Changes in fire regime break the legacy lock on successional trajectories in Alaskan boreal forest

Predicting plant community responses to changing environmental conditions is a key element of forecasting and mitigating the effects of global change. Disturbance can play an important role in these dynamics, by initiating cycles of secondary succession and generating opportunities for communities of long‐lived organisms to reorganize in alternative configurations. This study used landscape‐scale variations in environmental conditions, stand structure, and disturbance from an extreme fire year in Alaska to examine how these factors affected successional trajectories in boreal forests dominated by black spruce. Because fire intervals in interior Alaska are typically too short to allow relay succession, the initial cohorts of seedlings that recruit after fire largely determine future canopy composition. Consequently, in a dynamically stable landscape, postfire tree seedling composition should resemble that of the prefire forest stands, with little net change in tree composition after fire. Seedling recruitment data from 90 burned stands indicated that postfire establishment of black spruce was strongly linked to environmental conditions and was highest at sites that were moist and had high densities of prefire spruce. Although deciduous broadleaf trees were absent from most prefire stands, deciduous trees recruited from seed at many sites and were most abundant at sites where the fires burned severely, consuming much of the surface organic layer. Comparison of pre‐ and postfire tree composition in the burned stands indicated that the expected trajectory of black spruce self‐replacement was typical only at moist sites that burned with low fire severity. At severely burned sites, deciduous trees dominated the postfire tree seedling community, suggesting these sites will follow alternative, deciduous‐dominated trajectories of succession. Increases in the severity of boreal fires with climate warming may catalyze shifts to an increasingly deciduous‐dominated landscape, substantially altering landscape dynamics and ecosystem services in this part of the boreal forest.     

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.     

Macrocharcoal-Based Chronosequences Reveal Shifting Dominance of Conifer Boreal Forests Under Changing Fire Regime

Balsam fir (Abies balsamea) and black spruce (Picea mariana) forests are the main conifer forest types in the North American boreal zone. The coexistence of the two species as well as their respective canopy dominance in distinct stands raises questions about the long-term evolution from one forest type to the other in relation to environmental factors including climate and stand disturbance. We tested the hypothesis that repetitive fire events promote the succession of balsam fir forest to black spruce forest and vice versa. Postfire chronosequences of one black spruce (BSP) and one balsam fir (BFI) sites were reconstructed based on the botanical composition and ¹⁴C-dated soil macrocharcoals. The results support the hypothesis of a successional dynamics. The BSP site has been affected by fires for the last 7600 years, whereas the BFI site, after having been impacted by several fires during the first half of the Holocene, evolved in a fire-free environment for the last 4400 years. Periods of fire activity facilitated the dominance of black spruce forests. The cessation of fires around 4400 cal. years BP on BFI site marks the beginning of the transition from black spruce to balsam fir stands. This succession is a long process, due to the ability of black spruce to regenerate by layering in the absence of fire. The resulting balsam fir stands are ancient and precarious ecosystems, since fire generally leads to the return of black spruce. The increase in balsam fir to the detriment of black spruce in boreal forests is a response to a decrease in fire frequency.     

Effects of Soil Burn Severity on Post-Fire Tree Recruitment in Boreal Forest

Fire, which is the dominant disturbance in the boreal forest, creates substantial heterogeneity in soil burn severity at patch and landscape scales. We present results from five field experiments in Yukon Territory, Canada, and Alaska, USA that document the effects of soil burn severity on the germination and establishment of four common boreal trees: Picea glauca, Picea mariana, Pinus contorta subsp. latifolia, and Populus tremuloides. Burn severity had strong positive effects on seed germination and net seedling establishment after 3 years. Growth of transplanted seedlings was also significantly higher on severely burned soils. Our data and a synthesis of the literature indicated a consistent, steep decline in conifer establishment on organic soils at depths greater than 2.5 cm. A meta-analysis of seedling responses found no difference in the magnitude of severity effects on germination versus net establishment. There were, however, significant differences in establishment but not germination responses among deciduous trees, spruce, and pine, suggesting that small-seeded species experience greater mortality on lightly burned, organic soils than large-seeded species. Together, our analyses indicate that variations in burn severity can influence multiple aspects of forest stand structure, by affecting the density and composition of tree seedlings that establish after fire. These effects are predicted to be most important in moderately-drained forest stands, where a high potential variability in soil burn severity is coupled with strong severity effects on tree recruitment.     

