Post-fire Tree Research Articles

Variation in plant community composition and vegetation carbon pools a decade following a severe fire season in interior Alaska

AbstractQuestionsHow does fire severity, measured as depth of burn of ground layer fuels, control the regeneration of understorey species across black spruce‐dominated stands varying in pre‐fire organic layer depths? Are successional shifts from evergreen to deciduous understorey vegetation more likely to occur with greater depth of burn? Does a shift in understorey vegetation community towards more deciduous species influence carbon accumulation in vegetation biomass?LocationNorthern boreal forest, interior Alaska.MethodsWe sampled 32 stands in interior Alaska that burned in 2003 and 2004 in which depth of burn had been recorded soon after fire. In 2014 we characterized tree density, understorey vegetation composition, above‐ and below‐ground vegetation carbon pools, and a suite of environmental variables. We used ANOVA and multivariate redundancy analysis models to analyse the dominant controls on vegetation composition and carbon pools.ResultsFire severity was a strong control on post‐fire tree and non‐vascular species composition. Ten years post‐fire, sites that experienced deeper burning of organic layers had a higher abundance of deciduous tree species, fire‐adapted mosses, forbs and graminoids, and lower abundances of evergreen shrubs and Sphagnum mosses. Environmental variables (elevation, soil bulk density and mineral soil pH, moisture and temperature) served as important controls on tree and vascular understorey species composition. We found no evidence that carbon pools associated with recovering vegetation biomass were influenced by landscape position, fire severity or environmental variables.ConclusionsBoth fire frequency and depth of burn are projected to increase with a warmer climate in interior Alaska. These shifts in fire regime are expected to favour the regeneration of deciduous species over conifers, which have the potential to regulate successional pathways and large‐scale trends in albedo, which both ultimately could influence ecosystem–climate feedbacks and dampen future fire cycles. Our results show that understorey species composition is controlled by fire severity, suggesting that more severe burning causes a decline in the self‐replacement of conifer communities across a range of hydrologic settings. Overall, our results suggest a loss of resilience in black spruce forests in the face of a changing fire regime. However, we found no evidence that fire severity influences vegetation carbon pools 10 yrs post‐fire, suggesting that there is less landscape variation in biomass carbon pools than in vegetation composition and succession during early post‐fire succession.

Burn me twice, shame on who? Interactions between successive forest fires across a temperate mountain region.

Increasing rates of natural disturbances under a warming climate raise important questions about how multiple disturbances interact. Escalating wildfire activity in recent decades has resulted in some forests re-burning in short succession, but how the severity of one wildfire affects that of a subsequent wildfire is not fully understood. We used a field-validated, satellite-derived, burn-severity atlas to assess interactions between successive wildfires across the US Northern Rocky Mountains a 300,000-km2 region dominated by fire-prone forests. In areas that experienced two wildfires between 1984 and 2010, we asked: (1) How do overall frequency distributions of burn-severity classes compare between first and second fires? (2) In a given location, how does burn severity of the second fire relate to that of the first? (3) Do interactions between successive fires vary by forest zone or the interval between fires? (4) What factors increase the probability of burning twice as stand-replacing fire? Within the study area, 138,061ha burned twice between 1984 and 2010. Overall, frequency distributions of burn severity classes (low, moderate, high; quantified using relativized remote sensing indices) were similar between the first and second fires; however burn severity was 5-13% lower in second fires on average. Negative interactions between fires were most pronounced in lower-elevation forests and woodlands, when fire intervals were <10yr, and when burn severity was low in the first fire. When the first fire burned as high severity and fire intervals exceeded 10-12yr, burn-severity interactions switched from negative to positive, with high-severity fire begetting subsequent high-severity fire. Locations most likely to experience successive stand-replacing fires were high-elevation forests, which are adapted to high-severity fire, and areas conducive to abundant post-fire tree regeneration. Broadly similar severities among short-interval "re-burns" and other wildfires indicate that positive severity feedbacks, an oft-posited agent of ecosystem decline or state shift, are not an inevitable outcome of re-burning. Nonetheless, context-dependent shifts in both the magnitude and direction of wildfire interactions (associated with forest zone, initial burn-severity, and disturbance interval) illustrate complexities in disturbance interactions and can inform management and predictions of future system dynamics.

