In light of the increasingly severe and frequent wildfires in western U.S. forests, low-density forests could have enhanced resilience to stressors.
A study published in the journal Forest Ecology and Management on Jan. 18 found that the lack of competition between trees may be linked to their ecological resilience to disturbances such as recurrent fires, periodic severe droughts, insects and pathogens.
“So far, (resilience) has been so nebulous — it’s just kind of a concept,” said campus professor and study co-author Scott Stephens. “But if you want to quantify it, like we did in this paper, I think it helps people understand how to achieve (resilience).”
The study defines resilience as the ability of frequent-fire forests, or forests that have at least one fire every 30 years, to retain their inherent structure, composition and functional integrity in response to stressors.
According to Stephens, their research found that relative stand density index, or SDI, is an effective tool to help managers operationalize resilience in western U.S. frequent-fire forests. SDI has long been used for timber productive growth to measure the forest’s competitive environment since the 1940s, but has not historically been used to assess forest resiliency, Stephens added.
This study compared contemporary forest conditions to two historical datasets in the southern and central Sierra Nevada conducted in 1911. These data sets came from before the U.S. started to take wildfires “out of the system” through suppression and fuel reduction.
Additionally, the study found that between 1911 and 2011, tree densities increased by a factor of six to seven times on average while tree size was reduced by half.
The historical forests they studied had a lower density of trees, and consequently, there was less competition for resources between trees. In contrast, two-thirds of the contemporary forests they studied are in a “really severe competitive condition,” according to Stephens.
Stephens said the result not only applies to the Sierra Nevada but also all mixed conifer and ponderosa pine forests, among the most common forest types in California.
“Current management practices often prescribe conditions that maintain full competition to guide the development of desired forest conditions,” the study states. “Creating stands largely free of competition would require a fundamental rethinking of how frequent-fire forests can be managed for resilience.”
Stephens said restoration prescriptions since the 1990s are not “intense enough” to take on climate change and fire. He added that he would like to see restoration treatments be more aligned with future conditions such as climate change.
The number of trees removed and the number of prescribed fires have to change the forest condition today much more substantially than what is currently being done, Stephens said.
“When you look at what’s happening, right out the window anymore in terms of what’s happening to California forest,” Stephens said. “We’ve got to change the way we’re doing business because we are really in deep trouble.”