Increases In Fire Severity Research Articles

Increasing fire severity alters the species composition and decreases richness of seeds potentially dispersed by small mammals

AbstractSeed dispersal is a key process that influences the recruitment of plant species, yielding profound consequences on vegetation structure. This process can be affected by environmental disturbances, such as fires, leading to varied impacts on the agents involved, both dispersers and plants. We investigated the potential role of small mammals (rodents and marsupials) as seed dispersers in a Neotropical savanna (Brazilian Cerrado). We also evaluated the impact of fire severity on the seed dispersal services provided by these animals in gallery forests, 3 years after a major fire event. We live‐trapped small mammals in 14 sites, 7 of which were located in forests classified in three categories of fire severity (“unburned”, “low severity”, or “moderate severity”). We captured 13 mammal species, finding a total of 11,688 seeds in their feces (collected inside traps or handling bags). These seeds encompassed 28 plant species from at least 8 families. A multivariate analysis (NMDS) revealed differences among small mammal species in terms of potentially dispersed plants, with seeds of 18 plant species being consumed by no more than one small mammal species. An increase in fire severity affected the composition of plant species potentially dispersed, changed the relative role of forest small mammals as seed dispersers, and decreased observed richness of defecated seeds. We conclude that high‐severity wildfires affect highly relevant ecological processes, such as seed dispersal by animals. These effects could impact the recruitment dynamics of plant species and, consequently, the vegetation recovery and plant community composition in fire‐affected areas.Abstract in Portuguese is available with online material.

Fire refugia are robust across Western US forested ecoregions, 1986–2021

In the Western US, area burned and fire size have increased due to the influences of climate change, long-term fire suppression leading to higher fuel loads, and increased ignitions. However, evidence is less conclusive about increases in fire severity within these growing wildfire extents. Fires burn unevenly across landscapes, leaving islands of unburned or less impacted areas, known as fire refugia. Fire refugia may enhance post-fire ecosystem function and biodiversity by providing refuge to species and functioning as seed sources after fires. In this study, we evaluated whether the proportion and pattern of fire refugia within fire events have changed over time and across ecoregions. To do so, we processed all available Landsat 4–9 satellite imagery to identify fire refugia within the boundaries of large wildfires (405 ha+) in 16 forested ecoregions of the Western US. We found a significant change in % refugia from 1986–2021 only in one ecoregion—% refugia increased within fires in the Arizona/New Mexico Mountain ecoregion (AZ/NM). Excluding AZ/NM, we found no significant change in % refugia across the study area. Furthermore, we found no significant change in mean refugia patch size, patch density, or mean distance to refugia. As fire size increased, the amount of refugia increased proportionally. Evidence suggests that fires in AZ/NM had a higher proportion of reburns and, unlike the 15 other ecoregions, fires did not occur at higher elevation or within greener areas. We suggest several possibilities for why, with the exception of AZ/NM, ecoregions did not experience a significant change in the proportion and pattern of refugia. In summary, while area burned has increased over the past four decades, there are substantial and consistent patterns of refugia that could support post-fire recovery dependent on their spatial patterns and ability to function as seeds sources for neighboring burned patches.

Fuel build-up promotes an increase in fire severity of reburned areas in fire-prone ecosystems of the western Mediterranean Basin

BackgroundFire-vegetation feedbacks can modulate the global change effects conducive to extreme fire behavior and high fire severity of subsequent wildfires in reburn areas by altering the composition, flammability traits, and spatial arrangement of fuels. Repeated, high-severity wildfires at short return intervals may trigger long-term vegetation state transitions. However, empirical evidence about these feedbacks is absent in fire-prone ecosystems of the western Mediterranean Basin, where the response of fire activity has been enhanced by contemporary socioeconomic and land-use changes. Here, we evaluated whether fire severity differs between initial burns and subsequent wildfires in reburn areas (fire-free periods = 10–15 years) of maritime pine and Aleppo pine forests, holm oak woodlands, and shrublands in the western Mediterranean Basin, and whether there is a relationship between the severity of such interactive wildfire disturbances. We also tested how the type of ecosystem and changes in vegetation structure after the initial wildfires influence these relationships. We leveraged Landsat-based fire severity estimates for initial and last wildfires using the Relativized Burn Ratio (RBR) and Light Detection and Ranging (LiDAR) data acquired before the last wildfire.ResultsFire severity of the last wildfire was significantly higher than that of the initial wildfire for each dominant ecosystem type in reburn areas. These differences were very pronounced in maritime pine forests and shrublands. For consistency, the same patterns were evidenced for the fire severity in reburn and first-entry areas of the last wildfire for each dominant ecosystem type. Fire severity of the last wildfire in forests and woodlands (particularly maritime pine-dominated) raised with increasing severity of the previous wildfire to a greater extent than in shrublands. Pre-fire fuel density in the lower vegetation strata (up to 4 m high in maritime and Aleppo pine forests, as well as in shrublands, and up to 2 m high in holm oak forests) was significantly higher in reburn than in first-entry areas of the last wildfire.ConclusionsOur results suggest that land managers should promote more fire-resistant landscapes to high fire severity by minimizing fuel build-up and thus fire hazard through pre-fire fuel reduction treatments such as prescribed burning.

