Decades-old silvicultural treatments influence surface wildfire severity and post-fire nitrogen availability in a ponderosa pine forest

Wildfire severity and subsequent ecological effects may be influenced by prior land management, via modification of forest structure and lingering changes in fuels. In 2002, the Hayman wildfire burned as a low to moderate-severity surface fire through a 21-year pine regeneration experiment with two overstory harvest cuttings (shelterwood, seed-tree) and two site preparations (scarified, unscarified) that had been applied in a mature ponderosa pine forest in the montane zone of the Colorado Front Range in 1981. We used this event to examine how pre-fire fine fuels, surface-level burn severity and post-fire soil nitrogen-availability varied with pre-fire silvicultural treatments. Prior to the wildfire, litter cover was higher under both shelterwood and unscarified treatments than seed-tree and scarified treatments. Immediately after the fire in 2002, we assessed burn severity under 346 mature trees, around 502 planted saplings, and in 448 4 m² microplots nested within the original experimental treatments. In one-fourth of the microplots, we measured resin-bound soil nitrate and ammonium accumulated over the second and third post-fire growing season. Microplots burned less severely than bases of trees and saplings with only 6.8% of microplot area burned down to mineral soil as compared to >28% of tree and sapling bases. Sapling burn severity was highest in unscarified treatments but did not differ by overstory harvest. Microplot burn severity was higher under the densest overstory (shelterwood) and in unscarified treatments and was positively related to pre-fire litter/duff cover and negatively associated with pre-fire total plant cover, grass cover and distance to tree. In both years, resin-bound nitrate and ammonium (NH₄⁺-N) increased weakly with burn severity and NH₄⁺-N availability was higher in unscarified than scarified plots. The lasting effects of soil scarification and overstory harvest regime on modern patterns of surface burn severity after two decades underscores the importance of historic landuse and silviculture on fire behavior and ecological response. Unraveling causes of these patterns in burn severity may lead to more sustainable fire and forest management in ponderosa pine ecosystems.     

Influence of wildfire severity on riparian plant community heterogeneity in an Idaho,USA wilderness

Despite the increasing recognition of riparian zones as important ecotones that link terrestrial and aquatic ecosystems and of fire as a critical natural disturbance, much remains unknown regarding the influence of fire on stream-riparian ecosystems. To further this understanding, we evaluated the effects of mixed severity wildfire on riparian plant community structure and composition in headwater streams of the Big Creek Watershed of the Frank Church ‘River of No Return’ Wilderness of central Idaho. Five years after a large stand-replacing fire, we conducted riparian vegetation surveys at sixteen reaches across a range of burn types. Non-metric Multidimensional Scaling (NMS) and Multi-Response Permutation Procedure (MRPP) analyses showed an overall shift in community composition and structure between vegetation at unburned and severely burned reaches. Although total plant cover was significantly less at severely burned areas, recovery of the deciduous understory was apparent. Severely burned reaches were characterized by a marked increase in cheatgrass (Bromus tectorum). Reaches that were exposed to low-severity fire were indistinguishable from unburned reaches relative to vegetation community composition and structure, pointing to a possible disturbance threshold that may need to be crossed in order to alter riparian plant communities.     

Prescribed fires as ecological surrogates for wildfires: A stream and riparian perspective

Forest managers use prescribed fire to reduce wildfire risk and to provide resource benefits, yet little information is available on whether prescribed fires can function as ecological surrogates for wildfire in fire-prone landscapes. Information on impacts and benefits of this management tool on stream and riparian ecosystems is particularly lacking. We used a beyond-BACI (Before, After, Control, Impact) design to investigate the effects of a prescribed fire on a stream ecosystem and compared these findings to similar data collected after wildfire. For 3 years after prescribed fire treatment, we found no detectable changes in periphyton, macroinvertebrates, amphibians, fish, and riparian and stream habitats compared to data collected over the same time period in four unburned reference streams. Based on changes in fuels, plant and litter cover, and tree scorching, this prescribed fire was typical of those being implemented in ponderosa pine forests throughout the western U.S. However, we found that the extent and severity of riparian vegetation burned was substantially lower after prescribed fire compared to nearby wildfires. The early-season prescribed fire did not mimic the riparian or in-stream ecological effects observed following a nearby wildfire, even in catchments with burn extents similar to the prescribed fire. Little information exists on the effects of long-term fire exclusion from riparian forests, but a “prescribed fire regime” of repeatedly burning upland forests while excluding fire in adjacent riparian forests may eliminate an important natural disturbance from riparian and stream habitats.     

