Fire Severity Classes Research Articles

Estimating post‐fire organic soil depth in the Alaskan boreal forest using the Normalized Burn Ratio

As part of a long‐term moose browse/fire severity study, we used the Normalized Burn Ratio (NBR) with historic Landsat Thematic Mapper (TM) imagery to estimate fire severity from a 1983 wildfire in interior Alaska. Fire severity was estimated in the field by measuring the depth of the organic soil at 57 sites during the summer of 2006. Sites were selected for field sampling from five fire severity classes based on threshold NBR values. The linear relationship between post‐fire NBR and organic soil depth among sites within the burn was weak (r 2 = 0.26), and improved substantially (r 2 = 0.66) when restricted to non‐wetland black spruce sites. The relationship between NBR and aspen/willow counts was non‐linear. Sites with high densities of aspen stems consistently occurred in the high fire severity classes, and sites with high willow stem densities consistently occurred in the moderate fire severity class. However, NBR varied substantially from sites with low aspen or willow reproduction and therefore predicting aspen or willow regeneration based on post‐fire NBR values would be difficult.

Remote sensing of fire severity in the Blue Mountains: influence of vegetation type and inferring fire intensity

Fire intensity affects ecological and geophysical processes in fire-prone landscapes. We examined the potential for satellite imagery (Satellite Pour l’Observation de la Terre [SPOT2] and Landsat7) to detect and map fire severity patterns in a rugged landscape with variable vegetation near Sydney, Australia. A post-fire, vegetation-based indicator of fire intensity (burnt shrub branch tip diameters, representing the size of fuel consumed) was also used to explore whether fire severity patterns can be used to retrospectively infer patterns of fire intensity. Six severity classes (ranging from unburnt to complete crown consumption) were defined using aerial photograph interpretation and a field assessment across five vegetation types of varying height and complexity (sedge-swamp, heath, woodland, open forest, and tall forest). Using established Normalised Difference Vegetation Index (NDVI) differencing methodology, SPOT2 and Landsat7 imagery yielded similar broad-scale severity patterns across the study area. This was despite differences in image resolution (10 m and 30 m, respectively) and capture dates (2 months and 9 months apart, respectively). However, differences in the total areas mapped for some severity classes were found. In particular, there was reduced differentiation between unburnt and low-severity areas and between crown-scorched and crown-consumed areas when using the Landsat7 data. These differences were caused by fine-scale classification anomalies and were most likely associated with seasonal differences in vegetation condition (associated with time of image capture), post-fire movement of ash, resprouting of vegetation, and low sun elevation. Relationships between field severity class and NDVIdifference values revealed that vegetation type does influence the detection of fire severity using these types of satellite data: regression slopes were greater for woodland, forest, and tall forest data than for sedge-swamp and heath data. The effect of vegetation type on areas mapped in each fire severity class was examined but found to be minimal in the present study due to the uneven distribution of vegetation types in the study area (woodland and open forest cover 86% of the landscape). Field observations of burnt shrub branch tips, which were used as a surrogate for fire intensity, revealed that relationships between fire severity and fire intensity are confounded by vegetation type (mainly height). A method for inferring fire intensity from remotely sensed patterns of fire severity was proposed in which patterns of fire severity and vegetation type are combined.

Trembling aspen response to a mixed-severity wildfire in the Black Hills, South Dakota, USA

Trembling aspen (Populus tremuloides Michx.) regeneration dynamics including sprout production, growth, and clone size were measured to determine the effects of fire on small aspen clone persistence following a mixed-severity wildfire in the Black Hills, South Dakota. Four years postfire, 10 small, isolated aspen clones per low and high fire severity classes were compared with 10 unburned clones. Regardless of severity, fire did not cause an increase in the area occupied by individual aspen clones. Clones affected by high severity fire had the greatest suckering response producing an average of 31 930 sprouts·ha–1; more than double the sprout density in unburned clones and 67% greater than the sprout density in clones affected by low severity fire. Sprout growth in high severity clones was 135% and 60% greater than sprout growth in unburned and low severity clones. The succession of these clones to more shade-tolerant ponderosa pine was delayed in clones affected by high severity fire as high severity fire caused significant pine mortality within and surrounding the clone, whereas, without further disturbance, pine encroachment and dominance will likely continue in clones affected by low severity fire.

