Fire Rotation Research Articles

Simulating landscape-scale effects of fuels treatments in the Sierra Nevada, California, USA

In many coniferous forests of the western United States, wildland fuel accumulation and projected climate conditions increase the likelihood that fires will become larger and more intense. Fuels treatments and prescribed fire are widely recommended, but there is uncertainty regarding their ability to reduce the severity of subsequent fires at a landscape scale. Our objective was to investigate the interactions among landscape-scale fire regimes, fuels treatments and fire weather in the southern Sierra Nevada, California. We used a spatially dynamic model of wildfire, succession and fuels management to simulate long-term (50 years), broad-scale (across 2.2 × 106 ha) effects of fuels treatments. We simulated thin-from-below treatments followed by prescribed fire under current weather conditions and under more severe weather. Simulated fuels management minimised the mortality of large, old trees, maintained total landscape plant biomass and extended fire rotation, but effects varied based on elevation, type of treatment and fire regime. The simulated area treated had a greater effect than treatment intensity, and effects were strongest where more fires intersected treatments and when simulated weather conditions were more severe. In conclusion, fuels treatments in conifer forests potentially minimise the ecological effects of high-severity fire at a landscape scale provided that 8% of the landscape is treated every 5 years, especially if future fire weather conditions are more severe than those in recent years.

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.

Habitat selection by large herbivores in relation to fire at the Bontebok National Park (1974–2009): the effects of management changes

The Bontebok National Park has long been faced with the dilemma of reconciling the need for short-interval fires, which promote grazing for bontebok, with that for longer-interval fires to maintain plant diversity. We explored habitat selection by various large herbivores in relation to veld age (time since fire), different management regimes, and vegetation type. Taller grass grazers (Cape mountain zebra and red hartebeest) were introduced in the 1980s to prolong the usefulness of older veld to bontebok through a grazing succession. We found that all herbivores favoured young veld and largely avoided veld >5 years old. Zebra and hartebeest competed with bontebok in utilising young veld rather than grazing facilitation occurring. In 2004, the fire rotation was prolonged to favour plant diversity, resulting in reduced availability of young veld. In compensation, the bontebok stocking rate was reduced. Bontebok densities subsequently declined proportionally across veld ages, thereby averting high animal densities on the smaller area of young veld. Fire significantly influenced bontebok habitat use equally during the old and new burning regimes. Herbivores did not distinguish between the two predominant vegetation types at Bontebok National Park. However, previously disturbed areas with Cynodon dactylon lawns were favoured by most herbivores despite not being burnt.

Fire regimes, forest change, and self‐organization in an old‐growth mixed‐conifer forest, Yosemite National Park, USA

Fire is recognized as a keystone process in dry mixed-conifer forests that have been altered by decades of fire suppression, Restoration of fire disturbance to these forests is a guiding principle of resource management in the U.S. National Park Service. Policy implementation is often hindered by a poor understanding of forest conditions before fire exclusion, the characteristics of forest changes since excluding fire, and the influence of topographic or self-organizing controls on forest structure. In this study the spatial and temporal characteristics of fire regimes and forest structure are reconstructed in a 2125-ha mixed-conifer forest. Forests were multi-aged, burned frequently at low severity and fire-return interval, and forest structure did not vary with slope aspect, elevation, or slope position. Fire exclusion has caused an increase in forest density and basal area and a compositional shift to shade-tolerant and fire-intolerant species. The median point fire-return interval and extent of a fire was 10 yr and 115 ha, respectively. The pre-Euro-American settlement fire rotation of 13 yr increased to 378 yr after 1905. The position of fire scars within tree rings indicates that 79% of fires burned in the midsummer to fall period. The spatial pattern of burns exhibited self-organizing behavior. Area burned was 10-fold greater when an area had not been burned by the previous fire. Fires were frequent and widespread, but patches of similar aged trees were < 0.2 ha, suggesting small fire-caused canopy openings. Managers need to apply multiple burns at short intervals for a sustained period to reduce surface fuels and create small canopy openings characteristic of the reference forest. By coupling explicit reference conditions with consideration of current conditions and projected climate change, management activities can balance restoration and risk management.