Fire Severity Controlled Susceptibility to a 1940s Spruce Beetle Outbreak in Colorado, USA.

The frequency, magnitude, and size of forest disturbances are increasing globally. Much recent research has focused on how the occurrence of one disturbance may affect susceptibility to subsequent disturbances. While much has been learned about such linked disturbances, the strength of the interactions is likely to be contingent on the severity of disturbances as well as climatic conditions, both of which can affect disturbance intensity and tree resistance to disturbances. Subalpine forests in western Colorado were affected by extensive and severe wildfires in the late 19th century and an extensive and severe outbreak of spruce beetle (Dendroctonus rufipennis) in the 1940s. Previous research found that most, but not all, of the stands that burned and established following the late 19th century fires were not susceptible to the 1940s outbreak as beetles preferentially attack larger trees and stands in advanced stages of development. However, previous research also left open the possibility that some stands that burned and established following the 19th century fires may have been attacked during the 1940s outbreak. Understanding how strongly stand structure, as shaped by disturbances of varying severity, affected susceptibility to past outbreaks is important to provide a baseline for assessing the degree to which recent climate change may be relaxing the preferences of beetles for larger trees and for stands in latter stages of structural development and thereby changing the nature of linked disturbances. Here, dendroecological methods were used to study disturbance history and tree age of stands in the White River National Forest in Western Colorado that were identified in historical documents or remotely-sensed images as having burned in the 19th century and having been attacked by spruce beetle in the 1940s. Dendroecological reconstructions indicate that in young post-fire stands only old remnant trees that survived the otherwise stand-replacing fires were killed in the 1940s outbreak. No young post-fire trees (< ca. 128 years) were susceptible to the 1940s outbreak, implying that under the relatively cool and wet conditions of the mid-20th century, susceptibility to and spatial patterns of spruce beetle outbreak were most likely controlled by variations in severity of prior disturbance by fire. This study provides a baseline for comparing linked disturbances under the relatively warmer and drier conditions of recent (e.g. post-1990) outbreaks in order to assess how climate mitigates the degree to which pre-disturbance history and structure affect susceptibility to disturbances.

Experimental evidence for heat plume-induced cavitation and xylem deformation as a mechanism of rapid post-fire tree mortality.

Summary Recent work suggests that hydraulic mechanisms, rather than cambium necrosis, may account for rapid post‐fire tree mortality. We experimentally tested for xylem cavitation, as a result of exposure to high‐vapour‐deficit (D) heat plumes, and permanent xylem deformation, as a result of thermal softening of lignin, in two tree species differing in fire tolerance.We measured percentage loss of conductance (PLC) in distal branches that had been exposed to high‐D heat plumes or immersed in hot water baths (high temperature, but not D). Results were compared with predictions from a parameterized hydraulic model. Physical damage to the xylem was examined microscopically.Both species suffered c. 80% PLC when exposed to a 100°C plume. However, at 70°C, the fire‐sensitive Kiggelaria africana suffered lower PLC (49%) than the fire‐resistant Eucalytpus cladocalyx (80%). Model simulations suggested that differences in PLC between species were a result of greater hydraulic segmentation in E. cladocalyx. Kiggelaria africana suffered considerable PLC (59%), as a result of heat‐induced xylem deformation, in the water bath treatments, but E. cladocalyx did not.We suggest that a suite of ‘pyrohydraulic’ traits, including hydraulic segmentation and heat sensitivity of the xylem, may help to explain why some tree species experience rapid post‐fire mortality after low‐intensity fires and others do not.

What drives European beech (Fagus sylvatica L.) mortality after forest fires of varying severity?