Effects of fire severity on early development of understory vegetation

Four boreal mixedwood stands burned by the 1999 Black River wildfire in southeastern Manitoba were sampled to examine the effects of fire severity on early regeneration dynamics of understory vegetation. In each stand, three fire severity classes (scorched, lightly burned, and severely burned) were identified based on the degree of forest floor consumption, and six plots per severity class were randomly selected. Variation in fire severity significantly affected the initial regeneration of the understory plant community. Regeneration response after fire was largely controlled by interactions between fire severity and species' regeneration strategy. Establishment of invaders, seed bankers, and sprouters was best on severely burned, lightly burned, and scorched plots, respectively. Species richness and Shannon's diversity index was reduced by severe fire only in the first postfire year. However, the effects of fire severity on species abundance and composition persisted through the entire study period (1999–2002). Rapid changes in the understory plant community were only observed during the initial 3 postfire years, regardless of fire severity. At the end of the study, herbaceous plants were the most dominant component, with woody plants being a codominant component on scorched plots, and nonvascular plants being a codominant component on severely burned plots.

Measurement and prediction of post-fire erosion at the hillslope scale, Colorado Front Range

Post-fire soil erosion is of considerable concern because of the potential decline in site productivity and adverse effects on downstream resources. For the Colorado Front Range there is a paucity of post-fire erosion data and a corresponding lack of predictive models. This study measured hillslope-scale sediment production rates and site characteristics for three wild and three prescribed fires over two summers and one winter using 48 sediment fences. Over 90% of the sediment was generated by summer convective storms. Sediment production rates from recent, high-severity wildfires were 0.2–1.0 kg m–2 year–1. Mean sediment production rates from areas recently burned at moderate and low severity were only 0.02 and 0.005 kg m–2 year–1, respectively. For a given severity, sediment production rates from prescribed fires were generally lower than from wildfires, but there was considerable variability between plots and within fire severity classes. Fire severity, percent bare soil, rainfall erosivity, soil water repellency and soil texture explained 77% of the variability in sediment production rates, while a two-parameter model using percentage bare soil and rainfall erosivity explained 62% of the variability. Model validation confirmed the usefulness of these empirical models. The improved understanding of post-fire erosion rates can help guide forest management and post-fire rehabilitation efforts.

Initial response of understorey vegetation to fire severity and salvage‐logging in the southern boreal forest of Québec

Abstract. In this study we compared the effects of fire on understorey vegetation in the Québec southern boreal forest with effects of salvage‐logging (clear‐cutting after fire). All 61 400‐m2 sampling sites were controlled for overstorey composition (Deciduous, Mixed and Coniferous) and disturbance type, which consisted of three fire impact severity (FIS) classes (Light, Moderate and Extreme) and two harvesting techniques (Stem‐only and Whole‐tree Harvesting). Percent‐cover data of vegetation and post‐disturbance environmental characteristics were recorded in the field during the first two years after fire as well as soil texture. Ordination of fire alone demonstrated that, on Coniferous sites, fire initiates a succession whereby the understorey Coniferous sites approaches that of Deciduous‐Mixed sites, due to the release of the understorey from Sphagnum spp. dominance, this pattern being a function of FIS. On Deciduous‐Mixed stands, increased FIS resulted in a transition from herb to shrub dominance. Ordination of all five disturbance types showed that the impact of salvage‐logging on understorey composition was within the range of fire, but marginalized to the extreme end of the FIS spectrum. Variance partitioning demonstrated that overstorey and soil texture were the most important explanatory variables of fire alone, while disturbance type explained the largest independent fraction of understorey variation when salvage‐logging was introduced. Salvage‐logging also results in significant reductions in understorey abundance, richness and diversity, while indicator species analysis suggests that it favours mesoxerophytic to xeric species. Results are interpreted in light of shade‐tolerance dynamics, forest floor disturbance and soil moisture regimes. Implications for sustainable forest management are discussed.

The post-fire measurement of fire severity and intensity in the Christmas 2001 Sydney wildfires

Using pre- and post-fire satellite imagery from SPOT2, we examined the fire severity and intensity of the Christmas 2001 wildfires in the greater Sydney Basin, Australia. We computed a Normalised Difference Vegetation Index (NDVI) from the two satellite images captured before (November 2001) and after (January 2002) the wildfires, then subtracted the later from the former to produce a difference image (NDVIdiff) which was subsequently classified into six fire severity classes (unburnt, low, moderate, high, very high and extreme severity). We then tested the fire severity classification on 342 sample sites within the 225 000ha fire affected area using a qualitative visual assessment guide. We found that the NDVIdiff classification produced an accuracy of at least 88% (K hat = 0.86), with the greatest discrepancy being between the low and moderate classification. Knowledge of rate of spread over some of the affected area, coupled with a complete knowledge of fuel loads, was used to retrospectively model fire intensity, which in areas of extreme fire intensity, produced heat energy levels exceeding 70 000 kW m–1. Importantly, we found no positive effect of topography on fire severity, in fact finding an inverse relationship between slope and fire severity and no effect due to aspect. Further analysis showed that flat to moderate slopes less than 18° across all aspects suffered the greatest vegetal destruction, and there was no relationship between north-westerly aspects and fire severity. We also introduce a relatively simple method for estimating fuel load biomass using a combination of satellite image and rapid field assessment. We found 79% accuracy for this method based on 125 sample sites. It is postulated that this type of analysis can greatly improve our understanding of the spatial impact of fire, how natural areas within the fire ground were impacted, and how remote sensing and GIS technologies can be efficiently used in fire management planning and post-fire analysis.