Fire History within the Lower Elwha River Watershed, Olympic National Park, Washington

Abstract We used fire scars and tree ages to reconstruct a 440-year fire history in a portion of the Elwha River watershed, Olympic National Park, Washington. At least nine fire events occurred in the 1873 ha study area during the period 1568 to 2007. Reconstructed fires occurred in 1568, 1661, 1676, 1729, 1868, 1890, 1898, 1927, and 1977. A 140-year natural fire rotation and a 127-year mean fire return interval were calculated for the study area. Fires were more frequent on south aspects, at low elevations, and in Douglasfir forest types. Fire suppression has likely prevented fires larger than 20 ha over the past century although large fires were also absent during a 140-year period in the 18th and 19th centuries. Continued suppression may limit forest structural heterogeneity and the establishment of early seral species in this area.

Fireline Rotation Analysis

The concepts of fire line rotation and fire line extension are introduced as a complement to the usual rate of spread to describe the movement of surface fires. It is shown that both rotation and translation of the fire line elements contribute to increasing their length and reducing the curvature of the fire line. It is shown that in the case when the wind velocity is not parallel to the rate of spread vector, the transverse convective flux along the fire front induces a change of the rate of spread and consequently a rotation movement of the fire line. Using empirical formulations for the dependence of the rate of spread on wind velocity, a model for the rotational velocity of a wind and slope driven fire front is developed. The model is compared with experimental laboratory results for wind- and slope-induced extension and rotation of linear fire fronts.

Simulating dynamic and mixed-severity fire regimes: A process-based fire extension for LANDIS-II

Fire regimes result from reciprocal interactions between vegetation and fire that may be further affected by other disturbances, including climate, landform, and terrain. In this paper, we describe fire and fuel extensions for the forest landscape simulation model, LANDIS-II, that allow dynamic interactions among fire, vegetation, climate, and landscape structure, and incorporate realistic fire characteristics (shapes, distributions, and effects) that can vary within and between fire events. We demonstrate the capabilities of the new extensions using two case study examples with very different ecosystem characteristics: a boreal forest system from central Labrador, Canada, and a mixed conifer system from the Sierra Nevada Mountains (California, USA). In Labrador, comparison between the more complex dynamic fire extension and a classic fire simulator based on a simple fire size distribution showed little difference in terms of mean fire rotation and potential severity, but cumulative burn patterns created by the dynamic fire extension were more heterogeneous due to feedback between fuel types and fire behavior. Simulations in the Sierra Nevada indicated that burn patterns were responsive to topographic features, fuel types, and an extreme weather scenario, although the magnitude of responses depended on elevation. In both study areas, simulated fire size and resulting fire rotation intervals were moderately sensitive to parameters controlling the curvilinear response between fire spread and weather, as well as to the assumptions underlying the correlation between weather conditions and fire duration. Potential fire severity was more variable within the Sierra Nevada landscape and also was more sensitive to the correlation between weather conditions and fire duration. The fire modeling approach described here should be applicable to questions related to climate change and disturbance interactions, particularly within locations characterized by steep topography, where temporally or spatially dynamic vegetation significantly influences spread rates, where fire severity is variable, and where multiple disturbance types of varying severities are common.

Historical fire and multidecadal drought as context for piñon–juniper woodland restoration in western Colorado

Fire is known to structure tree populations, but the role of broad-scale climate variability is less clear. For example, the influence of climatic "teleconnections" (the relationship between oceanic-atmospheric fluctuations and anomalous weather patterns across broad scales) on forest age structure is relatively unexplored. We sampled semiarid piñon-juniper (Pinus edulis-Juniperus osteosperma) woodlands in western Colorado, U.S.A., to test the hypothesis that woodland age structures are shaped by climate, including links to oceanic-atmospheric fluctuations, and by past fires and livestock grazing. Low-severity surface fire was lacking, as fire scars were absent, and did not influence woodland densities, but stand-replacing fires served as long-rotation (>400-600 years), stand-initiating events. Old-growth stands (>300 years old) were found in 75% of plots, consistent with a long fire rotation. Juniper and piñon age structures suggest contrasting responses during the past several centuries to dry and wet episodes linked to the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO). Juniper density increased slightly during periods of drought, positive (warm) AMO (after approximately 10-year lag), and negative (cool) PDO. In contrast, piñon populations may still be recovering from a long, drought-filled period (AD 1620-1820), with pulses of recovery favored during cool AMO, warm PDO, and above-average moisture periods. Analysis of 20th-century tree establishment and instrumental climate data corroborate the long-term relationships between age structure and climate. After Euro-American settlement (AD 1881), livestock grazing reduced understory grasses and forbs, reducing competition with tree seedlings and facilitating climate-induced increases in piñons. Thus tree populations in these woodlands are in flux, affected by drought and wet periods linked to oceanic-atmospheric variability, Euro-American livestock grazing, and long-rotation, high-severity fires. Reductions in livestock grazing levels may aid ecological restoration efforts. However, given long-term fluctuations in tree density and composition, and expected further drought, thinning or burning to reduce tree populations may be misdirected.