Relationship between landscape structure and burn severity at the landscape and class levels in Samchuck, South Korea

Fuel treatments for reducing fire risk are necessarily tied to the landscape structure including forest composition and configuration. Thus understanding the relationships between landscape structure and burn severity is important for developing guidelines and management strategies for fire-resilient forests. The goal of this study was to investigate the relationship between landscape structure as described by spatial pattern metrics and burn severity at the landscape and class levels. In 2000, a mostly severe fire burned 16,210 ha of dense forest located in Samchuck on the east coast of the Korean peninsula. Spatial pattern metrics including patch density, largest patch index, mean shape index, area-weighted mean shape index, Euclidean nearest neighborhood distance, and Shannon's diversity index, as well as topographic characteristics of slope and elevation, were correlated with burn severity based on delta Normalized Burn Ratio (dNBR) assessments. Regression tree analysis was also carried out with the same variables to avoid spatial autocorrelation and to reveal the relative importance of variables to burn severity. The results of this study strongly suggest that both composition and configuration of the forest cover patches are closely tied to burn severity. In particular, both the correlation analysis and regression tree analysis indicated that the area of red pine tree forest cover was the most significant factor in explaining the variance of burn severity. Topography and spatial configuration of forest cover patches were also significantly related to burn severity. The heterogeneity of forests also had a significant influence on burn severity. To reduce fire risk and increase the fire resilience of forests, forest managers and agencies need to consider enhancing the heterogeneity of forests when implementing fuel treatment schemes. However, such fuel treatments for landscape structure may only be effective under moderate weather conditions.     

The role of disturbance severity and canopy closure on standing crop of understory plant species in ponderosa pine stands in northern Arizona,USA

Concerns about the long-term sustainability of overstocked dry conifer forests in western North America have provided impetus for treatments designed to enhance their productivity and native biodiversity. Dense forests are increasingly prone to large stand-replacing fires; yet, thinning and burning treatments, especially combined with other disturbances such as drought and grazing, may enhance populations of colonizing species, including a number of non-native species. Our study quantifies plant standing crop of major herbaceous species across contrasting stand structural types representing a range in disturbance severity in northern Arizona. The least disturbed unmanaged ponderosa pine stands had no non-native species, while non-native grasses constituted 7–11% of the understory plant standing crop in thinned and burned stands. Severely disturbed wildfire stands had a higher proportion of colonizing native species as well as non-native species than other structural types, and areas protected from grazing produced greater standing crop of native forbs compared to grazed unmanaged stands. Standing crop of understory plants in low basal area thinned and burned plots was similar to levels on wildfire plots, but was comprised of fewer non-native graminoids and native colonizing plants. Our results also indicate that size of canopy openings had a stronger influence on standing crop in low basal area plots, whereas tree density more strongly constrained understory plant standing crop in dense stands. These results imply that treatments resulting in clumped tree distribution and basal areas <10 m² ha⁻¹ will be more successful in restoring native understory plant biomass in dense stands. Multiple types and severity of disturbances, such as thinning, burning, grazing, and drought over short periods of time can create greater abundance of colonizing species. Spreading thinning and burning treatments over time may reduce the potential for non-native species colonization compared to immediately burning thinned stands.     

