Low-severity Surface Fires Research Articles

Low-severity fires in the boreal region: reproductive implications for black spruce stands in between stand-replacing fire events.

Stand-replacing crown fires are the most prevalent type of fire regime in boreal forests in North America. However, a substantial proportion of low-severity fires are found within fire perimeters. Here we aimed to investigate the effects of low-severity fires on the reproductive potential and seedling recruitment in boreal forests stands in between stand-replacing fire events. We recorded site and tree characteristics from 149 trees within twelve sites dominated by mature black spruce [Picea mariana (Mill.) B.S.P.] trees in the Northwest Territories, Canada. The presence of fire-scarred trees supported classification of sites as unburned or affected by low-severity fires in recent history. We used non-parametric tests to evaluate differences in site conditions between unburned and low-severity sites, and mixed effect models to evaluate differences in tree age, size, and reproductive traits among unburned trees and trees from low-severity sites. Results showed significantly higher density of dead black spruce trees in low-severity sites, and marginally significant higher presence of permafrost. Trees from low-severity fire sites were significantly older, exhibited significantly lower tree growth, and showed a tendency towards a higher probability of cone presence and percentage of open cones compared to trees from unburned sites. Surviving fire-scarred trees affected by more recent low-severity fires showed a tendency towards higher probability of cone presence and cone production. Density of black spruce seedlings significantly increased with recent low-severity fires. Trees in low-severity sites appeared to have escaped mortality from up to three fires, as indicated by fire scar records and their older ages. Shallow permafrost at low-severity sites may cause lower flammability, allowing areas to act as fire refugia. Low-severity surface fires temporarily enhanced the reproductive capacity of surviving trees and the density of seedlings, likely as a stress response to the fire event.

The response of epiphytic lichens on living and dead Pinus sylvestris to prescribed fires of varying severity

Prescribed burning can be used to restore forest ecosystems degraded by anthropogenic pressures. However, some species such as epiphytic lichens may be vulnerable to fire. We studied the effects of fire on epiphytic lichens on living and dead Pinus sylvestris (Scots pine) by surveying lichens up to two meters on the trunks 21 years after experimental and replicated prescribed burnings in pine-dominated boreal forests in Finland. We investigated three types of stands that had faced different levels of fire severity: 1) unburned semi-natural mature stands (i.e., control sites), 2) burned semi-natural mature stands, and 3) burned retention groups on clearcut sites. In the semi-natural mature stands, the fires were low-severity surface fires and induced negligible mortality in P. sylvestris. In the retention groups, the fires induced high tree mortality and made fresh deadwood the dominant substrate type. Low-severity fires reduced the occurrence of several species, but usually did not eliminate them. High-severity fires eliminated some species, but the abundant deadwood frequently hosted some species that were absent or infrequent on the unburned sites. Thus, the fires had only small effects on total species richness. However, fires substantially changed species composition: they reduced the occurrence of species associated with late-successional habitats, while increasing the occurrence of early-successional species. We also found fires to decrease the occurrence of microlichens and increase that of macrolichens. The magnitude of the compositional changes increased with fire severity. Our findings indicate that the high quantities of fresh deadwood in post-disturbance forests provide habitat only to a limited portion of deadwood-associated lichens. Fire can increase landscape-scale variation in epiphytic lichen communities, but we observed negative effects, which increased with fire severity, on lichen flora of conservation concern. Thus, we recommend prescribed burning to be applied primarily in sites with a history of intensive management. Fire should be avoided in sites with substrates that are fire-sensitive and important to lichen diversity.

Paleoecology provides context for conserving culturally and ecologically important pine forest and barrens communities.

