Median Fire Return Intervals Research Articles

Regional Drivers behind the Burning of Remanent Forests in Michoacán Avocado Belt, Central Mexico

The state of Michoacán in central Mexico supplies nearly 50% of the global avocado trade in a region known as the Michoacan Avocado Belt or Avocadoland. Fire has been a component associated with regional land-use change processes. We documented fire regime attributes for the period 2000–2017, discussed the use of fire related to the expansion of avocado orchards, and evaluated the role of atmospheric variables and human infrastructure. There was a mean of 276 fires covering 3287 ha of forest per year. Over 80% of the burned area was covered by pine and pine–oak forests, with a strong correlation of ignitions with the distance to urban settlements, roads, agricultural plots, and avocado orchards. There is a median fire return interval of 2–3 years, and the distance to avocado orchards and main roads was highly correlated with fire recurrence. Final users of the international marketing of this fruit may be unaware of the fire-related land-use changes, namely, the damage to biodiversity, forest health, and water bodies, as well as to producers’ well-being, behind the great demand for avocados. The present conditions of insecurity and social conflict must be addressed to guarantee, among other things, the conservation of these diverse forests.

An analysis of the recent fire regimes in the Angolan catchment of the Okavango Delta, Central Africa

BackgroundThis paper presents an analysis of fire regimes in the poorly studied Angolan catchment of the Okavango Delta in Botswana. We used MODIS data to examine the frequency and seasonality of fires over 20 years (from 2000 to 2020) in three dominant vegetation types (miombo woodlands, open woodlands and grasslands, and short closed to open bushlands), and in areas where people were present, and where they were absent.ResultsThe median fire return intervals for both open woodlands and grasslands and short bushlands were relatively short (1.9 and 2.2 years respectively). In miombo woodlands, fires were less frequent (median return periods of 4.5 years). Human population density had no discernible effect on the fire return intervals, but about 14% of the miombo woodlands experienced no fires over 20 years. Ongoing shifting cultivation within miombo woodlands has led to structural changes and the introduction of fire into this vegetation type where fires were rare or absent in the past. About 12% of the miombo did not burn during the period examined where people were present, whereas close to 20% of the sites remained unburnt where people were absent. This suggests that people did not change the fire return interval in any of the vegetation types studied, but that they altered the amount of the landscape that is flammable in miombo vegetation. Fires occurred between June and September, with a peak in the late dry season (August and September).ConclusionsHistorical research indicates that late dry-season fires are detrimental to miombo woodlands, and our analysis suggests that degradation in parts of the catchment has led to the introduction of fire to this previously fire-free and fire-sensitive vegetation type. Deforestation of miombo woodlands, and the consequent introduction of fire, is a cause for concern with respect to the ecological stability of the Okavango Delta. Managers should therefore aim to protect the remaining closed-canopy miombo stands from further clearing and to attempt to shift the timing of burns to the early dry season to reduce their intensity.

Forest Fire History in Amazonia Inferred From Intensive Soil Charcoal Sampling and Radiocarbon Dating

Fire has a historical role in tropical forests related to past climate and ancient land use spanning the Holocene; however, it is unclear from charcoal records how fire varied at different spatiotemporal scales and what sampling strategies are required to determine fire history and their effects. We evaluated fire variation in structurally intact, terra-firme Amazon forests, by intensive soil charcoal sampling from three replicate soil pits in sites in Guyana and northern and southern Peru. We used radiocarbon (14C) measurement to assess (1) locally, how the timing of fires represented in our sample varied across the surface of forest plots and with soil depth, (2) basin-wide, how the age of fires varies across climate and environmental gradients, and (3) how many samples are appropriate when applying the 14C approach to assess the date of last fire. Considering all 14C dates (n = 33), the most recent fires occurred at a similar time at each of the three sites (median ages: 728–851 cal years BP), indicating that in terms of fire disturbance at least, these forests could be considered old-growth. The number of unique fire events ranged from 1 to 4 per pit and from 4 to 6 per site. Based upon our sampling strategy, the N-Peru site—with the highest annual precipitation—had the most fire events. Median fire return intervals varied from 455 to 2,950 cal years BP among sites. Based on available dates, at least three samples (1 from the top of each of 3 pits) are required for the sampling to have a reasonable likelihood of capturing the most recent fire for forests with no history of a recent fire. The maximum fire return interval for two sites was shorter than the time since the last fire, suggesting that over the past ∼800 years these forests have undergone a longer fire-free period than the past 2,000–3,500 years. Our analysis from terra-firme forest soils helps to improve understanding of changes in fire regime, information necessary to evaluate post-fire legacies on modern vegetation and soil and to calibrate models to predict forest response to fire under climate change.

