition source. In contrast, the increased fire activity during the 20th century, which was concomitantly recorded by charcoal from all the sampled bogs and at all fire-scar sample sites, is attributed to human-set fires and is outside the range of variability characteristic of these ecosystems over many centuries and probably millennia.

Spatial and temporal corroboration of a fire‐scar‐based fire history in a frequently burned ponderosa pine forest

Fire scars are used widely to reconstruct historical fire regime parameters in forests around the world. Because fire scars provide incomplete records of past fire occurrence at discrete points in space, inferences must be made to reconstruct fire frequency and extent across landscapes using spatial networks of fire-scar samples. Assessing the relative accuracy of fire-scar fire history reconstructions has been hampered due to a lack of empirical comparisons with independent fire history data sources. We carried out such a comparison in a 2780-ha ponderosa pine forest on Mica Mountain in southern Arizona (USA) for the time period 1937-2000. Using documentary records of fire perimeter maps and ignition locations, we compared reconstructions of key spatial and temporal fire regime parameters developed from documentary fire maps and independently collected fire-scar data (n = 60 plots). We found that fire-scar data provided spatially representative and complete inventories of all major fire years (> 100 ha) in the study area but failed to detect most small fires. There was a strong linear relationship between the percentage of samples recording fire scars in a given year (i.e., fire-scar synchrony) and total area burned for that year (y = 0.0003x + 0.0087, r2 = 0.96). There was also strong spatial coherence between cumulative fire frequency maps interpolated from fire-scar data and ground-mapped fire perimeters. Widely reported fire frequency summary statistics varied little between fire history data sets: fire-scar natural fire rotations (NFR) differed by < 3 yr from documentary records (29.6 yr); mean fire return intervals (MFI) for large-fire years (i.e., > or = 25% of study area burned) were identical between data sets (25.5 yr); fire-scar MFIs for all fire years differed by 1.2 yr from documentary records. The known seasonal timing of past fires based on documentary records was furthermore reconstructed accurately by observing intra-annual ring position of fire scars and using knowledge of tree-ring growth phenology in the Southwest. Our results demonstrate clearly that representative landscape-scale fire histories can be reconstructed accurately from spatially distributed fire-scar samples.

El Niño–Southern Oscillation effect on a fire regime in northeastern Mexico has changed over time

The El Niño Southern Oscillation (ENSO) is a climate-forcing mechanism that has been shown to affect precipitation and the occurrence of wildfires in many parts of the world. In the southern United States and northern Mexico, warm events (El Niño) are associated with moist winter conditions and fewer fires, while cool events (La Niñia) tend to favor dry winters and more fires. We tested this relationship in a region of northeastern Mexico by characterizing the historical fire regime and climatic influences: Fire regimes were reconstructed from fire-scar samples collected from 100 trees in three high-elevation sites on Peña Nevada in southern Nuevo Le6n. The sites were approximately 25 ha each, and the site centers were approximately 1 km apart. The earliest recorded fire occurred in 1521 and the time period we used for analysis was 1645-1929. The sites were characterized by frequent surface fires before the 1920s. In the three sites, mean fire intervals ranged from 8.6 to 9.6 years (all fires) and 11.9 to 18.6 years (fires that scarred > or = 25% of recording trees). The per-tree mean fire return interval was 17 years, and all three sites burned in the same year seven times between 1774 and 1929. After 1929, fires were nearly eliminated in all sites, likely due to human causes. We found a temporal change in the association between ENSO events and fires; before the 1830s La Niña events were significantly associated with fire years, while after the 1830s this association was not significant. In 1998, when the most severe El Niño event of the past century occurred, the three sites experienced severe, stand-replacing fires that killed many trees that had survived multiple surface fires in the past. Prior to the 1830s, fires tended to occur during dry La Niña years, but since then both La Niña and El Niño have been associated with dry years in this region, especially during the last three decades. This result suggests that ENSO effects have changed over time in this location and that phases of ENSO are not consistent indicators of precipitation, fire occurrence, or fire behavior in this area of northeastern Mexico.

Fire regimes of the piñon–juniper woodlands of Big Bend National Park and the Davis Mountains, west Texas, USA

While piñon woodlands cover much of arid North America, surprisingly little is known about the role of fire in maintaining piñon forest structure and species composition. The lack of region-specific fire regime data for piñon–juniper woodlands presents a roadblock to managers striving to implement process-based management. This study characterized piñon–juniper fire regimes and forest stand dynamics in Big Bend National Park (BIBE) and the Davis Mountains Preserve of the Nature Conservancy (DMTNC) in west Texas. Mean fire return intervals were 36.5 and 11.2 years for BIBE and DMTNC, respectively. Point fire return intervals were 150 years at BIBE and 75 years at DMTNC. Tree regeneration in west Texas piñon–juniper woodlands occurred historically during favorable climatic conditions following fire years. The presence of multiple fire scars on our fire-scar samples and the multicohort stands of piñon suggested that low intensity fires were common. This study represents one of the few fire-scar-based fire regime studies for piñon–juniper woodlands. Our results differ from other studies in less topographically dissected landscapes that have identified stand-replacing fire as the dominant fire regime for piñon–juniper woodlands. This suggests that mixed-severity fire regimes are typical across southwestern piñon forests, and that topography is an important influence on fire frequency and intensity.

Dendrochronology-based fire history of mixed-conifer forests in the San Jacinto Mountains, California

The growing public awareness of the increasing number of large wildfires across forested landscapes, coupled with needs of resource base management has accelerated research into forest reference conditions and the historical role of fire in coniferous ecosystems. This work investigates historical fire regimes of mixed-conifer forests in the San Jacinto Mountains of southern California using fire-scar dendrochronology. As such this is the first reconstruction of fire history in the mixed-conifer forests of southern California using landscape-scale systematic-based fire-scar dendrochronology. The pre-historical fire size, seasonality, and frequency within these forests are reconstructed and demonstrated graphically, employing systematic sampling and Geographical Information System (GIS) reconstruction. A 250 m grid system was overlaid upon a 270 ha sample location, and fire-scar samples were collected from each of the grid intersection points. Fire-scar dendrochronology resulted in a 653 years long chronology, indicating a point mean fire return interval of 5.2 years, and an area wide grand mean fire interval of 32.2 years. The majority of fires occurred within latewood or at the ring boundary. Graphic modelling of fire events indicate three-quarters of all fires sampled were less than 6.25 ha in size, but burned over 50% of the area sampled during the period; only a small portion of fires were larger than 18 ha within the sample area. Use of systematic sampling is an important step in modeling long-term frequency and effects of fire on a landscape level, and is invaluable to the long-term management planning.