Historical Fire Frequency on Contrasting Slope Facets Along the Mckenzie River, Western Oregon Cascades

Fire has been a critical influence on forest dynamics over much of western North America, and knowledge of historical fire regimes provides a valuable reference point for guiding ecosystem management. Spatial and temporal variations in historical (1550–1997) fire frequency were quantified for a 145-km2 area straddling the McKenzie River in the central western Cascades of Oregon. Fire-scar and tree-origin years estimated from cut stumps in 63 clear-cut sites were used to approximately date fires. The estimated natural fire rotation was 162 years for the presettlement period (1550–1849). Natural fire rotation varied considerably over the period of record, ranging from 40 years for the Euro-American settlement period (1850–1924) to 504 years for the recent (1925–1997) fire-suppression period. Such temporal variation in fire regime may be attributed to a combination of climatic and anthropogenic influences. The study area also experienced 2 periods of widespread fire recorded at a majority of sample sites: one in the mid-1500s and the other from 1850 to 1925. Historical fire frequency has also varied spatially over at least 2 distinct scales. At the slope-facet scale, fire frequency was greatest on the generally south-facing slope to the north of the McKenzie River. At the finer scale of individual 4-ha sites, local topographic influences varied according to a site's broad geographical context north or south of the river. Results suggest that meaningful fire-regime characterization may require information derived from multiple spatial scales.

FIRE AND FOREST HISTORY AT MOUNT RUSHMORE

Mount Rushmore National Memorial in the Black Hills of South Dakota is known worldwide for its massive sculpture of four of the United States' most respected presidents. The Memorial landscape also is covered by extensive ponderosa pine (Pinus ponderosa) forest that has not burned in over a century. We compiled dendroecological and forest structural data from 29 plots across the 517-ha Memorial and used fire behavior modeling to reconstruct the historical fire regime and forest structure and compare them to current conditions. The historical fire regime is best characterized as one of low-severity surface fires with occasional (> 100 years) patches (< 100 ha) of passive crown fire. We estimate that only approximately 3.3% of the landscape burned as crown fire during 22 landscape fire years (recorded at > or = 25% of plots) between 1529 and 1893. The last landscape fire was in 1893. Mean fire intervals before 1893 varied depending on spatial scale, from 34 years based on scar-to-scar intervals on individual trees to 16 years between landscape fire years. Modal fire intervals were 11-15 years and did not vary with scale. Fire rotation (the time to burn an area the size of the study area) was estimated to be 30 years for surface fire and 800+ years for crown fire. The current forest is denser and contains more small trees, fewer large trees, lower canopy base heights, and greater canopy bulk density than a reconstructed historical (1870) forest. Fire behavior modeling using the NEXUS program suggests that surface fires would have dominated fire behavior in the 1870 forest during both moderate and severe weather conditions, while crown fire would dominate in the current forest especially under severe weather. Changes in the fire regime and forest structure at Mount Rushmore parallel those seen in ponderosa pine forests from the southwestern United States. Shifts from historical to current forest structure and the increased likelihood of crown fire justify the need for forest restoration before a catastrophic wildfire occurs and adversely impacts the ecological and aesthetic setting of the Mount Rushmore sculpture.