Pinus pinaster Ait. tree mortality following wildfire in Spain

Maritime pine (Pinus pinaster Ait.) is the tree species most affected by wildfire in the Iberian Peninsula. Prediction of the probability of fire-injured tree mortality is critical for management of burned areas, evaluation of the ecological and economic impact of wildfire and prescribed fire planning and application. Pine bark beetles (Scolytidae) frequently attack burned maritime pine stands and cause extensive post-fire mortality throughout the Iberian Peninsula. In the present study, maritime pine trees were monitored for three years following 14 wildfires in four ecotypes in Spain (11 fires in Galicia (Galician ecotype - NW Spain), one fire in Portillo (Meseta-Castellana ecotype - Central Spain), one fire in Rodenal (Rodenal ecotype - Central Spain), and one fire in Genalguacil (Sierra Bermeja ecotype - SW Spain)). Data on tree attributes, crown and bole injury, ground fire severity, Ips sp. presence and tree survival were obtained by examining 3085 trees. Logistic regression models for predicting the probability of delayed maritime pine mortality were developed by use of generalized estimated equations (GEE). An ample range of response to fire damage in mortality was evident among the four ecotypes and different models were fitted for each. The most important variables for predicting tree mortality were total crown volume damaged, presence of Ips sp. attack and cambium kill rating. The results highlight the extensive presence of Ips sp. in burned maritime pine forests and its importance in tree mortality process, the ample range of response of P. pinaster, in terms of post-fire mortality, as well as the need to develop site specific mortality models for the different ecotypes of this species following fire.     

Tree type and forest management effects on the structure of stream wood following wildfires

Wildfires are an increasingly common disturbance influencing wood recruitment to streams, and thereby affecting their physical and biological condition. Mediterranean countries such as Portugal, where more than 25% of the land area has burned since 1990, are ideal areas to study impacts of wildfire effects on streams. We evaluated the physical structure of 2206 downed wood pieces (DWP) across 27 first- to third-order streams in central Portugal, all of which had experienced recent wildfires. The streams flowed through monospecific upland forests of Eucalyptus, Maritime pines, or Cork oaks and were fringed by a mixture of riparian tree species. DWP structure differed between tree types and between burned and unburned pieces. Post-fire timber-production forests (Maritime pines and Eucalyptus) contributed a higher quantity of thinner, longer and straighter DWP to streams than Cork oak stands. Pieces from Maritime pines had more rootwads and branches than DWP from the other tree types. Pieces from Cork oak and riparian species generally had a bent form, were shorter and had no rootwads. Burned DWP in streams were often from riparian trees. Relative to unburned DWP, the burned DWP occurred more frequently, were larger and straighter, had branches less often, and were more decayed. With more complex branches, rootwads, and a larger diameter, inputs from burned Maritime pine forests are more likely to change stream hydraulics and habitat complexity, relative to inputs from Eucalyptus forests with their simpler structure. This study shows that, less than a decade after wildfires, structure of downed wood in and near streams is strongly influenced by wildfire, but also still reflects intrinsic species characteristics and respective silviculture practices, even after the effects of fire have been accounted for. Under an anticipated shift in landscape cover with higher shrubland proportions and more mixing of Maritime pine and Eucalyptus forests, our results suggest that instream large wood will become scarcer and more structurally homogeneous.     

Eight years of seasonal burning and herbicidal brush control influence sapling longleaf pine growth,understory vegetation,and the outcome of an ensuing wildfire

To study how fire or herbicide use influences longleaf pine (Pinus palustris Mill.) overstory and understory vegetation, five treatments were initiated in a 5–6-year-old longleaf pine stand: check, biennial arborescent plant control by directed herbicide application, and biennial burning in March, May, or July. The herbicide or prescribed fire treatments were applied in 1999, 2001, 2003, and 2005. All prescribed fires were intense and averaged 700 kJ/s/m of fire front across all 12 burns. Using pretreatment variables as covariates, longleaf pine survival and volume per hectare were significantly less on the three prescribed fire treatments than on checks. Least-square means in 2006 for survival were 70, 65, 64, 58, and 56% and volume per hectare was 129, 125, 65, 84, and 80 m³/ha on the check, herbicide, March-, May-, and July-burn treatments, respectively. A wildfire in March 2007 disproportionately killed pine trees on the study plots. In October 2007, pine volume per hectare was 85, 111, 68, 98, and 93 m³/ha and survival was 32, 41, 53, 57, and 55% on the check, herbicide, March-, May-, and July-burn treatments, respectively, after dropping trees that died through January 2009 from the database. Understory plant cover was also affected by treatment and the ensuing wildfire. In September 2006, herbaceous plant cover averaged 4% on the two unburned treatments and 42% on the three prescribed fire treatments. Seven months after the wildfire, herbaceous plant cover averaged 42% on the two previously unburned treatments and 50% on the three prescribed fire treatments. Before the wildfire, understory tree cover was significantly greater on checks (15%) than on the other four treatments (1.3%), but understory tree cover was similar across all five treatments 7 months after the wildfire averaging 1.1%. The greater apparent intensity of the wildfire on the previously unburned treatments most likely resulted from a greater accumulation of fuels on the check and herbicide plots that also collectively had a higher caloric content than fuels on the biennially prescribed burned plots. These results showed the destructive force of wildfire to overstory trees in unburned longleaf pine stands while also demonstrating the rejuvenating effects of wildfire within herbaceous plant communities. They caution for careful reintroduction of prescribed fire even if fire was excluded for less than a decade.     

