Yellowstone Fires of 1988 - Contributing Factors of The Fires

Contributing Factors of The Fires

Formal fire recordkeeping in Yellowstone began in 1931 when the Heart Lake Fire burned 18,000 acres (73 km2). Despite its small extent, it was the largest fire in the period from when the park was created until 1988. Research indicates that only 2 or 3 large fires occur on the Yellowstone Plateau every 1,000 years. The previous major Yellowstone fire was in the early to mid-18th century, well before the arrival of white explorers.

Yellowstone forests are dominated by the lodgepole pine and are eventually replaced by other tree species after 80 to 100 years as a forest matures. In Yellowstone, a very short growing season due to high altitude and relatively poor soils allows the lodgepole pine to reach ages approaching three hundred years before other trees such as Engelmann spruce and subalpine fir become established. The lodgepole pines found throughout the Yellowstone Plateau high country grow in uninterrupted dense stands averaging similar ages in various groupings. Though trees normally found in more mature forests do occur, they are not found in large stands as in other forests in the Rocky Mountains. By the 1980s much of the lodgepole pine forests in Yellowstone were between 200 and 250 years old and approaching the end of their lifecycle.

The mountain pine beetle killed a number of trees in the Greater Yellowstone Ecosystem from the late 1960s through the mid-1980s. This created a heterogeneous forest in which old surviving trees were mixed with younger trees of various ages. This mixed stand structure may have provided ladder fuels that contributed to the fires. In addition, the winter season of 1987–1988 was drier than usual, with the Greater Yellowstone Ecosystem receiving only 31% of the normal snowpack. However, April and May 1988 were very wet and the abundant moisture greatly aided grass and understory development. By June, the rainfall stopped and little was recorded in the entire Greater Yellowstone Ecosystem for the next four months. By July, the worst drought in the history of Yellowstone National Park began. Grasses and plants which grew well in the early summer soon turned to dry tinder. Relative humidity levels fell further, desiccating the forest. Fuel moisture content in dead and fallen timber was recorded as low as 5%. By mid-August, humidity levels were averaging below 20% and were recorded as low as 6% on one occasion. To compound the lack of rainfall, the majority of Yellowstone's soils are rhyolitic volcanic rocks and soils which have poor moisture retention. A series of strong but dry storm fronts also led to the rapid spread of a number of the largest fires.

Accumulated fuel, old forests, and unabated and exceptionally dry conditions spelled trouble for Yellowstone. However, foresters and fire ecologists predicted a normal fire season for the Greater Yellowstone Ecosystem and anticipated normal rainfall for July. History provided little evidence to suggest that 1988 would be any different than the previous 100 plus years the park had existed. But, as major fires started to break out throughout the Rocky Mountain region, the media began to take notice. Twenty small fires started in Yellowstone by July and of those, eleven went out on their own. The remainder were closely monitored in accordance with the prescribed natural fire policies. By July 15 fires throughout the entire Greater Yellowstone Ecosystem had burned 8,500 acres (34 km2) and though this was not out of the ordinary, media focus on fires raging throughout the American west influenced the decision by the park officials to initiate fire suppression efforts on July 15. Within a week after suppression efforts commenced, the fires had spread to include almost 100,000 acres (400 km2) on the parkland alone.