Climate influences on future fire severity: a synthesis of climate-fire interactions and impacts on fire regimes, high-severity fire, and forests in the western United States

Abstract Background Increases in fire activity and changes in fire regimes have been documented in recent decades across the western United States. Climate change is expected to continue to exacerbate impacts to forested ecosystems by increasing the frequency, size, and severity of wildfires across the western United States (US). Warming temperatures and shifting precipitation patterns are altering western landscapes and making them more susceptible to high-severity fire. Increases in large patches of high-severity fire can result in significant impacts to landscape processes and ecosystem function and changes to vegetation structure and composition. In this synthesis, we examine the predicted climatic influence on fire regimes and discuss the impacts on fire severity, vegetation dynamics, and the interactions between fire, vegetation, and climate. We describe predicted changes, impacts, and risks related to fire with climate change and discuss how management options may mitigate some impacts of predicted fire severity, and moderate some impacts to forests, carbon, and vegetation changes post fire. Results Climate change is increasing fire size, fire severity, and driving larger patches of high-severity fire. Many regions are predicted to experience an increase in fire severity where conditions are hotter and drier and changes in fire regimes are evident. Increased temperatures, drought conditions, fuels, and weather are important drivers of fire severity. Recent increases in fire severity are attributed to changes in climatic water deficit (CMD), vapor pressure deficit (VPD), evapotranspiration (ET), and fuels. Fire weather and vegetation species composition also influence fire severity. Future increases in fire severity are likely to impact forest resilience and increase the probability of forest type conversions in many ecosystems. Conclusions Increasing warming and drying trends are likely to cause more frequent and severe disturbances in many forested ecosystems in the near future. Large patches of high-severity fire have lasting legacies on vegetation composition and structure, and impacts on tree regeneration. In some ecosystems and under certain fire-weather conditions, restoration and fuel treatments may reduce the area burned at high severity and reduce conversions from forest to non-forest conditions, increasing forest resistance and resilience to wildland fire. Thinning and prescribed fire treatments can be effective at reducing the potential for crown fire, reducing fuels, and promoting forest resilience.

The magnitude and pace of photosynthetic recovery after wildfire in California ecosystems

Wildfire modifies the short- and long-term exchange of carbon between terrestrial ecosystems and the atmosphere, with impacts on ecosystem services such as carbon uptake. Dry western US forests historically experienced low-intensity, frequent fires, with patches across the landscape occupying different points in the fire-recovery trajectory. Contemporary perturbations, such as recent severe fires in California, could shift the historic stand-age distribution and impact the legacy of carbon uptake on the landscape. Here, we combine flux measurements of gross primary production (GPP) and chronosequence analysis using satellite remote sensing to investigate how the last century of fires in California impacted the dynamics of ecosystem carbon uptake on the fire-affected landscape. A GPP recovery trajectory curve of more than five thousand fires in forest ecosystems since 1919 indicated that fire reduced GPP by [Formula: see text] g C m[Formula: see text] y[Formula: see text]([Formula: see text]) in the first year after fire, with average recovery to prefire conditions after [Formula: see text] y. The largest fires in forested ecosystems reduced GPP by [Formula: see text] g C m[Formula: see text] y[Formula: see text] (n = 401) and took more than two decades to recover. Recent increases in fire severity and recovery time have led to nearly [Formula: see text] MMT CO[Formula: see text] (3-y rolling mean) in cumulative forgone carbon uptake due to the legacy of fires on the landscape, complicating the challenge of maintaining California's natural and working lands as a net carbon sink. Understanding these changes is paramount to weighing the costs and benefits associated with fuels management and ecosystem management for climate change mitigation.

