Changes in resilience to fire disturbance in lowland pine forest ecosystems

The effect of major disturbances on the structure of pine forests in the interfluve of the Chernaya and Pugai rivers has been retrospectively analyzed back to the late 19th century. The results show that intensive timber harvesting and fires produced a synergistic effect so that the natural fire regime in these forests has changed; in particular, the severity of fires and the frequency of stand-replacement fires have increased. The ecosystem of lowland pine forests, adapted to periodic fires, largely loses its resilience to fire disturbance if the age structure of pine populations becomes relatively homogeneous and communities at early succession stages gain significant prevalence in the vegetation. The main cause of the largest fire that occurred in pine forests of the Nizhni Novgorod Transvolga region (Nizhegorodskoe Zavolzhie) in 1972 was human activity rather than natural and climatic features of this territory.     

Transformation of the ground cover after surface fires and estimation of pyrogenic carbon emissions in the dark-coniferous forests of Central Siberia

Ground fuel loads and structure in dark-coniferous forests with the dominance of Siberian pine have been studied in the Central Siberian State Nature Biosphere Reserve located in the central-taiga zone of Central Siberia. The impacts of surface fires of various forms and severity on the living ground cover are examined. It is found that fires of low to moderate severity reduce ground fuel loads from 35–49 t/ha to 26–28 t/ha, while fires of moderate to high severity reduce them to 17–18 t/ha. Consumption of down woody debris varies from 3 to 29 t/ha, depending on the prefire fuel characteristics and fire form and severity. Steady fires spreading with the fire danger index PV-1 of 3919 ± 482 result in carbon emissions of 14.0 tC/ha from fires of low to moderate severity and 24.6 tC/ha from fires of moderate to high severity. The lowest carbon emissions (10.1 tC/ha) are noted for fast-moving fires spreading with PV-1 of 1167 ± 386.     

Lack of regeneration and climatic vulnerability to fire of Scots pine may induce vegetation shifts at the southern edge of its distribution

Aim Forest ecosystems dominated by fire‐sensitive species could suffer shifts in composition under altered crown fire regimes mediated by climate change. The aims of this study were to: (1) study the spatio‐temporal patterns and the climatic distribution of fires in Scots pine (Pinus sylvestris) forests during the last 31 years in north‐eastern Spain, (2) evaluate the climatic vulnerability to fire of these forests in Spain, (3) analyse the regeneration of Scots pine after fire, and (4) predict the mid‐term maintenance or replacement of Scots pine in burned areas. Location Catalonia (north‐eastern Spain): the southern distribution limit of Scots pine. Methods We characterized the spatio‐temporal and the climatic distribution of fires that occurred in Catalonia between 1979 and 2009. We used a generalized linear model to characterize the climatic vulnerability to fire of Scots pine in the whole of Spain. We assessed the regeneration of the species after crown fires in nine burned areas in Catalonia. The resulting data were integrated into a stochastic matrix model to predict the mid‐term maintenance or replacement of Scots pine in burned areas. Results During the last three decades, Scots pine forests distributed in dry sites were most affected by fire. Our assessment of the vulnerability to fire of Scots pine forests in Spain as a whole, based on climatic and topographical variables, showed that 32% of these forests are vulnerable to fire, and that this proportion could increase to 66% under a conservative climate change scenario. Field data showed almost no regeneration of Scots pine after crown fires, and a limited capacity to recolonize from unburned edges, even in relatively old fires, with 90% of recruits located in the first 25 m from the edge. This process could be delayed by the elapsed time for new recruits to achieve reproductive maturity, which we estimated to be c. 15 years. Finally, our matrix model predicted the replacement of burned Scots pine forests by oak (Quercus sp.) forests, shrublands or mixed resprouter forests. Main conclusions Increased vulnerability to fire of Scots pine forests under future, warmer conditions may result in vegetation shifts at the southern edge of the distribution of the species.     