Predicting the timing and amount of tree mortality after a forest fire is of paramount importance for post-fire management decisions, such as salvage logging or reforestation. Such knowledge is particularly needed in mountainous regions where forest stands often serve as protection against natural hazards (e.g., snow avalanches, rockfalls, landslides). In this paper, we focus on the drivers and timing of mortality in fire-injured beech trees (Fagus sylvatica L.) in mountain regions. We studied beech forests in the southwestern European Alps, which burned between 1970 and 2012. The results show that beech trees, which lack fire-resistance traits, experience increased mortality within the first two decades post-fire with a timing and amount strongly related to the burn severity. Beech mortality is fast and ubiquitous in high severity sites, whereas small- (DBH <12cm) and intermediate-diameter (DBH 12–36cm) trees face a higher risk to die in moderate-severity sites. Large-diameter trees mostly survive, representing a crucial ecological legacy for beech regeneration. Mortality remains low and at a level similar to unburnt beech forests for low burn severity sites. Beech trees diameter, the presence of fungal infestation and elevation are the most significant drivers of mortality. The risk of beech to die increases toward higher elevation and is higher for small-diameter than for large-diameter trees. In case of secondary fungi infestation beech faces generally a higher risk to die. Interestingly, fungi that initiate post-fire tree mortality differ from fungi occurring after mechanical injury. From a management point of view, the insights about the controls of post-fire mortality provided by this study should help in planning post-fire silvicultural measures in montane beech forests.

Effects of experimental fuel additions on fire intensity and severity: unexpected carbon resilience of a neotropical forest.

Global changes and associated droughts, heat waves, logging activities, and forest fragmentation may intensify fires in Amazonia by altering forest microclimate and fuel dynamics. To isolate the effects of fuel loads on fire behavior and fire-induced changes in forest carbon cycling, we manipulated fine fuel loads in a fire experiment located in southeast Amazonia. We predicted that a 50% increase in fine fuel loads would disproportionally increase fire intensity and severity (i.e., tree mortality and losses in carbon stocks) due to multiplicative effects of fine fuel loads on the rate of fire spread, fuel consumption, and burned area. The experiment followed a fully replicated randomized block design (N=6) comprised of unburned control plots and burned plots that were treated with and without fine fuel additions. The fuel addition treatment significantly increased burned area (+22%) and consequently canopy openness (+10%), fine fuel combustion (+5%), and mortality of individuals ≥5cm in diameter at breast height (dbh; +37%). Surprisingly, we observed nonsignificant effects of the fuel addition treatment on fireline intensity, and no significant differences among the three treatments for (i) mortality of large trees (≥30cm dbh), (ii) aboveground forest carbon stocks, and (iii) soil respiration. It was also surprising that postfire tree growth and wood increment were higher in the burned plots treated with fuels than in the unburned control. These results suggest that (i) fine fuel load accumulation increases the likelihood of larger understory fires and (ii) single, low-intensity fires weakly influence carbon cycling of this primary neotropical forest, although delayed postfire mortality of large trees may lower carbon stocks over the long term. Overall, our findings indicate that increased fine fuel loads alone are unlikely to create threshold conditions for high-intensity, catastrophic fires during nondrought years.

Local-scale and regional climate controls on historical fire regimes in the San Juan Mountains, Colorado