Stability in the patterns of long‐term development and growth of the Canadian spruce–moss forest

Aim The spruce–moss forest is the main forest ecosystem of the North American boreal forest. We used stand structure and fire data to examine the long‐term development and growth of the spruce–moss ecosystem. We evaluate the stability of the forest with time and the conditions needed for the continuing regeneration, growth and re‐establishment of black spruce (Picea mariana) trees. Location The study area occurs in Québec, Canada, and extends from 70°00′ to 72°00′ W and 47°30′ to 56°00′ N. Methods A spatial inventory of spruce–moss forest stands was performed along 34 transects. Nineteen spruce–moss forests were selected. A 500 m² quadrat at each site was used for radiocarbon and tree‐ring dating of time since last fire (TSLF). Size structure and tree regeneration in each stand were described based on diameter distribution of the dominant and co‐dominant tree species [black spruce and balsam fir (Abies balsamea)]. Results The TSLF of the studied forests ranges from 118 to 4870 cal. yr bp . Forests < 325 cal. yr bp are dominated by trees of the first post‐fire cohort and are not yet at equilibrium, whereas older forests show a reverse‐J diameter distribution typical of mature, old‐growth stands. The younger forests display faster height and radial growth‐rate patterns than the older forests, due to factors associated with long‐term forest development. Each of the stands examined established after severe fires that consumed all the soil organic material. Main conclusions Spruce–moss forests are able to self‐regenerate after fires that consume the organic layer, thus allowing seed regeneration at the soil surface. In the absence of fire the forests can remain in an equilibrium state. Once the forests mature, tree productivity eventually levels off and becomes stable. Further proof of the enduring stability of these forests, in between fire periods, lies in the ages of the stands. Stands with a TSLF of 325–4870 cal. yr bp all exhibited the same stand structure, tree growth rates and species characteristics. In the absence of fire, the spruce–moss forests are able to maintain themselves for thousands of years with no apparent degradation or change in forest type.     

Spatial and temporal variations in boreal forest fire frequency in northern Alberta

Abstract. Spatial and temporal variations in fire frequency in the boreal forest of Wood Buffalo National Park (WBNP) were assessed using forest stand age, fire scar and historical data. I test the hypotheses that (1) fire frequency is higher in jack pine forests and aspen forests than in black spruce forests and white spruce forests, (2) these variations in fire frequency can be related to the mean waterbreak distance (MWD) around a site and (3) fire frequency has changed over the past 300 years. The fire cycles (the time required to burn an area equal in size to the entire study area) in jack pine forests (39 years) and in aspen forests (39 years) were significantly shorter than those in black spruce forests (78 years) and in white spruce forests (96 years). The length of the fire cycle varies inversely with the MWD around a site, and the MWD was significantly higher in jack pine and aspen forests than in black or white spruce forests. It is suggested that covariations between soil type and the MWD influence, respectively, variations in forest dominant and fire frequency. A change in fire frequency at 1860 was apparent in the fire history for all of WBNP, the black spruce dominated stands, and the near and medium MWD classes. The fire cycle estimates for these classes were all significantly shorter during the period 1750 to 1859 (fire cycles = 25–49 years) than they were in the period 1860 to 1989 (fire cycles = 59–89 years). The possible roles of changes in climate and aboriginal burning practices in causing the temporal change in fire frequency are discussed.     

