Drought, Ocean Cycles and Big Subalpine Fires in Rocky Mountain National Park

Large-scale climate patterns play an important part in the park's fire regime. NPS.

The question

Climate shapes fire by creating fire-prone conditions such as drought. Knowing how fire has tracked climate helps predict how climate change may alter wildfire.

The study asked how large fires in subalpine forests relate to:

1. drought;
2. broad climate patterns driven by sea surface temperatures (SSTs), which swing between warm and cool phases — the El Niño–Southern Oscillation (ENSO) in the tropical Pacific, the Pacific Decadal Oscillation (PDO) in the north Pacific, and the Atlantic Multidecadal Oscillation (AMO) in the north Atlantic;
3. how those ocean temperatures vary over short (one to two years) and long (100+ years) time scales.

The research

Jason Sibold and Thomas Veblen of the University of Colorado at Boulder used dendroecology — tree cores and cross-sections of fire scars — plus vegetation mapping to reconstruct the fire history (the year and size of past fires) from 1650 to the present for 74,000 acres of subalpine forest in the southern part of the park.

They compared it with tree-ring width records. Ring widths track regional rainfall and temperature closely, so they are a good proxy for drought. The researchers tested big-fire years against drought, the ocean patterns, and all eight combinations of the three oscillations' phases.

How ocean temperatures affect Rocky Mountain climate

Findings

• Large subalpine fires happen in years of extreme regional drought, especially drought driven by these broad climate patterns.
• They do not depend on unusually wet years beforehand. In some lower forests, wet years grow the grasses and other fine fuels that fires need.
• In the park's subalpine forests, large fires depend only on severe drought in the fire year.
• That is the opposite of the lower-elevation ponderosa pine forests, where fires typically follow wet periods that build up fuel.

Sources

Based on "Subalpine Forest Fires and Climatic Variation," Rocky Mountain National Park, National Park Service, summarizing research by Jason Sibold and Thomas Veblen, University of Colorado at Boulder; a work of the United States government in the public domain. The ocean-temperature table was published as an image and is set here as text.