Initial response of understorey vegetation to fire severity and salvage-logging in the southern boreal forest of Québec

Abstract. In this study we compared the effects of fire on understorey vegetation in the Québec southern boreal forest with effects of salvage-logging (clear-cutting after fire). All 61 400-m2 sampling sites were controlled for overstorey composition (Deciduous, Mixed and Coniferous) and disturbance type, which consisted of three fire impact severity (FIS) classes (Light, Moderate and Extreme) and two harvesting techniques (Stem-only and Whole-tree Harvesting). Percent-cover data of vegetation and post-disturbance environmental characteristics were recorded in the field during the first two years after fire as well as soil texture. Ordination of fire alone demonstrated that, on Coniferous sites, fire initiates a succession whereby the understorey Coniferous sites approaches that of Deciduous-Mixed sites, due to the release of the understorey from Sphagnum spp. dominance, this pattern being a function of FIS. On Deciduous-Mixed stands, increased FIS resulted in a transition from herb to shrub dominance. Ordination of all five disturbance types showed that the impact of salvage-logging on understorey composition was within the range of fire, but marginalized to the extreme end of the FIS spectrum. Variance partitioning demonstrated that overstorey and soil texture were the most important explanatory variables of fire alone, while disturbance type explained the largest independent fraction of understorey variation when salvage-logging was introduced. Salvage-logging also results in significant reductions in understorey abundance, richness and diversity, while indicator species analysis suggests that it favours mesoxerophytic to xeric species. Results are interpreted in light of shade-tolerance dynamics, forest floor disturbance and soil moisture regimes. Implications for sustainable forest management are discussed.

Fire severity and seedling establishment in Pinus halepensis woodlands, eastern Iberian Peninsula

Given the observed heterogeneity in fire severity produced within wildfires, we asked to what extent this heterogeneity might affect post-fire regeneration. For this purpose, we studied the post-fire dynamics of Pinus halepensis (Aleppo pine) in the eastern Iberian Peninsula. Sampling was stratified on the basis of fire severity. We defined three fire severity classes based on the degree of consumption of the pine canopy. The results suggested that there is no clear relationship between seedling density and fire severity; however, mortality was lower and growth (height, shoot biomass and root biomass) was higher in the high severity class. These results can be explained by soil processes: Sites in the high fire severity class may have sustained higher fire intensities, resulting in higher soil organic matter mineralisation and higher ash deposition, and thus in higher post-fire soil fertility. This higher fertility would produce faster growth in pine seedlings. Independent of the severity class, seedling mortality was higher in quadrats (50 × 50 cm) with higher cover of the perennial grass Brachypodium retusum (Poaceae), suggesting a possible competitive effect. For all plots in all 3 severity classes, spatial analysis suggests an aggregate seedlings pattern, but with independence from the position of the adult (source) trees.

Influence of fire severity on plant regeneration by means of remote sensing imagery

In this paper we analyse the interactions between fire severity (plant damage) and plant regeneration after fire by means of remote sensing imagery and a field fire severity map. A severity map was constructed over a large fire (2692 ha) occurring in July 1994 in the Barcelona province (north-east of Spain). Seven severity classes were assigned to the apparent plant damage as a function of burning intensity. Several Landsat TM and MSS images from dates immediately before and after the fire were employed to monitor plant regeneration processes as well as to evaluate the relationship with fire severity observed in situ . Plant regeneration was monitored using NDVI measurements (average class values standardized with neighbour unburned control plots). Pre-fire NDVI measurements were extracted for every plant cover category (7), field fire severity class (7), and spatial cross-tabulation of both layers (33) and compared to post-fire values. NDVI decline due to fire was positively correlated with field fire severity class. Results show different patterns of recovery for each dominant species, severity class and combination of both factors. For all cases a significant negative correlation was found between damage and regeneration ability. This work leads to a better understanding of the influence of severity, a major fire regime parameter on plant regeneration, and may aid to manage restoration on areas burned under different fire severity levels.