Effects of Fire Severity and Woody Debris on Tree Regeneration for Exploratory Well Pads in Jack Pine (Pinus banksiana) Forests

Restoring anthropogenic footprints to pre-disturbance conditions or minimizing their long-term impacts is an important goal in conservation. Many footprints, particularly if left alone, have wide-ranging effects on biodiversity. In Canada, energy exploration footprints result in forest dissection and fragmentation contributing to declines in woodland caribou. Developing cost effective strategies to restore forests and thus conserving the woodland caribou habitat is a conservation priority. In this study, we compared the effects of wildfire and local variation in the amount of residual woody debris on natural regeneration in jack pine on exploratory well pads in Alberta’s boreal forest. Specifically, we investigated how footprint size, fire severity (overstory tree mortality), ground cover of fine and coarse woody debris, and adjacent stand characteristics (i.e., height, age, and cover), affected tree regeneration densities and height using negative binomial count and linear models (Gaussian), respectively. Regeneration density was 30% higher on exploratory well pads than adjacent forests, increased linearly with fire severity on the exploratory well pads (2.2% per 1% increase in fire severity), but non-linearly in adjacent forests (peaking at 51,000 stems/ha at 72% fire severity), and decreased with amount of woody debris on exploratory well pads (2.7% per 1% increase in woody debris cover). The height of regenerating trees on exploratory well pads decreased with fire severity (0.56 cm per 1% increase in fire severity) and was non-linearly related to coarse woody debris (peaking at 286 cm at 9.4% coarse woody debris cover). Heights of 3 and 5 m on exploratory well pads were predicted by 13- and 21-years post-fire, respectively. Our results demonstrate that wildfires can stimulate natural recovery of fire-adapted species, such as jack pine, on disturbances as large as exploratory well pads (500–1330 m2) and that the type and amount of woody debris affects these patterns.

The effects of wildfire severity and pyrodiversity on bat occupancy and diversity in fire-suppressed forests

Wildfire is an important ecological process that influences species’ occurrence and biodiversity generally. Its effect on bats is understudied, creating challenges for habitat management and species conservation as threats to the taxa worsen globally and within fire-prone ecosystems. We conducted acoustic surveys of wildfire areas during 2014–2017 in conifer forests of California’s Sierra Nevada Mountains. We tested effects of burn severity and its variation, or pyrodiversity, on occupancy and diversity for the 17-species bat community while accounting for imperfect detection. Occupancy rates increased with severity for at least 6 species and with pyrodiversity for at least 3. Two other species responded negatively to pyrodiversity. Individual species models predicted maximum occupancy rates across burn severity levels but only one species occurred most often in undisturbed areas. Species richness increased from approximately 8 species in unburned forests to 11 in pyrodiverse areas with moderate- to high-severity. Greater accessibility of foraging habitats, as well as increased habitat heterogeneity may explain positive responses to wildfire. Many bat species appear well adapted to wildfire, while a century of fire suppression and forest densification likely reduced habitat quality for the community generally. Relative to other taxa, bats may be somewhat resilient to increases in fire severity and size; trends which are expected to continue with accelerating climate change.

The species diversity × fire severity relationship is hump‐shaped in semiarid yellow pine and mixed conifer forests

AbstractThe combination of direct human influences and the effects of climate change are resulting in altered ecological disturbance regimes, and this is especially the case for wildfires. Many regions that historically experienced low–moderate severity fire regimes are seeing increased area burned at high severity as a result of interactions between high fuel loads and climate warming with a number of negative ecological effects. While ecosystem impacts of altered fire regimes have been examined in the literature, little is known of the effects of changing fire regimes on forest understory plant diversity even though understory taxa comprise the vast majority of forest plant species and play vital roles in overall ecosystem function. We examined understory plant diversity across gradients of wildfire severity in eight large wildfires in yellow pine and mixed conifer temperate forests of the Sierra Nevada, California, USA. We found a generally unimodal hump‐shaped relationship between local (alpha) plant diversity and fire severity. High‐severity burning resulted in lower local diversity as well as some homogenization of the flora at the regional scale. Fire severity class, post‐fire litter cover, and annual precipitation were the best predictors of understory species diversity. Our research suggests that increases in fire severity in systems historically characterized by low and moderate severity fire may lead to plant diversity losses. These findings indicate that global patterns of increasing fire size and severity may have important implications for biodiversity.

