When a severe fire burns down a Sierra Nevada forest, a common practice by forest managers is to kill the shrubs that soon start to grow, through herbicide applications. The goal is to help newly planted conifer seedlings (pines, etc.) grow and survive, and not be crowded by faster-growing shrubbery.
But killing the shrubs might actually be upping the wildfire risk for the recovering forest, putting all seedlings in danger. That's according to findings by WERC researchers Thomas McGinnis, Jon Keeley and their colleagues, presented in October at the 3rd Fire Behavior and Fuels Conference in Spokane, WA.
McGinnis and others studied areas that had experienced major burns ranging from 4 to 21 years ago, at Sequoia, Tahoe, Eldorado and Stanislaus National Forests. They wanted to know whether the post-fire practices such as applying herbicides had significant effects on subsequent fire risk, local plant life and available "dead fuel" loads -- the amount of accumulated dead plant matter that could feed a wildfire.
Using a computer model based on fire and forest data, the researchers found that that herbicide treatments in burn areas can lead to an overabundance of nonnative grass cover and nonnative grass species in a short period of time.
Although shrub growth is suppressed, the resulting grasses present a new danger -- increased risk of fire ignition. Fires fueled by either shrubs or grasses can wipe out a planation of seedlings, but nonnative grasses are more likely to be ignited, especially in areas with human activity.
"Spraying a minimum area around each tree for shrubs -- rather than an entire hillside -- might result in better protection for young conifers," says McGinnis. "You have to get young trees past their first few decades, when they're too small to withstand any fires. So during those years, minimizing ignition risk should be a priority, rather than minimizing potential fire intensity."
-- Ben Young Landis