Increasing western US forest wildfire activity: sensitivity to changes in the timing of spring

Abstract

Prior work shows western US forest wildfire activity increased abruptly in the mid-1980s. Large forest wildfires and areas burned in them have continued to increase over recent decades, with most of the increase in lightning-ignited fires. Northern US Rockies forests dominated early increases in wildfire activity, and still contributed 50% of the increase in large fires over the last decade. However, the percentage growth in wildfire activity in Pacific northwestern and southwestern US forests has rapidly increased over the last two decades. Wildfire numbers and burned area are also increasing in non-forest vegetation types. Wildfire activity appears strongly associated with warming and earlier spring snowmelt. Analysis of the drivers of forest wildfire sensitivity to changes in the timing of spring demonstrates that forests at elevations where the historical mean snow-free season ranged between two and four months, with relatively high cumulative warm-season actual evapotranspiration, have been most affected. Increases in large wildfires associated with earlier spring snowmelt scale exponentially with changes in moisture deficit, and moisture deficit changes can explain most of the spatial variability in forest wildfire regime response to the timing of spring.This article is part of the themed issue 'The interaction of fire and mankind'.

Keywords: climate; forest; wildfire.

Figure 1.

Human and lightning-ignited annual large forest fires, (a), grass and shrubland fires (b), forest burned area (c), and grass and shrub burned area (d), on Forest Service, Park Service and Indian Lands in the western US. Horizontal lines indicate decadal averages.

Figure 2.

Generalized Pareto distributions fit to log(fire size), by decade for forest fires (a), and grass and shrub fires (b).

Figure 3.

(a) Annual frequency of large (> 400 ha) western US forest wildfires (bars) and mean March through August temperature for the western US (line). Spearman's rank correlation between the two series is 0.69 (p < 0.001). (b) First principle component of centre timing of streamflow in snowmelt-dominated streams from Westerling et al. [1] (dashed line), and updated through 2012 (solid line). Low (pink shading), middle (no shading) and high (light blue shading) tercile values indicate early, mid- and late timing of spring snowmelt, respectively. (c) Annual time between first and last large-fire discovery and last large-fire control. This figure is an updated version of a previously published figure [1].

Figure 4.

Annual burned area by coarse vegetation type and snowmelt tercile for USFS, NPS and BIA wildfires (1973–2012) (a), and BLM wildfires (1980–2012) (b).

Figure 5.

(a) Standardized per cent change in forest-area weighted moisture deficit (Aδ) from early versus late snowmelt years by forest area and elevation plotted against mean snow-free season and April–August AET; legend: point colour indicates forest area, shape indicates elevation in 300 m bands and size indicates Aδ in standard deviations; (b) scatter plot of early snowmelt year minus late snowmelt year wildfires versus Aδ with regression fit to exp(Aδ) (line); (c) map of western US forest area: shading indicates elevation, colour indicates forest region.