Eastern redcedar dendrochronology links hill prairie decline with decoupling from climatic control of fire regime and reduced fire frequency1

Abstract Human-caused decreased fire frequency allows a shift toward woody vegetation in North American grasslands. Since the 1960s, Midwest hill prairies have declined in area due to invasion by Juniperus virginiana L. (eastern redcedar) and encroachment of deciduous woody vegetation. This process has been thought to result from reduced fire frequency but supportive data have not been available. We used tree-ring analysis of J. virginiana to develop chronologies of tree growth and fire scars since the mid-1800s at Fults Hill Prairie Nature Preserve, Monroe County, IL, and used them to understand effects of historical climate and changing fire regimes on loss of prairie habitat. Tree-ring analysis indicated spread of J. virginiana from escarpment habitat into prairie after 1960. Linear regression of time series data from aerial photos projected 80–100% loss of prairie cover before the year 2040. The J. virginiana fire scar chronology revealed a < 2-yr median fire return interval between 1850 and 1959. The...

Spatial and temporal variations of fire regimes in the Canadian Rocky Mountains and Foothills of southern Alberta

Like many fire-adapted ecosystems, decades of fire exclusion policy in the Rocky Mountains and Foothills natural regions of southern Alberta, Canada are raising concern over the loss of ecological integrity. Departure from historical conditions is evaluated using median fire return intervals (MdFRI) based on fire history data from the Subalpine (SUB), Montane (MT) and Upper Foothills (UF) natural subregions. Fire severity, seasonality and cause are also documented. Pre-1948 MdFRI ranged between 65 and 85 years in SUB, between 26 and 35 years in MT and was 39 years in UF. The fire exclusion era resulted in a critical departure of 197–223% in MT (MdFRI = 84–104 years). The departure in UF was 170% (MdFRI = 104 years), while regions of continuous fuels in SUB were departed by 129% (MdFRI = 149 years). The most rugged region of SUB is within its historical range of variation with a departure of 42% (MdFRI = 121 years). More mixed-severity burning took place in MT and UF. SUB and MT are in a lightning shadow pointing to a predominance of anthropogenic burning. A summer fire season prevails in SUB, but occurs from spring to fall elsewhere. These findings will assist in developing fire and forest management policies and adaptive strategies in the future.

Resistance of mixed subalpine forest to fire frequency changes: the ecological function of dwarf pine (Pinus mugo ssp. mugo)

The availability of fuel and climate are major factors responsible for forest fire activity over time. Here, we tested the hypothesis that forest ecosystems containing a high shrub biomass, which constitutes a fuel load, and affected by a warmer climate, which is associated with drier conditions and a longer fire season, are more prone to fire. Fire occurrence and woody vegetation histories were reconstructed for a subalpine site (Lago di Colbricon Inferiore) in the Dolomites, part of the eastern Italian Alps, for the past 13,000 years. The modern wet climate prevents fire in this area, in spite of the warm summers and an abundant biomass of dwarf pine (Pinus mugo) and three other conifer tree species (Pinus cembra, Picea abies, and Larix decidua). Past fire history reconstructed from sedimentary charcoal showed a median fire return interval of 140 years (30–735 yr fire−1), with a high variability (SD ± 170 years) throughout the Holocene, suggesting that the past environment was more favourable to fire than the modern one, probably due to a drier climate or to different fuel availability. The subalpine community containing P. mugo remained stable for the past 9000 years, despite the variability of the fire return interval. Interestingly, the fire frequency is higher at Lago di Colbricon than at sites in the western Alps that lack P. mugo, suggesting that this species tolerates fire disturbance. In fact, it probably favours the spread of fire due to its flammable biomass, prostrated form, and dense layering canopy, thus offsetting the influence of the wet climate. Since the 19th century, the removal of dwarf pine to promote subalpine grasslands may have suppressed fires in this region.