Fire, fuels and restoration of ponderosa pine–Douglas fir forests in the Rocky Mountains, USA

Aim Forest restoration in ponderosa pine and mixed ponderosa pine–Douglas fir forests in the US Rocky Mountains has been highly influenced by a historical model of frequent, low‐severity surface fires developed for the ponderosa pine forests of the Southwestern USA. A restoration model, based on this low‐severity fire model, focuses on thinning and prescribed burning to restore historical forest structure. However, in the US Rocky Mountains, research on fire history and forest structure, and early historical reports, suggest the low‐severity model may only apply in limited geographical areas. The aim of this article is to elaborate a new variable‐severity fire model and evaluate the applicability of this model, along with the low‐severity model, for the ponderosa pine–Douglas fir forests of the Rocky Mountains. Location Rocky Mountains, USA. Methods The geographical applicability of the two fire models is evaluated using historical records, fire histories and forest age‐structure analyses. Results Historical sources and tree‐ring reconstructions document that, near or before ad 1900, the low‐severity model may apply in dry, low‐elevation settings, but that fires naturally varied in severity in most of these forests. Low‐severity fires were common, but high‐severity fires also burned thousands of hectares. Tree regeneration increased after these high‐severity fires, and often attained densities much greater than those reconstructed for Southwestern ponderosa pine forests. Main conclusions Exclusion of fire has not clearly and uniformly increased fuels or shifted the fire type from low‐ to high‐severity fires. However, logging and livestock grazing have increased tree densities and risk of high‐severity fires in some areas. Restoration is likely to be most effective which seeks to (1) restore variability of fire, (2) reverse changes brought about by livestock grazing and logging, and (3) modify these land uses so that degradation is not repeated.     

Postfire recovery of the ground cover in pine forests of the Lower Angara region

This paper traces the dynamics of the living ground cover at the initial stage of pyrogenic succession (1–9 years) after different-intensity surface fires in the pine forests of the Lower Angara region. Depending on their intensities, fires have reduced the foliage cover and ground-cover biomass. The greatest changes occur in case of medium-intensity and high-intensity fires that change the horizontal structure of plant microgroups and lead to the death of moss-lichen layers.     

Lodgepole pine forest age class dynamics and susceptibility to mountain pine beetle attack

Because mountain pine beetle attack mature pine stands, an understanding of forest age class dynamics is important to managing forests within the distribution of the beetle. The assumed theoretical negative exponential forest age distribution provides an estimate when ecosystem dynamics are in equilibrium. This study investigates the dynamics of forest age distribution for non-equilibrium ecosystem dynamics, which result primarily from large and irregular stand-replacement fire disturbances that alter the forest age distribution. A model experiment using the SEM-LAND model on a 1 million ha lodgepole pine forest landscape was conducted to estimate how the proportion of susceptible area could be influenced by different fire regimes. The results of the simulation suggest that the temporal dynamics of the area susceptible to mountain pine beetle attack are complex and depend on the fire history of the study area, if the area is experiencing large and irregular stand-replacement fires. The age range of the lodgepole pine forest stands susceptible to mountain pine beetle attack might significantly affect the estimate of the area susceptible to attack.     

Post-fire transformation of microbial communities and invertebrate complexes in the pine forest soils,Central Siberia

We studied post-fire transformations in functional characteristics of soil microbial communities and invertebrate complexes in the central-taiga pine forests of Central Siberia. The study revealed that fires of any severity reduce the density and diversity of soil invertebrates and adversely affect the structure and functioning of the sandy podzol microbial complexes. Post-fire recovery of the density and structure of soil invertebrate complexes and the functioning of sandy podzol microbial communities depend on fire duration and severity, as well as dynamics of hydrothermal and trophic properties of the pine forest soils.     

Changing growth response to wildfire in old‐growth ponderosa pine trees in montane forests of north central Idaho

North American fire‐adapted forests are experiencing changes in fire frequency and climate. These novel conditions may alter postwildfire responses of fire‐adapted trees that survive fires, a topic that has received little attention. Historical, frequent, low‐intensity wildfire in many fire‐adapted forests is generally thought to have a positive effect on the growth and vigor of trees that survive fires. Whether such positive effects can persist under current and future climate conditions is not known. Here, we evaluate long‐term responses to recurrent 20th‐century fires in ponderosa pine, a fire‐adapted tree species, in unlogged forests in north central Idaho. We also examine short‐term responses to individual 20th‐century fires and evaluate whether these responses have changed over time and whether potential variability relates to climate variables and time since last fire. Growth responses were assessed by comparing tree‐ring measurements from trees in stands burned repeatedly during the 20th century at roughly the historical fire frequency with trees in paired control stands that had not burned for at least 70 years. Contrary to expectations, only one site showed significant increases in long‐term growth responses in burned stands compared with control stands. Short‐term responses showed a trend of increasing negative effects of wildfire (reduced diameter growth in the burned stand compared with the control stand) in recent years that had drier winters and springs. There was no effect of time since the previous fire on growth responses to fire. The possible relationships of novel climate conditions with negative tree growth responses in trees that survive fire are discussed. A trend of negative growth responses to wildfire in old‐growth forests could have important ramifications for forest productivity and carbon balance under future climate scenarios.     