In fire-prone ecosystems, knowledge of vegetation-fire-climate relationships and history of fire suppression and Indigenous cultural burning can inform discussions of how to use fire as a management tool, particularly as climate continues to change rapidly. On Wiisaakodewan-minis/Stockton Island in the Apostle Islands National Lakeshore of Wisconsin, USA, structural changes in a pine-dominated natural area containing a globally rare barrens community occurred after cessation of cultural burning by Indigenous Ojibwe people and the imposition of fire-suppression policies, leading to questions about the historical role of fire in this culturally and ecologically important area. To help better understand the ecological context needed to steward these pine forest and barrens communities, we developed paleoecological records of vegetation, fire, and hydrological change using pollen, charcoal, and testate amoebae preserved in peat and sediment cores collected from bog and lagoon sediments within the pine-dominated landscape. Results indicated that fire has been an integral part of Stockton Island ecology for at least 6000 years. Logging in the early-1900s led to persistent changes in island vegetation, and post-logging fires of the 1920s and 1930s were anomalous in the context of the past millennium, likely reflecting more severe and/or extensive burning than in the past. Before that, the composition and structure of pine forest and barrens changed little, perhaps due to regular low-severity surface fires, which may have occurred with a frequency consistent with Indigenous oral histories (~4-8 years). Higher-severity fire episodes, indicated by large charcoal peaks above background levels in the records, occurred predominantly during droughts, suggesting that more frequent or more intense droughts in the future may increase fire frequency and severity. Persistence of pine forest and barrens vegetation through past periods of climatic change indicates considerable ecological resistance and resilience. Future persistence in the face of climate changes outside this historical range of variability may depend in part on returning fire to these systems. This article is protected by copyright. All rights reserved.

Holocene wildfire and vegetation dynamics in Central Yakutia, Siberia, reconstructed from lake-sediment proxies

Wildfires play an essential role in the ecology of boreal forests. In eastern Siberia, fire activity has been increasing in recent years, challenging the livelihoods of local communities. Intensifying fire regimes also increase disturbance pressure on the boreal forests, which currently protect the permafrost beneath from accelerated degradation. However, long-term relationships between changes in fire regime and forest structure remain largely unknown. We assess past fire-vegetation feedbacks using sedimentary proxy records from Lake Satagay, Central Yakutia, Siberia, covering the past c. 10,800 years. Results from macroscopic and microscopic charcoal analyses indicate high amounts of burnt biomass during the Early Holocene, and that the present-day, low-severity surface fire regime has been in place since c. 4,500 years before present. A pollen-based quantitative reconstruction of vegetation cover and a terrestrial plant record based on sedimentary ancient DNA metabarcoding suggest a pronounced shift in forest structure toward the Late Holocene. Whereas the Early Holocene was characterized by postglacial open larch-birch woodlands, forest structure changed toward the modern, mixed larch-dominated closed-canopy forest during the Mid-Holocene. We propose a potential relationship between open woodlands and high amounts of burnt biomass, as well as a mediating effect of dense larch forest on the climate-driven intensification of fire regimes. Considering the anticipated increase in forest disturbances (droughts, insect invasions, and wildfires), higher tree mortality may force the modern state of the forest to shift toward an open woodland state comparable to the Early Holocene. Such a shift in forest structure may result in a positive feedback on currently intensifying wildfires. These new long-term data improve our understanding of millennial-scale fire regime changes and their relationships to changes of vegetation in Central Yakutia, where the local population is already being confronted with intensifying wildfire seasons.

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.

Cambio postincendio en la estructura y composición del estrato arbóreo y carga de combustibles en un bosque de Pinus douglasiana de México

