Northern Rockies Pyrogeography: an Example of Fire Atlas Utility

[摘要] We demonstrated the utility of digital fire atlases by analyzing forest fire extent across cold, dry, and mesic forests, within and outside federally designated wilderness areas during three different fire management periods: 1900 to 1934, 1935 to 1973, and 1974 to 2008. We updated an existing atlas with a 12 070 086 ha recording area in Idaho and Montana, USA, west of the Continental Divide, 81 % of which is forested. This updated atlas was derived from records maintained locally by 12 national forests and Glacier National Park. Within the cold, dry, and mesic forests that encompass 45 %, 44 %, and 11 % of the fire atlas recording area, respectively, we analyzed 3228 fire polygons (those ≥20 ha in extent and ≥75 % forested). We discovered that both fire extent and the number of fire polygons were greater in the north during the early period and greater in the south during the late period, but in all cases burned in proportion to land area. Over the 9 731 691 ha forested fire-atlas recording area, 36 % of 10 000 randomly located points burned at least once, 7 % burned twice, and fewer than 1 % burned three or more times. Of these same points, disproportionately more burned inside wilderness than outside. These points burned in proportion to land area by forest type and generally by slope, aspect, and elevation. Analysis revealed that despite extensive fires early and late in the twentieth century, area burned was likely still low relative to prior centuries, especially at low elevations and outside large wilderness areas. The fire atlas includes few fires <40 ha, and its perimeter accuracy is uncertain and likely historically inconsistent; even so, the perimeters are georeferenced and, because they include the entire twentieth century, can serve to bridge past and future fire regimes. Fire atlases are necessarily imperfect, but they remain useful for exploring the pyrogeography of modern fire regimes, including how the spatial distribution of fire varied through time with respect to landscape controls, fire management, and climate.

[学科分类] 生态、进化、行为和系统

[关键词] fire atlas;fire history;fire regimes;northern Rocky Mountains;wildfires