Ferocious California fires kill off aliens

By Nora Schultz A mild forest fire has got to be better than a mighty inferno, right? Not necessarily, according to a new study. In California’s chaparral shrublands, at least, the brightest blazes are the most effective at getting rid of invasive species because alien seeds can’t stand the heat. Jon Keeley at the US Geological Survey in Three Rivers, California, and colleagues analysed the relationship between fire severity and plant community structure before and after fires at 250 sites in the chaparral. High-intensity fires tend to occur in areas that have not burned for several decades. A year after these fierce blazes, few alien plants recolonise this site. But if the last fire was recent, more aliens prosper at the site. Keeley’s team found that the reduction in species diversity after huge fires was almost entirely due to the absence of many foreign species. Native shrubs recovered best after strong fires. If the last fire wasn’t long ago, fewer tall and dense native plants will have grown back, says Keeley. The researchers think that the invasive grasses they examined are sensitive to intense fires for two reasons. One, fewer will have thrived before the fire because native plants dominate the site after many years without a burn. Two, unlike the native shrubs that have seeds that withstand and are even stimulated to germinate by fire, alien grass seeds are destroyed. The results, says Keeley, suggest that prescribed shrubland burnings in California should be stopped unless the risks to humans from high intensity fire are very high. “Setting the shrubland on fire just to prevent the next big blaze does more harm than good for native-species diversity,” he says. Thomas Spies, a US Forest Service research scientist based in Corvallis, Oregon, says it is striking how sensitive invasive grasses are to intense chaparral fires, but warns that other forests may react differently. “In the dense forests of the Pacific Northwest, we find that after a severe fire we get lots of aliens, because they are weeds with good dispersal that make the most of the sudden openings in the canopy,” he says. Journal reference: Ecological Applications (DOI:
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