A spatially explicit model to predict future landscape composition of aspen woodlands under various management scenarios

Quaking aspen ( Populus tremuloides) is declining across the western United States. Aspen habitats are among the most diverse plant communities in this region and loss of these habitats can result in shifts in biodiversity, productivity, and hydrology across a range of spatial scales. Western aspen occurs on the majority of sites seral to conifer species, and long-term maintenance of these aspen woodlands requires periodic fire. Over the past century, fire intervals, extents, and intensities have been insufficient to regenerate aspen stands at historic rates; however the effects of various fire regimes and management scenarios on aspen vegetation dynamics at broad spatial and temporal scales are unexplored. Here we use field data, remotely sensed data, and fire atlas information to develop a spatially explicit landscape simulation model to assess the effects of current and historic wildfire regimes and prescribed burning programs on landscape vegetation composition across two mountain ranges in the Owyhee Plateau, Idaho. Model outputs depict the future structural makeup and species composition of the landscape at selected time steps under simulated management scenarios. We found that under current fire regimes and in the absence of management activities, loss of seral aspen stands will continue to occur over the next two centuries. However, a return to historic fire regimes (burning 12–14% of the modeled landscape per decade) would maintain the majority of aspen stands in early and mid seral woodland stages and minimizes the loss of aspen. A fire rotation of 70–80 years was estimated for the historic fire regime while the current fire regime resulted in a fire rotation of 340–450 years, underscoring the fact that fire is currently lacking in the system. Implementation of prescribed burning programs, treating aspen and young conifer woodlands according to historic fire occurrence probabilities, are predicted to prevent conifer dominance and loss of aspen stands.

Fire history of pinyon–juniper woodlands at upper ecotones with ponderosa pine forests in Arizona and New Mexico

We used maps of fire evidence, fire scar dendrochronology, forest age-structure analysis, and landscape analysis to investigate fire history at pinyon pine ( Pinus edulis Engelm.) – juniper ( Juniperus osteosperma (Torr.) Little, Juniperus scopulorum Sarg.) woodland – ponderosa pine ( Pinus ponderosa P. &amp; C. Lawson) forest ecotones in Arizona (Tusayan) and in New Mexico (Canjilon). Results showed that charred trees were not evenly distributed across vegetative communities but were significantly (p &lt; 0.001) more abundant than expected in ponderosa pine communities. Composite fire scar analysis indicated that surface fires occurred in ponderosa pine stands at both sites and burned at intervals of 7.2–11.1 years (WMPI; Weibull median probability interval). At Tusayan, landscape structure was fine grained, and maximum pinyon age was &gt;200 years across 80% of the site. At Canjilon, landscape pattern was relatively coarse, and most pinyon patches were 200–300 years old. Cumulative standing age distributions suggested pinyon–juniper fire rotations of 340 and 290 years at Tusayan and Canjilon, respectively. We concluded the following: (i) surface fires in ponderosa pine stands did not spread through pinyon–juniper communities at either site, (ii) fire evidence was prevalent across both sites, but old pinyon trees indicated that no widespread lethal fires had occurred in the last 300–400 years, and (iii) structurally heterogeneous landscapes suggested that historical pinyon–juniper fires were of limited extent but lethal in patches.

Fire Regime Attributes of Wildland Fires in Yosemite National Park, USA

Past attempts to suppress all fires in some western forests have altered historic fire regimes. Accumulated debris and dense understories of shade tolerant species coupled with a warmer climate have led to catastrophic wildfires. Prescribed fires and wildland fire use fires are used by land managers to reduce fuels and restore natural conditions. Little is known about how wildfires, prescribed fires, and wildland fire use fires differ in their fire regime attributes. We compared the attributes of start date, duration, 95th percentile burning index, size, fire rotation and fire return interval, and fire severity of 144 fires >40 ha that occurred in Yosemite National Park from 1974 through 2005, and mean patch size and patch squareness (an index of complexity) of 106 fires that occurred from 1984 through 2005. We used fire history, weather records, and the Relative differenced Normalized Burn Ratio (RdNBR) derived from satellite imagery to make the comparisons. Prescribed fires started both earlier and later than wildfires and wildland fire use fires, and had the shortest return intervals. Prescribed fires burned during periods with a lower 95th percentile burning index (an estimate of potential flame length and fire line intensity), and resulted in larger patches that were unchanged or that burned with low severity. Wildfires were the largest in size and had the largest percent area burned at moderate and high severity. Wildfires also had the largest moderate and high severity patches, and the high severity patches were the most square. The significant differences in fire regime attributes suggest that land managers seeking to restore natural fire regimes and forest composition and structure must consider more than the metrics of area burned or fuel loading.