Tropical and north Pacific teleconnections influence fire regimes in pine-dominated forests of north-eastern California, USA

Aim To assess the importance of drought and teleconnections from the tropical and north Pacific Ocean on historical fire regimes and vegetation dynamics in north‐eastern California. Location The 700 km² study area was on the leeward slope of the southern Cascade Mountains in north‐eastern California. Open forests of ponderosa pine (Pinus ponderosa var. ponderosa Laws.) and Jeffrey pine (P. jeffreyi Grev. & Balf) surround a network of grass and shrub‐dominated meadows that range in elevation from 1650 to 1750 m. Methods Fire regime characteristics (return interval, season and extent) were determined from crossdated fire scars and were compared with tree‐ring based reconstructions of precipitation and temperature and teleconnections for the period 1700–1849. The effect of drought on fire regimes was determined using a tree‐ring based proxy of climate from five published chronologies. The number of forest‐meadow units that burned was compared with published reconstructions of the El Niño/Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Results Landscape scale fires burned every 7–49 years in meadow‐edge forests and were influenced by variation in drought, the PDO and ENSO. These widespread fires burned during years that were dryer and warmer than normal that followed wetter and cooler years. Less widespread fires were not associated with this wet, then dry climate pattern. Widespread fires occurred during El Niño years, but fire extent was mediated by the phase of the PDO. Fires were most widespread when the PDO was in a warm or normal phase. Fire return intervals, season and extent varied at decadal to multi‐decadal time scales. In particular, an anomalously cool, wet period during the early 1800s resulted in widespread fires that occurred earlier in the year than fires before or after. Main conclusions Fire regimes in north‐eastern California were strongly influenced by regional and hemispheric‐scale climate variation. Fire regimes responded to variation that occurred in both the north and tropical Pacific. Near normal modes of the PDO may influence fire regimes more than extreme conditions. The prevalence of widespread teleconnection‐driven fires in the historic record suggests that variation in the Pacific Ocean was a key regulator of fire regimes through its influence on local fuel production and successional dynamics in north‐eastern California.     

Mesophication in temperate Europe: A dendrochronological reconstruction of tree succession and fires in a mixed deciduous stand in Białowieża Forest

1. The shift from shade‐intolerant species to shade‐tolerant mesophytic species in deciduous and mixed forests of the temperate zone is well described in studies from North America. This process has been termed mesophication and it has been linked to changes in fire regime. Fire suppression results in the cessation of establishment of heliophytic, fire‐dependent tree species such as oak (Quercus) and pine (Pinus). Due to the scarcity of old‐growth forests in Europe, data on long‐term compositional changes in mixed forests are very limited, as is the number of studies exploring whether fire played a role in shaping the dynamics.
2. The aim of this study was to reconstruct tree succession in a 43‐ha natural mixed deciduous forest stand in Bia?owie?a Forest (BF), Poland using dendrochronological methods. In addition, the presence of aboveground fire legacies (charred and fire‐scarred deadwood) enabled the fire history reconstruction.
3. Dendrochronological data revealed tree establishment (Quercus) back to the end of the 1500s and fires back to 1659. Under a regime of frequent fires until the end of the 18th century, only oak and pine regenerated, sporadically. A shift in the fire regime in the first half of the 19th century triggered oak and pine cohort regeneration, then gradually spruce (Picea) encroached. Under an increasingly dense canopy and less flammable conditions, regeneration of shade‐tolerant Carpinus, Tilia, and Acer began simultaneously with the cessation of oak and pine recruitment.
4. Synthesis. The study reports the first evidence of mesophication in temperate Europe and proves that fire was involved in shaping the long‐term dynamics of mixed deciduous forest ecosystems. Our data suggest that fire exclusion promoted a gradual recruitment of fire‐sensitive, shade‐tolerant species that inhibited the regeneration of oak and pine in BF.

