July 25, 2012
Tree-ring-derived climate data and analysis from the southern Colorado Plateau indicates that recent large-scale fires in the West are unusual in the long-term record, including during drought periods, leading researchers to conclude that fire-suppression and human activity are contributing factors.
The studied area includes the world’s largest continuous stand of ponderosa pine, stretching from Flagstaff, Ariz. into New Mexico. Large ponderosa pine forests have existed in the area for more than 10,000 years.
The study’s findings were published in The Holocene in an article, A 1416-year reconstruction of annual, multidecadal, and centennial variability in area burned for ponderosa pine forests of the southern Colorado Plateau region, Southwest USA.
Historical climate, forest fire patterns
Researchers constructed and analyzed a statistical model that encompassed 1,500 years of climate and fire patterns to test, in part, whether today’s dry, hot climate alone is causing the megafires, according to study co-author and fire anthropologist Christopher I. Roos of Southern Methodist University, Dallas.
Even when ancient climates varied from each other (one hotter and drier, the other cooler and wetter), the frequencies of year-to-year weather patterns that drive fire activity were similar, scientists found.
Consequently, today’s megafires in the southwestern U.S. are atypical in the historical record. Causes are not only modern climate change, but also human activity over the last century.
“The U.S. would not be experiencing massive large-canopy-killing crown fires today if human activities had not begun to suppress the low-severity surface fires that were so common more than a century ago,” said Roos.
Dense forests, accumulated fuels
Today’s extreme droughts caused by climate change probably would not cause megafires if not for a century of livestock grazing and firefighting, which have created more dense forests with accumulated logs and other fuels.
“If anything, what climate change reminds us is that it’s pretty urgent that we deal with the structural problems in the forests,” said Roos. “The forests may be equipped to handle the climate change, but not in the condition that they’re currently in. They haven’t been in that condition before.”
Fire-scar records & tree-ring data
Earlier research considered forest fire records from 1600 to the mid-1800s — the Little Ice Age — to understand current forest fire behavior. Those studies found that fires during the Little Ice Age occurred frequently in the grasses and downed needles on the surface of the forest floor, but stayed on the floor and didn’t burn into canopies.
However, the Little Ice Age was cooler and wetter than today. The Medieval Warm Period, from A.D. 800 to 1300, when the climate was hotter and drier (like today’s) may be a better period for comparison.
Tree rings are used to calculate a tree’s age and determine wet and dry weather patterns of moisture and drought. The best evidence for fire activity is the scarring on tree rings that indicates the occurrence of fires. While tree-ring data for climate are available for long periods, annual forest fire records don’t yet exist for the Medieval Warm Period.
Roos and Thomas W. Swetnam of the University of Arizona tested the Medieval Warm Period hypothesis by calibrating a statistical model that combined 200 years of Little Ice Age fire-scar data and nearly 1,500 years of climate data derived from existing tree rings, allowing them to predict what the annual fire activity would have been almost 1,500 years ago.
They discovered that the Medieval Warm Period was no different from the Little Ice Age in terms of what drives frequent low-severity surface fires: year-to-year moisture patterns.
Large canopy fires in the late 1800s
“It’s true that global warming is increasing the magnitude of the droughts we’re facing, but droughts were even more severe during the Medieval Warm Period,” said Roos. “It turns out that what’s driving the frequency of surface fires is having a couple wet years that allow grasses to grow continuously across the forest floor and then a dry year in which they can burn. We found a really strong statistical relationship between two or more wet years followed by a dry year, which produced lots of fires.”
“The best way to look at how fires may have varied — if climate were the only driver — is to do this type of modeling,” said Roos.
Fire-scar data for the region go back as far as the 1500s, but are most prevalent during the Little Ice Age period. Fire scientists have analyzed fire-scars from hundreds of trees from more than 100 locations in the Southwest.
Small, frequent fires kept forests healthy
In ancient forests, frequent, small fires swept the forest floor, often sparked by lightning. Larger fires of tens of thousands of acres burned and naturally extinguished. Fuel included grass, small trees, brush, bark, pine needles and fallen limbs on the ground.
“The fires cleaned up the understory, kept it very open, and made it resilient to climate changes because even if there was a really severe drought, there weren’t the big explosive fires that burn through the canopy because there were no fuels to take it up there,” said Roos. “The trees had adapted to frequent surface fires, and adult trees didn’t die from massive fire events because the fires burned on the surface and not in the canopy.”
The pattern of generally small, frequent fires changed by the late 1800s as the transcontinental railroad brought farmers, ranchers, cattle and sheep which grazed the forest floor, consuming the grasses that fueled small fires. Small saplings and brush grew up into dense, mature bushes and trees. Meanwhile, burning by Native Americans for horticulture and hunting, resulting in surface fires, ceased.
By the early 20th century, the U.S. Forest Service made fighting fires a key part of its mission and without continuous fuel, fires on the forest floor were suppressed.
“Many of our modern forests in central Arizona and New Mexico haven’t had a fire of any kind on them in 130 or 140 years,” said Roos. “That’s very different from the records of the ancient forests. The longest they would have gone without fires was 40 or 50 years, and even that length of time would have been exceptional.”