In ponderosa pine and mixed conifer ecosystems of the Southwestern US, regional-scale climate tends to synchronize fire years among study sites and increase fire extent or severity within a forest. At landscape scales (1–100km2), fire frequency and severity may also be influenced by local-scale differences in elevation and aspect, including fire barriers in rugged terrain. This study examines local-scale and regional controls of past fire regimes in three tributary watersheds of the San Juan Mountains of southwestern Colorado. In each watershed, we sampled fire-scarred trees and tree age structure and identified fire years and cohorts of post-fire tree recruitment. Fire frequency varied by aspect, and we observed the largest differences between the north- and south-facing aspects in the two largest basins. Fire severity was also different by aspect, with low-severity fire regimes on south-facing slopes and mixed-severity fire on north-facing slopes. The majority of fire years were unique, single-basin fires, indicating a lack of synchrony of fire years among basins, which we attributed to fire barriers and rugged terrain. We conducted Superposed Epoch Analysis (SEA) on two groups of fire years, to show average climate conditions before and during fire years. The first group were unique single-basin fires (n=27) and the second group included all years when synchronous fire burned in two or three basins (n=15). Both groups had significantly dry Palmer Drought Severity Index (PDSI) values during the fire year (p<0.01), while the second group also showed significantly wet conditions for two years prior to the fire year (p<0.01). Steep topography and fire barriers likely reduced the influence of regional climatic on the frequency and timing of fires for individual basins, though fires still burned during years of regional drought. Rugged terrain also contributed to asynchronous fire years among basins and longer fire return intervals compared with other ponderosa pine and mixed conifer forests in the region.

The carbon balance of reducing wildfire risk and restoring process: an analysis of 10-year post-treatment carbon dynamics in a mixed-conifer forest

Forests sequester carbon from the atmosphere, helping mitigate climate change. In fire-prone forests, burn events result in direct and indirect emissions of carbon. High fire-induced tree mortality can cause a transition from a carbon sink to source, but thinning and prescribed burning can reduce fire severity and carbon loss when wildfire occurs. However, treatment implementation requires carbon removal and emissions to reduce high-severity fire risk. The carbon removed and emitted during treatment may be resequestered by subsequent tree growth, although there is much uncertainty regarding the length of time required. To assess the long-term carbon dynamics of thinning and burning treatments, we quantified the 10-year post-treatment carbon stocks and 10-year net biome productivity (NBP) from a full-factorial experiment involving three levels of thinning and two levels of burning in a mixed-conifer forest in California’s Sierra Nevada. Our results indicate that (1) the understory thin treatment, that retained large trees, quickly recovered the initial carbon emissions (NBP = 31.4 ± 4.2 Mg C ha−1), (2) the carbon emitted from prescribed fire in the burn-only treatment was resequestered within the historical fire return interval (NBP = 32.8 ± 3.5 Mg C ha−1), and (3) the most effective treatment for reducing fire risk, understory thin and burn, had negative NBP (−6.0 ± 4.5 Mg C ha−1) because of post-fire large tree mortality. Understory thinning and prescribed burning can help stabilize forest carbon and restore ecosystem resilience, but this requires additional emissions beyond only thinning or only burning. Retaining additional mid-sized trees may reduce the carbon impacts of understory thinning and burning.

Tree survival scales to community-level effects following mixed-severity fire in a mixed-conifer forest

Identifying the drivers of tree mortality and survival is critical to developing conceptual and predictive models of fire effects on forest communities and landscapes. Individual tree characteristics (a function of species traits and tree size) govern tree- and community-scale mortality following fire, but mortality can also depend on tree density and effects arising from instantaneous extreme fire behavior. However, the relative importance and interaction of these factors are not well understood, especially for mixed-severity fire regimes. We sampled burned mixed-conifer forests dominated by western larch (Larix occidentalis) in the Bob Marshall Wilderness of Montana, U.S.A. We combined these field measurements with a remotely-sensed estimate of initial burn severity (dNBR) to test predictions about drivers of fire effects that produce heterogeneous post-fire tree and stand-level mortality. Tree survival 8–13years after fire depended on complex interactions between species, size, and initial burn severity. Western larch experienced much higher survival than other tree species across tree sizes. Predictably, less fire-tolerant species experienced much lower survival than western larch. These tree-level probabilities in survival scale up to govern community-level mortality through variability in species composition. Greater relative abundance of fire-tolerant larch was associated with reduced levels of mortality at community scales. Interestingly, higher tree densities were either uncorrelated with community-level mortality or associated with lower community-level mortality. Our results show that traits of individuals can govern fire effects from trees to communities, and give rise to highly variable fire effects characteristic of mixed-severity fire.