Chapter 12. A Wildfire and Emissions Policy Model for the Boise National Forest

Summary

We utilized the Boise National Forest's Hazard/Risk model, along with fire history records and fire behavior models, to estimate the current and anticipated levels of large wildfires and associated greenhouse gas and particulate emissions based on the forest condition and wildfire regime on the BNF. The model indicated that the forests at greatest risk of large, intense wildfires are the dense pondero-sa pine-Douglas-fir forests that make up over 1.1 million acres on the forest. We conclude that without an aggressive treatment program to reduce large areas of contiguous heavy fuel loadings the forest will be burned at an annual average rate of about 7.5% of the remaining at-risk forest. Using recent fire data to develop average patterns of intensity in wildfires within this forest type, we estimate that emissions will average around 1 million tons of carbon (C) per year over the next 20 years as the bulk of the ponderosa pine forests are burned. An aggressive treatment program featuring the removal of fuels where necessary, and prescribed fire as a means of re-introducing fire to these ecosystems, would result in a 30-50 percent reduction in the average annual wildfire experienced in the dense ponderosa pine forests, a 14-35% decrease in the average annual C emissions, and a 10-31% decrease in particulate emissions. We argue that the most effective way to curb emissions is with an aggressive treatment program linked to a landscape-based ecosystem management plan. This would have the effect of breaking up large contiguous landscape patterns so that fires become more patchy and diverse in their environmental impact, resulting in significantly reduced emissions as well as improved landscape diversity.     

Short-term effects of fire on soil properties inPinus densiflora stands

This study was performed to investigate a short time change (one week after fire) on soil properties due to the fire inPinus densiflora Sieb. et Zucc stands of the Kosung area in Kangwon Province in Korea. Twenty seven sampling plots [16 burned (8 low intensity fire, 8 high intensity fire) and 11 unburned plots] were chosen. Mineral soil samples from three depths (0–5, 5–15, and 15–25 cm) under the forest floor were collected. Forest fire in the area affected soil chemical properties. Soil pH, total nitrogen, available phosphorus, potassium, calcium, and magnesium in the surface soil (0–5 cm) of the burned area compared with the unburned area increased, but there was no marked change in the subsurface soil (5–25 cm). Organic matter, total nitrogen, available phosphorus, and exchangeable cations in the surface soil were generally lower in the high than in the low intensity fire areas. This indicates that these nutrients on the high intensity fire may be volatilized. The results suggest that change in soil chemical properties in the area was restricted mainly to the surface soil and was different between the high and the low intensity fire types.     

Effect of fire disturbance on active organic carbon of Larix gmelinii forest soil in Northeastern China

Active organic carbon in soil has high biological activity and plays an important role in forest soil ecosystem structure and function. Fire is an important disturbance factor in many forest ecosystems and occurs frequently over forested soils. However, little is known about its impact on soil active organic carbon(SAOC), which is important to the global carbon cycle. To investigate this issue, we studied the active organic carbon in soils in the Larix gmelinii forests of the Da Xing'an Mountains(Greater Xing'an Mountains) in Northeastern China, which had been burned by high-intensity wildfire in two different years(2002 and 2008). Soil samples were collected monthly during the 2011 growing season from over 12 sample plots in burned and unburned soils and then analyzed to examine the dynamics of SAOC. Our results showed that active organic carbon content changed greatly after fire disturbance in relation to the amount of time elapsed since the fire. There were significant differences in microbial biomass carbon, dissolved organic carbon, light fraction organic carbon, particulate organic carbon between burned and unburned sample plots in 2002 and 2008(p0.05). The correlations between active organic carbon and environmental factors such as water content, p H value and temperature of soils, and correlations between each carbon component changed after fire disturbance, also in relation to time since the fire. The seasonal dynamics of SAOC in all of the sample plots changed after fire disturbance; peak values appeared during the growing season. In plots burned in 2002 and 2008, the magnitude and occurrence time of peak values differed. Our findings provide basic data regarding the impact of fire disturbance on boreal forest soil-carbon cycling, carbon-balance mechanisms, and carbon contributions of forest ecosystem after wildfire disturbance.     