Context dependent fungal and bacterial soil community shifts in response to recent wildfires in the Southern Appalachian Mountains

Decades of fire suppression coupled with changing climatic conditions have increased the frequency and intensity of wildfires. The Southern Appalachia region of the United States is predicted to be particularly susceptible to climatic changes, with predicted increases in fire severity and occurrence. Following the record breaking fire season in 2016 in Southern Appalachia, we examined wildfire impacts on soil chemistry and below ground communities (fungi and bacteria – Illumina MiSeq) within two substrates (duff and soil) at two adjacent locations with similar plant communities (Great Smoky Mountains National Park – ‘Chimney Top 2’ Fire (GRSM) and Nantahala National Forest – ‘Cliffside’ Fire (NNF)) from replicate plots representing a range of fire severities (unburned, low severity, moderate severity, severe). Differing fire severities changed community composition, and fire severity played a stronger role in structuring bacterial communities than in structuring fungal communities. Further, fire impacts on soil communities and functional guilds responses were location- and substrate-specific with NNF responding more strongly to fire than GRSM. Additionally, using a novel analysis tool (Axis Weighted Ordination Distance – AWOrD), domain and location specific responses to wildfire severity are demonstrated. Taken together, our results suggest context-dependency in microbial responses to fire that must be accounted for to generate ecosystem-wide recovery predictions.

Forest Fire Severity Affects Host Plant Quality and Insect Herbivore Damage

Abstract Climate change models predict increased forest fire occurrence and severity in the near future. Forest fire disturbance affects multiple ecological interactions, but there is little evidence for how naturally-occurring fires affect plant quality and herbivore damage, which is important because plants and herbivorous insects comprise most of the diversity in natural ecosystems and are responsible for a variety of ecosystem services. We surveyed three fires in the Rocky Mountains to investigate the effects of fire severity on wax currant (Ribes cereum), an important source of food and cover for wildlife in Colorado. We measured plant quality and herbivore damage; we found that fire severity had a significant negative effect on both measures. Notably, high severity fires decreased herbivore damage by about 50%. Furthermore, we found that the effect of fire on insect herbivore damage is mostly direct, but that indirect effects mediated through changes in plant quality are also significant. Our results have important implications for the effects of climate-driven increases in fire severity on plant-insect interactions, illustrating strong direct and weaker indirect negative effects of fire severity in a forest ecosystem.

Altered fire regimes cause long-term lichen diversity losses.

Many global ecosystems have undergone shifts in fire regimes in recent decades, such as changes in fire size, frequency, and/or severity. Recent research shows that increases in fire size, frequency, and severity can lead to long-persisting deforestation, but the consequences of shifting fire regimes for biodiversity of other vegetative organisms (such as understory plants, fungi, and lichens) remain poorly understood. Understanding lichen responses to wildfire is particularly important because lichens play crucial roles in nutrient cycling and supporting wildlife in many ecosystems. Lichen responses to fire have been little studied, and most previous research has been limited to small geographic areas (e.g. studies of a single fire), making it difficult to establish generalizable patterns. To investigate long-term effects of fire severity on lichen communities, we sampled epiphytic lichen communities in 104 study plots across California's greater Sierra Nevada region in areas that burned in five wildfires, ranging from 4 to 16years prior to sampling. The conifer forest ecosystems we studied have undergone a notable increase in fire severity in recent decades, and we sample across the full gradient of fire severity to infer how shifting fire regimes may influence landscape-level biodiversity. We find that low-severity fire has little to no effect on lichen communities. Areas that burned at moderate and high severities, however, have significantly and progressively lower lichen richness and abundance. Importantly, we observe very little postfire lichen recolonization on burned substrates even more than 15years after fire. Our multivariate model suggests that the hotter, drier microclimates that occur after fire removes forest canopies may prevent lichen reestablishment, meaning that lichens are not likely to recolonize until mature trees regenerate. These findings suggest that altered fire regimes may cause broad and long-persisting landscape-scale biodiversity losses that could ultimately impact multiple trophic levels.