Contrasting patterns of tree mortality in late‐successional Picea abies stands in two areas in northern Fennoscandia

Question: What are tree mortality rates and how and why do they vary in late‐successional Picea abies‐dominated forests? Do observed tree mortality patterns allow comparative assessment of models of long‐term stand development? Location: Northern boreal Fennoscandia. Methods: We measured stand structure in 10 stands in two different areas. We determined age distributions and constructed a chronology of tree deaths by cross‐dating the years of death of randomly sampled dead trees. Results: The stands in the two areas had contrasting tree age distributions, despite similar live tree structure. In one area, stands were relatively even‐aged and originated following a stand‐replacing fire 317 years earlier. The stands in the second area had an uneven age structure and virtually no signs of past fires, suggesting a very long period since the last major disturbance. The younger stands were characterized by a high mortality rate and inter‐annual variation, which we attributed to senescence of the relatively even‐aged stands approaching the maximum age of P. abies. In contrast, the tree mortality rates in the older stands were low and relatively stable. Conclusions: Patterns of tree mortality were, to a large extent, dependent on the time since the last stand‐replacing disturbance, suggesting that northern boreal P. abies stands eventually reach a shifting mosaic state maintained through small‐scale dynamics, but the time needed to reach this state appears to be lengthy; even 300 years after a forest fire stands showed changes in patterns of tree mortality that were related to the developmental stage of the stands.     

Competition and facilitation between tree species change with stand development

Processes governing tree interspecific interactions, such as facilitation and competition, may vary in strength over time. This study tried to unveil them by performing dendrometrical analyses on black spruce Picea mariana, trembling aspen Populus tremuloides and jack pine Pinus banksiana trees from pure and mixed mature boreal forest stands in the Clay Belt of northwestern Quebec and on the tills of northwestern Ontario. We cored 1430 trees and cut 120 for stem analysis across all stand composition types, tree species and study regions. Aspen annual growth rate was initially higher when mixed with conifers, but then progressively decreased over time compared to pure aspen stands, while jack pine growth rate did not differ with black spruce presence throughout all stages of stand development. When mixed with aspen, black spruce showed a contrary response to aspen, i.e. an initial loss in growth but a positive gain later. On the richer clay soil of the Quebec Clay Belt region, however, both aspen and spruce responses in mixed stands reversed between 37 and 54 years. Overall, our results demonstrate that interspecific interactions were present and tended to change with stand development and among species. Our results also suggest that the nature of interspecific interactions may differ with soil nutrient availability.     

Fire Severity and Long-term Ecosystem Biomass Dynamics in Coniferous Boreal Forests of Eastern Canada

The objective of this study was to characterize the effects of soil burn severity and initial tree composition on long-term forest floor dynamics and ecosystem biomass partitioning within the Picea mariana [Mill.] BSP-feathermoss bioclimatic domain of northwestern Quebec. Changes in forest floor organic matter and ecosystem biomass partitioning were evaluated along a 2,355-year chronosequence of extant stands. Dendroecological and paleoecological methods were used to determine the time since the last fire, the soil burn severity of the last fire (high vs. low severity), and the post-fire tree composition of each stand (P. mariana vs. Pinus banksiana Lamb). In this paper, soil burn severity refers to the thickness of the organic matter layer accumulated above the mineral soil that was not burned by the last fire. In stands originating from high severity fires, the post-fire dominance by Pinus banksiana or P. mariana had little effect on the change in forest floor thickness and tree biomass. In contrast, stands established after low severity fires accumulated during the first century after fire 73% thicker forest floors and produced 50% less tree biomass than stands established after high severity fires. Standing tree biomass increased until approximately 100 years after high severity fires, and then decreased at a logarithmic rate in the millennial absence of fire. Forest floor thickness also showed a rapid initial accumulation rate, and continued to increase in the millennial absence of fire at a much slower rate. However, because forest floor density increased through time, the overall rate of increase in forest floor biomass (58 g m⁻² y⁻¹) remained constant for numerous centuries after fire (700 years). Although young stands (< 200 years) have more than 60% of ecosystem biomass locked-up in living biomass, older stands (> 200 years) sequester the majority (> 80%) of it in their forest floor. The results from this study illustrate that, under similar edaphic conditions, a single gradient related to time since disturbance is insufficient to account for the full spectrum of ecosystem biomass dynamics occurring in eastern boreal forests and highlights the importance of considering soil burn severity. Although fire severity induces diverging ecosystem biomass dynamics in the short term, the extended absence of fire brings about a convergence in terms of ecosystem biomass accumulation and partitioning.     

