Hundreds of miles of blue oak tree cover exclusive to California have vanished. Why?

Katie Camero
·3 min read

Sprinkled along the foothills of California’s Central Valley stand the iconic blue oak woodlands.

Towering up to 80 feet tall and some reaching over 400 years old, the trees are home to one of the most biologically diverse ecosystems in the state. But extreme drought and wildfires are forcing the woodlands into an uncertain future.

A new study conducted by U.S. Geological Survey researchers found that the historic drought of 2012-2016 alone caused nearly 490 square miles of tree cover loss — or the reduction of leaves and branches — in the blue oak woodlands.

That’s about 37% of the entire tree cover loss in the study’s 32-year period.

Put another way, it was a stressful couple of years for the trees.. Blue oak cover loss in 2015 and 2016 was 5.2 and 3.2 times greater, respectively, than the average annual tree cover loss in the entire study period.

Tree cover loss occurred even during periods without forest fires, with the greatest damage done during the driest and hottest years, according to the study published Tuesday in the journal Frontiers in Climate.

The findings don’t necessarily suggest the trees are dead. However these changes are indicators of tree mortality, reduced productivity, the release of carbon dioxide into the atmosphere and overall ecosystem degradation.

Researchers say the “alarming” amount of loss during California’s five-year drought, the hottest in over a century, reveals how vulnerable blue oaks are to extreme climate events, which are expected to become more frequent and severe over the years because of human-induced global warming.

It’s a positive feedback loop of destruction, the team suggests.

“Our findings signal a concerning future where multi-year droughts under elevated temperatures may trigger more severe forest die-off, pathogens, fire regimes and insect outbreaks,” the study reads. “Moreover, a drier and warmer post-fire environment in concert with high severity fires can severely inhibit forest recovery, thereby reducing resilience of ecosystems to fire.”

It’s not all gloom and doom. Researchers say their study can help in “identifying and prioritizing the most vulnerable areas of the woodlands” so experts can adapt conservation strategies as the climate changes.

The map shows the transition from (A) that shows land cover in California in 1985 before wildfires and drought consumed the area to (E) that shows land cover in 2016 post fires and drought, indicating tree cover loss.
The map shows the transition from (A) that shows land cover in California in 1985 before wildfires and drought consumed the area to (E) that shows land cover in 2016 post fires and drought, indicating tree cover loss.

A 2017 study limited to a small section of blue oak woodlands found that after California’s historic drought, 19% of standing blue oaks and 14% of all trees were killed. But little has been known about how wildfires affect the vulnerability of blue oak woodlands.

So, the team combined new data from the U.S. Geological Survey Land Change Monitoring, Assessment and Projection (LCMAP) project with climate and wildfire records between 1985 and 2016.

On top of the nearly 490 square miles of tree cover lost during the drought, the team also found about 238 square miles of tree cover “conditional change,” or the amount of tree cover that has been altered or degraded.

The more severe and frequent droughts become, the more trees die and generate fine fuels that exacerbate and spark more wildfires, the researchers said, as was the case in 2015 when over 880,000 acres burned, seven people died and more than 3,000 structures were damaged in California.

But the researchers are hopeful that fieldwork, together with models and satellite measurements that regularly monitor blue oak tree cover, can inspire new conservation strategies that will not only protect the blue oak woodlands, but also other ecosystems vulnerable to global warming in the West.