COS 80-1 - Drivers of Burn Severity Patterns in the Northern Cascade Range, Washington, USA

Wednesday, August 10, 2011: 1:30 PM
13, Austin Convention Center
C. Alina Cansler, College of Forest Resources, University of Washington, Seattle, WA and Donald McKenzie, Pacific Wildland Fire Sciences Lab, US Forest Service, Seattle, WA

We examined the influence of annual climate and topography on fire occurrence, size, severity, and within-fire severity pattern across 14,455 square kilometers of federally managed land in the northern Cascade Range of Washington State, USA. Landsat Thematic Mapper (LTM) data were used to quantify burn severity for all fires greater than 10 ha (n=125) that occurred during a 25-year period (1984-2008). Categorical burn-severity images were developed from an index of burn severity (Relative differenced Normalized Burn Ratio) derived from LTM data and parameterized with data from 639 field plots.


Our results show that fires in the northern Cascade Range respond both to local topographical controls and to large-scale annual climatic variation. Topographical complexity was positively correlated with patch density and negatively correlated with within-fire spatial aggregation, indicating that the within-fire severity mosaic reflects the underlying topographic complexity. Fire size was positively correlated with the proportion of area burned at high severity and with high-severity spatial aggregation, but severity, patch size, and patch interior increase more quickly with fire size in the north-east section of the study area, indicating that the relationship between area burned and spatial pattern depends on the local ecological setting. Warm and dry climate conditions, particularly July temperature, were positively correlated with fire occurrence, annual area burned, the proportion of area burned at high severity, and spatial aggregation of the high-severity class.

The relationships between climate drivers and fire-regime attributes identified in this study add nuance to the relationship between climate and area burned documented in previous research. Our results also indicate that in areas with high topographic complexity, the influence of climate on the spatial complexity of burn severity may be more limited than in areas with lower topographical complexity. A warmer and dryer future climate may override local controls, however, causing the size of fires and the spatial pattern of burn severity not to reflect local variation, but instead to be more homogenous, with larger contiguous areas of high severity. Conversely, the propensity for climate to override local controls is dependent on the local ecological context and the local physiographic setting; the greatest increases in fire size, severity, and spatial aggregation may occur in systems with more homogenous fuel patterns and less complex topography.

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