New study shows grass and shrubs -- not trees -- help to spread many catastrophic wildfires

Researchers built data sets that show where wildfires burned homes and indicate if those homes were rebuilt. (Map by Radeloff, UW, from Census Bureau, NASA, Forest Service data)

Grasses and shrubs are more often the cause of extensive wildfires, not forest fires "jumping from tree to tree," a new study published in the journal Science shows. "When people think of wildfires, they often think of huge forests burning," Elise Mahon of UW News reports. "But according to new research led by Volker Radeloff, a professor of forest and wildlife ecology at the University of Wisconsin, in the United States, the largest areas near humans burned by wildfires are grass and shrublands, not forests."

To look at the primary cause of catastrophic wildfires, Radeloff looked at areas "where people and wildlands meet, which are known as the Wildland Urban Interface, or WUI, these areas cover about 10% of land in the United States but are home to about one-third of the population," Mahon reports. "As Radeloff explains, many people enjoy living in these places because they like to be near the amenities of nature. But these areas are also hot spots for environmental conflicts like wildfires, the spread of diseases from animals, habitat fragmentation and loss of biodiversity."

In areas with population growth "such as the American sunbelt, more human development is expanding the WUI. Add to that a changing climate with warmer, drier conditions, and the likelihood rises that wildfires will affect humans more frequently," Mahon explains. "While the total area burned by grass and shrubland fires is much larger than forest fires, grass and shrublands are also much more widespread than forests. They burn and move faster than forests, meaning grassland fires can spread to a larger number of homes than a forest fire might."

Regardless of risk, many communities choose to expand. "Researchers found that the risk of wildfire in any kind of WUI vegetation is not deterring the development and rebuilding of homes in areas that have burned in the past," Mahon reports. "That's especially concerning for homes in grassland and shrubland because the vegetation, which can become fuel for fire, recovers much more quickly than a forest would. That means there's more fuel for a fire to burn again in the same grass and shrubland area more frequently."

To help build safer homes in these areas, "Radeloff believes learning from the homes that don't burn would be a step in the right direction for people that choose to rebuild after a fire," Mahon writes. "However, Radeloff says this burden shouldn't be entirely on the homeowner. He believes there is room for policymakers to influence how prepared a community is and where zoning should allow new housing developments."