Varying influence of climate and aspect as controls of montane forest fire regimes during the late Holocene, south‐eastern British Columbia, Canada

AbstractAimTop‐down (climatic) controls of fire occurrence are expected to homogenize fire regimes in a given area over long (millennial) temporal scales. Previous investigations in south‐eastern British Columbia have shown that bottom‐up (local site) factors can override long‐term climate as a dominant control. Here, we examine the interactions between fire regime controls using five 5000‐year‐long lake sediment records.LocationAll lakes are located in separate watersheds within a 550‐km2region ofEngelmann spruce–subalpine fir forest in theColumbia Mountains,BritishColumbia, Canada.MethodsSedimentary macroscopic (&gt; 150 μm) charcoal analysis was used to produce local fire history records. Fire events were identified by decomposing the charcoal concentration series using the programCharAnalysis. We analysed the temporal coherence of the reconstructed fire events and the distributions of the fire return intervals at all sites and pooled sites by their north‐facing or south‐facing aspect.ResultsSince 5000 cal. yrbp, fire frequency was highest between 4250 and 2750 cal. yrbpand lowest from 750 cal. yrbpto the present. Median fire return intervals were shorter on the warmer and drier south‐facing slopes (135–190 years) and longer on the cooler and moister north‐facing slopes (226–241 years). Significant synchrony existed between sites with similar aspect, but no synchrony was found at sites with opposing aspect, providing evidence for the importance of bottom‐up controls. Smoothed fire frequencies suggested that the influence of aspect varied throughout the period and that the importance of aspect could be overridden by other controls. The asynchrony between sites with opposite aspects suggests that local conditions for fire are important spatial controls on the fire regime.Main conclusionsAspect is an important bottom‐up control of fire regimes in mid‐elevation forests and its influence varies through time. The variability of climate–fire–vegetation interactions in the region needs to be investigated to understand the importance of top‐down and bottom‐up controls.

Historical fire regimes in a poorly understood, fire-prone ecosystem: eastern coastal fynbos

We characterised the historical fire regime (1900–2010) in eastern coastal fynbos shrublands, which occur in a poorly studied part of the Cape Floral Kingdom (CFK). Natural (lightning-ignited) fires dominated the fire regime. Fire seasonality decreased from west (Outeniqua region) to east (Tsitsikamma region) within the study area, and between the study area and further west in the CFK. This is consistent with a west–east climatic gradient in the CFK, where rainfall is concentrated in winter in the west, and evenly distributed across months in the east. Median fire return intervals (FRIs) (1980–2010) were broadly comparable to other fynbos areas but estimates varied widely depending on whether or not the data were censored (16–26 years with and 8–13 years without censoring). FRIs appeared to be shorter in the Tsitsikamma, where rainfall and plant growth rates are higher, than in the Outeniqua. The total area burnt annually has increased significantly since 1980, coinciding with an increase in weather conducive to fires, suggesting that fire regimes may be responding to climate change. Frequent recurrence of very large fires and the virtual absence of vegetation in older post-fire age classes are potential causes for concern in achieving fynbos conservation objectives.

A Summary of Fire Frequency Estimates for California Vegetation before Euro-American Settlement