     

Californian mixed-conifer forests under unmanaged fire regimes in the Sierra San Pedro Mártir, Baja California, Mexico

Aim This study appraises historical fire regimes for Californian mixed‐conifer forests of the Sierra San Pedro Mártir (SSPM). The SSPM represents the last remaining mixed‐conifer forest along the Pacific coast still subject to uncontrolled, periodic ground fire. Location The SSPM is a north–south trending fault bound range, centred on 31°N latitude, 100 km SE of Ensenada, Baja California. Methods We surveyed forests for composition, population structure, and historical dynamics both spatially and temporally over the past 65 years using repeat aerial photographs and ground sampling. Fire perimeter history was reconstructed based on time‐series aerial photographs dating from 1942 to 1991 and interpretable back to 1925. A total of 256 1‐ha sites randomly selected from aerial photographs were examined along a chronosequence for density and cover of canopy trees, density of snags and downed logs, and cover of non‐conifer trees and shrubs. Twenty‐four stands were sampled on‐the‐ground by a point‐centred quarter method which yielded data on tree density, basal area, frequency, importance value, and shrub and herb cover. Results Forests experience moderately intense understory fires that range in size to 6400 ha, as well as numerous smaller, low intensity burns with low cumulative spatial extent. SSPM forests average 25–45% cover and 65–145 trees per ha. Sapling densities were two to three times that of overstory trees. Size‐age distributions of trees ≥ 4 cm dbh indicate multi‐age stands with steady‐state dynamics. Stands are similar to Californian mixed conifer forests prior to the imposition of fire suppression policy. Livestock grazing does not appear to be suppressing conifer regeneration. Main conclusions Our spatially‐based reconstruction shows the open forest structure in SSPM to be a product of infrequent, intense surface fires with fire rotation periods of 52 years, rather than frequent, low intensity fires at intervals of 4–20 years proposed from California fire‐scar dendrochronology (FSD) studies. Ground fires in SSPM were intense enough to kill pole‐size trees and a significant number of overstory trees. We attribute long fire intervals to the gradual build‐up of subcontinuous shrub cover, conifer recruitment and litter accumulation. Differences from photo interpretation and FSD estimates are due to assumptions made with respect to site‐based (point) sampling of fire, and nonfractal fire intensities along fire size frequency distributions. Fire return intervals determined by FSD give undue importance to local burns which collectively use up little fuel, cover little area, and have little demographic impact on forests.     

Effects of fire season on flowering of forbs and shrubs in longleaf pine forests

Summary Effects of variation in fire season on flowering of forbs and shrubs were studied experimentally in two longleaf pine forest habitats in northern Florida, USA. Large, replicated plots were burned at different times of the year, and flowering on each plot was measured over the twelve months following fire. While fire season had little effect on the number of species flowering during the year following fire, fires during the growing season decreased average flowering duration per species and increased synchronization of peak flowering times within species relative to fires between growing seasons. Fires during the growing season also increased the dominance of fall flowering forbs and delayed peak fall flowering. Differences in flowering resulting from variation in fire season were related to seasonal changes in the morphology of clonal forbs, especially fall-flowering composites. Community level differences in flowering phenologies indicated that timing of fire relative to environmental cues that induced flowering was important in determining flowering synchrony among species within the ground cover of longleaf pine forests. Differences in fire season produced qualitatively similar effects on flowering phenologies in both habitats, indicating plant responses to variation in the timing of fires were not habitat specific.     

Modelling Variable Fire Severity in Boreal Forests: Effects of Fire Intensity and Stand Structure