En los bosques de pino con un régimen histórico de incendios superficiales frecuentes de baja severidad, bajo condiciones extremas del estado del tiempo o a consecuencia de la supresión del fuego, la severidad de los incendios puede incrementar. La respuesta de la vegetación a eventos de incendios severos se estudió en una cronosecuencia postincendio de 8 años, 28 años y 60 años en un bosque dominado por Pinus douglasiana en México. La densidad de árboles con diámetro > 2.5 cm y su área basal alcanzaron valores de 1095 árboles ha-1 ± 609 árboles ha-1 y 10.0 m2 ha-1 ± 2.2 m2 ha-1 a los 8 años; a los 28 años el área basal (44.6 m2 ha-1 ± 3.0 m2 ha-1) fue cercana a la de los rodales de 60 años (49.0 m2 ha-1 ± 6.4 m2 ha-1). Los pinos mantuvieron el mayor valor de importancia con el tiempo postincendio, se observó la formación de un subdosel de latifoliadas tolerantes a la sombra y un aumento en la riqueza y diversidad de especies; también se documentó que periodos largos sin incendios incrementan la carga de combustibles superficiales ligeros (66.0 Mg ha-1 ± 5.5 Mg ha-1 en los sitios de 28 años y 61.5 Mg ha-1 ± 4.6 Mg ha-1 en los sitios de 60 años) y modifican el comportamiento del fuego (velocidad de propagación de 1.3 m s-1 a 4.1 m s-1 y la longitud de la llama de 0.7 m a 1.7 m entre los rodales de 8 años y los de 28 años y 60 años combinados). El manejo de los bosques de pino debe considerar el balance entre objetivos de conservación biológica y mitigación del peligro de incendios de alta severidad.

Can wildland fire management alter 21st‐century subalpine fire and forests in Grand Teton National Park, Wyoming, USA?

In subalpine forests of the western United States that historically experienced infrequent, high-severity fire, whether fire management can shape 21st-century fire regimes and forest dynamics to meet natural resource objectives is not known. Managed wildfire use (i.e., allowing lightning-ignited fires to burn when risk is low instead of suppressing them) is one approach for maintaining natural fire regimes and fostering mosaics of forest structure, stand age, and tree-species composition, while protecting people and property. However, little guidance exists for where and when this strategy may be effective with climate change. We simulated most of the contiguous forest in Grand Teton National Park, Wyoming, USA to ask: (1) how would subalpine fires and forest structure be different if fires had not been suppressed during the last three decades? And (2) what is the relative influence of climate change vs. fire management strategy on future fire and forests? We contrasted fire and forests from 1989 to 2098 under two fire management scenarios (managed wildfire use and fire suppression), two general circulation models (CNRM-CM5 and GFDL-ESM2M), and two representative concentration pathways (8.5 and 4.5). We found little difference between management scenarios in the number, size, or severity of fires during the last three decades. With 21st-century warming, fire activity increased rapidly, particularly after 2050, and followed nearly identical trajectories in both management scenarios. Area burned per year between 2018 and 2099 was 1,700% greater than in the last three decades (1989-2017). Large areas of forest were abruptly lost; only 65% of the original 40,178ha of forest remained by 2098. However, forests stayed connected and fuels were abundant enough to support profound increases in burning through this century. Our results indicate that strategies emphasizing managed wildfire use, rather than suppression, will not alter climate-induced changes to fire and forests in subalpine landscapes of western North America. This suggests that managers may continue to have flexibility to strategically suppress subalpine fires without concern for long-term consequences, in distinct contrast with dry conifer forests of the Rocky Mountains and mixed conifer forest of California where maintaining low fuel loads is essential for sustaining frequent, low-severity surface fire regimes.

Nature of Forest Fires in Uttarakhand:Frequency, Size and Seasonal Patterns in Relation to Pre-Monsoonal Environment

Man-made forest fires in the traditionally populated zone (about 800-2000 m altitude) are common in much of the Central Himalaya, and are a major topic of environmental debate. This study based on an analysis of data of the State Forest Department at Uttarakhand on incidence of forest fires shows that these are high-frequency, low-severity surface fires of small size, largely determined by the moisture conditions of the pre-monsoon season (from March to mid-June), and the traditional practices of biomass collection by local people.