Effect of fire disturbance on active organic carbon of <Emphasis Type="Italic">Larix gmelinii</Emphasis> forest soil in Northeastern China

Active organic carbon in soil has high biological activity and plays an important role in forest soil ecosystem structure and function. Fire is an important disturbance factor in many forest ecosystems and occurs frequently over forested soils. However, little is known about its impact on soil active organic carbon (SAOC), which is important to the global carbon cycle. To investigate this issue, we studied the active organic carbon in soils in the Larix gmelinii forests of the Da Xing’an Mountains (Greater Xing’an Mountains) in Northeastern China, which had been burned by high-intensity wildfire in two different years (2002 and 2008). Soil samples were collected monthly during the 2011 growing season from over 12 sample plots in burned and unburned soils and then analyzed to examine the dynamics of SAOC. Our results showed that active organic carbon content changed greatly after fire disturbance in relation to the amount of time elapsed since the fire. There were significant differences in microbial biomass carbon, dissolved organic carbon, light fraction organic carbon, particulate organic carbon between burned and unburned sample plots in 2002 and 2008 (p < 0.05). The correlations between active organic carbon and environmental factors such as water content, pH value and temperature of soils, and correlations between each carbon component changed after fire disturbance, also in relation to time since the fire. The seasonal dynamics of SAOC in all of the sample plots changed after fire disturbance; peak values appeared during the growing season. In plots burned in 2002 and 2008, the magnitude and occurrence time of peak values differed. Our findings provide basic data regarding the impact of fire disturbance on boreal forest soil-carbon cycling, carbon-balance mechanisms, and carbon contributions of forest ecosystem after wildfire disturbance.     

Immediate effects of fire-severity on soil invertebrates in cut and uncut pine forests

We studied how variations in fire severity and the degree of cutting before burning affected soil invertebrates in a Pinus sylvestris forest in central Sweden. A varied depth of burn in the mor layer was obtained by exclusion of rain and addition of fuel in small plots (1 m×2 m) in clear-cut, selectively cut and uncut part of the forest before large-scale prescribed burning took place. Soil samples were taken from the plots immediately before, the day after, and two months after the fire. The overall mortality of invertebrates depended on the proportion of organic soil consumed by the fire, and for individual taxa it ranged between 59 and 100%. Invertebrates that lived deeper in soil suffered lesser mortality than those in the vegetation and litter layers did. Greater mobility in soil (Staphylinidae) or a thick cuticle (Oribatediae, Elateridae) may have contributed to the higher survival observed in these taxa. The beetles Atomaria pulchra (Cryptophagidae), Corticaria rubripes (Lathridiidae), and other fire-favoured insects colonised the burned forest the very day the fire burned. These species preferred the hard-burned plots and the uncut stand for colonisation. Sixty days after the fire, the abundance of invertebrates was lower in the burned cut stands compared to the burned uncut stand. The species composition of beetles in the burned stands was then characterised by a few very abundant fire-favoured species.     