Fire severity in relation to canopy composition within burned boreal mixedwood stands

Three boreal mixedwood stands burned by the 1999 Black River fire in southeastern Manitoba were sampled to determine if within-stand variation in fire severity was related to local variation in species composition. On each stand, three fire severity classes (scorched, lightly burned, and severely burned) were identified based on the impact of fire on the forest floor. Pre-fire basal areas and densities of coniferous and deciduous species associated with each fire severity class were determined. The severely burned class had significantly higher coniferous basal area (38.2 m 2/ha) and significantly lower deciduous basal area (7.0 m 2/ha) than the scorched and lightly burned classes. No significant differences in species composition were found between the scorched and lightly burned classes. Among 20 plots sampled on the severely burned class, 16 plots had coniferous basal area ≥30 m 2/ha but no plot had deciduous basal area ≥20 m 2/ha. Plots sampled on the severely burned class were either classified as softwood stands (16 plots) or softwood-dominated mixedwood stands (4 plots) according to percentage coniferous basal area.

Fire Severity and Tree Seedling Establishment in Abies Magnifica Forests, Southern Cascades, Oregon

To determine the relationship between fire severity and tree seedling establishment, we investigated landscape pattern and composition, fire history, and tree seedling establishment on recent natural burns in Shasta red fire (Abies magnifica var. shastensis) forests of Crater Lake National Park in the southern Oregon Cascade Range. Fire severity classes, defined by basal area mortality, were mapped on three burned sites (29‐197 ha). Variation in fire severity contributed to a landscape mosaic of patches with varying size, shape, forest stand structure, and tree species' composition. Post‐fire tree regeneration was numerically dominated by red fir in all fire severity classes, but seedling establishment and growth varied among severity classes. Density of red fir seedlings was highest in low‐ and moderate‐severity patches, and lower in high‐severity patches and unburned areas. Height growth of red fir seedling increases with fire severity. Variations in fire severity in red fir forests appear to be a major influence on post‐fire stand structure and species composition, successional pathways, and resulting landscape patterns. Our results suggest that drought may be an important mortality agent controlling red fir regeneration on these burned sites. Red fir seedling establishment occurs in pulses, usually 3‐4 yr after fire.

Conversion of dense lodgepole pine stands in west-central British Columbia into young lodgepole pine plantations using prescribed fire. 2. Effects of burning treatments on tree seedling establishment

The ecological effects of different treatments used to convert dense logdepole pine (Pinusconforta Dougl.) stands into young lodgepole pine plantations are being determined. The treatments used were bulldozing the trees and either broadcast burning the slash or bulldozing the slash into windrows, which were then burned. Burns were conducted under different fuel moisture conditions and state of fuel curing to achieve four classes of fire severity. Lodgepole pine seedling survival was affected by both site preparation and fire severity. Five years after outplanting, survival was significantly (p < 0.05) greater for areas between windrows (81%) than for areas beneath windrows (65%) or for broadcast-burned areas (67%). Survival was greatest after 5 years for low-severity burns in fresh slash (80%) compared with low-severity burns in cured slash and higher severity burns (67–69%). Seedling total height and height increment 5 years after outplanting were significantly greater (p < 0.05) in areas beneath windrows than in areas between windrows or in broadcast-burned areas. However, stem diameter was similar among all site preparation treatments. Fire severity had no effect on lodgepole pine total height, height increment, and basal diameter growth in any year after outplanting. Lodgepole pine seedling foliar nutrient levels were generally consistent with trends in seedling growth in that foliar concentrations generally tended to be higher when growth was higher (in the case of site preparation); however, no significant differences in growth were found (in the case of fire severity). This suggested that foliar nutrient levels could at least partly explain seedling growth results. Surface mineral soil (0–15 cm) chemistry exhibited similar trends to seedling foliar chemistry when site preparation treatments were compared, but not when fire severity classes were compared. This was attributed to foliar chemistry being controlled by factors other than soil chemistry alone.

Comparing the Prescribed Natural Fire Program With Presettlement Fires in the Selway-Bitterroot Wilderness

The severity and extent of recent fires (1979-1990) were compared with that of presettlement fires (pre-1935) by eight major forest types in the Selway-Bitterroot Wilderness (SBW) in Idaho and Montana. Presettlement fire intervals were determined for estimating area burned. Presettlement annual area burned for the entire SBW was 4,290 ha by stand replacement fire and 4,310 ha by nonlethal understory fire. All recent period fires>4 ha were mapped using a GIS system. Area burned was determined by the fire severity classes: crowning, lethal surface, nonlethal surface, and no burning. Annual area burned averaged 2,780 ha by stand replacement and 2,270 by understory fire. Area burned during presettlement was 1.7 times that during the recent period. During the presettlement period, area burned by stand replacement and nonlethal understory fire was 1.5 and 1.9 times greater respectively than during the recent period.