California fire regimes have been altered from those that occurred prior to Euro-American settlement, and are predicted to continue to change as global climates warm. Inclusion of fire as a landscape-level process is considered essential to successful ecological restoration in many ecosystems, and presettlement fire regimes provide foundational information for restoration or “realignment” of ecosystems as climate change and land use changes progress. The objective of our study was to provide an up-to-date, comprehensive summary of presettlement fire frequency estimates for California ecosystems dominated by woody plants, and to supply the basis for fire return interval departure (FRID) mapping and analysis in California. Using the LANDFIRE Biophysical Settings (BpS) vegetation-fire regime types as a framework, we used literature review and the outcomes of regional expert workshops to develop twenty-eight presettlement fire regime (PFR) groups based on similarity of their relationships with fire. We then conducted an exhaustive review of the published and unpublished literature pertaining to fire return intervals (FRIs) observed prior to significant Euro-American settlement in the twenty-eight PFRs, and summarized the values to provide a single estimate of the mean, median, mean minimum, and mean maximum FRI for each PFR.Much variability was evident among PFRs, with mean FRIs ranging from 11 yr to 610 yr, and median FRIs ranging from 7 yr to 610 yr; mean minimum FRIs ranged from 5 yr to 190 yr, and mean maximum FRIs ranged from 40 yr to 1440 yr. There was also high variability within many PFRs, and differences between minimum and maximum FRIs ranged from 32 yr to 1324 yr. Generally, median FRIs were lowest for productive drier forests such as yellow pine, dry and moist mixed conifer, and oak woodland (7 yr, 9 yr, 12 yr, and 12 yr, respectively). Median FRIs were highest for less productive woodlands such as pinyon-juniper (94 yr), high elevation types such as subalpine forest (132 yr), very dry types such as desert mixed shrub (610 yr), and productive moist forests such as spruce-hemlock (275 yr mean FRI). Our summary of California’s presettlement fire regimes should be a useful reference for scientists and resource managers, whether they are seeking a general estimate of the central tendency and variability of FRIs in a broadly defined vegetation type, background information for a planned restoration project or a mechanistic model of vegetation-fire interactions, or a list of literature pertaining to a specific vegetation type or geographic location.

A 400-year history of fires on lake islands in south-east Sweden

Island-lake ecosystems are suitable for testing scale dependence in forests disturbance theories thanks to differences in the potential for fire spread on islands and the mainland. We investigated past fire regime on the mainland and on islands in a large lake in south-east Sweden. We used dendrochronological methods to reconstruct fire disturbances on 18 small islands (0.04–24.1 ha) and in 43 sites in the surrounding 75-km2 landscape over the last 400 years. In the past, fires were frequent on both islands and mainland but not synchronised on an annual scale. Significant temporal changes occurred around the middle of the 18th century. Before 1750, fires were less frequent on islands than on the mainland (median fire return interval 58 v. 25 years respectively). However, an inversion of this pattern was observed during 1750–1860: islands showed even shorter fire intervals than mainland locations, suggesting additional and likely human-related source of ignitions (median fire return interval 15 v. 29 years respectively). A substantial decrease in fire activity in both islands and mainland was apparent in 1860–1890. We suggest that the present fire regime (the last 100 years) on the small islands is largely natural as fire suppression is not present there. The dynamic nature of the fire regime on islands still requires further studies: islands may, at times, attract lightning, humans with fire, or both.

Fire history and the establishment of oaks and maples in second-growth forests

We used dendrochronology to examine the influence of past fires on oak and maple establishment. Six study units were located in southern Ohio, where organized fire control began in 1923. After stand thinning in 2000, we collected basal cross sections from cut stumps of oak (n = 137) and maple (n = 204). The fire history of each unit was developed from the oaks, and both oak and maple establishment were examined in relation to fire history. Twenty-six fires were documented from 1870 to1933; thereafter, only two fires were identified. Weibull median fire-return intervals ranged from 9.1 to 11.3 years for the period ending 1935; mean fire occurrence probabilities (years/fires) for the same period ranged from 11.6 to 30.7 years. Among units, stand initiation began ca. 1845 to 1900, and virtually no oak recruitment was recorded after 1925. Most maples established after the cessation of fires. In several units, the last significant fire was followed immediately by a large pulse of maple establishment and the cessation of oak recruitment, indicating a direct relationship between fire cessation and a shift from oak to maple establishment.

Soil and sedimentary charcoal evidence for Holocene forest fires in an inland temperate rainforest, east-central British Columbia, Canada