Fire severity is more sensitive to low fuel moisture content on Calluna heathlands than on peat bogs

Moorland habitats dominated by the dwarf shrub Calluna vulgaris provide important ecosystem services. Drought is projected to intensify throughout their range, potentially leading to increased fire severity as moisture is a key control on severity. We studied the effect of low fuel moisture content (FMC) on fire severity by using 2×2m rain-out shelters prior to completing 19 experimental fires in two sites in Scotland (UK): a dry heath with thin organic soils and a raised bog with deep, saturated peat, both dominated by Calluna vulgaris. Reduced FMC of the moss and litter (M/L) layer at both sites, and the soil moisture of the dry heath, increased fire-induced consumption of the M/L layer and soil heating at both sites. Increase in fire severity was greater at the dry heath than at the raised bog, e.g. average maximum temperatures at the soil surface increased from 31°C to 189°C at the dry heath, but only from 10°C to 15°C at the raised bog. Substantial M/L layer consumption was observed when its FMC was below 150%. This led to larger seasonal and daily soil temperature fluctuation, particularly at the dry heath during warm months. The results suggest that low FMC following predicted changes in climate are likely to increase wildfire severity and that the impact on vegetation composition and carbon stores may be greater at heathlands than at peatlands. Managed burning aiming to minimise fire severity (e.g. ignition of the M/L layer and exposure to lethal temperatures of ericoid seeds) should be carried out when the FMC of the M/L layer is above 150% and the FMC of the soil is above 200–300%.

Moose alter the rate but not the trajectory of forest canopy succession after low and high severity fire in Alaska

Mammalian herbivory on palatable trees affects tree growth, forest composition, and forest succession. Antecedent effects of herbivores can be identified through remnants of dead stems and altered tree morphology as well as changes in tree ring patterns and growth. Increases in fire severity, particularly surface fuel combustion, in the boreal forest of western North America can cause a shift in the successional trajectory from coniferous to deciduous dominated forests, which may alter plant–animal interactions. We measured height and tree ring growth of the two dominant canopy tree species, trembling aspen (Populus tremuloides Michx.) and black spruce (Picea mariana (Mill.) BSP), in sites that experienced stand-replacing fire with deep versus shallow surface fuel combustion 20years ago. We also classified individual trees into a category of browsing damage based on external features and morphology. We hypothesized the effects of browsing to be contingent on fire severity. Using linear mixed effect models, we investigated the main and interactive effects of fire severity and browsing intensity on aspen growth. We also developed tree ring chronologies to test for growth releases in aspen and black spruce.Effects of moose browsing on aspen growth depended on fire severity (surface fuel combustion), with negative effects in high severity sites and no effects in low severity sites. Spruce growth showed no direct or indirect browsing effects, indicating moose have not altered the potential for spruce to reach the forest canopy. Aspen in severely burned sites showed abrupt growth releases in tree rings corresponding to changes in herbivore pressure and density. Height-growth projections indicated that moose slowed the rate of aspen growth and canopy dominance in severely burned areas by ten years, through repeated stunting of apical growth in aspen, without affecting the initial trajectory to an aspen-dominated canopy. Lightly burned areas, with their larger proportion of spruce biomass, slower aspen growth, and reduced browsing pressure during the first 20years after fire, will likely provide accessible aspen forage until >50years post-fire as sites return to spruce dominance. Heterogeneously burned forests could thus sustain high rates of available moose forage for a much longer period than previously reported. Our study highlights the importance of including fire severity when considering the impacts of large herbivores on tree growth and forest structure.

Post-fire soil functionality and microbial community structure in a Mediterranean shrubland subjected to experimental drought

Fire may cause significant alterations in soil properties. Post-fire soil dynamics can vary depending, among other factors, on rainfall patterns. However, little is known regarding variations in response to post-fire drought. This is relevant in arid and semiarid areas with poor soils, like much of the western Mediterranean. Furthermore, climate change projections in such areas anticipate reduced precipitation and longer annual drought periods, together with an increase in fire severity and frequency. This research evaluates the effects of experimental drought after fire on soil dynamics of a Cistus-Erica shrubland (Central Spain). A replicated (n=4) field experiment was conducted in which the total rainfall and its patterns were manipulated by means of a rain-out shelters and irrigation system. The treatments were: environmental control (natural rainfall), historical control (average rainfall, 2months drought), moderate drought (25% reduction of historical control, 5months drought) and severe drought (45% reduction, 7months drought). After one growing season under these rainfall treatments, the plots were burned. One set of unburned plots under natural rainfall served as an additional control. Soils were collected seasonally. Fire increased soil P and N availability. Post-fire drought treatments reduced available soil P but increased N concentration (mainly nitrate). Fire reduced available K irrespective of drought treatments. Fire reduced enzyme activities and carbon mineralization rate, a reduction that was higher in post-fire drought-treated soils. Fire decreased soil microbial biomass and the proportion of fungi, while that of actinomycetes increased. Post-fire drought decreased soil total microbial biomass and fungi, with bacteria becoming more abundant. Our results support that increasing drought after fire could compromise the resilience of Mediterranean ecosystems to fire.