Understorey fuel load estimation along two post-fire chronosequences of Pinus halepensis Mill. forests in Central Greece

Pinus halepensis forests are among the forest ecosystems in the Mediterranean Basin most affected by fire. Their distribution across lowland areas, in particular along the wildland–urban interface, increases the need to understand their ecology and responses to fire regime for their effective management. Apart from the extremely flammable tree layer, in several stands of these forests there is an increased fuel load attributed to the well-developed understorey of evergreen sclerophyllous shrubs. Taking into consideration that, in contrast with the long period required for full development of post-fire-regenerating pines, these shrubs resprout vigorously within the first post-fire weeks, it is important to explore the temporal trend of fuel accumulation to determine the risk of a second fire across a burned landscape. Two post-fire chronosequences of, in total, 12 P. halepensis stands were considered for sampling in Central Greece. The first chronosequence corresponds to pine stands characterized by the dominance of evergreen sclerophyllous shrubs in the understorey (Type 1) whereas the second chronosequence corresponds to pine stands where the cover of such shrubs was lower (Type 2). This study helps in understanding the fuel dynamics according to the type of P. halepensis forest stand and to anticipate future biomass growth. The proposed equations are simple tools, enabling land managers to estimate understorey total fuel load easily by visually recording the cover and height of the evergreen sclerophyllous shrub component, to justify understorey fuel reduction measures.     

Association of wildfire with tree health and numbers of pine bark beetles, reproduction weevils and their associates in Florida

Wildfires burned over 200,000 ha of forest lands in Florida from April to July 1998. This unique disturbance event provided a valuable opportunity to study the interactions of summer wildfires with the activity of pine feeding insects and their associates in the southeastern United States. We compared tree mortality with abundance of bark and ambrosia beetles, reproduction weevils and wood borers relative to fire severity. Over 27% of residual live trees in stands that experienced high fire severity died between October 1998 and May 1999. An additional 2–3% of trees that initially survived the fire died during the second year compared to <1% mortality in unburned stands. One year after the fire, more than 75% of the trees surviving in high fire severity stands had roots infected with one or more species of Leptographium and/or Graphium spp. and nearly 60% of the sampled roots were infected. No such fungi were recovered from roots of trees in unburned stands. Significantly, more root weevils, Hylobius pales and Pachylobius picovorus, were captured in unbaited pitfalls in the moderate and high fire severity stands than in the controls. Mean trap catches of Ips grandicollis, Dendroctonus terebrans and Hylastes salebrosus, three common bark beetles that feed on phloem tissue of pines, were lower in Lindgren traps in the fire-damaged areas than in the control stands. In contrast, catches of the ambrosia beetles, Xyleborus spp. and Monarthrum mali, were higher in burned stands than in control stands. The generalist predator, Temnochila virescens (Coleoptera: Trogositidae), showed a strong positive relationship between abundance and fire severity, while the flat bark beetle, Silvanus sp. (Coleoptera: Sylvanidae), exhibited the reverse trend. Our results show that most tree mortality occurred within 1 year of the fire. Ips or Dendroctonus bark beetle populations did not build up in dead and weakened trees and attack healthy trees in nearby areas. The prevalence of Leptographium spp. in roots may be a symptom of, or result in, weakened trees that may affect the trees’ susceptibility to bark beetles in the future.     

Distribution of saproxylic beetles in a recently burnt landscape of the northern boreal forest of Québec

This study evaluated the importance of burned habitat characteristics as well as the likely dispersal from specific habitats in the distribution of saproxylic beetles the same year as a fire occurred, in burned black spruce stands (Picea mariana [Mill] B.S.P.) in the northern boreal forest of Québec. The distribution of early post-fire saproxylic species was mainly driven by burned habitat attributes at the plot scale (0.04 ha), especially fire severity, suggesting that the effect of environment attributes can act at a relatively fine scale. Some xylophagous and most predaceous species were more abundant in severely burned stands whereas fire severity had the opposite effect on several common mycophagous species. The amount of newly fire-killed trees that could be used as breeding substrates in the burned stands had only a weak positive influence on these functional groups. The great majority of early saproxylic species were weakly associated with the distance from unburned forests or other recently burned patches that could act as potential “source habitats”. Indeed, these variables were of lesser importance than the attributes of the burned habitat. Woody debris that were already present in plots before the fire, potentially serving as local of source-populations for early colonizers, had virtually no influence on the local abundance of species. Many saproxylic species, including some true pyrophilous, clearly showed higher abundance as distance from unburned stands increased. This unexpected relation may reflect that dispersal of insects toward the burnt landscape very shortly after fire could be driven by the higher amount of volatiles released by severely burned forests, which are more likely as distance from unburned forest increased.     