Climatic influences on fire regimes in ponderosa pine forests of the Zuni Mountains, NM, USA

We characterized fire history and examined climate–fire relationships in dry ponderosa pine (Pinus ponderosa) forests in the Zuni Mountains of northwestern New Mexico. Our findings indicate that the historical wildfire regime for the study area was typified by high-frequency, low-severity surface fires. Climate–wildfire relationships were assessed using both Superposed Epoch Analysis (SEA) and Bivariate Event Analysis (BEA). SEA revealed that interannual variability of the El Niño–Southern Oscillation (ENSO) and the Palmer Drought Severity Index (PDSI) was a strong driver of widespread wildfires; wetter conditions often occurred one to two years prior to fire and were followed by drought during the fire year. BEA revealed statistically significant relationships only in the case of extreme PDSI events and widespread wildfire in the year of fire (t=0). No relationship was found between either the Pacific Decadal Oscillation (PDO) or the Atlantic Multidecadal Oscillation (AMO) and widespread fire occurrence, signifying that shorter-term (i.e. interannual) oscillations between wet and dry conditions, rather than longer-term climatic variability, were historically most conducive to fire. Contextualizing these findings under ongoing climate and land-use change is challenging. Full restoration to historical conditions may be neither possible nor desired, but targeted efforts to promote a frequent, low-severity wildfire regime are generally consistent with our understanding of historical fire activity in this forest type and may foster ecological, economic, and/or societal goals.

Changes of ground vegetation, soil chemical properties and microbiota following the surface fires in Scots pine forests

Abstract The aim of the study was to investigate the influence of low severity surface fires on the ground vegetation, soil chemical changes and soil microbiota in Scots pine stands on sandy soils (Arenosols). The study was conducted in the eastern part of Lithuania (55° 35′N, 26° 07′E). The annual investigations in 1–4-year-old burned sites showed that low severity surface fires mostly affected the above-ground part of the ground vegetation. The influence of surface fire on soil chemical properties and soil microbiota was minor. Only slight increases in pH and in the content of total N in soil organic layer were detected. Concentrations of mobile K2O and heavy metals (Cd, Cr, Cu, Mn, Zn and Fe) slightly increased in the mineral topsoil. The actinobacteria abundance increased in the soil organic layer and the mineral topsoil of the burned sites. The abundance of micromycetes decreased in the mineral topsoil after the surface fires.

Contrasting effects of wildfire and climate on radial growth of Pinus canariensis on windward and leeward slopes on Tenerife, Canary Islands

Little is known concerning the effects of wildfires on tree radial growth and their climatic response under contrasting regimes of fog water inputs on oceanic islands. On Tenerife, Canary Islands, windward slopes are humid with high-fog frequency due to influence of wet trade winds, while climate on leeward slopes is more arid. We used tree-ring records of Pinus canariensis Sweet ex Spreng. to quantify the effects of a fire of known date on radial growth and determine the main limiting climatic factors for growth. Radial growth patterns and their responsiveness to fire severity and climatic variation differed between windward and leeward slopes. Surface fire did not significantly impact growth, while crown fire caused short-term growth reduction, and even cessation, more pronounced on the windward slope. Growth rates, tree-ring common signal, and climate sensitivity were smaller on the windward slope, with cold winters, and summer water stress limiting growth. On the leeward slope, climate explained a greater amount of growth variation mainly due to negative effects of high October–December sea-level pressures causing dry winter conditions. Contrasting growth dynamics on both slopes may result from diverging physiological effects of water inputs and reduced radiation caused by fog drip. Our findings suggest that dating growth suppressions and absent rings are useful to date past high-severity crown fires in P. canariensis forests, in addition to ordinary fire scars dating indicative of low-severity surface fires.

Effects of fire frequency on prescribed fire behaviour and soil temperatures in dry dipterocarp forests

This study investigated how fire frequencies and fuel loads influence fire behaviour and soil heating in dry dipterocarp forests of the Huai Kha Khaeng Wildlife Sanctuary, Thailand. Fire behaviour and soil temperatures during burning were measured on a series of plots with different past fire frequencies ranging from unburned control, to rarely, infrequently and frequently burned, representing fire occurrences in 0, 1, 2 and 7 out of the past 10 years respectively. The pre-burning loads of fine fuel including grasses, herbs, shrubs, seedlings, saplings and litters increased with the length of the previous fire-free interval. The rate of spread, flame height, fireline intensity and maximum soil temperatures at any soil depths were not significantly different between the past burning regimes, so fires were classed as low-intensity and low-severity surface fire. The longest duration of heating with temperatures >60°C at ground level occurred at the rarely burned site (~14 min), followed by the infrequently burned site (~12 min) and the frequently burned site (~8 min). However, the duration of heating above any given critical temperature threshold at 2- and 5-cm soil depths was less than 1 min across all regimes. From a fuel management perspective, there does not appear to be a need to carry out prescribed burns more frequently than every 6–7 years, because fine fuel loads did not continue to accumulate substantially beyond 7 years after a fire.

