What does a “resilient” forest look like in California’s Sierra Nevada? A lot fewer trees than we’re used to, according to a study of frequent-fire forests from the University of California, Davis.

More than a century ago, Sierra Nevada forests faced almost no competition from neighboring trees for resources. The tree densities of the late 1800s would astonish most Californians today. Because of fire suppression, trees in current forests live alongside six to seven times as many trees as their ancestors did — competing for less water amid drier and hotter conditions. 

The study, published in the journal Forest Ecology and Management, suggests that low-density stands that largely eliminate tree competition are key to creating forests resilient to the multiple stressors of severe wildfire, drought, bark beetles and climate change. 

This approach would be a significant departure from current management strategies, which use competition among trees to direct forest development. 

Defining ‘resilience’

But first, the study asks: Just what does “resilience” even mean? Increasingly appearing in management plans, the term has been vague and difficult to quantify. The authors developed this working definition: “Resilience is a measure of the forest’s adaptability to a range of stresses and reflects the functional integrity of the ecosystem.”  

They also found that a common forestry tool — the Stand Density Index, or SDI — is effective for assessing a forest’s resilience.

“Resilient forests respond to a range of stressors, not just one,” said lead author Malcolm North, an affiliate professor of forest ecology with the UC Davis Department of Plant Sciences and a research ecologist with the U.S. Forest Service, Pacific Southwest Research Station. “‘Resistance’ is about surviving a particular stress, like fire — but there’s a lot more going on in these forests, particularly with the strain of climate change.”

Continue reading the article from UC Davis here.