Soil organic matter composition and quality across fire severity gradients in coniferous and deciduous forests of the southern boreal region

AbstractRecent patterns of prolonged regional drought in southern boreal forests of the Great Lakes region, USA, suggest that the ecological effects of disturbance by wildfire may become increasingly severe. Losses of forest soil organic matter (SOM) during fire can limit soil nutrient availability and forest regeneration. These processes are also influenced by the composition of postfire SOM. We sampled the forest floor layer (i.e., full organic horizon) and 0–10 cm mineral soil from stands dominated by coniferous (Pinus banksiana Lamb.) or deciduous (Populus tremuloides Michx.) species 1–2 months after the 2011 Pagami Creek wildfire in northern Minnesota. We used solid‐state 13C NMR to characterize SOM composition across a gradient of fire severity in both forest cover types. SOM composition was affected by fire, even when no statistically significant losses of total C stocks were evident. The most pronounced differences in SOM composition between burned and unburned reference areas occurred in the forest floor for both cover types. Carbohydrate stocks in forest floor and mineral horizons decreased with severity level in both cover types, whereas pyrogenic C stocks increased with severity in the coniferous forest floor and decreased in only the highest severity level in the deciduous forest floor. Loss of carbohydrate and lignin pools contributed to a decreased SOM stability index and increased decomposition index. Our results suggest that increases in fire severity expected to occur under future climate scenarios may lead to changes in SOM composition and dynamics with consequences for postfire forest recovery and C uptake.

Differences in land ownership, fire management objectives and source data matter: a reply to Hanson and Odion (2014)

We respond to Hanson and Odion (2014), who claim in this journal (vol. 23, no. 1, pp. 1–8) that their reanalysis of fire severity patterns in and around the Sierra Nevada refutes earlier work showing increases in fire severity in certain forest types over the last 3 decades. Hanson and Odion base their reanalysis on a highly inaccurate, very coarse-scale, and geographically misregistered vegetation map. Also, in contrast to the previous work, which was restricted to wildfires on Forest Service lands in forest types differentiated by their fire regimes, Hanson and Odion combine all types of fires on lands of all jurisdictions and stratify by very broad, unorthodox vegetation types that conjoin very different fire regimes. As such, their work does not constitute a test of the previous work. We present analyses that demonstrate sources of error associated with Hanson and Odion’s data and the analyses they perform, and explore how that error might confound their results. Fundamental and compounded problems in Hanson and Odion (2014) cast strong doubt on their conclusions.

Sources and sinks of carbon in boreal ecosystems of interior Alaska: A review

Abstract Boreal ecosystems store large quantities of carbon but are increasingly vulnerable to carbon loss due to disturbance and climate warming. The boreal region in Alaska and Canada, largely underlain by discontinuous permafrost, presents a challenging landscape for itemizing carbon sources and sinks in soil and vegetation. The roles of fire, forest succession, and the presence (or absence) of permafrost on carbon cycle, vegetation, and hydrologic processes have been the focus of multidisciplinary research in boreal ecosystems for the past 20 years. However, projections of a warming future climate, an increase in fire severity and extent, and the potential degradation of permafrost could lead to major landscape and carbon cycle changes over the next 20 to 50 years. To assist land managers in interior Alaska in adapting and managing for potential changes in the carbon cycle we developed this review paper by incorporating an overview of the climate, ecosystem processes, vegetation, and soil regimes. Our objective is to provide a synthesis of the most current carbon storage estimates and measurements to guide policy and land management decisions on how to best manage carbon sources and sinks. We surveyed estimates of aboveground and belowground carbon stocks for interior Alaska boreal ecosystems and summarized methane and carbon dioxide fluxes. These data have been converted into similar units to facilitate comparison across ecosystem compartments. We identify potential changes in the carbon cycle with climate change and human disturbance. A novel research question is how compounding disturbances affect carbon sources and sinks associated with boreal ecosystem processes. Finally, we provide recommendations to address the challenges facing land managers in efforts to manage carbon cycle processes. The results of this study can be used for carbon cycle management in other locations within the boreal biome which encompasses a broad distribution from 45° to 83° north.

