ALBUQUERQUE (AP) -- The Western United States could be hit by more frequent large forest fires as global warming continues, a study suggests.
``Many people believe the large, stand-replacing fires we've seen in more recent times have been unnatural,'' said University of New Mexico doctoral student Jennifer Pierce. ``What we found is that when you go back to other warm, dry periods in the past, there is evidence of the same sort of large, stand-replacing fires.''
She has spent four years researching charcoal deposits in the soils of central Idaho for evidence of forest fires from 900 to 1200, a global warming period known as the medieval climactic anomaly. The study found fires during those centuries were much larger than those of the Little Ice Age from 1300 to 1850.
Pierce and UNM geology professor Grant Meyer published a paper on the subject in the Nov. 4 issue of the journal Nature. Studies by Meyer in Yellowstone National Park found similar evidence, which he said strengthens his belief that large, catastrophic fires were more common during warmer periods in the past. ``Climate is a major control over fire, and if we want to better plan for future fire activity in the West, I think we should look at what happened in the past,'' Pierce said.
Large fires have increased in frequency over the past two decades in more northern forests in Alaska and northern Canada, which could be part of the general global warming and drought trend, Pierce said.
Previous studies of fire frequency in forests of the West in earlier centuries focused on damaged but still living trees. Those studies, looking back about 500 years, found evidence of frequent smaller wildfires during cooler, wetter temperatures, Pierce said.
Her research carbon-dated charcoal deposits from fires much older than those reflected by damaged living trees. The charcoal was laid down from burnt wood and mudslides during the medieval climate anomaly.
``I would definitely say that with the likelihood of extended and severe drought that these large, stand-replacing fires in the West will increase,'' Pierce said.
Meyer said the medieval climate anomaly was a time of severe fires in Yellowstone and Idaho _ two very different types of forests.
``That indicates that climate is probably a major controlling factor for fires,'' Meyer said.
Pierce said Western fire management strategies generally haven't added the potential effects of global warming into firefighting plans, ``and the administration doesn't necessarily acknowledge that global warming is going on,'' which could be a problem in the future if fires get worse.
The U.S. Forest Service has been criticized for dense forest underbrush, called ladder fuels because they tend to bring flames to the tops of forests, creating catastrophic fires.
However, Pierce said such conditions might be typical during sustained periods of drought.
``Our hypothesis is, with warmer and drier conditions in these Idaho forests you'd have a decrease in understory grass growth and an infilling of young pine and fir trees that could provide those ladder fuels,'' she said. ``Less water during a drought makes it harder for grass to grow, and since the trees don't have to compete with the grass, we tend to get more of them.''
The next step for study would be to look at a wider variety of areas, such as Southwestern ponderosa pine forests or brushland environments, to understand what's going on throughout the West, Pierce said.
``I think we really need to take a closer look at all these systems to establish what normal ranges of fire activity are,'' she said.
Meyer plans wider studies in the future, and said another graduate student plans to investigate charcoal deposits in New Mexico for similar data.