The Discriminatory Ability of Postfire Tree Mortality Logistic Regression Models

The Discriminatory Ability of Postfire Tree Mortality Logistic Regression Models

Positive coupling between growth and reproduction in young post-fire Aleppo pines depends on climate and site conditions

In fire- and drought-prone Mediterranean forests tree growth and regeneration depend on the moisture regime between fires. Therefore, post-fire tree regeneration will depend on moisture conditions and how they are altered by fire recurrence and climate warming. Aleppo pine forests are the most abundant Circum-Mediterranean ecosystems subjected to frequent wildfires and summer droughts. Because both stressors constrain their reproduction and growth patterns across diverse ecological conditions, these forests represent a suitable system to test how moisture availability drives post-fire regeneration. Aleppo pine is an obligate seeder species that reproduces at an early age after fire. Such precocious behaviour poses the question as to whether post-fire regeneration depends on moisture conditions and the coupling between female cone production and growth. Here, we evaluate if female cone production and radial growth are linked at the tree level in post-fire Aleppo pines by comparing a dry v. a very dry site and considering stands with three different tree densities in south-eastern Spain. We found that trees with higher basal areas produced more female cones and this positive association intensified as the water balance improved. Aleppo pines from the very dry site were more precocious in reproductive terms than pines of the same age from the dry site, but long-term cone production was lower at the very dry site. Lower tree density enhances the resilience of xeric post-fire Aleppo pine forests, in growth and reproduction terms, but this effect can be reversed by droughts. Overall, a more positive water balance improves post-fire regeneration by enhancing growth and cone production. Thus, silvicultural treatments such as thinning should be applied by taking into account the post-fire water balance to maximise growth and cone production in Aleppo pine stands. Our findings illustrate how climate warming could hamper post-fire tree regeneration by aggravating drought stress.

Fire severity and tree regeneration following bark beetle outbreaks: the role of outbreak stage and burning conditions

The degree to which recent bark beetle (Dendroctonus ponderosae) outbreaks may influence fire severity and postfire tree regeneration is of heightened interest to resource managers throughout western North America, but empirical data on actual fire effects are lacking. Outcomes may depend on burning conditions (i.e., weather during fire), outbreak severity, or intervals between outbreaks and subsequent fire. We studied recent fires that burned through green-attack/red-stage (outbreaks <3 years before fire) and gray-stage (outbreaks 3–15 years before fire) subalpine forests dominated by lodgepole pine (Pinus contorta var. latifolia) in Greater Yellowstone, Wyoming, USA, to determine if fire severity was linked to prefire beetle outbreak severity and whether these two disturbances produced compound ecological effects on postfire tree regeneration. With field data from 143 postfire plots that burned under different conditions, we assessed canopy and surface fire severity, and postfire tree seedling density against prefire outbreak severity. In the green-attack/red stage, several canopy fire-severity measures increased with prefire outbreak severity under moderate burning conditions. Under extreme conditions, few fire-severity measures were related to prefire outbreak severity, and effect sizes were of marginal biological significance. The percentage of tree stems and basal area killed by fire increased with more green-attack vs. red-stage trees (i.e., the earliest stages of outbreak). In the gray stage, by contrast, most fire-severity measures declined with increasing outbreak severity under moderate conditions, and fire severity was unrelated to outbreak severity under extreme burning conditions. Postfire lodgepole pine seedling regeneration was unrelated to prefire outbreak severity in either post-outbreak stage, but increased with prefire serotiny. Results suggest bark beetle outbreaks can affect fire severity in subalpine forests under moderate burning conditions, but have little effect on fire severity under extreme burning conditions when most large wildfires occur in this system. Thus, beetle outbreak severity was moderately linked to fire severity, but the strength and direction of the linkage depended on both endogenous (outbreak stage) and exogenous (fire weather) factors. Closely timed beetle outbreak and fire did not impart compound effects on tree regeneration, suggesting the presence of a canopy seedbank may enhance resilience to their combined effects.

Recent mountain pine beetle outbreaks, wildfire severity, and postfire tree regeneration in the US Northern Rockies.