Influence of Disturbance on Temperate Forest Productivity

Climate, tree species traits, and soil fertility are key controls on forest productivity. However, in most forest ecosystems, natural and human disturbances, such as wind throw, fire, and harvest, can also exert important and lasting direct and indirect influence over productivity. We used an ecosystem model, PnET-CN, to examine how disturbance type, intensity, and frequency influence net primary production (NPP) across a range of forest types from Minnesota and Wisconsin, USA. We assessed the importance of past disturbances on NPP, net N mineralization, foliar N, and leaf area index at 107 forest stands of differing types (aspen, jack pine, northern hardwood, black spruce) and disturbance history (fire, harvest) by comparing model simulations with observations. The model reasonably predicted differences among forest types in productivity, foliar N, leaf area index, and net N mineralization. Model simulations that included past disturbances minimally improved predictions compared to simulations without disturbance, suggesting the legacy of past disturbances played a minor role in influencing current forest productivity rates. Modeled NPP was more sensitive to the intensity of soil removal during a disturbance than the fraction of stand mortality or wood removal. Increasing crown fire frequency resulted in lower NPP, particularly for conifer forest types with longer leaf life spans and longer recovery times. These findings suggest that, over long time periods, moderate frequency disturbances are a relatively less important control on productivity than climate, soil, and species traits.     

Fire disturbance and forest structure in old‐growth mixed conifer forests in the northern Sierra Nevada,California

Question: This study evaluates how fire regimes influence stand structure and dynamics in old‐growth mixed conifer forests across a range of environmental settings. Location: A 2000‐ha area of mixed conifer forest on the west shore of Lake Tahoe in the northern Sierra Nevada, California. Methods: We quantified the age, size, and spatial structure of trees in 12 mixed conifer stands distributed across major topographic gradients. Fire history was reconstructed in each stand using fire scar dendrochronology. The influence of fire on stand structure was assessed by comparing the fire history with the age, size, and spatial structure of trees in a stand. Results: There was significant variation in species composition among stands, but not in the size, age and spatial patterning of trees. Stands had multiple size and age classes with clusters of similar aged trees occurring at scales of 113 ‐ 254 m². The frequency and severity of fires was also similar, and stands burned with low to moderate severity in the dormant season on average every 9–17 years. Most fires were not synchronized among stands except in very dry years. No fires have burned since ca. 1880. Conclusions: Fire and forest structure interact to perpetuate similar stand characteristics across a range of environmental settings. Fire occurrence is controlled primarily by spatial variation in fuel mosaics (e.g. patterns of abundance, fuel moisture, forest structure), but regional drought synchronizes fire in some years. Fire exclusion over the last 120 years has caused compositional and structural shifts in these mixed conifer forests.     

Effects of fire and spruce beetle outbreak legacies on the disturbance regime of a subalpine forest in Colorado

Aim There is increasing research attention being given to the role of interactions among natural disturbances in ecosystem processes. We studied the interactions between fire and spruce beetle (Dendroctonus rufipennis Kirkby) disturbances in a Colorado subalpine forest. The central questions of this research were: (1) How does fire history influence stand susceptibility to beetle outbreak? And conversely, (2) How does prior occurrence of a beetle outbreak influence stand susceptibility to subsequent fire? Methods We reconstructed the spatial disturbance history in a c. 4600 ha area by first identifying distinct patches in the landscape on aerial photographs. Then, in the field we determined the disturbance history of each patch by dating stand origin, fire scars, dates of mortality of dead trees, and releases on remnant trees. A geographical information system (GIS) was used to overlay disturbance by fire and spruce beetle. Results and main conclusions The majority of stands in the study area arose following large, infrequent, severe fires occurring in c. 1700, 1796 and 1880. The study area was also affected by a severe spruce beetle outbreak in the 1940s and a subsequent low‐severity fire. Stands that originated following stand‐replacing fire in the late nineteenth century were less affected by the beetle outbreak than older stands. Following the beetle outbreak, stands less affected by the outbreak were more affected by low‐severity fire than stands more severely affected by the outbreak. The reduced susceptibility to low‐severity fire possibly resulted from increased moisture on the forest floor following beetle outbreak. The landscape mosaic of this subalpine forest was strongly influenced by the interactions between fire and insect disturbances.     

