Wildfire, Climate and Geomorphic Response
Jen Pierce – 1-20-2015
Increasing temperatures and drought in the western US have driven increases in the size and frequency of wildfires; in the last decade, the frequency of large fires (>1000 acres) on federal lands has increased over five times, and the area burned in fires has increased over tenfold. The forests and rangelands of the Greater Yellowstone Ecosystem are especially prone to wildfire; climate change is projected to increase summer temperatures and decrease summer precipitation in the west, and a drier, warmer, and more variable climate will increase the risk of stand-replacing fires.
Ignitions are often human-caused and frequently occur in the Wildland Urban Interface (WUI), where life and property coexist with inherently fire-prone ecosystems. Along with the direct effect of fire, communities and infrastructure are also at risk from post-fire floods and debris flows. Following fires, reduced cohesion on hill slopes, combined with water repellant soils, make watersheds susceptible to large erosional events, even during average storms. Many regions and major cities in the western US, including the Colorado Front Range, southern California, Reno, Albuquerque, Salt Lake City, and Boise, are located at the base of mountain fronts. These locations provides scenic and valuable home sites within the foothills of the WUI; however, the combination of high fuel loads, ample ignitions, and steep topography put these communities at risk for fires and post-fire debris flows.
While infrastructure and alert systems are in place to warn residents about threats from hurricanes, floods and tornados, there is limited protection for communities in the ‘fire-plain.’ Part of this lack of preparation may stem from the belief that fires can be prevented or stopped; a perception that has been perpetuated by ‘Smokey Bear,’ and the generally successful interval of fire suppression during the 1960’s-1980’s. However, in the mid-1980’s, severe drought, rising temperatures, and early snowmelt have brought an era of ‘mega-fires’ to the American West.
An examination of the paleo-fire record shows that large, stand-replacing fires are not unique to recent decades. Records of fires and fire-related debris flows have been reconstructed from alluvial charcoal records in sites throughout Idaho, Yellowstone, and the southwest. These studies, which include a range of ecosystems from high elevation mixed conifer forests, mixed ponderosa pine and Douglas fir forests, and low elevation sagebrush steppe, reveal a complex history of fire over the Holocene (past 10,000 years). Importantly, these records show that ponderosa pine ecosystems burn ‘at both ends of the spectrum’; they burn frequently in low-severity surface fires, but also have burned severely in the past during times of prolonged drought. In drier systems (such as the Snake River Plain) where fuels are limited, fires burn during annual droughts within otherwise wet intervals. In high elevation systems (such as Yellowstone), forests are generally too wet to burn, but do burn severely during times of prolonged drought (such as during the fires of 1988).