New map IDs epicenter of Sierra Nevada tree mortality

The drought has killed an estimated 100 million trees in California since 2010. But it’s been uneven – some areas have been decimated, while others are unscathed. A new study tries to answer the question: What makes some trees more likely to die than others?

Researchers at the University of California-Davis sought to answer that question by first making a “heat map” of tree mortality in California. They used data from 2015 U.S. Forest Service aerial surveys to determine how many trees had died in a given area, then color-coded that area by how widespread the mortality was. Red means more trees died, light green means none did, and areas that are not inside the squares were not surveyed by the Forest Service.

Then they examined two factors: climate and forest structure.

The researchers looked specifically at how much water stress trees were experiencing, something that is determined by how hot and dry it is. When they ran their analysis, they found tree mortality was substantially higher in hotter, drier areas. That explains why the low elevation forests and those in the Southern Sierra Nevada were hit so much harder than higher elevation forests and those in the Northern Sierra, both of which are cooler and wetter. Indeed, in low elevation forests in the southern part of the range, there are more than 20,000 dead trees per square mile.

The second factor the researchers examined was how crowded the forest was. Was it open and park-like, or was it dense and overgrown with lots of small spindly trees? They found that in crowded forests with more, larger trees, each tree had access to less water, and mortality was higher.

Not surprisingly, forests that were both hot, dry and crowded were the epicenter of the tree mortality epidemic.

“You really get slammed when both factors are there at the same time,” said lead researcher Derek Young, a doctoral student at UC Davis.

But forest structure, unlike climate, is something forest managers can do something about. The researchers suggested using prescribed burns or selective logging to remove small trees before a drought in order to prevent big trees from dying during the dry spell. This may also help reduce the risk of severe wildfire.

Drought itself may also help thin the forests as millions of trees die. But according to co-author Jens Stevens, drought isn’t thinning out the forest the same way a forest manager would.

“It is impacting trees you’d want to leave and keep alive,” he said. Droughts kill both large and small trees instead of just removing spindly undergrowth.

The researchers say they hope their study will be used by forest managers to decide where to focus these kinds of preventative treatments. For example, managers could look for areas that are hot, dry and dense.

But U.S. Geological Survey forest ecologist Nate Stephenson, who was familiar with the work but not involved with it, said even more high resolution information may be necessary to truly be effective. Land managers want to know what slope or side of the road to burn, he said, not necessarily which type of forest.