Widespread tree mortality caused by outbreaks of native bark beetles (Circulionidae: Scolytinae) in recent decades has raised concern among scientists and forest managers about whether beetle outbreaks fuel more ecologically severe forest fires and impair postfire resilience. To investigate this question, we collected extensive field data following multiple fires that burned subalpine forests in 2011 throughout the Northern Rocky Mountains across a spectrum of prefire beetle outbreak severity, primarily from mountain pine beetle (Dendroctonus ponderosae). We found that recent (2001-2010) beetle outbreak severity was unrelated to most field measures of subsequent fire severity, which was instead driven primarily by extreme burning conditions (weather) and topography. In the red stage (0-2 y following beetle outbreak), fire severity was largely unaffected by prefire outbreak severity with few effects detected only under extreme burning conditions. In the gray stage (3-10 y following beetle outbreak), fire severity was largely unaffected by prefire outbreak severity under moderate conditions, but several measures related to surface fire severity increased with outbreak severity under extreme conditions. Initial postfire tree regeneration of the primary beetle host tree [lodgepole pine (Pinus contorta var. latifolia)] was not directly affected by prefire outbreak severity but was instead driven by the presence of a canopy seedbank and by fire severity. Recent beetle outbreaks in subalpine forests affected few measures of wildfire severity and did not hinder the ability of lodgepole pine forests to regenerate after fire, suggesting that resilience in subalpine forests is not necessarily impaired by recent mountain pine beetle outbreaks.

Effectiveness of fuel reduction treatments: Assessing metrics of forest resiliency and wildfire severity after the Wallow Fire, AZ

Landscape-scale wildfire has occurred in higher frequencies across the planet. Fuel reduction treatments to fire-adapted systems have been shown to reduce the impact to human values-at-risk. However, few studies have examined if these treatments contribute to ecosystem resilience, or the capacity of a system to absorb perturbation and return to a similar set of structures or processes. We defined short-term metrics of resiliency to test the hypothesis that fuel reduction treatments in mixed conifer forests increased a fire-adapted system’s resiliency to uncharacteristically severe wildfire. In addition, we tested the hypothesis that fuel reduction treatments reduced burn severity, thereby increasing protection for adjacent human communities. We examined a mixed conifer forested landscape in the southwestern U.S. that was burned by a landscape-scale “mega-fire” in 2011; fuel reduction treatments had been established around communities in the 10years prior to the fire. Fire effects were highly variable in both treated and untreated forests. However, analysis of resiliency metrics showed that: (a) treated units retained a higher proportion of large trees and had post-fire tree densities within the natural range of variability; (b) the understory herbaceous community had significantly higher cover of native grasses in the treated units, but no significant differences in nonnative cover between treated and untreated units; and (c) high-severity patch sizes were significantly larger in untreated stands and covered a larger proportion of the landscape than historical reference conditions. Fire severity, as defined by overstory mortality and basal area loss, was significantly lower in treated units; on average, trees killed per hectare in untreated units was six times the number of trees killed in treated units. Fuel reduction treatments simultaneously reduced fire severity and enhanced short-term metrics of ecosystem resiliency to uncharacteristically severe fire.

A 40-Year Record of Tree Establishment Following Chaining and Prescribed Fire Treatments in Singleleaf Pinyon (Pinus monophylla) and Utah Juniper (Juniperus osteosperma) Woodlands

