Fire Severity and Ecosytem Responses Following Crown Fires in California Shrublands

Abstract

Chaparral shrublands burn in large high-intensity crown fires. Managers interested in how these wildfires affect ecosystem processes generally rely on surrogate measures of fire intensity known as fire severity metrics. In shrublands burned in the autumn of 2003, a study of 250 sites investigated factors determining fire severity and ecosystem responses. Using structural equation modeling we show that stand age, prefire shrub density, and the shortest interval of the prior fire history had significant direct effects on fire severity, explaining > 50% of the variation in severity. Fire severity per se is of interest to resource managers primarily because it is presumed to be an indicator of important ecosystem processes such as vegetative regeneration, community recovery, and erosion. Fire severity contributed relatively little to explaining patterns of regeneration after fire. Two generalizations can be drawn: fire severity effects are mostly shortlived, i.e., by the second year they are greatly diminished, and fire severity may have opposite effects on different functional types. Species richness exhibited a negative relationship to fire severity in the first year, but fire severity impacts were substantially less in the second postfire year and varied by functional type. Much of this relationship was due to alien plants that are sensitive to high fire severity; at all scales from 1 to 1000 m2, the percentage of alien species in the postfire flora declined with increased fire severity. Other aspects of disturbance history are also important determinants of alien cover and richness as both increased with the number of times the site had burned and decreased with time since last fire. A substantial number of studies have shown that remote-sensing indices are correlated with field measurements of fire severity. Across our sites, absolute differenced normalized burn ratio (dNBR) was strongly correlated with field measures of fire severity and with fire history at a site but relative dNBR was not. Despite being correlated with fire severity, absolute dNBR showed little or no relationship with important ecosystem responses to wildfire such as shrub resprouting or total vegetative regeneration. These findings point to a critical need for further research on interpreting remote sensing indices as applied to postfire management of these shrublands.

Evaluating Spectral Indices for Assessing Fire Severity in Chaparral Ecosystems (Southern California) Using MODIS/ASTER (MASTER) Airborne Simulator Data

“…Knowledge of how well the index relates to fire severity in shrubland ecosystems is essential if the NBR is to be used as a land management tool in California chaparral ecosystems. Results confirm the reasonably good performance of the NBR in chaparral [20,39] and Mediterranean shrublands [18,32,33,42,43]. In studies in other ecosystems than shrublands assessing the correlation between several spectral indices and field data, Epting et al [26] ranked the NBR as the first index in single date and bi-temporal approaches.…”

“…The standard ATCOR4 desert aerosol model was chosen. The visible through SWIR bands (1-25) were processed to surface reflectance, the MIR bands (26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40) were not atmospherically corrected and the thermal bands (41)(42)(43)(44)(45)(46)(47)(48)(49)(50) were atmospherically corrected to surface radiance. The surface radiance of the thermal bands was then separated into surface temperature (T s ) and surface emissivity ( ) using the emissivity normalization method [54].…”

“…The Landsat NBR is used as a post-fire management tool in the USA and Canada, e.g., as operationally used by the Burned Area Emergency Rehabilitation (BAER) teams in the conterminous USA [12]. Numerous studies have demonstrated the usefulness of the index in the North American boreal and temperate regions [11,31,[36][37][38], however, far fewer studies have assessed its effectiveness in California chaparral shrublands [9,20,39], an ecosystem which is highly sensitive to burning [39][40][41]. The few studies in the California chaparral shrublands demonstrated that the NBR is reasonably well related to fire severity, however, none of them conducted an inter-indices comparison.…”

How Robust Are Burn Severity Indices When Applied in a New Region? Evaluation of Alternate Field-Based and Remote-Sensing Methods

“…CBI scores have stronger relationships with remotely sensed burn-severity indices than other common measurements of severity, such as fire-caused change in basal area or tree canopy cover [2]. In some systems, such as boreal forests [30] and chaparral [10], CBI does not capture burn-severity characteristics that have a strong relationship with ecosystem response to the fire, but in conifer forests in the western USA, the CBI corresponds well with field-based measurements of plant injury, fuel consumption, and tree mortality [2,3].…”

“…This classification scheme should be applicable to ecologically similar areas in the Cascade Range, from northern Oregon to southern British Columbia. No empirical validation of the relationship of the two burn severity indices with field data has been conducted in the Cascade Range of Washington and Oregon, although they have been conducted for many different vegetation types across North America [1,2,[7][8][9][10][11][12][13][14]. In the northern Cascade Range, the benefits of using remote sensing to quantify the severity and spatial pattern of fires are particularly strong.…”