It is becoming clear that fires in boreal forests are not uniformly stand-replacing. On the contrary, marked variation in fire severity, measured as tree mortality, has been found both within and among individual fires. It is important to understand the conditions under which this variation can arise. We integrated forest sample plot data, tree allometries and historical forest fire records within a diameter class-structured model of 1.0 ha patches of mono-specific black spruce and jack pine stands in northern Québec, Canada. The model accounts for crown fire initiation and vertical spread into the canopy. It uses empirical relations between fire intensity, scorch height, the percent of crown scorched and tree mortality to simulate fire severity, specifically the percent reduction in patch basal area due to fire-caused mortality. A random forest and a regression tree analysis of a large random sample of simulated fires were used to test for an effect of fireline intensity, stand structure, species composition and pyrogeographic regions on resultant severity. Severity increased with intensity and was lower for jack pine stands. The proportion of simulated fires that burned at high severity (e.g. >75% reduction in patch basal area) was 0.80 for black spruce and 0.11 for jack pine. We identified thresholds in intensity below which there was a marked sensitivity of simulated fire severity to stand structure, and to interactions between intensity and structure. We found no evidence for a residual effect of pyrogeographic region on simulated severity, after the effects of stand structure and species composition were accounted for. The model presented here was able to produce variation in fire severity under a range of fire intensity conditions. This suggests that variation in stand structure is one of the factors causing the observed variation in boreal fire severity.     

Forest fire occurrence increases the distribution of a scarce forest type in the Mediterranean Basin

Here we report how fire recurrence increases the distribution of a scarce forest type in NE Spain that is dominated by the resprouter tree species Arbutus unedo. We used a combination of GIS and field surveys to determine the effect of fire and pre-fire vegetation on the appearance of A. unedo forests. In the field, we also analyzed the factors that promote fire and lead to the appearance of A. unedo forests. Our results reveal an increased occurrence of A. unedo forests in NE Spain in recent years; this phenomenon was strongly related to fire recurrence and the vegetation type present prior to fire. Most Pinus halepensis forests that burned more than once gave rise to A. unedo forests. Our results indicate that these conversions were related to a reduction in pine density coupled with increases in the density and size of A. unedo trees due to recurrent fires. Given that fires are increasing in number and magnitude in the Mediterranean, we predict a major change in landscape structure and composition at the regional scale.     

云贵高原抚仙湖近13300年的花粉/炭屑记录*

抚仙湖是云贵高原著名的断陷深水湖,其沉积物蕴藏着流域地质历史时期丰富的环境信息。对钻取自该湖的900cm 湖泊沉积物岩芯进行花粉/炭屑分析及花粉数据的主成分分析表明,抚仙湖流域的植被、气候与火灾在过去的13 300年经历了5个阶段的变化：(1)13 300—10 400cal．a BP,植被以松林为主,伴有山地暗针叶林和常绿阔叶林,表明该时期气候较为冷湿,森林火灾多发,在后期随着温度和湿度的降低,森林火灾愈加频繁。(2)10 400—5 700cal．a BP,松林收缩,常绿阔叶林扩张,出现一定数量的落叶阔叶林,显示该时期气候偏暖偏干;此阶段早期随着气候变暖变干森林火灾的发生延续上阶段高发的状态,直到9 500cal．a BP后随着湿度的增加森林火灾明显减少。(3)5 700—1 800cal．a BP,松林变化较小,常绿/落叶阔叶林比重增大,首次出现了暖热性的枫香林,显示该时期暖湿的气候特征,火灾发生频率低。(4)1 800—500cal．a BP,松林扩张,阔叶林收缩,本阶段后期草本植被比重开始增加,显示该时期气候相对冷干,森林火灾发生频率较高。(5)500cal．a BP至今,松林收缩,落叶阔叶树种增多,草本植物花粉明显增多,显示该时期气候温凉偏干,森林火灾发生频率降低。     

Modelling responses of pine savannas to climate change and large‐scale disturbance

Global warming can potentially influence ecological communities through altered disturbance regimes in addition to increased temperatures. We investigate the response of pine savannas in the southeastern United States to global warming using a simple Lotka‐Volterra competition model together with predicted changes to fire and hurricane disturbance regimes with global climate change. In the southeastern United States, decreased frequency of both fires and hurricanes with global warming will shift pine savannas toward a forested state. A CO₂ fertilization effect that increases the growth rate of tree populations will also push southeastern landscapes from open savannas towards closed forests. Transient dynamics associated with climate driven changes in vegetation will last on the order of decades to a century. In our model, the sensitivity of savannas to relative changes in the frequency of fire versus hurricanes is linearly dependent on the growth rate and mortality of trees in fire and hurricane disturbances.     