Chaining and prescribed fire treatments have been widely applied throughout pinyon–juniper woodlands of the western United States in an effort to reduce tree cover and stimulate understory growth. Our objective was to quantify effects of treatment on woodland recovery rate and structure and the relative dominance of the two major tree species in our Great Basin study area, singleleaf pinyon (Pinus monophylla Torr. & Frém.) and Utah juniper (Juniperus osteosperma [Torr.] Little). We resampled plots after a 40-yr interval to evaluate species-specific differences in tree survivorship and establishment from posttreatment age structures. Tree age data were collected in 2008 within four chained sites in eastern Nevada, treated in 1958, 1962, 1968, and 1969 and originally sampled in 1971. The same data were collected at five prescribed burn sites treated in 1975 and originally sampled in 1976. All chained sites had greater juniper survival than pinyon survival immediately following treatment. Chained sites with higher tree survival also had the greatest amount of new tree establishment. During the interval between treatment and the 2008 sampling, approximately four more trees per hectare per year established following chaining than following fire. Postfire tree establishment only occurred for the initial 15 yr and was dominated by juniper. Establishment after chaining was dominated by juniper for the first 15 yr but by pinyon for 15–40 yr following treatment. Results support an earlier successional role for juniper than for pinyon, which is more dependent upon favorable microsites and facilitation from nurse shrubs. Repeated chaining at short intervals, or prescribed burning at infrequent intervals, will likely favor juniper dominance. Chaining at infrequent intervals (> 20–40 yr) will likely result in regained dominance of pinyon. Chaining treatments can be rapidly recolonized by trees and have the potential to create or amplify landscape-level shifts in tree species composition.

Salvage logging affects early post-fire tree composition in Canadian boreal forest

Salvage logging following fire has become increasingly used during the last few decades as a way to mitigate economic losses caused by fire. The removal of burned mature trees including their aerial seedbanks immediately after fire could have impacts on post-fire tree recruitment but specific effects are still unknown. We conducted a study of a fire in the boreal forest of Québec, Canada, to answer the following questions: Does fire severity influence tree compositional changes after fire? Does salvage logging affect these changes by favouring species that can reproduce vegetatively? Does salvage logging reduce among-sites heterogeneity relative to natural post-fire forests? To address these questions, we measured pre- and post-fire tree stocking (2years after fire) of burned forest of different pre-fire stand composition types both in salvaged and non-salvaged sites, whereas fire severity was measured in the latter only. Species composition was evaluated using a Principal Component Analysis, and mixed models were used to test the effects of canopy fire severity, residual organic layer thickness, stand type and salvage logging on pre- to post-fire composition changes. In non-salvaged sites, fire severity had a significant effect on composition changes, and the effect depended on stand type. Low fire severity favoured species that can reproduce vegetatively such as Populus tremuloides, whereas moderate and high fire severity favoured the aerial seedbank species such as Pinus banksiana and Picea mariana. We found no effect of residual organic layer thickness on post-fire composition changes. Salvage treatment had a significant effect on post-fire composition changes and the effect depended on stand type. Overall, salvage logging tended to favour species relying upon vegetative reproduction more than fire alone, which favoured P. banksiana. Although among-site heterogeneity after salvage logging was not smaller than after fire only, salvage alters species composition more than does natural fire alone. Since salvage logging and low severity portions of the burn, which are generally not salvaged, both favour tree species with vegetative reproduction, our results suggest that these species will increase their dominance throughout the landscape if burned forests are systematically salvaged. We suggest strategies that can help to attenuate this divergence from natural post-fire conditions.

Dynamics of structure and biological productivity of post-fire larch forests in the Northern Mongolia

Peculiarities of forming and growth of post-fire larch (Larix sibirica Ledeb.) forests at the southern range of their distribution in the Northern Mongolia were studied. Regularities of the stand structure and dynamics of biological productivity are analyzed and discussed in the paper. It has been proved that the structure of the organic mass of the post-fire herb-carex type larch tree stands at the southern border of forest vegetation distribution is closely related to their biometric indices as-age, density, and productivity. In comparison with tree stands from the other areas of forest vegetation, the total phytomass stock of larch phytocenoses appropriately increases by zonal gradient from forest-tundra border to the Transbaikalian southern taiga and northern regions of Mongolia. Regeneration of forest cover by edificator and formation of the original larch coenopopulation is a positive trend from an environmental point of view, because the Siberian larch in young and middle-age period in the area has a sufficiently high growth energy and rate of phytomass production, with more than twice exceeding mature forests in fixed carbon of the atmosphere and has positive values of carbon balance and total destruction of organic matter by the “input-output” parameters.