Fire Scarring Patterns in Sierra Nevada Wilderness Areas Burned by Multiple Wildland Fire Use Fires

Uncertainty associated with fire-scar reconstructions of historical fire occurrence has led to questioning both estimates of frequency derived from these methods and the inferences on fire regimes drawn from these estimates. Using information from multiple, naturally-occurring fires (referred to as wildland fire use (WFU) fires) in two Sierra Nevada wilderness areas, we identified forest structural, topographic, and fire characteristics influencing fire scarring in trees and conducted direct comparisons of fire-scar reconstructed fire extent and frequency to fire atlas-based estimates of fire extent and frequency. The most important factor influencing the probability of sampled Pinus jeffreyi trees scarring from WFU fires was the length of time since previous fire. When intervals between successive fires are short, probabilities of scarring were low. Tree basal area and aspect were also significant factors explaining observed pattern of tree scarring. In all WFU fires but one, the reconstructed extent of fires was substantially smaller than the fire atlas extent. As a result, fire-scar reconstructed estimates of fire rotation were much longer than fire atlas fire rotation. This information can provide some necessary insight in interpreting and accounting for uncertainty in fire-scar reconstructions for drier low- to mid-elevation forest types throughout the western United States.

Spatially explicit modeling of mixed-severity fire regimes and landscape dynamics

Simulation models of disturbance and succession are being increasingly applied to characterize landscape composition and dynamics under natural fire regimes, and to evaluate alternative management strategies for ecological restoration and fire hazard reduction. However, we have a limited understanding of how landscapes respond to changes in fire frequency, and about the sensitivity of model predictions to assumptions about successional pathways and fire behavior. We updated an existing landscape dynamics model (LADS) to simulate the complex interactions between forest dynamics, fire spread, and fire effects in dry forests of the interior Pacific Northwest. Experimental model runs were conducted on a hypothetical landscape at fire rotations ranging from 5 to 50 years. Three sensitivity analyses were carried out to explore the responses of landscape composition to (1) parameters characterizing succession and fire effects on vegetation, (2) the probability of fire spread into different successional stages, and (3) the size and spatial pattern of static fire refugia. The area of old open-canopy forests was highest at the shortest fire rotations, and was particularly sensitive to the probability of stand-replacement fire in open-canopy forests and to the fire-free period required for ingrowth to occur in open-canopy forests. The area of old closed-canopy forests increased with lengthening fire rotation, but always comprised a relatively small portion of the landscape (<10%). The area of old closed-canopy forests increased when fire spread was more rapid in open-canopy forests than in closed-canopy forests, and when the physical landscape incorporated large “fire refugia” with low fire spread rates. Old closed-canopy forests appear to comprise a relatively minor landscape component in mixed-severity fire regimes with fire rotations of 50 years or less. However, these results are sensitive to assumptions about the spatial interactions between fire spread, landscape vegetation patterns, and the underlying physical landscape.

Prehistoric fire area and emissions from California's forests, woodlands, shrublands, and grasslands

In the majority of US political settings wildland fire is still discussed as a negative force. Lacking from current wildfire discussions are estimates of the spatial extent of fire and their resultant emissions before the influences of Euro-American settlement and this is the focus of this work. We summarize the literature on fire history (fire rotation and fire return intervals) and past Native American burning practices to estimate past fire occurrence by vegetation type. Once past fire intervals were established they were divided into the area of each corresponding vegetation type to arrive at estimates of area burned annually. Finally, the First Order Fire Effects Model was used to estimate emissions. Approximately 1.8 million ha burned annually in California prehistorically (pre 1800). Our estimate of prehistoric annual area burned in California is 88% of the total annual wildfire area in the entire US during a decade (1994–2004) characterized as “extreme” regarding wildfires. The idea that US wildfire area of approximately two million ha annually is extreme is certainly a 20th or 21st century perspective. Skies were likely smoky much of the summer and fall in California during the prehistoric period. Increasing the spatial extent of fire in California is an important management objective. The best methods to significantly increase the area burned is to increase the use of wildland fire use (WFU) and appropriate management response (AMR) suppression fire in remote areas. Political support for increased use of WFU and AMR needs to occur at local, state, and federal levels because increasing the spatial scale of fire will increase smoke and inevitability, a few WFU or AMR fires will escape their predefined boundaries.

