A recent report by UC Davis gave a grim assessment on the health of forests around Lake Tahoe.
The threats facing them include white pine blister rust, an invasive fungus, and bark beetles, an insect that attacks trees. And both of these were exacerbated by the recent drought. From 2015 to 2016, tree mortality more than doubled, from 35,000 to 72,000.
Blister rust has hit sugar pine trees hard. They’re the largest species of pine, with enormous pine cones, and they grow at low elevations.
There is no cure for blister rust, but some trees are naturally resistant. For about 50 years, the U.S. Forest Service has run a program to reforest with pines that are resistant to the fungus. UC Davis scientists are closing in on a way to make the process a lot faster and cheaper.
Named for its rusty look, the blister rust infection starts at the pine needles and works its way through the branch. If the infection makes it to the trunk, a “canker” appears there and eventually spreads around the trunk, damaging the bark so nutrients and water can’t flow through and usually resulting in the tree’s death.
The infection progresses more quickly in young trees because their smaller size means the distance the fungus has to move to the trunk is shorter.
When blister rust ravages an area, some of the trees are unaffected. They are either resistant or just lucky. Between 3 percent to 5 percent of the trees in the forest possess genes that give them resistance to the blister rust.
The U.S. Forest Service collects seeds from trees that might be resistant and grows seedlings from them at a nursery in Placerville to plant at forest restorations, including sites once burned by fires. In the Tahoe Basin, the Forest Service works with the Sugar Pine Foundation to do this work.
It starts by retrieving a few pine cones from the enormous trees. They use a giant slingshot to knock the cones to the forest floor, said Maria Mircheva, executive director of the Sugar Pine Foundation.
They plant the seeds and wait for the seedlings grow and then try to infect them to see whether they’re able to resist the blister rust. This process takes years.
UC Davis researchers David Neale and Charles Langley knew it could go a lot faster if they could test the cones for blister rust resistance genes.
The first step was to sequence the sugar pine DNA. This itself was no small feat.
“It’s the current world record holder in terms of the largest genome … almost 10 times the size of the human genome,” Neale said.
Armed with genetic information, scientists went looking for genes that held the resistance. Neale said they’re on the verge of identifying resistance that’s controlled by one gene in the tree.
But there isn’t just one way for the tree to have the ability to resist blister rust. They’re also working to find a set of a few genes that provides a different path for sugar pines to thwart blister rust.
Neale likens the ability to screen for these genes in the pines to using a rapid diagnostic test like those in human medicine.
“You could snip a pine needle off a tree and five minutes later, it would tell you whether it was resistant … we’re just about ready to deliver that diagnostic.”
Not having to plant the seeds could save the Forest Service 15 years or more and tens of thousands of dollars for each tree, said Amanda De La Torre, a UC Davis scientist.
At the site of the Little Valley Fire, the Sugar Pine Foundation recently planted new seedlings where everything was charred black or brown.
“It just really feels like you’re creating life when you plant seedlings in the ashes,” Mircheva said.
And using genetics could make the process of getting trees to places like these a whole lot faster.