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.

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.

INFLUENCES OF SECONDARY DISTURBANCES ON LODGEPOLE PINE STAND DEVELOPMENT IN ROCKY MOUNTAIN NATIONAL PARK

Although high-severity fire is the primary type of disturbance shaping the structure of lodgepole pine (Pinus contorta) stands in the southern Rocky Mountains, many post-fire stands are also affected by blowdown, low-severity surface fires, and/or outbreaks of mountain pine beetle (MPB; Dendroctonus ponderosae). The ecological effects of these secondary disturbances are poorly understood but are potentially important in the context of managing for ecological restoration and fire hazard mitigation. We investigated the effects of blowdown, surface fires, and MPB outbreaks on demographic processes in post-fire lodgepole pine stands in Rocky Mountain National Park, Colorado, USA. We used dendroecological methods to reconstruct stand characteristics prior to and following secondary disturbances for paired stands with and without secondary disturbances. Surface fire events do not kill canopy trees or trigger pulses of recruitment and as such do not have detectable influences on stand development. In contrast, both MPB and blowdown kill canopy trees and trigger pulses of tree regeneration of lodgepole pine and subalpine fir (Abies lasiocarpa). The amount and species composition of post-disturbance regeneration is dependent on the severity of the disturbance and on the time since stand initiation. Secondary disturbances of higher severity (i.e., killing >50% of the canopy trees) that occur in younger post-fire stands favor new establishment of lodgepole pine. In contrast, secondary disturbances of lower severity in older stands (>250 years) trigger a pulse of establishment of subalpine fir. The results of this study demonstrate that the high tree densities characteristic of lodgepole pine stands in the southern Rockies (southern Wyoming to northern New Mexico) are the result of dense regeneration following stand-replacing fires and that surface fires had little or no thinning effect on tree densities. Thus, current high stand densities in the study area are not the result of suppression of surface fires. Moreover, the strong pulses of regeneration following forest thinning by MPB and blowdowns imply that, depending on the degree of thinning, thinning prescriptions to reduce fuels in the lodgepole pine forest type may have the unintended consequence of increasing ladder fuels 15-20 years following treatments.

How resilient are southwestern ponderosa pine forests after crown fires?

The exclusion of low-severity surface fire from ponderosa pine (Pinus ponderosa P. &amp; C. Lawson) forests of the Southwest has changed ecosystem structure and function such that severe crown fires are increasingly causing extensive stand mortality. This altered fire regime has resulted from the intersection of natural drought cycles with human activities that have suppressed natural fires for over a century. What is the trajectory of forest recovery after such fires? This study explores the regeneration response of ponderosa pine and other species to crown fires that occurred in the region from the late 1940s to the mid-1970s. We address two main questions: (1) What is the success of ponderosa regeneration and establishment, and (2) Can these sites, burned in stand-destroying fires, be "captured" by other species on the scale of decades? Two main trajectories of recovery were found: (1) establishment of unnaturally dense ponderosa pine stands vulnerable to further crown fire and (2) establishment of nonforested grass or shrub communities.