Simulating the effects of future fire regimes on western Canadian boreal forests

Abstract. Effects of future fire regimes on boreal tree species and plant functional types were studied in W Canada using a simulation approach. Present (1975–1990) and future (2080–2100) fire regimes were simulated using data from the Canadian Global Coupled Model (CGCM1). The long‐term effects of these fire regimes were simulated using a stand level, boreal fire effects model (BORFIRE) developed for this study. Changes in forest composition and biomass storage due to future altered fire regimes were determined by comparing the effects of present and future fire regimes on forest stands over a 400‐yr period. Differences in the two scenarios after 400 yr indicate shifting trends in forest composition and biomass that can be expected as a result of future changes in the fire regime. The ecological impacts of altered fire regimes are discussed in terms of general plant functional types. The Canadian Global Coupled Model showed more severe burning conditions under future fire regimes including fires with greater intensity, greater depth of burn and greater total fuel consumption. Shorter fire cycles estimated for the future generally favoured species which resprout (fire endurers) or store seed (fire evaders). Species with no direct fire survival traits (fire avoiders) declined under shorter fire cycles. The moderately thick barked trait of fire resisters provided little additional advantage in crown fire dominated boreal forests. Many species represent PFTs with multiple fire survival traits. The fire evader and avoider PFT was adaptable to the widest range of fire cycles. There was a general increase in biomass storage under the simulated future fire regimes caused by a shift in species composition towards fast‐growing re‐sprouting species. Long‐term biomass storage was lower in fire exclusion simulations because some stands were unable to reproduce in the absence of fire.     

What explains landscape patterns of tree mortality caused by bark beetle outbreaks in Greater Yellowstone?

Aim Bark beetle outbreaks have recently affected extensive areas of western North American forests, and factors explaining landscape patterns of tree mortality are poorly understood. The objective of this study was to determine the relative importance of stand structure, topography, soil characteristics, landscape context (the characteristics of the landscape surrounding the focal stand) and beetle pressure (the abundance of local beetle population eruptions around the focal stand a few years before the outbreak) to explain landscape patterns of tree mortality during outbreaks of three species: the mountain pine beetle, which attacks lodgepole pine and whitebark pine; the spruce beetle, which feeds on Engelmann spruce; and the Douglas‐fir beetle, which attacks Douglas‐fir. A second objective was to identify common variables that explain tree mortality among beetle–tree host pairings during outbreaks. Location Greater Yellowstone ecosystem, Wyoming, USA. Methods We used field surveys to quantify stand structure, soil characteristics and topography at the plot level in susceptible stands of each forest type showing different severities of infestation (0–98% mortality; n= 129 plots). We then used forest cover and beetle infestation maps derived from remote sensing to develop landscape context and beetle pressure metrics at different spatial scales. Plot‐level and landscape‐level variables were used to explain outbreak severity. Results Engelmann spruce and Douglas‐fir mortality were best predicted using landscape‐level variables alone. Lodgepole pine mortality was best predicted by both landscape‐level and plot‐level variables. Whitebark pine mortality was best – although poorly – predicted by plot‐level variables. Models including landscape context and beetle pressure were much better at predicting outbreak severity than models that only included plot‐level measures, except for whitebark pine. Main conclusions Landscape‐level variables, particularly beetle pressure, were the most consistent predictors of subsequent outbreak severity within susceptible stands of all four host species. These results may help forest managers identify vulnerable locations during ongoing outbreaks.     