Influence of recent bark beetle outbreak on fire severity and postfire tree regeneration in montane Douglas‐fir forests

Understanding how disturbances interact to shape ecosystems is a key challenge in ecology. In forests of western North America, the degree to which recent bark beetle outbreaks and subsequent fires may be linked (e.g., outbreak severity affects fire severity) and/ or whether these two disturbances produce compound effects on postfire succession is of widespread interest. These interactions remain unresolved, largely because field data from actual wildfires following beetle outbreaks are lacking. We studied the 2008 Gunbarrel Fire, which burned 27 200 ha in Douglas-fir (Pseudotsuga menziesii) forests that experienced a bark beetle outbreak 4-13 years prefire ("gray stage," after trees have died and needles have dropped), to determine whether outbreak severity influenced subsequent fire severity and postfire tree regeneration. In 85 sample plots we recorded prefire stand structure and outbreak severity; multiple measures of canopy and forest-floor fire severity; and postfire tree seedling density. Prefire outbreak severity was not related to any measure of fire severity except for mean bole scorch, which declined slightly with increasing outbreak severity. Instead, fire severity varied with topography and burning conditions (proxy for weather at time of fire). Postfire Douglas-fir regeneration was low, with tree seedlings absent in 65% of plots. Tree seedlings were abundant in plots of low fire severity that also had experienced low outbreak severity (mean = 1690 seedlings/ha), suggesting a dual filter on tree regeneration. Although bark beetles and fire collectively reduced live basal area to < 5% and increased snag density to > 2000% of pre-outbreak levels, the lack of relationship between beetle outbreak and fire severity suggests that these disturbances were not linked. Nonetheless, effects on postfire tree regeneration suggest compound disturbance interactions that contribute to the structural heterogeneity characteristic of mid/lower montane forests.

Post-fire survival and regeneration of Eucalyptus globulus in forest plantations in Portugal

Eucalyptus globulus is one of the most important pulpwood plantation species in the world, and nowadays it is present in most continents. Some of the regions where E. globulus plantations have been expanding have a high incidence of wildfires; therefore, knowing the factors affecting the fire resistance and resilience of this species is particularly important for forest management. This is the case of Portugal, where nearly 1.4million ha of land burned in the last decade and where E. globulus has become the most widespread tree species. In this paper we assessed the short-term post-fire responses of E. globulus trees in four sites affected by wildfires, and investigated their potential relationships with fire severity descriptors and with tree and stand characteristics. One year after fire, individual tree mortality was low (3.9%) but most trees (79.2%) were top-killed (i.e. had stem mortality). Both post-fire tree mortality and top-kill increased with fire severity (expressed by maximum char height and/or char severity rating). Moreover, top-kill was positively related with vegetation cover and the proportion of pine trees in the stand, and was negatively related with tree diameter and the proportion of other broadleaved trees in the stand. The most common post-fire regeneration type among the sampled trees was basal resprouting (89.2%), though 20.9% had epicormic resprouting. The number of basal resprouts increased with char height, top-kill and diameter of the parent tree, and decreased with slope. In terms of post-fire growth, the height of the dominant resprout increased with the number of resprouts and with char severity, while it decreased in drier southern aspects. Results showed that E. globulus is a fire-resilient species with a very high probability of surviving fire; however, in forest plantations where trees are usually felled in short rotations (thus with limited size), individuals have a high probability of being top-killed. The presented models may be useful to help managers on the assessment of post-fire production losses and regeneration potential in E. globulus plantations.

Influence of Gmelin larch trees on postfire restoration of forest communities in the permafrost zone of Central Siberia

This article shows the phytocoenotic role of individual postfire trees of the Gmelin larch under conditions of the permafrost zone of Central Siberia. On the basis of the vital state of the trees, the limits of their influence on the recovery of the species of the lower layers of phytocenosis are defined and the formation of the new stand generation is shown.