Accelerator radiocarbon dating of 147 charcoal samples recovered from colluvial and alluvial fan deposits at 29 sites was used to reconstruct the Holocene fire history of an inland temperate rainforest watershed in east-central British Columbia (BC), Canada. Radiocarbon dates ranged from 182 to 9558 cal. yr BP, with prominent peaks in the probability distribution of calibrated dates at e. 7100, 3900, 2300, 1600 and 250-1000 cal. yr BP. The inferred median fire return interval (FRI) was 800-1200 cal. yr, depending on the extent of inbuilt age errors resulting from charring of wood pre-dating actual fire ages. This FRI is likely an overestimate, as less severe events may not have created sufficient erosion and slope instability to preserve a record of charcoal in buried soils and slope deposits. Median time since fire was 467 cal. yr based on ages of the uppermost charcoal found at each site, but the severity of heart-rots in the dominant redcedars (Thuja plicata Donn ex D. Don) prevented independent confirmation of stand ages by dendrochronology. Sites with multiple charcoal-containing layers having similar radiocarbon ages can be explained with reference to contemporary post-fire mass-wasting processes. Peaks in fire-related sedimentation probability coincided broadly with periods of higher fire frequency c. 600-1000, 1300-2400 and 3500-4500 cal. yr BP inferred from sedimentary charcoal records at subalpine sites in southwestern BC. Correspondence with fire records from more distant sites in northwestern North America was less clear.

Fire History in Coast Redwood Stands in the Northeastern Santa Cruz Mountains, California

Fire regimes in coast redwood forests in the northeastern Santa Cruz Mountains were determined by ring counts from 48 coast redwood stumps, downed logs, and live trees. Degradation of remnant materials from post-harvest fires severely limited available fire scars in this region. The earliest recorded fire was recorded in approximately 1615 and the last fire recorded was in 1884. The Ohlone and early immigrants were probably the primary source of ignitions in this region. For all sites combined, the mean fire return interval (FRI) was 12.0 years; the median FRI was 10 years. There was no significant difference in FRI’s between plot aspects but there was a significant difference in MFI between the four sampled sites. The grand mean FRI for single trees (point) was 16.3 years. Past fire scars occurred most frequently in the latewood portion of the annual ring or during the dormant period. It is probable that the number of fires recorded in coast redwood trees is a subset of those fires that burned in adjacent grasslands and oak savannahs. Continued development of old-growth and young-growth coast redwood forests toward prehistoric conditions may be dependent of a fire regime where prescribed burning substitutes for the now-absent aboriginal ignitions.

Dendrochronology-based fire history of Jeffrey pine - mixed conifer forests in the Sierra San Pedro Martir, Mexico

Conifer forests in northwestern Mexico have not experienced systematic fire suppression or logging, making them unique in western North America. Fire regimes of Pinus jeffreyi Grev. &amp; Balf. mixed conifer forests in the Sierra San Pedro Martir, Baja California, Mexico, were determined by identifying 105 fire dates from 1034 fire scars in 105 specimens. Fires were recorded between 1521 and 1980 and median fire return intervals were less than 15 years at all compositing scales. Significant differences in mean fire return intervals were detected from 1700 to 1800, 1800 to 1900, and 1900 to 1997, most often at intermediate spatial compositing scales, and the proportion of trees scarred in the fires of the 1700s was significantly different from the fires of either the 1800s, the 1900s, or the combined post-1800 period. Superposed epoch analysis determined that moderate and large spatial scale fires occurred on significantly dry years during the length of the record, but before 1800, these fires were preceded by significantly higher precipitation 1 year before the fire. The dominance of earlywood fires in the Sierra San Pedro Martir is similar to the seasonality found in the southwest United States and is different from the western slope of the Sierra Nevada and Klamath Mountains of California.

Fire history differences in adjacent Jeffrey pine and upper montane forests in the eastern Sierra Nevada

Fire history and forest structural characteristics of adjacent Jeffrey pine (Pinus jeffreyi) and upper montane forests was investigated in the eastern Sierra Nevada at the University of California Valentine Natural Reserve. Jeffrey pine forests had lower canopy cover, higher amounts of fine fuels, and higher shrub cover when compared to upper montane forest that were dominated by red fir (Abies magnifica). Fire dates were determined using standard dendrochronolgy techniques from fire-scarred Jeffrey pine, lodgepole pine (Pinus contorta var. murrayana), red f ir, and western white pine (Pinus monticola) trees, snags, stumps, and downed logs. Fires were recorded from 1745 to 1889 and mean fire return intervals were 9 and 24.7 years for the Jeffrey pine and upper montane forest types, respectively. The median fire return interval was 9.0 years for Jeffrey pine and 24.0 years for upper montane forests. Significant differences were found in mean fire intervals and fire history distributions between the two similarly sized fire history plots even though they were only separated by approximately 100 m. This study suggests that fire regimes can vary over very fine spatial scales. Differences in fire regimes are likely due to differences in fuel beds and fire behavior.