Charcoal Reflectance Reveals Early Holocene Boreal Deciduous Forests Burned at High Intensities

Wildfire size, frequency, and severity are increasing in the Alaskan boreal forest in response to climate warming. One of the potential impacts of this changing fire regime is the alteration of successional trajectories, from black spruce to mixed stands dominated by aspen, a vegetation composition not experienced since the early Holocene. Such changes in vegetation composition may consequently alter the intensity of fires, influencing fire feedbacks to the ecosystem. Paleorecords document past wildfire-vegetation dynamics and as such, are imperative for our understanding of how these ecosystems will respond to future climate warming. For the first time, we have used reflectance measurements of macroscopic charcoal particles (>180μm) from an Alaskan lake-sediment record to estimate ancient charring temperatures (termed pyrolysis intensity). We demonstrate that pyrolysis intensity increased markedly from an interval of birch tundra 11 ky ago (mean 1.52%Ro; 485°C), to the expansion of trees on the landscape ∼10.5 ky ago, remaining high to the present (mean 3.54%Ro; 640°C) irrespective of stand composition. Despite differing flammabilities and adaptations to fire, the highest pyrolysis intensities derive from two intervals with distinct vegetation compositions. 1) the expansion of mixed aspen and spruce woodland at 10 cal. kyr BP, and 2) the establishment of black spruce, and the modern boreal forest at 4 cal. kyr BP. Based on our analysis, we infer that predicted expansion of deciduous trees into the boreal forest in the future could lead to high intensity, but low severity fires, potentially moderating future climate-fire feedbacks.     

Simulating the Effects of Fire Reintroduction Versus Continued Fire Absence on Forest Composition and Landscape Structure in the Boundary Waters Canoe Area, Northern Minnesota, USA

The Boundary Waters Canoe Area (BWCA) Wilderness of northern Minnesota, USA, ememplifies how fire management and natural disturbance determine forest composition and landscape structure at a broad scale. Historically, the BWCA (>400,000 ha) was subject to crown fires with a mean rotation period of 50–100 y. Fires often overlapped, creating a mosaic of differently aged stands with many stands burning frequently or, alternatively, escaping fire for several centuries. The BWCA may never have reached a steady-state (defined as a stable landscape age-class structure). In the early 1900s, a diminished fire regime began creating a more demographically diverse forest, characterized by increasingly uneven-aged stands. Shade-tolerant species typical of the region began replacing the shade-intolerant species that composed the fire-generated even-aged stands. Red pine (Pinus resinosa) stands are relatively uncommon in the BWCA today and are of special concern. The replacement of early-to-midsuccessional species is occurring at the scale of individual gaps, producing mixed-species multiaged forests. We used LANDIS, a spatially explicit forest landscape model, to investigate the long-term consequences of fire reintroduction or continuing fire absence on forest composition and landscape structure. Fire reintroduction was evaluated at three potential mean fire rotation periods (FRP): 50,100, and 300 y. Our model scenarios predict that if fire reintroduction mimics the natural fire regime (bracketed by FRP = 50 and 100 y), it will be most successful at preserving the original species composition and landscape structure, although jack pine (Pinus banksiana) may require special management. With limited fire reintroduction, all of the extant species are retained although species dominance and landscape structure will be substantially altered. If fire remains absent, many fire-dependent species will be lost as local dominants, including red pine. The landscape appears to be in a state of rapid change and a shift in management to promote fire may need to be implemented soon to prevent further deviation from historic, presettlement conditions.     

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.     