Persistent effects of fire severity on early successional forests in interior Alaska

There has been a recent increase in the frequency and extent of wildfires in interior Alaska, and this trend is predicted to continue under a warming climate. Although less well documented, corresponding increases in fire severity are expected. Previous research from boreal forests in Alaska and western Canada indicate that severe fire promotes the recruitment of deciduous tree species and decreases the relative abundance of black spruce (Picea mariana) immediately after fire. Here we extend these observations by (1) examining changes in patterns of aspen and spruce density and biomass that occurred during the first two decades of post-fire succession, and (2) comparing patterns of tree composition in relation to variations in post-fire organic layer depth in four burned black spruce forests in interior Alaska after 10–20 years of succession. We found that initial effects of fire severity on recruitment and establishment of aspen and black spruce were maintained by subsequent effects of organic layer depth and initial plant biomass on plant growth during post-fire succession. The proportional contribution of aspen (Populus tremuloides) to total stand biomass remained above 90% during the first and second decades of succession in severely burned sites, while in lightly burned sites the proportional contribution of aspen was reduced due to a 40-fold increase in spruce biomass in these sites. Relationships between organic layer depth and stem density and biomass were consistently negative for aspen, and positive or neutral for black spruce in all four burns. Our results suggest that initial effects of post-fire organic layer depths on deciduous recruitment are likely to translate into a prolonged phase of deciduous dominance during post-fire succession in severely burned stands. This shift in vegetation distribution has important implications for climate-albedo feedbacks, future fire regime, wildlife habitat quality and natural resources for indigenous subsistence activities in interior Alaska.

Fire severity in a northern Australian savanna landscape: the importance of time since previous fire

Using a detailed fire history collected over a 10-year period throughout a savanna landscape in northern Australia, we have addressed the question of whether fire severity, inferred from a semiquantitative fire severity index, increases with time since previous fire. There was a clear trend of fires becoming much more severe with increasing time since previous fire. Between 1 and 5 years following a fire, the probability of a subsequent fire being classified as ‘severe’ increased from 3 to 8% for early dry-season fires, and from 21 to 43% for late dry-season fires. It was clear that the strong increase in fire severity was not confined to the first 2–3 years following the previous fire, as previously suspected. These findings highlight the difficulty of reducing both fire frequency and severity in northern Australian savanna landscapes, as they imply that a negative feedback process exists between the two; that is, reducing fire frequency is likely to increase the severity of fires that do occur.

Climate, lightning ignitions, and fire severity in Yosemite National Park, California, USA

Continental-scale studies of western North America have attributed recent increases in annual area burned and fire size to a warming climate, but these studies have focussed on large fires and have left the issues of fire severity and ignition frequency unaddressed. Lightning ignitions, any of which could burn a large area given appropriate conditions for fire spread, could be the first indication of more frequent fire. We examined the relationship between snowpack and the ignition and size of fires that occurred in Yosemite National Park, California (area 3027 km2), between 1984 and 2005. During this period, 1870 fires burned 77 718 ha. Decreased spring snowpack exponentially increased the number of lightning-ignited fires. Snowpack mediated lightning-ignited fires by decreasing the proportion of lightning strikes that caused lightning-ignited fires and through fewer lightning strikes in years with deep snowpack. We also quantified fire severity for the 103 fires >40 ha with satellite fire-severity indices using 23 years of Landsat Thematic Mapper data. The proportion of the landscape that burned at higher severities and the complexity of higher-severity burn patches increased with the log10 of annual area burned. Using one snowpack forecast, we project that the number of lightning-ignited fires will increase 19.1% by 2020 to 2049 and the annual area burned at high severity will increase 21.9%. Climate-induced decreases in snowpack and the concomitant increase in fire severity suggest that existing assumptions may be understated – fires may become more frequent and more severe.