Historical and recent fire regimes in Piñon–Juniper woodlands on Mesa Verde, Colorado, USA

The fire history of Piñon–Juniper ( Pinus edulis– Juniperus osteosperma) woodlands in much of the southwestern United States is poorly understood, and as a result, fire management decisions are being made without a rigorous ecological underpinning. We investigated the historic fire regimes in Piñon–Juniper woodlands on the Mesa Verde cuesta utilizing stand and age structures. All Piñon trees in eight stands were aged and stand age was extrapolated to the surrounding landscapes using digital imagery, creating a time-since-fire map of the 1995 landscape. Six sampled stands were over 400 years, while two were between 200 and 300 years. Stand-replacing fire with a rotation of 400 years or longer characterized this Piñon–Juniper landscape before 1995; low-severity surface fires apparently have never been an important component of the fire regime in Mesa Verde. Superposed epoch analysis revealed that large fires occur following significantly low precipitation in May or in the winter (October–March) preceding a summer fire season. Since the mid-1990’s, a severe drought has characterized climate in this region, but the recent drought is similar to previous drought periods since 1950. A combination of canopy fuel build-up during two wet decades before 1995 and the current drought conditions has resulted in unprecedented fire activity (six large wildfires between 1996 and 2003) when compared with the reference period 1700–1900. We may be witnessing a unique period in the ecological history of the southwest, a period when vegetation patterns are being altered over extensive areas within a very short time. If the current drought continues, we ultimately may lose much of the old-growth Piñon–Juniper woodland in the southwest. We emphasize however, that these changes are not due to fire suppression or other direct human intervention but rather result from natural ecological responses to climatic variability. Therefore, our data provide no ecological justification for aggressive management activities such as mechanical fuel reduction or prescribed burning, except in the immediate vicinity of vulnerable cultural resources.

A fire history from tree rings in a high-elevation forest of Rocky Mountain National Park

Historical fire patterns in a subalpine forest of Rocky Mountain National Park were quantified from an analysis of forest stand ages and fire-scarred trees. A comparatively detailed sample of 3461 tree cores and 212 fire scars was collected from a 9200-ha study area north of Estes Park, Colorado. A total of 41 fire events were identified in the record. Annually precise fire dates, beginning in 1533, include 22 high-severity crown fires, 7 low-severity surface fires, and 8 mixed-severity events with both surface and crown fire components. Fire rotation was estimated for both surface fires (7587 years) and crown fires (346 years). Fire rotation did not appear to vary with fuel characteristics associated with topographical differences in the study area. Fires larger than 300 ha were few, but they determined a large proportion of the area burned since 1700 and were significantly correlated with a reconstructed index of summer drought. Low fire activity in the 20th century was associated with decreased severity and frequency of drought episodes. Long fire rotations preclude definitive conclusions regarding the effects of fire suppression in the 20th century, but relationships between high-severity fires, fuels, and drought suggest that climatic variability remains the primary influence on fire cycles in high-elevation ecosystems of the southern Rocky Mountains.

Determining effects of area burned and fire severity on carbon cycling and emissions in Siberia

The Russian boreal forest contains about 25% of the global terrestrial biomass, and even a higher percentage of the carbon stored in litter and soils. Fire burns large areas annually, much of it in low-severity surface fires – but data on fire area and impacts or extent of varying fire severity are poor. Changes in land use, cover, and disturbance patterns such as those predicted by global climate change models, have the potential to greatly alter current fire regimes in boreal forests and to significantly impact global carbon budgets. The extent and global importance of fires in the boreal zone have often been greatly underestimated. For the 1998 fire season we estimate from remote sensing data that about 13.3 million ha burned in Siberia. This is about 5 times higher than estimates from the Russian Aerial Forest Protection Service (Avialesookhrana) for the same period. We estimate that fires in the Russian boreal forest in 1998 constituted some 14–20% of average annual global carbon emissions from forest fires. Average annual emissions from boreal zone forests may be equivalent to 23–39% of regional fossil fuel emissions in Canada and Russia, respectively. But the lack of accurate data and models introduces large potential errors into these estimates. Improved monitoring and understanding of the landscape extent and severity of fires and effects of fire on carbon storage, air chemistry, vegetation dynamics and structure, and forest health and productivity are essential to provide inputs into global and regional models of carbon cycling and atmospheric chemistry.