Predicting the composition of Canadian southern boreal forest in different fire cycles

Abstract. Post-fire succession was reconstructed for a sector located in the southern part of the Québec boreal forest. Forest composition for different periods since fire was evaluated using a stand initiation map together with ecological maps representing both site conditions and stand types. Nine fires covering at least 100 ha and representing a chronosequence of more than 230 yr were used. Although a relatively clear successional pattern from deciduous to coniferous composition relating to time-since-fire was observed, Pinus banksiana stands showed an erratic distribution not related to succession but possibly to the pre-fire stand composition. A comparison with forest cover maps produced after a recent spruce budworm outbreak, showed that succession toward coniferous dominance appeared to be interrupted by spruce budworm (Choristoneura fumiferana) outbreaks which, by killing Abies balsamea, lead to a mixed deciduous forest composition. A simple empirical model based on a negative exponential distribution of age classes was developed to evaluate how changes in the fire cycle would affect the composition of the forest mosaic. The transition between deciduous dominance and coniferous dominance occurs in a fire cycle > 200 yr. Although pure deciduous stands tend to disappear during long fire cycles, the proportion of mixed stands remains relatively constant. Prediction of the forest composition for longer fire cycles is complicated by the interaction between post-fire composition and stand vulnerability to spruce budworm outbreaks.     

Influence of fire severity on stand development of Araucaria araucana–Nothofagus pumilio stands in the Andean cordillera of south‐central Chile

Fire is the prevalent disturbance in the Araucaria–Nothofagus forested landscape in south‐central Chile. Although both surface and stand‐replacing fires are known to characterize these ecosystems, the variability of fire severity in shaping forest structure has not previously been investigated in Araucaria–Nothofagus forests. Age structures of 16 stands, in which the ages of approximately 650 trees were determined, indicate that variability in fire severity and frequency is key to explaining the mosaic of forest patches across the Araucaria–Nothofagus landscape. High levels of tree mortality in moderate‐ to high‐severity fires followed by new establishment of Nothofagus pumilio typically result in stands characterized by one or two cohorts of this species. Large Araucaria trees are highly resistant to fire, and this species typically survives moderate‐ to high‐severity fires either as dispersed individuals or as small groups of multi‐aged trees. Small post‐fire cohorts of Araucaria may establish, depending on seed availability and the effects of subsequent fires. Araucaria's great longevity (often >700 years) and resistance to fire allow some individuals to survive fires that kill and then trigger new Nothofagus cohorts. Even in relatively mesic habitats, where fires are less frequent, the oldest Araucaria–Nothofagus pumilio stands originated after high‐severity fires. Overall, stand development patterns of subalpine Araucaria–N. pumilio forests are largely controlled by moderate‐ to high‐severity fires, and therefore tree regeneration dynamics is strongly dominated by a catastrophic regeneration mode.     

Examining forest resilience to changing fire frequency in a fire‐prone region of boreal forest

Future changes in climate are widely anticipated to increase fire frequency, particularly in boreal forests where extreme warming is expected to occur. Feedbacks between vegetation and fire may modify the direct effects of warming on fire activity and shape ecological responses to changing fire frequency. We investigate these interactions using extensive field data from the Boreal Shield of Saskatchewan, Canada, a region where >40% of the forest has burned in the past 30 years. We use geospatial and field data to assess the resistance and resilience of eight common vegetation states to frequent fire by quantifying the occurrence of short‐interval fires and their effect on recovery to a similar vegetation state. These empirical relationships are combined with data from published literature to parameterize a spatially explicit, state‐and‐transition simulation model of fire and forest succession. We use this model to ask if and how: (a) feedbacks between vegetation and wildfire may modify fire activity on the landscape, and (b) more frequent fire may affect landscape forest composition and age structure. Both field and GIS data suggest the probability of fire is low in the initial decades after fire, supporting the hypothesis that fuel accumulation may exert a negative feedback on fire frequency. Field observations of pre‐ and postfire composition indicate that switches in forest state are more likely in conifer stands that burn at a young age, supporting the hypothesis that resilience is lower in immature stands. Stands dominated by deciduous trees or jack pine were generally resilient to fire, while mixed conifer and well‐drained spruce forests were less resilient. However, simulation modeling suggests increased fire activity may result in large changes in forest age structure and composition, despite the feedbacks between vegetation–fire likely to occur with increased fire activity.