Fire regimes and forest changes in mid and upper montane forests of the southern Cascades, Lassen Volcanic National Park, California, U.S.A.

Abstract Aim Spatial and temporal variation in fire regime parameters and forest structure were assessed. Location A 2630‐ha area of mid‐ and upper montane forest in Lassen Volcanic National Park (LVNP). Methods Two hypotheses were tested concerned with fire‐vegetation relationships in southern Cascades forests: (1) fire regime parameters (return interval, season of burn, fire size, rotation period) vary by forest dominant, elevation and slope aspect; and (2) fire exclusion since 1905 has caused forest structural and compositional changes in both mid‐ and upper montane forests. The implications of the study for national park management are also discussed. Results Fire regime parameters varied by forest compositional group and elevation in LVNP. Median composite and point fire return intervals were shorter in low elevation Jeffrey pine (Pinus jeffreyi) (JP) (4–6 years, 16 years) and Jeffrey pine–white fir (Abies concolor) (JP‐WF) (5–10 years, 22 years) and longer in high elevation red fir (Abies magnifica)— western white pine (Pinus monticola) (RF‐WWP) forests (9–27 years, 70 years). Median fire return intervals were also shorter on east‐facing (6–9 years, 16.3 years) and longer on south‐ (11 years, 32.5 years) and west‐facing slopes (22–28 years, 54‐years) in all forests and in each forest composition group. Spatial patterns in fire rotation length were the same as those for fire return intervals. More growing season fires also occurred in JP (33.1%) and JP‐WF (17.5%) than in RF‐WWP (1.1%) forests. A dramatic decline in fire frequency occurred in all forests after 1905. Conclusions Changes in forest structure and composition occurred in both mid‐ and upper montane forests due to twentieth‐century fire exclusion. Forest density increased in JP and JP‐WF forests and white fir increased in JP‐WF forests and is now replacing Jeffrey pine. Forest density only increased in some RF‐WWP stands, but not others. Resource managers restoring fire to these now denser forests need to burn larger areas if fire is going to play its pre‐settlement role in montane forest dynamics.

Fire history and landscape dynamics in a late-successional reserve, Klamath Mountains, California, USA

The frequency, extent, and severity of fires strongly influence development patterns of forests dominated by Douglas-fir in the Pacific Northwest. Limited data on fire history and stand structure suggest that there is geographical variation in fire regimes and that this variation contributes to regional differences in stand and landscape structure. Managers need region-specific fire regime data to develop process-based management schemes to manage new late-successional reserves (LSR). This study quantifies fire regimes and stand structural patterns in a LSR in Douglas-fir-dominated forests in northern California. We analyzed tree species composition, structure (diameter, age), and fire scars from 75 plots in a 1570 ha area in the northern Klamath Mountains. Tree species composition varied with elevation and aspect, and median fire return intervals were similar (12–19 years) among species composition groups. However, median fire return intervals (FRI) were shorter on south- (8 years) and west-facing (13 years) slopes than on northern (15 years) or eastern (16.5 years) aspects. Fire return intervals also varied by historical period. Median FRIs were longer (21.8 years) during the suppression period (1905–1992) than in the settlement (1850–1904) (12.5 years) or presettlement (1627–1849) (14.5 years) period. The average burn area for a fire was 350 ha, and 16 fires larger than 500 ha burned between 1627 and 1992. Fire rotations varied by century from 15.5 to 25.5 years and were longest in the fire suppression period. Stand conditions were multi-aged, and Douglas-fir recruitment occurred after fire. Patterns of past fire severity, inferred from age-classes, indicate that upper slopes, ridgetops, and south- and west-facing slopes experienced more severe fires between 1850 and 1950 than lower slopes or east- and north-facing slopes. Implications are that lower slopes and north and east aspects are more likely than other topographic positions to sustain or promote long-term, late-successional conditions. Prescribed fire will likely be an integral component of management plans that successfully maintain natural processes and structures in newly established late-successional reserves in the Klamath Mountains.