BioEd Online

Climate change could increase the number of large wildfires in Yellowstone National Park, Wyoming, and the surrounding region in the coming decades. That could change the balance of species in some forests and convert others to meadows or grasslands, says a study published today in the Proceedings of the National Academy of Sciences¹.

In many parts of the western United States, especially those blanketed with conifer forests, raging wildfires are part of the landscape. They are typically large and strike any one spot infrequently, but can burn entire forests, leaving only charred stumps.

The area burned is strongly correlated with low rainfall, severe droughts and high temperature, says Monica Turner, a landscape ecologist at the University of Wisconsin–Madison. Many studies have hinted that climate change will alter the size, number and frequency of wildfires, but no one has known how big the changes will be until now.

Turner and her colleagues developed a model to estimate how climate change might affect the frequency of fires larger than 200 hectares in and around Yellowstone National Park, one of the most pristine and well-studied ecosystems in North America.

The researchers compared temperature and precipitation data between 1972 and 1999 with the frequency of large fires. They used this relationship to predict how fire occurrence and area burned will change from now to the end of the century in the Greater Yellowstone ecosystem, under three different climate models.

Bigger and more frequent

In 1972–99, years with a spring and summer temperature only 0.5° C above the average were rife with large fires, the team reports. That doesn't bode well, says Turner, because by the end of the century average spring and summer temperatures are expected to be between 4.5° and 5.5° C higher than they were between 1961 and 1990.

Although years with no large fires have been common in the past, the team's analyses suggest that by 2050, fires larger than 200 hectares will occur almost every year.

Before 1990, the fire rotation — the amount of time needed to burn an area equal to an entire landscape of interest — was more than 120 years in most of the Yellowstone ecosystem. But the model predicts that by the middle of this century, fire rotation will fall below 20 years for all but the most southeasterly portions of the ecosystem.

There hasn't been such a drastic change in fire regime in the last 10,000 years. Some areas now covered with trees could be transformed into grasslands or meadows, the researchers contend.

However, the exact effects of the change can't be predicted, and the outlook might not be so smoky after all. Cathy Whitlock, an ecologist at Montana State University in Bozeman who studies the effects of fire on the environment, says that some tree species common in Yellowstone are well adapted to fire. For example, the lodgepole pine (Pinus contorta) depends on blazes to help it regenerate.

"I suspect that lodgepole pine will have a happy future in Yellowstone even if fires become more frequent," says Whitlock.