Spatial and temporal variation in historic fire regimes in subalpine forests across the Colorado Front Range in Rocky Mountain National Park, Colorado, USA

Aim The historical variability of fire regimes must be understood in the context of drivers of the occurrence of fire operating at a range of spatial scales from local site conditions to broad‐scale climatic variation. In the present study we examine fire history and variations in the fire regime at multiple spatial and temporal scales for subalpine forests of Engelmann spruce–subalpine fir (Picea engelmannii, Abies lasiocarpa) and lodgepole pine (Pinus contorta) of the southern Rocky Mountains. Location The study area is the subalpine zone of spruce–fir and lodgepole pine forests in the southern sector of Rocky Mountain National Park (ROMO), Colorado, USA, which straddles the continental divide of the northern Colorado Front Range (40°20′ N and 105°40′ W). Methods We used a combination of dendroecological and Geographic Information System methods to reconstruct fire history, including fire year, severity and extent at the forest patch level, for c. 30,000 ha of subalpine forest. We aggregated fire history information at appropriate spatial scales to test for drivers of the fire regime at local, meso, and regional scales. Results The fire histories covered c. 30,000 ha of forest and were based on a total of 676 partial cross‐sections of fire‐scarred trees and 6152 tree‐core age samples. The subalpine forest fire regime of ROMO is dominated by infrequent, extensive, stand‐replacing fire events, whereas surface fires affected only 1–3% of the forested area. Main conclusions Local‐scale influences on fire regimes are reflected by differences in the relative proportions of stands of different ages between the lodgepole pine and spruce–fir forest types. Lodgepole pine stands all originated following fires in the last 400 years; in contrast, large areas of spruce–fir forests consisted of stands not affected by fire in the past 400 years. Meso‐scale influences on fire regimes are reflected by fewer but larger fires on the west vs. east side of the continental divide. These differences appear to be explained by less frequent and severe drought on the west side, and by the spread of fires from lower‐elevation mixed‐conifer montane forests on the east side. Regional‐scale climatic variation is the primary driver of infrequent, large fire events, but its effects are modulated by local‐ and meso‐scale abiotic and biotic factors. The low incidence of fire during the period of fire‐suppression policy in the twentieth century is not unique in comparison with the previous 300 years of fire history. There is no evidence that fire suppression has resulted in either the fire regime or current forest conditions being outside their historic ranges of variability during the past 400 years. Furthermore, in the context of fuel treatments to reduce fire hazard, regardless of restoration goals, the association of extremely large and severe fires with infrequent and exceptional drought calls into question the future effectiveness of tree thinning to mitigate fire hazard in the subalpine zone.     

Historical,Observed, and Modeled Wildfire Severity in Montane Forests of the Colorado Front Range

Large recent fires in the western U.S. have contributed to a perception that fire exclusion has caused an unprecedented occurrence of uncharacteristically severe fires, particularly in lower elevation dry pine forests. In the absence of long-term fire severity records, it is unknown how short-term trends compare to fire severity prior to 20^th century fire exclusion. This study compares historical (i.e. pre-1920) fire severity with observed modern fire severity and modeled potential fire behavior across 564,413 ha of montane forests of the Colorado Front Range. We used forest structure and tree-ring fire history to characterize fire severity at 232 sites and then modeled historical fire-severity across the entire study area using biophysical variables. Eighteen (7.8%) sites were characterized by low-severity fires and 214 (92.2%) by mixed-severity fires (i.e. including moderate- or high-severity fires). Difference in area of historical versus observed low-severity fire within nine recent (post-1999) large fire perimeters was greatest in lower montane forests. Only 16% of the study area recorded a shift from historical low severity to a higher potential for crown fire today. An historical fire regime of more frequent and low-severity fires at low elevations (<2260 m) supports a convergence of management goals of ecological restoration and fire hazard mitigation in those habitats. In contrast, at higher elevations mixed-severity fires were predominant historically and continue to be so today. Thinning treatments at higher elevations of the montane zone will not return the fire regime to an historic low-severity regime, and are of questionable effectiveness in preventing severe wildfires. Based on present-day fuels, predicted fire behavior under extreme fire weather continues to indicate a mixed-severity fire regime throughout most of the montane forest zone. Recent large wildfires in the Front Range are not fundamentally different from similar events that occurred historically under extreme weather conditions.     