Thermal characteristics of amphibian microhabitats in a fire-disturbed landscape

Disturbance has long been a central issue in amphibian conservation, often regarding negative effects of logging or other forest management activities, but some amphibians seem to prefer disturbed habitats. After documenting increased use of recently burned forests by boreal toads (Bufo boreas), we hypothesized that burned habitats provided improved thermal opportunities in terrestrial habitats. We tested this hypothesis by conducting a radio telemetry study of habitat use (reported previously) and by using physical models that simulated the temperature of adult toads. We deployed 108 physical models in and adjacent to a 1-year old burn using a fully-replicated design with three burn severities (unburned, partial, high severity) and four microhabitats (open surface, under vegetation, under log, in burrow). Model temperatures were compared to a range of preferred temperatures in published studies. We found 70% more observations within the preferred temperature range of B. boreas in forests burned with high severity than in unburned areas. Burned forest was warmer than unburned forest across all microhabitats, but the largest relative difference was in burrows, which averaged 3 °C warmer in high-severity burn areas and remained warmer though the night. More than twice as many observations were within the preferred temperature range in high-severity burrows than in unburned burrows. Areas burned with high severity were still warmer than unburned forest 3 years after the fire. Habitat use of toads during the concurrent radio telemetry study matched that predicted by the physical models. These results suggest there are fitness-linked benefits to toads using burned habitats, such as increased growth, fertility, and possibly disease resistance. However, increased soil temperatures that result from wildfire may be detrimental to other amphibian species that prefer cooler temperatures and stable environments. More broadly, our data illustrate the use of physical models to measure and interpret changes that amphibians may experience from disturbance, and highlight the need for research linking vital rates such as growth and survival to disturbance.     

A comparison of soil properties after contemporary wildfire and fire suppression

Forests that were subject to frequent wildfires, such as ponderosa pine/Douglas-fir forests, had fire-return intervals of approximately 6–24 years. However, fire suppression over the last century has increased the fire-return interval by a factor of 5 in these forests, possibly resulting in changes to the soil. The objective of this study was to determine if soils of recently burned areas (representative of the natural fire-return interval) have different properties relative to soils in areas without recent fire. To assess this, recent low-intensity, lightning-caused, spot wildfire areas were located within fire-suppressed stands of ponderosa pine/Douglas-fir of the central, eastern Cascade Mountains of Washington State. Soil horizon depths were measured, and samples collected by major genetic horizons. Samples were analyzed for pH, C, N, C/N ratio, cation exchange capacity (CEC), base saturation (%BS), hydrophobicity and extractable P. Results show very little difference in soil properties between sites burned by low-severity fires and those areas left unburned. Such minimal changes, from these low-severity fires, in soil properties from fire suppression suggest there has also been little change in soil processes.     

The influence of mountain pine beetle outbreaks and drought on severe wildfires in northwestern Colorado and southern Wyoming: A look at the past century

Outbreaks of bark beetles and drought both lead to concerns about increased fire risk, but the relative importance of these two factors is the subject of much debate. We examined how mountain pine beetle (MPB) outbreaks and drought have contributed to the fire regime of lodgepole pine forests in northwestern Colorado and adjacent areas of southern Wyoming over the past century. We used dendroecological methods to reconstruct the pre-fire history of MPB outbreaks in twenty lodgepole pine stands that had burned between 1939 and 2006 and in 20 nearby lodgepole pine stands that were otherwise similar but that had not burned. Our data represent c. 80% of all large fires that had occurred in lodgepole pine forests in this study area over the past century. We also compared Palmer Drought Severity Index (PDSI) and actual evapotranspiration (AET) values between fire years and non-fire years. Burned stands were no more likely to have been affected by outbreak prior to fires than were nearby unburned stands. However, PDSI and AET values were both lower during fire years than during non-fire years. This work indicates that climate has been more important than outbreaks to the fire regime of lodgepole pine forests in this region over the past century. Indeed, we found no detectable increase in the occurrence of high-severity fires following MPB outbreaks. Dry conditions, rather than changes in fuels associated with outbreaks, appear to be most limiting to the occurrence of severe fires in these forests.