Human and biophysical factors influencing modern fire disturbance in northern Wisconsin

Humans cause most wildfires in northern Wisconsin, but interactions between human and biophysical variables affecting fire starts and size are not well understood. We applied classification tree analyses to a 16-year fire database from northern Wisconsin to evaluate the relative importance of human v. biophysical variables affecting fire occurrence within (1) all cover types, and (2) within forest types in each of four different fire size groupings (all fires; fires ≥0.4 ha (1 acre); fires ≥4 ha (10 acres); fires ≥16 ha (40 acres)). Housing density was the most important indicator of fire observations. Increasing minimum fire size increased the relative importance of biophysical variables. Key biophysical variables included land cover type, soil moisture indicators, and an index of presettlement fire rotation associated with glacial landforms. Our results indicate the likelihood of fire starts is primarily influenced by human activity in northern Wisconsin, whereas biophysical factors determine whether those fire starts become large fires. Important interactions between human and biophysical variables were observed for nearly all fire types and size thresholds examined. Our results have implications for both ecological restoration and the management of fire risk within historically fire-prone systems currently experiencing rapid rural development.

A landscape model quantifies error in reconstructing fire history from scars

Reconstructing fire regimes from fire scars is widely used in fire ecology to understand the role of fire and to determine prescriptions for restoring fire, but systematic analyses of the accuracy of fire-regime reconstruction have never been done. Errors in reconstruction may lead to a misunderstanding of the role of fire or incorrect restoration prescriptions. Here, a stochastic landscape model is used to quantitatively assess the accuracy of a commonly used statistic, the composite fire interval (CFI), as an estimator of the population mean fire interval or fire rotation, which are the central parameters useful in comparing fire regimes. Seven factors, that may affect accuracy, are explored. Two control the fire regime, one controls the scarring process, and the other four define the sampling and analysis procedure. Results show that: (1) CFI can be from 0.15 to 5.0 times the population mean fire interval, a range of more than 30 times; (2) all factors, except population mean fire interval, significantly affect accuracy; (3) accuracy shows predictable patterns that could be useful in designing a sound sampling scheme and choosing a proper analysis method. However, the complexity of effects of these factors and the need for some prior knowledge of them make it difficult to design sampling and analysis procedures to accurately estimate the population mean fire interval.

Geographic analysis of natural fire rotation in the California Redwood Forest during the suppression era

A geographic information system (GIS) was used to analyze the effects of six physical variables (redwood sub-region, slope, aspect, elevation, distance from the coast, and moisture regime) on the natural fire rotation (NFR) of redwood (Sequoia sempervirens) forests between 1950 and 2003. NFR is defined as the years necessary for fires to burn over an area equal to that of the study area. This analysis relied on a spatial database of forest fire locations cataloged by California Department of Forestry and Fire Protection. The NFR for the California range of redwood forests was calculated as 778 years. The NFRs for north, central, and southern redwood sub-regions were 1,083, 717, and 551 years, respectively. The NFR of slope classes varied between 3,309 years for flat terrain and 423 years for steep slopes. The NFRs of north and south aspects were 796 years and 763 years, respectively. Elevation classes of 200 m illustrated a consistent decreasing trend in NFR values, 1,841 in the lowest elevation class and 53 years in the highest. Distance from the coast was classed in 5 km increments up to 50 km with NFR values of 648 years in the 5 km class and 1,148 years in the 50 km class. The NFRs of moisture regime classes were 1,183 years in the wettest class and 117 years for the driest class. Drier moisture regime classes were prevalent in the southern redwood range. The NFR calculated for redwood sub-regions, moisture regimes and elevation classes indicated that periodicity of fire in redwood stands decreased along a north-south, west-east gradient. The NFRs observed in these redwood variables substantiate results from previous work concerning the influence of a north-south, ocean-inland gradient on fire frequency in redwood forests.

Modeling the Impact of Climate and Vegetation on Fire Regimes in Mountain Landscapes

Assessing the long-term dynamics of mountain landscapes that are influenced by large-scale natural and anthropogenic disturbances and a changing climate is a complex subject. In this study, a landscape-level ecological model was modified to this end. We describe the structure and evaluation of the fire sub-model of the new landscape model LandClim, which was designed to simulate climate–fire–vegetation dynamics. We applied the model to an extended elevational gradient in the Colorado Front Range to test its ability to simulate vegetation composition and the strongly varying fire regime along the gradient. The simulated sequence of forest types along the gradient corresponded to the one observed, and the location of ecotones lay within the range of observed values. The model captured the range of observed fire rotations and reproduced realistic fire size distributions. Although the results are subject to considerable uncertainty, we conclude that LandClim can be used to explore the relative differences of fire regimes between strongly different climatic conditions.