Climatic influences on fire regimes in the northern Sierra Nevada mountains, Lake Tahoe Basin, Nevada, USA

Aim The goal of this study was to understand better the role of interannual and interdecadal climatic variation on local pre‐EuroAmerican settlement fire regimes in fire‐prone Jeffrey pine (Pinus jeffreyi Grev. & Balf.) dominated forests in the northern Sierra Nevada Mountains. Location Our study was conducted in a 6000‐ha area of contiguous mixed Jeffrey pine‐white fir (Abies concolor Gordon & Glend.) forest on the western slope of the Carson Range on the eastern shore of Lake Tahoe, Nevada. Methods Pre‐EuroAmerican settlement fire regimes (i.e. frequency, return interval, extent, season) were reconstructed in eight contiguous watersheds for a 200‐year period (1650–1850) from fire scars preserved in the annual growth rings of nineteenth century cut stumps and recently dead pre‐settlement Jeffrey pine trees. Superposed epoch analysis (SEA) and correlation analysis were used to examine relationships between tree ring‐based reconstructions of the Palmer Drought Severity Index (PDSI), Southern Oscillation Index (SOI), Pacific Decadal Oscillation (PDO) and pre‐EuroAmerican fire regimes in order to assess the influence of drought and equatorial and north Pacific teleconnections on fire occurrence and fire extent. Results For the entire period of record (1650–1850), wet conditions were characteristic of years without fires. In contrast, fire years were associated with drought. Drought intensity also influenced fire extent and the most widespread fires occurred in the driest years. Years with widespread fires were also preceded by wet conditions 3 years before the fire. Widespread fires were also associated with phase changes of the PDO, with the most widespread burns occurring when the phase changed from warm (positive) to cold (negative) conditions. Annual SOI and fire frequency or extent were not associated in our study. At decadal time scales, burning was more widespread during decades that were dryer and characterized by La Niña and negative PDO conditions. Interannual and interdecadal fire–climate relationships were not stable over time. From 1700 to 1775 there was no interannual relationship between drought, PDO, and fire frequency or extent. However, from 1775 to 1850, widespread fires were associated with dry years preceded by wet years. This period also had the strongest association between fire extent and the PDO. In contrast, fire–climate associations at interdecadal time scales were stronger in the earlier period than in the later period. The change from strong interdecadal to strong interannual climate influence was associated with a breakdown in decadal scale constructive relationships between PDO and SOI. Main conclusions Climate strongly influenced pre‐settlement pine forest fire regimes in northern Sierra Nevada. Both interannual and interdecadal climatic variation regulated conditions conducive to fire activity, and longer term changes in fire frequency and extent correspond with climate‐mediated changes observed in both the northern and southern hemispheres. The sensitivity of fire regimes to shifts in modes of climatic variability suggests that climate was a key regulator of pine forest ecosystem structure and dynamics before EuroAmerican settlement. An understanding of pre‐EuroAmerican fire–climate relationships may provide useful insights into how fire activity in contemporary forests may respond to future climatic variation.     

Post-Glacial Climate–Fire Interactions Control Tree Composition of Mesic Temperate Forests in Eastern North America

Stand-scale gap-phase dynamics is generally viewed as the main driver of development in mesic deciduous forests of the temperate biome. Soil charcoal of temperate forests in eastern North America are unnoticed in most surveys, thus explaining why fire is undervalued as a driver of forest succession. The extent to which gap-phase, fire, or other processes are responsible for the regeneration and maintenance of mesic deciduous forests is unknown because paleoecological evidence is lacking. We tested the fire-driven succession hypothesis on the development of this major forest type. Based on charcoal ¹⁴C dates of two sites, 44 and 55 fires occurred since early Holocene, with a mean interval of 170 to 215 years. The vegetation of both sites followed comparable post-glacial trajectories consisting of three distinct periods. Conifers dominated the two first periods during 5200–6000 years and were replaced by hardwoods–conifers over the last 3500 years. The first period was represented by boreal conifers, whereas the second period, dominated by white pine (Pinus strobus) forests, persisted during 3000–4300 years. The third period marked the development of hardwood (sugar maple, Acer saccharum) forests. Fires occurred continuously on the sites since early Holocene likely under dry conditions during the conifer periods and cooler and moister conditions during the hardwood–conifer period. Recurrent fires appear with climate as key drivers of the long-term dynamics of several temperate forests in eastern North America. Similar studies on other temperate forests should be pursued to test the hypothesis of climate–fire interactions influencing tree composition change.