Drivers of burn severity in the northern Cascade Range, Washington, USA

Abstract

I examined the influence of annual climate and topographical complexity on the occurrence, size, severity, and within-fire severity pattern of fires in the northern Cascade Range of Washington, USA. Landsat Thematic Mapper (LTM) data were used to quantify fire 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. My results show that the fire regime of the northern Cascade Range responds to annual climatic variation. Spring snowpack and summer temperature were negatively and positively correlated, respectively, with fire occurrence, and summer temperature was positively correlated with annual area burned, the proportion of landscape burned at high severity, and spatial aggregation of the high-severity class. Nevertheless, the within-fire severity mosaic reflects the underlying topographic complexity. Fires in areas with greater topographical complexity had increased spatial complexity of burn severity. Several recent studies in the western United States have documented a positive relationship between warm and dry climate and annual area burned. The relationship between climate drivers and fire-regime attributes identified in this study—a positive relationship between warm and dry conditions and the proportion of area burned at high severity and spatial aggregation of high severity patches—adds nuance to the climate-area burned relationship documented in previous studies.