High biodiversity oak woodlands like these in Humboldt County, California are imperiled by fire exclusion. Photo by Morgan Varner.
Oak woodlands are beautiful settings, and typically the most biologically diverse ecosystems in the Pacific Northwest. But they depend on frequent, low-severity fire to maintain their characteristic open canopies. After a century of land use change and exclusion of frequent fire, California’s oak woodlands have been disappearing. The main threat has been the invasion of competing conifer trees.
Morgan Varner studies the consequences of reintroducing fire to fire-excluded forests such as oak woodlands. He and several colleagues investigated the response of California black oak to repeated wildfires in Lassen National Forest, California. They examined shifts in tree community composition following two mixed-severity wildfires that burned through the same area in 2000 (Storrie Fire) and again in 2012 (Chips Fire).
They found that stand-replacing wildfire can trigger shifts in canopy dominance from conifers back to oaks. After the second fire, oaks in previously burned areas underwent vigorous regrowth, with a more robust response in the plots that burned under higher severity. At moderate and high reburn severities, conifer seedlings suffered complete mortality. In contrast, 97 percent of topkilled oaks resprouted from surviving rootstocks, reinforcing oak dominance.
However, the results of their study suggest that in California’s fire-excluded oak woodlands, a single wildfire, or two recurring wildfires, may be insufficient to restore historical oak woodland structure and function. In the past, black oak woodlands were regularly burned by tribes to maintain this cultural keystone species. But altered fire regimes make a return to frequent low-severity fires unlikely. As this study reveals, management of these biologically diverse oak stands will likely require supplemental fuel reduction treatments, including prescribed burning.